U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany

Scientific Investigations Map 3220

Prepared in cooperation with Cobb County, Georgia

Flood-Inundation Maps for Sweetwater Creek 

from Above the Confluence of Powder Springs 

Creek to the Interstate 20 Bridge, Cobb and 

Douglas Counties, Georgia
Cover. Measuring the flow of Sweetwater Creek over Interstate 20, September 22, 2009 

(photograph by Alan M. Cressler, U.S. Geological Survey).
Flood-Inundation Maps for Sweetwater Creek 

from Above the Confluence of Powder Springs 

Creek to the Interstate 20 Bridge, Cobb and 

Douglas Counties, Georgia

By Jonathan W. Musser

Prepared in cooperation with Cobb County, Georgia

Pamphlet to accompany 

Scientific Investigations Map 3220

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Musser, J.W., 2012, Flood-inundation maps for Sweetwater Creek from above the confluence of Powder Springs Creek 

to the Interstate 20 bridge, Cobb and Douglas Counties, Georgia: U.S. Geological Survey Scientific Investigations 

Map 3220, 10 p. pamphlet, 21 sheets; available online at http://pubs.usgs.gov/sim/3220/.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction ....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................4

Study Area Description........................................................................................................................4

Previous Studies ...................................................................................................................................4

Constructing Water-Surface Profiles.........................................................................................................6

Hydrologic and Steady-Flow Data .....................................................................................................6

Topographic/Bathymetric Data ..........................................................................................................6

Energy Loss Factors .............................................................................................................................6

Model Calibration and Performance .................................................................................................7

Development of Water-Surface Profiles...........................................................................................7

Inundation Mapping ......................................................................................................................................8

Sweetwater Creek, Georgia, Flood-Inundation Maps on the Internet ........................................8

Disclaimer for Flood-Inundation Maps .............................................................................................9

Uncertainties and Limitations Regarding Use of Flood-Inundation Maps ..................................9

Summary..........................................................................................................................................................9

References Cited..........................................................................................................................................10

Figures

1. Map showing location of study reach for Sweetwater Creek and location of 

U.S. Geological Survey streamgages and National Weather Service (NWS) 

forecast site...................................................................................................................................3

2. Map showing Sweetwater Creek bridge crossings, model extent, and direction 

of surface-water flow ..................................................................................................................5

Tables

1. Site information for U.S. Geological Survey streamgages in the 

Sweetwater Creek basin near Austell, Georgia ......................................................................2

2. Comparison of hydraulic-model output and surveyed high-water mark 

elevations from the flood of February 4, 1982, Sweetwater Creek, Georgia.......................7

3. Stages (and water-surface elevations) with corresponding discharge estimates

for selected simulated water-surface profiles at selected locations for the 

Sweetwater Creek near Austell, Georgia, streamgage (02337000)......................................7

4. Stream stages and water-surface elevations at Sweetwater Creek near 

Austell, Georgia, streamgage (02337000), with reference to corresponding 

map-sheet numbers in this report..............................................................................................8
iv

Sheets (Each sheet has individual pdf and jpg files available for download. 

For links, see http://pubs.usgs.gov/sim/3220/.)

1–21. Maps showing flood-Inundation of Sweetwater Creek in Cobb and 

Douglas Counties, Georgia, corresponding to a gage height and an 

elevation (NAVD 88) as listed below at U.S. Geological Survey 

streamgage Sweetwater Creek near Austell, Georgia (02337000)—

1. Gage height of 12.0 feet and an elevation of 869.3 feet

2. Gage height of 13.0 feet and an elevation of 870.3 feet

3. Gage height of 14.0 feet and an elevation of 871.3 feet

4. Gage height of 15.0 feet and an elevation of 872.3 feet

5. Gage height of 16.0 feet and an elevation of 873.3 feet

6. Gage height of 17.0 feet and an elevation of 874.3 feet

7. Gage height of 18.0 feet and an elevation of 875.3 feet

8. Gage height of 19.0 feet and an elevation of 876.3 feet

9. Gage height of 20.0 feet and an elevation of 877.3 feet

10. Gage height of 21.0 feet and an elevation of 878.3 feet

11. Gage height of 22.0 feet and an elevation of 879.3 feet

12. Gage height of 23.0 feet and an elevation of 880.3 feet

13. Gage height of 24.0 feet and an elevation of 881.3 feet

14. Gage height of 25.0 feet and an elevation of 882.3 feet

15. Gage height of 26.0 feet and an elevation of 883.3 feet

16. Gage height of 27.0 feet and an elevation of 884.3 feet

17. Gage height of 28.0 feet and an elevation of 885.3 feet

18. Gage height of 29.0 feet and an elevation of 886.3 feet

19. Gage height of 30.0 feet and an elevation of 887.3 feet

20. Gage height of 31.0 feet and an elevation of 888.3 feet

21. Gage height of 32.0 feet and an elevation of 889.3 feet
v

Acronyms and Additional Abbreviations

AHPS Advanced Hydrologic Prediction Service

DFIRM Digital Flood Insurance Rate Map

FEMA Federal Emergency Management Agency

GIS geographic information system

NED National Elevation Dataset 

NWS National Weather Service

USGS U.S. Geological Survey

Conversion Factors and Datums

Multiply By To obtain

Length

inch 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 0.0929 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to the distance above NAVD 88.

Stage, as used in this report, is the elevation of the water surface above an arbitrary datum 

established at the gage (gage datum).

Acknowledgments

The author wishes to thank Cobb County for funding operation and maintenance of 

streamgages used for this study. Special thanks are given to the National Weather Service 

for their continued support to the U.S. Geological Survey flood-inundation mapping initiative.

Flood-Inundation Maps for Sweetwater Creek 

from Above the Confluence of Powder Springs 

Creek to the Interstate 20 Bridge, Cobb and 

Douglas Counties, Georgia

By Jonathan W. Musser

Abstract

Digital flood-inundation maps for a 10.5-mile reach of 

Sweetwater Creek, from about 1,800 feet above the confluence of Powder Springs Creek to about 160 feet below the 

Interstate 20 bridge, were developed by the U.S. Geological 

Survey (USGS) in cooperation with Cobb County, Georgia. 

The inundation maps, which can be accessed through the 

USGS Flood Inundation Mapping Science Web site at 

http://water.usgs.gov/osw/flood_inundation/, depict estimates 

of the areal extent and depth of flooding corresponding 

to selected water levels (stages) at the USGS streamgage 

at Sweetwater Creek near Austell, Georgia (02337000). 

Current stage at this USGS streamgage may be obtained at 

http://waterdata.usgs.gov/ and can be used in conjunction with 

these maps to estimate near real-time areas of inundation. The 

National Weather Service (NWS) is incorporating results from 

this study into the Advanced Hydrologic Prediction Service 

(AHPS) flood-warning system (http:/water.weather.gov/ahps/). 

The NWS forecasts flood hydrographs at many places 

that commonly are collocated at USGS streamgages. The 

forecasted peak-stage information for the USGS streamgage at 

Sweetwater Creek near Austell (02337000), which is available 

through the AHPS Web site, may be used in conjunction with 

the maps developed in this study to show predicted areas of 

flood inundation.

A one-dimensional step-backwater model was developed 

using the U.S. Army Corps of Engineers HEC–RAS software 

for Sweetwater Creek and was used to compute flood profiles 

for a 10.5-mile reach of the creek. The model was calibrated 

using the most current stage-discharge relations at the 

Sweetwater Creek near Austell streamgage (02337000), as 

well as high-water marks collected during annual peak-flow 

events in 1982 and 2009. The hydraulic model was then used 

to determine 21 water-surface profiles for flood stages at the 

Sweetwater Creek streamgage at 1-foot intervals referenced 

to the streamgage datum and ranging from just above bankfull 

stage (12.0 feet) to approximately 1.2 feet above the highest 

recorded water level at the streamgage (32.0 feet). The 

simulated water-surface profiles were then combined with 

a geographic information system digital elevation model—

derived from contour data (8-foot horizontal resolution), in 

Cobb County, and USGS National Elevation Dataset (31-foot 

horizontal resolution), in Douglas County—to delineate the 

area flooded for each 1-foot increment of stream stage.

The availability of these maps, when combined with 

real-time information regarding current stage from USGS 

streamgages and forecasted stream stages from the NWS, 

provides emergency management personnel and residents 

with critical information during flood-response activities, such 

as evacuations and road closures, as well as for post-flood 

recovery efforts.
2 Flood-Inundation Maps for Sweetwater Creek, Cobb and Douglas Counties, Georgia

Introduction 

Cobb and Douglas Counties, Georgia (Ga.), are suburban 

communities with populations of 688,078 and 132,403, 

respectively, in 2010 (U.S. Census Bureau, 2011; fig. 1). 

Land use within the flood plain of Sweetwater Creek is widely 

varied with a mix of residential, recreational, and commercial 

structures, and areas of agriculture, grass fields, wetlands, 

and forest. In recent years, Cobb and Douglas Counties 

have experienced severe flooding that has caused substantial 

damage along Sweetwater Creek. The floods of July 2005 and 

September 2009 are two such notable events. Austell, Ga., 

which is in the southern part of Cobb County, was particularly 

affected by the 2009 event.

Prior to this study, Cobb and Douglas County officials 

and officials from the City of Austell have relied on several 

information sources (available on the Internet) to make 

decisions on how best to alert the public and mitigate 

flood damages along Sweetwater Creek. One source of 

information is the Federal Emergency Management Agency 

(FEMA) Digital Flood Insurance Rate Map (DFIRM) 

(Federal Emergency Management Agency, 2011). A second 

source of information is the U.S. Geological Survey 

(USGS) streamgage—Sweetwater Creek near Austell, Ga. 

(02337000)—from which current or historical water levels 

(stage) can be obtained (table 1). Stage is the height of the 

water surface above an arbitrary datum established at the gage 

(gage datum). A third source is the National Weather Service 

(NWS) forecast of peak stage at the USGS Sweetwater Creek 

near Austell streamgage (forecast site AUSG1) through the 

Advanced Hydrologic Prediction Service (AHPS) Web site at 

http:/water.weather.gov/ahps/. The USGS has six additional 

gages located within the Sweetwater Creek basin where 

current and historical stages can be obtained.

Although USGS current stage and NWS forecast stage 

information is particularly useful for residents in the immediate vicinity of a streamgage, it is of limited use to residents 

farther upstream or downstream because the water-surface 

elevation is not constant along the entire stream channel. 

Also, FEMA and State emergency management mitigation 

teams or property owners typically lack information related 

to water depth at locations other than locations near USGS 

streamgages or NWS flood-forecast points. To help guide 

the general public in taking individual safety precautions 

and provide local officials with a tool to help efficiently 

manage emergency flood operations and flood-mitigation 

efforts, digital flood-inundation maps for a 10.5-mile reach 

of Sweetwater Creek, from about 1,800 feet above the 

confluence of Powder Springs Creek to about 160 feet below 

the Interstate 20 bridge, were developed by the USGS in 

cooperation with Cobb County, Georgia.

Table 1. Site information for U.S. Geological Survey streamgages in the Sweetwater Creek basin near Austell, Georgia.

[USGS, U.S. Geological Survey; Ga., Georgia; current year, year of publication; ft, feet; mi, mile; see fig. 1 for station locations]

Station name

USGS 

station

number

Drainage 

area, in 

square miles

Latitude Longitude

Period of record

Maximum 

recorded stage 

(degrees, minutes, seconds) at gage and date

Sweetwater Creek near 

Austell, Ga.

02337000 246 33°46'35" 84°36'56" 1937 to current year 30.82 ft

Sept. 22, 2009

Sweetwater Creek at 

Brownsville Road, near 

Powder Springs, Ga.

02336840 102 33°49'38" 84°43'11" 2007 to current year 31.40 ft

Sept. 21, 2009

Sweetwater Creek 0.05 mi 

upstream of Norfolk 

Southern Railroad bridge 

at Austell, Ga.

02336910 157 33°49'04" 84°39'00" 2010 to current year 14.52 ft

Apr. 16, 2011

Sweetwater Creek below 

Austell, Ga.

02337040 262 33°43'15" 84°36'54" 1998 to current year 38.0 ft

Sept. 22, 2009

Powder Springs Creek near 

Powder Springs, Ga.

02336870 17.3 33°51'33" 84°41'17" 2007 to current year 19.91 ft

Sept. 21, 2009

Noses Creek at Powder 

Springs Road, Powder 

Springs, Ga.

02336968 44.5 33°51'33" 84°39'10" 1998 to current year 23.21 ft

Sept. 22, 2009

Olley Creek at Clay Road, 

near Austell, Ga.

02336986 13.5 33°50'10" 84°37'54" 2007 to current year 27.39 ft

Sept. 22, 2009
Introduction 3

Sweetwater

Creek

Chattahoochee River

CARROLL

COUNTY

COBB COUNTY

PAULDING

COUNTY

DOUGLAS

COUNTY

CARROLL

COUNTY

COBB COUNTY

PAULDING

COUNTY

DOUGLAS

COUNTY

02336910

02337000

AUSG1

Noses Creek

Olley 

Mill Creek

Creek

Powder Springs Cr

Creek

Sweetwater

Chattahoochee River

85°00'W 84°50'W 84°40'W

34°00'N

33°50'N

Chattahoochee

GEORGIA

River

02336986

02336968

02336870

02336840

02337040

Base modified from U.S. Geological Survey

1:100,000 and 1-24,000-scale digital data

EXPLANATION

USGS streamgage and identifier

USGS streamgage and NWS 

 forecast site and identifiers

Piedmont physiographic province

Subbasin boundary

Sweetwater Creek Basin boundary 02337000

AUSG1

Area of map

in figure 2

Area of map below

0 5 10 MILES

0 5 10 KILOMETERS

Figure 1.

02336870

Figure 1. Location of study reach for Sweetwater Creek and location of U.S. Geological 

Survey streamgages and National Weather Service (NWS) forecast site.
4 Flood-Inundation Maps for Sweetwater Creek, Cobb and Douglas Counties, Georgia

Purpose and Scope

The purpose of this report is to describe the development 

of a series of estimated flood-inundation maps for Sweetwater 

Creek in Cobb and Douglas Counties, Ga. The maps and other 

flood information are available on the USGS Flood Inundation Mapping Science Web site at http://water.usgs.gov/osw/

flood_inundation/ and the previously mentioned NWS AHPS 

Web site. Internet users can select estimated inundation maps 

that correspond to (1) current stages at the USGS streamgage 

near Austell (02337000), (2) the NWS forecasted peak stage, 

or (3) other desired stream stages.

The scope of the study was limited to a 10.5-mile reach 

of Sweetwater Creek from about 1,800 feet (ft) above the 

confluence of Sweetwater Creek and Powder Springs Creek 

to about 160 ft downstream of the Interstate 20 bridge (fig. 2). 

Tasks specific to development of the flood-inundation maps 

were (1) analysis of the flow and stage data collected at two 

streamgages, Sweetwater Creek near Austell (02337000) and 

Sweetwater Creek at Brownsville Road near Powder Springs 

(02336840) (table 1), as well as flow and stage data recorded 

at numerous locations along the reach during the 1982 and 

2009 peak-flow events; (2) collection of topographic data 

and geometric data (for structures, bridges, and the stream 

channel) throughout the study reach; (3) determination of 

energy-loss factors (roughness coefficients) in the stream 

channel and flood plain; (4) computation of water-surface 

profiles using the U.S. Army Corps of Engineers HEC–RAS 

computer program (U.S. Army Corps of Engineers, Hydrologic Engineering Center, 2010); (5) production of estimated 

flood-inundation maps at various stream stages using the 

U.S. Army Corps of Engineers HEC–GeoRAS computer 

program (U.S. Army Corps of Engineers, Hydrologic Engineering Center, 2009) and a geographic information system 

(GIS); and (6) development of a Web-site interface that links 

to USGS real-time streamgage information and (or) NWS 

forecasted peak stage to facilitate the display of user-selected 

flood-inundation maps on the Internet.

Methods used generally are cited from previously 

published reports (Bales and others, 2007; Whitehead and 

Ostheimer, 2009). If techniques varied substantially from 

previously documented methods because of local hydrologic 

conditions or available data, they are described in detail in this 

report. Maps were produced for water levels referenced to the 

stage at Sweetwater Creek near Austell (02337000) and ranged 

from approximately bankfull (12.0 ft) to above the maximum 

observed water level at the streamgage (32.0 ft). 

Study Area Description

Sweetwater Creek is in northern Georgia in the Piedmont 

physiographic province (Clark and Zisa, 1976). The drainage 

area ranges from 128 square miles (mi2) at the junction of 

Powder Springs Creek to 246 mi2 at the Sweetwater Creek 

near Austell streamgage (02337000), the downstream extent of 

the study reach (fig. 2). The headwaters of Sweetwater Creek 

originate in Paulding County (fig. 1). Sweetwater Creek and its 

tributaries generally flow eastward through Paulding, Carroll, 

Cobb, and Douglas Counties until the junction with Olley 

Creek. From there, the flow direction is generally southward 

until Sweetwater Creek joins with the Chattahoochee River. 

Four major tributaries flow into Sweetwater Creek. From 

upstream to downstream, they are Powder Springs Creek, 

Noses Creek, Olley Creek, and Butternut Creek. The USGS 

monitors flow on Powder Springs Creek, Noses Creek, and 

Olley Creek with the following gages: Powder Springs Creek 

near Powder Springs (02336870), Noses Creek at Powder 

Springs Road, Powder Springs (02336968), and Olley Creek at 

Clay Road, near Austell (02336986). In addition, Sweetwater 

Creek below Austell (02337040) is located on Sweetwater 

Creek near where it flows into the Chattahoochee River (fig. 1).

The basin terrain is gently rolling with stream valleys 

that are fairly deep and narrow and lie 100 to 200 ft below the 

narrow, rounded stream divides (Clark and Zisa, 1976). The 

study reach is about 10.5 miles (mi) long, with an average 

channel slope of about 1.9 feet per mile (ft/mi). The land 

that is contiguous to the study reach is mostly classified as 

developed, with some areas classified as forest. Some large 

areas of woody wetlands are along Sweetwater Creek and its 

tributaries (Fry and others, 2011). The main channel within the 

study reach has nine major road bridges and two rail bridges 

that cross the channel and the adjacent flood plain. 

Previous Studies

The current DFIRM for Cobb County was published 

June 18, 2010, and the DFIRM for Douglas County was 

published August 18, 2009 (Federal Emergency Management 

Agency, 2011). The DFIRMs were used to check the inundation extent for discharges that are close to the 1-percent and 

0.2-percent annual exceedance probability. Two HEC–RAS 

models of Sweetwater Creek were obtained in 2011 from Sam 

Crampton at Dewberry, an engineering firm contracted by 

FEMA. These models were used in the development of the 

DFIRMs. The two HEC-RAS models were combined for use 

in this study, and the cross sections, stream channel lengths, 

and bridge details within the study area were used as a starting 

point for creation of the model used in this study.

Similar studies that provide flood-inundation maps for a 

range of stream stages were completed in Albany, Ga., for the 

Flint River (Musser and Dyar, 2007) and in Atlanta, Ga., for 

Peachtree Creek (Musser, 2012). The methods for the Flint 

River model used a finite-element two-dimensional model, 

which differed from those of the Sweetwater Creek model. 

The methods used for the Peachtree Creek model were similar 

to those used for the Sweetwater Creek model.
Introduction 5

Noses Creek

Olley Creek

Powder Springs Creek

Butternut Creek

84°40'W 84°39'W 84°38'W 84°37'W

33°50'N

33°49'N

33°48'N

33°47'N

INTERSTATE 20

SKYVIEW DRIVE

THORNTON ROAD

OLD ALABAMA ROAD

 VETERANS

 MEMORIAL 

HIGHWAY

NORFOLK SOUTHERN 

RAILROAD 

(DOWNSTREAM)

OLD MARIETTA ROAD

NORFOLK SOUTHERN

RAILROAD (UPSTREAM)

POWDER SPRINGS ROAD

CH JAMES PARKWAY

WESTSIDE ROAD

0 0.5 1 MILE

0 0.5 1 KILOMETER

COBB COUNTY

DOUGLAS COUNTY

Austell

02336910

02337000

AUSG1

02336986

Base modified from U.S. Geological Survey

1:24,000 National Hydrography Dataset

Figure 2.

EXPLANATION

Direction of surface-water flow

Modeled inundation area boundary

Road

Railway

Bridge

Sweetwater Creek modeled section

Tributary

02336910 USGS streamgage and identifier

USGS streamgage and NWS 

 forecast site and identifiers

02337000

AUSG1

Figure 2. Sweetwater Creek bridge crossings, model extent, and direction of surface-water flow.
6 Flood-Inundation Maps for Sweetwater Creek, Cobb and Douglas Counties, Georgia

Constructing Water-Surface Profiles

The water-surface profiles used to produce the 

21 flood-inundation maps in this study were computed using 

HEC–RAS, version 4.1.0 (U.S. Army Corps of Engineers, 

Hydrologic Engineering Center, 2010). HEC–RAS is a 

one-dimensional step-backwater model for simulation of 

water-surface profiles with steady-state (gradually varied) or 

unsteady-state flow computation options. For this study, the 

HEC–RAS analysis was completed by using the steady-state 

flow computation option.

Hydrologic and Steady-Flow Data

The hydrologic network in the study area consists of two 

streamgages on the study reach and one streamgage upstream 

of the study reach (fig. 1; table 1). The Sweetwater Creek near 

Austell (02337000) and Sweetwater Creek at Brownsville 

Road, near Powder Springs (02336840) streamgages were 

used in this study to determine flow values for HEC–RAS 

along the study reach. The streamgage identified as Sweetwater Creek 0.05 mi upstream of Norfolk Southern Railroad 

bridge at Austell (02336910) was installed in 2010, and 

the defined stage-discharge relation (as of 2012) does not 

extend beyond bankfull stage into the flood zone. Water level 

(stage) is measured continuously, and continuous records of 

streamflow are computed at all of the sites. All water-surface 

elevations are referenced to North American Vertical Datum of 

1988 (NAVD 88). The gages are equipped with satellite radio 

transmitters that allow data to be transmitted routinely and 

made available on the Internet within an hour of collection. 

The gaging stations on Sweetwater Creek 0.05 mi upstream of 

Norfolk Southern Railroad bridge at Austell (02336910) and 

Sweetwater Creek at Brownsville Road, near Powder Springs 

also are equipped with a recording rain gage. Longitudinal 

water-surface profiles from high-water marks along the main 

channel were documented following floods in 1982 and 2009. 

The 1982 high-water marks were used for model calibration. 

The 2009 high-water marks were affected by large amounts of 

debris at the bridges, so they were of limited use in calibration. 

High-water marks were not available for the 2005 flood.

Steady-flow data consisted of flow regime, boundary 

conditions (stage and discharge values), and peak discharge 

information. Steady-flow data for the study reach were 

obtained by using the stage-discharge relation at the Sweetwater Creek near Austell streamgage (02337000) and the 

Sweetwater Creek at Brownsville Road, near Powder Springs 

(02336480) streamgage as well as the historic peak stage and 

flow information available for both of these streamgages. 

Steady-flow data were also obtained from historical peak stage 

and flow information available at various bridge crossings 

along Sweetwater Creek for the 1982 annual peak event.

Topographic/Bathymetric Data

The cross sections from the HEC–RAS models, obtained 

from Dewberry, were used as a starting point for the model 

used in this study. Hereafter, the term “original” will be used 

to describe the model as obtained from Dewberry. Additional 

cross sections were added to the model where the distance 

between original cross sections was more than 500 ft. The 

elevations of the in-channel portion of the new cross sections 

were calculated by interpolating between original cross 

sections. The overbank elevations of the new cross sections 

were calculated from a raster elevation dataset. The source 

for the raster elevation in Cobb County was an 8-ft by 8-ft 

cell raster from photogrammatically derived contours. The 

source for the raster elevation in Douglas County is the USGS 

1/3-Arc Second National Elevation Dataset (NED), with a cell 

size of approximately 31 ft by 31 ft. The NED was resampled 

to 8 ft by 8 ft to match the resolution of the Cobb County data. 

Additional elevation data in the area of the Old Alabama Road 

bridge were obtained by survey. The survey data were collected 

using a Topcon GTS-210 Total Station and a survey rod. 

Various drainage structures (bridges, culverts, roadway 

embankments, levees, and dams) in and along the stream 

affect or have the potential to affect water-surface elevations 

during floods along the stream. To properly account for these 

features in the model, structural dimensions for 11 bridges 

were measured and surveyed in the field as part of the original 

HEC–RAS model. The bridges in the original HEC–RAS 

model were visually verified during site visits. The bridges 

from upstream to downstream were Westside Road, C.H. 

James Parkway, Norfolk Southern Railroad (upstream), 

Powder Springs Road, Old Marietta Road, Norfolk Southern 

Railroad (downstream), Veterans Memorial Highway, 

Old Alabama Road, Thornton Road, Skyview Drive, and 

Interstate 20 (fig. 2). A detailed description of the methods 

used to acquire and process the topographic and bathymetric 

data can be found in Bales and others (2007). 

Energy Loss Factors

Field observations and high-resolution aerial photographs were used to select initial (precalibration) Manning’s 

roughness coefficients (“n” values) for energy (friction) 

loss calculations. The final Manning’s n values ranged from 

0.075 to 0.095 for the main channel. The Manning’s n values 

ranged from 0.033 (grass) to 0.26 (forested areas with heavy 

undergrowth) for the overbank areas modeled in this analysis. 

The Manning’s n values from the original model were not used 

because they did not match the land-cover characteristics in 

many locations. Additionally, more flows were simulated than 

in the original HEC–RAS model. 
Constructing Water-Surface Profiles 7

Model Calibration and Performance

The hydraulic model was calibrated to the most current 

(2011) stage-discharge relation at the Sweetwater Creek 

near Austell streamgage (02337000) and to high-water 

marks collected during the 1982 and 2009 annual peak-flow 

events. The estimated peak discharge for the 1982 flood was 

10,700 cubic feet per second (ft3/s) at an estimated stage of 

about 19.90 ft at the gage. For the 2009 flood, the estimated 

peak discharge was 31,500 ft3/s at an estimated stage of about 

30.82 ft. Model calibration was accomplished by adjusting 

Manning’s n values until the results of the hydraulic computations closely agreed with the known flood discharge and stage 

values for the 1982 flood profile. During the 2009 flood, a 

large amount of debris accumulated on many bridges resulting 

in higher water surfaces than would be expected for the flow. 

The simulated water surface was between 0.4 and 2.0 ft 

lower than the measured high-water marks from the 2009 

flood. Differences between measured and simulated water 

levels for models calibrated to the 1982 flood were between 

– 0.1 and +0.1 ft (table 2). The boundaries of the DFIRM flood 

zones were similar to the boundaries of the modeled inundated 

areas from the 21-ft stage Sweetwater Creek flood-inundation 

model. These results demonstrate that the model is capable of 

simulating accurate water levels over a wide range of flows in 

the basin. Details on techniques used in model development 

and calibration can be found in Bales and others (2007).

Development of Water-Surface Profiles

Profiles were developed for 21 stages at 1.0-ft intervals 

between 12.0 and 32.0 ft, as referenced to the Sweetwater 

Creek near Austell streamgage (02337000). Discharges at 

the streamgage corresponding to the various stages were 

obtained from the most current stage-discharge relation 

(rating no. 22.1). Discharges for all profiles were calculated 

based on a ratio of streamflow to drainage area for historical 

events. Table 3 lists results for stages at 2-ft increments from 

12 to 32 ft. A relation of yield (flow per drainage area) and 

drainage area was calculated based on the historical discharges 

from Sweetwater Creek at Brownsville Road, near Powder 

Springs (02336840) downstream to Sweetwater Creek near 

Austell (02337000). This relation was then used to determine 

discharge estimates along the model reach for the 21 stages. 

Stages and discharge estimates for selected locations and 

profiles are shown in table 3. 

Table 2. Comparison of hydraulic-model output and surveyed 

high-water mark elevations from the flood of February 4, 1982, 

Sweetwater Creek, Georgia.

[Values listed in feet above North American Vertical Datum of 1988]

Location along 

Sweetwater Creek

High-water 

mark 

elevation

Modeled 

water-surface 

elevations

Elevation 

difference

Skyview Drive 880.6 880.6 0.0

Old Alabama Road 888.7 888.7 0.0

Veterans Memorial 

Highway

891.2 891.1 –0.1

Norfolk Southern 

Railway 

(upstream)

893.6 893.5 –0.1

Westside Road 897.7 897.8 +0.1

Table 3. Stages (and water-surface elevations) with corresponding discharge estimates for selected simulated water-surface profiles 

at selected locations for the Sweetwater Creek near Austell, Georgia, streamgage (02337000).

[Values 12.0–32.0 represent stage, in feet above gage datum. Values in parentheses (869.3–889.3) are water-surface elevations, in feet above North American 

Vertical Datum 1988; Ga., Georgia]

Location Water-surface elevations and discharge estimates

Stage, in feet

Sweetwater Creek 

near Austell, Ga. 

12.0 

(869.3)

14.0

(871.3)

16.0 

(873.3)

18.0

(875.3)

20.0 

(877.3)

22.0 

(879.3)

24.0 

(881.3)

26.0 

(883.3)

28.0 

(885.3)

30.0 

(887.3)

32.0 

(889.3)

Discharge, in cubic feet per second

Below Powder 

Springs Creek

3,560 4,390 5,330 6,890 9,190 11,800 15,000 18,800 23,200 28,200 33,500

Below Noses Creek 4,080 5,030 6,090 7,800 10,100 12,900 16,100 19,900 24,100 28,900 33,800

Below Olley Creek 4,230 5,220 6,320 8,060 10,400 13,200 16,400 20,200 24,400 29,100 33,900

At Old Alabama Road 4,300 5,300 6,420 8,180 10,600 13,300 16,600 20,300 24,500 29,200 34,000

Sweetwater Creek 

near Austell 

streamgage

4,430 5,460 6,600 8,400 10,700 13,600 16,800 20,500 24,700 29,300 34,000
8 Flood-Inundation Maps for Sweetwater Creek, Cobb and Douglas Counties, Georgia

Inundation Mapping

Flood-inundation maps were created based on the 

Sweetwater Creek near Austell streamgage (02337000), 

which has been designated as an NWS flood-forecast point 

(AUSG1). The maps were created in a GIS by combining 

the water-surface profiles and digital elevation model data. 

The digital elevation model data were derived in two parts. 

The Cobb County elevation data are from an 8-ft by 8-ft 

cell raster from photogrammatically derived contours. The 

Douglas County elevation data are from the USGS 1/3-Arc 

Second NED, with a cell size of approximately 31 ft by 31 ft. 

Estimated flood-inundation boundaries for each simulated 

profile were developed with HEC–GeoRAS software (U.S. 

Army Corps of Engineers, Hydrologic Engineering Center, 

2009). HEC–GeoRAS is a set of procedures, tools, and 

utilities for processing geospatial data in ArcGIS (Esri, 2010) 

by using a graphical user interface (Whitehead and Ostheimer, 

2009). The interface allows the preparation of geometric data 

for import into HEC–RAS and processes simulation results 

exported from HEC–RAS (U.S. Army Corps of Engineers, 

Hydrologic Engineering Center, 2010). HEC–GeoRAS results 

were modified to ensure a hydraulically reasonable transition 

of the boundary between modeled cross sections relative to the 

contour data for the land surface (Whitehead and Ostheimer, 

2009). The resulting inundation maps have a vertical accuracy 

of about 0.5 ft in Cobb County and 2.0 ft in Douglas County. 

The maps show estimated flood-inundated areas overlain on 

high-resolution, georeferenced, aerial photographs of the study 

area for each of the water-surface profiles that were generated 

by the hydraulic model. The bed slope of Sweetwater Creek is 

fairly consistent from Powder Springs Creek to Old Alabama 

Road. The bed slope is then steeper downstream to Thornton 

Road where the bed slope becomes flatter to the Sweetwater 

Creek near Austell streamgage (02337000) at Interstate 20. 

The reach of Sweetwater Creek included in the inundation 

maps is about 10.5 mi long. The sheet numbers corresponding 

to each 1-ft stage increment at the Sweetwater Creek 

streamgage are shown in table 4.

The flood-inundation maps could be extended further 

upstream once additional data are collected to extend the 

stage-discharge relation into the flood zone at the Sweetwater 

Creek 0.05 mi upstream of Norfolk Southern Railroad 

bridge at Austell, Ga., streamgage (02336910). Additionally 

flood-inundation maps could be created based on the other 

streamgages within the Sweetwater Creek basin.

Sweetwater Creek, Georgia, Flood-Inundation 

Maps on the Internet

A USGS Flood Inundation Mapping Science World Wide 

Web portal (http://water.usgs.gov/osw/flood_inundation/) has 

been established by the USGS to provide estimated floodinundation information to the public. The maps from this 

study are available in several commonly used electronic file 

formats that can be downloaded from that portal. Each stream 

reach displayed on the Web site contains links to the USGS 

National Water Information System graphs of the current 

stage and streamflow at the USGS Sweetwater Creek near 

Austell streamgage (02337000), to which the inundation maps 

are referenced. A link also is provided to the NWS AHPS 

Web site (http:/water.weather.gov/ahps/) so that the user can 

obtain applicable information on forecasted peak stage. The 

estimated flood-inundation maps are displayed in sufficient 

detail to note the extent of flooding with respect to individual 

structures so that preparations for flooding and decisions for 

emergency response can be performed efficiently.

Table 4. Stream stages and water-surface elevations at Sweetwater Creek near Austell, Georgia, streamgage (02337000), with 

reference to corresponding map-sheet numbers in this report.

[Values 12.0–32.0 represent stage, in feet above gage datum. Values in parentheses (869.3–889.3) represent water-surface elevation in feet above North 

American Vertical Datum 1988]

Sweetwater Creek near Austell

Stage (watersurface 

elevation)

12.0

(869.3)

13.0

(870.3)

14.0

(871.3)

15.0

(872.3)

16.0

(873.3)

17.0

(874.3)

18.0

(875.3)

19.0

(876.3)

20.0

(877.3)

21.0

(878.3)

22.0

(879.3)

Sheet number 1 2 3 4 5 6 7 8 9 10 11

Stage (watersurface 

elevation)

23.0

(880.3)

24.0

(881.3)

25.0

(882.3)

26.0

(883.3)

27.0

(884.3)

28.0

(885.3)

29.0

(886.3)

30.0

(887.3)

31.0

(888.3)

32.0

(889.3)

Sheet number 12 13 14 15 16 17 18 19 20 21
Summary 9

Disclaimer for Flood-Inundation Maps

Inundated areas shown should not be used for navigation, 

regulatory, permitting, or other legal purposes. The USGS 

provides these maps “as-is” for a quick reference, emergency 

planning tool but assumes no legal liability or responsibility 

resulting from the use of this information.

Uncertainties and Limitations Regarding 

Use of Flood-Inundation Maps

Although the flood-inundation maps represent the boundaries of inundated areas with a distinct line, some uncertainty 

is associated with these maps. The flood boundaries shown 

were estimated based on water stages and streamflows at 

selected USGS streamgages. Water-surface elevations along 

the stream reaches were estimated by steady-state hydraulic 

modeling, assuming unobstructed flow, and by using streamflows and hydrologic conditions anticipated at the USGS 

streamgage(s). The hydraulic model reflects the land-cover 

characteristics and any bridge, dam, levee, or other hydraulic 

structures existing as of September 2011. Unique meteorological factors (timing and distribution of precipitation) may 

cause actual streamflows along the modeled reach to vary from 

those assumed during a flood, which may lead to deviations in 

the water-surface elevations and inundation boundaries shown. 

Additional areas may be flooded due to unanticipated conditions such as changes in the streambed elevation or roughness, 

backwater into major tributaries along a main stem river, or 

backwater from localized debris or ice jams. The accuracy of 

the floodwater extent portrayed on these maps will vary with 

the accuracy of the digital elevation model used to simulate 

the land surface. Additional uncertainties and limitations 

pertinent to this study are described elsewhere in this report.

If this series of flood-inundation maps will be used in 

conjunction with National Weather Service (NWS) river forecasts, the user should be aware of additional uncertainties that 

may be inherent or factored into NWS forecast procedures. 

The NWS uses forecast models to estimate the quantity and 

timing of water flowing through selected stream reaches in 

the United States. These forecast models (1) estimate the 

amount of runoff generated by precipitation and snowmelt, 

(2) simulate the movement of floodwater as it proceeds 

downstream, and (3) predict the flow and stage (and watersurface elevation) for the stream at a given location (AHPS 

forecast point) throughout the forecast period (every 6 hours 

and 3 to 5 days out in many locations). For more information 

on AHPS forecasts, please see http://water.weather.gov/ahps/

pcpn_and_river_forecasting.pdf. 

Summary

A series of estimated flood-inundation maps were 

developed by the U.S. Geological Survey (USGS), in 

cooperation with Cobb County, Georgia, for a 10.5-mile reach 

of Sweetwater Creek from about 1,800 feet above the confluence with Powder Springs Creek to about 160 feet below the 

Interstate 20 bridge. These maps, available at a USGS Web 

portal, in conjunction with the real-time stage data from the 

USGS streamgage at Sweetwater Creek near Austell, Georgia 

(02337000), and National Weather Service flood-stage forecasts, can help to guide the general public in taking individual 

safety precautions and can provide local officials with a tool 

to help efficiently manage emergency flood operations and 

flood-mitigation efforts. 

The maps were developed using the U.S. Army Corps 

of Engineers HEC–RAS and HEC–GeoRAS computer 

programs to simulate water-surface profiles and to delineate 

estimated flood-inundation areas for selected stream stages. 

The maps show estimated flood-inundation areas overlain on 

high-resolution, georeferenced, aerial photographs of the study 

area for 1.0-foot increments of stream stage between 12.0 and 

32.0 feet (gage datum) at the Sweetwater Creek near Austell, 

Georgia, streamgage (02337000). 
10 Flood-Inundation Maps for Sweetwater Creek, Cobb and Douglas Counties, Georgia

References Cited

Bales, J.D., Wagner, C.R., Tighe, K.C., and Terziotti, Silvia, 

2007, LiDAR-derived flood-inundation maps for realtime flood-mapping applications, Tar River Basin, North 

Carolina: U.S. Geological Survey Scientific Investigations 

Report 2007–5032, 42 p.

 Clark, W.Z., Jr., and Zisa, A.C., 1976, Physiographic map of 

Georgia: Georgia Geologic Survey, scale 1:2,000,000. 

Esri, ArcGIS Desktop: Version 9.3.1: accessed August 2010 

at http://www.esri.com/.

Federal Emergency Management Agency, Digital Flood 

Insurance Rate Map: accessed March 2011 at 

http://www.msc.fema.gov/.

Fry, J.A., Xian, G., Jin, S., Dewitz, J.A., Homer, C.G., Yang, 

L., Barnes, C.A., Herold, N.D., and Wickham, J.D., 2011, 

Completion of the 2006 National Land Cover Database for 

the conterminous United States: Photogrammetric Engineering & Remote Sensing, v. 77, no. 9, p. 858–864.

Musser, J.W., 2012, Flood-inundation maps for Peachtree 

Creek from the Norfolk Southern Railway bridge to the 

Moores Mill Road NW bridge, Atlanta, Georgia: U.S. 

Geological Survey Scientific Investigations Map 3189, 9 p., 

50 sheets, available online at http://pubs.usgs.gov/sim/3189/.

Musser, J.W., and Dyar, T.R., 2007, Two-dimensional floodinundation model of the Flint River at Albany, Georgia: 

U.S. Geological Survey Scientific Investigations 

Report 2007–5107, 49 p., available online at 

http://pubs.usgs.gov/sir/2007/5107/.

U.S. Army Corps of Engineers, Hydrologic Engineering 

Center, 2009, HEC–GeoRAS, GIS Tools for Support of 

HEC–RAS using ArcGIS: User’s Manual, version 4.2 

[variously paged]. 

U.S. Army Corps of Engineers, Hydrologic Engineering 

Center, 2010, HEC–RAS River Analysis System: Hydraulic 

Reference Manual, version 4.1 [variously paged].

U.S. Census Bureau, State and County QuickFacts: accessed 

November 2011, at http://quickfacts.census.gov/qfd/index.html.

Whitehead, M.T., and Ostheimer, C.J., 2009, Development 

of a flood-warning system and flood-inundation mapping 

for the Blanchard River in Findlay, Ohio: U.S. Geological 

Survey Scientific Investigations Report 2008–5234, 9 p.
Manuscript approved on June 26, 2012

Edited by Kimberly A. Waltenbaugh

Illustrations and layout by Caryl J. Wipperfurth

Science Publishing Network, Raleigh PSC

For more information about this publication, contact:

USGS Georgia Water Science Center

3039 Amwiler Road

Atlanta, GA 30360

telephone: 770-903-9100

http://ga.water.usgs.gov/
Musser—Flood-Inundation Maps for Sweetwater Creek, Cobb and Douglas Counties, Georgia—Scientific Investigations Map 3220