Fluvial Processes and Landforms Fluvial Processes and

Fluvial Processes and Landforms
Chapter
16
Fluvial Processes and Landforms
Landforms shaped by running water are described as
fluvial landforms; they develop from the fluvial processes of
overland flow and stream flow.
Weathering and mass wasting operate in concert with
overland flow, providing the rock and mineral fragments
that are carried into stream systems.
Fluvial Processes and Landforms
Erosional and Depositional Landforms
All agents of denudation perform the geological activities of erosion,
transportation, and deposition.
These are sequential landforms, which because they are shaped by
progressive removal of the bedrock mass, are also erosional landforms.
Fragments of soil, regolith, and bedrock are transported and deposited
elsewhere to make an entirely different set of surface features,
collectively referred to as depositional landforms.
1
Slope Erosion
Fluvial action starts with overland flow, which consists of a
thin film of water or tiny rivulets that move across the
ground.
Excess rain and melting snow can also reach the ground
ground,
either as throughfall that drips from the vegetation or as
stemflow that trickles along branches and down stems and
trunks.
Slope Erosion
The time required to initiate overland flow depends on the
antecedent conditions, which determine the amount of water in
the soil prior to a precipitation event.
p
and behind
Some will also be held in small depressions
obstructions, such as little twigs and leaves, through surface
retention and depression storage.
Slope Erosion
Once the storage capacities of the soil and surface depressions
are exceeded, the water will begin to move downslope in a
process called infiltration excess overland flow.
y saturated from a p
previous rainstorm and
If the soil is fully
depression storage is filled, then runoff will occur immediately
when another rainstorm begins through the process of saturation
overland flow.
2
Slope Erosion
Hortonian overland flow (unsaturated overland flow)
develops when rainfall intensity exceeds the rate at which
water can be absorbed by the soil, even though the soil is
dry.
Hortonian overland is the principle stormflow generation
process in thinly vegetated watersheds in arid and semiarid regions under intense rainfall, especially where surface
sealing can limit soil infiltration capacity.
Slope Erosion
As it moves across the surface, overland flow will entrain
small particles of mineral matter, beginning the first stage in
fluvial erosion.
At this stage
stage, the water is not concentrated in a well
welldefined channel, and movement of particles occurs as
sheet erosion.
3
Slope Erosion
As the water moves across the surface, it exerts a tractive
force on the loose soil particles, which become entrained in
the flow.
The size of the particles that can be transported depends
on the gradient of the surface, the volume of water that
moves across it, and the degree to which the particles are
bound by plant roots or held down by a mat of leaves.
Soil particles carried in runoff typically range from about
0.001 to 1.0 mm in diameter.
Slope Erosion
Properties that determine erodibility, such as soil aggregation
and shear strength, show systematic seasonal variation and are
strongly affected by climatic factors, including rainfall distribution
and frost action.
Under equilibrium conditions, some soil is removed each year,
but it is replaced by material freshly weathered from the
underlying regolith.
Soil scientists refer to this continual replacement process as the
geologic norm.
Average Annual Runoff in Canada
4
Slope Erosion
Accelerated Erosion
The rate of soil erosion may be greatly increased by human
activities or by natural events of unusual magnitude
(removal of the plant cover by fire, logging, or other
disturbances); this results in a state of accelerated erosion
erosion.
The surface soil is removed much faster than it can be
formed, progressively exposing the underlying soil layers.
5
Slope Erosion
Accelerated Erosion
Eroded soil materials reach the base of the slope, where the
gradient lessens as it merges with the valley floor (particles
come to
t restt and
d accumulate
l t as colluvium).
ll i )
Because colluvial If not deposited as colluvium, the sediment
carried by overland flow ultimately reaches a stream and is
transported downstream, where it may accumulate as alluvium
in layers on the valley floor.
Slope Erosion
Accelerated Erosion
The quantity of sediment that overland flow removes from
an area in a given period of time is called sediment yield.
Estimates of sediment yield - Revised Universal Soil Loss
Equation (RUSLE):
A = R × K × LS × C × P
Slope Erosion
Accelerated Erosion
A = R × K × LS × C × P
A = estimated average soil loss in tonnes per hectare per year,
R = rainfall-runoff erosivity factor based on the average erosive force of the annual
rainfall,
rainfall
K = soil erodibility,
LS = slope-length factor that represents a ratio of soil loss under specified conditions to a
“standard” slope,
C = cover management factor, used to determine the effectiveness of soil and vegetation
management systems on erosion rates,
P = support practice factor that reflects the impact of practices designed to reduce the
amount and rate of the water runoff and thus reduce the amount of erosion.
6
Slope Erosion
Rilling and Gullying
Where the land is steeply sloping, runoff from torrential rains can cut
more pronounced channels through the process of rill erosion.
In this way
way, a series of closely spaced channels can be scored into the
soil over a short period of time.
If the rills are not destroyed by soil tillage, they may join together into
larger channels.
These rapidly deepen and soon become gullies—steep-walled
trenches whose upper ends grow progressively upslope.
7
The Work of Streams
Stream erosion is the progressive removal of mineral
material from the floor and sides of the channel, whether
bedrock or regolith.
Stream transportation consists of movement of the eroded
particles dragged over the stream bed
bed, suspended in the
body of the stream, or held in solution as ions.
Stream deposition is the accumulation of transported
particles on the stream bed and flood plain or on the floor
of a lake, where it may be temporarily held until it is carried
to the oceans.
The Work of Streams
Stream Erosion
The force of the flowing water not only sets up a dragging
action on the bed and banks, but also causes particles to
strike against these surfaces.
Dragging and impact can easily erode alluvial materials,
such as gravel, sand, and silt.
This form of erosion, called hydraulic action, can excavate
enormous quantities of channel material in a short time.
The Work of Streams
Stream Erosion
The force of flowing water is greatly affected by friction
between the water and the channel bed and sides, and so
p according
g to the length
g of the
is related to channel shape
wetted perimeter.
The wetted perimeter is the portion of the cross-sectional
area of a river that is in contact with the channel substrate.
8
The Work of Streams
Stream Erosion
The average flow velocity in a stream channel can be
calculated from Manning’s equation based on channel
gradient and the roughness of the sand, gravel, and cobble
surfaces over which the water flows:
V = 1.49/n (R2/3S1/2)
where V is the velocity (m s-1), R is the hydraulic radius in metres
calculated by dividing the cross-sectional area of the channel by the
wetted perimeter, S is the gradient of the energy line in metres per
metre (m m-1), and n is the coefficient of roughness, specifically known
as Manning’s roughness coefficient (function of friction along the
channel bed).
9
The Work of Streams
Stream Transportation
Stream load refers to the materials carried or transported by a stream.
Stream transport occurs in three ways (depends on particle size).
Finer particles, such as clay and silt, are carried in suspension
(suspended load).
Sand, gravel, and larger particles move as bed load by rolling,
bouncing, or sliding along the channel
floor.
Dissolved matter is transported invisibly in the form of chemical ions.
10
The Work of Streams
Capacity of a Stream to Transport Load
The maximum solid load of debris that a stream can carry at a
given discharge is a measure of stream capacity.
This load is usually recorded in units of metric tonnes per day
passing downstream at a given location.
Total solid load includes both bed load and suspended load.
A stream’s capacity to carry load increases as its velocity
increases, because the swifter the current, the more intense the
turbulence.
11
The Work of Streams
Stream Deposition
When bed load increases and exceeds the transporting capacity of the
stream, the sediment accumulates as river bars of sand, gravel, and
pebbles.
These deposits raise the elevation of the stream bed, a process called
aggradation.
The development of distinctive channel forms reflects this type of
adjustment to flow regime and sediment load.
The three basic channel patterns generally recognized
are straight, meandering, and braided.
The Work of Streams
Aggregation and Flood Plain Development
Sediment introduced at any point in a stream will be
gradually spread along its whole length and slowly build up
the land surface through aggradation.
River channels of unconsolidated materials can be easily
and continually reworked by changing flow regimes.
The dynamic nature of the flood plain landscape is
exemplified by the changing shape and position of
meanders.
12
The Work of Streams
Aggregation and Flood Plain Development
The shapes and positions of meanders change over time
as the outer banks erode and deposition builds up the inner
banks (related to different rates of erosion and deposition
within the curving channel).
The deepest and fastest flowing water is represented by
the thalweg.
The Work of Streams
Aggregation and Flood Plain Development
The current slowest on the inside of each bend –
deposition results in the accumulation of long, curving
deposits of sediment called point bars.
On the outside of the bend, the bank is undercut and
collapses - shortens the river course and leaves a
meander loop to form a cutoff (followed by deposition of
silt and sand across the ends of the abandoned channel,
producing an oxbow lake).
13
The Work of Streams
Aggregation and Flood Plain Development
Meandering rivers widen flood plains, so that broad level areas
lie on both sides of the river channel.
plains bring
g an infusion of dissolved mineral
Floods on flood p
nutrients, which help retain the high natural fertility of flood plain
soils.
As the current in the overbank flow slackens (during floods), the
coarser particles are deposited in a zone adjacent to the channel
- an accumulation of higher land on either side of the channel
known as a natural levee.
14
15
The Work of Streams
Aggregation and Flood Plain Development
A meandering river can create a sequence of terraces
(alluvial terraces) when it moves back and forth across the
valley, progressively cutting into deeper alluvial fill.
As each terrace is cut into the original sediments, the river
leaves a relic flood plain at a higher elevation.
16
Stream Gradation
Over time, the gradients of the stream segments that make
up the drainage network tend to adjust so that they carry
the average load of sediment that they receive from slopes
and inflowing channels (equilibrium).
A stream
t
in
i this
thi equilibrium
ilib i
condition
diti iis called
ll d a graded
d d
stream.
17
Stream Gradation
Landscape Evolution of a Graded Stream
Regional changes occur throughout a landscape during the stream
gradation process - initially waterfalls and rapids occur in stretches of a
stream with steep gradients (capacity of the stream exceeds the load
supplied to it - little or no alluvium accumulates in the channels)
channels).
As a stream gradient diminishes, erosion reduces the gradient of the
channel segment so the slope more closely approximates the average
gradient of that section of the stream.
Its capacity to move bed load also decreases - the load supplied to the
fluvial system eventually matches its capacity to transport it, and the
major river achieves its graded profile.
Stream Gradation
Landscape Evolution of a Graded Stream
As lateral cutting continues, the width of the flood plain increases and
the channel develops the sweeping meandering course.
The flood plain widens into a continuous belt of flat land in the valley
floor.
Flood plain development reduces the frequency with which the river
attacks and undermines the adjacent valley wall.
Weathering, mass wasting, and overland flow act to reduce the
steepness of the valley side slopes.
18
19
Stream Gradation
Waterfalls
The stream gradation process smoothes the profile of a stream by
draining lakes and removing falls and rapids (large waterfalls on major
rivers are comparatively rare).
Faulting and dislocation of large crustal blocks have caused
spectacular waterfalls on several rivers (Victoria Falls on the Zambezi
River in East Africa).
Another class of large waterfalls involves new river channels resulting
from past glacial activity (erosion and deposition by vast moving ice
sheets greatly disrupted drainage patterns).
20
Stream Gradation
Entrenched Meanders
When tectonic activity causes regional uplift of a land surface, the
gradient of pre-existing stream channels increases; this rejuvenates the
river system, which continues to erode the bedrock with renewed
energy.
Slow but considerable uplift can lead to the formation of incised
meanders:
Entrenched meanders are symmetrical in cross-section with little
difference in gradient on the opposing valley sides.
Asymmetrical ingrown meanders occur when uplift of the land is slow incision is less rapid and the river can cut laterally as well as vertically.
21
The Geographic Cycle
The sequence of landscape development (first described
by William Morris Davis in 1899) is called the geographic
cycle - assumes that landscapes are a function of structure,
process, and time – stages:
Youthful
Mature
Old Age (peneplain)
Rejuvenation
Fluvial Processes in Arid Climates
Although deserts have low precipitation, running water
actually forms many landforms in these arid regions.
A specific location in a dry desert may experience heavy
rain only once in several years and, at these times, the
stream
t
channels
h
l carry water
t and
d work
k as agents
t off
erosion, transportation, and deposition.
Fluvial processes are especially effective in shaping desert
landforms because the sparse vegetation cover offers little
or no protection to the land surface, allowing large
quantities of rock debris to be washed into the streams
from the barren slopes.
22
A Flash Flood
Fluvial Processes in Arid Climates
Alluvial Fans
One common landform (in arid climates) built by braided, aggrading
streams is the alluvial fan, which develops as a low cone of sands and
gravels.
Often adjacent fans coalesce to form a bajada,
bajada or apron of coarse
sediment along the foot of a mountain scarp.
In regions of internal drainage, where there is no outflow to the sea,
arid basin can become a closed system for the transport of sediment
(only the hydrologic system is open, with water entering as precipitation
and leaving as evaporation).
Consequently, fine sediment and precipitated salts accumulate in the
basin to form a level salt flat or playa (some playas form a salt lake).
23
A Look Ahead
Chapter 17 discusses the role of wind in desert and coastal
environments.
The erosional and depositional features created by ice
action are discussed in Chapter 18
18.
24