brantley county, georgia - Georgia Flood Mapping Program

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brantley county, georgia - Georgia Flood Mapping Program
BRANTLEY COUNTY,
GEORGIA
AND INCORPORATED AREAS
Community
Number
Community Name
BRANTLEY COUNTY
(UNINCORPORATED AREAS)
HOBOKEN, CITY OF
130012
130013
NAHUNTA, CITY OF
130014
Brantley County
EFFECTIVE:
September 25, 2009
Federal Emergency Management Agency
FLOOD INSURANCE STUDY NUMBER
13025CV000A
NOTICE TO
FLOOD INSURANCE STUDY USERS
Communities participating in the National Flood Insurance Program have established repositories of
flood hazard data for floodplain management and flood insurance purposes. This Flood Insurance
Study may not contain all data available within the repository. It is advisable to contact the community
repository for any additional data.
This preliminary revised Flood Insurance Study contains profiles presented at a reduced scale to
minimize reproduction costs. All profiles will be included and printed at full scale in the final
published report.
Part or all of this Flood Insurance Study may be revised and republished at any time. In addition, part
of this Flood Insurance Study may be revised by the Letter of Map Revision process, which does not
involve republication or redistribution of the Flood Insurance Study. It is, therefore, the responsibility
of the user to consult with community officials and to check the community repository to obtain the
most current Flood Insurance Study components.
Initial Countywide FIS Effective Date: September 25, 2009
TABLE OF CONTENTS
Page
1.0 INTRODUCTION ........................................................................................................................... 1 1.1 Purpose of Study ................................................................................................................. 1 1.2 Authority and Acknowledgments ....................................................................................... 1 1.3 Coordination ....................................................................................................................... 2 2.0 AREA STUDIED ............................................................................................................................ 2 2.1 Scope of Study .................................................................................................................... 2 2.2 Community Description ...................................................................................................... 3 2.3 Principal Flood Problems.................................................................................................... 5 2.4 Flood Protection Measures ................................................................................................. 6 3.0 ENGINEERING METHODS .......................................................................................................... 6 3.1 Hydrologic Analyses........................................................................................................... 6 3.2 Hydraulic Analyses ............................................................................................................. 9 3.3 Vertical Datum .................................................................................................................. 10 4.0 FLOODPLAIN MANAGEMENT APPLICATIONS ................................................................... 10 4.1 Floodplain Boundaries ...................................................................................................... 11 4.2 Floodways ......................................................................................................................... 11 5.0 INSURANCE APPLICATION ..................................................................................................... 12 6.0 FLOOD INSURANCE RATE MAP ............................................................................................. 14 7.0 OTHER STUDIES ......................................................................................................................... 14 8.0 LOCATION OF DATA ................................................................................................................. 15 9.0 BIBLIOGRAPHY AND REFERENCES ...................................................................................... 17 FIGURES
Figure 1. Floodway Schematic ............................................................................................................... 12
TABLES
Table 1: Scope of Study ........................................................................................................................... 3 Table 2: Summary of Discharges ............................................................................................................. 8 Table 3: Floodway Data Table ............................................................................................................... 13 Table 4: Community Map History ......................................................................................................... 16 i
TABLE OF CONTENTS (Cont’d)
EXHIBITS
Exhibit 1 – Flood Profiles
Buffalo Creek
Panels
01P-02P
Little Buffalo Creek
Panels
03P-04P
Little Buffalo Creek Tributary
Panel
05P
Satilla River
Panels
06P-08P
Tributary A
Panel
09P
Tributary B
Panel
10P
Tributary No. 1
Panels
11P-12P
Exhibit 2 – Flood Insurance Rate Map Index (Published Separately)
Flood Insurance Rate Maps (Published Separately)
ii
FLOOD INSURANCE STUDY
BRANTLEY, GEORGIA AND INCORPORATED AREAS
1.0
INTRODUCTION
1.1
Purpose of Study
This Flood Insurance Study (FIS) revises and updates information on the existence and
severity of flood hazards in the geographic area of Brantley, Georgia, including the Cities
of Hoboken and Nahunta and the unincorporated areas of Brantley (referred to
collectively herein as Brantley), and aids in the administration of the National Flood
Insurance Act of 1968 and the Flood Disaster Protection Act of 1973. This study has
developed flood-risk data for various areas of the community that will be used to
establish actuarial flood insurance rates and to assist the community in its efforts to
promote sound floodplain management. Minimum floodplain management requirements
for participation in the National Flood Insurance Program (NFIP) are set forth in the
Code of Federal Regulations at 44 CFR, 60.3.
In some States or communities, floodplain management criteria or regulations may exist
that are more restrictive or comprehensive than the minimum Federal requirements. In
such cases, the more restrictive criteria take precedence, and the State (or other
jurisdictional agency) will be able to explain them.
1.2
Authority and Acknowledgments
The sources of authority for this FIS report are the National Flood Insurance Act of 1968
and the Flood Disaster Protection Act of 1973.
For this countywide FIS, new hydrologic and hydraulic analyses were prepared by
Watershed Concepts, a Division of HSMM AECOM, for the Georgia Department of
Natural Resources (Georgia DNR), under Contract No. 761-80189. This study was
completed in September 2009. The histories of the individual communities before the
first countywide study are presented below.
Brantley County (Unincorporated Areas)
The hydrologic and hydraulic analyses for the previous study for the
unincorporated areas of Brantley County were performed by the U.S. Army
Corps of Engineers (USACE) (the Study Contractor) for the Federal Emergency
Management
Agency
(FEMA),
under
Inter-Agency
Agreement
No.
EMW-85-E-1822, Project Order No. 1. That study was completed in January
1987 (Reference 1).
1
1.3
Coordination
An initial Consultation Coordination Officer’s (CCO) meeting was held with
representatives of the communities, the Federal Emergency Management Agency
(FEMA), Georgia DNR, and the study contractors to explain the nature and purpose of
the FIS and to identify the streams to be studied by detailed methods. A final CCO
meeting is held with representatives of the communities, FEMA, and the study
contractors to review the results of the study.
For this countywide FIS, the initial CCO meeting was held on January 26, 2007. A final
CCO meeting was held on November 19, 2008. The meetings were attended by
representatives of the communities, the Georgia DNR, FEMA, and the study contractor.
All problems raised at the meeting have been addressed.
The history of the coordination activities for the individual communities before the first
countywide meeting is presented below.
Brantley County (Unincorporated Areas)
On January 5, 1985 an initial CCO meeting was held with representatives of the Study
Contractor and Brantley County. On November 19, 1987, the results of that FIS were
reviewed and accepted at a final coordination meeting and attended by representatives of
the Study Contractor, FEMA, and the community.
2.0
AREA STUDIED
2.1
Scope of Study
This FIS report covers the geographic area of Brantley, Georgia, including the
incorporated communities listed in Section 1.1.
No new detailed studies have been performed as part of this countywide study. Streams
previously studied by detailed methods are presented in Table 1.
2
Table 1: Scope of Study
Flooding Source
Limits of Detailed Study Area
Buffalo Creek
From approximately 1,600 feet downstream of County
Route 92 to Railroad crossing
Little Buffalo Creek
From the confluence with Buffalo Creek to
approximately 0.6 mile upstream of County Route 76
Little Buffalo Creek Tributary
From the confluence with Little Buffalo Creek to a
point upstream approximately 2,000 feet upstream of
Road G
Satilla River
From US Highway 84 to a point approximately 1.9 miles
upstream of County Route 188
Satilla River
From State Route 15 to County Boundary
Tributary A
From the confluence with Tributary No. 1 to a point
approximately 960 feet upstream of Road L
Tributary B
From the confluence with Tributary A to US Highway
84
Tributary No. 1
From the confluence with Buffalo Creek to US
Highway 84
Information on flooding caused by overflow of Buffalo Creek form its confluence with
the Satilla River to the CSX Railroad; Little Buffalo Creek form its confluence with
Buffalo Creek to the Brantley-Charlton county line; Tributary No. 1 from its confluence
with Buffalo Creek to U.S. Highway 84; Tributary A from its confluence with Tributary
No. 1 to State Highway 301; and Tributary B from its confluence with Tributary A to
U.S. Highway 84 was obtained from a USACE Flood Plain Information Report
(Reference 1).
Approximate analyses were used to study those areas having a low development potential
or minimal flood hazards. The scope and methods of study were proposed to, and agreed
upon, by FEMA, the Georgia DNR, Brantley, and the Study Contractor.
2.2
Community Description
Brantley covers approximately 447 square miles and is located in the southeastern portion
of the State of Georgia. The county is bounded on the northwest by Pierce County, on the
northeast by Wayne County, on the east by Glynn County, on the southeast by Camden
County, on the southwest by Charlton County, and on the west by Ware County. The
2000 population of Brantley was reported to be 14,629 (Reference 2).
Economic activities in Brantley County include apparel manufacture, timber harvesting,
farming, metal working, and electric motor repair. In 1982, about 90 percent of the land
in the county was in commercial forests (Reference 3). In 2004, 40% of the population
was involved in services, 36% in government services, and 26% in goods production
(Reference 4).
3
Brantley C o u n t y is situated on the lower Atlantic Coastal Plain. The Georgia portion
of this physiographic region is composed of broad, nearly level marine terraces that
gradually decline in elevation seaward. Brantley County contains portions of three
principal terraces. From west to east these terraces and their general elevations are the
Okefenokee or Sunderland, 100 to 150 feet National Geodetic Vertical Datum of 1929
(NGVD); the Penholoway, 70 to 75 feet NGVD; and the Talbot, 40 to 45 feet NGVD
(Reference 5). The terraces show little evidence of erosion so the edges are
distinguished in places by sandy shoreline ridges or scarps (Reference 6).
The slight relief in the area consists of low, broad ridges and rolling sandy ridges. The
broad ridges and slight depressions between ridges contain ponds, swamps, and
sluggish drainage ways. The remnants of a bar formation known as Trail Ridge extend
northward through the western part of the county. Trail Ridge separates the Okefenokee
and Penholoway Terraces and its southern extent forms the eastern boundary of the
Okefenokee Swamp, which lies south and west of Brantley County.
Soils of the Brantley County area are mainly sands and sandy loams. In swamps these
soils are covered by alluvial muck and peat (Reference 7). Natural vegetation in the
county consists of mixed hardwoods and softwoods.
Brantley County has warm, humid summers and mild winters with only a few days of
below freezing temperatures. Spring and fall are generally mild and sunny. Mean
temperatures (1895 to 1993) in Division 9 of Georgia (of which Brantley County is
part) range from a low of 51 degrees Fahrenheit (°F) in January to a high of 81°F in July
(Reference 8). The average annual temperature is approximately 67°F and the mean
annual precipitation in Division 9 is 50.2 inches.
About half of the annual rainfall occurs during the period from June through
September. Precipitation during the summer months is usually due to convection
activity that causes short, sometimes intense, showers and thunderstorms. From
October through May much of the rainfall is due to frontal disturbances passing over
the area. Brantley County is infrequently subjected to the very heavy rains associated
with tropical storms or hurricanes originating in the Atlantic Ocean, Caribbean Sea, or
the Gulf of Mexico (Reference 9).
Except for a small area in the southwest that drains toward the Okefenokee Swamp,
most of Brantley County lies within the watershed of the Satilla River. The Satilla
River arises in Ben Hill County and flows generally east to the Atlantic Ocean. In
Brantley County, it makes up the northwestern border of the county. About midway
along the northern border, the river enters the county, joining the Little Satilla River
before turning south on a course that takes it nearly through the length of the county.
The Satilla River in Brantley County has a wide swampy floodplain that is situated
about 35 to 60 feet below the general level of the terraces. Descent to the river
floodplain is over gradually sloping terrain. Tributary streams have cut into the slopes
bordering the floodplain and into the higher ridges on the terraces.
The Little Satilla River originates to the northwest of the county and flows southeast,
defining a portion of the boundary between Brantley and Pierce Counties. The Little
4
Satilla River then enters northeastern Brantley County and flows eastward to join the
Satilla River.
Other major streams in Brantley County that flow into the Satilla River include Big
Creek and its tributaries Mill Creek and South Prong Big Creek, and Buffalo Creek and
its tributaries Little Buffalo Creek and Tiger Bay. The Big Creek system, which drains
northward into the Satilla River, is located in the extreme eastern part of the county.
Buffalo Creek drains a large portion of central Brantley County as it flows in an arc-like
pattern northward, eastward, and southward until it joins the Satilla River. The Satilla
River is also fed by a large number of minor creeks, some of which drain extensive areas
of swamp.
The two largest urban areas in Brantley County are the City of Hoboken, whose 2000
population was reported to be 463, and the City of Nahunta, the county seat, whose 2000
population was reported to 930 (Reference 2). Only a small amount of residential and
commercial development has taken place in the floodplain in the unincorporated areas of
Brantley County. Most of the development that could be affected by flooding is located
in the floodplain of the Satilla River or in the upper drainage area of Buffalo Creek.
2.3
Principal Flood Problems
Flood crest elevations have been recorded on the Satilla River at the U.S. Route 84
crossing since 1930 (Reference 10). Based on information provided by local residents,
the highest flood known to have occurred, at least since 1862, took place in September
1929, when the Satilla River, at U.S. Route 84, reached on elevation 41.99 feet NGVD.
The maximum discharge during this flood was calculated to be 110,000 cubic feet per
second (cfs). The second highest known flood peak occurred on April 6, 1948. On this
date, the flood crest at U.S. Route 84 was 38.69 feet NGVD, and the maximum discharge
was 68,100 cfs. The discharge for both of these floods exceeded the 100-year frequency
discharge. Other record floods since 1930 for the Satilla River at the same location are
the flood of September 5, 1949, which had a discharge of 33,200 cfs and reached an
elevation of 34.39 feet NGVD, and the flood of October 3, 1954, which had a discharge
of 34,600 cfs and reached an elevation of 34.59 feet NGVD. The 1949 and 1954 floods
are estimated to have a frequency greater than the 10-year flood event.
The record high water and flooding along the Satilla River, Buffalo Creek, and its
tributaries in Brantley County in the fall of 1929 were caused by two storms that passed
over the area during the period September 25 through October 3 (Reference 11).
Approximately 7 inches of rain fell on Brantley County during the first storm and about 4
inches during the second, a disturbance that was rated as a tropical storm. Damaging
floods owing to high rainfall in the area were also reported to have occurred in January
1947, November 1948, September 1953, March 1959, April 1960, and April 1969
(Reference 11). Most flooding in Brantley Count; is confined to forest lands bordering
streams and swamps. Only a few residences and other buildings are located in areas
subject to significant flood damage. A very small amount of farmland is located in floodprone areas.
5
2.4
Flood Protection Measures
Nonstructural measures consisting of floodplain building restrictions and land use
controls are used to reduce flood damages. Some small farm and forest dams with limited
storage capacity have been constructed on headwater streams, but the county has no
major flood control structures.
3.0
ENGINEERING METHODS
For the flooding sources studied by detailed methods in the community, standard hydrologic and
hydraulic study methods were used to determine the flood-hazard data required for this study.
Flood events of a magnitude that is expected to be equaled or exceeded once on the average
during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having
special significance for floodplain management and for flood insurance rates. These events,
commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and
0.2-percent chance, respectively, of being equaled or exceeded during any year. Although the
recurrence interval represents the long-term, average period between floods of a specific
magnitude, rare floods could occur at short intervals or even within the same year. The risk of
experiencing a rare flood increases when periods greater than 1 year are considered. For
example, the risk of having a flood that equals or exceeds the 1-percent-annual-chance flood in
any 50-year period is approximately 40 percent (4 in 10); for any 90-year period, the risk
increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding
potentials based on conditions existing in the community at the time of completion of this study.
Maps and flood elevations will be amended periodically to reflect future changes.
3.1
Hydrologic Analyses
Hydrologic analyses were carried out to establish peak discharge-frequency relationships
for each flooding source studied by detailed methods affecting the community.
Flow frequencies for the Satilla River were based on a statistical analysis of United
States Geological Survey (USGS) gage data. These data were analyzed in accordance
with criteria outlined in U.S. Bulletin No. 17B (Reference 12). Frequency-discharge
data were based on a USACE computer program, Flood Flow Frequency Analysis
(Reference 13).
Drainage areas were obtained from an open file report listing drainage areas for
Georgia streams (Reference 14).
T h e H E C F l o o d F l o w Frequency Analysis program used the period of record from
1931 to 1984 on gage no. 02228000 on the Satilla River at U.S. Route 84 for t h e reach
of the Satilla River between U.S. Route 84 and Happy Hollow, and the period of record
from 1937 to 1984 on gage no. 02226500 on the Satilla River near the City of
Waycross for the reach of the Satilla River between the State Route 15 bridge and the
western county boundary (Reference 10). Flow frequency curves were developed for
the periods of record at these gages. The log-Pearson Type III distribution was used in
the computation of the frequency curves.
6
Discharges for the 10-, 2-, 1-, and 0.2-percent-annual-chance probabilities in the 4.7-mile
reach of the Satilla River between Atkinson and Happy Hollow were obtained directly
from the flow frequency curve.
Computed flows derived from the Flood Frequency Analysis Program were used to
develop regression equations for the desired 10-, 2-, 1-, and 0.2-percent-annual-chance
probabilities in the 13.6-mile reach of the Satilla River between the State Route 15
Bridge and the western county boundary. The equations were developed using the
CORPS M0001 computer program (Reference 15). Flows for ungaged sites were then
computed using the regression equations.
Peak discharge-drainage area relationships for streams studied by detailed methods are
shown in Table 2: Summary of Discharges.
7
Table 2: Summary of Discharges
Flooding Source and Location
Drainage Area
(Square miles)
Peak Discharges (Cubic Feet per Second)
10-percent
2-percent
1-percent
0.2-percent
BUFFALO CREEK
At County Route 84
63
At State Route 301
59
*
*
*
*
6,440
*
*
5,880
5,600
*
*
6,160
*
*
LITTLE BUFFALO CREEK
At County Route 88
57
At County Route 76
49
*
*
At U.S. Route 84
2,790
31,900
51,800
61,000
84,300
At State Route 15
1,350
20,800
33,880
40,790
53,260
At County Route 91
13
5
*
*
2,500
At Road K
*
*
1,350
*
*
6
*
*
1,400
*
SATILLA RIVER
TRIBUTARY NO. 1
TRIBUTARY A
At Road L
*Data Not Available
8
3.2
Hydraulic Analyses
Analyses of the hydraulic characteristics of flooding from the sources studied were
carried out to provide estimates of the elevations of floods of the selected recurrence
intervals. Users should be aware that flood elevations shown on the FIRM represent
rounded whole-foot elevations and may not exactly reflect the elevations shown on the
Flood Profiles or in the Floodway Data tables in the FIS report. Flood elevations shown
on the FIRM are primarily intended for flood insurance rating purposes. For construction
and/or floodplain management purposes, users are cautioned to use the flood elevation
data presented in this FIS in conjunction with the data shown on the FIRM.
Photogrammetric methods were employed to obtain cross-section data in overbanks for
the backwater analyses (Reference 16). Below-water sections were obtained by field
survey. All bridges were field surveyed to obtain elevation data and structural
geometry.
Locations of selected cross sections used in the hydraulic analyses are shown on the
Flood Profiles. For stream segments for which a floodway was computed (Section 4.2),
selected cross-section locations are also shown on the FIRM.
Water-surface elevations of floods of the selected recurrence intervals were computed
using the USACE HEC-2 step-backwater computer program (Reference 17).
Channel and overbank roughness factors (Manning's "n") used in the hydraulic
computations were chosen by engineering judgment and were based on field
observations of the river and floodplain areas. The channel "n" values for the Satilla
River ranged from 0.050 to 0.13, and overbank "n" values ranged from 0.092 to 0.25.
For the HEC-2 program in the segment of the Satilla River upstream from the U.S.
Route 84 Bridge, starting water-surface elevations were determined by the slope-area
method. The hydraulic analysis was then calibrated to match starting water-surface
elevations with rating curve elevations calculated for USGS gage no. 02228000
(Reference 10).
The HEC-2 program starting water-surface elevations for the segment of the Satilla
River upstream from State Route 15 were based on an analysis of records for USGS
gage no. 2226582 (Reference 10). This gage is located at the State Route 15 bridge and
has a period of record from 1974 to the present. Water-surface profiles on the Satilla
River were then extended upstream beyond the western county boundary and calibrated
to match the rating curve calculated for USGS gage no. 0226500 near the City of
Waycross (Reference 10). Land-use and land-cover data were obtained from field
surveys.
The hydraulic analyses for this study were based on unobstructed flow. The flood
elevations shown on the Flood Profiles (Exhibit 1) are thus considered valid only if
hydraulic structures remain unobstructed, operate properly, and do not fail.
9
3.3
Vertical Datum
All FIS reports and FIRMs are referenced to a specific vertical datum. The vertical
datum provides a starting point against which flood, ground, and structure elevations can
be referenced and compared. Until recently, the standard vertical datum used for newly
created or revised FIS reports and FIRMs was the National Geodetic Vertical Datum of
1929 (NGVD). With the completion of the North American Vertical Datum of 1988
(NAVD), many FIS reports and FIRMs are now prepared using NAVD as the referenced
vertical datum.
Flood elevations shown in this FIS report and on the FIRM are referenced to the
NAVD88. These flood elevations must be compared to structure and ground elevations
referenced to the same vertical datum. Some of the data used in this revision were taken
from the prior effective FIS reports and FIRMs and adjusted to NAVD88. The datum
conversion factor from NGVD29 to NAVD88 in Brantley is negative 1.01 feet.
For information regarding conversion between the NGVD and NAVD, visit the National
Geodetic Survey website at www.ngs.noaa.gov, or contact the National Geodetic Survey
at the following address:
NGS Information Services
NOAA, N/NGS12
National Geodetic Survey
SSMC-3, #9202
1315 East-West Highway
Silver Spring, Maryland 20910-3282
(301) 713-3242
Temporary vertical monuments are often established during the preparation of a flood
hazard analysis for the purpose of establishing local vertical control. Although these
monuments are not shown on the FIRM, they may be found in the Technical Support
Data Notebook associated with the FIS report and FIRM for this community. Interested
individuals may contact FEMA to access these data.
4.0
FLOODPLAIN MANAGEMENT APPLICATIONS
The NFIP encourages State and local governments to adopt sound floodplain management
programs. To assist in this endeavor, each FIS report provides 1-percent-annual-chance
floodplain data, which may include a combination of the following: 10-, 2-, 1-, and 0.2-percentannual-chance flood elevations; delineations of the 1- and 0.2-percent-annual-chance floodplains;
and a 1-percent-annual-chance floodway. This information is presented on the FIRM and in
many components of the FIS report, including Flood Profiles, Floodway Data tables, and
Summary of Stillwater Elevation tables. Users should reference the data presented in the FIS
report as well as additional information that may be available at the local community map
repository before making flood elevation and/or floodplain boundary determinations.
10
4.1
Floodplain Boundaries
To provide a national standard without regional discrimination, the 1-percent-annualchance flood has been adopted by FEMA as the base flood for floodplain management
purposes. The 0.2-percent-annual-chance flood is employed to indicate additional areas
of flood risk in the community. For each stream studied by detailed methods, the 1- and
0.2-percent-annual-chance floodplain boundaries have been delineated using the flood
elevations determined at each cross section. For each stream studied by approximate
methods, the 1-percent-annual-chance floodplain boundaries have been delineated by
interpolation using 10-foot topographic mapping developed from USGS DEM data.
The 1- and 0.2-percent-annual-chance floodplain boundaries are shown on the FIRM. On
this map, the 1-percent-annual-chance floodplain boundary corresponds to the boundary
of the areas of special flood hazards (Zones A and AE), and the 0.2-percent-annualchance floodplain boundary corresponds to the boundary of areas of moderate flood
hazards. In cases where the 1- and 0.2-percent-annual-chance floodplain boundaries are
close together, only the 1-percent-annual-chance floodplain boundary has been shown.
Small areas within the floodplain boundaries may lie above the flood elevations, but
cannot be shown due to limitations of the map scale and/or lack of detailed topographic
data.
For the streams studied by approximate methods, only the 1-percent-annual-chance
floodplain boundary is shown on the FIRM.
4.2
Floodways
Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity,
increases flood heights and velocities, and increases flood hazards in areas beyond the
encroachment itself. One aspect of floodplain management involves balancing the
economic gain from floodplain development against the resulting increase in flood
hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities
in this aspect of floodplain management. Under this concept, the area of the 1-percentannual-chance floodplain is divided into a floodway and a floodway fringe. The
floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept
free of encroachment so that the base flood can be carried without substantial increases in
flood heights. Minimum Federal standards limit such increases to 1 foot, provided that
hazardous velocities are not produced.
The area between the floodway and 1-percent-annual-chance floodplain boundaries is
termed the floodway fringe. The floodway fringe encompasses the portion of the
floodplain that could be completely obstructed without increasing the water-surface
elevation (WSEL) of the base flood more than 1 foot at any point. Typical relationships
between the floodway and the floodway fringe and their significance to floodplain
development are shown in Figure 1.
11
Figure 1: Floodway Schematic
The floodway presented in this FIS was computed for certain stream segments on the
basis of equal conveyance reduction from each side of the floodplain. Floodway widths
were computed at cross sections. Between cross sections, the floodway boundaries were
interpolated. The results of the floodway computations are tabulated for selected cross
sections (Table 3). The computed floodway is shown on the FIRM (Exhibit 2). In cases
where the floodway and 1-percent-annual-chance floodplain boundaries are either close
together or collinear, only the floodway boundary is shown. Portions of the floodway for
the Satilla River extend beyond the county boundary.
Along streams where floodways have not been computed, the community must ensure
that the cumulative effect of development in the floodplain will not cause more than a
1.0-foot increase in the base flood elevations at any point within the county.
5.0
INSURANCE APPLICATION
For flood insurance rating purposes, flood insurance zone designations are assigned to a
community based on the results of the engineering analyses. These zones are as follows:
Zone A
Zone A is the flood insurance rate zone that corresponds to the 1-percent-annual-chance
floodplains that are determined in the FIS report by approximate methods. Because
detailed hydraulic analyses are not performed for such areas, no base
(1-percent-annual-chance) flood elevations (BFEs) or depths are shown within this zone.
12
FLOODING SOURCE
CROSS SECTION
BASE FLOOD
WATER-SURFACE ELEVATION
(FEET NAVD 88)
FLOODWAY
DISTANCE1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
148.92
150.08
152.89
154.06
155.51
156.94
157.19
157.51
157.84
158.29
159.18
160.74
162.78
6,223
5,125
4,535
4,028
3,125
4,718
4,319
3,447
2,309
1,629
3,780
4,806
4,206
41,298
34,727
36,319
30,453
19,307
36,455
42,247
31,535
18,324
10,318
43,160
61,291
37,239
2
MEAN
VELOCITY
(FEET PER
SECOND)
REGULATORY
WITHOUT
FLOODWAY
WITH
FLOODWAY
INCREASE
1.0
1.2
1.1
1.3
2.1
1.1
1.0
1.3
2.2
4.0
0.9
0.7
1.1
69.7
70.4
72.4
73.4
75.9
77.8
78.1
78.3
78.8
80.8
82.3
82.6
83.0
69.7
70.4
72.4
73.4
75.9
77.8
78.1
78.3
78.8
80.8
82.3
82.6
83.0
70.7
71.4
73.4
74.4
76.9
78.8
79.1
79.3
79.8
81.4
83.2
83.6
84.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.6
0.9
1.0
1.0
SATILLA RIVER
A-I3
J
K
L
M
N
O
P
Q
R
S
T
U
V
1
Miles Above Mouth
Width extends beyond County Boundary
3
Floodway not calculated
2
FEDERAL EMERGENCY MANAGEMENT AGENCY
TABLE 3
FLOODWAY DATA
BRANTLEY COUNTY, GA
AND INCORPORATED AREAS
SATILLA RIVER
Zone AE
Zone AE is the flood insurance rate zone that corresponds to the 1-percent-annual-chance
floodplains that are determined in the FIS report by detailed methods. Whole-foot BFEs
derived from the detailed hydraulic analyses are shown at selected intervals within this
zone.
Zone X
Zone X is the flood insurance rate zone that corresponds to areas outside the
0.2-percent-annual-chance floodplain, areas within the 0.2-percent-annual-chance
floodplain, areas of 1-percent-annual-chance flooding where average depths are less than
1 foot, areas of 1-percent-annual-chance flooding where the contributing drainage area is
less than 1 square mile (sq. mi.), and areas protected from the base flood by levees. No
BFEs or depths are shown within this zone.
6.0
FLOOD INSURANCE RATE MAP
The FIRM is designed for flood insurance and floodplain management applications.
For flood insurance applications, the map designates flood insurance rate zones as described in
Section 5.0 and, in the 1-percent-annual-chance floodplains that were studied by detailed
methods, shows selected whole-foot BFEs or average depths. Insurance agents use zones and
BFEs in conjunction with information on structures and their contents to assign premium rates for
flood insurance policies.
For floodplain management applications, the map shows by tints, screens, and symbols, the 1percent-annual-chance floodplains and the locations of selected cross sections used in the
hydraulic analyses.
The countywide FIRM presents flooding information for the entire geographic area of Brantley.
Previously, FIRMs were prepared for each incorporated community and the unincorporated areas
of the County identified as flood-prone. This countywide FIRM also includes flood-hazard
information that was presented separately on Flood Boundary and Floodway Maps (FBFMs),
where applicable. Historical data relating to the maps prepared for each community are presented
in Table 4: Community Map History.
7.0
OTHER STUDIES
A FIS report has previously been prepared for the unincorporated areas of Brantley County
(Reference 1). FIRMs were published for the Cities of Nahunta and Hoboken, Georgia
(Reference 18) (Reference 19).
FIS reports published for Glynn, Camden, and Wayne Counties, Georgia are in agreement with
this study (Reference 20) (Reference 21) (Reference 22).
14
FIS reports in progress for Charlton, Pierce, and Ware Counties, Georgia are in agreement with
this study. Initial water-surface elevations in this study for the Satilla River above the State Route
15 Bridge were calibrated to match rating curve elevations derived from USGS gage data at the
bridge. Therefore, water-surface elevations in this study do not match starting water-surface
elevations calculated by slope-area method for the Satilla River at the Brantley-Ware county line
in the Ware County FIS study.
This FIS report either supersedes or is compatible with all previous studies published on streams
studied in this report and should be considered authoritative for the purposes of the NFIP.
8.0
LOCATION OF DATA
Information concerning the pertinent data used in the preparation of this study can be obtained by
contacting Federal Insurance and Mitigation Division, FEMA Region IV, Koger-Center —
Rutgers Building, 3003 Chamblee Tucker Road, Atlanta, GA 30341.
15
INITIAL
IDENTIFICATION
FLOOD HAZARD
BOUNDARY MAP
REVISIONS DATE
FIRM
EFFECTIVE DATE
FIRM
REVISIONS DATE
April 14, 1978
---
September 30, 1988
September 25, 2009
Hoboken, City of
August 30, 1974
January 23, 1976
April 15, 1986
September 25, 2009
Nahunta, City of
May 17, 1975
January 30, 1976
September 4, 1985
September 25, 2009
COMMUNITY
NAME
Brantley County
(Unincorporated Areas)
TABLE 4
FEDERAL EMERGENCY MANAGEMENT AGENCY
BRANTLEY COUNTY, GA
AND INCORPORATED AREAS
COMMUNITY MAP HISTORY
9.0
BIBLIOGRAPHY AND REFERENCES
1. Federal Emergency Management Agency. Flood Insurance Study, Brantley County, Georgia,
Unincorporated Areas. Washington, D.C. : s.n., September 30, 1988.
2. U.S. Department of Commerce, Bureau of the Census. State and County Quick Facts. [Online] 2008.
http://quickfacts.census.gov.
3. Bachtel, Douglas C., ed. The Georgia County Guide. s.l. : Cooperative Extension Service, College
of Agriculture, The University of Georgia, Athens, Georgia, 1984.
4. U.S. Department of Commerce, Bureau of the Census. County Snapshots. [Online] 2008.
http://www.dca.state.ga.us/CountySnapshotsNet.
5. Johnson, Sydney A., et. al. An Ecological Survey of the Coastal Region of Georgia. Scientific
Monograph Series No. 3. Washington, D.C. : National Park Service, 1974.
6. Fenneman, Nevin M. Physiography of the Eastern United States. New York : McGraw-Hill Book
Company, 1938.
7. Akioka, Lorena M., ed. Georgia Statistical Abstract, 1984-85. s.l. : Division of Research, College of
Business Administration, The University of Georgia, Athens, Georgia, 1984.
8.
University
of
Georgia,
http://climate.engr.uga.edu/info.html.
State
Climate
Office.
???
[Online]
2008.
9. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National
Climatic Data Center. Tropical Cyclones of the North Atlantic, 1871-1980. Asheville, North Carolina :
s.n., revised July 1981.
10. U.S. Department of the Interior, Geological Survey. Floods in Georgia, Magnitude and Frequency.
Doraville, Georgia : s.n., October 1979.
11. U.S. Department of the Army, Corps of Engineers, Savannah District. Floodplain Information:
Lower Buffalo Creek and Its Tributaies, Nahunta and Brantley County, Georgia. Savannah, Georgia :
s.n., October, 1972.
12. U.S. Department of Commerce, Geological Survey, Interagency Advisory Committee on Water
Data, Office of Water Data Coordination, Hydrology Subcommittee, Bulletin No. 17b. Guidelines for
Determining Flood Flow Frequency. September 1981, revised March 1982.
13. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center. Flood Flow
Frequency Analysis, 723-X6-L7550, Generalized Computer Program. Davis, California : s.n., 1972.
14. U.S. Department of the Interior, Geological Survey, Open-file Report. Drainage Area Data for
Georgia Streams. Atlanta, Georgia : s.n., 1959.
15. U.S. Army Corps of Engineers. CORPS Computer Program M0001, Curve Fitting and Statistical
Analysis . no date.
16. Continental Aerial Surveys, Inc. Orthophotographs of the Satilla River. Brantley County, Georgia
Scale 1:4800. Alcoa, Tennessee : s.n., January 1985.
17. U.S. Army Corps of Engineers, Hydrologic Engineering Center. HEC-2 Water-Surface Profiles,
Generalized Computer Program. Davis, California : s.n., January 1985.
18. Federal Emergency Management Agency. Flood Insurance Rate Map, City of Nahunta, Brantley
17
County, Georgia. September, 1985.
19. —. Flood Insurance Rate Map, City of Hoboken, Brantley County, Georgia. April 1986.
20. —. Flood Insurance Study, Glynn County, Georgia, and Incorporated Areas. Washington, D.C. :
s.n., September 6, 2006.
21. —. Flood Insurance Study, Camden County, Georgia and Incorporated Areas. Washington, D.C. :
s.n., September 30, 1988.
22. —. Flood Insurance Study, Wayne County, Georgia, Unincorporated Areas. Washington, D.C. :
s.n., January 3, 1980.
18