Prepared in cooperation with the City of Central, Louisiana

Flood-Inundation Maps for the Amite and Comite Rivers From 

State Highway 64 To U.S. Highway 190 at Central, Louisiana

Scientific Investigations Report 2019–5028

U.S. Department of the Interior

U.S. Geological Survey
Cover. Upper left: Flood-inundation map for the Amite and Comite Rivers at Central, Louisiana, 

corresponding to stages of 59.0 ft and 35.0 ft at the U.S. Geological Survey streamgages Amite 

River at Magnolia, La., and Comite River near Comite, La., respectively. Further details provided on 

figure 8 of this report. 

Lower right: Photograph showing backwater flooding across Florida Boulevard near the Amite 

River Bridge in Denham Springs, Louisiana. Photograph by James Fountain, U.S. Geological Survey.
Flood-Inundation Maps for the Amite and 

Comite Rivers From State Highway 64 To 

U.S. Highway 190 at Central, Louisiana

By John B. Storm

Prepared in cooperation with the City of Central, Louisiana

Scientific Investigations Report 2019–5028

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

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

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

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

Suggested citation:

Storm, J.B., 2019, Flood-inundation maps for the Amite and Comite Rivers from State Highway 64 to U.S. 

Highway 190 at Central, Louisiana: U.S. Geological Survey Scientific Investigations Report 2019–5028, 20 p, 

https://doi.org/10.3133/sir20195028

ISSN 2328-0328 (online)
iii

Acknowledgments

The author wishes to thank the Louisiana Department of Transportation and Development, the 

U.S. Army Corps of Engineers-New Orleans District, the East Baton Rouge Parish-Department of 

Public Works, and the City of Central, Louisiana, for funding the operation and maintenance of 

the multiple streamgages used for this study. Special thanks are given to Richard McComas and 

Wesley Crosby with the U.S. Army Corps of Engineers-Vicksburg District for their guidance in 

two-dimensional (2D) modeling. Special thanks are also given to the National Weather Service 

for their continued support of the U.S. Geological Survey Flood Inundation Mapping Program.

v

Contents

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

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

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

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

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

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

Hydraulic Model Development and Flood-Inundation Map Library Creation......................................4

Hydrologic Data.....................................................................................................................................5

Hydraulic Model Development...........................................................................................................7

Topographic and Bathymetric Data..........................................................................................8

Hydraulic Structures .................................................................................................................10

Energy-Loss Factors..................................................................................................................11

Hydraulic Model Calibration ....................................................................................................11

Development of Water-Surface Profiles................................................................................11

Development of Flood-Inundation Maps ........................................................................................15

Flood-Inundation Map Viewer.................................................................................................17

Disclaimer for Flood-Inundation Maps .................................................................................17

Uncertainties and Limitations Regarding Use of Flood-Inundation Maps ......................17

Summary........................................................................................................................................................17

References Cited..........................................................................................................................................19

Figures

1. Map showing location of study reach for the Amite and Comite Rivers at 

Central, Louisiana, and location of U.S. Geological Survey streamgages and 

National Weather Service forecast sites .................................................................................2

2. Graph showing corresponding annual peak stages 1964–2018 for Amite River 

at Magnolia, Louisiana and Comite River near Comite, La....................................................5

3. Graph showing correlation of corresponding annual peak streamflows, 

1964–2018, for Amite River at Magnolia, Louisiana and Amite River at Denham 

Springs, La .....................................................................................................................................7

4. Graph showing correlation of corresponding annual peak streamflows, 

1964–2018, for Comite River near Comite, Louisiana and Amite River at Denham 

Springs, La .....................................................................................................................................9

5. Graph showing correlation of corresponding annual peak stages, 1964–2018, for

Comite River near Comite, Louisiana, and Amite River at Denham Springs, La.................9

6. Map showing location of study reach for the Amite and Comite Rivers at Central,

Louisiana, and location of U.S. Geological Survey streamgages, National Weather

Service forecast sites, and high-water mark locations observed following the

August 2016 flood........................................................................................................................13

7. Map merger location for a flood scenario of 48 ft at the Amite River at Magnolia,

Louisiana and 28 ft at the Comite River near Comite, La......................................................16

8. Flood-inundation map for the Amite and Comite Rivers at Central, La., corresponding 

to stages of 59.0 ft and 35.0 ft at the U.S. Geological Survey streamgages Amite 

River at Magnolia, La. and Comite River near Comite, La., respectively...........................18
vi

Tables

1. U.S. Geological Survey streamgage information for Amite and Comite Rivers at

Central, Louisiana .........................................................................................................................3

2. Peak discharge estimates for the 1.0-percent annual exceedance probability for

selected locations on the Amite River in Louisiana................................................................5

3. Estimated discharges for corresponding stages and water-surface elevations at

selected locations, used in the hydraulic model of the Amite and Comite Rivers at

Central, Louisiana .........................................................................................................................6

4. Regression estimated stage correlations between Amite River at Magnolia,

Louisiana and Amite River at Denham Springs, La.................................................................8

5. Range-of-stage correlation for Comite River near Comite, Louisiana and Amite

River at Denham Springs, La.....................................................................................................10

6. Range-of-stage correlation for Amite River at Magnolia, Louisiana and Comite

River near Comite, La .................................................................................................................10

7. Calibration of model to water-surface elevations at U.S. Geological Survey

streamgages along the Amite and Comite Rivers, Louisiana, for the floods of 

March 2016 and August 2016....................................................................................................12

8. Calibration of model to water-surface elevations at select high-water mark

locations within the study reach along the Amite and Comite Rivers, Louisiana, 

for the flood of August 2016 ......................................................................................................14

9. Calibration of model to target water-surface elevations at U.S. Geological 

Survey streamgages at Amite River at Magnolia, Louisiana and Comite River

near Comite, La ...........................................................................................................................15
vii

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in) 2.54 centimeter (cm)

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)

Datum

Vertical coordinate information is referenced to (1) stage, the height above an arbitrary datum 

established at a streamgage, and (2) elevation, the height above the North American Vertical 

Datum of 1988 (NAVD 88).

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

Abbreviations

2D two-dimensional

AHPS Advanced Hydrologic Prediction Service

DEM digital elevation model

FEMA Federal Emergency Management Agency

FESWMS-2DH Finite Element Surface-Water Modeling System: Two-Dimensional Flow 

in a Horizontal Plane

FIM flood inundation mapper

FIS flood insurance study

GIS geographic information system

GPS Global Positioning System

HEC-RAS Hydrologic Engineering Center River Analysis System

lidar light detection and ranging

NOAA National Oceanic and Atmospheric Administration

NWIS National Water Information System

NWS National Weather Service

RTN real-time network

SMS Surface-Water Modeling System

USACE U.S. Army Corps of Engineers

USGS U.S. Geological Survey 
Flood-Inundation Maps for the Amite and Comite 

Rivers From State Highway 64 To U.S. Highway 190 

at Central, Louisiana 

By John B. Storm

Abstract

Flood-inundation maps for a 14.5-mile reach of the 

Amite River and a 20.2-mile reach of the Comite River from 

State Highway 64 to U.S. Highway 190 were created by the 

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

of Central, Louisiana. These maps, which can be accessed 

through an interactive mapper at the USGS Flood Inundation 

Mapping Program website and from a companion USGS 

data release, depict estimates of the areal extent and depth of 

flooding corresponding to selected water levels (stages) at the 

USGS streamgages Amite River at Magnolia, La., (07377300) 

and Comite River near Comite, La. (07378000). 

Flood profiles were computed for the Amite and 

Comite River reaches by using the two-dimensional (2D), 

finite-volume numerical modeling options in the U.S. Army 

Corps of Engineers Hydrologic Engineering Center’s River 

Analysis System (USACE HEC-RAS) software version 5.0.3. 

Models were calibrated to the current (2018) stage-discharge 

relations at the Amite River at Magnolia, La., and Comite 

River near Comite, La., streamgages, water-surface profiles 

from the March and August 2016 floods, and documented 

high-water marks from the flood of August 2016. 

The hydraulic models were used to compute 

37 individual water-surface profiles (21 for the Amite River 

and 16 for the Comite River) at 1.0-foot intervals ranging 

from the National Weather Service flood stage to the highest 

peak on record at the two streamgages. The 37 simulated 

water-surface profiles were used with a light detection and 

ranging-derived digital elevation model to delineate the flood 

extent and associated depth at each water level. The delineated 

areas (inundation maps) were merged into 127 combinations 

or possible flooding scenarios based on annual peak stage 

information from the two streamgaging stations.

The availability of these maps, along with real-time 

data delivered via the internet, will provide emergency 

management personnel and residents with information that is 

critical for flood-response activities such as evacuations and 

road closures, as well as for recovery efforts after floods.

Introduction 

The City of Central, Louisiana, is located in East Baton 

Rouge Parish (fig. 1) above the confluence of the Amite and 

Comite Rivers and has an estimated population of 26,864 (U.S. 

Census Bureau, 2010). The Amite River forms the eastern 

boundary of the city and the Comite River forms its western 

boundary. The city’s location and low-lying floodplains 

make it susceptible to flooding from each river individually 

and combined. Backwater-affected flows along the Comite 

River can exist for approximately 8.7 miles (mi) from its 

confluence to the streamgage at Comite River near Comite, 

La. (station number 07378000), depending upon the level of 

the Amite River. The U.S. Geological Survey (USGS) began 

monitoring the Amite River at Denham Springs, La. (station 

number 07378500), in 1938 just below the confluence with the 

Comite River at U.S. Highway 190. Based on current flood 

stage categories established by the National Weather Service 

(NWS), the Amite River at Denham Springs, La. (07378500), 

streamgage has exceeded flood stage at least 57 times since 

1938 (NWS, 2018a). In 2016, Central, La., experienced 

two major flood events including the peak of record flood 

of August 2016. This flood was the result of a slow-moving 

inland tropical depression releasing record amounts of rainfall 

across southern Louisiana, with amounts exceeding 31 inches 

(in.) in some areas (Di Liberto, 2016). The April 1983 flood 

had been considered the peak of record flood prior to the 

August 2016 flood in the Central, La., area. The August 2016 

flood peak eclipsed the April 1983 peak by 2.2 feet (ft) at the 

Comite River at Comite, La., streamgage, 7.6 ft at the Amite 

River at Magnolia, La., and 4.7 ft at the Denham Springs, 

La., streamgages. Damages throughout Louisiana resulting 

from the August flood event were estimated to be $10 billion 

and resulted in at least 13 fatalities (National Oceanic and 

Atmospheric Administration [NOAA], 2016). 

Prior to this study, the city’s emergency responders relied 

on several online information sources to make decisions 

on how to best alert the public and mitigate flood damages. 

These sources all have static information that are limited in 
2 Flood-Inundation Maps for the Amite and Comite Rivers From State Hwy. 64 To U.S. Hwy. 190 at Central, Louisiana

#

#

#

#

#

#

O

#

O

#

O

!

!

!

£

£

07377750

07377754

07377760

07377870

07378050 07378000

07378500

07377300

EAST BATON ROUGE 

PARISH

LIVINGSTON 

PARISH

ILLINOIS CENTRAL RAILROAD

190

190

16

67

64

EAST BATON ROUGE 

PARISH

LIVINGSTON 

PARISH

JOOR RD

HOOPER ROAD

COMITE DR

DYER RD

SPRINGS

ROAD

BRIDGE

ROAD

MAGNOLIA

GREENWELL

CENTRAL

THRUWAY

Amite

River

Amite River

River

Comite

Bayou

Hub

Central

Baton 

Rouge

Denham 

Springs

Zachary

Baker

Greenwell 

Springs

91°08' 91°06' 91°04' 91°02' 91° 90°58' 90°56'

30°38'

30°36'

30°34'

30°32'

30'

30°28'

Base map hydrography from the 1:24,000-scale 

National Hydrography Dataset, Parish and city 

boundaries from U.S. Census Bureau, 

Transportation routes from U.S. Census Bureau

EXPLANATION

U.S. Geological Survey streamgage and 

 National Weather Service forecast 

 site and identifier

# U.S. Geological Survey streamgage 

 and identifier

Streams

Central city boundary

Study area boundary

Parish boundary

07377750

07378500#

0 1 2 MILES O

0 1 2 KILOMETERS

Study 

area

LOUISIANA

"

"

"

"

"

"

Figure 1. Location of study reach for the Amite and Comite Rivers at Central, Louisiana, and location of 

U.S. Geological Survey streamgages and National Weather Service forecast sites.
Introduction 3

their application during an emergency. One source is the Federal 

Emergency Management Agency (FEMA) flood insurance 

study (FIS) for East Baton Rouge Parish and incorporated areas 

(FEMA, 2012). A second source of information is the eight USGS 

streamgages located within the study reach (table 1). Current and 

historical information for these streamgages—including water 

levels, discharges, and annual peak flows—can be obtained 

from the USGS National Water Information System (NWIS) 

Web interface (USGS, 2018a). A third source of flood-related 

information is the National Weather Service Advanced 

Hydrologic Prediction Service (AHPS), which displays the USGS 

stage data and also issues forecasts of stage for the Amite River 

at Denham Springs, the Amite River at Magnolia, and the Comite 

River near Comite Joor Road (NWS, 2018b).

Although the current stage at a USGS streamgage is 

particularly useful for residents in the immediate vicinity of a 

streamgage, it is of limited use to residents farther upstream or 

downstream because of the sloping water surface. Knowledge 

of a water level at a streamgage is difficult to translate into 

depth and areal extent of flooding at points distant from the 

streamgage. To address these data gaps that exist away from a 

streamgage, a library of interactive flood-inundation maps that 

are referenced to the stages recorded at the USGS streamgage 

can be produced that cover the entire reach/study area. 

In 2017, the USGS, in cooperation with the City 

of Central, initiated a study to produce a library of 

flood-inundation maps for the Amite and Comite Rivers at 

Central, La. By referring to the maps, emergency responders 

can discern the severity of flooding (depth of water and areal 

extent), identify roads that are or will soon be flooded, and 

make notification or evacuation plans for some distance 

upstream and downstream from a USGS streamgage. In 

addition, the capability to visualize the potential extent of 

flooding can motivate residents to take precautions and heed 

warnings that they previously might have disregarded. 

Purpose and Scope

The purpose of this report is to describe and document 

the development of a series of estimated flood-inundation 

maps that represent flooding scenarios for the Amite and 

Comite Rivers at Central, La. The flood-inundation maps 

along with USGS real-time stage data and NWS flood-forecast 

data are made available for interactive viewing, planning, 

and/or decision support through the USGS Flood Inundation 

Mapping Program (FIM) at https://www.usgs.gov/missionareas/water-resources/science/flood-inundation-mapping-fimprogram?qt-science_center_objects=0#qt-science_center_

objects. Flood extent polygons and depth grids are publicly 

available from Heal and Storm (2019). 

Table 1. U.S. Geological Survey streamgage information for Amite and Comite Rivers at Central, Louisiana.

[Station location is shown in figure 1. mi2, square mile; ft, foot; NAVD 88, North American Vertical Datum of 1988; ft3/s, cubic foot per second; NA, not available]

Station name Station 

number

Drainage 

area, 

(mi2)

Latitude Longitude Datum 

(ft, NAVD 88)

Period of 

peak-flow 

record 

(water years1)

Maximum 

recorded stage 

(ft), and elevation 

(ft, NAVD 88) 

and date

Maximum 

discharge (ft3/s) 

and date

Amite River at 

Magnolia, La.

07377300 884 300 32’ 05” 900 58’ 50” −1.43 1949–83; 

1993–2018

58.56 (57.13)

August 13, 2016

202,000

August 13, 2016

Amite River at Denham 

Springs, La.

07378500 1,280 300 27’ 50’ 900 59’ 25” −1.35 1939–2018 46.20 (44.85)

August 14, 2016

266,000

August 14, 2016

Comite River near 

Zachary, La.

07377750 230 300 38’ 35” 910 05’ 40” 58.58 1951–62 31.99 (90.57) 

August 13, 2016

NA

Comite River near 

Baker, La.

07377754 2238 300 35’ 46’ 910 05’ 39” 48.20 NA 26.91 (75.12) 

August 13, 2016

NA

Comite River at Comite 

Drive near Baton 

Rouge, La.

07377760 2261 300 33’ 29.1” 910 05’ 53.4” 36.43 NA 29.36 (65.79) 

August 13, 2016

NA

Comite River at Hooper 

Road near Baton 

Rouge, La.

07377870 2271 300 31’ 50” 910 05’ 37” 28.49 NA 31.96 (60.45) 

August 14, 2016

NA

Comite River near 

Comite, La.

07378000 284 300 30’ 45” 910 04’ 25” 21.51 1944–2018 34.22 (55.73) 

August 14, 2016

71,000

August 14, 2016

Comite River at 

Greenwell Springs 

Road near Baton 

Rouge, La.

07378050 2304 300 30’ 20” 910 02’ 24” −0.95 NA 54.06 (53.11) 

August 14, 2016

NA

1

Water year is the 12-month period from October 1 of one year through September 30 of the following year and is designated by the calendar year in 

which it ends.

2

Drainage area not published; computed by using ESRI ArcGIS.
4 Flood-Inundation Maps for the Amite and Comite Rivers From State Hwy. 64 To U.S. Hwy. 190 at Central, Louisiana

Flood inundation map extents for this study were required 

to incorporate the city limit boundaries of Central, La. These 

extents were expanded to also include inundated tributaries 

outside of the city boundaries to at least the first bridge 

crossing when possible. Model boundary extents included 

an even larger area of the stream reaches. The scope of the 

maps included a 14.5-mi reach of the Amite River from the 

northern city limit boundary of Central above the community 

of Greenwell Springs, La., to U.S. Highway 190 at Denham 

Springs, La., and a 20.2-mi reach of the Comite River from 

State Highway 64 near Zachary, La., to its confluence with the 

Amite River above U.S. Highway 190 (fig. 1). 

Incremental flood-inundation maps were created for 

possible flooding scenarios referenced to the recorded stages 

at the USGS streamgages Amite River at Magnolia, La. 

(07377300), and Comite River near Comite, La. (07378000) 

(USGS, 2018a). The inundation maps cover a range in stage 

on the Amite River from 39 to 59 ft, gage datum. The Amite 

River range begins 9 ft below the NWS flood stage, defined 

as the stage at which a rise in water levels begins to create a 

hazard to lives, property, and commerce (NWS, 2018c). The 

purpose for starting the range below the flood stage at the 

Amite River streamgage is to account for possible coincident 

flooding on the Comite River based on annual peak stage 

comparisons (fig. 2). The inundation maps also cover a range 

in stage from 20 to 35 ft, gage datum on the Comite River. The 

20-ft stage corresponds to the flood stage at the Comite River 

streamgage as defined by the NWS. The maximum stages 

covered by the inundation maps correspond to approximately 

the highest recorded water level at the respective streamgages 

during the peak of record flood of August 2016.

Study Area Description

The Amite River has its headwaters in southwestern 

Mississippi and flows for approximately 117 mi to its confluence 

with Lake Maurepas in southeastern Louisiana. The Amite River 

Basin has a total drainage area of approximately 2,200 square 

miles (mi2) and includes the Comite River, contributing 

approximately 334 mi2 of drainage. Major urban areas within 

the basin include Baton Rouge, Central, Zachary, and Denham 

Springs (U.S. Army Corps of Engineers [USACE], 2012). 

Located within the East Gulf Coast and Mississippi Alluvial 

Plain sections of the Coastal Plain Province (Renken, 1998), 

the basin transitions from the Southern Pine Hills characterized 

by rolling hills and narrow, well-defined floodplains in the 

upper third of the basin to alluvial floodplains with seasonally 

inundated bottomland hardwoods and tupelo swamps in the 

lower third (Gulf Engineers and Consultants, Inc., 2015).

The Amite River reach length within the study area is 

about 14.5 mi, average channel bed slope is approximately 

1.5 feet per mile (ft/mi), and top of bank widths average 

about 410 ft. The length of the Comite River reach within 

the study area is about 20.2 mi, average channel bed slope is 

approximately 2.9 ft/mi, and top of bank widths average about 

300 ft. There are 2 major road crossings of the Amite River, 

7 major road crossings of the Comite River, and 1 railroad 

crossing both rivers within the study area.

Previous Studies

The current FIS for East Baton Rouge Parish 

(FEMA, 2012) was completed by FTN/Taylor Joint 

Venture in 2008 and revised by Risk Assessment, Mapping, 

and Planning Partners in 2012. The FIS presents estimates 

of the peak discharges with 1-percent annual exceedance 

probabilities (table 2) and their associated water-surface 

profiles for the Amite River. Comite River flood profiles 

for the 0.2-, 1.0-, and 10-percent annual exceedance 

probabilities are also presented in the current FIS, but 

the associated discharges are not presented and could not 

be determined. 

Hydraulic Model Development and 

Flood-Inundation Map Library Creation

The USGS has standardized procedures for creating 

flood-inundation maps (USGS, 2018b) so that the process and 

products are consistent among USGS offices. Tasks specific to 

the development of the flood inundation maps for Central, La., 

were as follows:

• Upgrade USGS streamgage 07377300 to a stage/

discharge gaging station.

• Review current and historical hydrologic data at USGS 

streamgaging stations 07377300, 07378000, and 

07378500.

• Collect topographic data for cross sections at major 

bridge crossings within the study area.

• Estimate energy-loss factors (roughness coefficients) in 

the stream channels and floodplains,

• Develop and calibrate two-dimensional (2D) hydraulic 

models by using USACE Hydrologic Engineering 

Center-River Analysis System (HEC-RAS) modeling 

software (Brunner, 2016a).

• Compute incremental water-surface profiles along 

the Amite and Comite River reaches by using the 

calibrated 2D hydraulic models.

• Produce incremental estimated flood extent and 

associated depth layers by using the HEC-RAS Mapper 

module within the modeling software (Brunner, 2016a) 

and geographic information system (GIS) software.

• Merge the incremental flood extent and depth layers 

into polygon feature classes and depth rasters to 

represent 127 possible flood-inundation map scenarios 

between the two study reaches.

• Serve and display derived inundation map scenarios in 

the USGS Flood Inundation Mapper.
Hydraulic Model Development and Flood-Inundation Map Library Creation 5

Amite River at Magnolia, La. (07377300) stage, in feet

15

20

25

30

35

35 40 45 50 55 60

Comite River near Comite, LA. (07378000) stage, in feet

Annual peak stages

Flood stage

EXPLANATION

Figure 2. Corresponding annual peak stages 1964–2018 for Amite River at Magnolia, Louisiana (07377300), and 

Comite River near Comite, La. (07378000).

Table 2. Peak discharge estimates for the 1.0-percent annual exceedance probability for selected 

locations on the Amite River in Louisiana.

[ft3/s, cubic foot per second; mi2, square mile; n.d., no data. Data from Federal Emergency Management Agency (2012)]

Location on Amite River Drainage area (mi2) Discharge 

estimate (ft3

/s)

Confluence of Comite River 1,286 140,138 

Confluence of Hub Bayou 910 116,685 

At State Route 64 n.d. 114,409 

Hydrologic Data

The study-area hydrologic network contains eight USGS 

streamgages (fig. 1; table 1) composed of two streamgages 

on the Amite River and six streamgages on the Comite 

River. Five of the streamgages on the Comite River measure 

continuous stage only with the Comite River near Comite, 

La. (07378000), being the only station computing continuous 

streamflow (discharge). Both gaging stations on the Amite 

River measure stage and compute continuous streamflow. The 

Amite River at Magnolia, La. (07377300), streamgage was 

upgraded in January 2017 to compute continuous streamflow 

and is being upgraded to a flood forecast gaging station by the 

NWS at the writing of this report (2018). Stage is measured 

every 15 minutes, transmitted hourly by a satellite radio in the 

streamgage, and made available from NWIS (USGS, 2018a). 

Stage data from these streamgages are referenced to a local 

datum but can be converted to water-surface elevations 

referenced to the North American Vertical Datum of 1988 

(NAVD 88) by adding the local datum (table 1). Continuous 

records of streamflow are computed from a stage-discharge 

relation at Amite River at Magnolia, La. (07377300) and are 

available from NWIS (USGS, 2018a). 

Variable backwater effects exist on the Comite River 

above its confluence with the Amite River. The water-surface 

elevation at the confluence is considered the control, and 

continuous streamflow computations at the Comite River near 

Comite, La., streamgage are complex. Continuous streamflow 

computations incorporate the stage-discharge rating with 

water-surface slope below the gage (Rantz and others, 1982) 

and are available from NWIS (USGS, 2018a). 
6 Flood-Inundation Maps for the Amite and Comite Rivers From State Hwy. 64 To U.S. Hwy. 190 at Central, Louisiana

The peak flows used in the model simulations (table 3) 

were taken from preliminary stage-discharge relations that 

were developed after the August 2016 flood event. These flows 

differ slightly from the approved (effective) ratings because 

of adjustments for smoothing of the rating and rounding. 

Differences in discharges are less than 3 percent and are 

therefore considered within the range of acceptable modeling 

accuracy requirements and correspond with target stages. The 

stage-discharge ratings in effect at the time of this report are 

as follows:

07378000 Comite River near Comite, La. 

Rating 32.0 effective October 1, 2017

07377300 Amite River at Magnolia, La. 

Rating 2.0 effective October 1, 2017

The August 2016 floodflow peaks were estimated 

outside of this study effort by using the two-dimensional, 

depth-integrated, surface-water flow model Finite Element 

Surface-Water Modeling System: Two-Dimensional Flow 

in a Horizontal Plane (FESWMS-2DH) (Froehlich, 1989) 

in conjunction with Surface-Water Modeling System 

(SMS) (Aquaveo, 2018). SMS was used to construct a 

finite element grid for the study reach, and FESWMS-2DH 

was the computational engine for all model simulations. 

Input floodflows were increased incrementally until 

simulated water-surface elevation matched recorded or 

surveyed elevations. Rating curves were extended through 

the August 2016 flood peak estimates by using straight 

line extensions.

Historical USGS stage and discharge data from the 

Amite River at Denham Springs, La., (07378500) streamgage 

were used to develop a flood-scenario relationship between 

the Amite River at Magnolia, La. (07377300), and Comite 

River near Comite, La. (07378000), streamgages. A linear 

regression of corresponding annual peak streamflow values 

from 1964 to 2018 at the two Amite River streamgaging 

stations has a coefficient of determination (R2

) value of 

0.98 and a standard deviation of 4,560 cubic feet per 

second (ft3

/s) (fig. 3). A comparative stage table at even 1-ft 

increments can be estimated by using the linear regression 

equation and stage-discharge ratings at the gages (table 4). 

A linear regression of corresponding annual peak streamflow 

Table 3. Estimated discharges for corresponding stages and water-surface elevations at selected locations, used in the hydraulic 

model of the Amite and Comite Rivers at Central, Louisiana.

[ft, foot; NAVD 88, North American Vertical Datum of 1988; ft3/s, cubic foot per second]

07377300 Amite River at Magnolia, La. 07378000 Comite River near Comite, La.

Stage 

(ft gage datum)

Target 

water-surface 

elevation 

(ft NAVD 88)

Discharge (ft3

/s) Stage 

(ft gage datum)

Target 

water-surface 

elevation 

(ft NAVD 88)

Discharge (ft3/s)

39.0 37.57 15,000 20.0 41.51 9,210 

40.0 38.57 17,200 21.0 42.51 9,920 

41.0 39.57 20,000 22.0 43.51 10,600 

42.0 40.57 23,000 23.0 44.51 11,400 

43.0 41.57 26,400 24.0 45.51 12,600 

44.0 42.57 30,100 25.0 46.51 14,200 

45.0 43.57 34,600 26.0 47.51 16,900 

46.0 44.57 39,800 27.0 48.51 20,000 

47.0 45.57 45,400 28.0 49.51 25,000 

48.0 46.57 51,900 29.0 50.51 29,500 

49.0 47.57 59,000 30.0 51.51 35,000 

50.0 48.57 67,200 31.0 52.51 42,000 

51.0 49.57 76,000 32.0 53.51 49,000 

52.0 50.57 86,000 33.0 54.51 58,000 

53.0 51.57 98,000 34.0 55.51 70,000 

54.0 52.57 112,000 35.0 56.51 82,000 

55.0 53.57 127,000

56.0 54.57 146,000

57.0 55.57 167,000

58.0 56.57 191,000

59.0 57.57 218,000
Hydraulic Model Development and Flood-Inundation Map Library Creation 7

Annual peak discharges

EXPLANATION

0

50,000

100,000

150,000

200,000

250,000

0 50,000 100,000 150,000 200,000 250,000 300,000

y = 0.743x - 445.11

R2

 = 0.9839

Amite River at Denham Springs, La. (07378500) discharge, in cubic feet per second

Amite River at Magnolia, La. (07377300) discharge, in cubic feet per second

Figure 3. Correlation of corresponding annual peak streamflows, 1964–2018, for Amite River at Magnolia, 

Louisiana (07377300), and Amite River at Denham Springs, La. (07378500).

values for the same time period, but for the Comite River 

near Comite, La. (07378000), and Amite River at Denham 

Springs, La. (07378500), showed poor correlation (R2

=0.61) 

and was indicative of backwater or storage effects (fig. 4). 

Another comparison was made with the same two streamgages 

and time period but with corresponding annual peak stage 

values. From this dataset, upper and lower envelope curves 

could be assumed for the range of stages that were possible 

at the Comite River streamgage for a single stage at the 

Amite River streamgage (fig. 5). A 1-ft “safety factor” was 

added both above and below the upper and lower envelope 

curves to account for outliers, and a comparative incremental 

range-of-stage table was developed for the streamgages 

(table 5). A corresponding table of 127 possible flooding 

scenarios (table 6) between the Amite River at Magnolia, La. 

(07377300), and Comite River near Comite, La. (07378000), 

streamgages was then created by combining the results of 

tables 4 and 5. 

Hydraulic Model Development

The Amite and Comite River reaches within the study 

area are characterized by low slopes, wide floodplains, 

variable backwater effects, and intermingling flows during 

extreme events. A 2D hydraulic model is better suited for 

analyzing these types of complex hydraulic characteristics, 

particularly where flow directions vary across a floodplain. 

For this study, individual 2D hydraulic models were developed 

for the Amite and Comite River reaches by using HEC-RAS 

version 5.0.3 (USACE, 2018). Individual 2D models of each 

river reach were chosen over a 2D model of the entire study 

area for the following reasons:

1. Modeling the entire study area would require 

trial-and-error adjustments of the flood hydrograph timing 

to account for backwater along the Comite River reach 

for all 127 possible flooding scenarios shown in table 6.

2. Model run times were extremely long (5 to 6 hours) 

for larger floodflows and would require multiple 

trial-and-error calibration runs for each of the 127 

possible flooding scenarios.

3. GIS clean-up of 127 flood inundation maps would be 

very time consuming.

Flow computations for the individual 2D models 

occur within an area defined by a computational mesh that 

is directly connected to external boundary conditions and 

overlays a digital elevation model (DEM) layer. For this 

study, a computational mesh of 200-ft × 200-ft grid cells was 

created. Barriers to flow, such as roadways or high ground, 

were delineated as break lines within the mesh. A finer grid 

of 10-ft × 10-ft cells align along these break lines and then 

gradually transition back to the original mesh dimensions. 
8 Flood-Inundation Maps for the Amite and Comite Rivers From State Hwy. 64 To U.S. Hwy. 190 at Central, Louisiana

Table 4. Regression estimated stage correlations between Amite River at Magnolia, Louisiana (07377300), and Amite River at Denham 

Springs, La. (07378500).

[ft, foot; ft3/s, cubic foot per second]

07378500 07377300 07377300

Stage

(ft gage datum)

Discharge 

(ft3

/s)

Regression 

calculated 

discharge

(ft3

/s)

Rounded 

discharge (ft3

/s)

 Stage

(ft gage datum)

Adjusted stage

(ft gage datum)

Discharge 

(ft3/s)

28.0 20,400 14,712 14,700 38.83 39.0 15,000 

29.0 25,000 18,130 18,100 40.31 40.0 17,200 

30.0 29,800 21,696 21,700 41.56 41.0 20,000 

30.0 29,800 21,696 21,700 41.56 42.0 23,000 

31.0 35,000 25,560 25,600 42.76 43.0 26,400 

32.0 40,400 29,572 29,600 43.86 44.0 30,100 

33.0 46,100 33,807 33,800 44.82 45.0 34,600 

1

34.0 52,000 38,191 38,200 45.70 46.0 39,800 

35.0 58,200 42,797 42,800 46.54 46.0 39,800 

36.0 64,600 47,553 47,600 47.34 47.0 45,400 

37.0 71,200 52,456 52,500 48.08 48.0 51,900 

1

38.0 78,100 57,583 57,600 48.81 49.0 59,000 

39.0 85,100 62,784 62,800 49.47 49.0 59,000 

40.0 92,400 68,208 68,200 50.12 50.0 67,200 

41.0 102,000 75,341 75,300 50.92 51.0 76,000 

42.0 113,000 83,514 83,500 51.76 52.0 86,000 

43.0 130,000 96,145 96,100 52.85 53.0 98,000 

44.0 154,000 113,977 114,000 54.15 54.0 112,000 

44.6 170,900 126,534 126,000 54.94 55.0 127,000 

45.0 187,000 138,496 138,000 55.60 56.0 146,000 

45.7 228,500 169,330 169,000 57.09 57.0 169,000 

46.0 250,000 185,305 185,000 57.77 58.0 191,000 

46.4 294,600 218,443 218,000 59.00 59.0 218,000 

1

Not used. The next higher stage produced similar results and was considered more conservative. 

Water-surface profiles are simulated with a 2D unsteady flow 

equations solver by using an Implicit Finite Volume algorithm 

to solve user-specified Saint-Venant or Diffusion Wave 

equations (Brunner, 2016b). The Diffusion Wave equations, 

which were chosen for this study, allow the software to run 

faster and have greater stability properties. The Finite Volume 

algorithm also provides improved stability and robustness over 

traditional finite difference and finite element techniques. The 

algorithm can also handle subcritical, supercritical, and mixed 

flow regimes.

Topographic and Bathymetric Data

All topographic data used in this study were referenced 

vertically to NAVD 88 and horizontally to the North American 

Datum of 1983. The base terrain elevation data used by the 

HEC-RAS models were obtained from the Louisiana State 

University Department of Geography and Anthropology 

(2018). Elevation data were retrieved as a DEM derived from 

light detection and ranging (lidar) data collected and processed 

in 2001 during Phase 1 of the Louisiana Statewide LiDAR 

Project (Cunningham and others, 2004). The original lidar data 

have horizontal resolution of 16.40 ft and vertical accuracy 

of 0.75 ft at a 95-percent confidence level. Green band lidar 

was not used in the data collection effort, so no elevation 

data below the water surface were collected. Supplementary 

channel elevation data were collected by USGS field crews 

to define the channel bottom at select locations in the study 

reach. Cross-sectional depths were measured at the upstream 

and downstream side of nine bridge crossings by tapedowns 

from the handrails at 10-ft increments to define the channel 

elevation between the top of banks. USGS surveying standards 
Hydraulic Model Development and Flood-Inundation Map Library Creation 9

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

y = 0.202x + 7,434.3

R2

 = 0.6078

0 50,000 100,000 150,000 200,000 250,000 300,000

Amite River at Denham Springs, La. (07378500) discharge, in cubic feet per second

Comite River near Comite, LA. (07378000) stage, in cubic feet per second

Annual peak discharges

EXPLANATION

Figure 4. Correlation of corresponding annual peak streamflows, 1964–2018, for Comite River near Comite, 

Louisiana (07378000), and Amite River at Denham Springs, La. (07378500).

y = 0.9577x - 11.742 

y = 0.6555x + 3.9298

15

17

19

21

23

25

27

29

31

33

35

25 30 35 40 45 50

Amite River at Denham Springs, La. (07378500), stage in feet

Comite River near Comite, LA. (07378000) stage, in feet

Annual peak stage

Flood stage

Upper/lower stage envelope

EXPLANATION

Louisiana, (07378000) and Amite River at Denham Springs, La. (07378500).

Figure 5. Correlation of corresponding annual peak stages, 1964–2018, for Comite River near Comite, 
10 Flood-Inundation Maps for the Amite and Comite Rivers From State Hwy. 64 To U.S. Hwy. 190 at Central, Louisiana

Table 5. Range-of-stage correlation for Comite River near 

Comite, Louisiana (07378000), and Amite River at Denham Springs, 

La. (07378500).

[ft, foot]

07378500 07378000

Stage 

(ft gage datum)

Range in stage 

(ft gage datum)

28.0 20.0 to 24.0

29.0 20.0 to 24.0

30.0 20.0 to 25.0

31.0 20.0 to 25.0

32.0 20.0 to 26.0

33.0 20.0 to 27.0

35.0 21.0 to 28.0

36.0 22.0 to 29.0

37.0 23.0 to 29.0

39.0 25.0 to 31.0

40.0 26.0 to 31.0

41.0 27.0 to 32.0

42.0 28.0 to 33.0

43.0 28.0 to 33.0

44.0 29.0 to 34.0

44.6 30.0 to 34.0

45.0 30.0 to 34.0

45.7 31.0 to 35.0

46.0 31.0 to 35.0

46.4 32.0 to 35.0

and protocols (Rydlund and Densmore, 2012) were followed 

by using a differential Global Positioning System (GPS) 

device with real-time network technology (RTN) to establish 

horizontal locations and the elevation of the water surface 

and bridge handrails at each surveyed cross section. A 

“channel-only” DEM was created by interpolating channel 

shape and channel-bottom elevations between cross sections 

by using the HEC-RAS Mapper and by following a series 

of steps outlined in the HEC-RAS 2D Modeling User’s 

Manual (Brunner, 2016a). An improved terrain model could 

then be created within the HEC-RAS Mapper by combining 

the original lidar DEM with the “channel-only” DEM 

(Brunner, 2016a).

Hydraulic Structures

Ten hydraulic structures, consisting of nine road 

crossings (Magnolia Bridge Road, U.S. Highway 190, State 

Highway 64, Dyer Road, Comite Drive, Hooper Road, Joor 

Road, Greenwell Springs Road, and Central Thruway), and 

a railroad crossing (Illinois Central Railroad) can affect 

Table 6. Range-of-stage correlation for Amite River at Magnolia, 

Louisiana (07377300), and Comite River near Comite, La. (07378000).

[ft, foot]

07378500 07377300 07378000

Stage 

(ft gage datum)

Stage 

(ft gage datum)

Range in stage 

(ft gage datum)

28.0 39.0 20.0 to 24.0

29.0 40.0 20.0 to 24.0

30.0 41.0 20.0 to 25.0

30.0 42.0 20.0 to 25.0

31.0 43.0 20.0 to 25.0

32.0 44.0 20.0 to 26.0

33.0 45.0 20.0 to 27.0

35.0 46.0 21.0 to 28.0

36.0 47.0 22.0 to 29.0

37.0 48.0 23.0 to 29.0

39.0 49.0 25.0 to 31.0

40.0 50.0 26.0 to 31.0

41.0 51.0 27.0 to 32.0

42.0 52.0 28.0 to 33.0

43.0 53.0 28.0 to 33.0

44.0 54.0 29.0 to 34.0

44.6 55.0 30.0 to 34.0

45.0 56.0 30.0 to 34.0

45.7 57.0 31.0 to 35.0

46.0 58.0 31.0 to 35.0

46.4 59.0 32.0 to 35.0

water-surface elevations during floods along the stream. A 

primary limitation of the HEC-RAS 2D model is the lack of 

capability to incorporate bridge hydraulic routines into its 

flow computations. As a result, no hydraulic structures are 

defined, and flows were modeled with a simple narrowing 

of the channel or constriction at the crossings. Roadways 

approaching these structures tend to be lower in elevation 

than the bridge structure because of the wide floodplains and 

low slopes associated with the Amite and Comite Rivers. It 

is assumed that during extreme events, when floodflows are 

affected by the bridges, water will have already broken over 

the roadways and effects to the water surface elevation will be 

localized near the bridge. Break lines were delineated along 

all major roadways within the computational mesh by creating 

a refined grid cell size of 10 ft × 10 ft along the roadway 

centerlines. This refined grid better defines the elevation 

change in the DEM and diverts or blocks flow paths when the 

roadway elevation is higher than the upstream water surface 

and eliminates “leakage” across the road. The grid cell size 

transitions gradually back to the original 200-ft × 200-ft mesh 

dimensions as distance is gained away from the break lines.
Hydraulic Model Development and Flood-Inundation Map Library Creation 11

Energy-Loss Factors

The Amite and Comite Rivers are wide, low-gradient 

channels made up of sand and silt bed material and have 

steep banks. Floodplains vary in their composition and 

include dense bottom land forests, open agricultural areas, 

and medium and high-density urbanization. All of these 

characteristics are factors that contribute to the frictional 

resistance on flow.

Hydraulic analyses require the estimation of energy 

losses that result from frictional resistance exerted by a 

channel on flow. These energy losses are quantified by 

the Manning’s roughness coefficient (“n” value). Initial 

(precalibration) n values were selected based on field 

observations and high-resolution aerial photographs 

(Arcement and Schneider, 1989). 

As part of the calibration process, the initial n values 

were varied by flow and adjusted until the differences between 

simulated and observed water-surface elevations at the 

streamgages and high watermarks were minimized. The final 

n values for the main channel of the Amite and Comite Rivers 

ranged from 0.026 to 0.055 and 0.030 to 0.058, respectively. 

Floodplain area n values modeled in this analysis ranged 

from 0.07 to 0.20. 

Hydraulic Model Calibration

The HEC-RAS 2D flow models for the Amite and Comite 

River reaches were calibrated to water-surface profiles for the 

March and August 2016 floods, to surveyed high-water marks 

from the August 2016 flood, and to the current stage-discharge 

relations at the Amite River at Magnolia, La. (07377300), and 

Comite River near Comite, La. (07378000), streamgages. 

Upstream boundary conditions of the 2D flow area for 

both river reaches were unit value hydrographs with 30-minute 

time-step intervals. The hydrographs were held steady once 

they reached the target discharge to simulate a steady-state 

condition. The downstream boundary condition of the 2D 

flow area was the current stage-discharge rating at the Amite 

River at Denham Springs, La. (07378500), streamgage. 

Flow data consisted of boundary conditions and peak 

flows that produced average water-surface elevations at the 

corresponding streamgage that matched target water-surface 

elevations (see “Hydrologic Data” for more on the peak flows 

used in the model). Lesser tributaries entering the Comite and 

Amite River reaches were considered to be small enough to 

assume that inflows would have already peaked prior to the 

main-stem flood peak arrival and that the tributary would be in 

backwater. Therefore, the streamgage-derived discharges were 

not adjusted for tributary inflows. During extreme flooding 

conditions, some of these tributaries can become conduits 

to convey Amite and Comite River floodplain flow. Model 

calibrations were accomplished by adjusting Manning’s n

values until the results of the hydraulic computations closely 

agreed with the observed water-surface elevations for given 

flows. Absolute differences between observed and modeled 

water-surface elevations for the March and August 2016 

floods along the Amite River reach were equal to or less 

than 0.13 ft and 0.07 ft, respectively (table 7). Absolute 

differences between observed and modeled water-surface 

elevations for the March and August 2016 floods along the 

Comite River reach were equal to or less than 0.34 ft and 

0.20 ft, respectively (table 7). Absolute differences between 

surveyed and modeled high-water mark elevations for the 

August 2016 flood (fig. 6) were equal to or less than 1.4 ft 

(table 8). Absolute differences between observed and modeled 

water-surface elevations for the 21 simulated flows at the 

Amite River at Magnolia, La. (07377300), streamgage were 

equal to or less than 0.16 ft (table 9). Absolute differences 

between observed and modeled water-surface elevations for 

the 16 simulated flows at the Comite River near Comite, La. 

(07378000), streamgage were equal to or less than 0.18 ft 

(table 9). The results demonstrated that the models are capable 

of simulating accurate water levels over a wide range of flows 

in the basin. 

Development of Water-Surface Profiles

The calibrated 2D hydraulic models were used to 

generate water-surface profiles at 1-ft increments for 21 river 

stages between 39 and 59 ft at the Amite River at Magnolia, 

La. (07377300), streamgage and 16 river stages between 

20 and 35 ft at the Comite River near Comite, La. (07378000), 

streamgage. Discharges corresponding to the target stages 

were obtained from the current (2018) stage-discharge 

relations shown in table 3. Tributaries along the Amite and 

Comite River reaches were considered to be in backwater, and 

modeled discharges were not adjusted.

Incremental flood profiles for flow values along the 

Comite River reach model were calculated as if there was no 

flow on the Amite River above the confluence. Roughness 

coefficients were adjusted until the average simulated water 

surface at the Comite River near Comite, La. (07378000), 

streamgage matched the targeted stage within 0.2 ft (table 7). 

The model produced accurate water-surface profiles above the 

Comite River gage but can underestimate profile elevations as 

distance increases downstream from the streamgage because 

of the absence of control from the Amite River.

 A second 2D model was created containing floodflows 

for both river reaches. Incremental flood profiles for the Amite 

River reach were calculated by using the correlated flow values 

determined from the regression analysis results summarized 

in table 4. Tributary flow from the Comite River was included 

in this model as the difference in flow values between the 

two Amite River streamgages. Roughness coefficients were 

adjusted until the simulated water surface at the Amite River 

at Magnolia, La. (07377300), streamgage matched the targeted 

stage within 0.2 ft (table 7). Comite River channel roughness 

coefficients were adjusted until the average simulated water 

surface at the Comite River near Comite, La. (07378000), 

streamgage approximately matched the lowest stage of 

the corresponding Comite River stage range. This is the 

corresponding lower envelope stage (table 6). 
12 Flood-Inundation Maps for the Amite and Comite Rivers From State Hwy. 64 To U.S. Hwy. 190 at Central, Louisiana Table 7. Calibration of model to water-surface elevations at U.S. Geological Survey streamgages along the Amite and Comite Rivers, Louisiana, for the floods of March 2016 and August 2016. [ft3/s, cubic foot per second; ft, foot; NAVD 88, North American Vertical Datum of 1988; --, no calibration value; **, gage overtopped, four high-water marks collected with a 1.3-ft range, omitted for this study] Station name Station number March 2016 flood August 2016 flood Discharge (ft3/s) Stage (ft gage datum) Water surface elevation (ft NAVD 88) Model water-surface elevation (ft NAVD 88) Elevation difference (ft) Discharge (ft3/s) Stage (ft gage datum) Water surface elevation (ft NAVD 88) Model water-surface elevation (ft NAVD 88) Elevation differ- ence (ft) Amite River at Magnolia, La. 07377300 46,000 47.1 45.67 45.54 −0.13 202,000 58.56 57.13 57.08 −0.05 Amite River at Denham Springs, La. 07378500 65,200 36.09 34.74 34.82 0.08 266,000 46.2 44.85 44.78 −0.07 Comite River near Zachary, La. 07377750 -- -- -- 81.95 -- -- 31.99 90.57 90.59 0.02 Comite River near Baker, La. 07377754 -- 18.78 66.99 66.94 −0.05 -- 26.91 75.12 75.06 −0.06 Comite River at Comite Drive near Baton Rouge, La. 07377760 -- 22.18 58.61 58.64 0.03 -- 29.36 65.79 65.87 0.08 Comite River at Hooper Road near Baton Rouge, La. 07377870 -- 24.13 52.62 52.55 −0.07 -- 31.96 60.45 60.25 −0.20 Comite River near Comite, La. 07378000 17,400 26.18 47.69 47.68 0.01 71,000 34.22 55.73 55.78 0.05 Comite River at Greenwell Springs Road near Baton Rouge, La. 07378050 -- 43.65 42.7 42.36 −0.34 -- ** ** 51.53 --
Hydraulic Model Development and Flood-Inundation Map Library Creation 13

Figure 6. Location of study reach for the Amite and Comite Rivers at Central, Louisiana, and location of U.S. 

Geological Survey streamgages, National Weather Service forecast sites, and high-water mark locations observed 

following the August 2016 flood.

0 1 2 MILES

0 1 2 KILOMETERS

E

E

E

E

E

E

E

E

E

E

E

E

E

E E

E

E

E

E

E

E

E

E E

E

E

E

E

E

E

E

E 9

8

7

6

5

4

3

2

1

33

32

31

30

29

28

27

26

25 24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

"

"

"

"

"

"

#

#

#

#

#

#

O

#

O

#

O

!

!

!

£

£

07377750

07377754

07377760

07377870

07378050 07378000

07378500

07377300

EAST BATON ROUGE 

PARISH

LIVINGSTON 

PARISH

ILLINOIS CENTRAL RAILROAD

190

190

16

67

64

EAST BATON ROUGE 

PARISH

LIVINGSTON 

PARISH

JOOR RD

HOOPER ROAD

COMITE DR

DYER RD

SPRINGS

ROAD

BRIDGE

ROAD

MAGNOLIA

GREENWELL

CENTRAL

THRUWAY

Amite

River

River

Comite

Amite River

Bayou

Hub

Central

Baton 

Rouge

Denham 

Springs

Zachary

Baker

Greenwell 

Springs

Study 

area

LOUISIANA

91°08' 91°06' 91°04' 91°02' 91° 90°58' 90°56'

30°38'

30°36'

30°34'

30°32'

30'

30°28'

Base map hydrography from the 1:24,000-scale 

National Hydrography Dataset, Parish and city 

boundaries from U.S. Census Bureau, 

Transportation routes from U.S. Census Bureau

EXPLANATION

Streams

Central city boundary

Study area boundary

Parish boundary

29 E High-water mark and number

U.S. Geological Survey streamgage and 

 National Weather Service forecast 

 site and identifier

# U.S. Geological Survey streamgage 

 and identifier

07377750

07378500#

O
14 Flood-Inundation Maps for the Amite and Comite Rivers From State Hwy. 64 To U.S. Hwy. 190 at Central, Louisiana

Table 8. Calibration of model to water-surface elevations at select high-water mark locations within the study reach along the Amite 

and Comite Rivers, Louisiana, for the flood of August 2016.

[HWM, high-water mark; ft, foot; NAVD 88, North American Vertical Datum of 1988]

HWM 

location

(see fig. 6)

HWM 

ID number Latitude Longitude HWM type HWM quality

HWM 

elevation 

(ft NAVD 88)

Model 

water-surface 

elevation 

(ft NAVD 88)

Elevation 

difference 

(ft NAVD 88)

1 15367 30.600 −90.997 Seed line Good: +/− 0.10 ft 69.6 69.3 −0.3

2 15381 30.573 −90.994 Seed line Good: +/− 0.10 ft 65.3 65.4 0.1

3 15516 30.556 −90.967 Mud Good: +/− 0.10 ft 60 60.7 0.7

4 15518 30.556 −90.967 Mud Good: +/− 0.10 ft 60 60.8 0.8

5 15557 30.510 −90.984 Seed line Good: +/− 0.10 ft 52.2 52.8 0.6

6 15565 30.527 −90.992 Seed line Good: +/− 0.10 ft 55.1 55.4 0.3

7 15568 30.541 −91.006 Seed line Excellent: +/− 0.05 ft 57.2 57.9 0.7

8 15220 30.496 −90.946 Seed line Excellent: +/− 0.05 ft 48 48.2 0.2

9 15272 30.503 −90.941 Seed line Excellent: +/− 0.05 ft 51.9 51.3 −0.6

10 15278 30.544 −90.953 Seed line Excellent: +/− 0.05 ft 56.8 57.2 0.4

11 15419 30.516 −90.947 Seed line Excellent: +/− 0.05 ft 52.2 51.4 −0.8

12 15451 30.528 −90.962 Seed line Excellent: +/− 0.05 ft 56.6 55.7 −0.9

13 15457 30.522 −90.951 Seed line Good: +/− 0.10 ft 52.8 52.4 −0.4

14 15465 30.485 −90.934 Seed line Excellent: +/− 0.05 ft 46.1 46 −0.1

15 15241 30.498 −91.018 Seed line Excellent: +/− 0.05 ft 49.6 49.8 0.2

16 15289 30.497 −91.036 Seed line Excellent: +/− 0.05 ft 50.9 50.8 −0.1

17 15299 30.472 −91.026 Seed line Good: +/− 0.10 ft 47.4 47.6 0.2

18 15300 30.491 −91.084 Seed line Excellent: +/− 0.05 ft 53.9 54.1 0.2

19 15305 30.533 −91.073 Seed line Excellent: +/− 0.05 ft 61.3 61.2 −0.1

20 15319 30.561 −91.072 Seed line Excellent: +/− 0.05 ft 63.9 65.3 1.4

21 15393 30.627 −91.111 Seed line Excellent: +/− 0.05 ft 83.9 84.3 0.4

22 15397 30.591 −91.122 Seed line Good: +/− 0.10 ft 73.6 73.8 0.2

23 15404 30.532 −91.065 Seed line Good: +/− 0.10 ft 59.2 57.8 −1.4

24 15409 30.515 −91.049 Stain line Excellent: +/− 0.05 ft 54.4 54 −0.4

25 15430 30.516 −90.994 Debris Good: +/− 0.10 ft 51.3 51.4 0.1

26 15460 30.598 −91.074 Seed line Excellent: +/− 0.05 ft 76.1 75.8 −0.3

27 15503 30.489 −91.066 Stain line Good: +/− 0.10 ft 52.6 53.6 1

28 15547 30.482 −91.008 Seed line Excellent: +/− 0.05 ft 47.1 47 −0.1

29 15211 30.471 −91.005 Seed line Excellent: +/− 0.05 ft 46.1 46.2 0.1

30 15226 30.479 −91.004 Seed line Excellent: +/− 0.05 ft 46.8 46.6 −0.2

31 15260 30.491 −90.993 Seed line Excellent: +/− 0.05 ft 46.8 47.2 0.4

32 15327 30.644 −91.083 Seed line Fair: +/− 0.20 ft 91.6 91.1 −0.5

33 15555 30.508 −90.999 Seed line Good: +/− 0.10 ft 49.7 49.5 −0.2
Hydraulic Model Development and Flood-Inundation Map Library Creation 15

Table 9. Calibration of model to target water-surface elevations at U.S. Geological Survey streamgages at Amite River at Magnolia, 

Louisiana (07377300), and Comite River near Comite, La. (07378000).

[ft, foot; ft3/s, cubic foot per second; NAVD 88, North American Vertical Datum of 1988]

07377300 Amite River at Magnolia, La. 07378000 Comite River near Comite, La.

Stage

(ft gage 

datum)

Discharge 

(ft3

/s)

Target 

water-surface 

elevation 

(ft NAVD 88)

Model 

water-surface 

elevation 

(ft NAVD 88)

Elevation 

difference 

(ft)

Stage

(ft gage 

datum)

Discharge 

(ft3

/s)

Target 

water-surface 

elevation 

(ft NAVD 88)

Model 

water-surface 

elevation

(ft NAVD 88)

Elevation 

difference 

(ft)

39.0 15,000 37.57 37.63 0.06 20.0 9,210 41.51 41.6 0.09

40.0 17,200 38.57 38.7 0.13 21.0 9,920 42.51 42.59 0.08

41.0 20,000 39.57 39.72 0.15 22.0 10,600 43.51 43.4 −0.11

42.0 23,000 40.57 40.71 0.14 23.0 11,400 44.51 44.4 −0.11

43.0 26,400 41.57 41.44 −0.13 24.0 12,600 45.51 45.69 0.18

44.0 30,100 42.57 42.47 −0.10 25.0 14,200 46.51 46.53 0.02

45.0 34,600 43.57 43.49 −0.08 26.0 16,900 47.51 47.46 −0.05

46.0 39,800 44.57 44.5 −0.07 27.0 20,000 48.51 48.63 0.12

47.0 45,400 45.57 45.45 −0.12 28.0 25,000 49.51 49.62 0.11

48.0 51,900 46.57 46.45 −0.12 29.0 29,500 50.51 50.63 0.12

49.0 59,000 47.57 47.46 −0.11 30.0 35,000 51.51 51.63 0.12

50.0 67,200 48.57 48.45 −0.12 31.0 42,000 52.51 52.65 0.14

51.0 76,000 49.57 49.48 −0.09 32.0 49,000 53.51 53.66 0.15

52.0 86,000 50.57 50.52 −0.05 33.0 58,000 54.51 54.58 0.07

53.0 98,000 51.57 51.67 0.10 34.0 70,000 55.51 55.49 −0.02

54.0 112,000 52.57 52.72 0.15 35.0 82,000 56.51 56.37 −0.14

55.0 127,000 53.57 53.7 0.13

56.0 146,000 54.57 54.71 0.14

57.0 167,000 55.57 55.73 0.16

58.0 191,000 56.57 56.7 0.13

59.0 218,000 57.57 57.5 −0.07

Comparing the resulting Comite River reach profiles from 

the two individual 2D models determines the estimated point 

of backwater control along the Comite River stream reach for 

that particular flood scenario. Further discussion related to 

determining this location is provided in the following section.

Development of Flood-Inundation Maps

The HEC-RAS Mapper module was used to create 

GIS shapefiles of estimated inundation extent polygons and 

associated depth grids for each water-surface profile generated 

by the 2D models. Shapefiles were created for 1-ft incremental 

profiles ranging from 39 to 59 ft at the Amite River at 

Magnolia, La. (07377300), streamgage and from 20 ft to 35 ft 

at the Comite River near Comite, La. (07378000), streamgage. 

The shapefiles were exported from HEC-RAS and then 

edited by using the ArcMap application of ArcGIS to remove 

erroneous areas of inundation (Esri, 2018). Any inundated 

areas that were detached from the main channel were 

examined to identify subsurface connections with the main 

river, such as through culverts under roadways. Where such 

connections existed, the mapped inundated areas were retained 

in their respective flood maps; otherwise, the erroneously 

delineated parts of the flood extent were deleted.

A total of 127 possible flooding scenarios were shown 

to exist between the Amite and Comite River reaches within 

the study area (table 6). Flood-inundation maps depicting 

the estimated areal extent and flood depths were created 

by combining each scenario’s shapefiles at their point of 

overlap on the Comite River reach by using ArcMap. For 

each flooding scenario, the point of overlap was determined 

by subtracting the Amite River model depth grid file from the 

corresponding Comite River model depth grid file to produce 

a grid file of depth-differences. This file was contoured at 

0.5-ft intervals to determine where the line of zero-difference 

was located along the Comite River reach (fig. 7). The areal 

extent polygon shapefiles of the flood scenario were clipped 

and merged along this line and the depth grid shapefiles 

were clipped and mosaiced along this line as well to produce 

seamless flood-inundation maps. 
16 Flood-Inundation Maps for the Amite and Comite Rivers From State Hwy. 64 To U.S. Hwy. 190 at Central, Louisiana

Figure 7. Merger location for a flood scenario of 48 ft at the Amite River at Magnolia, Louisiana (07377300), and 28 ft 

at the Comite River near Comite, La. (07378000).

0 0.5 1 MILES

0 0.5 1 KILOMETERS

8

7

2

9

1

3

4

5

6

0

7.5

8.5

0.5

-2

2.5

1.5

4.5

6.5

3.5

5.5

9.5

12

15

13

10.5

14.5

13.5

15.5

12.5

5

6.5

7.5

7.5

7.5

9.5

8

8

3

4

-1.5

1

9.5

9

8

1

-1.5

7

-1

10

8.5

2.5

2

11

-0.5

-0.5

3

0

91°

30'

30°28'

Inundation maps merger location

Base modified from U.S. Geological Survey digital data, 1:30,000, 2011

Lambert Conformal Conic projection

State Plane Louisiana South FIPS 1702 Ft US

North American Datum of 1983

7.5

EXPLANATION

Comite model inundation at 28 feet

Amite model inundation at 48 feet

Depth difference contour and number.

Contour interval 0.5 foot

Depth differences between models
Summary 17

The flood-inundation maps can be overlaid on 

georeferenced, aerial photographs of the study area for 

determining affected structures at different flooding scenarios. 

Bridge surfaces are displayed as inundated regardless of 

the actual water-surface elevation in relation to the lowest 

structural chord of the bridge or the bridge deck. The flood map 

corresponding to the highest simulated water-surface profile 

scenario, a stage of 59 ft at the Amite River at Magnolia, La. 

(07377300), streamgage and a stage of 35 ft at the Comite 

River near Comite, La. (07378000), is presented in figure 8.

Flood-Inundation Map Viewer

The FIM Program website (USGS, 2018b) has been 

established to make USGS-derived flood-inundation data 

available to the public. An interactive viewer presents map 

libraries over user-selected base maps and provides detailed 

information on flood extents and depths for modeled stream 

reaches. The estimated flood-inundation maps are presented 

in the interactive viewer so that preparations for flooding and 

decisions for emergency response can be performed for the 

mapped area. Depending on the flood magnitude, roadways are 

shown as shaded (inundated and likely impassable) or not shaded 

(dry and passable) to facilitate emergency planning and use. 

Bridges are shaded—that is, shown as inundated—regardless of 

the flood magnitude. A shaded building should not be interpreted 

to mean that the structure is completely submerged, but rather 

that bare earth surfaces near the building are inundated. In 

these instances, the water depth (as indicated in the mapping 

application by clicking with the cursor over an inundated 

area) near the building would be an estimate of the water level 

inside the structure, unless flood proofing measures had been 

implemented. The interactive flood inundation map library 

for the Amite and Comite Rivers at the city of Central, La., is 

located at https://www.usgs.gov/mission-areas/water-resources/

science/flood-inundation-mapping-fim-program?qt-science_

center_objects=0#qt-science_center_objects. The map libraries 

are downloadable in formats that may be used in other software 

packages and are available to the public in an accessible, 

machine-readable format from Heal and Storm (2019).

Disclaimer for Flood-Inundation Maps 

The flood-inundation maps should not be used for 

navigation, regulatory, permitting, or other legal purposes. 

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

emergency planning tool but assumes no legal liability or 

responsibility resulting from the use of this information.

Uncertainties and Limitations Regarding Use of 

Flood-Inundation Maps 

Although the flood-inundation maps represent the 

boundaries of inundated areas with a distinct line, some 

uncertainty is associated with these maps. The flood 

boundaries shown were estimated on the basis of water stages 

and streamflows at selected USGS streamgages. Water-surface 

elevations along the stream reaches were estimated by 

steady-state hydraulic modeling, assuming unobstructed 

flow, and used streamflows and hydrologic conditions 

anticipated at the USGS streamgages. The scenario-based 

modeling approach produces many potential flooding scenario 

estimates that have not previously occurred or been measured. 

Backwater affects along the Comite River can vary and may 

not be fully represented by this modeling approach. The 

hydraulic model reflects the land-cover characteristics and 

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

as of May 2018. 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 by 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. 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 (and 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: https://water.weather.gov/ahps/pcpn_

and_river_forecasting.pdf.

Summary

A library of digital flood-inundation maps was developed 

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

the city of Central, Louisiana, for a 14.5-mile (mi) reach of 

the Amite River and a 20.2-mi reach of the Comite River 

that extend approximately from State Highway 64 to U.S. 

Highway 190. The flood-inundation maps were developed 

by using 2D modeling options in the U.S. Army Corps of 

Engineers’ Hydrologic Engineering Center-River Analysis 

System (HEC–RAS, version 5.0.3) software to compute watersurface profiles and delineate estimated inundation extents 

for selected stream stages referenced to the Amite River at 

Magnolia, La. (07377300), and Comite River near Comite, 

La. (07378000), streamgages. Flood-inundation layers were 

created at 1.0-foot (ft) increments ranging from 39 to 59 ft, 

gage datum, at the Amite River streamgage and from 20 to 

35 ft, gage datum, at the Comite River streamgage. These 
18 Flood-Inundation Maps for the Amite and Comite Rivers From State Hwy. 64 To U.S. Hwy. 190 at Central, Louisiana

Figure 8. Flood-inundation map for the Amite and Comite Rivers at Central, La., corresponding to stages of 59.0 ft 

and 35.0 ft at the U.S. Geological Survey streamgages Amite River at Magnolia, La. (07377300), and Comite River near 

Comite, La. (07378000), respectively.

#

#

#

#

#

07377750

07377754

07377760

07377870

07378050

#

O

#

O

#

O

07377300

07378000

07378500

0 1 2 MILES

0 1 2 KILOMETERS

Study 

area

LOUISIANA

"

"

"

"

"

"

!

!

!

£

£

EAST BATON ROUGE 

PARISH

LIVINGSTON 

PARISH

THRUWAY

CENTRAL

ILLINOIS CENTRAL RAILROAD

190

190

16

67

64

EAST BATON ROUGE 

PARISH

LIVINGSTON 

PARISH

JOOR RD

HOOPER ROAD

COMITE DR

DYER RD

SPRINGS

ROAD

BRIDGE

ROAD

MAGNOLIA

GREENWELL

THRUWAY

CENTRAL

Amite

River

River

Comite

Amite River

Bayou

Hub

Central

Baton 

Rouge

Denham 

Springs

Zachary

Baker

Greenwell 

Springs

91°08' 91°06' 91°04' 91°02' 91° 90°58' 90°56'

30°38'

30°36'

30°34'

30°32'

30'

30°28'

Base map hydrography from the 1:24,000-scale 

National Hydrography Dataset, Parish and city 

boundaries from U.S. Census Bureau, 

Transportation routes from U.S. Census Bureau

EXPLANATION

Streams

Central city boundary

Study area boundary

Parish boundary

Flood-inundation area

U.S. Geological Survey streamgage and 

 National Weather Service forecast 

 site and identifier

# U.S. Geological Survey streamgage 

 and identifier

07377750

07378500#

O
References Cited 19

layers were seamlessly combined by using ArcGIS to define 

estimated inundation extents and flood depths for 127 different 

flood scenarios that are possible between the two river 

reaches based on historical stage and flow data. The maps are 

downloadable in formats that may be used in other software 

packages and are available to the public. The flood-inundation 

maps are also presented on the interactive USGS Flood 

Inundation Mapping Program website along with USGS 

real-time streamgage data and National Weather Service flood 

forecast information to assist the public and local officials 

during a flood emergency.

References Cited

Aquaveo, 2018, Surface-Water Modeling System, Version 

13.0: Aquaveo website accessed April 2019, at https://www.

aquaveo.com/software/sms-surface-water-modeling-systemintroduction

Arcement, G.J., and Schneider, V.R., 1989, Guide for selecting Manning’s roughness coefficients for natural channels 

and floodplains: U.S. Geological Survey Water-Supply 

Paper 2339, 38 p.

Brunner, G.W., 2016a, U.S. Army Corps of Engineers 

Hydrologic Engineering Center-River Analysis System (HEC-RAS)—2D modeling user’s manual (Version 

5.0): Davis, Calif., U.S. Army Corps of Engineers report 

CPD-68A, 171 p.

Brunner, G.W., 2016b, U.S. Army Corps of Engineers 

Hydrologic Engineering Center-River Analysis System 

(HEC-RAS)—Hydraulic reference manual (Version 

5.0): Davis, Calif., U.S. Army Corps of Engineers report 

CPD-69, 547 p. 

Cunningham, R., Gisclair, D., and Craig, J., 2004, The 

Louisiana statewide LiDAR project: Accessed October 

2018, at http://atlas.lsu.edu/central/la_lidar_project.pdf.

Di Liberto, T., 2016, August 2016 extreme rain and 

floods along the Gulf Coast: Accessed August 2018, at 

https://www.climate.gov/news-features/event-tracker/

august-2016-extreme-rain-and-floods-along-gulf-coast

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Flood insurance study, East Baton Rouge Parish, Louisiana, 

and incorporated areas: Washington D.C., Federal 

Emergency Management Agency report 41 p., 108 pl.

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Surface-Water Modeling System—Two-dimensional flow 

in a horizontal plane - user’s manual: Federal Highway 

Administration Report FHWA-RD-88-177, 285 p.

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Floodplain Management Plan for the Amite River Basin 

Drainage and Water Conservation District: Baton Rouge, 

La., G.E.C., Inc., 77 p.

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for the Amite and Comite Rivers from State Highway 64 

to U.S. Highway 190—City of Central, Louisiana: U.S. 

Geological Survey data release, https://doi.org/10.5066/

P9PQKSYF.

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and Anthropology, 2018, Atlas—The Louisiana statewide GIS: Baton Rouge, La., accessed August 2018, at 

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hydrologic prediction service: National Weather Service 

web page accessed August 2018, at https://water.weather.

gov/ahps2/hydrograph.php?wfo=lix&gage=denl1.

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hydrologic prediction service: National Weather 

Service web page accessed August 2018, at 

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

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accessed August 2018, at http://w1.weather.gov/glossary/

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Rantz, S.E., and others, 1982, Measurement and computation 

of streamflow—Volume 2. Computation of Discharge: 

U.S. Geological Survey Water-Supply Paper 2175, 631 p. 

Renken, R.A., 1998, Groundwater Atlas of the United 

States—Arkansas, Louisiana, and Mississippi: U.S. 

Geological Survey Hydrologic Atlas 730–F.

Rydlund, P.H., Jr., and Densmore, B.K., 2012, Methods of 

practice and guidelines for using survey-grade global 

navigation satellite systems (GNSS) to establish vertical 

datum in the United States Geological Survey: U.S. 

Geological Survey Techniques and Methods, book 11, 

chap. D1, 102 p. with appendixes.

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Basin and tributaries, Louisiana, Comite River Diversion 

Project Review Plan: U.S. Army Corps of Engineers 

New Orleans District report, accessed August 2018, at 

http://www.mvn.usace.army.mil/Portals/56/docs/PD/

PeerReview/Comiteapprovedplan.pdf
20 Flood-Inundation Maps for the Amite and Comite Rivers From State Hwy. 64 To U.S. Hwy. 190 at Central, Louisiana

U.S. Army Corps of Engineers, [USACE] 2018, HEC-RAS 

River Analysis System, Version 5.0.3: U.S. Army Corps of 

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flood_inundation.
For more information about this publication, contact

Director 

Lower Mississippi Gulf Water Science Center

U.S. Geological Survey

640 Grassmere Park, Ste 100 

Nashville, TN 37211

(615) 837-4700

For additional information visit 

https://www.usgs.gov/centers/lmg-water

Publishing support provided by

Lafayette Publishing Service Center
J.B. Storm—Flood-Inundation Maps for the Amite and Comite Rivers From State Highway 64 To U.S. Highway 190 at Central, Louisiana—Scientific Investigations Report 2019–5028

ISSN 2328–0328 (online)

https://doi.org/10.3133/sir20195028