analysis of streambank erosion along the lower tombigbee river

Transcription

Watershed Physical Processes Research Unit
National Sedimentation Laboratory
Oxford, Mississippi
ANALYSIS OF STREAMBANK EROSION ALONG THE
LOWER TOMBIGBEE RIVER, ALABAMA
By Natasha Bankhead, Andrew Simon and Danny Klimetz
USDA-ARS National Sedimentation Laboratory
Research Report Number 62
May 2008
Prepared by
U.S. Department of Agriculture – Agricultural Research Service
National Sedimentation Laboratory
Watershed Physical Processes Research Unit
For
Alabama Clean Water Partnership
May 2008
Stability Analysis: The Lower Tombigbee River
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ARS Designated Representative and Project Manager:
Carlos V. Alonso
Technical Direction, Data Analysis:
Andrew Simon and Natasha Bankhead
Report Preparation:
Natasha Bankhead and Andrew Simon
Mapping, GIS and Interactive CD:
Danny Klimetz
Field Data Collection and Data Processing:
Brian Bell, Edward Jennings, Ibrahim Tabanca, Danny Klimetz, Lauren Klimetz,
and Lee Patterson
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EXECUTIVE SUMMARY
This study presents a preliminary evaluation of bank-stability conditions along the Lower
Tombigbee River. Landowners and local agencies have been aware of significant erosion
of streambanks in several Alabama counties along the Lower Tombigbee River, since the
early 1980’s. Bank erosion has led to loss of land and property, and so in September
2005, the Alabama Clean Water Partnership began a project to focus on these issues of
land loss along the Lower Tombigbee River, to improve understanding of potential
causes and mitigation strategies. The overall objective of this study was to evaluate the
extent and severity of streambank erosion along the Lower Tombigbee River between
river miles 72 and 259. More specifically, the study aimed to identify locations of bank
instability and determine rates and magnitudes of channel changes. A sub-objective was
to determine under what conditions streambanks become unstable (critical conditions),
and to investigate the timing and frequency of such conditions.
The Tombigbee and Black Warrior Rivers have undergone significant changes since
1895, involving three separate phases of modifications and periods of dam construction
(original 17 locks and dams on the Tombigbee-Black Warrior system, modernization of
these locks and dams between 1937 and 1965, and finally the construction of the TennTom Waterway in the 1970’s and early 1980’s). Morphological changes of a river tend to
occur most rapidly after a disturbance, with the effects diminishing with space and time.
As such, any changes that are currently occurring within the study reach could be a result
of channel responses to a combination of all the previous channel modifications that have
occurred. Effects caused by the older dams may now be more muted than the effects of
more recent disturbance. However, distinguishing potential effects of the TenneesseeTombigbee Waterway on the study reach of the Lower Tombigbee River may be difficult
as the flow of water and sediment, and resulting channel morphology have already been
affected and continue to be affected by the impoundments in the system that pre-date the
Tenn-Tom Waterway.
To learn more about the Lower Tombigbee River in its current state, it was important to
examine the present geomorphology of the river bed and banks, and where necessary, to
collect geotechnical data regarding the resistance of the bank material for stability
analysis. In addition, available USGS gage data were analyzed along with historic sets of
aerial photographs, to establish past conditions, and rates and patterns of change in the
river system. Magnitudes and extents of bank erosion in this study were determined over
a 29-year period (1974-2003) based on interpretation of a series of aerial photographs
with resolutions ranging form 0.5 meters to 1.5 meters. Banks that have recently failed
were determined by aerial reconnaissance of the reach and represent more than 50% of all
banks. Some reaches had widened as much as 60 m over the period but rates show a clear
spatial distribution upstream and downstream of the dams. Rates generally are low in
reaches just upstream of the structures because water is held, thus providing a confining
force that acts to support the bank. Maximum rates occur some distance downstream
from each structure where post-dam channel incision has increased bank heights. An
increased rate of average and peak flows over the period, similarly contribute to greater
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instability. The increase in flows, however, seems to be the result of a trend of increasing
precipitation over the last century. Operation of the dams has shifted the timing of flows
by storing water during the spring so it is available for navigation in the summer months.
The highest flows generally occur in the winter months now. Still, the average rate of
channel widening was determined to be about 1.2 m/y over the entire study reach (river
miles 72-259). This rate is in excess of the rate that would be expected in stable, alluvial
streams, where bank erosion would be negligible. This rate of widening also represents a
loss of 2580 acres of land adjacent to the channel over the 29-year period from 1974 –
2003.
Five reaches (at RM 75.3, 114.7, 161.9, 190.9 and 239.7) were also investigated in detail
to determine critical conditions for bank instability and the frequency that these
conditions occur. Geotechnical and erodibility tests of the bank materials were conducted
to provide data for simulations of bank stability using the Bank-Stability and Toe-Erosion
Model (BSTEM) developed by the National Sedimentation Laboratory. Critical
conditions occurred when water table heights were high. The flow percentiles associated
with the simulated critical water-table heights all represent relatively high flows. In
general though, the critical conditions occur with discharges that are exceeded about 20%
of the time, representing a fairly frequent occurrence. The reinforcing effect of top bank
grasses resulted in slightly less frequent, higher critical discharges compared to
simulations without any vegetative cover. The effect of mature, woody vegetation on
bank-stability was shown to be much more pronounced, and for two sites led to a
situation where no critical conditions existed (RM 75.3 and RM 239.7).
Results of a more detailed set of modeling runs for the site at RM 114.7 were carried out
using a discretized annual hydrograph for a particularly wet year (1991). The annual
hydrograph was used to model iteratively toe erosion and bank stability over a year, to
estimate the volume of material eroded and the number of failure events under i) current
conditions, and ii) under a series of mitigated conditions, representing a broad range of
options and costs. These results indicated that bank failure frequency and failure volumes
cannot be eradicated entirely, but can be significantly reduced, from 11 failures to 3, and
from almost 55,000 m3 to about 3,200 m3 for the year modeled, depending on the
mitigation strategy selected. Recognizing that these findings represent only one site, we
feel confident that similar results would be obtained at other sites within the study reach.
The fact that banks are particularly high, and that high flows are maintained in some
reaches by structures exacerbates the bank-stability problems. Areas of dredging would
also be less prone to stabilization because of renewed deepening of the channel in these
reaches.
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CONTENTS
EXECUTIVE SUMMARY ................................................................................................ ii
CONTENTS....................................................................................................................... iv
LIST OF ILLUSTRATIONS............................................................................................. vi
LIST OF TABLES........................................................................................................... viii
UNIT CONVERSION TABLE ......................................................................................... ix
LIST OF ABBREVIATIONS AND UNITS ...................................................................... x
1. INTRODUCTION, PURPOSE AND SCOPE................................................................ 1
1.1 A summary of effects of dams on channel forms and processes ............................. 2
1.2 Study objectives ........................................................................................................ 4
2. STUDY AREA ............................................................................................................... 5
2.1 A History of Channel Modifications on the Tombigbee River................................. 6
2.1.1 Development of the Black Warrior-Tombigbee Waterway............................... 6
2.1.2 Development of the Tennessee-Tombigbee Waterway ..................................... 6
2.1.3 Summary ............................................................................................................ 7
2.1.3 Geology............................................................................................................ 10
3. FUNDAMENTALS of BANK STABILITY................................................................ 11
4. METHODOLOGY ....................................................................................................... 13
4.1 Air Reconnaissance Survey .................................................................................... 13
4.1.1
Rapid Geomorphic Assessments: RGA’s ................................................. 13
4.2 Analysis of Geographic Trends of Channel Conditions using Aerial Photography
and Geographical Information Systems (GIS) Layers.................................................. 16
4.3 Gauging-Station Analysis ....................................................................................... 16
4.3.1 Specific Gage Analysis using Water-Surface Elevation.................................. 16
4.3.2
Analysis of Discharge Records................................................................. 17
4.3.3 Analysis of Precipitation Data ......................................................................... 19
4.4 Testing of Bank Materials....................................................................................... 20
4.4.1
Geotechnical Data Collection – Borehole Shear Tests ............................. 21
4.4.2 Hydraulic Forces and Resistance ..................................................................... 22
4.5 Quantifying streambank stability: The Bank Stability and Toe Erosion Model
(BSTEM)....................................................................................................................... 23
4.6 Determining the timing and frequency of critical conditions, and modeling rates of
bank retreat.................................................................................................................... 24
5. RESULTS .................................................................................................................... 26
5.1 Trends in Flow and Precipitation............................................................................ 26
5.1.1 Total Annual Discharge ................................................................................... 26
5.1.2 Mean-Annual Precipitation and Water Yield ................................................. 28
5.1.3 Timing and Distribution of Flows.................................................................... 29
5.1.4 Variations in Annual-Peak Discharge.............................................................. 33
5.2 Channel Changes at Gauging Stations.................................................................... 34
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5.3 Changes and Trends in Channel Width .................................................................. 35
5.3.1 Locations and amounts of dredging in the study reach.................................... 40
5.3.2 Explanations for trends and locations of high bank erosion. ........................... 41
5.4 Bank-Stability Analysis .......................................................................................... 42
5.4.1 Determining Critical Bank-Stability Conditions ............................................. 43
5.4.2 Frequency and duration of high-flow events and consequences for bank
instability................................................................................................................... 47
5.5 Simulations of Potential Mitigation Techniques to Reduce Streambank Erosion and
Widening....................................................................................................................... 51
6. SUMMARY and CONCLUSIONS .............................................................................. 59
6.1 Suggestions for future work.................................................................................... 60
7. REFERENCES ............................................................................................................. 62
APPENDIX A................................................................................................................... 65
Bank stratigraphy, geotechnical data and channel cross-sections for each site studied in
detail
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LIST OF ILLUSTRATIONS
Figure 1. Location map showing the study reach on the Tombigbee River between River
miles 72 and 259. ........................................................................................................ 4
Figure 2. Present day locations of locks and dams on Tennessee-Tombigbee Waterway
and Black Warrior - Tombigbee Waterway. The study reach for this project is
highlighted in red. ....................................................................................................... 9
Figure 3. Channel stability ranking scheme used to conduct rapid geomorphic
assessments (RGA’s). The channel stability index is the sum of the values obtained
for the nine criteria.................................................................................................... 15
Figure 4. Location of rain gages in the watershed of the Tennessee-Tombigbee River that
were used for precipitation analysis.......................................................................... 19
Figure 6. Schematic representation of borehole shear tester (BST) used to determine
cohesive and frictional strengths of in situ streambank materials. Modified from
Thorne et al., 1981. ................................................................................................... 21
Figure 7. Discretized hydrograph for 90th percentile flow year at Coffeeville Dam,
showing the six steady-state, rectangular shaped flow events modeled in BSTEM. 25
Figure 8. Annual trends in total discharge along the Tombigbee River for specific periods
of record (Top) and since 1961 (Bottom). Note the difference in trends. ................ 27
Figure 9. Precipitation trends for the Tennessee-Tombigbee River Watershed from 1885
to present................................................................................................................... 28
Figure 10. Trends in water yield (discharge per unit of precipitation) for specific periods
of record (Top) and since 1961 (Bottom) along the Tombigbee River. ................... 29
Figure 11. Difference in mean-daily flows pre- and post-waterway for a given calendar
day. Note the increased flows during the winter and the decreased flows during the
spring......................................................................................................................... 30
Figure 12. Distribution of mean-daily flows pre-and post-waterway at Gainesville Dam
(Top), Demopolis Dam (Middle) and Coffeeville Dam (Bottom)............................ 32
Figure 13. Changes in annual-peak flows at Gainesville (upper left), Demopolis (upper
right), and Coffeeville (lower right). Trend lines are displayed for pre- and postwaterway periods, as well as for the entire period of record. ................................... 33
Figure 14. Changes in water surface elevation over time for gage 02449000 Tombigbee
River near Gainesville. This gage represents the upper boundary of the study reach
even though it is upstream of the last site in the study reach.................................... 34
Figure 15. Variations in channel width along study reach................................................ 37
Figure 16. Changes in channel width (five-point moving average) between 1974 and
2003, and percent of reach failing estimated from aerial reconnaissance. ............... 37
Figure 17. Five-point moving average of widening rates along the study reach with the
percent of each reach with recent bank failures........................................................ 38
Figure 18. Raw data on widening rates at 2 mile intervals............................................... 38
Figure 19. Percent of reach failing from River Mile 72 to 259 of the Lower Tombigbee
River.......................................................................................................................... 39
Figure 20. Volume and locations of dredging along the study reach, including locations
of tributary mouths marked in green......................................................................... 40
Figure 21. Generalized trends in widening rates and prospective rationale for why
widening rates vary this upstream and downstream of the structures. The range of
widening rates is also given: Low rates 0.2-0.3 m/y; Hogh rates 2.0 – 3.0 m/y....... 42
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Figure 22. Bank geometry and photographs for sites at river miles 160.8 (Left), 114.7
(Middle) and 190.8 (Right). ...................................................................................... 43
Figure 23. Example of model output from BSTEM for critical bank-stability conditions
(Fs = 1.00) at river mile 114.7. Note that the blue triangle represents the elevation of
the water table. .......................................................................................................... 44
Figure 24. Figure XX. Results of iterative modeling at RM 114.7 for bank condition 1:
Existing conditions with no mitigation. .................................................................... 54
Figure 25. Results of iterative modeling at RM 114.7 for bank condition 2: Rock
placement at the bank toe.......................................................................................... 55
placement at the bank toe and 5-year old woody vegetation on the bank top. ......... 56
placement at the bank toe, 5-year old woody vegetation on the bank top, grading the
bank to a slope of 45o (1:1), and 5-year old vegetation on the bank face. ............... 57
Figure 28. Summary of iterative modeling results for alternative mitigation strategies
showing the number and volumes of failures for each bank condition. ................... 58
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LIST OF TABLES
Table 1. Locks and dams built to replace the original system of 17 locks and dams on the
Tombigbee and Black-Warrior Rivers between 1895 and 1915................................. 6
Table 2. Dams located within or directly affecting the study reach between river miles
259 and 72................................................................................................................... 8
Table 3. Geology of the Tombigbee River Basin (Alabama Clean Water Partnership
Website, accessed, 2008) .......................................................................................... 10
Table 4. List of USGS gages and available data within the study reach on the Lower
Tombigbee River. The gages highlighted in blue were used for the analysis of
discharge. .................................................................................................................. 18
Table 5. Vegetation descriptions and additional cohesion due to roots added to the top
meter of the streambank at each site. ........................................................................ 45
Table 6. Critical bank-stability conditions without vegetation (the general case)............ 46
Table 7. Critical bank-stability conditions with existing vegetation. “-“ denotes there are
no critical conditions (bank is stable). ...................................................................... 46
Table 8. Percentiles of mean-daily flow at gage 02469761, Coffeeville, AL, 1960 – 2007,
compared to the critical discharge for bank failure at sites 75.3 and 114.7 (A);
Percentiles at gage 02467000, Demopolis, AL, 1928 – 2007, compared to the critical
discharge for bank failure at sites 161.9 and 190.9 (B); and Percentiles at gage
02447025, Heflin (Gainesville), 1978 – 2007, compared to the critical discharge for
bank failure at site 239.7(C). Numbers in green represent critical discharge values
with vegetation and numbers in black represent critical discharge values without
vegetation. Note that critical conditions with mature, woody vegetation are not
apparent for sites at river miles 75.3 and 239.7. ....................................................... 49
Table 9. Frequency (in average number of occurrences per year) of critical discharges
maintained for 5- and 10-day periods for simulation cases without vegetation. Data
are separated into pre- and post-waterway and combined for both periods. ............ 50
Table 10. Frequency (in average number of occurrences per year) of critical discharges
maintained for 5- and 10-day periods for simulation cases with vegetation. Data are
separated into pre- and post-waterway and combined for both periods. .................. 50
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UNIT CONVERSION TABLE
METRIC
1 meter
1 kilometer
1 cms
USDA sites located at River Mile (RM)
75.3
114.7
161.9
190.9
239.7
ENGLISH
3.2803 feet
0.6214 miles
35.31 cfs
USDA sites located at River Kilometer (RKm)
121.2
184.6
260.6
307.2
385.8
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LIST OF ABBREVIATIONS AND UNITS
a
BMP
BST
c’
ca
F
Fs
g
k
RGA
RM
Sb
Sr
TTW
USGS
W
β
ε
φ’
φb
γw
µa
µw
ρs
ρw
σ
τ*
τc
τe
τo
ψ
Exponent assumed to equal 1.0
Best-Management Practice
Borehole Shear Test device
Effective cohesion, in kilopascals; kPa
Apparent cohesion, in kilopascals; kPa
Driving force acting on bank material, in Newtons; N
Factor of Safety, a ratio
Acceleration due to gravity, in meters per square second; 9.81 m/s2
Erodibility coefficient in cubic meters per Newton second: m3/N-s
Rapid Geomorphic Assessment
River mile
Bed slope, in meters per meter; m/m
Shear strength, in kilopascals; kPa
Tennessee-Tombigbee Waterway
United States Geologic Survey
Weight of failure block, in Newtons; N
Angle of the failure plane, in degrees
Rate of scour, in meters per second; ms-1
The angle of internal friction, in degrees
The angle that describes the increase in shear strength due to an increase in
matric suction, in degrees
Unit weight of water, in Newtons per cubic meter; 9810 N/m3
Soil pore air pressure, in kilopascals; kPa
Soil pore water pressure, in kilopascals; kPa
Sediment density, in kilograms per cubic meter; 2.65 kg/m3
Water density, in kilograms per cubic meter; 1 kg/m3
Normal Stress, in kilopascals; kPa
Dimensionless critical shear stress
Critical shear stress, in Pascals; Pa
Excess shear stress, in Pascals; Pa
Average boundary shear stress, in Pascals; Pa
Matric suction, the difference between air pressure and water pressure (µa - µw),
in kilopascals, kPa
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1. INTRODUCTION, PURPOSE AND SCOPE
Landowners and local agencies have been aware of significant erosion of streambanks in
several Alabama counties along the Lower Tombigbee River, since the early 1980’s.
Bank erosion has led to loss of land and property. In September 2005, the Alabama Clean
Water Partnership began a project to focus on these issues of land loss along the Lower
Tombigbee River, to improve understanding of potential causes and mitigation strategies.
Channel instability results from a disruption to the dynamic equilibrium (or balance)
between available stream energy and erosional resistance of the bed and bank materials
present. (Lane, 1955; Bull, 1979; Simon, 1995). Disturbances can be classified as direct
(e.g. dam construction, channelization) or indirect (e.g. urbanization, land-use change)
and can be due to natural events or human interference. Alluvial channels respond to
disturbances by altering aspects of their morphology, hydraulic characteristics and
sediment load to attain a new dynamic equilibrium, termed a quasi-equilibrium.
Responses to disturbances may be short (days) or long (centuries) and may be local or
system wide.
Channels can respond to a situation where excess energy is available in the system, by
widening and/or incising. The adjustment of width (streambank erosion) through masswasting processes is an important mechanism of energy dissipation in alluvial streams.
Streambank retreat occurs by a combination of hydraulic scour at the toe of the bank, and
geotechnical failure of the streambank. Streambanks fail when the driving forces acting
on them exceed their resisting forces. Driving forces are controlled by bank height and
slope, the saturation of the bank, and the surcharge imposed by any objects on top of the
bank. Resisting forces are determined by the geotechnical properties of the bank
materials, and the presence of root networks of riparian vegetation. Banks become more
unstable as they become higher, steeper, and more saturated, or as cohesion of the bank
material decreases.
Identifying locations of channel instability is relatively easy in the field, but identifying
the cause of instability can be difficult, and requires multiple lines of investigation and
analysis (Simon, 1995). Analysis of trends in mean daily discharge, peak annual
discharge, precipitation, and bed elevation can be carried out using USGS gage data and
can identify morphologic changes or potential changes to the balance of force and
resistance in the system over a period of time. In addition, it is important to determine the
timing and location of potential disturbances to the fluvial system being studied. In this
case, the Tombigbee River, and one of its main tributaries, the Black Warrior River, have
undergone significant modifications over the past century through the construction of
various locks and dams. In particular, it is important to keep in mind that although parts
of this study aimed to investigate how completion of the Tennessee-Tombigbee
Waterway in December 1984 affected subsequent channel changes along the Lower
Tombigbee River, in fact, the current lock and dam at Demopolis (in the middle of the
study reach) has been in place since 1954; at Coffeeville since 1960. Because these
structures can represent a barrier to water and sediment, downstream channel adjustments
have probably been underway for a long period of time and identifying the cause of
changes since 1984 may not be practical.
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1.1 A summary of effects of dams on channel forms and processes
The regulation of a river through dam construction can significantly affect downstream
discharge and channel morphology in a number of ways. Dams act as sediment traps with
sediment being deposited upstream of the dam in the reservoir, and cleaner, sediment
starved water being released downstream of the dam. Alluvial rivers tend to erode and
lower their beds downstream of a dam (Richards, 1982; Williams and Wolman, 1984),
with the amount of degradation decreasing non-linearly downstream of the dam. The
effect of a dam is also expected to diminish with time. In most cases therefore, maximum
bed degradation is seen directly downstream of the dam, where the energy available for
sediment transport is most out of balance with the pre-dammed system, with the
generalized trend of degradation decreasing progressively downstream of the dam. Local
variations in bed degradation are however likely and variations about this decreasing
trend are often seen (Williams and Wolman, 1984). A regulated river may travel many
kilometers before its suspended sediment load reaches the value upstream of the dam, or
other factors listed above return to an equilibrium.
The downstream effects of a dam can extend until there is 1. local control of bed
elevation (e.g. bedrock), 2. downstream control of base level (ocean, lake, larger river or
another dam), 3. decrease in flow competence (flattening of slope by degradation,
expansion of channel width resulting in decreased depth and redistributed flow
velocities), 4. infusion of sufficient sediment to restore the balance of incoming and
outgoing sediment in a reach, and/or 5. growth of vegetation (Williams and Wolman,
1984). Williams and Wolman (1984) also note that the downstream location of zero bed
degradation ranged from several to about 2 000 channel widths (4 to 125 km) for the data
they examined, depending on factors 1-5 listed above.
Changes in channel width tend to extend further downstream than bed degradation and
are also harder to predict. Channel width can increase, decrease or stay the same
downstream of a dam (Williams and Wolman, 1984), depending upon a number of
factors including but not limited to, the bed and bank materials, flow regime, and type
and density of riparian vegetation. In the case of some rivers, reduction in channel
migration rates have been seen after dam construction (Shields et al., 2002). Friedman et
al. (1998) suggested that the effects of dams on large rivers vary with the pre-existing
planform of the river. As such, regulation of floods by dams tends to cause braided rivers
to narrow, while meandering rivers experience little change in width, but do tend to see a
reduction in migration rates.
Deposition is also a common problem in regulated rivers because regulation of high
magnitude floods artificially slows sediment transport, but sediment emanating from
tributaries may be unaffected. The steeper tributaries commonly therefore introduce
coarse sediments to the main stem of the channel, which are deposited as the tributaries
lose competence on entering the main stem, leading to aggradation. In the absence of
dams, floods on the main stem would periodically rework this aggraded material.
Tributary confluences in regulated rivers are thus often found to be locations of
significant sediment accumulation (Petts, 1984). Tributary sediment yields can also be
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increased following dam closure, due to the lowering of base level in some cases which
can induce tributary rejuvenation (Petts, 1984).
Dams are built for different purposes (e.g. flood regulation, hydropower, water
conservation, navigation), and their effects on the magnitude and duration of flow
releases can therefore vary greatly. In this case, the Tennessee-Tombigbee Waterway
locks and dams and the original dams built on the Black Warrior-Tombigbee Waterway
were designed to allow for easy navigation of the river rather than for flood control.
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1.2 Study objectives
The overall objective of this study was to evaluate the extent and severity of streambank
erosion along the Lower Tombigbee River between river miles 72 and 259. More
specifically, the study aimed to identify locations of bank instability and determine rates
and magnitudes of channel changes. A sub-objective was to determine under what
conditions streambanks become unstable (critical conditions), and to investigate the
timing and frequency of such conditions. In part, this will be examined using historical
flow records obtained from USGS gauging stations in the reach.
Figure 1. Location map showing the study reach on the Tombigbee River between
River miles 72 and 259.
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2. STUDY AREA
The Tombigbee River occupies the western edge of the Mobile River Basin, occupying
an area of approximately 35656 square kilometers (13 767 square miles) within the states
of Alabama and Mississippi (19925 square kilometers (7 693 square miles) in Alabama
and 15 734 square kilometers (6 075 square miles) in Mississippi). Its headwaters emerge
out of the Fall Line Hills and Black Prairie Belt Districts of the Coastal Plain
Physiographic Province adjacent to the Tennessee River Valley in northeastern
Mississippi and northern Alabama, gathering flow from four major rivers: Buttahatchee,
Noxubee, and Sucarnoochee from the west, and the Sipsey from the northeast. The main
stem of the river joins with the Black Warrior at Demopolis, Alabama and then continues
due south to join the Alabama River, south of Jackson, Alabama and drains into the
Mobile River, which flows into the Gulf of Mexico. The Tombigbee River Basin includes
two sub-basin components: Upper and Lower Tombigbee River subbasins. The Upper
Tombigbee River and its sub-basin run mostly in a north-south direction in western
central Alabama. It reaches north from its confluence with the Black Warrior River near
Demopolis, AL well into Mississippi. The Lower Tombigbee River and its subbasin run
from the confluence of the Upper Tombigbee and Black Warrior Rivers in a southerly
direction to the confluence with the Alabama River in southeastern Alabama.
The river provides one of the principal routes of commercial navigation in the southern
United States, navigable along much of its length through locks and connected at its
upper end to the Tennessee River by the Tennessee-Tombigbee Waterway. There are a
total of 12 locks and dams on the Tombigbee River with four of these located in
Alabama. Ten of these 12 dams are a part of the 377 kilomater (234-mile) TennesseeTombigbee Waterway system that connects Pickwick Lake on the Tennessee River with
the Tombigbee River. The two locks in the Tennessee-Tombigbee system located in
Alabama (Belville and Heflin, built in the late 1970s), open the waterway to Demopolis
Lake and the Demopolis Lock and Dam where it meets up with the Black Warrior –
Tombigbee navigational project, which connects the Black Warrior River as far north as
the City of Birmingham. South of Demopolis on the main stem of the lower Tombigbee
River the Coffeeville Lock and Dam constitutes the first impoundment on the Tombigbee
and the Black Warrior Tombigbee project. They all fall under the jurisdiction of the
Army Corps of Engineers and serve navigational and recreational purposes. Heflin lock
and dam is the structure located upstream of the study reach for this project, with
Demopolis Dam and Coffeeville Dam being located within the study reach.
The Tombigbee River basin is located within the physiographic province referred to as
the Coastal Plain. This part of Alabama formed in the shallow waters that have covered
most of the central continent throughout geologic history. Generally, the geology of the
Tombigbee River basin consists of Cretaceous chalk and sediments and clastic sediments
with porous limestone from the Oligocene, Eocene, Paleocene periods (Robinson, 2003).
The official soil of the state of Alabama is the Bama soil series. A typical Bama soil
profile consists of a topsoil of about 0.13 m (five inches) of dark brown fine sandy loam;
a 0.15 m (six inch) subsurface of fine sandy loam; and a red clay loam and sandy clay
loam subsoil to 1.5 m (60 inches) or more. They generally parallel major river systems,
forming in thick deposits of loamy fluvial or marine sediments.
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2.1 A History of Channel Modifications on the Tombigbee River
2.1.1 Development of the Black Warrior-Tombigbee Waterway
The first major modifications to affect the Tombigbee River were carried out on the
Lower Tombigbee subbasin. The Black Warrior River merges with the Tombigbee River
just above Demopolis Dam, and the Tombigbee merges with the Alabama near
Coffeeville to form the Mobile River. It in turn flows into Mobile Bay on the Gulf of
Mexico. These rivers have historically played a vital role in the economic development of
their basins, with explorers, then traders and settlers using the rivers as water highways.
In the nineteenth century, cultivation of cotton and the invention of the paddlewheel
steamboat stimulated trade on the rivers. However, navigation was difficult in places due
to hazards such as sandbars, fallen trees and debris jams. As a result the first plans to
improve the rivers for navigation were approved in 1875. The authorized project
provided for a navigable channel extending from the mouth of the Tombigbee River, 45
miles above Mobile, to the vicinity of Birmingham, via the Tombigbee and Black
Warrior Rivers. Between 1895 and 1915, a system of 17 locks and dams was constructed
between Mobile and Birmingham, allowing adequate passage for the steam-powered tow
boats of the era. A modernization program of the locks and dams on the Black WarriorTombigbee Waterway began in 1937 to expand the navigable channel to a depth of 2.74
m (nine feet) by a width of 60.96 m (200 feet), allowing tows of up to eight standard
barges to be accommodated at all locks. To enable this modernization of the original
system, the following dams were constructed (Table 1). Each new dam replaced a set of
three of the original locks and dams, except Holt Lock and Dam which replaced four
original locks and dams. Dates and information were obtained from USACE (2008).
Locations of present day locks and dams are shown in Figure 2.
Table 1. Locks and dams built to replace the original system of 17 locks and dams
on the Tombigbee and Black-Warrior Rivers between 1895 and 1915.
Lock and Dam
William Bacon Oliver
Demopolis
Armistead L. Selden
Coffeeville
Holt
Date opened
August 1939
August 1954
October 1957
August 1960
June 1966
Additional Comments
Near Tuscaloosa
Work continued until 1965
2.1.2 Development of the Tennessee-Tombigbee Waterway
The Upper Tombigbee River sub-basin, north of Demopolis has also undergone dramatic
engineering work in the past forty years. The first suggestion of a waterway to join the
Tennessee and Tombigbee Rivers to allow easier navigation to the Gulf of Mexico came
as far back as the 18th century, by French explorers who believed that such a link was
necessary to develop that part of the south (Tennessee-Tombigbee Waterway
Development Authority, 2008). However, serious attention was not really paid to the idea
7
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until after the Civil War, by which time projects designed to improve navigation of the
nation’s rivers were gaining more political support (Stine, 1993). The first engineering
investigation of the waterway was conducted during the Grant Administration in 187475, but the study concluded that although the U.S. Corps of Engineers could build such a
project using a total of 43 locks and a channel 1.22 m (four feet) deep, its commercial
limitations made it impractical. Other studies were conducted by the Corps in 1913, 1923,
1935, 1938 and 1945 that eventually led to congressional approval of the waterway in
1946 (Tennessee-Tombigbee Waterway Development Authority, 2007). However, strong
opposition from key members of the Congress from other regions of the nation and from
the railroad industry, prevented funds for the project from being appropriated until 1971
(Stine, 1993). After 12 years of construction the Tennessee- Tombigbee Waterway was
completed on December 12, 1984. The Waterway begins at its northern end at Pickwick
Lake on the Tennessee River, flows through northeast Mississippi and west Alabama, and
connects with the established Black Warrior-Tombigbee Waterway at Demopolis,
Alabama. The main features of the 377 km (234-mile) long Tenn-Tom are 10 locks and
dams and a 46.7 km (29-mile) man-made canal (Figure 2). The 10 locks raise or lower
barges and boats a total of 103.9 m (341 feet), the difference in elevation between the two
ends of the waterway.
Spillways operated on the Tennesse-Tombigbee Waterway are used primarily for
navigation purposes, rather than for flood control. They do however have the capability
of allowing small pool fluctuations for mosquito control, fish propagation and
recreational activities (Underwood and Imsand, 1985). During periods of high flow,
headwater elevations at the dams are maintained by opening the gates until the tailwater
rises to within 0.6 meters (1.97 feet) of normal pool headwater elevation. At this flow
condition, all the dam gates are opened, and if the headwater continues to rise,
uncontrolled overflow takes place (Underwood and Imsand, 1985).
2.1.3 Summary
The Tombigbee and Black Warrior Rivers have undergone significant changes since
1895, involving three separate phases of modifications and periods of dam construction
(original 17 locks and dams on the Tombigbee-Black Warrior system, modernization of
these locks and dams between 1937 and 1965, and finally the construction of the TennTom Waterway in the 1970’s and early 1980’s). Morphological changes of a river tend to
occur most rapidly after a disturbance, with the effects diminishing with space and time.
As such, any changes that are currently occurring within the study reach could be a result
of channel responses to a combination of all the previous channel modifications that have
occurred. Effects caused by the older dams may now be more muted than the effects of
more recent disturbance. However, distinguishing potential effects of the TenneesseeTombigbee Waterway on the study reach of the Lower Tombigbee River may be difficult
as the flow of water and sediment, and resulting channel morphology have already been
affected and continue to be affected by the impoundments in the system that pre-dated the
Tenn-Tom Waterway. In terms of this study, the dams that are of particular interest are
shown in the following table (Table 2), along with their date of construction. These dates
should be kept in mind when analyzing the flow and bank widening trends in later
sections.
8
____________________________________________________________________________________________________________
Table 2. Dams located within or directly affecting the study reach between river
miles 259 and 72.
Dam
Heflin (Gainesville) Lock and Dam
Date Opened River Mile
Late 1970’s
266
Demopolis Lock and Dam
1954
213
Coffeeville Lock and Dam
1960
117
9
____________________________________________________________________________________________________________
Figure 2. Present day locations of locks and dams on Tennessee-Tombigbee Waterway
and Black Warrior - Tombigbee Waterway. The study reach for this project is highlighted
in red.
10
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2.1.3 Geology
This part of Alabama formed in the shallow waters that have covered most of the central
continent throughout geologic history. Generally, the geology of the Tombigbee River
basin consists of Cretaceous chalk and sediments and clastic sediments with porous
limestone from the Oligocene, Eocene, and Paleocene periods (Robinson, 2003). In
places, the chalk is exposed as white, steep banks along the river. A listing of the types of
rocks and their formation of origin can be found in Table 3 (Alabama Clean Water
Partnership Website, accessed, 2008).
Table 3. Geology of the Tombigbee River Basin (Alabama Clean Water Partnership
Website, accessed, 2008)
Rock Type
Formation
Sand, Clay, Silt, Mud
Alluvial, coastal, low terrace deposits
Sand, Clay
Providence sand
Sand, Clay
Coker Formation
Sand, Gravel, Clay
Gordo Formation
Sand, Clay
Eutaw Formation
Chalk, Marl, Clay
Demopolis chalk
Limestone, Silt, Sand
Clayton Formation
Clay, Claystone, Sand
Nanafalia Formation
Silt, Clay, Sand
Tuscahoma Formation
Clay, Limestone, Sand
Jackson Group undifferentiated
Sand, Limestone, Marl, Clay
Oligocene Series undifferentiated
Gravelly, Sand, Clay
Miocene Series undifferentiated
11
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3. FUNDAMENTALS of BANK STABILITY
Conceptual models of bank retreat and the delivery of bank sediments to the flow
emphasize the importance of interactions between hydraulic forces acting at the bed and
bank toe, and gravitational forces acting on in situ bank materials (Carson and Kirkby,
1972; Thorne, 1982; Simon et al., 1991). Failure occurs when erosion of the bank toe
and possibly the channel bed adjacent to the bank increase the height and angle of the
bank to the point that gravitational forces exceed the shear strength of the bank material.
After failure, failed bank materials may be delivered directly to the flow and deposited as
bed material, dispersed as wash load, or deposited along the toe of the bank as intact
blocks, or as smaller, dispersed aggregates (Simon et al., 1991).
Bank materials do not maintain a constant shear strength (resistance to failure)
throughout the year. Strength varies with the moisture content of the bank and the
elevation of the saturated zone in the bank mass. The wetter the bank and the higher the
water table, the weaker the bank mass becomes and the more prone it is to failure. Bank
failures, however, do not occur frequently during high flows because the water in the
channel is providing a buttressing, or confining force to the bank mass. This is true even
though it is during high-flow events that the bank may be undercut by hydraulic forces. It
is upon recession of the flow when the bank loses the confining force but still maintains a
high degree of saturation when it is most likely to fail. This is why changes in flow
regime can be very important in determining trends of bank stability over time.
Analyzing streambank stability is a matter of characterizing the gravitational forces
acting on the bank and the geotechnical strength of the in situ bank material. Field data
are required to quantify those parameters controlling this balance between force and
resistance. If we initially envision a channel deepened by bed degradation in which the
streambanks have not yet begun to fail, the gravitational force acting on the bank cannot
overcome the resistance (shear strength) of the in situ bank material. Shear strength is a
combination of frictional forces represented by the angle of internal friction (φ’), and
effective cohesion (c’). Pore-water pressures in the bank serve to reduce the frictional
component of shear strength. A factor of safety (Fs) is expressed then as the ratio
between the resisting and driving forces. A value of unity (or the critical case) indicates
the driving forces are equal to the resisting forces and that failure is imminent.
The forces resisting failure on the saturated part of the failure surface are defined by the
Mohr-Coulomb equation:
Sr = c’ + (σ - µ) tan φ’
(1)
where µ is the pore pressure and φ’ is the angle of internal friction.
The geotechnical driving force is given by the term:
F = W sinβ
(2)
12
____________________________________________________________________________________________________________
where, F = driving force acting on bank material (N), W = weight of failure block (N),
and β = angle of the failure plane (degrees).
In the part of the streambank above the “normal” level of the groundwater table, bank
materials are unsaturated, pores are filled with water and with air, and pore-water
pressure is negative. The difference (µa - µw) between the air pressure (µa) and the water
pressure in the pores (µw) represents matric-suction (ψ). This force acts to increase the
shear strength of the material and with effective cohesion produces apparent cohesion
(ca). The increase in shear strength due to an increase in matric suction is described by
the angle φ b. This effect has been incorporated into the standard Mohr-Coulomb
equation normally used for saturated soils by Fredlund et al. (1978), with a maximum
value of φ’ under saturated conditions (Fredlund and Rahardjo, 1993). The effect of
matric suction on shear strength is reflected in the apparent or total cohesion (ca) term:
ca = c’ + (µa - µw) tan φ b = c’ + ψ tan φ b
(3)
As can be seen from equation 1, negative pore-water pressures (positive matric suction;
ψ) in the unsaturated zone provide for cohesion greater than the effective cohesion, and
thus, greater shearing resistance. This is often manifest in steeper bank slopes than would
be indicated by φ’.
Thus, for the unsaturated part of the failure surface the resisting forces as modified by
Fredlund et al. (1978) are used:
Sr = c’ + (σ- µa) tan φ’ + (µa-µw) tan φb
(4)
where Sr is shear strength (kPa), c’ is effective cohesion (kPa), σ is normal stress (kPa), µa is
pore air pressure (kPa), µw is pore-water pressure (kPa), (µa-µw) is matric suction, or negative
pore-water pressure (kPa), and tan φb is the rate of increase in shear strength with increasing
matric suction.
13
____________________________________________________________________________________________________________
4. METHODOLOGY
To learn more about the Lower Tombigbee River in its current state, it was important to
examine the present geomorphology of the river bed and banks, and where necessary, to
collect geotechnical data regarding the resistance of the bank material for stability
analysis. In addition, available USGS gage data were analyzed along with historic sets of
aerial photographs, to establish past conditions, and rates and patterns of change in the
river system. Rivers are continually changing, dynamic systems. However, it is the rates
and magnitudes of these changes that help determine whether a river is stable or unstable.
For the purpose of this study, stability is defined in geomorphic terms; that is, a dynamic
equilibrium that can transport all of the sediment delivered to it from upstream without
altering its dimensions over a period of years. That is not to say that the stream is static
but that the short-term, local processes of scour and fill, erosion and deposition, are
balanced through a reach such that the stream does not widen, narrow, degrade of
aggrade.
4.1 Air Reconnaissance Survey
The length of the study reach (from river mile 72 to 259) was photographed from a lowflying helicopter using a high-speed video camera. From the air it was possible to
characterize active geomorphic processes and relative stability along different sections of
the study reach, for example, by observing bank failures, and areas of significant
aggradation. Locations were identified from mile markers posted along the river. Rapid
geomorphic assessments (RGAs) were conducted approximately every 2 river miles.
Information from the RGAs was used to estimate channel stability by looking at the
percentage of banks failing in a particular reach. Results provided information on the
magnitude, distribution and extent of bank instabilities along the reach to map critical
locations and discern any system-wide trends.
4.1.1
Rapid Geomorphic Assessments: RGA’s
A modified version of the Rapid Geomorphic Assessment tool (Simon, 1995; Simon and
Klimetz, 2008) was used to assess channel stability throughout the study reach. This
approach was used as the method allows for a very rapid analysis of many sites, and
highlights the important processes occurring at each site, enabling assignment of stages
of channel evolution. RGAs utilize diagnostic criteria of channel form to infer dominant
channel processes and the magnitude of channel instabilities through a series of nine
questions. Granted, evaluations of this sort do not include an evaluation of watershed or
upland conditions; however, stream channels act as conduits for energy, flow and
materials as they move through the watershed and will reflect a balance or imbalance in
the delivery of sediment. RGAs provide an efficient method of assessing in-stream
geomorphic conditions, enabling the rapid characterization and stability of any given
channel.
14
____________________________________________________________________________________________________________
Generally, the RGA procedure consists of five steps to be completed on site:
1. Determine the ‘reach’. The ‘reach’ is described as the length of channel covering
6-20 channel widths, thus is scale dependent and covers at least two pool-riffle
sequences.
2. Take photographs looking upstream, downstream and across the reach; for quality
assurance and quality control purposes. Photographs are used with RGA forms to
review the field evaluation
3. Make observations of channel conditions and diagnostic criteria listed on the
channel-stability ranking scheme.
4. Sample bed material.
5. Perform a survey of thalweg, or water surface if the water is too deep to wade.
Bed or water surface slope is then calculated over at least two pool-riffle
sequences.
In this case, however, the RGA methodology was used simply to establish the
longitudinal extent of recent streambank failures in each 2 mile reach. (see highlighted
part of field data sheet in Figure 3). This was quantified as the percent of the reach failing
as estimated from the video taken during the air reconnaissance flight. These percentages
are broken into classes (0-10, 11-25, 25-50, 51-75 and 76-100; Figure 3) and used as a
measure of the severity of bank instability and when mapped, the extent of that
instability. Bed sampling and stages of channel evolution were not evaluated for this
particular study reach.
15
____________________________________________________________________________________________________________
CHANNEL-STABILITY RANKING SCHEME
River_________________________
Site Identifier____________________________________
Date _____________ Time_______ Crew _______________ Samples Taken_________________________
Pictures (circle) U/S D/S X-section
1. Primary bed material
Bedrock
Boulder/Cobble
0
1
2. Bed/bank protection
Yes
No
(with)
Slope__________
Gravel
2
1 bank
Sand
3
Pattern:
Meandering
Straight
Braided
Silt Clay
4
2 banks
protected
0
1
2
3
3. Degree of incision (Relative elevation of "normal" low water; floodplain/terrace @ 100%)
0-10%
11-25% 26-50% 51-75%
76-100%
4
3
2
1
0
4. Degree of constriction (Relative decrease in top-bank width from up to downstream)
0-10%
11-25% 26-50% 51-75%
76-100%
0
1
2
3
4
5. Stream bank erosion (Each bank)
None
Fluvial Mass wasting (failures)
Left 0
1
2
Right 0
1
2
6. Stream bank instability (Percent of each bank failing)
0-10%
11-25% 26-50% 51-75%
76-100%
Left 0
0.5
1
1.5
2
Right 0
0.5
1
1.5
2
7. Established riparian woody-vegetative cover (Each bank)
0-10%
11-25% 26-50% 51-75%
76-100%
Left 2
1.5
1
0.5
0
Right 2
1.5
1
0.5
0
8. Occurrence of bank accretion (Percent of each bank with fluvial deposition)
0-10%
11-25% 26-50% 51-75%
76-100%
Left 2
1.5
1
0.5
0
Right 2
1.5
1
0.5
0
9. Stage of channel evolution
I
II
III
IV
V
VI
0
1
2
4
3
1.5
Figure 3. Channel stability ranking scheme used to conduct rapid geomorphic
assessments (RGA’s). The channel stability index is the sum of the values obtained
for the nine criteria.
16
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4.2 Analysis of Geographic Trends of Channel Conditions using Aerial Photography
and Geographical Information Systems (GIS) Layers
Aerial photographs of the study reach for four different periods were used to quantify
changes in channel width over a 33-year period (1974-2005) spanning a timeframe from
pre-Tennessee Tombigbee Waterway to 20 years after the opening of the Waterway.
Photograph resolution ranged from 0.5 to 1.5 meters per pixel. Banklines were digitized
in a GIS format from each set of aerial photographs: 1974, 1985, 1999 (bank line only
available) and 2005. The resulting sets of digitized maps were overlain to calculate
amounts of bank retreat along the study reach. For each set of images, bank lines were
drawn at the top-bank edge. For most of the images this bank-top edge was obvious, but
in the few cases where bank vegetation was too dense, or where shadows obscured the
views, channel width values were omitted from the analysis for this particular set of
photographs. The images were rectified and viewed in ArcView 3.3 to measure changes
in channel widths at each 2 mile interval. Rates of bank retreat were obtained by dividing
the differences in width between successive photographs by the time period represented
by the two sets of photos. Geographical Information Systems Layers were also used in
order to represent graphically the percent of reach failing data collected from aerial
reconnaissance.
Results of bank-width change in the aerial photo analysis and the analysis of current
percent of reach failing were compared, and areas identified by Tasks 3.1 and 3.2 as
being particularly active then provided guidance for the selection of sites where
geotechnical testing and bank-stability modeling was conducted as part of Tasks 3.4 and
3.5. One limitation of the method used was that undercut banks could not be seen from
the air photographs. However, given enough erosion over time, undercut banks will
become unstable and fail, the result of which is visible from aerial photographs. Given
the lengths of time between the sets of photographs used, the average bank widening for a
given reach should still be sufficient to show widening trends over time.
4.3 Gauging-Station Analysis
Historical data from U.S. Geological Survey (USGS) gauging stations along the reach
were used to identify changes in channel morphology over the period of record. This
technique, known as “specific gauge analysis” involves an examination of the watersurface width and elevation at various discharges over time. Results of such analyses
provide information on any temporal trends in channel dimensions.
4.3.1 Specific Gage Analysis using Water-Surface Elevation
One way to examine the vertical stability of a channel over a period of time is to
determine whether the elevation of the water surface at a specific discharge is changing
with time. A low-flow discharge is used for this analysis. A decrease in elevation over a
period of years indicates channel-bed erosion over the period. A lack of discernable
17
____________________________________________________________________________________________________________
change indicates relative bed stability. Using gage height (m) and discharge data (m3/s)
from USGS discharge-measurement data (USGS form 9-207) for a given site, a relation
is established for each year of available data. The resulting equation for each year was
solved for a low-flow discharge (10th percentile flow) to calculate the gage height at that
specific discharge for each given year. This gage height was then added to the elevation
of the gage from the USGS Site Inventory website: (http://waterdata.usgs.gov/nwis/si)
and a plot of water surface elevation over time was created. The plot of water-surface
elevation over time helps to identify trends in aggradation (filling) or degradation
(incising) of the channel bed.
4.3.2 Analysis of Discharge Records
To examine if and how the frequency of flows along the Lower Tombigbee River has
varied over the period of study, mean-daily discharge data from the USGS were
downloaded from http://nwis.waterdata.usgs.gov/usa/nwis/discharge for gages at
Gainesville, Demopolis and Coffeeville dams. Mean-daily data were used to calculate
mean-annual discharge values at each gage over the period of record of the gage. Where
possible, the data were split out into pre- and post Tennessee-Tombigbee Waterway time
periods, and general trends over the periods of record analyzed. Mean-daily data were
also used to investigate changes to the timing of flows during a given calendar year and
to identify changes due to Waterway operation. In addition, mean-daily data were used to
calculate and plot percent exceedance graphs, showing the percentage of time a certain
discharge is equaled or exceeded. This information is potentially important to understand
the behavior of the channel banks. Again, these data were split into pre- and postwaterway dates so that potential differences in flow magnitudes and timing could be
ascertained. Peak discharge measurements for each year were also analyzed to see if any
trends existed.
18
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Table 4. List of USGS gages and available data within the study reach on the Lower
Tombigbee River. The gages highlighted in blue were used for the analysis of
discharge.
GAGE
NUMBER
02449500
02467000
02467001
02469525
02469761
02469762
02470000
02470050
02449000
GAGE NAME
TOMBIGBEE
RIVER AT EPES,
AL
TOMBIGBEE R
AT DEMOPOLIS
L&D NEAR
COATOPA, AL
TOMBIGBEE
RIVER BL
DEMOPOLIS
L&D NEAR
COATOPA AL
TOMBIGBEE
RIVER NEAR
NANAFALIA,
AL.
TOMBIGBEE R
AT
COFFEEVILLE
L&D NR
COFFEEVILLE,
AL
TOMBIGBEE R
BL
COFFEEVILLE
L&D NEAR
COFFEEVILLE,
AL
TOMBIGBEE
RIVER NEAR
LEROY, AL
TOMBIGBEE
RIVER AT
STEAMPLANT
NR LEROY, AL
TOMBIGBEE
RIVER AT
GAINESVILLE,
AL
PERIOD
OF
RECORD
(daily
data)
PERIOD OF
RECORD
(stream
measurement)
DRAINAGE
AREA (sq
km)
AVAILABLE
DATA
LOCATION
1901-1945
1983-1983
14,371
No gauge
height,
discharge
At a bridge
1973-2007
1983-1990
24,759
Gauge height
and discharge
On dam
1971-2003
NO DATA
24,759
Gauge height,
no discharge
On dam
1990-2007
NO DATA
28,142
Gauge height,
no discharge
At a bridge
1960-2007
1966-2007
29,639
Gauge height
and discharge
On dam
1971-2007
NO DATA
29,639
Gauge height,
no discharge
On dam
1928-1960
NO DATA
30,521
No gauge
height,
discharge
A little north
of Jackson
2000-2007
NO DATA
30,770
Gauge height,
no discharge
At Jackson
AL
1938-1978
1959-1989
13,891
No gauge
height,
discharge
-
19
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4.3.3 Analysis of Precipitation Data
Any potential changes in discharge values in the Tennesse-Tombigbee Waterway and
Tombigbee River could be caused simply by changing amounts of precipitation over the
study period. Daily precipitation data were obtained for the rain gages shown in Figure 4,
to determine trends in precipitation since 1885. Annual values for each year of record
were averaged for all sites. A water yield for each year of record was then calculated by
dividing average, annual discharge (m3/s) by annual rainfall (mm). This represents the
amount of flow per unit of precipitation. A flat relation would indicate that the Waterway
has not had a discernable effect on discharge on an annual basis. In contrast, a sloping
relation would indicate that the Waterway has resulted in changes in the annual flow
regime.
Figure 4. Location of rain gages in the watershed of the Tennessee-Tombigbee River
that were used for precipitation analysis.
20
____________________________________________________________________________________________________________
4.4 Testing of Bank Materials
As bank stability is a function of the strength of the bank material to resist collapse under
gravity, measurements of the components of shearing resistance (or shear strength) were
required. In addition, tests of the resistance of the bank-toe materials to erosion by
flowing water were carried out using a submerged jet-test device (Hanson, 1990). In situ
tests of the shear strength of bank materials at five unstable sites were conducted using a
borehole shear-test device (BST; Lohnes and Handy, 1968)). Site selection was based on
information obtained during the reconnaissance phase and from information provided by
the Alabama Clean Water Partnership (Sections 3.1 and 3.2). Data obtained in the field
were used as inputs to the Bank-Stability and Toe Erosion Model (BSTEM; Simon et al.,
1999) to determine critical conditions for bank stability. This is described further in the
next section (3.5).
Testing was carried out at sites at river miles
75.3, 114.7, 161.9, 190.9, and 239.7 (Figure 5).
Sections 3.4.1 and 3.4.2 describe the theory behind
BST and submerged jet tests, and how they are
conducted in the field. At each site, a description of
the soil layers in the bank was obtained using
samples taken from a borehole dug to the point of
saturation within the bank. Soil samples were taken
at each change in material type, and were brought
back to the laboratory to be analyzed for particle
size, soil moisture, and bulk density. BST tests were
conducted in situ in each soil layer. Jet tests were
carried out in situ on the exposed toe material at the
base of the bank at each site to obtain estimates of
hydraulic resistance. Each jet test was also
accompanied by a soil sample collected to establish
particle size for this material. The soil layers
identified at each site can be seen in Appendix A.
Figure 5. Map showing location of unstable
sites selected for geotechnical testing.
21
____________________________________________________________________________________________________________
4.4.1 Geotechnical Data Collection – Borehole Shear Tests
To model bank stability at selected reaches of the Lower Tombigbee River using
BSTEM, the banks within each reach were characterized. Representative sites were
chosen along the study reach. Bank surveys at each site were also conducted. To gather
data on the internal shear strength properties of the banks, in-situ Borehole Shear Test
(BSTs) devices were used.
To properly determine the resistance of cohesive materials to erosion by mass movement,
data must be acquired on those characteristics that control shear strength; that is cohesion,
angle of internal friction, pore-water pressure, and bulk unit weight. Cohesion and
friction angle data can be obtained from standard laboratory testing (triaxial shear or
unconfined compression tests), or by in-situ testing with a borehole shear-test (BST)
device (Lohnes and Handy 1968; Thorne et al. 1981; Little et al. 1982; Lutenegger and
Hallberg 1981). The BST provides direct, drained shear-strength tests on the walls of a
borehole (Figure 6). Advantages of the instrument include:
1. The test is performed in situ and testing is, therefore, performed on undisturbed
material.
2. Cohesion and friction angle are evaluated separately with the cohesion value
representing apparent cohesion (ca). Effective cohesion (c’) is then obtained by adjusting
ca according to measured pore-water pressure and φ b.
3. A number of separate trials are run at the same sample depth to produce single values
of cohesion and friction angle based on a standard Mohr-Coulomb failure envelope.
4. Data and results obtained from the instrument are plotted and calculated on site,
allowing for repetition if results are unreasonable; and
5. Tests can be carried out at various depths in the bank to locate weak strata (Thorne et
al. 1981).
Figure 6. Schematic representation of borehole shear tester (BST) used to determine
cohesive and frictional strengths of in situ streambank materials. Modified from
Thorne et al., 1981.
22
____________________________________________________________________________________________________________
At each testing depth, a small core of known volume was removed and sealed to be
returned to the laboratory. The samples were weighed, dried and weighed again to obtain
values of moisture content and bulk unit weight, both required for analysis of streambank
stability.
4.4.2 Hydraulic Forces and Resistance
A submerged jet-test device was used in situ to estimate the resistance of materials to
hydraulic forces in fine-grained materials (Hanson 1990; 1991; Hanson and Simon,
2001). The device shoots a jet of water at a known head onto the streambed causing it to
erode at a given rate. As the toe material erodes, the distance between the jet and the bed
increases, resulting in a decrease in applied shear stress. Theoretically, the rate of erosion
beneath the jet decreases asymptotically with time to zero. A critical shear stress for the
material can then be calculated from the field data as that shear stress where there is no
erosion.
The rate of scour ε (ms-1) is assumed to be proportional to the shear stress in excess of a
critical shear stress and is expressed as:
ε = k (τo - τc) a
(5)
where k = erodibility coefficient (m3/N-s), τo = average boundary shear stress (Pa), τc =
critical shear stress, and a = exponent assumed to equal 1.0. The quantity (τo − τc) =
excess shear stress (Pa).
Average boundary shear stress, representing the stress applied by flowing water along the
edge of the bank is calculated from channel geometry and stage data collected at the sites
as:
τo = γ R S
(6)
where γ = unit weight of water (N/m3), R = hydraulic radius (m), and S is channel
gradient (m/m). An inverse relation between τc and k occurs when soils exhibiting a low
τc have a high k or when soils having a high τc have a low k.
The measure of material resistance to hydraulic stresses is a function of both τc and k.
Based on observations from across the United States, k can be estimated as a function of
τc (Hanson and Simon, 2001). This is generalized to:
k = 0.1 τc – 0.5
(7)
Erosion of cohesive bank-toe materials, such as those along the Tenn-Tom reach being
studied, can then be calculated using equations 5 and 6. It should be noted however, that
the relation from Hanson and Simon (2001) given in Eq. 7 has an r2 value of 0.6,
indicating a considerable amount of scatter about the regression line. Using the regression
in Eq. 7 to obtain values for τc and k in the absence of field data may therefore lead to a
23
____________________________________________________________________________________________________________
degree of error in these variables. Critical shear stress of these types of materials can then
be calculated using conventional (Shields-type) techniques as a function of particle size
and weight.
The magnitude of bank-toe erosion and bank steepening by hydraulic forces is calculated
using a bank-toe erosion algorithm incorporated into the Bank Stability and Toe-Erosion
Model (BSTEM, Simon et al., 2000). The algorithm calculates the hydraulic forces
acting on the bank face for a particular flow event. Flows are discretized as simple,
rectangular hydrographs with the user specifying flow depth, channel gradient and the
duration of the flow. The boundary shear stress exerted by the flow on each node, is
determined by dividing the flow area at a cross section into segments that are affected
only by the roughness on the bank or on the bed and then further subdividing to
determine the flow area affected by the roughness on each node. The line dividing the
bed- and bank-affected segments is assumed to bisect the average bank angle and the
average bank toe angle. The hydraulic radius of the flow on this segment is the area of the
segment (A) divided by the wetted perimeter of the segment (Pn), and S is the channel
slope. An average erosion distance is computed by comparing the boundary shear stress
with critical shear stress and erodibilty for each node for the specified duration of the
flow.
4.5 Quantifying streambank stability: The Bank Stability and Toe Erosion Model
(BSTEM)
A bank-stability and toe-erosion model (BSTEM) developed by the USDA-ARS National
Sedimentation Laboratory was used to model current bank-stability conditions and to
determine stable-bank configurations (Simon et al., 2000). BSTEM was developed for
use with multi-layered banks with complex geometries. Data collected at field sites, in
addition to flow data from USGS gages were used to model a range of typical flow
conditions ranging from low summer flows (<100 cms), to large springtime events (up to
6000 cms).
Bank stability was analyzed using the limit-equilibrium method, based on static
equilibrium of forces and/or moments (Bishop, 1955). Streambank failure occurs when
gravitational forces that tend to move soil downslope exceed those forces that resist
movement (driving forces exceed resisting forces). The potential for failure is usually
expressed by a factor of safety (FS), defined as the ratio of resisting to driving forces.
BSTEM performs stability analysis of planar-slip and cantilever ailures and accounts for
the important driving and resisting forces that control bank stability. Bank geometry, soil
shear-strength (effective cohesion, c', and angle of internal friction, φ'), pore-water
pressure, confining pressure, and mechanical and hydrologic effects of riparian
vegetation are model inputs, used to numerically determine the critical conditions for
bank stability, for example. in terms of bank height, angle, and degree of bank saturation.
24
____________________________________________________________________________________________________________
4.6 Determining the timing and frequency of critical conditions, and modeling rates
of bank retreat.
BSTEM was used to determine the critical conditions for bank failure at each of the five
sites examined in detail. In each case the bank profile was entered, along with the
geotechnical data collected in the field. A first set of model runs was conducted in which
water table height, flow depth, and the size and geometry of the failure block were varied
to find the most critical bank condition for that site. In all cases investigated, the most
critical bank condition occurred when the water table was higher than the level of the
flow in the channel (ie. bank pore-water pressures were high, but confining force from
the flow in the channel had been removed). Such conditions are likely during drawdown
after a high flow event. The bank is only likely to become saturated to the level of highest
flow, however, if the flow remains high for several consecutive days, and/or the bank’s
antecedent moisture conditions were already high from a series of rainfall events. To
account for this, the discharge relating to the critical water table height was established at
each site, and the mean daily data from USGS gages were analyzed to see how many
times discharge events of that magnitude, and lasting 5-days or greater and 10-days or
greater occurred during the period of record before the Tenn-Tom Waterway opened and
after its opening. Critical conditions were found assuming first that no riparian vegetation
was present and second using estimates of root-reinforcement obtained using descriptions
of the vegetation present at each field site. Root reinforcement was estimated using the
root-reinforcement model, RipRoot (Pollen and Simon, 2005; Pollen, 2007).
A second set of model runs was also carried out for one of the sites (RM 114.7) to
examine rates of bank retreat during a high flow year, and to see how different mitigation
strategies might affect this rate. Mean-daily flow records for the gage closest to the site
(Coffeeville dam) were plotted for the entire available data record. The hydrograph for
the 90th percentile flow year (1991) was selected from the period of record (1961 – 2006)
to model a high flow year and estimate worst-case flow conditions for that particular site.
The 1991 hydrograph was discretized into a series of steady-state rectangular-shaped
discharge events (Figure 7) which were iterated through using the following approach to
run the toe erosion and bank stability algorithms in BSTEM:
1. The effects of the first flow event was simulated using the toe-erosion sub model
to determine the amount (if any) of hydraulic erosion and the change in geometry
in the bank-toe-region.
2. The new geometry was exported into the bank-stability sub-model to test for the
relative stability of the bank.
a. If the factor of safety (Fs) was greater than 1.0, geometry was not updated
and the next flow event was simulated.
b. If Fs was less than 1.0, failure was simulated and the resulting failure
plane became the geometry of the bank for simulation of toe erosion for
the next flow event in the series.
c. If the next flow event had an elevation lower than the previous one, the
bank-stability sub-model was run again using the new flow elevation to
25
____________________________________________________________________________________________________________
test for stability under drawdown conditions. If Fs was less than 1.0,
failure was simulated and the new bank geometry was exported into the
toe-erosion sub-model for the next flow event.
3. The next flow event in the series was simulated.
8000
MEAN DAILY DISCHARGE (cms)
1991 Mean Daily Discharge
7000
Discretized Hydrograph for
Modeling in BSTEM
6000
5000
4000
3000
2000
1000
0
1/1
2/20
4/11
5/31
7/20
9/8
10/28
12/17
DATE
Figure 7. Discretized hydrograph for 90th percentile flow year at Coffeeville Dam,
showing the six steady-state, rectangular shaped flow events modeled in BSTEM.
Four different sets of runs were carried out for site 114.7 using this approach. First, the
model was run with no bank protection. Second, rock was added along the entire length of
the bank toe. Third, rock was added to the entire length of bank toe, and 5-year-old River
birch and Eastern Sycamore trees were added to the top of the bank. Fourth, the bank
angle was decreased to see what effect that might have on bank retreat rates.
26
____________________________________________________________________________________________________________
5. RESULTS
To determine the current stability conditions of the Lower Tombigbee River and the way
the river’s morphology has changed before, during construction of, and after construction
of the Tennessee-Tombigbee Waterway, a combination of approaches were used. Current
conditions were obtained from RGA’s and aerial reconnaisance, while historical
conditions and changes in channel morphology were analyzed using time-series aerial
photography and USGS gauging-station data. To compliment the RGA analysis of
current conditions, bank-stability modeling was used to identify critical conditions for
stability at five representative sections along the study reach.
5.1 Trends in Flow and Precipitation
Mean daily discharge, and peak annual discharge data were analyzed for the time periods
between the air photographs to see if flow trends have varied over the extent of the gage
records, and pre- and post- Tenn-Tom Waterway construction. USGS gage data were
available for a number of gages on the Lower Tombigbee River (Table 4 in
methodology), but only gages 02449000 (Tombigbee River at Gainesville, AL),
02467000 (Tombigbee River at Demopolis, AL) and 02469761 (Tombigbee River at
Coffeeville, AL) had the correct data to analyze mean-daily discharge data and peakdischarge data.
Analysis of discharge records was carried out because it was important to establish if and
when, the magnitudes and timing of flows have changed along the Lower Tombigbee
River, so that processes acting on the bed and banks could be identified correctly. As
with all the analyses in this report involving gauging-station data, the dataset was
analyzed as a whole period of record to look at the long term trends, and also separated
into pre-and post- waterway construction (start of data set – 1974 and 1985 – end of data
set, respectively).
5.1.1 Total Annual Discharge
Mean-daily discharge data were used to calculate the total discharge during each calendar
year. The results of this analysis showed increases in total discharge as indicated by the
positive gradients of the trend lines in Figure 8 (Top), although the lines for Gainesville
and Coffeeville are only slightly positive. Annual rates of increase above the upstream
end of the study reach (Gainesville) were small about 130 m3/s/y. A similar value was
obtained downstream at Coffeeville Dam (about 120 m3/s/y). The gage at Demopolis in
the middle of the study reach, however, showed an order of magnitude greater rate of
increase (about 1200 m3/s/y). Given that the Tombigbee River at Demopolis is receiving
flow from Gainesville, which showed no trend over time, this indicates that the
increasing total discharge at Demopolis may be due to greater flow volumes emanating
from the Black Warrior River system. These calculated rates, however, are problematic
27
____________________________________________________________________________________________________________
in that the increase at Demopolis does not seem to transfer downstream to Coffeeville. It
is more likely that the absolute values of theses trends are, at least in part due to
differences in the length of record for each gage. If these same data are re-plotted from
1961 to present (where data are available for each gage) a different picture emerges. In
this case (Figure 8 (Bottom)), there is a slight trend of decreasing total discharge at the
upstream end of the reach at Gainesville (-355 m3/s/y) countered by a larger increase at
Demopolis (530 m3/s/y), resulting in a very mild increase at Coffeeville (133 m3/s/y).
This is more likely the correct interpretation in that the difference between the increase at
Demopolis and the decrease at Gainesville is similar to the resulting downstream increase
in total discharge at Coffeeville, with the additional water being provided through the
Black Warrior River System.
a)
TOTAL DISCHARGE, IN CUBIC METERS PER SECOND
700000
GAINSVILLE
600000
DEMOPOLIS
500000
COFFEEVILLE
400000
y = 133x + 47034
300000
200000
y = 1190x - 2E+06
100000
y = 121x - 113753
0
1939
1949
1959
1969
1979
1989
1999
YEAR
b)
TOTAL DISCHARGE, IN CUBIC METERS PER SECOND
700000
GAINSVILLE
600000
DEMOPOLIS
500000
COFFEEVILLE
400000
y = 133x + 47034
300000
200000
y = 530x - 785252
100000
y = -355x + 834146
0
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
YEAR
Figure 8. Annual trends in total discharge along the Tombigbee River for specific
periods of record (a) and since 1961 (b). Note the difference in trends.
28
____________________________________________________________________________________________________________
5.1.2 Mean-Annual Precipitation and Water Yield
The next step in the analysis was to evaluate the precipitation data to see if the observed
changes in total discharge could in part be related to changes precipitation over the same
period. Precipitation records for the Tennessee-Tombigbee River Watershed date back as
far as 1885. The data show that there has been a trend of increasing precipitation since
1885 with precipitation increasing approximately 240 mm (approximately 9 inches)
(Figure 9). This value equates to an average increase in precipitation of about 60 mm/y
(2.4 inches) in the past 30 years alone. To establish if the slight increase in total discharge
of water each year over the periods of record could be explained by increases in
precipitation, an analysis of water yields (flow discharge per unit of precipitation) was
conducted. A water yield was calculated by dividing the total volume of water at each
gage for each year (the sum of the mean daily discharge data) by the average annual
precipitation for the corresponding year (using data from all available rain gages in the
drainage basin). The results (Figure 10) show that the trends of water yield are either
decreasing slightly with time or, in the case of Demopolis, are flatter than the trends for
total discharge shown in Figure 8. This flattening of the trend line for Demopolis
suggests that the amount of discharge, per unit of precipitation has either slightly
decreased or remained fairly steady over the period of record investigated. This indicates
that the increase in total discharge over the period can be explained in large part by the
increasing trend in precipitation (that is most obvious at Demopolis as it receives water
from the Black Warrior River), representing the bulk of the drainage area in the entire
Tombigbee River System.
2000
MEAN ANNUAL PRECIPITATION,
IN MILLIMETERS
Tennessee-Tombigbee River Watershed
y = 2.27x + 1173
1800
1600
1400
1200
1000
800
600
1885 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005
YEAR
Figure 9. Precipitation trends for the Tennessee-Tombigbee River Watershed from
1885 to present. Red line shows the five-year moving average.
29
____________________________________________________________________________________________________________
a)
WATER YIELD,
IN CUBIC METERS PER SECOND PER MILLIMETER
OF PRCIPITATION
350.000
GAINSVILLE
DEMOPOLIS
300.000
COFFEEVILLE
250.000
y = -0.182x + 576
200.000
y = 0.337x - 485
150.000
y = -0.0959x + 278
100.000
50.000
0.000
1939
1949
1959
1969
1979
1989
1999
YEAR
b)
WATER YIELD,
IN CUBIC METERS PER SECOND PER MILLIMETER
OF PRCIPITATION
350.000
GAINSVILLE
DEMOPOLIS
300.000
COFFEEVILLE
250.000
y = -0.182x + 576
200.000
y = 0.119x - 52.0
150.000
y = -0.383x + 848
100.000
50.000
0.000
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
YEAR
Figure 10. Trends in water yield (discharge per unit of precipitation) for specific
periods of record (a) and since 1961 (b) along the Tombigbee River.
5.1.3 Timing and Distribution of Flows
The timing and distribution of flows were also examined. To find any differences in
timing of flow events pre- and post- waterway construction, mean-daily discharge data
were used to establish the mean-daily discharge on a given calendar day (ie. an average
discharge was calculated for January 1st, January 2nd etc). This was carried out for the pre
30
____________________________________________________________________________________________________________
waterway period (beginning of available data to 1974) and the post waterway period
(after waterway opened in 1985 to the end of the data set). The difference between the
means for each calendar day was then calculated and plotted in Figure 12. Values less
than zero indicate periods when there was, on average, lower discharge after waterway
construction than before. Values greater than zero indicate periods when discharge values
were higher following waterway construction than before. The results clearly show that
for all three of the gages (Gainesville, Demopolis and Coffeeville), a redistribution of
flows has occurred since waterway construction as follows:
• winters flows are higher, although less so at Gainesville;
• springs flows are lower; and
• summers flows fluctuate between slightly more and slightly less discharge than
pre-waterway conditions.
The data seem to suggest that during the spring, water is held back behind the dams to
store water for the summer when flows naturally become lower and navigation thus
becomes more difficult. By storing water in the spring, it can be released when necessary
in summer months to maintain a flow deep enough for navigation. During the winter any
excess water not used during the summer is then released and the cycle begins again. The
magnitudes of the differences pre- and post- waterway can also be seen to get larger with
progression to each downstream dam.
DIFFERENCE IN MEAN-DAILY DISCHARGE
FROM PRE WATERWAY CONDITIONS,
IN CUBIC METERS PER SECOND
800
600
400
200
0
-200
-400
Gainesville dam
Demopolis dam
Coffeeville dam
-600
-800
-1000
-1200
1/1
1/26 2/20 3/16 4/10
5/5
5/30 6/24 7/19 8/13
9/7
10/2 10/27 11/21 12/16
DATE
Figure 11. Difference in mean-daily flows pre- and post-waterway for a given
calendar day. Note the increased flows during the winter and the decreased flows
during the spring.
31
____________________________________________________________________________________________________________
Having evaluated total discharge and the timing of flows, mean-daily discharge data were
used to investigate the distribution of low, moderate and high flows before and after
waterway construction. These data are expressed in terms of the percentage of time a
given discharge is equaled or exceeded (Figure 12). Results show that for a given lowflow exceedance value (say 99%), that flows were lower at Gainesville and Coffeeville,
and higher at Demopolis than pre-waterway. This is in agreement with the water yield
results discussed previously. In the medium-flow range (represented by the 50%
exceedance value), discharges are higher for all stations, and a given medium discharge
occurs more frequently post-waterway. A similar trend exists for the high flows at
Gainesville and Demopolis, with more frequent and higher discharges post-waterway.
This trend, however, is largely masked at Coffeeville.
The increased occurrence of greater high-flow discharges can have an impact on
streambank stability insomuch as high flows tend to scour the bank-toe more readily. In
addition, if high flows are more frequent and maintained for longer periods of time, this
could lead to a greater frequency of bank saturation and reductions in the strength of the
bank materials to resist failure. This idea will be investigated further in the section on
critical streambank conditions.
32
PERCENTAGE OF TIME DISCHARGE IS
EQUALLED OR EXCEEDED
____________________________________________________________________________________________________________
99.99
99.9
99
90
70
50
30
Pre waterway
Post waterway
10
1
0.1
0.01
1
10
100
1000
10000
DISCHARGE, IN CUBIC METERS PER SECOND
99.99
99.9
99
90
70
50
30
Pre waterway (corrected)
Post waterway
Calculated exceedance
10
1
0.1
0.01
0.1
1
10
100
1000
10000
100000
99.99
99.9
99
90
Pre waterway
Post waterway
70
50
30
10
1
0.1
0.01
1
10
100
1000
10000
Figure 12. Distribution of mean-daily flows pre-and post-waterway at Gainesville
Dam (Top), Demopolis Dam (Middle) and Coffeeville Dam (Bottom). The
‘corrected’ line for Demopolis shows as adjusted line as gage data at the lower end
was erroneous.
33
____________________________________________________________________________________________________________
5.1.4 Variations in Annual-Peak Discharge
10000
10000
8000
8000
6000
6000
4000
2000
0
1940
1960
1980
2000
DATE
10000
8000
PEAK ANNUAL DISCHARGE IN CMS
In keeping with the results of the precipitation and flow-duration analyses, annual-peak
discharges have increased over the periods of gage record (Figure 13). At Gainesville,
there has been a 75% increase in peak discharges since 1940, from about 2000 to 3500
m3/s, while at Demopolis there has been a 67% increase, from about 3000 to 5000 m3/s
since 1930. The increase in peak discharges at Coffeeville appears significantly less
(about 10%) than at the upstream gages but this is due, in part, to the shorter gage record
which starts in 1961. Bearing in mind the increase in precipitation over the past century,
one would expect greater percentage increases at the stations with longer record. Still, the
increase in peak discharges at Gainesville and Demopolis are greater than at Coffeeville,
even over the last 45 years.
(a)
(b)
4000
2000
0
1930
8000
4000
4000
2000
2000
1960
1980
1990
10000
6000
1940
1970
DATE
6000
0
1950
0
2000
1930
1950
DATE
1970
1990
DATE
(c)
10000
8000
Figure 13. Changes in annual-peak flows
at Gainesville (upper left, (a)), Demopolis
(upper right, (b)), and Coffeeville (lower
right, (c)). Trend lines are displayed for
pre- and post-waterway periods, as well
as for the entire period of record.
6000
4000
2000
0
1960
1970
1980
1990
2000
1990
2000
DATE
10000
8000
6000
4000
2000
0
1960
1970
1980
DATE
34
____________________________________________________________________________________________________________
5.2 Channel Changes at Gauging Stations
Only one gage in the study reach had sufficient data to analyze changes in bed elevation
over time (02449000 Tombigbee River at Gainesville) because this analysis is based on
comparisons of hydraulic data collected during field measurements of discharge. A lowflow discharge representing the flow that is exceeded 90% of the time was selected for
use to evaluate changes in bed level with time.
Changes in water surface elevation over time at gage 02449000 (Figure 14) show that
until 1972, bed elevation at the gage location was stable. From 1972 to 1978, a period
that coincides with construction work in the reach, the channel experienced a period of
scour (approximately 1 m) followed by a period of fill that returned the bed elevation to
its 1972 elevation. Between 1978 and 1984 the channel incised just over 2.0 m before
being partially filled again by 1989. Still, the streambed incision that lasted about six
years was probably due to sediment being trapped at the dam site. This is a typical
channel response downstream from dams (Williams and Wolman, 1984). The incision
process probably progressed some distance downstream, deepening the channel albeit to
a lesser extent. A lack of sufficient data prevented this analysis from being extended
beyond 1989 at Gainesville, and at all for the other gage sites.
Similar responses downstream from Demopolis and Coffeeville probably occurred
immediately following closure of those dams in 1954 and 1960, respectively. In general
terms, incision leads to increased bank heights and a consequent reduction in streambank
stability downstream from each dam.
24.00
23.50
ELEVATION ABOVE SEA LEVEL,
IN METERS
90th Percentile Discharge
23.00
22.50
22.00
21.50
21.00
20.50
20.00
19.50
19.00
1955
1960
1965
1970
1975
1980
1985
1990
1995
YEARS
Figure 14. Changes in water surface elevation over time for gage 02449000
Tombigbee River near Gainesville. This gage represents the upper boundary of the
study reach even though it is upstream of the last site in the study reach.
35
____________________________________________________________________________________________________________
5.3 Changes and Trends in Channel Width
Amounts and rates of channel widening over the study reach were determined by
comparing channel widths obtained from aerial photographs that were available for the
years 1974, 1985, and 2003. A GIS layer showing bank lines was also available for 1999,
thereby providing three periods; 1974-1985, 1985-1999, and 1999-2003.
For each date, channel width increases with distance downstream along the study reach
from 125-175 m above Demopolis Dam to between 175 and 275 meters at the lower end
of the study reach (Figure 15). This trend is common and occurs because a river’s
drainage area and thus discharge increase downstream. It can also be seen that channel
widths have generally increased over the 29-year period (1974-2003) (Figure 15).
A considerable amount of widening has occurred along the length of the study reach. The
Tombigbee River has widened up to 85 meters in the period from 1974 to 2003 (Figures
15 and 16). Interpretations as to the severity of widening from the different time periods
in Figure 16 is somewhat misleading because the plotted lines represent different lengths
of time. Dividing by the number of years in each period provides a widening rate for the
period. Results show that in most reaches, widening rates were substantially higher
between 1974-1985 than between 1985-2003 (Figure 17). This is not the case, however,
just downstream from Demopolis Dam where the reverse is true. The average rate of
widening (calculated at two-mile increments) over the entire 29-year period is
approximately 1.2 m/y but was as high as 2 – 3 m/y in some reaches just downstream
from the dams (Figure 17). When the data is split into pre-and post-waterway periods it
was found that between 1974 and 1985, the average rate of channel widening was 1.4
m/y and from 1985 to 2003, the average rate of widening decreased to approximately 1.0
m/y. Still, these values represent widening rates in excess of what would occur in “stable”
alluvial streams where bank erosion and channel widening would be largely
imperceptible.
Areas of high bank erosion were seen in certain locations with a spatial trend being seen
between the dams in the study reach (Figure 16 and 17). Directly downstream of
Demopolis Dam bank erosion rates were low, but increased up to approximately RM 190.
Downstream of RM 190 trends of bank erosion decreased towards Coffeeville Dam, with
bank erosion increasing again a few miles upstream of the dam. Below Coffeeville Dam,
bank erosion rates were high, and then decreased downstream to RM 80. The percent of
reach failing obtained from aerial reconnaissance shows similar patterns, although a
stretch of the river between RM 140 and RM 170 was highlighted as being an area of
high percent of reach failing, although bank erosion rates obtained from aerial
photographs showed bank erosion to be decreasing in this reach. Figure 18 shows the raw
data on widening rates for every 2 mile increment of the study reach.
A map generated from the aerial reconnaissance data and showing the percent of each
reach that has recently experienced recent bank failures can be seen in Figure 19. This
map and the data shown in Figure 17 show the areas of greatest bank erosion to be
located approximately:
36
____________________________________________________________________________________________________________
•
•
•
•
33 miles upstream of Demopolis Dam at RM 245,
Downstream from Demopolis Dam between RM 185 and 200,
Between Demopolis and Coffeeville Dams from RM 141 to 167, and
Immediately downstream of Coffeeville Dam from RM 97 to 113.
Over the entire study reach results of the aerial reconnaissance indicate that about 52 %
of all banks had failed recently.
37
____________________________________________________________________________________________________________
325
CHANNEL WIDTH , IN METERS
Downstream
Upstream
1974
1985
1999
2003
275
225
175
125
Coffeeville
Dam
Demopolis
Dam
75
70
80
90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260
RIVER MILE
Figure 15. Variations in channel width along study reach. A break in the data
occurred in the reach downstream of Coffeeville Dam because the top bank edge
could not be defined from the air photographs at one site.
100.00
Coffeeville
Dam
90.00
CHANGE IN CHANNEL WIDTH,
IN METERS
80.00
1985-2003
1974-1985
1974-2003
Percent of reach failing
Demopolis
Dam
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
70
80
90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260
RIVER MILE
Figure 16. Changes in channel width (five-point moving average) between 1974 and
2003, and percent of reach failing estimated from aerial reconnaissance.
38
____________________________________________________________________________________________________________
Downstream
4
Upstream
Widening 1974-1985
Widening 1985-2003
Widening 1974-2003
Percent of Reach Failing
70
60
50
2
40
30
1
20
10
Coffeeville
Dam
0
Demopolis
Dam
0
70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260
RIVER MILE
Figure 17. Five-point moving average of widening rates along the study reach with
the percent of each reach with recent bank failures.
6
Downstream
Upstream
Widening rate1974-1985
Widening rate 1985-2003
Widening rate 1974-2003
WIDENING RATE, IN M/Y
5
4
3
2
1
0
70
80
90
100
110
120
130
140
150
160
170
180
190
200
RIVER MILE
Figure 18. Raw data on widening rates at 2 mile intervals.
210
220
230
240
250
260
PERCENT OF REACH FAILING
3
WIDENING RATE,
IN METERS PER YEAR
80
39
____________________________________________________________________________________________________________
Figure 19. Percent of reach failing from River Mile 72 to 259 of the Lower
Tombigbee River.
40
____________________________________________________________________________________________________________
5.3.1 Locations and amounts of dredging in the study reach
Dredging is carried out regularly along the study reach for navigational purposes.
Locations and amounts of dredged material (in gallons per cubic yard). are shown in
Figure 20. Also marked on the figure are the locations of Coffeeville and Demopolis
dams (in black) and the river miles at which tributaries have their confluences with the
Tombigbee River (in green). In some cases the locations that have frequently been
dredged are at or just below a tributary confluence, for example at RM 110.13 (Santa
Bogue Creek) and RM 123.12 (Okatuppa Creek) the first two tributaries downstream of
Coffeeville dam. In addition, dredging has also been necessary near the confluences of
Double Creek and Cotohaga Creek, the second and third confluences downstream of
Demopolis Dam.
Downstream
Demopolis
Dam
Coffeeville
Dam
Upstream
8000000
DREDGED MATERIAL (gcy)
7000000
6000000
5000000
4000000
3000000
2000000
1000000
0
70
80
90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260
RIVER MILE
Figure 20. Volume and locations of dredging along the study reach, including
locations of tributary mouths marked in green.
It is not surprising that dredging is required in the vicinity of tributaries that enter just
downstream of the structures. Assuming that the Tombigbee River incised downstream of
the structures following closure of the dams, tributaries entering these reaches would
have much steeper slopes and would undergo upstream-migrating incision. These
tributaries would then deliver much larger quantities of sediment to the Tombigbee River
than previously. In these cases, deltas often form at tributary mouths that would
subsequently require dredging to maintain depths for navigation. In terms of bank
stability, however, the regular dredging would help to maintain higher bank heights and
potentially, bank instability. In fact, the two reaches mentioned above, have some of the
greatest amounts of widening in the study reach over the 29-year period.
41
____________________________________________________________________________________________________________
5.3.2 Explanations for trends and locations of high bank erosion.
The apparent spatial trends of changes in channel width (in m) and widening rates (in
m/y) are related to the dams at Gainesville, Demopolis and Coffeeville because these
structures alter the transfer of water and sediment downstream. The classic work by
Williams and Wolman (1984) on the downstream effects of dams on alluvial rivers
describes how the trapping of sediment on the upstream side of dams leads to bed erosion
downstream of the structure as the river attempts to balance the energy available for
sediment transport with the amount of sediment that is being delivered to it from
upstream. The greatest amount of incision, therefore, is expected just downstream the
structure. As the river entrains sediment from the bed there is less of an imbalance
between the available energy and the sediment contained in the flow. As a result, bed
incision decreases with distance downstream and, at a point, with time. Because incision
leads to increases in bank heights, bank instability is likely to be greatest in the reaches
just downstream from the structures. If the streambed in the reach immediately
downstream from the dam is either protected with rock or much wider than the “normal”
channel, this trend of incision and consequent widening may be shifted somewhat
downstream. This seems to be the case on the Lower Tombigbee River. In addition, the
streambanks just downstream from the dams tend to be graded to a stable slope and
protected with rip rap.
The trends of raw widening data shown in Figures 15-18 are characterized below in a
schematic drawing that relates how the dams have affected widening rates in the study
reach (Figure 21). Not only are the hydraulic and sediment-transport affects of the dams
important in terms of potential bed incision, but so are the affects on pore-water pressure
distributions and the confining forces operating on the banks. Specific processes that
control bank stability are summarized for areas of low and high widening. It is clear from
the systematic longitudinal trends in widening rates around each dam (Figure 21) that
these structures control spatial trends of channel widening and have impacted streambank
stability in the study reach. Without the structures and given the increase in precipitation
and flows throughout the last century one would expect that widening rates would be
generally increasing with time. This is not the case, however, as the greatest period of
widening was between 1974 and 1985, with less widening between 1985 and 2003. In
part, this could be related to the large flows during the spring of 1979. It seems, however,
that the greatest impact of the structures on widening occurred in the 1974 – 1985 period
with regular dredging activities exacerbating these affects locally.
42
____________________________________________________________________________________________________________
High Widening Rates
• Higher shear stress
• Low confining force
• Moderate pore pressure
• Drawdown pore pressure
• Bed scour
Low Widening Rates
• Low shear stress
• High confining force
• High pore pressure
• No drawdown
• No bed scour
m/y
1974-85 1985-2003
3.0
0.2-0.3
2.0
0.3
Coffeeville Dam
Demopolis Dam
Figure 21. Generalized trends in widening rates and prospective rationale for why
widening rates vary this upstream and downstream of the structures. The range of
widening rates is also given: Low rates 0.2-0.3 m/y; High rates 2.0 – 3.0 m/y.
5.4 Bank-Stability Analysis
Bank-stability analyses were carried out for five representative sites along the study
reach, to quantify bank stability conditions for a range of steady state flow and water
table heights. Results of these analyses can be used to determine critical conditions for
stability and under what conditions the banks could be made unstable. Bank-stability
modeling was conducted using the Bank-Stability and Toe-Erosion Model (BSTEM)
(Simon et al., 2000) and accounts for all of the important processes acting on the banks of
the Lower Tombigbee River with the exception of how wave action enhances bank-toe
erosion. Geotechnical properties of the banks were obtained from BST results and soil
coring during testing. Average values attributed to each bank layer are shown in
Appendix A. Each cross section was surveyed to provide bank geometry coordinates as
input to BSTEM. The bank stability algorithms in BSTEM have been successfully tested
previously under a broad range on geotechnical and hydrologic conditions with and
without vegetation (Simon et al., 2000; Simon and Collison, 2002; Simon et al. 2006;
Pollen-Bankhead et al., 2007; Langendoen and Simon, 2008).
43
____________________________________________________________________________________________________________
5.4.1 Determining Critical Bank-Stability Conditions
Model runs were carried out at varying flow depths and water table heights to simulate
the full range of bank-stability conditions between worst- and best-case scenarios.
Photographs and bank geometries for three of the sites are shown for example in Figure
22. Best case conditions generally occur when flow depth and water-table heights are
both low, or when the water table is low and flow depth is high (confining force from the
flow provides stability). Conversely, worst-case conditions for bank stability generally
occur when the water-table height is high and flow depth is low. Such conditions tend to
occur during the falling limb of a hydrograph, when the banks have become saturated,
and the flow recedes, removing the confining force from the flow, and decreasing bank
stability. Instability is indicated by the factor of safety (Fs) representing the resisting
forces divided by the driving forces) of 1.0 or less. A value greater than 1.0 indicates
stability where the resisting forces are greater than the gravitational driving forces.
Model runs were carried out with and without the vegetation assemblages recorded at
each site to obtain a range of conditions for sites of similar geometry and bank materials
with varying levels of riparian growth. Values for cohesion due to riparian root networks
was estimated using the root-reinforcement model, RipRoot (Pollen and Simon, 2005;
Pollen 2007), based on the species assemblage recorded at each site. Values ranged from
1.18 kPa for site 161.9 which just had a grass layer, to 31.6 kPa at RM 239.7, which was
covered in mature deciduous trees (Table 5).
101
100
99
98
99
97
96
95
97
94
93
95
92
91
93
90
89
88
91
87
89
4989
4990
4991
4992
4993
4994
4995
4996
4997
4998
85
4988
86
4990.5
4990
4992
4994
4996
4998
4995.5
5000.5
5005.5
5010.5
5015.5
5000
Figure 22. Bank geometry and photographs for sites at river miles 160.8 (Left), 114.7
(Middle) and 190.8 (Right).
For each of the five sites, critical conditions were found, in some cases for multiple
failure block geometries (Figure 23). In each critical case flow stage was low and water
table height was higher than flow stage (ie. a worst case drawdown condition). For sites
at river miles 75.3, 114.7, 161.9, and 190.9, two critical water-table heights were found,
44
____________________________________________________________________________________________________________
corresponding to two different failure block geometries. At RM 239.7, just one critical
failure block geometry was found. Results in Tables 6 and 7 show the critical water-table
heights at each site, and the estimated discharge that corresponds to a water-surface
elevation equal to those water table heights. This was done because of the assumption
that a given water-table height can be associated with a flow elevation if that flow is
maintained for at least several days. A raised water table in the bank materials results in a
loss of matric suction and an increase in positive pore-water pressures, both conducive to
bank instability. An estimate of the discharge associated with that flow elevation and
assumed water-table height is then used in a later analysis to determine the frequency that
this flow occurs. Discharge was estimated using USGS gage data from the dam upstream
of each site, and using known discharges and water surface elevations taken on two visits
to each site. The critical conditions, however, still assumed that flow elevations in the
channel were low, as on the receding limb of the hydrograph that produced the high water
table at its peak.
100.00
114.7
98.00
Water table depth (m) below bank top
6.67
Use water table
Input own pore pressures (kPa)
bank profile
96.00
ELEVATION (M)
base of layer 1
94.00
base of layer 2
92.00
base of layer 3
90.00
base of layer 4
88.00
Own Pore
Pressures
-4.00
kPa
Layer 1
Pore Pressure From
Water Table
-63.47
-34.04
-8.00
Layer 2
10.00
Layer 3
5.69
10.00
Layer 4
22.86
10.00
Layer 5
37.57
failure plane
86.00
water surface
84.00
82.00
4970.00
water table
4980.00
4990.00
5000.00
STATION (M)
5010.00
5020.00
5030.00
Factor of Safety
1.00
Unstable
Figure 23. Example of model output from BSTEM for critical bank-stability
conditions (Fs = 1.00) at river mile 114.7. Note that the blue triangle represents the
elevation of the water table.
45
____________________________________________________________________________________________________________
Table 5. Vegetation descriptions and additional cohesion due to roots added to the
top meter of the streambank at each site.
Site
(River Miles)
Vegetation Description
75.3
Approximately 20-yearold mixed deciduous
trees, with no vegetation
on the bank face
114.7
Layer of short grass on
bank top, with sparse
young trees (3-10 yearsold) on shallower angled
parts of the bank face
and toe
161.9
Layer of short grass on
bank top, with no
vegetation on the bank
face
190.9
Layer of short turf grass
on bank top, with sparse
young riparian trees and
a couple of large mature
deciduous trees
239.7
Mature deciduous trees
with sparse shrub
understorey
Species Assemblage Composition
Cohesion due to roots
added to top meter of
bank
20 yr-old Eastern Sycamore (50%)
25.1 kPa ± 5.3
20 yr-old River Birch (50%)
Turf grass (90%)
1.24 kPa ± 0.26
8 yr-old Black Willow (10%)
Turf grass (100%)
1.18 kPa ± 0.25
Turf grass (85%)
2.89 kPa ± 0.63
31.6 kPa ± 6.63
46
____________________________________________________________________________________________________________
Table 6. Critical bank-stability conditions without vegetation (the general case).
USGS GAGE
DATA USED
DISCHARGE AT
DAM
SITE
CRITIAL WATER
TABLE DEPTH
BELOW BANK TOP
(m)
SHEAR SURFACE
EMERGENCE POINT
BELOW BANK TOP
(m)
FAILURE
PLANE ANGLE
(degrees)
PREDICTED DISCHARGE
AT CRITICAL
WATER TABLE HEIGHT
(cms)
2469761
2469761
COFFEEVILLE
COFFEEVILLE
75.3
75.3
1.24
3.30
1.60
5.00
40
40
1204
746
2469761
2469761
COFFEEVILLE
COFFEEVILLE
114.7
114.7
4.90
6.70
3.29
9.50
60
50
1186
958
2467000
2467000
DEMOPOLIS
DEMOPOLIS
161.9
161.9
2.75
6.75
2.26
6.25
50
45
2765
1026
2467000
2467000
DEMOPOLIS
DEMOPOLIS
190.9
190.9
3.40
3.80
5.95
8.25
35
30
1877
1802
2447025
HEFLIN
239.7
3.90
3.60
55
2244
Table 7. Critical bank-stability conditions with existing vegetation. “-“ denotes there
are no critical conditions (bank is stable).
USGS GAGE
DATA USED
DISCHARGE AT
DAM
SITE
CRITIAL WATER
TABLE DEPTH
BELOW BANK TOP
(m)
SHEAR SURFACE
EMERGENCE POINT
BELOW BANK TOP
(m)
FAILURE
PLANE ANGLE
(degrees)
PREDICTED DISCHARGE
AT CRITICAL
WATER TABLE HEIGHT
(cms)
2469761
2469761
COFFEEVILLE
COFFEEVILLE
75.30
75.30
-
-
-
-
2469761
2469761
COFFEEVILLE
COFFEEVILLE
114.70
114.70
4.80
6.67
3.29
9.50
60
50
1186
958
2467000
2467000
DEMOPOLIS
DEMOPOLIS
161.90
161.90
2.45
6.7
2.26
6.25
50
45
2765
1026
2467000
2467000
DEMOPOLIS
DEMOPOLIS
190.90
190.90
3.18
3.67
5.95
8.25
35
30
1877
1802
2447025
HEFLIN
239.70
-
-
-
-
The tables above (Tables 6 and 7), showing the discharges required to raise water table to
critical heights show distinct differences with and without the addition of riparian
vegetation. For sites 75.3 and 239.7 where mature deciduous trees were present on the
bank top, the addition of cohesion due to roots increased bank factor of safety sufficiently
to maintain stable conditions even when the bank was fully saturated. Estimated
discharges required for critical conditions ranged from 750 to 2250 cms for scenarios
with no vegetation and from 950 cms for scenarios with vegetation. The root networks of
riparian species have a reinforcing effect on the bank, allowing vegetated banks to
withstand higher, less frequent discharges and associated water-table heights before
reaching critical conditions for bank failure.
47
____________________________________________________________________________________________________________
5.4.2 Frequency and duration of high-flow events and consequences for bank
instability.
Once estimates for discharges relating to critical water-table heights for each site had
been established, USGS gage records were used to determine how often discharges of
this magnitude occur. First the discharges were compared against mean daily data to see
what percentile of the full range of flows that the critical discharge represents (Table 8).
In this case, a high percentile (ie. 99%) is associated with a high flow, exceeded only 1%
of the time. In contrast, a flow percentile of 1% represents a relatively low flow
(exceeded 99% of the time).
The flow percentiles associated with the simulated critical water-table heights all
represent relatively high flows, with the lowest flow percentile being 65% for the site at
RM 75.3. In general though, the critical discharges occur even less frequently with
percentile values of about 80% or more. This translates to flows that are exceeded about
20% of the time. The lower the percentile value, the more frequent that critical bankstability conditions are likely to occur at a particular site. The reinforcing effect of top
bank grasses is apparent from the data in Table 8 with slightly less frequent (0.5%),
higher discharges associated with creating pore-water pressure conditions that are critical
for bank stability. The effect of mature, woody vegetation on bank-stability can be much
more pronounced as is indicated by the lack of critical conditions at sites 75.3 and 239.7
when the effects of vegetation are included (Table 8).
The assumption that a given water-surface elevation can be associated with an equal
water-table elevation is because if the flow is held at that elevation for an extended period
(days to weeks), lateral infiltration of water into the bank can occur, raising the watertable to that elevation. The result is a reduction in the shear strength (resistance) of the
bank material to failure. To quantify this effect and to provide support for this
assumption, it was necessary to estimate the amount of time it would take for river water
to laterally infiltrate into the bank mass. We used a conservative infiltration rate for silty
materials (sands would be faster) of 10-6 m/s. If this were the case, infiltration and
saturation of a zone 1.0 m into the bank can take place in about 10 days; 5 days for 0.5 m
while river stage remains high over that period. Thus, shallow failures would be
associated with 5-day durations and deeper failures with the 10-day durations. Infiltration
from precipitation was not taken into account in these model runs.
To quantify this effect and to investigate any differences between pre- and post-waterway
conditions, an analysis of the flow record at the stream gages associated with each
research site was conducted. The frequency that the critical discharges were maintained
for 5 and 10 consecutive days were calculated with results expressed as the average
number of occurrences per year (Tables 9 and 10). Critical conditions are shown to occur
as often as 3.6 times per year for the shallow, non-vegetated case at RM 75.3 but drops to
0 occurrences with the mature, riparian cover. It is interesting to note that in the nonvegetated (general) cases (5- and 10-day), the frequency of critical conditions increases
with distance downstream, in support of the general trend of increasing channel width
48
____________________________________________________________________________________________________________
and widening rates downstream that were discussed in previous sections. There is little
difference, between the occurrence of critical conditions pre- and post-waterway. In all
but the case at RM 161.9, the frequency of the 5- and 10-day duration critical events is
less than before 1985. It is doubtful, however, whether these differences are statistically
significant given the level of uncertainty in estimating the water-surface and water-table
elevations. It is also questionable as to whether differences are likely to appear between
the time periods used (pre 1985 and 1985-2003) because both periods represent periods
of channel disturbance where flows were being controlled by structures.
49
___________________________________________________________________________________________________________
Table 8. Percentiles of mean-daily flow at gage 02469761, Coffeeville, AL, 1960 – 2007, compared to the critical discharge for
bank failure at sites 75.3 and 114.7 (A); Percentiles at gage 02467000, Demopolis, AL, 1928 – 2007, compared to the critical
discharge for bank failure at sites 161.9 and 190.9 (B); and Percentiles at gage 02447025, Heflin (Gainesville), 1978 – 2007,
compared to the critical discharge for bank failure at site 239.7(C). Numbers in green represent critical discharge values with
vegetation and numbers in black represent critical discharge values without vegetation. Note that critical conditions with
mature, woody vegetation are not apparent for sites at river miles 75.3 and 239.7.
A)
Percentiles
0.100
0.250
0.500
0.600
0.650
0.700
0.725
0.750
0.775
0.780
0.800
0.900
0.950
0.990
B)
Q
(cms)
91
165
411
592
708
861
951
1059
1189
1226
1345
2283
3228
4587
Critical Q
for 75.3
Critical Q
for 114.7
746
958
962
1204
1186
1198
Percentiles
0.100
0.250
0.500
0.750
0.785
0.790
0.800
0.850
0.890
0.895
0.900
0.910
0.950
0.960
0.965
0.990
C)
Q
(cms)
65
116
294
875
1025
1051
1104
1407
1764
1824
1877
1985
2554
2778
2888
4106
Critical Q
for 161.9
Critical Q
for 190.9
1026
1048
1802
1826
1877
1919
2765
2895
Percentiles
0.1
0.25
0.5
0.75
0.9
0.95
0.96
0.97
0.98
0.99
Q
(cms)
21
52
139
360
850
1385
1560
1808
2147
2917
Critical Q
for 239.7
2244
50
___________________________________________________________________________________________________________
Table 9. Frequency (in average number of occurrences per year) of critical discharges maintained for 5- and 10-day periods
for simulation cases without vegetation. Data are separated into pre- and post-waterway and combined for both periods.
USGS GAGE
DATA USED
DISCHARGE AT
DAM
SITE
NUMBER OF TIMES Q > CRITICAL Q
5 days or more
PER YEAR
PRE TT
POST TT
10 days or more
PER YEAR
PRE TT
POST TT
BOTH
BOTH
2469761
2469761
COFFEEVILLE
COFFEEVILLE
75.3
75.3
2.92
3.71
2.91
3.50
2.91
3.61
1.88
2.92
1.68
2.32
1.78
2.63
2469761
2469761
COFFEEVILLE
COFFEEVILLE
114.7
114.7
3.08
3.50
2.91
3.23
3.00
3.37
1.92
2.75
1.77
2.14
1.85
2.46
2467000
2467000
DEMOPOLIS
DEMOPOLIS
161.9
161.9
0.62
2.64
0.59
3.00
0.61
2.76
0.29
1.91
0.18
1.77
0.25
1.87
2467000
2467000
DEMOPOLIS
DEMOPOLIS
190.9
190.9
1.64
1.64
1.27
1.23
1.52
1.51
1.16
1.18
0.36
0.41
0.90
0.93
2447025
HEFLIN
239.7
0.43
0.00
0.10
0.14
0.00
0.03
Table 10. Frequency (in average number of occurrences per year) of critical discharges maintained for 5- and 10-day periods
for simulation cases with vegetation. Data are separated into pre- and post-waterway and combined for both periods.
USGS GAGE
DATA USED
DISCHARGE AT
DAM
SITE
5 days or more
PER YEAR
PRE TT
POST TT
BOTH
10 days or more
PER YEAR
PRE TT
POST TT
BOTH
2469761
2469761
COFFEEVILLE
COFFEEVILLE
75.3
75.3
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
2469761
2469761
COFFEEVILLE
COFFEEVILLE
114.7
114.7
3.08
3.50
2.91
3.23
3.00
3.37
1.88
2.71
1.68
2.09
1.78
2.41
2467000
2467000
DEMOPOLIS
DEMOPOLIS
161.9
161.9
0.49
2.62
0.41
3.00
0.46
2.75
0.22
1.87
0.14
1.77
0.19
1.84
2467000
2467000
DEMOPOLIS
DEMOPOLIS
190.9
190.9
1.56
1.64
1.23
1.23
1.45
1.51
1.13
1.18
0.36
0.36
0.88
0.91
2447025
HEFLIN
239.7
0.00
0.00
0.00
0.00
0.00
0.00
51
___________________________________________________________________________________________________________
5.5 Simulations of Potential Mitigation Techniques to Reduce Streambank Erosion
and Widening
Previous sections have shown that streambank erosion is prevalent throughout the studied
reach of the Lower Tombigbee River with more than 50% of all banks experiencing
recent bank failures. Associated with this erosion is the loss of land and property. Taking
the average widening rate of 1.2m/y over the 29-year period of air-photo analysis and
multiplying by the length of the study reach (187 miles; 301 km) provides us with an
estimate of the total land loss over the period. This is equivalent to 1044 hectares, or
about 2580 acres. Given this considerable amount of land loss from bank instabilities, it
is reasonable to investigate potential strategies that could be used to reduce the magnitude
and frequency of bank failures along the study reach.
To reduce bank instability it is convenient to think again in terms of the driving and
resisting forces that lead to bank failures. Mitigation measures, therefore, should aim to
reduce bank-toe erosion by increasing the resistance of the bank toe and/or by reducing
the shear stress acting in this region. These measures might include the placement of rock
or the planting of woody vegetation. Practitioners have had considerable success using
rock structures known as “bendway wiers” that deflect flow away from the edge of the
bank back into the center of the channel. This technique may have added benefits on the
Lower Tombigbee River by creating greater shear stresses and, therefore, sediment
transport in the center of the channel. With regard to increasing stability of the upper part
of the bank alternative strategies might include the planting of woody vegetation to
increase shear strength by root reinforcement and/or grading the bank to a flatter slope.
These measures represent but a few of the types of strategies that could be employed and
are not meant as an exhaustive listing.
As an example, a series of alternative strategies to reduce the magnitude and frequency of
bank failures was simulated for the site at RM 114.7. Given that the BSTEM model
simulates failures in two dimensions (height and width), a reach length of 100 m was
assumed to provide results in m3. The simulations were conducted in such a way as to be
able to quantify the reduction in the frequency of failures and the volume of material
delivered to the channel by bank failures. Using mean-daily discharges from a high-flow
year (1991) to represent a worst-case flow conditions (Figure 7) and a bed slope of
0.000088 m/m, the toe-erosion and bank-stability sub-models were run iteratively for the
six major flow events of that year. The iterative modeling was carried out for each of the
following four bank conditions:
1. Existing conditions, no mitigation;
2. Rock placement along the bank toe;
3. Rock placement at the bank toe and 5-year old woody vegetation on the bank top;
and
4. Rock placement at the bank toe, 5-year old woody vegetation on the bank top,
grading the bank to a 45o (1:1) slope, and 5-year old woody vegetation on the regraded slope.
52
___________________________________________________________________________________________________________
For rock-toe protection, 25.6 cm-sized rock was used (default value for boulders). Fiveyear old woody vegetation was selected as a reasonable age for planted specimens where
root reinforcement effects become significant. As the vegetation ages, the additional
cohesion provided by the roots would increase.
For each set of conditions the total number of bank failures and the volume associated
with each failure was summed and then compared to the other alternatives to quantify the
effectiveness of each treatment. It is important to recognize that both the absolute
frequency and volume of failures likely represents an overestimate of what actually took
place during 1991. This is because once failure is simulated, the model does not account
for the fate of this material, which may often be deposited at the bank toe, providing a
buttressing (stabilizing) effect and serving to build-up the bank toe region. What is
relevant is the relative differences between the exiting case (no mitigation) and the
various alternatives.
Overall, this approach proved very successful in evaluating the effectiveness of each
treatment. For the initial case with existing bank conditions, 11 failures were simulated,
resulting in about 55,000 m3 of eroded bank sediment (Figure 24). Although the number
of bank failures is only reduced by one (to 10) for the case with toe protection, the
amount of lateral retreat and volume of failed material is drastically reduced (by about
500%) to about 9500 m3 (Figure 25). This is because the toe protection does not allow the
bank to be undercut at its base, thereby reducing the size of subsequent failures. The
addition of bank-top vegetation provides additional cohesive strength to the top 1.0 m of
the bank and resulted in a further reduction of failure frequency (to 8) and failure volume
(8500 m3) (Figure 26). This affect would probably be more pronounced if older
specimens were simulated because of greater root density and diameters. Alternative 4
which includes rock at the bank toe, grading the bank slope to 1:1 and placing woody
vegetation on the bank top and face, greatly reduces failure frequency (to 3) and shows
the smallest failure volume of all the cases (about 3200 m3) (Figure 27). Here, the
combined effects of the bank protection measures are:
•
•
•
•
Increased resistance to hydraulic erosion at the bank toe from the rock placement,
providing a critical shear stress of about 249 Pa;
Increased resistance to hydraulic erosion on the bank face due to the below
ground effects of the planted trees which provide an estimated increase in critical
shear stress of 17 Pa;
Decreased driving force for geotechnical failure associated with flattening the
bank slope to 1:1;
Increase resistance to bank failure by increasing the shear strength of the top 1.0
m through root reinforcement.
Results of this series of modeling runs for the site at RM 114.7 indicates that bank failure
frequency and failure volumes cannot be eradicated entirely, but can be significantly
reduced, from 11 failures to 3, and from almost 55,000m3 to about 3,200m3. Results from
each of the alternative strategies are summarized in tabular and graphic form in Figure
28). Obviously these treatments represent a broad range of options and costs.
53
___________________________________________________________________________________________________________
Recognizing that these findings represent only one site, we feel confident that similar
results would be obtained at other sites within the study reach. The fact that banks are
particularly high, and that high flows are maintained in some reaches by structures
exacerbates the bank-stability problems. Areas of dredging would also be less prone to
stabilization because of renewed deepening of the channel in these reaches. Finally, it
should be recognized that the alternatives that were simulated and presented in this
section, although representing a range of strategies is not meant to be an inclusive list of
all possible avenues of mitigation.
54
METERS
___________________________________________________________________________________________________________
100.0
98.0
96.0
94.0
92.0
90.0
88.0
86.0
84.0
82.0
4900.0
BEFORE
AFT ER
4920.0
4940.0
4960.0
4980.0
5000.0
5020.0
5040.0
METERS
1991 MODELING WITH NO BANK PROTECTION
DEPTH BELOW
TOP BANK
EVENT
HOURS DISCHARGE
NUMBER
DEPTH OF
FLOW
FLOW
ELEVATION
AVERAGE SHEAR
STRESS
Fs
kPa
TOE EROSION
AMOUNT
2
(m )
m
VOLUME OF
FAILURE
3
m
95.9
30.5
0.0
1.1
0.92
3.06
0.61
0.45
-
10.01
11.18
3.56
-
4227
9844
1498
-
1.34
>1
0.71
-
7.66
-
6864
-
23.9
5.4
3.87
1.31
1.92
-
-
-
7.5
45.0
12.1
0.93
1.11
0.84
-
6.38
-
2143
4101
-
6.6
5.3
40.5
1.66
0.4
0.83
0.15
-
3.33
8.39
8.61
7244
5590
6531
9.6
5.6
31.4
3.34
0.61
1.26
0.86
-
4.89
2.78
-
5553
1318
-
TOTALS:
294.68
66.79
54913
cms
m
m
m
120
120
6000
4000
0.00
1.65
14.49
12.84
98.5
96.8
48
800
7.95
6.54
90.5
7.2
2.2
2
120
48
3300
1030
2.28
6.13
12.21
8.36
96.2
92.4
4.7
4.2
7.2
1.1
3
288
48
3350
1700
2.24
3.72
12.26
10.77
96.3
94.8
5.7
6.1
4
456
4800
0.93
13.56
97.6
6.9
48
2600
2.91
11.58
95.6
192
3600
2.01
12.48
96.5
48
500
10.32
4.17
88.2
168
2200
3.27
11.22
95.2
48
1200
4.79
9.70
93.7
1
5
6
Fs AT
DRAWDOWN
WIDTH OF
FAILURE
Figure 24. Results of iterative modeling at RM 114.7 for bank condition 1: Existing
conditions with no mitigation.
55
METERS
___________________________________________________________________________________________________________
100.0
98.0
96.0
94.0
92.0
90.0
88.0
86.0
84.0
82.0
4900.0
BEFORE
AFT ER
4920.0
4940.0
4960.0
4980.0
5000.0
5020.0
5040.0
METERS
1991 MODELING WITH ROCK AT BANK TOE
DEPTH BELOW TOP
EVENT
HOURS
DISCHARGE
BANK
cms
m
m
m
kPa
TOE
EROSION
(m2)
120
6000
0
14.49
98.5
120
48
4000
800
1.65
7.95
12.84
6.54
96.8
90.5
7.6
5.9
2.1
24.94
5.09
-
0.45
1.4
1.28
2
120
48
3300
1030
2.28
6.133
12.21
8.357
96.2
92.4
5.3
3.5
4.35
-
3
288
3350
2.235
12.255
96.3
48
1700
3.72
10.77
94.8
6.8
4.6
456
4800
0.93
13.56
97.6
48
2600
2.91
11.58
5
192
48
3600
500
2.01
10.32
6
168
48
2200
1200
3.27
4.79
NUMBER
1
4
DEPTH OF
FLOW
FLOW
AVERAGE SHEAR
ELEVATION
STRESS
m
VOLUME OF
FAILURE
m3
0.73
0.23
-
6.6
3.68
2
-
1694
932
886
-
1.2
0.96
0.48
-
1.38
off face
711
741
15.64
0.03
0.87
1.69
0.57
-
1..16
2.13
-
875
639
-
95.6
6.3
6.5
8.29
-
0.69
2.08
0.61
-
off face
5.07
-
7.4
2551
-
12.48
4.17
96.5
88.2
7.1
2.46
-
2.78
3.74
0.58
-
1.49
-
446
-
11.22
9.7
95.2
93.7
6.3
5.4
-
2.35
2.25
1.63
-
-
-
TOTALS:
58.34
22.35
9482.4
Fs
Fs AT
DRAWDOWN
WIDTH OF
FAILURE
placement at the bank toe.
56
METERS
___________________________________________________________________________________________________________
100.0
98.0
96.0
94.0
92.0
90.0
88.0
86.0
84.0
82.0
4900.0
BEFORE
AFT ER
4920.0
4940.0
4960.0
4980.0
5000.0
5020.0
5040.0
METERS
1991 MODELING WITH ROCK AT TOE AND 5 YR-OLD VEGETATION ON BANK TOP
DEPTH BELOW
DEPTH OF
FLOW
AVERAGE SHEAR
EVENT HOURS DISCHARGE
TOP BANK
FLOW
ELEVATION
STRESS
cms
m
m
m
kPa
TOE EROSION
AMOUNT
(m2)
120
6000
0
14.49
98.5
120
4000
1.65
12.84
96.8
48
800
7.95
6.54
90.5
7.9
6.1
2.0
24.76
7.28
-
0.52
0.72
2.11
2
120
48
3300
1030
2.28
6.133
12.21
8.357
96.2
92.4
5.8
3.5
10.29
-
3
288
3350
2.235
12.255
96.3
48
1700
3.72
10.77
94.8
6.3
4.7
4
456
48
4800
2600
0.93
2.91
13.56
11.58
97.6
95.6
5
192
48
3600
500
2.01
10.32
12.48
4.17
6
168
48
2200
1200
3.27
4.79
11.22
9.7
NUMBER
1
m
VOLUME OF
FAILURE
m3
0.9
2.48
-
6.46
3.78
failure on face
-
3190
1832
42
-
1.68
1.82
1.25
-
-
-
6.55
0.62
0.82
1.47
0.8
-
1.12
1.8
-
1029
540
-
5.8
-
12.65
-
0.97
-
0.27
failure on face
2.4
223
1223
96.5
88.2
5.5
1.34
3.81
-
1.61
3.2
0.45
-
1.4
-
421
-
95.2
93.7
3.8
1.7
4.33
-
1.67
1.6
1.23
-
-
-
TOTALS:
70.29
16.96
8500
Fs
Fs AT
DRAWDOWN
placement at the bank toe and 5-year old woody vegetation on the bank top.
WIDTH OF
FAILURE
57
METERS
___________________________________________________________________________________________________________
100.0
98.0
96.0
94.0
92.0
90.0
88.0
86.0
84.0
82.0
4920.0
BEFORE
GRADED
AFT ER
4940.0
4960.0
4980.0
5000.0
5020.0
5040.0
METERS
1991 MODELING WITH ROCK AT TOE AND 5 YR-OLD VEGETATION ON BANK TOP, plant cuttings (17Pa) on 45 degree graded bank
DEPTH BELOW
DEPTH OF
FLOW
AVERAGE SHEAR TOE EROSION
TOP BANK
FLOW
ELEVATION
STRESS
AMOUNT
EVENT HOURS DISCHARGE
Fs
NUMBER
cms
m
m
m
kPa
(m2)
Fs AT
DRAWDOWN
WIDTH OF
FAILURE
VOLUME OF
FAILURE
m
m3
1
120
120
48
6000
4000
800
0
1.65
7.95
14.49
12.84
6.54
98.5
96.8
90.5
7.6
6.9
2.8
-
3.15
1.15
2.31
1.19
0.44
-
3.17
-
951
-
2
120
48
3300
1030
2.28
6.133
12.21
8.357
96.2
92.4
6.7
4.2
-
4.85
1.46
0.71
-
2.51
-
1162
-
3
288
48
3350
1700
2.235
3.72
12.255
10.77
96.3
94.8
6.4
5.5
-
3.6
3.08
2.06
-
-
-
4
456
48
4800
2600
0.93
2.91
13.56
11.58
97.6
95.6
7.3
6.0
-
6.15
3.11
2.74
-
-
-
5
192
48
3600
500
2.01
10.32
12.48
4.17
96.5
88.2
6.6
1.74
-
3.9
5.82
0.94
-
2.85
-
1063
-
6
168
48
2200
1200
3.27
4.79
11.22
9.7
95.2
93.7
5.5
5.0
-
3.46
3.31
2.47
-
-
-
TOTALS:
0
8.53
3176
placement at the bank toe, 5-year old woody vegetation on the bank top, grading the
bank to a slope of 45 degrees (1:1), and 5-year old vegetation on the bank face.
58
___________________________________________________________________________________________________________
NUMBER OF FAILURES
VOLUME OF FAILED
MATERIAL ALONG 100m REACH
m3
NO MITIGATION
11
54913
WITH ROCK AT TOE
10
9482
WITH ROCK AT TOE,
5 YR-OLD SAPLINGS ON BANK TOP
8
8500
3
3176
MITIGATION STRATEGY
WITH ROCK AT TOE,
5 YR-OLD SAPLINGS ON BANK TOP,
BANK FACE GRADED TO 45 DEGREES
AND PLANT CUTTINGS ON BANK FACE
100.0
98.0
96.0
METERS
94.0
92.0
90.0
BEFORE
AFT ER WIT H VEGET AT ION AND ROCK T OE
88.0
AFT ER WIT H ROCK T OE
86.0
AFT ER WIT H NO T REAT MENT
84.0
82.0
4860.0
AFT ER WIT H ROCK T OE, VEGET AT ION ON T OP AND
PLANT CUT T INGS ON GRADED FACE
4880.0
4900.0
4920.0
4940.0
4960.0
4980.0
5000.0
5020.0
METERS
Figure 28. Summary of iterative modeling results for alternative mitigation
strategies showing the number and volumes of failures for each bank condition.
5040.0
59
___________________________________________________________________________________________________________
6. SUMMARY and CONCLUSIONS
This study presents a preliminary evaluation of bank-stability conditions along the Lower
Tombigbee River. For this reason, we focused on providing a general overview of the
magnitude and distribution of bank-stability problems along the river and how this
instability might be related to the activities in the operation of the Tennessee-Tombigbee
Waterway. Two primary issues made this determination particularly complex. The first
was that any effects on the study reach by the waterway per se are muted due to the
presence of Demopolis and Coffeeville Dams which alter the transfer of flow and
sediment, and pre-date the waterway. The second issue was that the Tombigbee River has
been controlled by a series of dams for more than a century and channel adjustments to
these kinds of disturbances generally occur rapidly at first, and then attenuate with time.
The construction of Gainesville (Heflin) Lock and Dam in the late 1970s upstream of the
study reach, and associated, intermittent dredging activities represent the only direct
modifications that impact the Lower Tombigbee and are related to the waterway itself.
Still it should be emphasized that dams have profound effects on alluvial channels such
as the Lower Tombigbee River, generally by trapping sediment on their upstream side,
resulting in incision downstream.
Bank erosion by mass failure is often the result of hydraulic erosion of the bank toe that
steepens the bank, leading to subsequent failure by gravity. These processes are
exacerbated in a channel that has been deepened, either by bed erosion or by dredging.
Magnitudes and extents of bank erosion in this study were determined over a 29-year
period (1974-2003) based on interpretation of a series of aerial photographs. Banks that
have recently failed were determined by aerial reconnaissance of the reach and represent
more than 50% of all banks. Some reaches had widened as much as 60 m over the period
but rates show a clear spatial distribution upstream and downstream of the dams. Rates
generally are low in reaches just upstream of the structures because water is held, thus
supporting the bank. Maximum rates occur some distance downstream from each
structure where post-dam channel incision has increased bank heights. An increased rate
of average and peak flows over the period, similarly contribute to greater instability. The
increase in flows, however, seems to be the result of a trend of increasing precipitation
over the last century. Operation of the dams has shifted the timing of flows by storing
water during the spring so it is available for navigation in the summer months. The
highest flows generally occur in the winter months. Still, the average rate of channel
widening was determined to be about 1.2 m/y over the entire study reach (river miles 72259). This rate is in excess of the rate that would be expected in stable, alluvial streams of
the region and represents a loss of 2580 acres of land adjacent to the channel.
Five reaches (at RM 75.3, 114.7, 161.9, 190.9 and 239.7) were investigated in detail to
determine critical conditions for bank instability and the frequency that these conditions
occur. Geotechnical and erodibility tests of the bank materials were conducted to provide
data for simulations of bank stability using the Bank-Stability and Toe-Erosion Model
(BSTEM) developed by the National Sedimentation Laboratory. Critical conditions
occurred when water table heights were high. The flow percentiles associated with the
simulated critical water-table heights all represent relatively high flows. In general
60
___________________________________________________________________________________________________________
though, the critical conditions occur with discharges that are exceeded about 20% of the
time, representing a fairly frequent occurrence.
Given the considerable amount of land loss from bank instabilities, we investigated
potential strategies that could be used to reduce the magnitude and frequency of bank
failures along a study reach (RM 114.7). To reduce bank instability mitigation measures
should aim to reduce bank-toe erosion by increasing the resistance of the bank toe and/or
by reducing the shear stress acting in this region. Iterative modeling was carried out for
each of the following four bank conditions for a high-flow year (1991) to determine
differences in the frequency and total volume of failed material:
1. Existing conditions, no mitigation;
2. Rock placement along the bank toe;
3. Rock placement at the bank toe and 5-year old woody vegetation on the bank top;
and
4. Rock placement at the bank toe, 5-year old woody vegetation on the bank top,
grading the bank to a 45o (1:1) slope, and 5-year old woody vegetation on the regraded slope.
Results of this series of modeling runs for the site at RM 114.7 indicates that bank failure
frequency and failure volumes cannot be eradicated entirely, but can be significantly
reduced, from 11 failures to 3, and from almost 55,000 m3 to about 3,200 m3. Obviously
these treatments represent a broad range of options and costs. Recognizing that these
findings represent only one site, we feel confident that similar results would be obtained
at other sites within the study reach. The fact that banks are particularly high, and that
high flows are maintained in some reaches by structures exacerbates the bank-stability
problems. Areas of dredging would also be less prone to stabilization because of renewed
deepening of the channel in these reaches. Finally, it should be recognized that the
alternatives that were simulated are not meant to be an inclusive list of all possible
avenues of mitigation.
6.1 Suggestions for future work
As this work represented a preliminary investigation, there were only limited resources
available to conduct detailed data-collection activities at specific areas of severe bank
instability. Because of this, the rigorous bank-stability modeling could only be conducted
at five sites. To provide greater confidence in the determination of critical bank-stability
conditions and the effects of potential mitigation measure, additional unstable sites
should be investigated. The study reach should also be extended upstream to Gainesville
(Heflin) Dam where we suspect that particularly unstable conditions are prevalent. In
addition, an enhanced version of BSTEM that includes a groundwater model is nearing
completion and should be available in the near future. This enhanced version would allow
for a more accurate assessment of the role of changes in bank strength, and, therefore,
bank stability, due to fluctuating flow elevations, and may help provide a flowmanagement strategy to reduce the frequency of critical conditions. A critical element
61
___________________________________________________________________________________________________________
that was not incorporated in this study was the effects of wave action (from boats) on
bank-toe erosion and the consequent bank failure. We propose that a future study should
include the development of a submodel for BSTEM that incorporates this process. This
would be the first of its kind and represent a major improvement in the prediction of bank
instability along the Lower Tombigbee River and other navigable waterways.
62
___________________________________________________________________________________________________________
7. REFERENCES
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and approaches for engineering management. Earth Surface Processes and Landforms,
20: 611 – 628.
Stine, J.K. 1993. Mixing the Waters: Environment, Politics and the Building of the
Tennessee-Tombigbee Waterway. University of Akron Press, Akron, Ohio. pp336.
Tennessee-Tombigbee Waterway Development Authority,
http://www.tenntom.org/organizations/ttworgsttwda.html, accessed March 2008.
Thorne, C. R., Murphey, J. B. and Little, W. C., 1981. Stream Channel Stability,
Appendix D, Bank Stability and Bank Material Properties in the Bluffline Streams of
Northwest Mississippi. U.S. Department of Agriculture, Agricultural Research Service,
National Sedimentation Laboratory. Oxford, MS. 227 p.
Thorne, C.R., 1982. Processes and Mechanisms of River Bank Erosion. In, Hey, R.D.,
Bathurst, J.C. and Thorne, C.R., (Eds.). Gravel-Bed Rivers, John Wiley and Sons,
Chichester, England. 227-271 p.
Underwood, K.D. and Imsand, F.D. 1985. Hydrology, Hydraulic and Sediment
Considerations of the Tennessee-Tombigbee Waterway. Environ. Geol. Water Sci., 7(12): 69-90.
USACE 2008, http://tenntom.sam.usace.army.mil/, accessed March 2008.
Williams, G.P and Wolman, M.P. 1984. Downstream effects of dams on alluvial rivers.
U.S. Geol. Surv., Prof. Pap., Vol/Issue: 1286
65
___________________________________________________________________________________________________________
APPENDIX A
Bank stratigraphy, geotechnical data and channel cross-sections for each site
studied in detail
66
___________________________________________________________________________________________________________
RM 75.3
0
Brown sandy loam
0.95 m
0.8 m
1.10 m
Bulk Sample at 0.8 m:
Sand 74.6 % Silt/Clay 25.4 %
Apparent Cohesion = 3.48 kPa
Effective Cohesion = 1.98 kPa
Friction Angle = 31.9 o
Matric Suction = 8.6 kPa
Saturated Unit Weight = 17.3 kN/m3
Representative BST data
Gravel 0.1 % Sand 80.4 %
Silt/Clay 19.5%
70
y = 0.6222x + 3.4836
2
R = 0.9778
60
Brown gray mottled clay
2.14 m
Apparent Cohesion =6.24 kPa
Effective Cohesion =3.19 kPa
Shear Stress in kPa
50
2.00 m
40
30
Depth = 0.95m
Left Bank
20
10
0
0
20
40
60
80
100
120
100
120
Normal Stress in kPa
4.00 m
50
y = 0.37x + 7.3
45
Depth = 0.35m
into toe
Left Toe
Shear Stress in kPa
40
6.00 m
35
2
R = 0.9956
30
25
20
15
Brown/gray mottled clay at toe
6.85 m
Apparent Cohesion = 7.3 kPa
Effective Cohesion = 6.33 kPa
Avg. Critical Shear Stress = 5.99 Pa
Avg. Erodibility Coefficient =3.10 m3/N-s
10
5
0
0
20
40
60
80
67
___________________________________________________________________________________________________________
RM 114.7
1.1 m
2.00 m
Gravel/red sand
Brown Loam 1.5 m
60
Gravel 1.95 % Sand 80.9 %
Silt/Clay 17.1%
y = 0.59x + 4.9
2
R = 0.9847
50
Shear Stress in kPa
0
0.4 m
40
30
Depth = 1.5m
Right Bank
20
4.00 m
10
5.5 m
0
0
10
20
30
6.00 m
Brown Loam with clay
7.3 m
10.00 m
Saturated Unit Weight = 17.29
Gravel 1.95% Sand 86.3 %
Silt/Clay 13.7 %
11.5 m
12.00 m
14.00 m
50
60
70
80
90
70
y = 0.54x + 3.9
60
Depth = 7.3m
Right Bank
50
Shear Stress in kPa
8.00 m
40
2
R = 0.972
40
30
20
10
Blue/gray sandy clay
Avg. Critical Shear Stress =
0.02 Pa
Avg. Erodibility Coeffecient =
15.7 m3/N-s
0
0
20
40
60
80
100
120
68
___________________________________________________________________________________________________________
RM 161.9
0
Brown Loam 0.70 m
35
y = 0.2544x + 5.3397
2
R = 0.9792
30
25
Shear Stress in kPa
Sand 69.1 % Silt/Clay 31.0%
20
15
Depth = 0.7m
Right Bank
10
2.6 m
5
Brown Sand
0
0
20
40
60
80
100
120
4.26 m
Brown Loam with clay
4.60 m
Saturated Unit Weight = 17.9kN/m3
60
y = 0.47x + 11.3
2
Shear Stress in kPa
Brown Sand
R = 0.9915
50
40
30
20
Depth = 4.6m
Right Bank
10
0
0
20
40
60
80
100
69
___________________________________________________________________________________________________________
RM 190.9
0
Brown sand w/gravel
2.00 m
Silt/Clay 6.39%
Silt/Clay 9.29 %
Brown silty sand
1.5 m
4.00 m
60
y = 0.5x + 9.6667
50
Shear Stress in kPa
1.3 m
2
R = 0.9868
40
30
20
Depth = 1.5m
Right Bank
10
0
0
10
20
30
6.00 m
40
50
60
70
80
90
8.00 m
60
8.7 m
y = 0.3591x + 7.9654
Saturated Unit Weight =
17.19kN/m3
40
12.00 m
14.00 m
2.94 Pa
Avg. Erodibility Coefficient =
1.89 m3/N-s
Shear Stress in kPa
10.00 m
Brown gray mottled clay
8.9 m
50
2
R = 0.9981
30
20
Depth = 8.9m
Right Bank
10
0
0
20
40
60
80
100
120
140
70
___________________________________________________________________________________________________________
RM 239.7
0
BST at 1.3 m
Orange silty sand
1.7 m
2.00 m
Silt/Clay 15.9%
Silt/Clay 5.44 %
Orange sand, moves to tan
with depth
70
y = 0.6221x + 6.4351
4.00 m
2
R = 0.9946
60
Shear Stress in kPa
50
6.00 m
6.4 m
White clay/shale
40
30
20
Depth = 1.3m
Right Bank
10
8.00 m
10.00 m
12.00 m
Bulk Sample from toe:
67.0 Pa
Avg. Erodibility Coefficient =
0.10 m3/N-s
0
0
20
40
60
80
100

analysis of streambank erosion along the lower tombigbee river

Transcription

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