U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany 

Scientific Investigations Map 3242

Prepared in cooperation with City of Hopkinsville, Kentucky, Community Development Services

Flood-Inundation Maps for an 8.9-Mile Reach of the South 

Fork Little River at Hopkinsville, Kentucky

Flood-Inundation Maps for an 8.9-Mile 

Reach of the South Fork Little River at 

Hopkinsville, Kentucky

By Jeremiah G. Lant 

Prepared in cooperation with City of Hopkinsville, Kentucky, 

Community Development Services

Pamphlet to accompany

Scientific Investigations Map 3242 

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: 2013

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as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Lant, J.G., 2012, Flood-inundation maps for an 8.9-mile reach of the South Fork Little River at Hopkinsville, Kentucky: 

U.S. Geological Survey Scientific Investigations Map 3242, 13 map sheets, 8-p. pamphlet. 
iii

Acknowledgments

The author wishes to thank the many local, State, and Federal agencies that have cooperated in and (or) funded for the operation and maintenance of the streamgages used for this 

study, especially the City of Hopkinsville. Special thanks are given to the City of Hopkinsville Community Development Services for their cooperation in this study and to the National 

Weather Service for their continued support to the USGS flood-inundation mapping initiative. 
iv
v

Contents

Acknowledgments........................................................................................................................................iii

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

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

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

Study Area Description........................................................................................................................2

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

Constructing Water-Surface Profiles.........................................................................................................4

Hydrologic and Steady-Flow Data .....................................................................................................4

Topographic/Bathymetric Data ..........................................................................................................4

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

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

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

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

South Fork Little River, Kentucky, Flood-Inundation Maps on the Internet.................................6

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

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

Summary..........................................................................................................................................................7

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

Figures

1. Map showing location of study reach for the South Fork Little River at Hopkinsville, 

Kentucky, and location of USGS streamgages........................................................................3

Tables

1. USGS streamgage information for study basin, South Fork Little River, Kentucky............2

2. 1.0-percent annual exceedance probability peak-discharge estimate and 

drainage area on the South Fork Little River............................................................................5

3. Comparison of hydraulic-model output and flood events June 21, November 21, 

and December 5, 2011, South Fork Little River, Kentucky. .....................................................5

4. Stages (elevations), in feet, with corresponding discharge estimates at USGS 

station 03437495 (South Fork Little River at Highway 68 By-Pass at Hopkinsville, 

Kentucky) for selected simulated water-surface profiles. ....................................................6
vi

Sheets

[Sheets and additional materials are available online at http://pubs.usgs.gov/sim/2007/3242/.]

1–13. Flood-inundation map for Hopkinsville, Kentucky corresponding to a stage of feet (noted 

below) and an elevations in feet (NAVD 88) at U.S. Geological Survey Streamgage 

Number 03437495 South Fork Little River at Highway 68 By-Pass—

1. 10.00 Feet and an elevation of 530.00 feet

2. 11.00 Feet and an elevation of 531.00 feet

3. 12.00 Feet and an elevation of 532.00 feet

4. 13.00 Feet and an elevation of 533.00 feet

5. 14.00 Feet and an elevation of 534.00 feet

6. 15.00 Feet and an elevation of 535.00 feet

7. 16.00 Feet and an elevation of 536.00 feet

8. 17.00 Feet and an elevation of 537.00 feet

9. 18.00 Feet and an elevation of 538.00 feet

10. 19.00 Feet and an elevation of 539.00 feet

11. 20.00 Feet and an elevation of 540.00 feet

12. 21.00 Feet and an elevation of 541.00 feet

13. 21.50 Feet and an elevation of 541.50 feet

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

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).
Flood-Inundation Maps for an 8.9-Mile Reach of the 

South Fork Little River at Hopkinsville, Kentucky

By Jeremiah G. Lant 

Abstract

Digital flood-inundation maps for an 8.9-mile reach 

of South Fork Little River at Hopkinsville, Kentucky, were 

created by the U.S. Geological Survey (USGS) in cooperation with the City of Hopkinsville Community Development 

Services. 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 

South Fork Little River at Highway 68 By-Pass at Hopkinsville, Kentucky (station no. 03437495). Current conditions for 

the USGS streamgage may be obtained online at the USGS 

National Water Information System site (http://waterdata.

usgs.gov/nwis/inventory?agency_code=USGS&site_

no=03437495). In addition, the information has been provided 

to the National Weather Service (NWS) for incorporation into 

their Advanced Hydrologic Prediction Service flood warning 

system (http:/water.weather.gov/ahps/). The NWS forecasts 

flood hydrographs at many places that are often co-located at 

USGS streamgages. The forecasted peak-stage information, 

also available on the Internet, may be used in conjunction with 

the maps developed in this study to show predicted areas of 

flood inundation. 

In this study, flood profiles were computed for the South 

Fork Little River reach by using HEC-RAS, a one-dimensional step-backwater model developed by the U.S. Army 

Corps of Engineers. The hydraulic model was calibrated by 

using the most current (2012) stage-discharge relation at the 

South Fork Little River at Highway 68 By-Pass at Hopkinsville, Kentucky, streamgage and measurements collected during recent flood events. The calibrated model was then used 

to calculate 13 water-surface profiles for a sequence of flood 

stages, most at 1-foot intervals, referenced to the streamgage 

datum and ranging from a stage near bankfull to the estimated 

elevation of the 1.0-percent annual exceedance probability 

flood at the streamgage. To delineate the flooded area at each 

interval flood stage, the simulated water-surface profiles were 

combined with a Digital Elevation Model (DEM) of the study 

area by using Geographic Information System (GIS) software. 

The DEM consisted of bare-earth elevations within the study 

area and was derived from a Light Detection And Ranging 

(LiDAR) dataset having a 3.28-foot horizontal resolution. 

These flood-inundation maps, along with online information regarding current stages from USGS streamgage and forecasted stages from the NWS, provide emergency management 

and local residents with critical information for flood response 

activities such as evacuations, road closures, and post-flood 

recovery efforts.

Introduction

The City of Hopkinsville, Kentucky, is an urban community with an estimated population of 31,577 (U.S. Census 

Bureau, 2010). Hopkinsville is the largest incorporated city in 

Christian County, Kentucky. Christian County has experienced 

18 major flood events within the last 100 years, most notably 

in March 1997, when major flooding caused over $75 million 

in damages and destroyed more than 450 homes (City of Hopkinsville, 2012). The majority of flood damages have occurred 

along the North Fork Little River and the South Fork Little 

River, which both flow through the city. Flood plains within 

Hopkinsville are moderately developed and contain a mix of 

rural, residential, and commercial buildings.

Prior to this study, Hopkinsville officials relied on several 

information sources (all of which are available on the Internet) 

to make decisions on how to best alert the public and mitigate 

flood damages. One source is the September 17, 2008, Federal 

Emergency Management Agency (FEMA) Flood Insurance 

Study (FIS) for Christian County and Incorporated Areas 

(Federal Emergency Management Agency, 2008). A second 

source of information consists of two USGS streamgages 

that lie within the city limits of Hopkinsville: South Fork 

Little River at Highway 68 By-Pass at Hopkinsville, Kentucky (station no. 03437495), and South Fork Little River at 

Hopkinsville, Kentucky (station no. 03437500), from which 

current and historical water levels (stage) can be obtained. A 

third source is the National Weather Service forecast of peak 

stage at the USGS Highway 68 By-Pass streamgage (station 

no. 03437495) through the Advanced Hydrologic Prediction 

Service (AHPS) Web site (NWS forecast point HLBK2). 

Although current USGS stage and NWS forecasted stage 
2 Flood-Inundation Maps for an 8.9-Mile Reach of the South Fork Little River at Hopkinsville, Kentucky

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, State emergency management mitigation teams, and 

property owners typically lack information related to water 

depth at locations other than near a USGS streamgage or a 

NWS flood-forecast point. 

Purpose and Scope

The purpose of this report is to describe the development of a series of estimated flood-inundation maps for the 

South Fork Little River near Hopkinsville, Kentucky. The 

flood-inundation maps and other useful flood information are 

available on the USGS Flood Inundation Mapping Science 

Web site (http://water.usgs.gov/osw/flood_inundation/) and 

on the NWS AHPS Web site (http://water.weather.gov/ahps/). 

Through these online applications, Internet users can select 

estimated inundation maps that correspond to various flood 

stages that occur at a USGS streamgage or an NWS forecast 

point.

The scope of the study was limited to the South Fork 

Little River reach bounded by the U.S. Highway 68 By-Pass, 

on the east by the section known locally as Dr. Martin Luther 

King Jr. Way and on the south by the section known locally 

as Eagle Way (fig. 1). To develop flood-inundation maps, a 

hydraulic model of the South Fork Little River reach was 

built to provide water-surface profiles for various flood stages. 

The following steps were taken to construct the water-surface 

profiles: (1) examination of current and historical data from 

streamgages on the South Fork Little River (table 1); (2) collection of hydrologic and steady-flow data; (3) collection of 

topographic data, geometric data (for structures/bridges), and 

bathymetric data throughout the study reach; (4) determination 

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

channel and flood plain; (5) model calibration and evaluation, and (6) computation of water-surface profiles by means 

of HEC-RAS, the U.S. Army Corps of Engineers’ hydraulic 

modeling software (U.S. Army Corps of Engineers, 2010). 

Flood-inundation maps were produced from the results 

of the modeled water-surface profiles. Production of estimated 

flood-inundation maps corresponding to targeted water-surface 

elevations was completed by using HEC-GeoRas, a software 

tool from the U.S. Army Corps of Engineers. HEC-GeoRas 

is a tool used for processing geospatial data in ArcGIS, a 

Geographic Information System (U.S. Army Corps of Engineers, 2009). The NWS AHPS Web site links USGS real-time 

streamgage information and NWS forecasted peak stage to 

facilitate the online display of user-selected flood-inundation 

maps. 

Methods used are generally cited from previously 

published reports. If techniques varied significantly 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 station 03437495 and 

ranging from near bankfull (10 ft; 530 ft above NAVD 88) to 

the elevation of the 1.0-percent annual exceedance probability 

flood at the streamgage (21.5 ft; 541.5 ft above NAVD 88). 

Study Area Description

The South Fork Little River is in southwestern Kentucky 

in what is locally called the Mississippi Plateau or Pennyrile 

Region. The drainage area ranges from 35.9 mi2 at station 

03437495 (upstream extent of the study reach) to 49 mi2 at the 

downstream extent of the study reach. The headwaters originate in unincorporated Christian County, and the river generally flows southwestward before entering the city limits. There 

is no major tributary to the South Fork Little River that joins 

the main stem as it flows through Hopkinsville, but there are 

two minor tributaries, South Fork Little River Tributary and 

Rock Bridge Branch, that join the main stem approximately 4 

and 5 mi, respectively, south of Hopkinsville and outside the 

study reach. The basin terrain changes gradually from steep to 

gently rolling hills northeast of the city to gently rolling hills 

to flat southwest of the city. The study reach is approximately 

8.9 mi long, has an average top-of-bank channel width of 

about 50 ft, and has an average channel slope of 4 ft/mi. The 

Station 

name

Station 

number

Drainage 

area (mi2)

Latitude

(degrees, 

minutes, 

seconds)

Longitude

(degrees, 

minutes, 

seconds)

Period 

of record

Maximum recorded 

stage at streamgage 

(ft above NAVD 88)

 and date

South Fork 

Little River at 

Hwy.68 By-Pass

03437495 35.9 36°50′58′′ N. 87°25′44′′ W. Oct. 2009 to current 

year (2012)

19.95 ft (539.95 ft) 

May 3, 2011.

South Fork 

Little River at 

Hopkinsville, 

Kentucky

03437500 46.5 36°50′22′′ N. 87°28′53′′ W. Oct. 1949 to 

Sept.1973; 

Jan. 1998 to 

current year (2012)

18.83 ft (538.83 ft) 

Aug. 30, 2005.

Table 1. USGS streamgage information for study basin, South Fork Little River, Kentucky.

[mi2, square miles; ft, feet; NAVD 88, North American Vertical Datum of 1988]
Figure 1. Location of study reach for the South Fork Little River at Hopkinsville, Kentucky, and location of USGS streamgages. The National WService forecast point (HLBK2) is at the USGS streamgage at Highway 68 By-Pass at Hopkinsville, Kentucky (station no. 03437495). 

KE

Study 

!

87°29' 87°28' 87°27' 87°26'

36°51'36°50'36°49'

0 0.25 0.5 1 Mile

0 0.25 0.5 1 Kilometer

68

Flow arrow—Indicates di

 water flow Centerline South Fork Littl

Limits of study areaUSGS streamgage and num

03437495

EXPLANATION

U.S. route marker

03437500

034

41A

41

68

(U.S. 68 By-Pass)

Edward T. Breathitt Pennyrile Parkway Dr. Martin Luther King Jr. Way

Eagle Way

Hopkinsville

South Fork Little River

Projection: Lambert Conformal ConicState Plane Coordinate System, Kentucky, FIPS, 1600North American Datum of 1983 (NAD83)Orthophotography from National Agriculture Imagery Program, 2006
4 Flood-Inundation Maps for an 8.9-Mile Reach of the South Fork Little River at Hopkinsville, Kentucky

basin is still undergoing suburban and urban development; 

population increased 4.9 percent (from 30,089 to 31,577) 

between 2000 and 2010 (U.S. Census Bureau, 2010). The 

main channel within the study reach has seven major road 

crossings or other structures that lie within the channel or the 

adjacent flood plain. 

Previous Studies 

The current FIS for Christian County (Federal Emergency 

Management Agency, 2008) was completed by the U.S. Army 

Corps of Engineers (USACE) Nashville District and AMEC 

Earth & Environmental Inc. in 2007. The FIS provided information on water-surface profiles and associated flood plain 

maps for the South Fork Little River at 1.0- and 0.2-percent 

annual exceedance probabilities. The estimated peak discharge 

for flooding at the 1.0-percent annual exceedance probability 

on the South Fork Little River near South Fork Little River at 

Highway 68 By-Pass at Hopkinsville, Kentucky (station no. 

03437495), as shown in table 2 for the study reach, is from 

the FEMA FIS (2008). The City of Hopkinsville provided the 

hydraulic model used in the FIS, which contained bathymetric 

and structure datasets for most of the study area. 

Constructing Water-Surface Profiles

The water-surface profiles used to produce the 13 

flood-inundation maps in this study were computed by using 

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

2010). HEC-RAS simulates the flow of water through open 

channels and computes water-surface profiles. 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. The HEC-RAS 

analysis for this study was done by using the steady-state flow 

computation option.

Hydrologic and Steady-Flow Data

The study-area hydrologic network consists of two USGS 

streamgages (fig. 1; table 1). The streamgage on South Fork 

Little River at Highway 68 By-Pass at Hopkinsville, Kentucky 

(station no. 03437495) has a continuous record of measured 

stage (water level) and computed streamflow. The streamgage 

on South Fork Little River at Hopkinsville, Kentucky (station 

no. 03437500) has a discontinuous record of computed streamflow (Table 1.); however, the streamgage has a continuous 

record of measured stage from 1998 to the present (2012). All 

measured stages and corresponding water-surface elevations 

are referenced to North American Vertical Datum of 1988 

(NAVD 88). The USGS streamgages within the study reach 

are equipped with satellite radio transmitters that allow data 

to be transmitted in near real time. Flow and stage data for a 

particular streamgage can be accessed online from the USGS 

at http://waterdata.usgs.gov/in/nwis/current/?type=flow. Both 

streamgages are also equipped with recording tipping-bucket 

rain gages. 

Steady-flow data necessary for the hydraulic model 

consisted of flow regime, boundary conditions (either known 

water-surface elevation associated with a discharge measurement, critical depth, normal depth, or streamgage rating-curve 

value), and peak-discharge information. The hydraulic model 

developed contains a normal depth boundary condition using 

an average channel slope within the study domain. The steadyflow data for the study reach were obtained from the FIS, field 

measurements of streamflow made at station 03437495, and 

the most current (2012) stage-discharge relation at station 

03437495. 

To construct a flood-inundation map of the 1.0-percent 

annual exceedance probability flood along the South Fork 

Little River, flow data from the FIS for Christian County (Federal Emergency Management Agency, 2008) and recent (2010) 

topographic data were used. The flow data from the FIS for 

Christian County were derived by using a hydrologic model 

developed by the U.S. Army Corps of Engineers Nashville 

District (Federal Emergency Management Agency, 2008). 

Topographic/Bathymetric Data

LiDAR data near Hopkinsville were furnished by the 

Kentucky Energy and Environment Cabinet (Carey Johnson, 

Kentucky Energy and Environment Cabinet, written commun., 

2012). LiDAR provides high-resolution topographic data 

for models and other applications. The LiDAR data used in 

the study had a horizontal resolution of 3.28 ft and a vertical 

accuracy of 0.30 ft and were used to create a Digital Elevation 

Model dataset of the study area. The LiDAR data were collected and postprocessed by Aero-Metric, Inc. (2010). 

River bathymetry and structure data in and along the 

South Fork Little River were furnished by the City of Hopkinsville (D. Herndon, City of Hopskinsville Community 

Services and Planning, written commun., 2012). Most of 

the bathymetric and structure datasets within the study area 

originated from the FIS for Christian County (Federal Emergency Management Agency, 2008). The FIS datasets included 

channel and structure surveys of the South Fork Little River 

within the mapped river reach (fig. 1), excluding the bridge on 

the Highway 68 By-Pass. The City of Hopkinsville conducted 

surveys of the bridge on Highway 68 By-Pass and of the 

South Fork Little River approximately 500 ft downstream and 

upstream from the bridge (D. Herndon, City of Hopskinsville 

Community Services and Planning, written commun., 2012). 

The structure datasets were checked and validated with the 

City of Hopkinsville (D. Herndon, City of Hopskinsville 

Community Services and Planning, written commun., 2012) 
Constructing Water-Surface Profiles 5

to ensure that no newly built structures were within the study 

area. 

HEC-GeoRAS and ArcGIS were used to extract flood 

plain elevations from the LiDAR-derived DEM at selected 

cross sections along the South Fork Little River for use in the 

HEC-RAS model. The flood plain elevations were merged 

with the river-bathymetry and structure datasets during the 

development of the HEC-RAS model. 

Energy-Loss Factors

USGS field observations and photographs, along with 

high-resolution aerial photographs furnished by the City of 

Hopkinsville (D.Herndon, City of Hopskinsville Community 

Services and Planning, written commun., 2012), were used to 

select initial (precalibration) channel and flood plain friction 

coefficients. These friction coefficients, commonly called 

Manning’s roughness coefficients or Manning’s n values, 

account for energy (friction) loss in model calculations. The 

final Manning’s n values used ranged from 0.04 to 0.065 for 

the main channel and 0.1 to 0.15 for the overbank areas; these 

values were selected according to the channel and flood plain 

characteristics described by Chow (1959). The main channel, 

especially in the upstream study area domain, and overbank 

areas throughout the entire study area domain are highly 

vegetated.

Model Calibration and Performance

The hydraulic model was calibrated to the most current 

stage-discharge relation at station 03437495 (South Fork Little 

River at Highway 68 By-Pass at Hopkinsville, Kentucky). 

The hydraulic model was also calibrated to measurements 

collected during flood events on June 21, November 21, and 

December 5, 2011. Station 03437500 (South Fork Little River 

at Hopkinsville, Kentucky) was used as an additional calibration point for each flood event. These flood events cover a 

range of calculated peak discharges at station 03437495: for 

June 21, a peak discharge of 659 ft3/s at a measured stage of 

11.92 ft; for November 21, a peak discharge of 1,270 ft3/s 

at a measured stage of 15.17 ft; and for December 5, a peak 

discharge of 2,100 ft3/s at a measured stage of 17.89 ft. Model 

calibration was accomplished by adjusting Manning’s n values 

until the results of the hydraulic computations closely agreed 

with the known flood discharge and stage values; Manning’s 

n values were also verified to fall within realistic bounds, as 

described by Chow (1959). Differences between measured and 

simulated water levels for specified flows from the established 

stage-discharge relation were less than 0.5 ft. Differences 

between measured and simulated water levels for models 

calibrated to flood events were less than 1 ft (table 3). These 

calibration results demonstrate that the model is capable of 

simulating accurate water levels over a wide range of flows in 

the basin. 

Drainage area (mi2) Discharge estimate (ft3/s)

700 feet downstream from USGS streamgage 

South Fork Little River at Highway 68 By-Pass

35.9 9,870

Date USGS streamgage 

(station number)

Peak water-surface 

elevation (ft)

Model water-surface 

elevation (ft)

Elevation 

difference (ft)

June 21, 2011 03437495 531.94 531.92 -0.02

03437500 526.30 526.27 -0.03

November 21, 2011 03437495 535.17 535.19 +0.02

03437500 528.21 529.15 +0.94

December 5, 2011 03437495 537.89 538.14 +0.25

03437500 532.39 532.42 +0.03

Table 2. 1.0-percent annual exceedance probability peak-discharge estimate and drainage area on the 

South Fork Little River (from Federal Emergency Management Agency, 2008).

[mi2, square miles; ft3/s, cubic feet per second]

Table 3. Comparison of hydraulic-model output and flood events June 21, November 21, and December 5, 2011, 

South Fork Little River, Kentucky.
6 Flood-Inundation Maps for an 8.9-Mile Reach of the South Fork Little River at Hopkinsville, Kentucky

Development of Water-Surface Profiles

Profiles were developed for 12 stages at 1-ft intervals between 10 ft and 21 ft and 1 stage at the 1.0-percent 

annual exceedance probability flood as referenced to station 

03437495. Discharges corresponding to the various stages 

were obtained from the most current stage-discharge relation 

(rating number 2.0) at station 03437495 (South Fork Little 

River at Highway 68 By-Pass at Hopkinsville, Kentucky). 

Discharges for all profiles (table 4) were selected with 

the assumption that, within the 8.9-mi study reach, there are 

no significant inflows. The calibration results of the two flood 

events revealed that no significant flow increases downstream 

from USGS station 03437495 needed to be added to the model 

as had been added in the FIS (table 2); the FIS model, however, was not calibrated to recent flood events. The discharges 

were estimated to be uniform and steady throughout the study 

reach. The water-surface elevation and the corresponding 

discharge estimate used for the 1.0-percent annual exceedance probability flood along the South Fork Little River were 

obtained from the FIS for Christian County (Federal Emergency Management Agency, 2008).

Inundation Mapping

Flood-inundation maps were created for a river reach 

near Hopkinsville, Kentucky, by using two USGS streamgage 

sites, one of which, station 03437495 (South Fork Little River 

at Highway 68 By-Pass at Hopkinsville, Kentucky) is an NWS 

flood-forecast point. The inundation maps were created in a 

GIS by combining the water-surface profiles from the hydraulic model and the DEM data. The DEM data were derived 

from 3.28-ft horizontal resolution LiDAR data with a vertical 

accuracy of 0.30 ft, obtained from Aero-Metric, Inc. (2010). 

Estimated flood-inundation datasets for each simulated watersurface 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 data in ArcGIS by use of a graphical user interface. The 

Table 4. Stages (elevations), with corresponding discharge estimates at USGS station 03437495 (South Fork Little River at 

Highway 68 By-Pass at Hopkinsville, Kentucky) for selected simulated water-surface profiles.

[NAVD 88, North American Vertical Datum of 1988]

Stage, in feet above gage datum (elevation, in feet, NAVD 88) associated with the 

indicated discharge value, in cubic feet per second

Location 10.00 

(530.00)

11.00 

(531.00)

12.00 

(532.00)

13.00 

(533.00)

14.00 

(534.00)

15.00 

(535.00)

16.00 

(536.00)

17.00 

(537.00)

18.00 

(538.00)

19.00 

(539.00)

20.00 

(540.00)

21.00 

(541.00)

21.50 

(541.50)

Highway 68 

By-Pass

388 515 668 840 1,030 1,240 1,470 1,760 2,150 3,000 5,440 7,800 9,870

Sheet no. 1 2 3 4 5 6 7 8 9 10 11 12 13

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). The datasets produced by using HEC-GeoRAS included flood-inundation polygons and water depth grids. The procedure used 

to create the datasets for each simulated water-surface profile 

included creating a bounding polygon that defined the analysis 

extent for mapping the inundation. Then, surfaces containing 

the water-surface elevation for each simulated water-surface 

profile were created using the analysis extent. Lastly, the DEM 

was subtracted from each surface containing the water-surface 

elevation for each simulated water-surface profile, resulting in 

the final water depth grids. The water depth grids were then 

converted to the final flood-inundation polygons representing the inundation extent. The 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 maps show estimated flood-inundated areas overlaid on high-resolution, georeferenced aerial photographs of 

the study area for each water-surface profile generated by the 

hydraulic model.

South Fork Little River, Kentucky, FloodInundation Maps on the Internet

The flood-inundation maps and current study documentation are available online at the U.S. Geological Survey 

Publications Warehouse (http://pubs.usgs.gov/sim/2012/3242). 

Also, a Flood Inundation Mapping Science website has been 

established to provide a portal for USGS flood-inundation 

study information to the public at http://water.usgs.gov/osw/

flood_inundation/. That web portal has a link (http://wim.usgs.

gov/FIMI/FloodInundationMapper.html) to interactive online 

map libraries that can be downloaded in several commonlyused electronic file formats. In the latter link, each stream 

reach displayed contains further links to NWISWeb graphs of 

the current stage and streamflow at USGS station 03437495 

to which the inundation maps are referenced. A link also is 
Summary 7

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. Roadways and bridges were closely 

reviewed and are shown as shaded (inundated and likely 

impassable) or not shaded (dry and passable) to facilitate 

emergency planning and use. However, buildings which are 

shaded do not reflect inundation but denote that bare earth 

surfaces in the vicinity of the buildings are inundated. When 

the water depth (as indicated in the Web Mapping Application 

by holding the cursor over an inundated area) in the vicinity 

of the building of interest exceeds that building’s height, the 

structure can be considered fully submerged.

Disclaimer for Flood-Inundation Maps

Inundated areas shown should not be used for navigation, 

regulatory, permitting, or other legal purposes. The USGS 

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

planning tool but assumes no legal liability or responsibility 

resulting from the use of this information.

Uncertainties and Limitations for Use of FloodInundation 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 on the basis of stages (water-surface elevations) and streamflows at selected USGS streamgages. Watersurface elevations along the stream reaches were estimated by 

steady-state 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 August 2012. 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 small tributaries along 

the study reach, 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 DEM used to simulate 

the land surface. 

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. 

Summary

A series of estimated flood-inundation maps were developed in cooperation with the City of Hopkinsville Community 

Development Services for the South Fork Little River at Hopkinsville, Kentucky. These maps, available through a USGS 

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

the USGS streamgage 03437495 (South Fork Little River at 

Highway 68 By-Pass at Hopkinsville, Kentucky) and National 

Weather Service flood-stage forecasts, will help to guide the 

general public in taking individual safety precautions and will 

provide city officials with a tool to efficiently manage emergency flood operations and flood mitigation efforts. 

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

of Engineers’ HEC-RAS and HEC-GeoRAS programs to compute water-surface profiles and to delineate estimated floodinundation areas for selected stream stages. The maps show 

estimated flood-inundation areas overlaid on high-resolution, 

georeferenced aerial photographs of the study area for stream 

stages between 10 ft and 21.5 ft at station 03437495. 
8 Flood-Inundation Maps for an 8.9-Mile Reach of the South Fork Little River at Hopkinsville, Kentucky

References Cited

Aero-Metric, Inc., 2010, NRCS IL-KY LIDAR task order 

: 4020 Technology Parkway, Sheboygan, Wisconsin 

53082-0449.

Chow, V.T., 1959, Open-channel hydraulics: New York, 

McGraw-Hill, 680 p.

City of Hopkinsville, 2012, City of Hopkinsville City 

Clerk—Miscellaneous information, accessed September 

12, 2012, at http://www.hopkinsvilleky.us/city-departments/

administration/city-clerk/miscellaneous-information.

Federal Emergency Management Agency, 2008, Flood insurance study for Christian County, Kentucky: Washington 

D.C., Flood Insurance Study 21047CV000A.

U.S. Army Corps of Engineers, Hydrologic Engineering 

Center, 2009, HEC-GeoRAS, GIS tools for support of HECRAS using ArcGIS, User’s manual, version 4.2, available 

at http://www.hec.usace.army.mil/software/hec-ras/hecgeoras_downloads.html.

U.S. Army Corps of Engineers, Hydrologic Engineering 

Center, 2010, HEC-RAS River Analysis System, Hydraulic 

Reference Manual, version 4.1, available at http://www.hec.

usace.army.mil/software/hec-ras/hecras-download.html.

U.S. Census Bureau, 2010, 2010 Census interactive population 

search, KY—Hopkinsville town: Washington, D.C., available at http://quickfacts.census.gov/qfd/states/21/2137918.

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., 

11 sheets.

Lant —Flood-Inundation Maps for an 8.9-Mile Reach of the South Fork Little River at Hopkinsville, Kentucky—Scientific Investigations Map 3242