Pamphlet to accompany

Scientific Investigations Map 3189

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the City of Atlanta, Georgia

Flood-Inundation Maps for Peachtree Creek 

from the Norfolk Southern Railway Bridge to the 

Moores Mill Road NW Bridge, Atlanta, Georgia
Cover. Surveying a Peachtree Creek cross section at the Howell Mill Road NW bridge 

(photograph by Paul D. Ankcorn, U.S. Geological Survey).
Flood-Inundation Maps for Peachtree Creek 

from the Norfolk Southern Railway Bridge to the 

Moores Mill Road NW Bridge, Atlanta, Georgia

By Jonathan W. Musser

Prepared in cooperation with the City of Atlanta, Georgia

Scientific Investigations Map 3189

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 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/.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................2

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

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

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

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

Topographic and Bathymetric Data...................................................................................................5

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

Model Calibration and Performance .................................................................................................6

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

Inundation Mapping ......................................................................................................................................6

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

Uncertainties and Limitations Regarding Use of Flood Inundation Maps...................................8

Disclaimer for Flood Inundation Maps..............................................................................................8

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

References Cited............................................................................................................................................9

Figures

1. Map showing locations of study reach for Peachtree Creek, U.S. Geological Survey 

streamgages, and National Weather Service forecast site..................................................3

2. Map showing Peachtree Creek modeled section with model extent, flow-ratios, 

and bridge crossings....................................................................................................................4

Tables

1. U.S. Geological Survey streamgage and miscellaneous site information 

for study basin, Peachtree Creek at Atlanta, Georgia (02336300), and 

Chattahoochee River at Georgia 280 (02336490) .....................................................................2

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

at Peachtree Creek at Atlanta, Georgia, streamgage (02336300).........................................6

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

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

Peachtree Creek at Atlanta, Georgia, streamgage (02336300) .............................................7

4. Sheet number reference based on stages (water-surface elevations) at 

Peachtree Creek at Atlanta, Georgia streamgage (02336300) and Chattahoochee 

River at Georgia 280, near Atlanta, Georgia, streamgage (02336490)..................................7
iv

Sheets (Each sheet has individual pdf and jpg files available for download. For links, see 

http://pubs.usgs.gov/sim/3189/.)

1–10. Maps showing flood-inundation of Peachtree Creek in Atlanta, GA, corresponding 

to a gage height and an elevation (NAVD 88) as listed below at U.S. Geological Survey 

streamgage Peachtree Creek at Atlanta, GA (02336300), and a gage height of 26.4 feet 

and an elevation of 763.0 feet (NAVD 88) at U.S. Geological Survey streamgage 

Chattahoochee River at GA 280, near Atlanta, GA (02336490)

1. Gage height of 15.0 feet and an elevation of 779.2 feet

2. Gage height 16.0 feet and an elevation of 780.2 feet

3. Gage height 17.0 feet and an elevation of 781.2 feet

4. Gage height 18.0 feet and an elevation of 782.2 feet

5. Gage height 19.0 feet and an elevation of 783.2 feet

6. Gage height 20.0 feet and an elevation of 784.2 feet

7. Gage height 21.0 feet and an elevation of 785.2 feet

8. Gage height 22.0 feet and an elevation of 786.2 feet

9. Gage height 23.0 feet and an elevation of 787.2 feet

10. Gage height 24.0 feet and an elevation of 788.2 feet

11–20. Maps showing flood-inundation of Peachtree Creek in Atlanta, GA, corresponding 

to a gage height and an elevation (NAVD 88) as listed below at U.S. Geological Survey 

streamgage Peachtree Creek at Atlanta, GA (02336300), and a gage height of 29.4 feet 

and an elevation of 766.0 feet (NAVD 88) at U.S. Geological Survey streamgage 

Chattahoochee River at GA 280, near Atlanta, GA (02336490)

11. Gage height 15.0 feet and an elevation of 779.2 feet

12. Gage height 16.0 feet and an elevation of 780.2 feet

13. Gage height 17.0 feet and an elevation of 781.2 feet

14. Gage height 18.0 feet and an elevation of 782.2 feet

15. Gage height 19.0 feet and an elevation of 783.2 feet

16. Gage height 20.0 feet and an elevation of 784.2 feet

17. Gage height 21.0 feet and an elevation of 785.2 feet

18. Gage height 22.0 feet and an elevation of 786.2 feet

19. Gage height 23.0 feet and an elevation of 787.2 feet

20. Gage height 24.0 feet and an elevation of 788.2 feet

21–30. Maps showing flood-inundation of Peachtree Creek in Atlanta, GA, corresponding 

to a gage height and an elevation (NAVD 88) as listed below at U.S. Geological Survey 

streamgage Peachtree Creek at Atlanta, GA (02336300), and a gage height of 32.4 feet 

and an elevation of 769.0 feet (NAVD 88) at U.S. Geological Survey streamgage 

Chattahoochee River at GA 280, near Atlanta, GA (02336490)

21. Gage height 15.0 feet and an elevation of 779.2 feet

22. Gage height 16.0 feet and an elevation of 780.2 feet

23. Gage height 17.0 feet and an elevation of 781.2 feet

24. Gage height 18.0 feet and an elevation of 782.2 feet

25. Gage height 19.0 feet and an elevation of 783.2 feet

26. Gage height 20.0 feet and an elevation of 784.2 feet

27. Gage height 21.0 feet and an elevation of 785.2 feet

28. Gage height 22.0 feet and an elevation of 786.2 feet

29. Gage height 23.0 feet and an elevation of 787.2 feet

30. Gage height 24.0 feet and an elevation of 788.2 feet
v

31–40. Maps showing flood-inundation of Peachtree Creek in Atlanta, GA, corresponding 

to a gage height and an elevation (NAVD 88) as listed below at U.S. Geological Survey 

streamgage Peachtree Creek at Atlanta, GA (02336300), and a gage height of 35.4 feet 

and an elevation of 772.0 feet (NAVD 88) at U.S. Geological Survey streamgage 

Chattahoochee River at GA 280, near Atlanta, GA (02336490)

31. Gage height 15.0 feet and an elevation of 779.2 feet

32. Gage height 16.0 feet and an elevation of 780.2 feet

33. Gage height 17.0 feet and an elevation of 781.2 feet

34. Gage height 18.0 feet and an elevation of 782.2 feet

35. Gage height 19.0 feet and an elevation of 783.2 feet

36. Gage height 20.0 feet and an elevation of 784.2 feet

37. Gage height 21.0 feet and an elevation of 785.2 feet

38. Gage height 22.0 feet and an elevation of 786.2 feet

39. Gage height 23.0 feet and an elevation of 787.2 feet

40. Gage height 24.0 feet and an elevation of 788.2 feet

41–50. Maps showing flood-inundation of Peachtree Creek in Atlanta, GA, corresponding 

to a gage height and an elevation (NAVD 88) as listed below at U.S. Geological Survey 

streamgage Peachtree Creek at Atlanta, GA (02336300), and a gage height of 38.4 feet 

and an elevation of 775.0 feet (NAVD 88) at U.S. Geological Survey streamgage 

Chattahoochee River at GA 280, near Atlanta, GA (02336490)

41. Gage height 15.0 feet and an elevation of 779.2 feet

42. Gage height 16.0 feet and an elevation of 780.2 feet

43. Gage height 17.0 feet and an elevation of 781.2 feet

44. Gage height 18.0 feet and an elevation of 782.2 feet

45. Gage height 19.0 feet and an elevation of 783.2 feet

46. Gage height 20.0 feet and an elevation of 784.2 feet

47. Gage height 21.0 feet and an elevation of 785.2 feet

48. Gage height 22.0 feet and an elevation of 786.2 feet

49. Gage height 23.0 feet and an elevation of 787.2 feet

50. Gage height 24.0 feet and an elevation of 788.2 feet
Acknowledgments

The author wishes to thank the City of Atlanta, and the U.S. Army Corps of Engineers, 

Mobile District, for funding the operation and maintenance of the gages used for this study. 

Special thanks are given to the National Weather Service for their continued support to the 

USGS flood-inundation mapping initiative.

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

MARTA Metropolitan Atlanta Rapid Transit Authority

NE northeast

NW northwest

NWS National Weather Service

USGS U.S. Geological Survey

Conversion Factors and Datums

Multiply By To obtain

Length

inch 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 0.0929 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2)

Flow rate

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

Hydraulic gradient

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

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

(NAVD 88).

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

Elevation, as used in this report, refers to the distance above the vertical datum.

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

established at the gage (gage datum).
Flood-Inundation Maps for Peachtree Creek 

from the Norfolk Southern Railway Bridge to the 

Moores Mill Road NW Bridge, Atlanta, Georgia

By Jonathan W. Musser

Abstract

Digital flood-inundation maps for a 5.5-mile reach of the 

Peachtree Creek from the Norfolk Southern Railway bridge to 

the Moores Mill Road NW bridge, were developed by the 

U.S. Geological Survey (USGS) in cooperation with the 

City of Atlanta, 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 Peachtree Creek at Atlanta, Georgia (02336300) 

and the USGS streamgage at Chattahoochee River at Georgia 

280, near Atlanta, Georgia (02336490). Current water level 

(stage) at these USGS streamgages 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 Peachtree Creek, which is available through the AHPS Web 

site, may be used in conjunction with the maps developed in 

this study to show predicted areas of flood inundation.

A one-dimensional step-backwater model was developed 

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

for a 6.5-mile reach of Peachtree Creek and was used to 

compute flood profiles for a 5.5-mile reach of the creek. The 

model was calibrated using the most current stage-discharge 

relations at the Peachtree Creek at Atlanta, Georgia, streamgage 

(02336300), and the Chattahoochee River at Georgia 280, near 

Atlanta, Georgia, streamgage (02336490) as well as high water 

marks collected during the 2010 annual peak flow event. The 

hydraulic model was then used to determine 50 water-surface 

profiles. The profiles are for 10 flood stages at the Peachtree 

Creek streamgage at 1-foot intervals referenced to the 

streamgage datum and ranging from just above bankfull stage 

(15.0 feet) to approximately the highest recorded water level 

at the streamgage (24.0 feet). At each stage on Peachtree 

Creek, five stages at the Chattahoochee River streamgage, 

from 26.4 feet to 38.4 feet in 3-foot intervals, were used to 

determine backwater effects. 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 0.3-foot vertical and 

16.4-foot horizontal resolution—to delineate the area flooded 

for each 1-foot increment of stream stage.

The availability of these maps, when combined with 

real-time information regarding current stage from USGS 

streamgages and forecasted stream stages from the NWS, 

provide emergency management personnel and residents 

with critical information during flood response activities, 

such as evacuations and road closures as well as for 

postflood-recovery efforts.
2 Flood-Inundation Maps for Peachtree Creek, Atlanta, Georgia

Introduction

The City of Atlanta, Georgia (Ga.) (fig. 1), is a large 

urban community with an estimated population of 486,411 in 

2006 (U.S. Bureau of Census, 2011). Flood plains within the 

city are highly developed with a mix of residential, recreational, and commercial structures. In recent years, Atlanta 

has experienced severe flooding that has caused substantial 

damage along Peachtree Creek. The floods of September 2004 

and September 2009 are two such notable events. 

Prior to this study, Atlanta officials have relied on several 

information sources (available on the Internet) to make decisions on the best way to alert the public and to mitigate flood 

damages along Peachtree Creek. One source of information is 

the Federal Emergency Management Agency (FEMA) Digital 

Flood Insurance Rate Map (DFIRM) (Federal Emergency 

Management Agency, 2010). A second source of information 

is the U.S. Geological Survey (USGS) streamgage—Peachtree 

Creek at Atlanta, Georgia (02336300)—from which current 

or historical water levels (stage) can be obtained. Stage is 

the elevation 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 Peachtree Creek at Atlanta streamgage (forecast site 

AANG1) through the Advanced Hydrologic Prediction Service 

(AHPS) Web site at http:/water.weather.gov/ahps/.

Although USGS current stage and NWS forecast stage 

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

farther upstream or downstream, because the water-surface 

elevation is not constant along the entire stream channel. 

Also, FEMA and State emergency management mitigation 

teams or property owners typically lack information related to 

water depth at locations other than locations near the USGS 

streamgages or the NWS flood-forecast points. 

Purpose and Scope

The purpose of this report is to describe the development 

of a series of estimated flood-inundation maps for Peachtree 

Creek in Atlanta, Ga. The maps and other useful flood 

information are available on the USGS Flood Inundation 

Mapping Science Web site at http://water.usgs.gov/osw/

flood_inundation/ and the NWS AHPS Web site. Internet 

users can select estimated inundation maps that correspond 

to (1) current stages at the USGS streamgages, (2) the NWS 

forecasted peak stage, or (3) other desired stream stages.

The scope of the study was limited to a 6.5-mile (mi) 

reach of Peachtree Creek between the junction of North Fork 

Peachtree Creek and South Fork Peachtree Creek to about 

2,100 feet (ft) downstream of the Moores Mill Road NW 

bridge (fig. 2). Results are presented for a 5.5-mi subsection of 

this stream reach extending from the Norfolk Southern Railway 

bridge to the Moores Mill Road NW bridge. Tasks specific to 

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

of the flow and stage data collected at two streamgages, one 

on Peachtree Creek and one on the Chattahoochee River 

(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, 2010); (5) production of estimated floodinundation maps at various stream stages using the U.S. Army 

Corps of Engineers HEC–GeoRAS computer program (U.S. 

Army Corps of Engineers, 2009) and a geographic information 

system (GIS); and (6) development of an online interface that 

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

forecasted peak stage to facilitate the display of user-selected 

flood-inundation maps on the Internet.

Table 1. U.S. Geological Survey streamgage and miscellaneous site information for study basin, Peachtree Creek at Atlanta, Georgia 

(02336300), and Chattahoochee River at Georgia 280 (02336490).

Station name Station

number

Drainage 

area, in 

square 

miles

Latitude Longitude Period of record

Maximum recorded 

water-surface 

elevation (stage) 

at gage and date

Peachtree Creek at 

Atlanta, Georgia

02336300 86.8 33°49'10" 84°24'28" 1956 to current year 23.70 feet

September 21, 2009

Chattahoochee River 

at Georgia 280 

02336490 1,590 33°49'01" 84°28'48" 1973 to current year 35.98 feet

September 21, 2009
Introduction 3

Area of map in figure 2

Peachtree

Creek

Area of map below

DEKALB

COUNTY

GWINNETT

COUNTY

FULTON

COUNTY

Atlanta

Atlanta

Chattahoochee

Chattahoochee

River

Chattahoochee

River

S Fork

Nancy Creek

Nancy Creek

North Fork

South Fork

Peachtree Creek

Peachtree Creek

Peachtree Creek

GEORG IA

River

N Fork

02336490

02336000

VING1

02336300

AANG1

84°30'W 84°25'W 84°20'W 84°15'W

33°55'N

33°50'N

33°45'N

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

EXPLANATION

Figure 1.

USGS streamgage and identifier

USGS streamgage and NWS 

 forecast site and identifiers

N

Base modified from 

U.S. Geological Survey

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

digital data

Base modified from U.S. Geological Survey

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

Piedmont physiographic province

Subbasin boundary

02336000

VING1

02336490

Peachtree Creek Basin boundary

Figure 1. Locations of study reach for Peachtree Creek, U.S. Geological Survey streamgages, and National Weather 

Service forecast site.
4 Flood-Inundation Maps for Peachtree Creek, Atlanta, Georgia

1.039

1.004

1.000

0.993

0.967

0.958

0.919

0.904

0.861

1.050

1.018

1.009

84°26' 84°25' 84°24' 84°23' 84°22'

33°50'

33°49'

0 2,500 5,000 FEET

0 500 1,000 1,500 METERS

Woodall Creek

Nancy Creek

Clear Creek

N Fork

Peachtree

Creek

S Fork

Peachtree

Creek

Unnamed tributary at

Cross Creek Golf Course Unnamed tributary at

Cross Creek Golf Course

MOORES

MILL ROAD

NW

NORTHSIDE DRIVE NW

MILL ROAD

HOWELL

NW

BOHLER ROAD NW

PEACHTREE ROAD NE

NORFOLK

SOUTHERN

RAILWAY

PIEDMONT ROAD NE

Bobby Jones 

Golf Course

bridges

MARTA

bridges

(3)

75

Base modified from U.S. Geological Survey

1:24,000 National Hydrography Dataset

Figure 2.

EXPLANATION

Flow-ratio point and value

Direction of surface-water flow

Modeled inundation 

 area boundary

0.993

Road

Railway

 Bridge

Peachtree Creek 

 modeled section

Tributary

USGS streamgage and NWS 

 forecast site and identifiers

02336300

AANG1

02336300

AANG1

MARTA (rail

 mass transit) 

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 

water-surface elevation (stage) at Peachtree Creek at Atlanta 

(02336300) and ranged from approximately bankfull (15.0 ft) to 

the maximum observed water level at the streamgage (24.0 ft). 

The stage at Chattahoochee River at Georgia 280 (GA 280), 

near Atlanta (02336490) was used as a downstream boundary 

condition to simulate backwater effect on Peachtree Creek from 

the Chattahoochee River. The stage at the Chattahoochee River 

streamgage ranged from 26.4 to 38.4 ft in 3.0 ft intervals.

Study Area Description

Peachtree Creek is in northern Georgia in the Piedmont 

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

area ranges from 68.9 square miles (mi2) at the junction of 

North Fork Peachtree Creek and South Fork Peachtree Creek 

to 86.8 mi2 at the Peachtree Creek at Atlanta streamgage 

(02336300) to 92.6 mi2 downstream of the Moores Mill 

Road NW bridge, the downstream extent of the study reach 

(fig. 2). The headwaters of North Fork Peachtree Creek 

originate in Gwinnett County, and the headwaters for South 

Fork Peachtree Creek originate in DeKalb County (fig. 1). 

The two forks generally flow westward before entering the 

Atlanta city limits and join to form the main stem of Peachtree 

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 6.5-mi long, with an average channel 

slope of 4.8 feet per mile (ft/mi). The land contiguous to the 

study reach is mostly classified as urban or developed, with 

some small areas classified as forest (Fry and others, 2011). 

The main channel within the study reach has seven major road 

bridges, four rail bridges, and two small golf cart bridges that 

cross the channel and the adjacent flood plain. 

Figure 2. Peachtree Creek modeled section with model extent, flow-ratios, and bridge crossings.
Constructing Water-Surface Profiles 5

Previous Studies

The current DFIRM for Atlanta was published 

June 18, 2010 (Federal Emergency Management Agency, 

2010), and was used to help select the location and length of 

cross sections in the HEC-RAS model developed for this study. 

A similar study was completed in Albany, Ga., that provides 

flood-inundation maps for a range of streamgage stages on the 

Flint River (Musser and Dyar, 2007). The methods used for the 

Albany model differed from that of Peachtree Creek because 

a finite-element two-dimensional model was used. Publication 

products are similar to those of this study, including this 

pamphlet and publication to the NWS AHPS Web site.

Constructing Water-Surface Profiles

The water-surface profiles used to produce the 

50 flood-inundation maps in this study were computed using 

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

2010). HEC–RAS is a one-dimensional step-backwater model 

for simulation of water-surface profiles with steady-state 

(gradually varied) or unsteady-state flow computation options. 

For this study the HEC–RAS analysis was steady-state.

Hydrologic and Steady Flow Data

The study area hydrologic network consists of one 

streamgage on the study reach and one streamgage on the 

Chattahoochee River used for modeling backwater effects 

(fig. 1; table 1). Water level (stage) is measured continuously 

at all of the sites and continuous records of streamflow 

are computed at both sites. All water-surface elevations 

are referenced to North American Vertical Datum of 1988 

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

transmitters that allow data to be transmitted routinely and 

made available on the Internet within an hour of collection. 

One of the sites also is equipped with a recording rain gage. 

Longitudinal water-surface profiles from high-water marks 

along the main channel were documented following floods in 

2010 and also were used for model calibration.

Steady-flow data consisted of flow regime, boundary 

conditions (either normal depth, or a backwater elevation 

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

the study reach were obtained from previous storm events and 

from the stage-discharge relation at the Peachtree Creek at 

Atlanta streamgage (02336300).

Topographic and Bathymetric Data

Channel cross sections were developed from USGS field 

surveys that were conducted in May and June of 2010. These 

cross sections provide detailed elevation data in the channel 

below the water surface, on the stream banks, and up into the 

edge of the flood plain. The distance surveyed into the flood 

plain varied and depended on the steepness and the amount of 

undergrowth. The survey data were collected using a Topcon 

GTS-210 Total Station and a survey rod. Light Detection 

and Ranging (LiDAR) data with vertical resolution of 0.3 ft 

were used to provide digital elevation data for the parts 

of the cross sections that were beyond the surveyed areas. 

The LiDAR data were collected in 2006, by 3001 Inc., The 

Geospatial Company, and postprocessing was completed in 

November 2007 (3001 Inc., The Geospatial Company, 2007). 

Benchmarks and reference points at the streamgage were used 

for vertical control when available. At bridge sites where no 

vertical control points were available, multiple points were 

selected from aerial photography, and the elevations at these 

points were extracted from the LiDAR data. The survey 

elevations were adjusted to NAVD 88 by minimizing the 

differences between the elevations from the LiDAR data and 

the adjusted elevation of the corresponding survey points. The 

differences in elevation between the survey and the LiDAR 

values at each matching location were averaged. This average 

offset difference was then applied to all survey points to 

register the survey elevations to NAVD 88.

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 13 bridges were 

measured and surveyed in the field concurrently with the stream 

channel surveys. The bridges from upstream to downstream 

were Piedmont Road NE, three Metropolitan Atlanta Rapid 

Transit Authority (MARTA) bridges, Norfolk Southern Railway, 

Peachtree Road NE, two golf cart bridges on the Bobby Jones 

Golf Course, Northside Drive NW, Howell Mill Road NW, 

Interstate 75, Bohler Road NW, and Moores Mill Road NW 

(fig. 2). Two of the MARTA bridges were adjacent to each other 

and treated as one bridge in the model. 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 

0.029 to 0.054 for the main channel, except for two cross 

sections where the model became unstable at high flows and 

the channel Manning’s n value was increased to 0.07 or 0.09 

to smooth the flow profile. In addition, at one location the 

Manning’s n value was 0.08 where a large concrete bridge 

pier remnant was directly in the flow path downstream of the 

Norfolk Southern Railway bridge, causing turbulence that was 

accounted for by way of an increased n value. The Manning’s 

n values ranged from 0.029 (golf courses) to 0.14 (forested 

areas with heavy undergrowth) for the overbank areas 

modeled in this analysis. 
6 Flood-Inundation Maps for Peachtree Creek, Atlanta, Georgia

Model Calibration and Performance

The hydraulic model was calibrated to the most current 

stage-discharge relation at the Peachtree Creek at Atlanta 

streamgage (02336300) along with high water marks collected 

during the 2010 annual peak flow event. The estimated peak 

discharge for the 2010 flood was 8,050 cubic feet per second 

(ft3/s) at an estimated stage of about 19.17 ft at the gage. 

Model calibration was accomplished by adjusting Manning’s 

n values until the results of the hydraulic computations 

closely agreed with the known flood discharge and stage 

values. Differences between measured and simulated water 

levels for models calibrated to gage heights were between 

–0.09 and +0 .07 ft (table 2). High water marks located 

upstream of Peachtree Road NE and the Norfolk Southern 

Railway bridge were about 0.2 ft higher than the modeled 

elevation from the 2010 flood. During the field investigation, 

debris was noted at the Peachtree Road NE bridge, which 

could account for this difference in water-surface elevation. 

The model assumes no debris in the stream channel. The 

boundaries of the DFIRM flood zones were similar to the 

modeled inundated areas for the 22- and 23-ft Peachtree 

Creek gage height inundation model at all gage heights at 

the Chattahoochee River gage. In some areas, the DFIRM 

extended beyond the modeled inundated area. In these areas, 

the base-flood elevation from the DFIRM was lower than 

the ground surface from the LiDAR elevation data. These 

discrepancies can be attributed to the use of different sources 

and different resolutions for the ground-surface elevation. The 

results demonstrate that the model is capable of simulating 

accurate water levels over a wide range of flows in the 

basin. Details on techniques used in model development and 

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

Development of Water-Surface Profiles

Profiles were developed for a total of 10 stages at 

1.0-ft intervals between 15.0 and 24.0 ft as referenced to the 

Peachtree Creek at Atlanta streamgage (02336300). Discharges 

corresponding to the various stages were obtained from the 

most current stage-discharge relation (rating no. 25.0). 

Discharges for all profiles (table 3) were calculated 

based on the gage discharges and on observations during 

three historical flood events in 1964, 1966, and 1976. During 

those events, peak discharges were estimated at all of the 

road bridges along Peachtree Creek. An average flow ratio 

for the three events was calculated between the discharge 

at each bridge and the discharge at the gage. If a tributary 

joined Peachtree Creek just upstream of a bridge, then the 

flow ratio at that bridge was assigned to the first cross section 

downstream from the tributary. For tributaries not located near 

a bridge, the flow ratio was determined by a linear interpolation based on cross-section distance along the creek between 

the previously assigned upstream and downstream near-bridge 

cross sections (fig. 2). Flow ratios were determined at the 

upstream end of the model and below each tributary. Model 

flows for each gage height were determined by multiplying 

the computed flow ratio at each location by the flow at the 

Peachtree Creek streamgage. Using the ratio of the historical 

flows along Peachtree Creek resulted in higher flows upstream 

of the gage and lower flows downstream of the gage than 

using a direct drainage area to flow calculation.

Backwater effects from the Chattahoochee River were 

accounted for in the model by adjusting the downstream 

boundary condition based on the stage of the Chattahoochee 

River at the GA 280 near Atlanta streamgage (02336490). 

The five backwater stages ranged from 26.4 to 38.4 ft in 

3.0-ft intervals. Combining the 5 backwater stages with 

each of the 10 stages on Peachtree Creek produced a total of 

50 flood-inundation maps. The backwater effect on Peachtree 

Creek from the Chattahoochee River at GA 280, near Atlanta 

streamgage (02336490) was calculated by measuring the 

water-surface slope upstream to the Chattahoochee River at 

Atlanta streamgage (02336000).

Inundation Mapping

Flood-inundation maps were created based on the 

Peachtree Creek at Atlanta streamgage (02336300), which has 

been designated a NWS flood-forecast point (AANG1). The 

maps were created in a GIS by combining the water-surface 

profiles and digital elevation model data. The digital elevation 

model data were derived from 16.4-ft horizontal resolution 

LiDAR data with a vertical resolution of 0.3 ft obtained from 

Fulton County (3001 Inc., The Geospatial Company, 2007). 

Estimated flood-inundation boundaries for each simulated 

profile were developed with HEC–GeoRAS software 

(U.S. Army Corps of Engineers, 2009). HEC–GeoRAS is a 

set of procedures, tools, and utilities for processing geospatial 

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

streamgage (02336300).

[Values listed in feet]

Gage

height

Actual 

water-surface 

elevations

Modeled

water-surface 

elevations

Elevation

difference

15.00 779.23 779.30 +0.07

16.00 780.23 780.27 +0.04

17.00 781.23 781.26 +0.03

18.00 782.23 782.22 –0.01

19.00 783.23 783.24 +0.01

20.00 784.23 784.24 +0.01

21.00 785.23 785.23 0.00

22.00 786.23 786.25 +0.02

23.00 787.23 787.14 –0.09

24.00 788.23 788.28 +0.05
Inundation Mapping 7

data in ArcGIS (ESRI) 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, 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 0.4 ft. The maps show 

estimated flood-inundated areas overlaid 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 Peachtree Creek is fairly consistent 

throughout the study reach, except where the slope increases 

upstream of the Norfolk Southern Railway bridge. Because 

of the increase in bed slope, the inundation maps were 

discontinued at this bridge (fig. 2). At the downstream end of 

the model, inundation maps discontinued just downstream 

of Moores Mill Road NW because of the lack of verification 

data for the model and the backwater effects from the 

Chattahoochee River. The reach of Peachtree Creek included 

in the inundation maps is about 5.5-mi long from the 6.5-mi 

reach that was modeled. The sheet numbers corresponding to 

each gage height at the Peachtree Creek and Chattahoochee 

River streamgages are shown in table 4.

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

selected locations for the Peachtree Creek at Atlanta, Georgia, streamgage (02336300).

[Values 15.0–24.0 represent stage heights, in feet above gage datum. Values in parentheses (779.2–788.2) represent elevation in feet above North American 

Vertical Datum of 1988]

Location Water-surface elevations and discharge estimates

Stage, in feet

Peachtree Creek at 

Atlanta, Georgia

15.0

(779.2)

16.0

(780.2)

17.0

(781.2)

18.0

(782.2)

19.0

(783.2)

20.0

(784.2)

21.0

(785.2)

22.0

(786.2)

23.0

(787.2)

24.0

(788.2)

Discharge, in cubic feet per second

Upstream end of 

the model

4,300 4,880 5,490 6,140 6,820 7,530 8,610 10,300 14,200 18,100

Below Clear Creek 4,590 5,210 5,860 6,550 7,280 8,030 9,190 11,000 15,200 19,300

Peachtree Creek 

at Atlanta, 

5,000 5,670 6,380 7,130 7,920 8,740 10,000 12,000 16,500 21,000

Below two unnamed 

tributaries at Cross 

Creek Golf Course

5,090 5,770 6,500 7,260 8,070 8,900 10,200 12,200 16,800 21,400

Below Woodall Creek 5,200 5,890 6,630 7,400 8,230 9,080 10,400 12,500 17,100 21,800

Table 4. Sheet number reference based on stages (water-surface elevations) at Peachtree Creek at Atlanta, Georgia, streamgage 

(02336300) and Chattahoochee River at Georgia 280, near Atlanta, Georgia, streamgage (02336490).

[Values 15.0–24.0 represent stage heights, in feet above gage datum. Values in parentheses (779.2–788.2) represent elevation in feet above North American 

Vertical Datum of 1988]

Chattahoochee 

River at 

Georgia 280

Peachtree Creek at Atlanta, Georgia

15.0

(779.2)

16.0

(780.2)

17.0

(781.2)

18.0

(782.2)

19.0

(783.2)

20.0

(784.2)

21.0

(785.2)

22.0

(786.2)

23.0

(787.2)

24.0

(788.2)

26.4

(763.0) 1 2 3 4 5 6 7 8 9 10

29.4

(766.0) 11 12 13 14 15 16 17 18 19 20

32.4

(769.0) 21 22 23 24 25 26 27 28 29 30

35.4

(772.0) 31 32 33 34 35 36 37 38 39 40

38.4

(775.0) 41 42 43 44 45 46 47 48 49 50
8 Flood-Inundation Maps for Peachtree Creek, Atlanta, Georgia

Peachtree Creek, Georgia Flood-Inundation 

Maps on the Internet

A USGS Flood Inundation Mapping Science World Wide 

Web portal was established by the USGS to provide estimated 

flood-inundation 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. The Web 

portal URL is http://water.usgs.gov/osw/flood_inundation/.

Each stream reach displayed on the Web site contains links 

to the USGS National Water Information System Web graphs 

of the current stage and streamflow at the USGS streamgage 

Peachtree Creek at Atlanta (02336300), to which the inundation maps are referenced. A link also is provided to the 

NWS AHPS 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.

Uncertainties and Limitations Regarding 

Use of Flood Inundation Maps

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

associated with these maps. The flood boundaries shown were 

estimated based on water stages (water-surface elevations) 

and streamflows at selected USGS streamgages. Water-surface 

elevations along the stream reaches were estimated by steadystate hydraulic modeling, assuming unobstructed flow, and 

using streamflows and hydrologic conditions anticipated at 

the USGS streamgage(s). The hydraulic model reflects the 

land-cover characteristics and any bridge, dam, levee, or 

other hydraulic structures existing as of March 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 in the document accompanying this set of flood inundation map sheets.

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

conjunction with 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 (water-surface 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. 

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.

Summary

A series of estimated flood-inundation maps were 

developed in cooperation with the City of Atlanta for a 5.5-mile 

reach of Peachtree Creek between the Norfolk Southern 

Railway bridge and the Moores Mill Road NW bridge. These 

maps, available at a U.S. Geological Survey (USGS) Web 

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

USGS streamgage at Peachtree Creek at Atlanta, Georgia 

(02336300), real-time stage data from the Chattahoochee 

River at Georgia 280, near Atlanta, Georgia (02336490), and 

National Weather Service flood-stage forecasts, will help to 

guide the general public in taking individual safety precautions 

and will 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 programs to 

compute water-surface profiles and to delineate estimated 

flood-inundation areas for selected stream stages. The maps 

show estimated flood-inundation areas overlaid on highresolution, georeferenced, aerial photographs of the study 

area for stream stages between 15.0 and 24.0 feet (gage 

datum) at the Peachtree Creek at Atlanta, Georgia, streamgage 

(02336300) and stream stages between 26.4 and 38.4 feet 

(gage datum) at the Chattahoochee River at Georgia 280, 

near Atlanta, Georgia, streamgage (02336490). 
References Cited 9

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., available online at http://pubs.

usgs.gov/sir/2007/5032/.

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

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

ESRI, ArcGIS Desktop: Version 9.3.1, accessed August 2010 

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

Federal Emergency Management Agency, Digital Flood 

Insurance Rate Map: Accessed June 2010 at 

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

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

Barnes, C., Herold, N., and Wickham, J., 2011, 

Completion of the 2006 National Land Cover Database 

for the conterminous United States, PE&RS, v. 77, 

no. 9, p. 858–864.

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., Web-only publication at 

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

3001 Inc., The Geospatial Company, 2007, Ground Control 

Survey Report GPS Survey for LiDAR Control, Fulton 

County, Georgia: 58 p.

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, Atlanta (city) QuickFacts from the 

U.S. Census Bureau: Accessed March 2011 at http://

quickfacts.census.gov/qfd/states/13/1304000.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., 

available online at http://pubs.usgs.gov/sir/2008/5234/.
Manuscript approved on November 10, 2011

Edited by Twila D. Wilson

Illustrations and layout by Caryl J. Wipperfurth

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 Peachtree Creek, Atlanta, Georgia—Scientific Investigations Map 3189