U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2019–5009

Prepared in cooperation with Gwinnett County, Georgia

Flood-Inundation Maps for the Yellow River From River Drive 

to Centerville Highway, Gwinnett County, Georgia
Front cover: Photograph of U.S. Geological Survey personnel at the streamgage Yellow River at Georgia 124, near Lithonia, Georgia (02207120). Photograph by 

Karen Stull, U.S. Geological Survey, September 2009.

Back cover: Photograph of the streamgage Yellow River near Snellville, Georgia (02206500). Photograph by Jonathan Musser, U.S. Geological Survey, February 2018.
Flood-Inundation Maps for the Yellow River 

From River Drive to Centerville Highway, 

Gwinnett County, Georgia

By Jonathan W. Musser

Prepared in cooperation with Gwinnett County, Georgia

Scientific Investigations Report 2019–5009

U.S. Department of the Interior

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

DAVID BERNHARDT, Acting Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2019

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Suggested citation:

Musser, J.W., 2019, Flood-inundation maps for the Yellow River from River Drive to Centerville Highway, 

Gwinnett County, Georgia: U.S. Geological Survey Scientific Investigations Report 2019–5009, 15 p., 

https://doi.org/10.3133/sir20195009.

ISSN 2328-0328 (online)
iii

Acknowledgments

The author wishes to thank Gwinnett County, Georgia, for funding the operation and 

maintenance of the two streamgages used for this study, for sharing information used in 

this report, and for its continued involvement in the U.S. Geological Survey Cooperative 

Water Program. Thank you to Tony Gotvald and Jeff Morris for their technical reviews of this 

report. Special thanks to the National Weather Service for their continued support to the 

U.S. Geological Survey flood-inundation mapping initiative.
iv

Contents

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

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

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

Methods..................................................................................................................................................3

Study Area Description........................................................................................................................5

Previous Studies ...................................................................................................................................5

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

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

Topographic-Bathymetric Data ..........................................................................................................7

Energy-Loss Factors.............................................................................................................................7

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

Development of Water-Surface Profiles...........................................................................................8

Flood-Inundation Mapping...........................................................................................................................8

Yellow River, Georgia, Flood-Inundation Maps on the Internet ..................................................11

Disclaimer For Flood-Inundation Maps...........................................................................................13

Uncertainties and Limitations Regarding Use of Flood-Inundation Maps ................................13

Summary........................................................................................................................................................13

References Cited..........................................................................................................................................14
v

Figures

1. Map showing location of study reach for the Yellow River and locations of 

U.S. Geological Survey streamgages and National Weather Service forecast 

sites, Gwinnett County, Georgia.................................................................................................2

2. Map showing Yellow River bridge crossings, mapped river extent, floodinundation mapping boundary, high-water marks from September 2009, and 

direction of surface-water flow, Gwinnett County, Georgia..................................................4

3. Map showing National Weather Service radar rainfall estimates for 

September 18–22, 2009, for the Yellow River Basin, Gwinnett County, Georgia.................9

4. Map showing locations of streets inundated by the hydraulic model of the Yellow 

River, Gwinnett County, Georgia...............................................................................................12

Tables

1. Site information for selected U.S. Geological Survey streamgages on the Yellow 

River in Gwinnett County, Georgia.............................................................................................5

2. Stages (and water-surface elevations) with corresponding streamflow estimates 

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

River near Snellville, Georgia (02206500), streamgage ..........................................................6

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

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

River at Georgia 124, near Lithonia, Georgia (02207120), streamgage.................................6

4. Comparison of high-water mark elevations and modeled water-surface 

elevations on the Yellow River, Gwinnett County, Georgia, for simulated and 

constant flow...............................................................................................................................10

5. Comparison of modeled water-surface elevations and rating curve watersurface elevations at the Yellow River near Snellville, Georgia (02206500), 

streamgage, for the 9.0-mile reach of the Yellow River from 0.5-mile upstream 

from River Drive to Stone Mountain Highway, Gwinnett County, Ga.................................10

6. Comparison of modeled water-surface elevations and rating curve watersurface elevations at the Yellow River near Snellville, Georgia (02206500), 

streamgage, based on stage and flows at the Yellow River at Ga. 124, near 

Lithonia, Ga. (02207120), streamgage for the 7.4-mile reach of the Yellow River 

from Stone Mountain Highway to Centerville Highway, Gwinnett County, Ga.................11
vi

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 0.09290 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)

Datum

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 the vertical datum.

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

at the gage (gage datum).

Abbreviations

AHPS Advanced Hydrologic Prediction Service 

DEM digital elevation model

DFIRM Digital Flood Insurance Rate Map

FEMA Federal Emergency Management Agency

GIS geographic information system

HEC–RAS Hydrologic Engineering Center’s River Analysis System

HWM high-water mark

lidar light detection and ranging

NWISWeb National Water Information System Web Interface

NWS National Weather Service

USGS U.S. Geological Survey
Flood-Inundation Maps for the Yellow River From River 

Drive to Centerville Highway, Gwinnett County, Georgia

By Jonathan W. Musser

Abstract

Digital flood-inundation maps for a 16.4-mile reach 

of the Yellow River in Gwinnett County, Georgia, from 

0.5 mile upstream from River Drive to Centerville Highway 

(Georgia State Route 124) were developed to depict estimates 

of the areal extent and depth of flooding corresponding 

to selected water levels (stages) at two U.S. Geological 

Survey (USGS) streamgages in the mapped area. The maps 

for the 9.0-mile reach from 0.5 mile upstream from River 

Drive to Stone Mountain Highway (U.S. Route 78) are 

referenced to the streamgage Yellow River near Snellville, Ga. 

(station 02206500), and the maps for the 7.4-mile reach 

from Stone Mountain Highway to Centerville Highway are 

referenced to the streamgage Yellow River at Ga. 124, near 

Lithonia, Ga. (02207120). Real-time stage information from 

these streamgages can be used with these maps to estimate 

near real-time areas of inundation. The forecasted peak-stage 

information for the USGS streamgages Yellow River near 

Snellville, Ga. (02206500), and Yellow River at Ga. 124, near 

Lithonia, Ga. (02207120), can be used in conjunction with 

the maps developed for 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 Hydrologic 

Engineering Center’s River Analysis System (HEC–RAS) 

software for the Yellow River and was used to compute 

flood profiles for a 16.4-mile reach of the Yellow River. The 

hydraulic model was then used to simulate 16 water-surface 

profiles at 1.0-foot (ft) intervals at the Yellow River near 

Snellville streamgage and 17 water-surface profiles at 1.0-ft 

intervals at the Yellow River near Lithonia streamgage. At the 

Yellow River near Snellville streamgage, the profiles ranged 

from a stage of 18.0 ft, which is 819.1 ft above the North 

American Vertical Datum of 1988 (NAVD 88), to a stage 

of 33.0 ft, which is 834.1 ft above NAVD 88. At the Yellow 

River near Lithonia streamgage, the profiles ranged from 

the National Weather Service action stage of 13.0 ft, which 

is 732.5 ft above NAVD 88, to a stage of 29.0 ft, which is 

748.5 ft above NAVD 88. 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-ft horizontal resolution—to 

delineate the area flooded at each 1.0-ft interval of stream 

stage for both streamgages.

Introduction 

Gwinnett County is a suburban county in northern 

Georgia with the cities of Lilburn and Snellville located in the 

southern part of the county (fig. 1). The estimated population 

of Gwinnett County was 920,260 in 2017 (U.S. Census 

Bureau, 2018). Within the study area, the Yellow River flood 

plain is primarily forest with some areas of woody wetlands. 

The Yellow River Basin in Gwinnett County is 74.4 percent 

developed land, 20.4 percent forest, 1.6 percent wetlands, 

0.8 percent open water, and 2.8 percent other land, which 

includes barren, pasture, and shrub/scrub (Homer and others, 

2015). The Yellow River generally flows southward through 

Gwinnett County (fig. 1). Peak flood flows of greater than 

7,000 cubic feet per second (ft3/s) were recorded at the 

U.S. Geological Survey (USGS) streamgage Yellow River 

near Snellville, Georgia (station 02206500), in 1948, 1956, 

1961, 1990, 1995, and 1998 (the streamgage was not in 

operation from 2002 to 2017 [table 1]); the largest peak flow 

was recorded on October 5, 1995, at 12,800 ft3/s. Peak flood 

flows of greater than 7,000 ft3/s were recorded at the USGS 

streamgage Yellow River at Ga. 124, near Lithonia, Ga. 

(station 02207120), in 2005, 2009, and 2015; the largest peak 

flow was recorded on September 22, 2009, at 16,500 ft3/s 

(U.S. Geological Survey, 2018).
2 Flood-Inundation Maps for the Yellow River From River Drive to Centerville Highway, Gwinnett County, Georgia

#

# Snellville

Lilburn

GWINNETT

DEKALB

ROCKDALE

02206500

YESG1

02207120

YELG1

Yellow

River

02207120

 YELG1

EXPLANATION

#

Modeled part of Yellow River

Yellow River

U.S. Geological Survey streamgage and number

with National Weather Service

site identification

AL GA

FL

TN NC

SC

84°10'

33°55'

33°50'

33°45'

84°5' 84°0'

Hydrography from the 1:24,000-scale National Hydrography Dataset

Georgia State Plane West projection

City boundaries from U.S. Census Bureau

0 2 4 MILES

0 2 4 KILOMETERS

Map area

Figure 1. Location of study reach for the Yellow River and locations of U.S. Geological Survey streamgages and National Weather 

Service forecast sites, Gwinnett County, Georgia.
Introduction 3

Gwinnett County officials have historically relied on 

several information sources to decide how to alert the public 

and mitigate flood damages along the Yellow River. One 

source was the Federal Emergency Management Agency 

(FEMA) Digital Flood Insurance Rate Map (DFIRM) (Federal 

Emergency Management Agency, 2017). A second source 

was the USGS streamgages (table 1) Yellow River near 

Snellville, Ga. (02206500), and Yellow River at Ga. 124, 

near Lithonia, Ga. (02207120). From these sites, current and 

historical water levels (stage) can be obtained through the 

USGS National Water Information System Web Interface 

(NWISWeb; https://doi.org/10.5066/F7P55KJN). Stage is 

the height of the water surface above an arbitrary datum 

established at the gage (gage datum). A third source of 

usable information was the National Weather Service (NWS) 

Yellow River near Northeast Lithonia streamgage (NWS 

YELG1; colocated with USGS 02207120) and the NWS 

Yellow River near Snellville streamgage (NWS YESG1; 

colocated with USGS 02206500), available through the 

Advanced Hydrologic Prediction Service (AHPS) web page 

(https://water.weather.gov/ahps/).

Although USGS real-time stages are useful for residents 

in the immediate vicinity of a streamgage, the data are of 

limited use to residents upstream or downstream from the gage 

because water-surface elevation is not constant along an entire 

stream channel. Likewise, depth varies locally along stream 

channels, and FEMA and State emergency management 

mitigation teams or property owners typically lack information 

on water depth in areas not near USGS streamgages or 

NWS flood-forecast points. To help the general public take 

individual safety precautions and give local officials a tool 

to help manage emergency flood operations and mitigation 

efforts, two series of digital flood-inundation maps for a 

16.4-mile (mi) reach of the Yellow River were developed by 

the USGS in cooperation with Gwinnett County, Ga.

Purpose and Scope

This report describes the development of estimated 

flood-inundation maps for the Yellow River in Gwinnett 

County, Ga. Inundation maps and other flood-related data are 

available on the USGS Flood Inundation Mapping Science 

web page (https://water.usgs.gov/osw/flood_inundation/). 

Users can select estimated inundation maps at or near (1) the 

current stage at the USGS streamgage near Snellville, Ga. 

(02206500), or the USGS streamgage near Lithonia, Ga. 

(02207120), (2) the NWS forecasted peak stage, or (3) other 

desired stages at either USGS streamgage.

The study covers a 16.4-mi reach of the Yellow River 

from 0.5 mi upstream from River Drive to Centerville 

Highway (Georgia State Route 124) (fig. 2). Two sets of 

inundation maps were created on the basis of data from the 

two USGS streamgages within the reach. The first set of 

inundation maps is for a 9.0-mi reach of the Yellow River 

from 0.5 mi upstream from River Drive to Stone Mountain 

Highway (U.S. Route 78) and is based on the streamgage 

Yellow River near Snellville, Ga. (02206500). The second set 

of inundation maps is for a 7.4-mi reach of the Yellow River 

from Stone Mountain Highway to Centerville Highway and 

is based on the streamgage Yellow River at Ga. 124, near 

Lithonia, Ga. (02207120). 

Development of the flood-inundation maps included 

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

USGS streamgages (table 1); (2) collection of topographic and 

geometric data for flood plains and bridges; (3) determination 

of energy-loss factors (roughness coefficients) in the stream 

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

profiles using the Hydrologic Engineering Center’s River 

Analysis System (HEC–RAS) computer program, created 

by the Hydrologic Engineering Center at the U.S. Army 

Corps of Engineers Institute for Water Resources (U.S. Army 

Corps of Engineers, Hydrologic Engineering Center, 2018); 

(5) production of two sets of estimated flood-inundation maps 

based on simulated stream stages at the Yellow River near 

Snellville, Ga. (02206500), and Yellow River at Ga. 124, near 

Lithonia, Ga. (02207120), streamgages, using the geographic 

information system (GIS) program ArcGIS Desktop (Esri, 

2018a); and (6) development of a web interface that links to 

USGS real-time streamgage information.

Methods

Most methods used here are cited from published reports 

(Bales and others, 2007; Whitehead and Ostheimer, 2009). If 

techniques varied substantially because of local hydrologic 

conditions or available data, the variances are described in 

detail within this report. Inundation maps for 16 water-surface 

profiles referenced to the stage and water-surface elevation at 

the streamgage Yellow River near Snellville, Ga. (02206500), 

and 17 water-surface profiles referenced to the stage and 

water-surface elevation at the streamgage Yellow River at 

Ga. 124, near Lithonia, Ga. (02207120), were produced. 

The profiles for the Yellow River near Snellville streamgage 

ranged from a stage of 18.0 feet (ft), which is 819.1 ft above 

the North American Vertical Datum of 1988 (NAVD 88), 

to a stage of 33.0 ft, which is 834.1 ft above NAVD 88. 

The profiles for the Yellow River at Ga. 124, near Lithonia 

streamgage ranged from the NWS action stage of 13.0 ft, 

which is 732.5 ft above NAVD 88, to a stage of 29.0 ft, which 

is 748.5 ft above NAVD 88 and 5.0 ft above major flood stage 

(National Weather Service, 2018). 
4 Flood-Inundation Maps for the Yellow River From River Drive to Centerville Highway, Gwinnett County, Georgia

Figure 2. Yellow River bridge crossings, mapped river extent, flood-inundation mapping boundary, high-water marks from September 

2009, and direction of surface-water flow, Gwinnett County, Georgia.

#

#

Area of steep

riverbed slope

Area of steep

riverbed slope

FIVE FO RKS TRICKUM R D

KILLIAN HILL ROAD

STONE MOUNTAIN HIGHWAY

ANNISTOWN ROAD

Centerville Creek

CENTERVILLE HIGHWAY

RIVER

DRIVE

Garner C r eek

EE

E

E

E

E

E

E

E

EE D1_21C

D1_21A

D1_20C

D1_19C

D1_18A

D1_17B

GWINNETT

Map area

DEKALB WALTON

FULTON

ROCKDALE

02207120

EXPLANATION

Modeled part of Yellow River #

Flood-inundation mapping

boundary

Direction of surface water flow

U.S. Geological Survey streamgage

and number

High-water mark from September 2009

and number

D1_15C

D1_14C

D1_13C

D1_13B

D1_16A

D1_16B

Turkey Cre e k Watso n Creek

Pounds C reek

Jacks Cree

Y

k

ellow

River

D1_21C E

02207120

02206500

Sweetwater

Creek

Hydrography from the 1:24,000-scale National Hydrography Dataset

Georgia State Plane West projection

Base map image is the intellectual property of Esri and is used herein

under license. Copyright © 2018 Esri and its licensors. All rights reserved.

0 1 2 MILES

0 1 2 KILOMETERS

84°8' 84°7' 84°6' 84°5' 84°4' 84°3' 84°2' 84°1'

33°53'

33°52'

33°51'

33°50'

33°49'

33°48'

33°47'
Constructing Water-Surface Profiles 5

Study Area Description

The Yellow River is located in northern Georgia in the 

Piedmont physiographic province (Clark and Zisa, 1976). 

The drainage area of the Yellow River is 55.1 square miles 

(mi2) at the upstream end of the study reach, upstream from 

where Sweetwater Creek joins with the Yellow River. The 

drainage area of Sweetwater Creek where it joins the Yellow 

River is 70.7 mi2. The drainage area of the Yellow River is 

136 mi2 at the Yellow River near Snellville, Ga. (02206500), 

streamgage, 148 mi2 at Stone Mountain Highway, and 161 mi2

at the Yellow River at Ga. 124, near Lithonia, Ga. (02207120), 

streamgage, which is at the downstream end of the study reach 

(fig. 2). The study reach of the Yellow River is located entirely 

in Gwinnett County, Ga. (fig. 1). The Yellow River generally 

flows south through Gwinnett County between the cities of 

Lilburn and Snellville and flows into DeKalb County near the 

lower end of the study reach. The major tributaries flowing 

into the Yellow River along the study reach are Sweetwater 

Creek, Turkey Creek, Watson Creek, Garner Creek, Pounds 

Creek, Jacks Creek, and Centerville Creek. The Yellow 

River 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 

Yellow River study reach is about 16.4 mi long and is fairly 

consistent in slope and width, except for two areas of steep 

riverbed slope downstream from Stone Mountain Highway. 

The average channel slope over the entire reach is about 

6.6 feet per mile. The land contiguous to the study reach is 

classified as a mix of developed and forest, with the immediate 

flood plain area being mostly forest. Between Annistown 

Road and Centerville Highway, the land is mostly classified 

as forest (Homer and others, 2015). Within the study reach, 

six road bridges cross the main channel and the adjacent flood 

plain (fig. 2).

Previous Studies

Similar studies that provide flood-inundation maps for 

a range of stream stages in Georgia were completed by the 

USGS in Albany, Ga., for the Flint River (Musser and Dyar, 

2007); Atlanta, Ga., for Peachtree Creek (Musser, 2012a); 

Gwinnett County, Ga., for Suwanee Creek (Musser, 2012b); 

Cobb County, Ga., for Sweetwater Creek (Musser, 2012c); 

Alpharetta and Roswell, Ga., for Big Creek (Musser, 2015a); 

DeKalb County, Ga., for South Fork Peachtree Creek (Musser, 

2015b); and Lowndes County, Ga., for the Withlacoochee 

River (Musser, 2018). The methods for the Flint River 

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

differed from those used to develop the Yellow River model. 

The methods used for the Peachtree Creek, Suwanee Creek, 

Sweetwater Creek, Big Creek, South Fork Peachtree Creek, 

and the Withlacoochee River models were similar to those 

used to develop the Yellow River model. Additionally, the 

current—as of 2019—DFIRM for Gwinnett County, Ga., was 

published on June 4, 2017 (Federal Emergency Management 

Agency, 2017), but it was not used in this study, other than to 

define a general area of the flood plain for modeling purposes.

Constructing Water-Surface Profiles

The water-surface profiles used to produce the 

33 flood-inundation maps in this study were computed using 

HEC–RAS, version 5.0.3 (U.S. Army Corps of Engineers, 

Hydrologic Engineering Center, 2018). HEC–RAS is a onedimensional, step-backwater model used for simulation of 

water-surface profiles with gradually varied, steady-state, 

or unsteady-state flow computation options. Inputs into the 

HEC–RAS model include cross sections representing the land 

surface, bridge geometry, roughness coefficients, and multiple 

flow values. For this study, the HEC–RAS analysis was 

completed by using the steady-state flow computation option.

Table 1. Site information for selected U.S. Geological Survey streamgages on the Yellow River in Gwinnett County, Georgia.

[Data for this table were compiled from U.S. Geological Survey (2018). USGS, U.S. Geological Survey; ft, foot; ft3/s, cubic foot per second]

USGS station 

number 

(fig. 1)

Station name Drainage area 

(square miles)

Latitude 

(degrees, 

minutes, seconds)

Longitude 

(degrees, 

minutes, seconds)

Period of record

Maximum recorded stage 

and corresponding flow 

at streamgage and date

02206500 Yellow River near 

Snellville, Ga.

136 33°51'11" 84°04'42" 1943 to 2002 and 

2017 to current 

year (2019)

19.75 ft*

12,800 ft3/s

Oct. 5, 1995

02207120 Yellow River at 

Ga. 124, near 

Lithonia, Ga.

161 33°46'20" 84°03'28" 2002 to current 

year (2019)

27.47 ft

16,500 ft3/s

Sept. 22, 2009

*

Stage at a previous gage datum.
6 Flood-Inundation Maps for the Yellow River From River Drive to Centerville Highway, Gwinnett County, Georgia

Hydrologic and Steady-Flow Data

The hydrologic network in the study area consists of two 

USGS streamgages operated within the study reach (fig. 1; 

table 1). Water level (stage) is measured continuously at each 

site, and continuous records of streamflow are computed. 

Stage is the height of the water surface above an arbitrary 

datum established at the gage (gage datum). All water-surface 

elevations are referenced to NAVD 88. The streamgages are 

equipped with satellite radio transmitters that allow data to 

be transmitted routinely and made available online within 

1 hour of collection. The streamgages are also equipped with 

recording rain gages. Steady-flow data for the study reach 

were calibrated in the model using the rating curve (stagestreamflow or stage-discharge relation) at the streamgages 

Yellow River near Snellville, Ga. (02206500), and Yellow 

River at Ga. 124, near Lithonia, Ga. (02207120). Downstream 

boundary conditions in the model were set using the rating 

curve at the Yellow River near Lithonia streamgage.

The flows at Yellow River near Snellville, Ga. 

(02206500), were used for the reach from 0.5 mi upstream 

from River Drive to Stone Mountain Highway. The flows 

at Yellow River at Ga. 124, near Lithonia, Ga. (02207120), 

were used for the reach from Stone Mountain Highway to 

Centerville Highway. Flow-change points were added into 

the HEC–RAS model at stream tributaries in each reach 

where the drainage area increased abruptly. The ratio of the 

drainage area at these points, with the drainage area at each 

streamgage raised to the power of 0.54 multiplied by the flow 

at the streamgage, is the model flow at these points (Gotvald 

and Knaak, 2011). Tables 2 and 3 show model flow values 

for selected locations. The Yellow River near Snellville, Ga. 

(02206500), streamgage was operational and at a different 

datum from 1943 to 2002. In 2017, the current streamgage 

(as of 2019) was installed on a new bridge with a different 

structure, which changed the out-of-bank topography in the 

flood plain.

Table 2. Stages (and water-surface elevations) with corresponding streamflow estimates for selected simulated water-surface 

profiles at selected locations for the Yellow River near Snellville, Georgia (02206500), streamgage.

Location of streamflow 

estimate for the Yellow River

Stage, in feet above gage datum

(Water-surface elevation, in feet above North American Vertical Datum of 1988)

18.0

(819.1)

20.0

(821.1)

22.0

(823.1)

24.0

(825.1)

26.0

(827.1)

28.0

(829.1)

30.0

(831.1)

32.0

(833.1)

33.0

(834.1)

Streamflow, in cubic feet per second

0.5 miles above River Drive 2,800 3,610 4,430 5,320 6,270 7,280 8,360 9,520 10,100

Below Sweetwater Creek 4,380 5,640 6,930 8,310 9,800 11,400 13,100 14,900 15,700

Below Turkey Creek 4,490 5,780 7,100 8,520 10,000 11,700 13,400 15,300 16,100

Near Snellville streamgage (02206500) 4,580 5,890 7,230 8,680 10,200 11,900 13,700 15,600 16,400

Below Garner Creek 4,690 6,030 7,410 8,890 10,500 12,200 14,000 15,900 16,800

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

profiles at selected locations for the Yellow River at Georgia 124, near Lithonia, Georgia (02207120), streamgage.

Location of streamflow 

estimate for the Yellow River

Stage, in feet above gage datum

(Water-surface elevation, in feet above North American Vertical Datum of 1988)

13.0

(732.5)

15.0

(734.5)

17.0

(736.5)

19.0

(738.5)

21.0

(740.5)

23.0

(742.5)

25.0

(744.5)

27.0

(746.5)

29.0

(748.5)

Streamflow, in cubic feet per second

Below Pounds Creek 5,070 6,410 7,770 9,230 10,800 12,400 14,100 15,900 17,700

Below Jacks Creek 5,220 6,590 8,000 9,500 11,100 12,800 14,500 16,400 18,200

Near Lithonia streamgage (02207120) 5,320 6,720 8,150 9,680 11,300 13,000 14,800 16,700 18,600
Constructing Water-Surface Profiles 7

Topographic-Bathymetric Data

Two HEC–RAS models for the Yellow River were 

obtained from Gwinnett County. Parts of the models were 

combined to create one model for the study reach. Additional 

cross sections were added to the USGS model where the 

distance between two cross sections was more than 500 ft 

in the original model and where the original cross sections 

were not long enough to extend across the entire flood plain. 

The in-channel elevations of these new cross sections were 

calculated by extracting the in-channel part of an original 

cross section and lowering or raising it on the basis of the 

channel slope between two adjacent original cross sections. 

The overbank elevations of the new cross sections were 

calculated from a raster elevation dataset. The source for 

the raster elevation was a 5.0-ft by 5.0-ft cell raster digital 

elevation model (DEM) derived from light detection and 

ranging (lidar) data collected by Gwinnett County in 2015. 

The additional cross sections for HEC–RAS were created from 

the DEM using the HEC–GeoRAS software (Esri, 2018b) 

within ArcGIS. HEC–GeoRAS is a set of tools developed 

for ArcGIS to translate data to and from HEC–RAS. The 

HEC–RAS model covers a 16.4-mi reach of the Yellow 

River from 0.5-mi upstream from River Drive to Centerville 

Highway. Downstream stream lengths were computed 

within HEC–GeoRAS. 

Various drainage modifying structures (bridges, culverts, 

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

stream can affect water-surface elevations during floods. To 

account for these structural features, the structural dimensions 

of six bridges were included in the model. From upstream to 

downstream on the Yellow River, the bridges are River Drive, 

Five Forks Trickum Road, Killian Hill Road, Stone Mountain 

Highway, Annistown Road, and Centerville Highway (fig. 2). 

The geometries of these bridges were already present in 

the HEC–RAS model; however, the bridges on Five Forks 

Trickum Road and Killian Hill Road had changed since the 

model was created. To account for the changes, the structural 

dimensions of these two bridges were measured and surveyed 

in the field. Using data obtained from the surveys, the bridges 

were replaced in the HEC–RAS model. A footbridge is located 

just upstream from the Stone Mountain Highway bridge, but 

on the basis of visual inspection, it was determined that the 

bottom of the footbridge is higher than the bottom of the Stone 

Mountain Highway bridge and there are no piers in the river 

to impede flow. The footbridge was not included in the model 

except to adjust the ineffective flow areas to account for the 

change in topography on the ends of the footbridge. A detailed 

description of the methods used to acquire and process the 

topographic and bathymetric data is provided in Bales and 

others (2007). 

Energy-Loss Factors

Field observations and land-cover data were used to 

select initial (precalibration) Manning’s roughness coefficients 

(n values) for energy (friction) loss calculations (Coon, 1998). 

The final Manning’s n values ranged from 0.04 to 0.11 for the 

main channel. The Manning’s n values ranged from 0.06 to 

0.17 for the overbank areas modeled in the reach.

Model Calibration and Performance

 The hydraulic model was calibrated using the most 

current (2018) stage-discharge relations at the Yellow River 

near Snellville, Ga. (02206500), and Yellow River at Ga. 124, 

near Lithonia, Ga. (02207120), streamgages in conjunction 

with adjusting the Manning’s n value for the channel and 

overbank areas until the results of the hydraulic computations 

closely agreed with the expected water-surface elevations at 

the Yellow River near Snellville, Ga. (02206500), streamgage 

and high-water marks (HWMs) from September 2009, which 

were obtained from Gwinnett County (fig. 2). 

The initial calibration of the model was to the HWMs 

from September 2009. The Manning’s n values were 

adjusted until the water-surface elevations closely matched 

the elevations from the HWMs. Many of the HWMs were 

collected within 300 ft of other HWMs, and some recorded 

elevations were not consistent with nearby HWMs. When 

inconsistencies were found, the description and quality of the 

HWMs were evaluated to determine which HWMs would be 

used for calibration. Of the 23 HWMs along the study reach, 

12 were used for the model calibration. 

The initial calibration of the upstream end of the model 

to the three HWMs near River Drive resulted in model 

elevations between 0.9 and 1.4 ft lower than the HWMs for 

the same Manning’s n values used in other parts of the model. 

Table 4 shows the differences between the HWM elevations 

and the modeled water-surface elevations. Because the Yellow 

River near Snellville, Ga. (02206500), streamgage was not 

operating during August 2009, the flows used in the model 

for this calibration are based on the August 2009 peak flow at 

the Yellow River at Ga. 124, near Lithonia, Ga. (02207120), 

streamgage. The rainfall radar estimate downloaded from 

the NWS on September 23, 2009, for September 18–22, 

2009 (fig. 3), shows higher rainfall totals in the Yellow 

River Basin upstream from the Yellow River near 

Snellville, Ga. (02206500), streamgage than downstream 

from the streamgage. Because of this rainfall distribution, 

the actual flow in the Yellow River near River Drive during 

the September 2009 flood event is likely higher than what 

would be predicted using the drainage area ratio from Gotvald 

and Knaak (2011). To test the calibration, a constant flow of 

17,100 ft3/s was used in the model. This flow corresponds 

to the peak gage height at the Yellow River at Ga. 124, near 

Lithonia, Ga. (02207120), streamgage in September 2009 

based on the current rating. This flow value is larger than 

the 16,500 ft3/s peak that was recorded in 2009 because the 

streamgage rating changed between 2009 and 2019 and the 

current rating is used in the model (established September 30, 

2013). With this constant flow, the modeled water-surface 
8 Flood-Inundation Maps for the Yellow River From River Drive to Centerville Highway, Gwinnett County, Georgia

elevations at the HWMs near River Drive ranged from 0.3 

to 0.7 ft higher than the elevation of the HWMs. Thus, the 

actual streamflow in the Yellow River near River Drive 

during the September 2009 flood event was likely between 

the 15,300 ft3/s determined by the drainage area ratio method 

and the 17,100 ft3/s peak at the Yellow River at Ga. 124, near 

Lithonia, Ga. (02207120), streamgage.

On the basis of the calibration of the model to the 

September 2009 HWMs, the simulated flow in the model at 

the Yellow River near Snellville, Ga. (02206500), streamgage 

was 15,600 ft3/s and the water-surface elevation was 833.2 ft, 

which corresponds to a streamgage height of 32.1 ft. This flow 

is higher than any flow previously recorded at the streamgage, 

so the rating curve was adjusted to this simulated streamgage 

height and flow and extended with a straight-line extension to 

a streamgage height of 33.0 ft.

After the model was calibrated to the September 2009 

HWMs, the model was further calibrated to various stages and 

flows by adjusting the Manning’s n values for the channel. 

For the 9.0-mi reach from 0.5 mi upstream from River 

Drive to Stone Mountain Highway, the flow values in the 

model were based on the Yellow River near Snellville, Ga. 

(02206500), streamgage and calibrated to the rating curve at 

that streamgage. Table 5 shows the modeled water-surface 

elevations and the water-surface elevations from the rating 

curve at the Yellow River near Snellville, Ga. (02206500), 

streamgage for this section of the model. For the 7.4-mi 

reach between Stone Mountain Highway and Centerville 

Highway, the flow values in the model were based on flows at 

the Yellow River at Ga. 124, near Lithonia, Ga. (02207120), 

streamgage. These flow values were extended upstream, 

using the drainage area ratio method, to the Yellow River near 

Snellville, Ga. (02206500), streamgage, where they were used 

for calibration to the water-surface elevations corresponding 

to the flows in the model. Table 6 shows the modeled watersurface elevations and the water-surface elevations from 

the rating curve at the Yellow River near Snellville, Ga. 

(02206500), streamgage. 

Development of Water-Surface Profiles

Profiles were developed for 16 stages at 1.0-ft intervals 

between 18.0 and 33.0 ft referenced to the Yellow River 

near Snellville, Ga. (02206500), streamgage. Streamflows 

corresponding to profiles at water-surface elevations were 

obtained from the most current (2018) rating curve in use 

at the streamgage (rating no. 14.0, effective December 20, 

2017; U.S. Geological Survey, 2018) for the Yellow River 

between 0.5 mi upstream from River Drive to Stone Mountain 

Highway. Profiles were developed for 17 stages at 1.0-ft 

intervals between 13.0 and 29.0 ft referenced to the Yellow 

River at Ga. 124, near Lithonia, Ga. (02207120), streamgage. 

Streamflows corresponding to profiles at water-surface 

elevations were obtained from the most current (2018) rating 

curve in use at the streamgage (rating no. 8.0, effective 

December 14, 2016; U.S. Geological Survey, 2018). The 

streamflow values for all profiles were adjusted along the 

Yellow River, primarily at junctions with smaller tributaries. 

Simulated stage and streamflow estimates for selected 

locations and profiles are provided in tables 2 and 3. The 

downstream boundary conditions were set as the water-surface 

elevation based on the stage at the Yellow River at Ga. 124, 

near Lithonia, Ga. (02207120), streamgage. All streamflow 

and water-elevation data used for the model are available in 

U.S. Geological Survey (2018). 

Flood-Inundation Mapping

Flood-inundation GIS layers were created on the basis 

of simulated water-surface elevations at the Yellow River 

near Snellville, Ga. (02206500; NWS designation YESG1), 

streamgage for each 1.0-ft interval in stage from 18.0 ft to 

33.0 ft and the Yellow River at Ga. 124, near Lithonia, Ga. 

(02207120; NWS designation YELG1), streamgage for each 

1.0-ft interval in stage from 13.0 ft to 29.0 ft. A polygon layer 

showing the inundated area and a raster showing the depth 

of inundation were created. Additionally, boundary polygon 

layers showing the extent of the inundation area were created 

for both streamgages. The layers were created in ArcGIS by 

exporting a 5.0-ft by 5.0-ft cell depth raster from HEC–RAS 

and selecting all cells with a depth greater than 0.05 ft. The 

resulting polygon inundated area was cleaned up by deleting 

all polygons smaller than 400 square feet (ft2) and filling all 

holes (non-inundated areas) in the inundated layer smaller than 

400 ft2. Any polygon pieces not directly connected to the river 

were deleted, except where there was a documented culvert or 

bridge. Although these inundated areas separated by a culvert 

or bridge were included in the inundation maps, there is a large 

amount of uncertainty in these areas because of the interaction 

of the flow through the culvert or bridge and backwater from 

the Yellow River. Thus, these areas are marked as “Area of 

uncertainty” on the online Flood Inundation Mapper. Also, the 

tributaries of the Yellow River, which are shown as inundated, 

are a result of backwater effects from the Yellow River. At the 

lower flow values of the Yellow River, the tributaries may not 

be shown as inundated, but it is likely water is still flowing 

in them. Boundary layers were created to limit the extent of 

the inundation polygons and depth rasters to the Yellow River 

from 0.5-mi upstream from River Drive to Stone Mountain 

Highway and from Stone Mountain Highway to Centerville 

Highway. Though the depth rasters are rounded to the nearest 

0.1 ft, the resulting inundation maps have a vertical accuracy 

of about 0.5 ft.
Flood-Inundation Mapping 9

Figure 3. National Weather Service radar rainfall estimates for September 18–22, 2009, for the Yellow River Basin, Gwinnett 

County, Georgia.

EXPLANATION

Total rainfall September 18–22, 2009,

in inches

0 to 2

Greater than 2 to 4

Greater than 4 to 6

Greater than 6 to 8

Greater than 8 to 10

Greater than 10 to 12

Greater than 12 to 14

GWINNETT

DEKALB

FULTON

WALTON

FORSYTH HALL

BARROW

ROCKDALE

Modeled part of Yellow River

Yellow River watershed boundary

U.S. Geological Survey streamgage

and number

Map area

120

141

140

236

20

20

316

120

20

316

317

23

29

78

29

985 85

85

285

02206500

02207120

02207120

124

124

10

78

Hydrography from the 1:24,000-scale National Hydrography Dataset

Georgia State Plane West projection

Base map image is the intellectual property of Esri and is used herein

under license. Copyright © 2018 Esri and its licensors. All rights reserved.

34°0'

33°55'

33°50'

33°45'

84°15' 84°10' 84°5' 84°0' 83°55'

0 2 4 8 MILES

0 2 4 8 KILOMETERS
10 Flood-Inundation Maps for the Yellow River From River Drive to Centerville Highway, Gwinnett County, Georgia

Table 4. Comparison of high-water mark (HWM) elevations and modeled water-surface elevations on the Yellow River, Gwinnett 

County, Georgia, for simulated and constant flow.

[Elevation is listed in feet above the North American Vertical Datum of 1988. ft, foot; —, no value]

Gwinnett County 

HWM 

identification

HWM 

type

HWM 

quality1

HWM 

elevation

(ft)

Modeled watersurface elevation 

for simulated flow

(ft)

Elevation 

difference

(ft)

Modeled watersurface elevation 

for constant flow

(ft)

Elevation 

difference

(ft)

D1_21C Debris Moderate 748.5 748.6 0.1 — —

D1_21A Debris line Good 748.9 748.7 −0.2 — —

D1_19C Mud line Good 762.2 762.0 −0.2 — —

D1_20C Debris line Moderate 781.6 781.9 0.3 — —

D1_18A Mud line Good 818.3 818.3 0.0 — —

D1_17B Mud line Moderate 823.4 823.1 −0.3 — —

D1_15C Debris Good 846.2 845.9 −0.3 — —

D1_14C Mud line Moderate 854.4 853.5 −0.9 855.1 0.7

D1_16A and D1_16B Water line Good 856.5 855.1 −1.4 856.8 0.3

D1_13B and D1_13C Debris Good 856.6 855.3 −1.3 857.0 0.4

1

High-water mark quality is from Gwinnett County and is not representative of U.S. Geological Survey quality standards.

Table 5. Comparison of modeled water-surface elevations and rating curve water-surface elevations at the Yellow River near 

Snellville, Georgia (02206500), streamgage, for the 9.0-mile reach of the Yellow River from 0.5-mile upstream from River Drive to Stone 

Mountain Highway, Gwinnett County, Ga.

[Stage is listed in feet above the gage datum. Elevation is listed in feet above the North American Vertical Datum of 1988. ft, foot]

Stage at Yellow River 

near Snellville, Ga. 

(02206500)

(ft)

Modeled water-surface elevation 

at Yellow River near Snellville, Ga. 

(02206500)

(ft)

Rating curve water-surface 

elevation at Yellow River 

near Snellville, Ga. (02206500)

(ft)

Elevation 

difference

(ft)

18.0 819.1 819.6 0.5

19.0 820.1 820.5 0.4

20.0 821.1 821.3 0.2

21.0 822.1 822.1 0.0

22.0 823.1 822.8 −0.3

23.0 824.1 824.5 0.4

24.0 825.1 825.1 0.0

25.0 826.1 825.8 −0.3

26.0 827.1 827.3 0.2

27.0 828.1 828 −0.1

28.0 829.1 829.4 0.3

29.0 830.1 830.0 −0.1

30.0 831.1 831.3 0.2

31.0 832.1 831.9 −0.2

32.0 833.1 833.2 0.1

33.0 834.1 833.7 −0.4
Flood-Inundation Mapping 11

The final step in the creation of the inundation polygons 

was to remove the area of the non-inundated bridges from the 

inundation polygon. None of the simulated stages inundated 

the six major roads crossing the Yellow River: River Drive, 

Five Forks Trickum Road, Killian Hill Road, Stone Mountain 

Highway, Annistown Road, and Centerville Highway, but 

some of the flows did reach the low part of the bridge deck 

at three bridges. For the Yellow River near Snellville, Ga. 

(02206500), streamgage, the River Drive bridge was reached 

at a stage of 30 ft, and the Five Forks Trickum Drive bridge 

was reached at a stage of 31 ft. For the Yellow River at 

Ga. 124, near Lithonia, Ga. (02207120), streamgage, the Stone 

Mountain Highway bridge was reached at a stage of 23 ft 

and was nearly overtopped at 29 ft. Numerous small streets 

were inundated at various simulated stages. The locations 

of these streets are shown in figure 4 along with the stage at 

which the streets would be inundated according to the model. 

The 33 inundation polygons, 33 depth rasters, 2 boundary 

polygons, and all associated metadata are available for 

download in Musser (2019).

Yellow River, Georgia, Flood-Inundation Maps 

on the Internet

The USGS Flood Inundation Mapping Program website 

(https://water.usgs.gov/osw/flood_inundation/) was established 

by the USGS to provide estimated flood-inundation 

information to the public. The GIS layers from this study are 

viewable on the Flood Inundation Mapper (available from the 

above link) over selected base maps linked from the website. 

The GIS layers can also be downloaded in commonly used 

electronic file formats. Each stream reach displayed on the 

website contains links to the NWISWeb graphs of the current 

stage and streamflow at the streamgages Yellow River near 

Snellville, Ga. (02206500), and Yellow River at Ga. 124, 

near Lithonia, Ga. (02207120), to which the inundation maps 

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

web page (https://water.weather.gov/ahps/). The estimated 

flood-inundation maps are displayed in sufficient detail to 

note the extent of flooding concerning individual structures 

so preparations for flooding and decisions about emergency 

responses can be completed efficiently.

Table 6. Comparison of modeled water-surface elevations and rating curve water-surface elevations at the Yellow River near 

Snellville, Georgia (02206500), streamgage, based on stage and flows at the Yellow River at Ga. 124, near Lithonia, Ga. (02207120), 

streamgage for the 7.4-mile reach of the Yellow River from Stone Mountain Highway to Centerville Highway, Gwinnett County, Ga.

[Stage is listed in feet above the gage datum. Elevation is listed in feet above the North American Vertical Datum of 1988. ft, foot]

Stage at Yellow River 

at Ga. 124, near Lithonia, Ga. 

(02207120) 

(ft)

Modeled water-surface elevation 

at Yellow River near 

Snellville, Ga. (02206500) 

(ft)

Rating curve water-surface 

elevation at Yellow River near 

Snellville, Ga. (02206500) 

(ft)

Elevation 

difference 

(ft)

13.0 819.6 819.9 0.3

14.0 820.5 820.8 0.3

15.0 821.5 821.6 0.1

16.0 821.7 822.3 0.6

17.0 822.4 823 0.6

18.0 824.3 824.6 0.3

19.0 825.3 825.3 0.0

20.0 826.3 825.9 −0.4

21.0 827.2 827.4 0.2

22.0 828.2 828 −0.2

23.0 829.1 829.4 0.3

24.0 830 830.0 0.0

25.0 830.9 831.2 0.3

26.0 831.8 831.7 −0.1

27.0 832.8 832.9 0.1

28.0 833.6 833.5 −0.1

29.0 834.7 834.1 −0.6
12 Flood-Inundation Maps for the Yellow River From River Drive to Centerville Highway, Gwinnett County, Georgia

#

#

2

21

02207120

GWINNETT

DEKALB WALTON

FULTON

ROCKDALE

BARROW

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

CENTERVILLE HIGHWAY

Map area

EXPLANATION

#

Modeled part of Yellow River

Flood-inundation mapping boundary

U.S. Geological Survey streamgage and number

Location of inundated street and

map identification number

02207120

7

8

6

1

23

19

11

15

14

13

12

10

02206500

9

5

4

3

22 20

18

17

16

STONE MOUNTAIN HIGHWAY

KILLIAN HILL ROAD

F IVE FORKS TRICKUM RD

RIVER

DRIVE

ANNISTOWN ROAD

26

25 24

28

27

1

2

3

4

5

6

7

8

9

10

18

20

22

23

23

24

25

25

27

27

Rolling River Drive

Riverside Drive

Connemara Trace

Rae Place

Tamer Lane

Oleander Drive

East River Bend Drive

Tom Smith Road

Finger Creek

Gwens Trail

12

13

14

15

16

17

18

19

20

21

22

23

28

28

28

28

28

29

31

31

31

32

32

32

Larkview Drive

Rangewood Drive

River Club Drive

Rivermist Drive

Silver Creek Run

Vinyard Way

Gavenwood Drive

Timber Glen Court

Vinyard Way

Lake Lucerne Road

Sean Way

Yellowstone Way

11 27 Yellow River Water

Reclamation Driveway

24

25

26

27

28

18

21

22

24

27

Spain Road

Juhan Road

Juhan Road

Carlene Way

Moorings Parkway

! 26

84°6' 84°5' 84°4' 84°3'

33°54'

33°53'

33°52'

33°51'

33°50'

33°49'

33°48'

33°47'

33°46'

Hydrography from the 1:24,000-scale 

National Hydrography Dataset

Georgia State Plane West projection

Base map image is the intellectual property

of Esri and is used herein under license.

Copyright © 2018 Esri and its licensors.

All rights reserved.

0 1 2 MILES

0 1 2 KILOMETERS

Streets inundated by the hydraulic model based on the

Yellow River near Snellvile, Georgia (02206500), streamgage

Streets inundated by the hydraulic model based on the

Yellow River near Georgia 124, near Lithonia Georgia (02207120),

streamgage

Map

identification

number

Street

name

Stage of first

inundation, in feet

above gage datum

Figure 4. Locations of streets inundated by the hydraulic model of the Yellow River, Gwinnett County, Georgia.
Summary 13

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 use as quick-reference 

emergency planning tools but assumes no legal liability or 

responsibility resulting from the use of this information.

Uncertainties and Limitations Regarding Use of 

Flood-Inundation Maps

The flood boundaries shown were estimated on the basis 

of water stages and streamflows at the USGS streamflow 

gaging stations (Yellow River near Snellville, Ga. [02206500], 

or Yellow River at Ga. 124, near Lithonia, Ga. [02207120]), 

steady-state hydraulic modeling (assuming unobstructed flow), 

and a DEM. The hydraulic model reflects the land-cover 

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

structures existing in March 2018. Unique meteorological 

factors (timing and distribution of storms) could cause actual 

streamflows along the modeled reach to vary from those 

assumed during a flood, which may lead to deviations from 

the water-surface elevations and inundation boundaries 

shown. Additional areas could be flooded due to unanticipated 

backwater from major tributaries along the main stem or 

localized debris- or ice-jams. Inundated areas shown should 

not be used for navigation, regulatory, permitting, or other 

legal purposes. Although the USGS intends to make this 

server available 24 hours a day, 7 days a week, timely delivery 

of data and products from this server through the internet is 

not guaranteed. The USGS provides these maps “as-is” for 

use as quick-reference emergency planning tools but assumes 

no legal liability or responsibility resulting from the use of 

this information.

If this series of flood-inundation maps is to be used in 

conjunction with NWS river forecasts, the user should be 

aware that additional uncertainties may be inherent or factored 

into NWS forecast procedures. The NWS uses river forecast 

models to estimate the quantity and timing of water flowing 

through selected river reaches in the United States. These 

forecast models (1) estimate the amount of runoff generated 

by a precipitation event, (2) compute how the water moves 

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

forecast point) throughout the forecast period (3 to 5 days, 

at 6-hour intervals in many locations). For information on 

AHPS forecasts, please visit the NWS Precipitation and River 

Forecasting overview web page (https://water.weather.gov/

ahps/pcpn_and_river_forecasting.pdf). 

Summary

A series of estimated geographic information system 

(GIS) flood-inundation layers was developed by the 

U.S. Geological Survey (USGS), in cooperation with Gwinnett 

County, Georgia, for a 16.4-mile (mi) reach of the Yellow 

River, which includes a 9.0-mi reach from 0.5 mi upstream 

from River Drive to Stone Mountain Highway (U.S. Route 

78) and a 7.4-mi reach from Stone Mountain Highway to 

Centerville Highway (Georgia State Route 124). These 

GIS flood-inundation layers, available at the USGS Flood 

Inundation Mapping Program website, in conjunction with 

the real-time stage data from the USGS streamgages Yellow 

River near Snellville, Ga. (02206500), and Yellow River at 

Ga. 124, near Lithonia, Ga. (02207120), and National Weather 

Service flood-stage forecasts, can help the general public take 

individual safety precautions and provide local officials with 

a tool for managing emergency flood operations and floodmitigation efforts. 

The GIS flood-inundation layers were developed using 

the U.S. Army Corps of Engineers Hydrologic Engineering 

Center’s River Analysis System (HEC–RAS) computer 

program to compute water-surface profiles and delineate 

estimated flood-inundation areas for selected stream stages. 

The layers show estimated flood-inundation areas overlain on 

selected base maps of the upper 9.0-mi reach of the study area 

for 1.0-foot (ft) increments of stage between 18.0 and 33.0 ft 

(water-surface elevation of 819.1 to 834.1 ft above the North 

American Vertical Datum of 1988 [NAVD 88]) at the Yellow 

River near Snellville, Ga. (02206500), streamgage and the 

lower 7.4-mi reach for 1.0-ft increments of stage between 13.0 

and 29.0 ft (water-surface elevation of 732.5 to 748.5 ft above 

NAVD 88) at the Yellow River at Ga. 124, near Lithonia, Ga. 

(02207120), streamgage.
14 Flood-Inundation Maps for the Yellow River From River Drive to Centerville Highway, Gwinnett County, Georgia

References Cited

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

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. [Also available at 

https://doi.org/10.3133/sir20075032.]

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

of Georgia: Atlanta, Ga., Georgia Geologic and Water 

Resources Division, scale 1:2,000,000.

Coon, W.F., 1998, Estimation of roughness coefficients 

for natural stream channels with vegetated banks: 

U.S. Geological Survey Water Supply Paper 2441, 133 p. 

[Also available at https://doi.org/10.3133/wsp2441.]

Esri, 2018a, ArcGIS Desktop (ver. 10.5.1): Esri ArcGIS 

Desktop web page, accessed April 4, 2018, at 

http://desktop.arcgis.com/en/.

Esri, 2018b, HEC–GeoRAS software (ver. 10.5): Esri 

Downloads web page, accessed April 4, 2018, at 

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For more information about this publication, contact:

Director, South Atlantic Water Science Center

U.S. Geological Survey

720 Gracern Road

Stephenson Center, Suite 129

Columbia, SC 29210

Or visit the South Atlantic Water Science Center website at

https://www.usgs.gov/water/southatlantic/

Publishing support provided by the U.S. Geological Survey 

Science Publishing Network, Reston and Sacramento Publishing 

Service Centers
Musser—Flood-Inundation Maps for the Yellow River From River Drive to Centerville Highway, Gwinnett County, Georgia—SIR 2019–5009

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20195009