U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany

Scientific Investigations Map 3196

Prepared in cooperation with the

New Hampshire Department of Homeland Security and Emergency Management

Flood-Inundation Maps for the Suncook River in Epsom, 

Pembroke, Allenstown, and Chichester, New Hampshire
Cover. Photograph looking downstream at the Suncook River avulsion channel through the sand pit, Epsom, New Hampshire.
Flood-Inundation Maps for the Suncook 

River in Epsom, Pembroke, Allenstown, 

and Chichester, New Hampshire

By Robert H. Flynn, Craig M. Johnston, and Laura Hayes

Prepared in cooperation with the 

New Hampshire Department of Homeland Security and Emergency Management

Pamphlet to accompany

Scientific Investigations Map 3196

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Flynn, R.H., Johnston, C.M. and Hayes, Laura, 2012, Flood-inundation maps for the Suncook River in Epsom, 

Pembroke, Allenstown, and Chichester, New Hampshire: U.S. Geological Survey Scientific Investigations Map 3196, 

10 p. pamphlet, 20 sheets, scale 1:24,000.
iii

Acknowledgments

The authors wish to thank the New Hampshire Department of Environmental Services for 

cooperative funding for the operation and maintenance of the Suncook River streamgage in 

North Chichester, N.H. Special thanks go to the New Hampshire Department of Homeland 

Security and Emergency Management for their cooperation in this study and to the National 

Weather Service for their continued support of the U.S. Geological Survey flood-inundation 

mapping initiative.

The authors would like to acknowledge that the cost of generating the 1- and 4-ft contour 

interval data used in the creation of the flood mapping for this study was funded through a 

New Hampshire Department of Safety–Bureau of Emergency Management grant (Emergency 

Management Performance Grant) with in-kind matching funding provided by the New Hampshire 

Department of Environmental Services through its dam-maintenance efforts in the Suncook 

River watershed.
iv

Contents

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

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

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

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

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

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

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

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

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

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

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

Development of Water-Surface Profiles ...........................................................................................5

Inundation Mapping ......................................................................................................................................5

Availability of Suncook River, New Hampshire, Flood-Inundation Maps ...................................8

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

Uncertainty Associated with Inundation Maps ...............................................................................8

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

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

Figures

 1. Map showing location of study reach for the Suncook River, National Weather 

Service forecast site, and U.S. Geological Survey streamgages, Suncook River, 

New Hampshire ............................................................................................................................2

Tables

 1. Description of U.S. Geological Survey streamgage, Suncook River at Depot Road, 

North Chichester, New Hampshire ............................................................................................3

 2. Miscellaneous town information along study reach, Suncook River, 

New Hampshire ............................................................................................................................4

 3. 1.0-Percent annual exceedance probability peak-discharge estimates, 

drainage areas, and percentage of total discharge for selected locations 

on the Suncook River, New Hampshire ....................................................................................4

 4. Comparison of hydraulic-model output and surveyed high-water mark elevations 

from the flood of April 16–18, 2007; Suncook River, New Hampshire ..................................6

 5. Stream stages (elevations) with corresponding estimated discharges at selected 

locations on the Suncook River, New Hampshire, for 10 flood-inundation HEC–RAS 

water-surface profiles .................................................................................................................7
v

Sheets

 1. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 

Allenstown, New Hampshire, for a flood corresponding to a stream stage of 7 feet 

at the U.S. Geological Survey streamgage at Suncook River at North Chichester, 

New Hampshire (Station 01089500)—Northern Region

 2. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 

Allenstown, New Hampshire, for a flood corresponding to a stream stage of 7 feet 

at the U.S. Geological Survey streamgage at Suncook River at North Chichester, 

New Hampshire (Station 01089500)—Southern Region

 3. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 

Allenstown, New Hampshire, for a flood corresponding to a stream stage of 9 feet 

at the U.S. Geological Survey streamgage at Suncook River at North Chichester, 

New Hampshire (Station 01089500)—Northern Region

 4. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 

Allenstown, New Hampshire, for a flood corresponding to a stream stage of 9 feet 

at the U.S. Geological Survey streamgage at Suncook River at North Chichester, 

New Hampshire (Station 01089500)—Southern Region

 5. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 

Allenstown, New Hampshire, for a flood corresponding to a stream stage of 11 feet 

at the U.S. Geological Survey streamgage at Suncook River at North Chichester, 

New Hampshire (Station 01089500)—Northern Region

 6. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 

Allenstown, New Hampshire, for a flood corresponding to a stream stage of 11 feet 

at the U.S. Geological Survey streamgage at Suncook River at North Chichester, 

New Hampshire (Station 01089500)—Southern Region

 7. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 

Allenstown, New Hampshire, for a flood corresponding to a stream stage of 12 feet 

at the U.S. Geological Survey streamgage at Suncook River at North Chichester, 

New Hampshire (Station 01089500)—Northern Region

 8. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 

Allenstown, New Hampshire, for a flood corresponding to a stream stage of 12 feet 

at the U.S. Geological Survey streamgage at Suncook River at North Chichester, 

New Hampshire (Station 01089500)—Southern Region

 9. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 

Allenstown, New Hampshire, for a flood corresponding to a stream stage of 13 feet 

at the U.S. Geological Survey streamgage at Suncook River at North Chichester, 

New Hampshire (Station 01089500)—Northern Region

 10. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 

Allenstown, New Hampshire, for a flood corresponding to a stream stage of 13 feet 

at the U.S. Geological Survey streamgage at Suncook River at North Chichester, 

New Hampshire (Station 01089500)—Southern Region

 11. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 

Allenstown, New Hampshire, for a flood corresponding to a stream stage of 14 feet 

at the U.S. Geological Survey streamgage at Suncook River at North Chichester, 

New Hampshire (Station 01089500)—Northern Region

 12. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 

Allenstown, New Hampshire, for a flood corresponding to a stream stage of 14 feet 

at the U.S. Geological Survey streamgage at Suncook River at North Chichester, 

New Hampshire (Station 01089500)—Southern Region
vi

 13. Flood-inundation map of the Suncook River in Chichester, Epsom, Pembroke, and 
vii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2

) 0.09290 square meter (m2)

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate

cubic foot per second (ft3

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

Hydraulic gradient

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

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

(NAVD 88).

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

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

(This page intentionally left blank.)
Flood-Inundation Maps for the Suncook River in Epsom, 

Pembroke, Allenstown, and Chichester, New Hampshire

By Robert H. Flynn, Craig M. Johnston, and Laura Hayes

Abstract

Digital flood-inundation maps for a 16.5-mile reach of the 

Suncook River in Epsom, Pembroke, Allenstown, and Chichester, N.H., from the confluence with the Merrimack River to 

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

01089500 at Depot Road in North Chichester, N.H., were 

created by the USGS in cooperation with the New Hampshire 

Department of Homeland Security and Emergency Management. The inundation maps presented in this report depict 

estimates of the areal extent and depth of flooding corresponding to selected water levels (stages) at the USGS streamgage at 

Suncook River at North Chichester, N.H. (station 01089500). 

The current conditions at the USGS streamgage may be 

obtained on the Internet (http://waterdata.usgs.gov/nh/nwis/

uv/?site_no=01089500&PARAmeter_cd=00065,00060). The 

National Weather Service forecasts flood hydrographs at many 

places that are often collocated with USGS streamgages. 

Forecasted peak-stage information is available on the Internet 

at the National Weather Service (NWS) Advanced Hydrologic Prediction Service (AHPS) flood-warning system site 

(http://water.weather.gov/ahps/) and may be used in conjunction with the maps developed in this study to show predicted 

areas of flood inundation.

These maps along with real-time stream stage data from 

the USGS Suncook River streamgage (station 01089500) and 

forecasted stream stage from the NWS will provide emergency 

management personnel and residents with information that is 

critical for flood-response activities, such as evacuations, road 

closures, disaster declarations, and post-flood recovery. The 

maps, along with current stream-stage data from the USGS 

Suncook River streamgage and forecasted stream-stage data 

from the NWS, can be accessed at the USGS Flood Inundation Mapping Science Web site http://water.usgs.gov/osw/

flood_inundation/.

Introduction

The towns of Epsom, Pembroke, Allenstown, and Chichester, New Hampshire (N.H.), (fig. 1) are rural communities 

with estimated populations of 4,609, 7,344, 4,957, and 2,583, 

respectively, in 2009 (New Hampshire Employment Security, 

Economic and Labor Market Information Bureau, 2010a–d). 

These towns have experienced severe flooding numerous 

times, most notably in 1936, 2006, 2007, and 2010. Flooding 

of the Suncook River and several tributaries (Little Suncook 

River, Leighton Brook, and Bear Brook) caused damage to 

property and infrastructure along the river. Flood plains within 

these towns are moderately developed and contain a mix of 

residential and commercial structures.

Prior to 2011 town officials relied on several information sources (all of which are available on the Web) to make 

decisions on how best to alert the public and mitigate flood 

damages. One source is the Federal Emergency Management 

Agency (FEMA) Flood Insurance Studies (FIS) for Epsom, 

Pembroke, Allenstown, and Chichester, N.H. (U.S. Department 

of Housing and Urban Development Federal Insurance Administration, 1978a–d). A second source of information is the U.S. 

Geological Survey (USGS) streamgage, Suncook River at 

Depot Road in North Chichester, N.H. (station 01089500; table 

1). Data on current and historical water levels (stage) are available for this streamgage. A third source is the National Weather 

Service (NWS) forecasts of flood hydrographs showing peak 

stage at the USGS streamgage. These forecasts are available 

through the NWS Advanced Hydrologic Prediction Service 

(AHPS) site (http://water.weather.gov/ahps/). Although USGS 

current stage and NWS forecast stage information is useful 

for residents in the immediate vicinity of the streamgage, it 

is of limited use to residents farther upstream or downstream 

because the water-surface elevation is not constant along the 

entire stream channel. Also, FEMA and state emergency management mitigation teams or property owners typically lack 

information related to the depth of flood waters other than near 

USGS streamgages or NWS flood-forecast points. To provide 

information on floods in areas downstream from the USGS 

streamgage, the USGS, in cooperation with the New Hampshire Department of Homeland Security and Emergency Management, conducted a study to produce digital flood-inundations maps for a 16.5-mile stretch of the Suncook River. These 

digital flood-inundation maps show flood-water extent and 

depth on the land surface and are tied to USGS and NWS flood 

forecast sites to enable officials to make timely operational and 
2 Flood-Inundation Maps for the Suncook River in Epsom, Pembroke, Allenstown, and Chichester, New Hampshire

Figure 1. Location of study reach for the Suncook River, National Weather Service forecast site, and U.S. Geological 

Survey streamgages, Suncook River, New Hampshire.

71°

44°

43°

45°

Concord

Manchester

MAINE

MASSACHUSETTS

VERMONT

CANADA

72°

NEW

HAMPSHIRE

Manchester

Merrimack River

Soucook River

Sun

cook River

Merrimack River

Winnipesaukee Lake

Village of

Suncook

Chichester

01089500

01089925

Danbury Hill

Andover

Franklin

Northfield

Canterbury Loudon

Pittsfield

Chichester

Epsom

Allenstown Hooksett Dunbarton

Bow

Hopkinton Henniker

Bradford

Newbury

Sutton

Warner

Webster

Salisbury

Wilmot

New

London

Boscawen

Pembroke

Concord

ROCKINGHAM

STRAFFORD

BELKNAP

GRAFTON

SULLIVAN MERRIMACK

HILLSBOROUGH

CHESHIRE

MASSACHUSETTS

VERMONT

NEW HAMPSHIRE

NEW HAMPSHIRE

MAINE

EXPLANATION

Towns bordering Suncook 

River study reach

Limit of study area

U.S. Geological Survey 

streamgage and identifier

National Weather Service

forecast site

01089500

10 20 KILOMETERS 

0 10 20 MILES

0

72 ̊ 71 ̊

43 ̊

43 ̊

30”

Base from U.S. Geological Survey digital line graphs
Introduction 3

Table 1. Description of U.S. Geological Survey streamgage, Suncook River at Depot Road, North Chichester, New Hampshire.

[mi2, square miles; ft, feet]

Station name Station 

number

Drainage 

area 

(mi2

)

Latitude Longitude Period of record

Maximum recorded 

elevation (stage) at gage 

(ft above gage datum) and 

date

Suncook River at 01089500 154.4 N 43°15’24” W 71°22’12” May 1918 to September 1920, 15.97 ft on April 17, 2007

Depot Road in June 1921 to November 1927, 

North Chiches- November 1928 to September 

ter, N.H. 1970, November 2007 to 2011

public safety decisions during floods. The flood-inundation 

maps show estimated flood-inundated areas overlaid on highresolution, georeferenced, aerial photographs of the study area 

for each of the water-surface profiles generated by the HECRAS hydraulic model.

Purpose and Scope

The purpose of this report is to describe the development 

of flood-inundation maps for the Suncook River at Epsom, 

Pembroke, Allenstown, and Chichester, N.H. The maps and 

other flood information are available on the USGS Flood Inundation Mapping Science Web site. Internet users can select 

estimated inundation maps that correspond closely to (1) current stages at the USGS streamgage, (2) the NWS forecasted 

peak stage, and (3) other desired stream stages.

The scope of the study was limited to the Suncook River 

between its confluence with the Merrimack River and the 

USGS streamgage 01089500 at Suncook River at Depot Road 

in North Chichester, N.H. Tasks specific to development of 

the maps were the (1) collection of topographic/bathymetric 

data and geometric data (for structures/bridges) throughout the 

study reach, (2) determination of energy-loss factors (roughness coefficients) in the stream channel and flood plain and 

collection of steady-flow data, (3) computation of water-surface profiles using the U.S. Army Corps of Engineers HEC–

RAS computer model, (4) production of flood-inundation 

maps at various stream stages using the U.S. Army Corps of 

Engineers HEC–GeoRas computer program and a geographic 

information system (ArcGIS), and (5) development by USGS 

of a Web interface that links to USGS real-time streamgage 

information and NWS forecasted peak stage to facilitate the 

display of user-selected flood-inundation maps on the Internet.

Maps were produced for water levels referenced to the 

water-surface elevation (stage) at the USGS streamgage at 

Suncook River at Depot Road in North Chichester, N.H.; the 

water-surface elevations ranged from the NWS-determined 

“flood stage” of 7.0 ft to the water-surface elevation (stage) of 

the 0.2-percent annual exceedance probability flow at 18.0 ft, 

at the Suncook River streamgage.

Suncook River at N.H. 28, near Suncook, N.H., 

streamgage number 01089925 was installed in August of 

2011. This streamgage records stage only, and data from 

this streamgage were not used in this study. The location of 

streamgage on the figures and map sheets is for informational 

purposes only.

Study Area Description

The Suncook River is in south-central New Hampshire 

in the New England physiographic province, New England 

Upland physiographic section. The drainage areas range 

from 154.4 mi2 for the Suncook River at the North Chichester streamgage to 255.7 mi2

 for the study reach downstream 

extent at the Suncook River confluence with the Merrimack 

River. The river’s headwaters originate in Belknap County, 

south of Lake Winnipesaukee, and the river flows in a generally southward direction for approximately 39 miles. There 

are three major tributaries to the Suncook River in the study 

area: Bear Brook, Little Suncook River, and Leighton Brook. 

The basin terrain is moderately hilly. The study reach is 

approximately 16.5 mi long and has an average top-of-bank 

channel width of about 282 ft and an average channel slope 

of 9.1 ft/mi. Although the land contiguous to the study reach 

is primarily forest or cropland, the basin is being developed 

with new houses and commercial businesses. The cumulative 

population for the four towns in the study area has increased 

7.83 percent to 19,493 between 2000 and 2009 (New Hampshire Employment Security, Economic and Labor Market 

Information Bureau, 2009a–d). Population and land-use information for each of the towns in the study area are presented 

in table 2. Along the study reach, there are five major roads, 

seven bridges, and six dams that cross the channel and the 

adjacent flood plain.
4 Flood-Inundation Maps for the Suncook River in Epsom, Pembroke, Allenstown, and Chichester, New Hampshire

Previous Studies

The current FISs for Epsom, Pembroke, Allenstown, and 

Chichester (U.S. Department of Housing and Urban Development Federal Insurance Administration,1978a–d) were completed by Hamilton Engineering Associates, Inc., and Anderson-Nichols and Co. in 1977 and 1978, respectively. Those 

studies provided information on the 0.2-, 1-, 2-, and 10-percent annual exceedance probability water-surface profiles 

and associated flood-plain maps for the Suncook River, Little 

Suncook River, and Sanders Brook. Estimates of the peak discharges for the 50-, 20-, 10-, 4-, 2-, 1-, and 0.2-percent annual 

exceedance probability floods along the Suncook River were 

described in Flood Study of the Suncook River in Epsom, 

Pembroke, and Allenstown, New Hampshire, 2009 (Flynn, 

2010). Estimates of the peak discharge with a 1-percent annual 

exceedance probability along the Suncook River are shown in 

table 3. The peak discharge with a 1-percent annual exceedance probability at the Suncook River streamgage (9,820 ft3

/s) 

corresponds to an elevation of 15.34 ft referenced to the gage 

datum.

Constructing Water-Surface Profiles

The water-surface profiles used to produce the floodinundation maps in this study were computed using HEC– RAS, version 4.1.0 (U.S. Army Corps of Engineers, 2010). 

HEC–RAS is a one-dimensional step-backwater model used 

Table 2. Miscellaneous town information along study reach, Suncook River, New Hampshire.

[Data from the New Hampshire Employment Security, Economic and Labor Market Information Bureau (2009a–d); mi2, square miles; %, percent]

Characteristic

Town

Epsom Pembroke Allenstown Chichester

Land area (mi2

) 34.5 22.6 20.5 21.2

2009 Population density 133.6 324.9 241.8 121.8

2009 Residential land (%) 85.6 78.3 82.5 85.6

2009 Commercial land (%) 12.6 15.9 14.3 13.0

2009 Public utilities and other land use (%) 1.8 5.8 3.2 1.4

2000 Population 4,051 6,917 4,854 2,259

2009 Population 4,609 7,344 4,957 2,583

2000–2009 Population percent increase 13.8 6.2 2.1 14.3

Table 3. 1.0-Percent annual exceedance probability peak-discharge estimates, drainage areas, and percentage of total discharge for 

selected locations on the Suncook River, New Hampshire.

[Data from Flynn (2010); mi2, square miles; ft3/s, cubic feet per second]

Location on Suncook River

Drainage 

area 

(mi2

)

Estimated peak 

discharge 

(ft3/s)

Percentage 

of total 

discharge

At Suncook River streamgage 01089500

At U.S. Route 4

Downstream from Little Suncook River

At Short falls Road bridge

Upstream from Bear Brook

Downstream from Bear Brook

At U.S. Route 28 bridge (site of new USGS streamgage 01089925)

At U.S. Route 3 bridge

At confluence with Merrimack River

154.4

161.2

200.9

210.1

219.3

236.4

240.1

255.2

255.7

9,820

10,200

12,600

13,100

13,700

14,700

14,900

15,700

15,800

62

65

80

83

87

93

94

99

100
Inundation Mapping 5

to simulate water-surface profiles with steady-state (gradually varied) or unsteady-state flow computation options. The 

HEC–RAS analysis for this study was completed using the 

steady-state flow computation option.

Hydrologic and Steady-Flow Data

The hydrologic network in the study area consists of one 

streamgage (fig. 1; table 1). USGS streamgage 01089500, 

Suncook River at Depot Road in North Chichester, Merrimack 

County, was active from 1919 to 1920, 1922 to 1927, 1929 to 

1970. In November of 2007, the Suncook River streamgage 

was reactivated. The water level (stage) is measured continuously at the streamgage, and a continuous record of streamflow is computed. All water-surface elevations are referenced 

to gage datum and to North American Vertical Datum of 

1988 (NAVD 88). The gage is equipped with a satellite radio 

transmitter that allows data to be transmitted routinely to the 

Internet within an hour of collection. 

Steady-flow data consist of flow regime, boundary conditions (normal depth and streamgage rating-curve values), and 

peak discharge. The steady-flow data for the study reach were 

determined for the Suncook River streamgage 01089500 using 

discharge values with known stages obtained from streamflow 

measurements and stage-discharge relations (Flynn, 2010). In 

addition, after the flood of April 16–18, 2007, high-water-mark 

data were collected at the then inoperative streamgage site and 

downstream to the Suncook River’s confluence with the Merrimack River (Flynn, 2010 and 2008). These high-water-mark 

data were used for HEC–RAS model calibration. 

Topographic/Bathymetric Data

 Channel cross sections were developed from USGS field 

surveys that were conducted in the summer of 2008. These 

cross sections, which provide detailed valley and channel 

elevation data, were obtained using a total station surveying 

instrument with elevations determined using closed level-loop 

surveying techniques from either a known reference datum 

or from a reference datum established using a Leica System 

1200 Global Positioning System (GPS) with a 3-millimeter 

(mm; 0.01-ft) horizontal accuracy and 10-mm (0.03-ft) vertical accuracy in static mode (Leica Geosystems, 2008). In 

addition, some cross-sectional data were collected between 

the Short Falls Road bridge and the Buck Street dam using a 

GPS in conjunction with an Acoustic Doppler Current Profiler 

(ADCP) (Flynn, 2008).

Various man-made drainage structures (bridges, culverts, 

roadway embankments, and dams) along the stream affect or 

have the potential to affect water-surface elevations during 

floods. To properly account for these features in the HEC–

RAS model, structural dimensions for seven bridges and six 

dams were measured and surveyed in the field concurrently 

with the stream-channel surveys. 

Energy Loss Factors

Field observations were used to select initial (pre-calibration) Manning’s roughness coefficients (“n” values) for energy 

(friction) loss calculations. The final Manning’s n values used 

ranged from 0.02 to 0.09 for the main channel and 0.02 to 0.12 

for the overbank areas modeled. 

Model Calibration and Performance

The HEC-RAS hydraulic model was calibrated to the 

most current stage-discharge relation at the Suncook River 

streamgage and to high-water marks from the flood of April 

16–18, 2007 (table 4). The estimated peak discharge from 

high-water mark data for the April 16–18, 2007, flood was 

10,600 ft3/s (Flynn, 2008) with an annual exceedance probability of 0.2- to 1-percent and an estimated stage at the 

streamgage of 15.97 ft, local datum (344.75 ft, NAVD 88). 

The estimated peak discharge from high-water marks for 

the flood of May 13–16, 2006, is 7,600 ft3

/s (Olson, 2007), 

the annual exceedance probability is 2 to 4 percent, and the 

estimated stage at the streamgage is 13.55 ft, local datum 

(342.33 ft, NAVD 88). The mean absolute difference between 

the field-measured high-water marks (HWM) of the May 2007 

flood, which are rated as “good,” and simulated water levels 

at the same locations (as modeled in HEC–RAS) is 0.36 ft 

(4.3 in.). Discrepancies between the model output and the 

field-collected HWM data are due to model assumptions and 

(or) interpretation of the location of the HWM in the field. 

Development of Water-Surface Profiles

Profiles were developed for 10 stages from the NWS 

“flood stage” of 7 ft to the 0.2-percent annual exceedance 

probability water-surface elevation (stage) of 18 ft, as referenced to the Suncook River streamgage (station 01089500). 

These 10 water-surface profile flood stages are 7, 9, 11, 12, 13, 

14, 15, 16, 17, and 18 feet. Discharges for profiles of all 10 

stages from the NWS “flood stage” of 7 ft to 18 ft, as referenced to the Suncook River streamgage (station 01089500), 

at locations downstream from the streamgage are presented in 

table 5. Discharges for the 1- and 0.2-percent annual exceedance probability flood profiles are presented in Flynn (2010).

Inundation Mapping

Flood-inundation maps were created in a geographic 

information system (GIS) by combining the water-surface 

profiles and digital-elevation (DEM) model data. The DEM 

data were derived from 1-meter (3.28-ft) horizontal resolution 

Light Detection and Ranging (LiDAR) data (available for the 

downstream half of the study reach) with a vertical accuracy 
6 Flood-Inundation Maps for the Suncook River in Epsom, Pembroke, Allenstown, and Chichester, New Hampshire

Table 4. Comparison of hydraulic-model output and surveyed high-water mark elevations from the flood of April 16–18, 2007, Suncook 

River, New Hampshire.

[NAVD 88, North American Vertical Datum of 1988; mi, mile; ft, feet]

River station 

(mi)

Latitude 

(N, decimal degrees)

Longitude 

(W, decimal degrees)

High-water mark 

elevation 

(ft, NAVD88)

Simulated watersurface elevation 

(ft, NAVD88)

Elevation 

difference 

(ft)

16.44

13.30

9.81

9.76

9.32

8.56

7.05

6.81

6.67

6.10

5.68

5.50

3.55

2.35

43.25667

43.22705

43.20235

43.20207

43.19566

43.18979

43.17599

43.17284

43.17179

43.16594

43.16063

43.15884

43.14331

43.1366

71.3700

71.35659

71.38183

71.383

71.38341

71.38192

71.39454

71.39644

71.39785

71.40455

71.40569

71.4063

71.42523

71.43085

344.37

337.08

309.22

308.06

307.5

305.53

302.81

302.03

301.71

300.82

299.22

296.54

292.41

291.04

344.5

337.4

309.29

308.31

307.37

305.45

302.74

302.24

301.74

301.26

300.59

295.42

292.44

291.57

0.13

0.32

0.07

0.25

-0.13

-0.08

-0.07

0.21

0.03

0.44

1.37

-1.12

0.03

0.53

of 0.001 meter (0.03 in), obtained from U.S. National Geospatial-Intelligence Agency (2011), in combination with 1- and 

4-ft contour-interval data created from aerial photogrammetry 

collected in May 2007 (Eastern Topographics, 2007) and in 

April 2008 (Eastern Topographics, 2008). The 1-ft contourinterval data (Eastern Topographics, 2007) were created for 

the reach of the Suncook River from U.S. Route 4 to approximately 0.25 mi downstream from the Short Falls Road bridge. 

These 1-ft contour-interval data extend 660 ft on either side of 

this reach. The 4-ft contour-interval data (Eastern Topographics, 2008) were created for the reach of the Suncook River 

from the Short Falls Road bridge downstream to the confluence with the Merrimack River and from the U.S. Route 4 

bridge upstream to the Suncook River streamgage. These 4-ft 

contour-interval data extend 1,000 ft on either side of the Suncook River. In flood-inundation areas beyond the extent of the 

available LiDAR and 1- and 4-ft contour-interval data, 10-ft 

contour interval USGS 15-minute quadrangle topographic 

Digital Raster Graphics (DRG) produced at a scale of 1:24,000 

and referenced to the horizontal datum of North American 

Datum of 1983 (NAD 83) and the vertical datum of National 

Geodetic Vertical Datum of 1929 (NGVD 29) (U.S. Geological Survey, 1967a-b, 1968) were used to delineate the extent 

of flooding. The horizontal accuracy of the DRGs is half of the 

contour interval (±5 ft). The base maps for the delineation of 

the flood-inundation areas were created using black and white 

aerial photographic imagery collected by Eastern Topographics (2007) in May 2007 (post-avulsion; Flynn, 2008) along the 

Suncook River and 1-ft-resolution color aerial photographic 

imagery of southeastern New Hampshire (New Hampshire 

Department of Transportation, 2006) collected for the New 

Hampshire Department of Transportation (NHDOT) in May 

2005 (pre-avulsion) for areas outside the extent of the Eastern 

Topographics (2007) imagery. 

Estimated flood-inundation boundaries for each simulated profile were developed with HEC–GeoRAS software 

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

set of procedures, tools, and utilities for processing geospatial 

data in ArcGIS by using a graphical user interface (Whitehead 

and Ostheimer, 2009). The interface allows the preparation 

of geometric data for import into HEC–RAS and processes 

simulation results exported from HEC–RAS (U.S. Army 

Corps of Engineers, 2010). The HEC–GeoRAS results were 

then 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). Based on least accurate data set of the 4-ft 

contour interval, the resulting inundation maps have a vertical 

accuracy of about 0.2 ft (2.4 in). 

The inundation maps depict estimates of the areal extent 

of flooding corresponding to selected water levels (stages) 

at the USGS Suncook River streamgage. The areal extent of 

flooding corresponding to selected stages at the streamgage 

was, in some areas, beyond the available LiDAR or 1- and 

4-ft contour-interval data. Therefore, the aerial extent of 

flooding in these locations was determined using USGS 

15-minute quadrangle topographic Digital Raster Graphics 

(DRG) maps. 
Inundation Mapping 

7

Table 5. Stream stages (elevations) with corresponding estimated discharges at selected locations on the Suncook River, New Hampshire, for 10 flood-inundation HEC–RAS water-surface profiles.[NAVD88, North American Vertical Datum of 1988; mi2, square miles; ft3/s, cubic feet per second]

Stage, in feet above gage datum 

(elevation, in feet, NAVD 88) 

Drainage 7.00 9.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00

Location area 

(335.78) (337.78) (339.78) (340.78) (341.78) (342.78) (343.78) (344.78) (345.78) (346.78) (mi2)

Discharge 

(ft3)

USGS Suncook River 154.4 1,240 1,960 3,380 4,600 5,900 7,300 9,000 10,600 12,800 15,300 Streamgage 01089500

At U.S. Route 4 161.2 1,290 2,040 3,520 4,790 6,140 7,600 9,370 11,040 13,330 15,930

Downstream of Little Sun- 200.9 1,590 2,510 4,330 5,890 7,560 9,350 11,530 13,580 16,390 19,600 cook River

At Short Falls Road 210.1 1,660 2,620 4,510 6,150 7,880 9,750 12,020 14,160 17,100 20,440

Upstream of Bear Brook 219.3 1,730 2,730 4,700 6,400 8,210 10,150 12,520 14,740 17,800 21,280

Downstream of Bear Brook 236.4 1,850 2,930 5,040 6,870 8,810 10,890 13,430 15,820 19,100 22,830

At U.S. Route 28 bridge (site 

of new USGS streamgage 240.1 1,880 2,970 5,120 6,970 8,930 11,050 13,630 16,050 19,380 23,17001089925)

At U.S. Route 3 bridge 255.2 1,990 3,140 5,420 7,380 9,460 11,710 14,430 17,000 20,530 24,540

At mouth of Suncook River 255.7 1,990 3,150 5,430 7,390 9,480 11,730 14,460 17,030 20,570 24,580
8 Flood-Inundation Maps for the Suncook River in Epsom, Pembroke, Allenstown, and Chichester, New Hampshire

Availability of Suncook River, New Hampshire, 

Flood-Inundation Maps 

A USGS Flood Inundation Mapping Science World Wide 

Web portal (http://water.usgs.gov/osw/flood_inundation/) has 

been established by the USGS to provide estimated floodinundation information to the public. These maps show the 

aerial extent of flooding, as well as inundation area depth of 

flooding, and are available in several commonly used electronic file formats that can be downloaded from that portal. 

Each stream reach displayed on the Web site contains links 

to National Water Information System graphs of the current 

stage and the streamflow at USGS Suncook River streamgage 

to which the inundation maps are referenced. A link also 

is provided to the NWS Advanced Hydrologic Prediction 

Service (AHPS) site (http:/water.weather.gov/ahps/) so that 

the user can obtain information on forecasted peak stage. The 

estimated flood-inundation maps are displayed in sufficient 

detail to note the extent of flooding with respect to individual 

structures so that preparations for flooding and decisions for 

emergency response can be performed efficiently.

Disclaimer for Flood-Inundation Maps 

Inundated areas shown should not be used for navigation, 

regulatory, permitting, or other legal purposes. The USGS 

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

planning tool but assumes no legal liability or responsibility 

resulting from the use of this information.

Uncertainty Associated with Inundation Maps

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

is associated with these maps. The flood boundaries shown 

were estimated based on water stages (water-surface elevations) and streamflows at USGS Suncook River streamgage 

01089500. Water-surface elevations along the stream reach 

were estimated by steady-state hydraulic modeling, assuming unobstructed flow, and using discharges and hydrologic 

conditions anticipated at USGS streamgage 01089500. The 

hydraulic model reflects the land-cover characteristics and any 

bridge, dam, levee, or other hydraulic structures existing as of 

July 2011. Since these flood-inundation maps were prepared, 

the Buck Street dams (Sheets 1–20) have been removed (October 2011). Flood-inundation areas and elevations on these 

maps in the vicinity of the dams may not now reflect current 

conditions. Unique meteorological factors (timing and distribution of precipitation) may cause actual discharges along the 

modeled reach to vary from those values assumed during a 

flood, which may lead to deviations in the water-surface elevations and inundation boundaries shown. Additional areas may 

be flooded due to unanticipated conditions such as changes 

in the streambed elevation or roughness, backwater into 

major tributaries along a main stem river, or backwater from 

localized debris or ice jams. The accuracy of the floodwater 

extent portrayed on these maps will vary with the accuracy of 

the digital elevation model used to simulate the land surface. 

Elevation data were generated from aerial photogrammetry 

and LiDAR, and as such, the below-water-surface elevations 

are approximations; therefore, flood-inundation map depths in 

the river channel may not be accurate. 

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

conjunction with National Weather Service (NWS) river forecasts, the user needs to 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 or 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 flood-inundation maps were developed using 

estimated flood profiles for a 16.5-mile reach of the Suncook River between its confluence with the Merrimack River 

and the USGS streamgage at Suncook River at Depot Road 

in North Chichester, N.H. The study was conducted by the 

USGS in cooperation with the New Hampshire Department 

of Homeland Security and Emergency Management. These 

maps, available at a USGS Web portal, in conjunction with the 

real-time stage data from the USGS streamgage at Suncook 

River (station 01089500) and National Weather Service floodstage forecasts, can help to guide the general public in taking 

individual safety precautions and provide city officials with 

a tool to efficiently manage emergency flood operations and 

flood-mitigation efforts. 

In this study, inundation maps were computed for the 

Suncook River study reach 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 model was 

calibrated using the most current stage-discharge relations at 

the Suncook River at North Chichester, N.H., streamgage and 

documented high-water marks from the flood of May 2007. 

The hydraulic model was then used to determine 10 watersurface profiles for flood stages at 7, 9, 11, 12, 13, 14, 15, 16, 

17, and 18 feet, referenced to the Suncook River streamgage 

datum and ranging from less than bankfull flow (the flow 

associated with the incipient elevation on the bank where 

flooding begins which is approximately the 50-percent annual 

exceedance probability) to the 0.2-percent annual exceedance 
References Cited 9

probability flood which is greater than the highest recorded 

water level at the streamgage. The water-surface elevations for 

the 1- and 0.2-percent annual exceedance probabilities (also 

referred to as the 100- and 500-year recurrence interval floods) 

are 15.34 ft and 18 ft, respectively, at the streamgage.

The simulated water-surface profiles were combined 

with a geographic information system digital elevation model 

(DEM) to create inundation area maps. The DEM was derived 

from Light Detection and Ranging (LiDAR) data having a 

0.03-ft vertical and less than 3.28-ft horizontal resolution in 

conjunction with 1-ft and 4-ft contour-interval aerial photogrammetric data having a ±0.2-ft vertical and ±0.2-ft horizontal accuracy (Eastern Topographics, 2007 and 2008) in order to 

delineate the flooded areas for each profile. Flood-inundation 

maps were developed showing the estimated flood-inundation 

areas overlaid on high-resolution, georeferenced, aerial photographs of the study area for the stream stages of 7 ft to 18 ft, 

local datum, at the Suncook River streamgage. The estimated 

inundation areas were shaded to give a general indication of 

depth of water at any point. In flood-inundation areas beyond 

the extent of the available LiDAR and 1-ft and 4-ft contourinterval data, 10-ft contour-interval USGS 15-minute quadrangle topographic Digital Raster Graphics (DRG) produced 

at a scale of 1:24,000 and referenced to the horizontal datum 

North American Datum of 1983 (NAD 83) and the vertical 

datum National Geodetic Vertical Datum of 1929 (NGVD 29) 

were used to delineate the extent of flooding. The horizontal 

accuracy of the Digital Raster Graphics is half of the contour 

interval (±5 ft). 

References Cited

Eastern Topographics, 2007, Photogrammetric ground control 

report for the Town of Epsom, N.H., Suncook River area, 

Epsom, N.H.: R10B/10343{3}-1, Imagery acquired on May 

12, 2007, September 13, 2007, NGVD 29, 1-foot contourinterval, scale 1:12,000.

Eastern Topographics, 2008, Photogrammetric ground 

control report for the Towns of Epsom and Allenstown, 

N.H., Suncook River Area, Epsom and Allenstown, N.H.: 

R10A/10573, Imagery acquired on April 18, 2008 and May 

27, 2008, NGVD 29, 4-ft. contour-interval, scale 1:12,000.

Flynn, R.H., 2008, Flood of April 2007 in New Hampshire: 

U.S. Geological Survey Scientific Investigations Report 

2008–5120, 48 p. (available online at http://pubs.usgs.gov/

sir/2008/5120/).

Flynn, R.H., 2010, Flood study of the Suncook River in 

Epsom, Pembroke, and Allenstown, New Hampshire, 2009: 

U.S. Geological Survey Scientific Investigations Report 

2010–5127, 64 p. (available online at http://pubs.usgs.gov/

sir/2010/5127/).

Leica Geosystems, 2008, Leica GPS1200, GPS1200 Surveying Systems, accessed August 28, 2010, at http://www.leicageosystems.com/en/Products-Leica-System-1200_4580.htm.

New Hampshire Department of Transportation, 2006, 2005 

1-ft Color Aerial Imagery-Southeast N.H., Imagery acquired 

at an elevation of approximately 9,800 ft. above mean sea 

level on May 11, 2005.

New Hampshire Employment Security, Economic and Labor 

Market Information Bureau, 2009a, Community profiles 

(Allenstown, N.H.), accessed January 12, 2010, at http://

www.nh.gov/nh es/elmi/htmlprofiles/pdfs/allenstown.pdf.

New Hampshire Employment Security, Economic and Labor 

Market Information Bureau, 2009b, Community profiles 

(Chichester, N.H.), accessed January 12, 2010, at http://

www.nh.gov/nh es/elmi/htmlprofiles/pdfs/chichester.pdf.

New Hampshire Employment Security, Economic and Labor 

Market Information Bureau, 2009c, Community profiles 

(Epsom, N.H.), accessed January 12, 2010, at http://www.

nh.gov/nh es/elmi/htmlprofiles/pdfs/epsom.pdf.

New Hampshire Employment Security, Economic and Labor 

Market Information Bureau, 2009d, Community profiles 

(Pembroke, N.H.), accessed January 12, 2010, at http://

www.nh.gov/nh es/elmi/htmlprofiles/pdfs/pembroke.pdf.

Olson, S. A., 2007, Flood of May 2006 in New Hampshire: 

U.S. Geological Survey Open-File Report 2007–1122, 32 p. 

(Available at http://pubs.water.usgs.gov/ofr2007-1122.)

U.S. Army Corps of Engineers, Hydrologic Engineering 

Center, 2009, HEC-GeoRAS, GIS Tools for Support of 

HEC–RAS using ArcGIS, User’s Manual, version 4.2 [variously paged]. 

U.S. Army Corps of Engineers, Hydrologic Engineering 

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

Reference Manual, version 4.1 [variously paged].

U.S. Department of Housing and Urban Development Federal 

Insurance Administration, 1978a, Flood-Insurance Study, 

Town of Allenstown, Merrimack County, New Hampshire: 

Washington, D.C., 16 p.

U.S. Department of Housing and Urban Development Federal 

Insurance Administration, 1978b, Flood-Insurance Study, 

Town of Chichester, Merrimack County, New Hampshire: 

Washington, D.C., 16 p.

U.S. Department of Housing and Urban Development Federal 

Insurance Administration, 1978c, Flood-Insurance Study, 

Town of Epsom, Merrimack County, New Hampshire: 

Washington, D.C., 19 p.
10 Flood-Inundation Maps for the Suncook River in Epsom, Pembroke, Allenstown, and Chichester, New Hampshire

U.S. Department of Housing and Urban Development Federal 

Insurance Administration, 1978d, Flood-Insurance Study, 

Town of Pembroke, Merrimack County, New Hampshire: 

Washington, D.C., 17 p.

U.S. Geological Survey, 1967a, Photoinspected 1974, Gossville, New Hampshire 7.5-minute series quadrangle map: 

U.S. Geological Survey Topographic Map, 10-ft contour 

interval, scale 1:24,000.

U.S. Geological Survey, 1967b, Photoinspected 1985, Suncook, New Hampshire 7.5-minute series quadrangle map: 

U.S. Geological Survey Topographic Map, 10-ft contour 

interval, scale 1:24,000.

U.S. Geological Survey, 1968, Photorevised 1985, Manchester 

North, New Hampshire 7.5-minute series quadrangle map: 

U.S. Geological Survey Topographic Map, 10-ft contour 

interval, scale 1:24,000.

U.S. National Geospatial-Intelligence Agency, 2011, Concord_LiDAR , accessed February 10, 2011 at https://www1.

nga.mil/Pages/Default.aspx.

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.
Publishing support provided by:

West Trenton Publishing Service Center

For more information concerning this publication, contact:

Director, New Hampshire Water Science Center

U.S. Geological Survey

331 Commerce Way

Pembroke, NJ 03275

Or visit the New Hampshire Water Science Center web site at:

http://nh.water.usgs.gov/
Flynn and others—Flood-Inundation Maps for the Suncook River in Epsom, Pembroke, Allenstown, and Chichester, New Hampshire—Scientific Investigations Map 3196