Prepared in cooperation with Gwinnett County, Georgia

Flood-Inundation Maps for Suwanee Creek from the 

Confluence of Ivy Creek to the Noblin Ridge Drive Bridge, 

Gwinnett County, Georgia

Pamphlet to accompany

Scientific Investigations Map 3226

U.S. Department of the Interior

U.S. Geological Survey
Cover. Suwanee Creek flooding greenway trail, September 20, 2009 (photograph from 

City of Suwanee, Georgia).
Flood-Inundation Maps for Suwanee Creek from the 

Confluence of Ivy Creek to the Noblin Ridge Drive Bridge, 

Gwinnett County, Georgia

By Jonathan W. Musser

Prepared in cooperation with Gwinnett County, Georgia

Pamphlet to accompany 

Scientific Investigations Map 3226

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 Suwanee Creek from the confluence of Ivy Creek to the Noblin Ridge 

Drive bridge, Gwinnett County, Georgia: U.S. Geological Survey Scientific Investigations Map 3226, 8 p. pamphlet, 

19 sheets; available online at http://pubs.usgs.gov/sim/3226/.
iii

Figures

1. Map showing location of study reach for Suwanee Creek and location of U.S. 

Geological Survey streamgage and National Weather Service forecast site ...................2

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

surface-water flow.......................................................................................................................4

Contents

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

Introduction ....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................3

Study Area Description........................................................................................................................3

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

Constructing Water-Surface Profiles.........................................................................................................5

Hydrologic and Steady-Flow Data .....................................................................................................5

Topographic/Bathymetric Data ..........................................................................................................5

Energy Loss Factors .............................................................................................................................5

Model Calibration and Performance .................................................................................................5

Development of Water-Surface Profiles...........................................................................................6

Inundation Mapping ......................................................................................................................................7

Suwanee Creek, Georgia, Flood-Inundation Maps on the Internet .............................................7

Disclaimer for Flood-Inundation Maps .............................................................................................7

Uncertainties and Limitations Regarding Use of Flood-Inundation Maps ..................................7

Summary..........................................................................................................................................................8

References Cited............................................................................................................................................8

Tables

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

Suwanee Creek basin at Suwanee, Georgia ...........................................................................3

2. Comparison of hydraulic-model output and water-surface elevations at 

the Suwanee Creek at Suwanee, Georgia, streamgage........................................................6

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

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

Suwanee Creek at Suwanee, Georgia, streamgage...............................................................6

4. Stream stages (and water-surface elevations) at the Suwanee Creek at 

Suwanee, Georgia, streamgage, with reference to corresponding map-sheet 

numbers in this report..................................................................................................................7
iv

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

For links, see http://pubs.usgs.gov/sim/3226/.)

1–19. Maps showing flood-inundation of Suwanee Creek in Gwinnett County, 

Georgia, corresponding to a gage height and an elevation (NAVD 88) as 

listed below at U.S. Geological Survey streamgage Suwanee Creek at 

Suwanee, Georgia (02334885)—

1. Gage height of 7.0 feet and an elevation of 916.9 feet

2. Gage height of 7.5 feet and an elevation of 917.4 feet

3. Gage height of 8.0 feet and an elevation of 917.9 feet

4. Gage height of 8.5 feet and an elevation of 918.4 feet

5. Gage height of 9.0 feet and an elevation of 918.9 feet

6. Gage height of 9.5 feet and an elevation of 919.4 feet

7. Gage height of 10.0 feet and an elevation of 919.9 feet

8. Gage height of 10.5 feet and an elevation of 920.4 feet

9. Gage height of 11.0 feet and an elevation of 920.9 feet

10. Gage height of 11.5 feet and an elevation of 921.4 feet

11. Gage height of 12.0 feet and an elevation of 921.9 feet

12. Gage height of 12.5 feet and an elevation of 922.4 feet

13. Gage height of 13.0 feet and an elevation of 922.9 feet

14. Gage height of 13.5 feet and an elevation of 923.4 feet

15. Gage height of 14.0 feet and an elevation of 923.9 feet

16. Gage height of 14.5 feet and an elevation of 924.4 feet

17. Gage height of 15.0 feet and an elevation of 924.9 feet

18. Gage height of 15.5 feet and an elevation of 925.4 feet

19. Gage height of 16.0 feet and an elevation of 925.9 feet
v

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).

Acronyms and Additional Abbreviations

AHPS Advanced Hydrologic Prediction Service

DFIRM Digital Flood Insurance Rate Map

FEMA Federal Emergency Management Agency

GIS geographic information system

LiDAR light detection and ranging

NED National Elevation Dataset 

NWS National Weather Service

RMSE root-mean-square error 

USGS U.S. Geological Survey

Acknowledgments

The author wishes to thank Gwinnett 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 Suwanee Creek from the 

Confluence of Ivy Creek to the Noblin Ridge Drive Bridge, 

Gwinnett County, Georgia

By Jonathan W. Musser

Abstract

Digital flood-inundation maps for a 6.9-mile reach of 

Suwanee Creek, from the confluence of Ivy Creek to the 

Noblin Ridge Drive bridge, were developed by the U.S. 

Geological Survey (USGS) in cooperation with Gwinnett 

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 Suwanee Creek at Suwanee, Georgia 

(02334885). 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 Suwanee Creek at Suwanee 

(02334885), 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 Suwanee Creek and was used to compute flood profiles 

for a 6.9-mile reach of the creek. The model was calibrated 

using the most current stage-discharge relations at the 

Suwanee Creek at Suwanee streamgage (02334885). The 

hydraulic model was then used to determine 19 water-surface 

profiles for flood stages at the Suwanee Creek streamgage 

at 0.5-foot intervals referenced to the streamgage. The 

profiles ranged from just above bankfull stage (7.0 feet) to 

approximately 1.7 feet above the highest recorded water level 

at the streamgage (16.0 feet). The simulated water-surface 

profiles were then combined with a geographic information 

system digital elevation model—derived from light detection 

and ranging (LiDAR) data having a 5.0-foot horizontal 

resolution—to delineate the area flooded for each 0.5-foot 

increment of stream stage.

The availability of these maps, when combined with 

real-time stage information from USGS streamgages and 

forecasted stream stage 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.

Introduction 

Gwinnett County, Georgia (Ga.), is a suburban community with a population of 805,321 in 2010 (U.S. Census 

Bureau, 2012; fig. 1). Development on flood plains within the 

county is varied with a mix of residential, recreational, and 

commercial structures, and areas of agriculture, grass fields, 

wetlands, and forest. Suwanee Creek flows through Suwanee, 

Ga., which is in the northwestern part of Gwinnett County. 

In September of 2009, Gwinnett County experienced record 

flooding along Suwanee Creek. 

Prior to this study, Gwinnett County and City of 

Suwanee officials 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 Suwanee 

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 (table 1)—Suwanee Creek at 

Suwanee, Ga. (02334885)—from which current or historical 

water levels (stage) can be obtained. 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 Suwanee Creek 

at Suwanee streamgage (forecast site SWEG1) through the 

Advanced Hydrologic Prediction Service (AHPS) Web site 

at http://water.weather.gov/ahps/. 
2 Flood-Inundation Maps for Suwanee Creek, Gwinnett County, Georgia

Chattahoochee

GEORGIA

River

Base modified from U.S. Geological Survey

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

EXPLANATION

USGS streamgage and NWS 

 forecast site and identifiers

Piedmont physiographic province

Subbasin boundary

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

streamgage and National Weather Service forecast site.

N

GWINNETT

COUNTY

HALL

COUNTY

Lake Sidney Lanier

 Lake 

 Sidney

Lanier

Suwanee Creek

Ivy Creek

Chattahoochee River

02334885

SWEG1

02334885

SWEG1

Area of map in figure 2

Area of map below

84°05'W 84°00'W 83°55'W

34°05'N

34°00'N

0 1 2 3 4 MILES

0 2 31 4 KILOMETERS

Suwanee

GWINNETT COUNTY

HALL

COUNTY

River

Chattahoochee

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

streamgage and National Weather Service (NWS) forecast site.
Introduction 3

Although USGS current stage and NWS forecast stage 

data are particularly useful for residents in the immediate 

vicinity of a streamgage, the data are 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 6.9-mile (mi) 

reach of Suwanee Creek were developed by the USGS in 

cooperation with Gwinnett County, Ga.

Purpose and Scope

The purpose of this report is to describe the development 

of a series of estimated flood-inundation maps for Suwanee 

Creek in Gwinnett County, 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 

would be close to (1) current stage at the USGS streamgage at 

Suwanee (02334885), (2) the NWS forecasted peak stage, or 

(3) other desired stream stages.

The scope of the study was limited to a 6.9-mi reach of 

Suwanee Creek from about 440 feet (ft) above the confluence 

of Suwanee Creek and Ivy Creek to about 930 ft downstream 

of the Noblin Ridge Drive bridge (fig. 2). Tasks specific to 

development of the flood-inundation maps were (1) analysis of 

the flow and stage data collected at one streamgage, Suwanee 

Creek at Suwanee (02334885) (table 1); (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 userselected 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 Suwanee Creek at Suwanee (02334885) and ranged 

from approximately bankfull (7.0 ft) to above the maximum 

observed water level at the streamgage (16.0 ft). 

Study Area Description

Suwanee Creek is in northern Georgia in the Piedmont 

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

area ranges from 39.2 square miles (mi2) at the confluence 

of Ivy Creek to 47.0 mi2 at the Suwanee Creek at Suwanee 

streamgage (02334885) to 50.7 mi2 below Noblin Ridge 

Drive at the downstream extent of the study reach (fig. 2). The 

headwaters of Suwanee Creek originate in Gwinnett and Hall 

Counties (fig. 1). Suwanee Creek and its tributaries generally 

flow southwestward through Gwinnett County until Suwanee 

Creek joins with the Chattahoochee River. Ivy Creek is the 

one major tributary that flows into Suwanee Creek. 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 6.9 mi long, with an average channel slope of 

about 7.2 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 Suwanee Creek and its tributaries (Fry and others, 

2011). The main channel within the study reach has six major 

road bridges, two footbridges, and one railroad bridge that 

cross the channel and the adjacent flood plain. 

Table 1. Site information for the U.S. Geological Survey streamgage in the Suwanee Creek basin at Suwanee, Georgia.

[USGS, U.S. Geological Survey; Ga., Georgia; ft, foot; current year, year of publication; see fig. 1 for station location]

Station name

USGS

station

number

Drainage area

(square miles)

Latitude Longitude Period of

record

Maximum 

recorded stage 

(degrees, minutes, seconds) at gage and date

Suwanee Creek at 

Suwanee, Ga.

02334885 47.0 34°01'56" 84°05'22" 1985 to

current year

14.30 ft

Sept. 21, 2009
4 Flood-Inundation Maps for Suwanee Creek, Gwinnett County, Georgia

Ivy Creek

Chattahoochee River

84°06'W 84°05'W 84°04'W 84°03'W

34°03'N

34°2'N

0 0.5 1 MILE

0 0.5 1 KILOMETER

SUWANEE ROAD

LAWRENCEVILLE 

MARTIN FARM ROAD

MCGINNIS FERRY ROAD

BUFORD HIGHWAY

NORFOLK SOUTHERN RAILROAD

PEACHTREE INDUSTRIAL BOULEVARD

NOBLIN

RIDGE

DRIVE

GWINNETT

COUNTY

Suwanee

02334885

SWEG1

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

Foot

Bridge

Suwanee Creek modeled section

Tributary

USGS streamgage and NWS 

 forecast site and identifiers

02334885

SWEG1

Flood verification,

 September 2009 

Swift Atlanta 

AES Clean Technology

Innovating Machining Solutions

Gwinnett Collision

Figure 2. Suwanee Creek bridge crossings, model extent, and direction of surface-water flow.

Previous Studies

The current DFIRM for Gwinnett County was published 

June 18, 2010 (Federal Emergency Management Agency, 

2011). The DFIRM was used to check the inundation extent 

for stages that are close to the 1-percent and 0.2-percent 

annual exceedance probability. A HEC–RAS model of 

Suwanee Creek, which was used in the publication of the 

DFIRM, was obtained in 2011 from Gwinnett County 

(Gwinnett County, 2011). The cross sections, stream channel, 

and bridge details within the study area from the HEC–RAS 

model 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 Suwanee Creek model. The 

methods used for the Peachtree Creek model were similar to 

those used for the Suwanee Creek model.
Constructing Water-Surface Profiles 5

Constructing Water-Surface Profiles

The water-surface profiles used to produce the 

19 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 gradually varied, steady-state (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 

one streamgage on the study reach (fig. 1; table 1). Water 

level (stage) is measured continuously at this site, and 

continuous records of streamflow are computed. All watersurface elevations are referenced to North American Vertical 

Datum of 1988 (NAVD 88). The gage is equipped with a 

satellite radio transmitter that allows data to be transmitted 

routinely and made available on the Internet within an hour 

of collection. Suwanee Creek at Suwanee (02334885) is 

also equipped with a recording rain gage. Steady-flow data 

consisted of flow regime, boundary conditions (stage and 

discharge value), and peak discharge information. Steady-flow 

data for the study reach were calibrated in the model using 

the stage-discharge relation at the Suwanee Creek at Suwanee 

streamgage (02334885). 

Topographic/Bathymetric Data

The cross sections from the HEC–RAS DFIRM model, 

obtained from Gwinnett County (Gwinnett County, 2011), 

were used as a starting point for the model used in this study. 

Hereafter, the term “DFIRM” will be used to describe the 

model as obtained from Gwinnett County. Additional cross 

sections were added to the USGS model where the distance 

between cross sections was more than 500 ft in the DFIRM 

model. The in-channel elevations of the new cross sections for 

the USGS model were calculated by extracting the in-channel 

part of an original cross section and lowering or raising it 

based on the channel slope between original cross sections. 

The overbank elevations of the new cross sections for the 

USGS model were calculated from a raster elevation dataset. 

The source for the raster elevation was a 5.0-ft by 5.0-ft cell 

raster from light detection and ranging (LiDAR) data obtained 

from Gwinnett County. The root-mean-square error (RMSE) 

of the LiDAR vertical data is about 0.61 ft in bare areas and 

about 1.21 ft in vegetated areas. The flow lengths between 

cross sections were recomputed after the new cross sections 

were added. Additionally, all of the ineffective flow areas were 

recalculated for all of the cross sections. 

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 nine bridges 

were measured and surveyed in the field. The bridge geometry 

was in the DFIRM HEC–RAS model. Site visits were made 

to all of the bridges to confirm the accuracy of the bridge 

geometry. The bridges from upstream to downstream were a 

footbridge, Lawrenceville Suwanee Road, a footbridge, Martin 

Farm Road, McGinnis Ferry Road, Buford Highway, Norfolk 

Southern Railroad, Peachtree Industrial Boulevard, and Noblin 

Ridge Drive (fig. 2). The upstream footbridge was included 

in the model, but the downstream footbridge is located just 

downstream of Lawrenceville Suwanee Road and so would 

have negligible effect on the streamflow. 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 0.04 to 0.11 for the 

main channel. The Manning’s n values ranged from 0.03 (grass 

and roads) to 0.18 (forested areas with heavy undergrowth) for 

the overbank areas modeled in this analysis.

Model Calibration and Performance

The hydraulic model was calibrated to the most 

current (2011) stage-discharge relation at the Suwanee 

Creek at Suwanee streamgage (02334885). No high-water 

marks were available along Suwanee Creek, so the only 

calibration point to a water-surface elevation is at the 

streamgage. The maximum recorded peak discharge was on 

September 21, 2009, and was 7,870 cubic feet per second 

(ft3

/s) at a stage of about 14.30 ft. Though no high-water marks 

were obtained, the extent of this flood closely matched the 

14.5-ft stage in the model at four business locations. At Swift 

Atlanta, which is located approximately 1 mi upstream of the 

streamgage, the flood was reported to have reached the top of 

the curb in their back parking lot. At AES Clean Technology 

on Martin Farm Road, the flood extent was reported to be 

approximately 30 ft uphill on the road from a public parking 

lot on the south side of the road. In the model, the distance 

is approximately 34 ft. At two businesses on the downstream 

side of Lawrenceville Suwanee Road, Innovative Machining 

Solutions and Gwinnett Collision, the reported flood extent on 

their property is close to the 14.5-ft stage model. At Gwinnett 

Collision, the flood came within a few feet of a retaining wall, 

which matches the model.
6 Flood-Inundation Maps for Suwanee Creek, Gwinnett County, Georgia

Model calibration in the vicinity of the gage was 

accomplished by adjusting Manning’s n value of the channel 

until the results of the hydraulic computations closely agreed 

with the known stage-discharge relation. Differences between 

measured and simulated water levels for models calibrated to 

stages 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 for the 16-ft Suwanee Creek 

stage in the USGS inundation model. 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 19 stages at 0.5-ft intervals 

between 7.0 and 16.0 ft as referenced to the Suwanee Creek at 

Suwanee streamgage (02334885). Discharges corresponding to 

the various stages were obtained from the most current stagedischarge relation (rating no. 14.1, effective June 2, 2009). 

Discharges for all profiles were calculated based on a ratio of 

streamflow to drainage area. Stages and discharge estimates 

for selected locations and profiles are shown in table 3.

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

profiles at selected locations for the Suwanee Creek at Suwanee, Georgia, streamgage (02334885).

[Values 7.0–16.0 represent stage, in feet above gage datum. Values in parentheses (916.9–925.9) represent water-surface elevations, in feet above 

North American Vertical Datum of 1988]

Location Stage (and water-surface elevations) and discharge estimates

Stage (and water-surface elevations), in feet

Suwanee Creek at 

Suwanee, Georgia

7.0

(916.9)

8.0

(917.9)

9.0

(918.9)

10.0

(919.9)

11.0

(920.9)

12.0

(921.9)

13.0

(922.9)

14.0

(923.9)

15.0

(924.9)

16.0

(925.9)

Discharge, in cubic feet per second

Below Ivy Creek 647 850 1,250 1,830 2,560 3,480 4,600 5,920 7,450 9,150

Below unnamed tributary 

downstream of 

Lawrenceville 

Suwanee Road

690 906 1,340 1,950 2,730 3,710 4,900 6,310 7,940 9,750

Below unnamed tributary 

upstream of McGinnis 

Ferry Road

725 952 1,410 2,050 2,870 3,900 5,150 6,640 8,340 10,200

Suwanee Creek at 

Suwanee streamgage

800 1050 1,550 2,260 3,160 4,300 5,680 7,320 9,200 11,300

Below unnamed tributary 

upstream of Peachtree 

Industrial Boulevard

835 1100 1,620 2,360 3,300 4,490 5,930 7,640 9,610 11,800

Table 2. Comparison of hydraulic-model output and watersurface elevations at the Suwanee Creek at Suwanee, Georgia, 

streamgage (02334885).

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

Stage

Actual watersurface

elevations

Modeled 

water-surface 

elevations

Elevation

difference

7.0 916.9 917.0 +0.1

7.5 917.4 917.4 0.0

8.0 917.9 917.8 –0.1

8.5 918.4 918.4 0.0

9.0 918.9 918.8 –0.1

9.5 919.4 919.4 0.0

10.0 919.9 919.8 –0.1

10.5 920.4 920.3 –0.1

11.0 920.9 920.8 –0.1

11.5 921.4 921.4 0.0

12.0 921.9 921.8 –0.1

12.5 922.4 922.4 0.0

13.0 922.9 922.9 0.0

13.5 923.4 923.4 0.0

14.0 923.9 923.8 –0.1

14.5 924.4 924.3 –0.1

15.0 924.9 925.0 +0.1

15.5 925.4 925.5 +0.1

16.0 925.9 925.8 –0.1
Inundation Mapping 7

Inundation Mapping

Flood-inundation maps were created based on the 

Suwanee Creek at Suwanee streamgage (02334885), which 

has been designated as a NWS flood-forecast point (SWEG1). 

The maps were created in a GIS by combining the watersurface profiles and digital elevation model data. The digital 

elevation model data are from a 5.0-ft by 5.0-ft cell raster 

derived from LiDAR data obtained from Gwinnett County. 

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. 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 Suwanee 

Creek is fairly consistent along the 6.9-mi modeled reach. The 

sheet numbers corresponding to each 0.5-ft stage increment at 

the Suwanee Creek streamgage are shown in table 4.

Suwanee 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 

Table 4. Stream stages (and water-surface elevations) at the Suwanee Creek at Suwanee, Georgia, streamgage (02334885), with 

reference to corresponding map-sheet numbers in this report.

[Values 7.0–16.0 represent stage, in feet above gage datum. Values in parentheses (916.9–925.9) represent water-surface elevation in feet above 

North American Vertical Datum of 1988]

Suwanee Creek at Suwanee

Stage (watersurface 

elevation)

7.0

(916.9)

7.5

(917.4)

8.0

(917.9)

8.5

(918.4)

9.0

(918.9)

9.5

(919.4)

10.0

(919.9)

10.5

(920.4)

11.0

(920.9)

11.5

(921.4)

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

Stage (watersurface 

elevation)

12.0

(921.9)

12.5

(922.4)

13.0

(922.9)

13.5

(923.4)

14.0

(923.9)

14.5

(924.4)

15.0

(924.9)

15.5

(925.4)

16.0

(925.9)

Sheet number 11 12 13 14 15 16 17 18 19

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 Suwanee Creek at Suwanee 

streamgage (02334885), 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.

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 stage and 

discharge at the Suwanee Creek at Suwanee, Ga. (02334885) 

streamgage. 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. 
8 Flood-Inundation Maps for Suwanee Creek, Gwinnett County, Georgia

The hydraulic model reflects the land-cover characteristics and 

any bridge, dam, levee, or other hydraulic structures existing 

as of October 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 Gwinnett County, Georgia, for a 6.9-mile reach of 

Suwanee Creek from 440 feet above the confluence with Ivy 

Creek to about 930 feet below the Noblin Ridge Drive bridge. 

These maps, available at a USGS Web portal, in conjunction 

with the real-time stage data from the USGS streamgage at 

Suwanee Creek at Suwanee, Georgia (02334885), 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 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 compute 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 0.5-foot increments of stream stage between 

7.0 and 16.0 feet (gage datum) at the Suwanee Creek at 

Suwanee, Georgia, streamgage (02334885). 

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 [2010], ArcGIS Desktop: Version 9.3.1: Accessed 

August 2010 at http://www.esri.com.

Federal Emergency Management Agency [2011], 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.

Gwinnett County [2011], Flood Information Portal: Accessed 

April 2011 at http://www.gwinnettfloodplain.com/gwinnett/

welcome.htm.

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 

flood-inundation 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 [2012], State and County QuickFacts: 

Accessed January 2012 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 July 25, 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 Suwanee Creek, Gwinnett County, Georgia—Scientific Investigations Map 3226