U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011–5088

Prepared in cooperation with the Federal Emergency Management Agency and 

the New Hampshire Department of Environmental Services

Analysis of the Transport of Sediment by the 

Suncook River in Epsom, Pembroke, and 

Allenstown, New Hampshire, after the 

May 2006 Flood
Cover. Photograph looking upstream at the Suncook River avulsion channel through the sand pit, Epsom, New Hampshire.

(Photograph taken October 9, 2008)
Analysis of the Transport of Sediment by 

the Suncook River in Epsom, Pembroke, 

and Allenstown, New Hampshire, after the 

May 2006 Flood

By Robert H. Flynn

Prepared in cooperation with the Federal Emergency Management Agency and 

the New Hampshire Department of Environmental Services

Scientific Investigations Report 2011–5088

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: 2011

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Flynn, R.H., 2011, Analysis of the transport of sediment by the Suncook River in Epsom, Pembroke, and Allenstown, 

New Hampshire, after the May 2006 flood: U.S. Geological Survey Scientific Investigations Report 2011–5088, 73 p. 

(Also available at http://pubs.usgs.gov/sir/2011/5088.)
iii

Acknowledgments

Support to generate the 1- and 4-foot contour-interval data used in the creation of the 

flood maps for this study was provided by Eastern Topographics through a New Hampshire 

Department of Safety-Bureau of Emergency Management Grant (Emergency Management 

Performance Grant) with in-kind cooperation provided by the New Hampshire Department of 

Environmental Services through its dam-maintenance efforts in the Suncook River watershed.

The author would like to express appreciation to the following U.S. Geological Survey (USGS) 

personnel who assisted with collection of data in this report—Thor Smith and Chandlee 

Keirstead for sediment and flow data collection, Rick Kiah for Acoustic Doppler Current Profiler 

(ADCP) data collection and postprocessing, James Degnan and Thor Smith for geophysical 

data collection in the former sand pit and Scott Olson, Heather Manzi, and Glenn Berwick for 

collection of field-surveyed elevation data. The author would also like to express appreciation to 

the USGS Kentucky Water Science Center Sediment Laboratory for the analyses of suspended-, 

bedload- and streambed-sediment samples.
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v

Contents

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

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

Purpose and Scope ..............................................................................................................................4

Previous Investigations........................................................................................................................4

Description of Study Reach ................................................................................................................5

Geohydrology of the Suncook River..................................................................................................7

Characterizing Sediment Transport and the River Streambed....................................................12

Channel Equilibrium...................................................................................................................13

Stream Temperature..................................................................................................................14

Bankfull Flow as Effective Discharge.....................................................................................15

Aggradation and Degradation .................................................................................................15

Methods of Analyses...................................................................................................................................15

Channel-Geometry Data ....................................................................................................................16

Contour Data........................................................................................................................................16

Streamflow and Temperature Data..................................................................................................16

Streambed and Sediment Sampling ................................................................................................16

Simulation of Flow and of Sediment Transport..............................................................................17

Initial- and Boundary-Condition Data.....................................................................................18

Hydrologic Data..........................................................................................................................18

Hydraulic Data............................................................................................................................18

Calibration of the HEC–RAS Hydraulic Model ...............................................................................18

HEC–RAS Sediment-Transport Function Selection.......................................................................19

Limitations of the HEC–RAS Model..................................................................................................19

Markov Chain Analyses of Flow.......................................................................................................20

Geophysical Data................................................................................................................................21

Channel Morphology and Stream Processes .........................................................................................21

Lateral Migration of Streambed .......................................................................................................21

Distribution of Sediment Grain Size.................................................................................................22

Bankfull Channel Dimensions...........................................................................................................25

HEC–RAS Sediment-Transport Model.............................................................................................25

Flood-Generated Shear Stress and the Potential for Entrainment....................................30

Model and Field-Data Sediment-Transport Curves..............................................................34

Simulating Sediment Transport to Investigate the Impact of Future Sediment 

Loads on Flood Flows ..................................................................................................40

Electrical Resistivity Data..................................................................................................................43

Sediment Load and its Effect on Streambed and Flood Elevations.....................................................43

Summary........................................................................................................................................................48

References Cited..........................................................................................................................................50

Appendix 1. Particle-Size Gradation and Median Diameter (d50) of Streambed Sediment 

Samples Collected at 30 Locations in the Suncook River .......................................................70
vi

Figures

1. Map showing extent of the sediment-transport study of the Suncook River in Epsom, 

Pembroke, and Allenstown, New Hampshire, 2009–10 .........................................................3

2. Preavulsion aerial imagery showing the new and abandoned channels resulting from 

the May 2006 Suncook River avulsion, Epsom, New Hampshire..........................................4

3. Map showing overview of the Suncook River study reach in Epsom, Pembroke, and 

Allenstown, New Hampshire ......................................................................................................6

4A–D. Aerial imagery showing locations and saturated thicknesses of four stratified-drift 

aquifers, location of sediment-transport-model cross sections, and locations of 

30 streambed-material collection sites along the Suncook River in Epsom, Pembroke, 

and Allenstown, New Hampshire ..............................................................................................8

5A–B. A, May 2007 and B, April 2008 aerial imagery of the avulsion and former sand pit 

showing the May 2007 channel location and geophysical-survey cross sections 

(lines 1–4) and model cross sections CR through DD (1:25,000).........................................23

6A–C. Graphs showing A, streambed-sediment grain-size distribution at six cross sections 

on the Suncook River upstream of the May 2006 avulsion, B, streambed-sediment 

grain-size distribution at ten cross sections on the Suncook River between the 

May 2006 avulsion and Short Falls Road Bridge, and C, streambed-sediment 

grain-size distribution at ten cross sections on the Suncook River downstream of the 

Short Falls Road Bridge .............................................................................................................24

7. Graph showing median sediment grain sizes (d50) for the Suncook River in Epsom, 

Pembroke, and Allenstown, New Hampshire ........................................................................25

8. Graphs showing A, bankfull-flow mean depth and B, bankfull-flow top width of the 

Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire...........................26

9. Graph showing Suncook River streambed thalweg elevation between the Buck 

Street Dams and the Short Falls Road Bridge as surveyed on June 11, 2009, and as 

model-predicted using the Laursen (Copeland) sediment-transport function.................32

10. Graph showing measured vs. simulated streambed thalweg-elevation changes for 

the Suncook River from the Buck Street Dams to the Short Falls Road Bridge, Epsom 

and Pembroke, New Hampshire, June 11, 2009 ....................................................................33

11A. Graph showing distribution of streambed shear stress in the Suncook River for the 

50-, 10- and 1-percent annual exceedence probability floods, and of critical 

streambed shear stress based on the d50 (cross sections A to BL) ...................................35

11B. Graph showing distribution of streambed shear stress in the Suncook River for the 

50-, 10- and 1-percent annual exceedence probability flood, and of critical 

streambed shear stress based on the d50 (cross sections BL to DW) ...............................35

12A–B. Scatterplots showing sediment-transport curve for Suncook River at A, Route 3 

bridge, and B, Route 28 bridge .................................................................................................38

12C–D. Scatterplots showing sediment-transport curve for Suncook River at C, Short Falls 

Road Bridge, and D, Route 4 bridge.........................................................................................39

13A–B. Graphs showing thalweg elevations of the Suncook River streambed from data published in May 2008 and in 1978 Flood Insurance Studies and from model predictions 

for end of water year 2010 in A, cross sections A to AI, and B, cross sections 

AJ to BT........................................................................................................................................41

13C–D. Graphs showing thalweg elevations of the Suncook River streambed from data published in May 2008 and in 1978 Flood Insurance Studies and from model predictions 

for end of water year 2010 in C, cross sections BU to DQ, and D, cross sections 

DR to DW......................................................................................................................................42
vii

Tables

1. Population information for the towns in the Suncook River study area..............................7

2. Bankfull-flow mean depth and top width of the Suncook River in Epsom, Pembroke, 

and Allenstown, New Hampshire ............................................................................................27

3. Field-collected total sediment loads and estimates of total sediment loads from 

sediment-transport curves........................................................................................................29

4. Streambed thalweg elevations for the Suncook River, New Hampshire, study reach, 

surveyed (2008, 2009) and modeled (2009) using the Hydrologic Engineering Center 

River Analysis System (HEC–RAS) during a 400-day calibration, May 8, 2008–

June 11, 2009 ...............................................................................................................................31

5. Daily mean shear stress at four total sediment-load collection locations, Suncook 

River, Epsom to Allenstown, New Hampshire, during May 2008–end of water 

year 2009 ......................................................................................................................................32

6. Distribution of streambed shear stress in the Suncook River for the 50-, 10- and 

1-percent annual exceedence probability flood ...................................................................36

7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-

predicted flood elevations for 2008 and the end of water year 2010 for the 50-, 10-, 

2-, 1-, and 0.2-percent annual exceedence probability floods and streambed 

elevations at each cross section .............................................................................................54

8. Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted changes 

in water-surface elevation and in thalweg elevation above and below the avulsion 

and between Short Falls Road and Buck Street Dams from 2008 through the end of 

water year 2010...........................................................................................................................43

9. Suncook River mean and median of the daily sediment discharges for water 

year 2009 using the Laursen (Copeland) function. ................................................................45

14A–B. Cross sections showing electrical resistivity geophysical survey transect for 

A, line 2, and B, line 4 .................................................................................................................44

15. Graph showing Suncook River mean and median of the daily sediment discharges 

for water year 2009 using the Laursen (Copeland) function................................................47
viii

Conversion Factors, Datum, and Abbreviations

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2)

Volume

cubic yard (yd3) 0.7646 cubic meter (m3)

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

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

mile per hour (mi/h) 1.609 kilometer per hour (km/h)

Hydraulic gradient

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

Pressure

pound per square foot (lb/ft2) 0.04788 kilopascal (kPa)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F–32)/1.8

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27) 

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

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29) and the North American Vertical Datum of 1988 (NAVD 88).
ix

Acronyms

ADCP acoustic doppler current profiler

ASCE American Society of Civil Engineers

CCO Consultation Coordination Officer

CDP census-designated place

FEMA Federal Emergency Management Agency

FIRM flood insurance rate map

FIS flood insurance study

GPS global positioning system

HEC–RAS Hydrologic Engineering Center–River Analysis System

NFIP National Flood Insurance Program

NHDES New Hampshire Department of Environmental Services

NHDOT New Hampshire Department of Transportation

NWIS National Water Information System

RMSE root mean square error

USACE U.S. Army Corps of Engineers

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey
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Analysis of the Transport of Sediment by the 

Suncook River in Epsom, Pembroke, and Allenstown, 

New Hampshire, after the May 2006 Flood

By Robert H. Flynn

Abstract

During May 13–16, 2006, rainfall in excess of 8.8 inches 

flooded central and southern New Hampshire. On May 15, 

2006, a breach in a bank of the Suncook River in Epsom, New 

Hampshire, caused the river to follow a new path. In order 

to assess and predict the effect of the sediment in, and the 

subsequent flooding on, the river and flood plain, a study by 

the U.S. Geological Survey (USGS) characterizing sediment 

transport in the Suncook River was undertaken in cooperation 

with the Federal Emergency Management Agency (FEMA) 

and the New Hampshire Department of Environmental 

Services (NHDES).

The U.S. Army Corps of Engineers (USACE) 

Hydrologic Engineering Center–River Analysis System 

(HEC–RAS) model was used to simulate flow and the 

transport of noncohesive sediments in the Suncook River 

from the upstream corporate limit of Epsom to the river’s 

confluence with the Merrimack River in the Village of 

Suncook (Allenstown and Pembroke, N.H.), a distance of 

approximately 16 miles. In addition to determining total 

sediment loads, analyses in this study reflect flooding 

potentials for selected recurrence intervals that are based 

on the Suncook River streamgage flow data (streamgage 

01089500) and on streambed elevations predicted by HEC–

RAS for the end of water year 2010 (September 30, 2010) in 

the communities of Epsom, Pembroke, and Allenstown.

This report presents changes in streambed and watersurface elevations predicted by the HEC–RAS model using 

data through the end of water year 2010 for the 50-, 10-, 2-, 1-, 

0.2-percent annual exceedence probabilities (2-, 10-, 50-, 100-, 

and 500-year recurrence-interval floods, respectively), calculated daily and annual total sediment loads, and a determination of aggrading and degrading stream reaches. The model 

was calibrated and evaluated for a 400-day span from May 8, 

2008 through June 11, 2009; these two dates coincided with 

field collection of stream cross-sectional elevation data. Seven 

sediment-transport functions were evaluated in the model 

with the Laursen (Copeland) sediment-transport function best 

describing the sediment load, transport behavior, and changes 

in streambed elevation for the specified spatial and temporal 

conditions of the 400-day calibration period. 

Simulation results from the model and field-collected 

sediment data indicate that, downstream of the avulsion 

channel, for the average daily mean flow during the study 

period, approximately 100 to 400 tons per day of sediment 

(varying with daily mean flow) was moving past the Short 

Falls Road Bridge over the Suncook River in Epsom, while 

approximately 0.05 to 0.5 tons per day of sediment was 

moving past the Route 28 bridge in Pembroke and Allenstown, 

and approximately 1 to 10 tons per day was moving past the 

Route 3 bridge in Pembroke and Allenstown. Changes in 

water-surface elevation that the model predicted for the end 

of water year 2010 to be a result of changes in streambed 

elevation ranged from a mean increase of 0.20 feet (ft) for 

the 50-percent annual exceedence-probability flood (2-year 

recurrence-interval flood) due to an average thalweg increase 

of 0.88 ft between the Short Falls Road Bridge and the Buck 

Street Dams in Pembroke and Allenstown to a mean decrease 

of 0.41 ft for the 50-percent annual exceedence-probability 

flood due to an average thalweg decrease of 0.49 ft above the 

avulsion in Epsom.

An analysis of shear stress (force created by a fluid acting 

on sediment particles) was undertaken to determine potential 

areas of erosion and deposition. Based on the median grain 

size (d50) and shear stress analysis, the study found that in general, for floods greater than the 50-percent annual exceedence 

probability flood, the shear stress in the streambed is greater 

than the critical shear stress in much of the river study reach. 

The result is an expectation of streambed-sediment movement 

and erosion even at high exceedence-probability events, pending although the stream ultimately attains equilibrium through 

stream-stabilization measures or the adjustment of the river 

over time. The potential for aggradation in the Suncook River 

is greatest in the reach downstream of the avulsion. Specifically, these reaches are (1) downstream of the former sand 

pit from adjacent to Round Pond to downstream of the flood 

chute at the large meander bends, and (2) downstream of the 

Short Falls Road Bridge to approximately 3,800 ft upstream of 

the Route 28 bridge. The potential for degradation—net lowering of the streambed—is greatest for the reach upstream of the 

avulsion to the Route 4 bridge.
2 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Introduction

The Suncook River drains a group of lakes to the 

south of Lake Winnipesaukee in New Hampshire and flows 

southwest for approximately 30 miles (mi) to the confluence 

with the Merrimack River at a location between Concord and 

Manchester, New Hampshire (fig. 1). During May 13–16, 

2006, significant flooding occurred in central and southern 

New Hampshire as a result of rainfall in excess of 8.8 inches 

(in.) (measured in Concord). On May 15, a breach in the bank 

of the Suncook River south of U.S. Route 4 and east of State 

Route 28 caused the Suncook River in Epsom, N.H., to follow 

a new path through an abandoned sand pit (fig. 2). Before 

this flood, the Suncook River flowed southward around Bear 

Island in two stream reaches that rejoined west of Round 

Pond. The breach resulted in a new channel upstream of 

Bear Island and two dams at the north end of the island. The 

new channel rejoins the old channel to the east of the island. 

After May 16, there was no flow in the former western and 

northeastern reaches of the Suncook River around Bear Island. 

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

the New Hampshire Department of Environmental Services 

(NHDES) and the Federal Emergency Management Agency 

(FEMA), undertook a study of the Suncook River 2006 flood. 

A sedimentation model was created to aid State and local 

authorities in predicting sediment levels and riverbed stability 

and movement under various flow scenarios, and equally to 

assist future restoration efforts by furthering the understanding 

of the effect of sediment transport on the environment by use 

of data from an established streamgage in the study area. 

The USGS streamgage 01089500, Suncook River at 

Depot Road in North Chichester, Merrimack County, New 

Hampshire (fig. 1) was active during 1919–20, 1922–27, 

and 1929–70 (only peak flow data were collected from 

1971 through 1977). On November 9, 2007, the Suncook 

River streamgage was reactivated. On the basis of surveyed 

high-water marks, the peak flow for May 13–16, 2006, was 

determined to be the third highest flood (7,600 cubic feet per 

second (ft3/s)) at the Suncook River streamgage (Olson, 2007). 

Less than a year after the 2006 breach, during April 16–18, 

2007, southeastern New Hampshire experienced severe flooding due to a storm that stalled off the New England coast. The 

peak flow of April 2007 (10,600 ft3/s) was the second highest flood recorded at the Suncook River streamgage (Flynn, 

2008). The highest flood of record at the Suncook River 

streamgage (12,900 ft3/s) occurred in March 1936.

The new channel through the abandoned sand pit has 

impacted the natural level of sediment induction into the 

river. Shortly after the breach, Chad Wittkop (New Hampshire 

Geological Survey, written commun., 2006) estimated that 

150,000 cubic yards (yd3) of sediment was introduced into the 

Suncook River as a result of the new channel path through 

the sand pit and adjacent wetlands. Following the May 2006 

breach, layers of silt and sand as much as 5 feet (ft) thick 

were deposited downstream of the new avulsion (abandonment of a river channel and formation of a new river channel), 

potentially changing designated flood zones and increasing 

sediment loads. The transported sediment decreased overall 

channel depth downstream of the sand pit with a mean aggradation (net raising of the streambed) of 0.88 ft between the 

Short Falls Road Bridge and the Buck Street Dams, leading to 

an increase in the elevation of the 50-, 10-, 2-, 1-, and 

0.2-percent annual exceedance-probability floods. In the avulsion area and upstream, downcutting of the stream continues 

(2011) to occur due to the river’s increased gradient as a result 

of the reduction in river length and the convergence of flow 

from two river reaches into one. As the gradient increases, a 

higher erosion rate of the river’s banks can be expected. 

Quasi-unsteady-state flow approximates a continuous 

hydrograph with a series of discrete steady flow profiles (for 

example, sequential steady-state) (USACE, 2008c). For each 

record in the flow series, flow remains constant over a specified time window for transport (for example, mean daily flow 

is assumed for 24 hours) and, as in the case of the sedimenttransport model, bathymetry is updated. Steady-state flow 

refers to the condition where fluid properties at a point in the 

channel do not change over time.

Following verification of the steady-state flow model and 

of a 400-day quasi-unsteady-state calibration model for sediment transport, the response of the sediment-transport model 

was simulated from May 2008 (date of acquisition of channel 

cross-section data) through the end of water year (WY) 2010 

(September 30, 2010). Based on Markov chain analyses, daily 

mean flow and water-temperature data at the Suncook River 

streamgage for WY 2009 were selected to represent daily 

mean flow and water temperature for WY 2010 in the model. 

The resulting quasi-unsteady-state streambed elevations, 

modeled through the end of WY 2010 to determine the effects 

of sediment aggradation and degradation on the streambed, 

were then imported into a steady-state HEC–RAS model to 

estimate changes to the 50-, 10-, 2- 1-, and 0.2-percent annual 

exceedence-probability (2-, 10-, 50-, 100-, and 500-year 

recurrence-interval, respectively) flood elevations as reported 

by Flynn (2010).

An understanding of sediment-transport characteristics 

is important for applications that include flow regime change, 

channel-restoration efforts, and predictions of the effects of 

land-use changes (Wilcock, 2001). In order to establish river 

equilibrium for river-corridor protection and restoration, 

knowledge of the relation between flow and sediment transport and an assessment of the ability of the existing channel to 

transport sediment is critical. The measurement of sediment 

load and particle size moving through a reach at various flows 

for development of sediment-discharge curves (curves showing the relation between streamflow and sediment discharge 

for a specific sediment-sampling site) is important for rivercorridor protection and restoration design. Once the conditions 

required for sediment transport are understood, a determination of the channel dimension, pattern, and profile required 

for sufficient transport of the expected sediment supply can be 

made (Vermont Agency of Natural Resources, 2009).
Introduction 3

Figure 1. Extent of the sediment-transport study of the Suncook River in Epsom, Pembroke, and Allenstown, New 

Hampshire, 2009–10. 

10 20 KILOMETERS 

10 20 MILES

ROCKINGHAM

STRAFFORD

BELKNAP

GRAFTON

SULLIVAN

CHESHIRE

MERRIMACK

HILLSBOROUGH

EXPLANATION

Base from U.S. Geological Survey digital line graphs

71°

44°

43°

45°

Concord

Manchester

Maine

Massachusetts

Vermont

CANADA

72°

New

Hampshire

Manchester

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

MASSACHUSETTS

VERMONT

0

0

72 ̊ 71 ̊

43 ̊

43 ̊30”

Merrimack River

Soucook River

Suncook River

Merrimack River

Suncook River study area

NEW HAMPSHIRE

NEW HAMPSHIRE

MAINE

Winnipesaukee

Lake

Chichester

Village of

Suncook

Endpoint of study reach

U.S. Geological Survey Suncook River

streamgage, at North Chichester, N.H.

(01089500)
4 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Purpose and Scope

This report describes the results of the study to assess 

sediment aggradation and degradation in the Suncook River; 

to quantify sediment loads for calibration of the sedimenttransport components of deposition and erosion, and of 

deposition in a quasi-unsteady-state sediment-transport HEC–

RAS model; to study the effect of the stream avulsion on 

sediment transport; and to predict the impact of sediment on 

selected recurrence-interval floods in the Suncook River from 

its confluence with the Merrimack River in Allenstown to the 

upstream limit of Epsom, N.H. The report also describes the 

collection of suspended-, bedload-, and streambed-sediment 

samples to characterize sediment sources and transport under 

varied flow conditions and to provide information for determining the impact of sediment on property, infrastructure, 

flood elevations, and aquatic life and habitat.

This report documents the development and calibration 

of a one-dimensional sediment-transport model based on 

the Hydrologic Engineering Center–River Analysis System 

(HEC–RAS), version 4.0.0 (USACE, 2008a), which was 

used for a 15.9-mile (mi) reach of the Suncook River from 

its confluence with the Merrimack River to the upstream 

corporate limit of Epsom, N.H. This reach includes the 

location of the May 2006 avulsion. The steady-state HEC–

RAS flow model that was used to develop a quasi-unsteadystate sediment-transport model to calibrate stream-channel 

cross sections and April 2007 high-water floodmark data 

(Flynn, 2010). The one-dimensional quasi-unsteady-state 

HEC–RAS sediment-transport model used in this study 

simulates a moveable streambed, calculates sediment 

transport, and averages sediment deposition or aggradation 

across all of the cross sections in the study reach. 

Previous Investigations

Previous hydrologic and geomorphic studies of the 

Suncook River include a stream-restoration study for the 

Suncook River (Vanasse Hangen Brustlin (VHB) Inc., and 

others, 2008); a characterization of sediment wave-induced 

channel evolution in the river (Perignon, 2008); and a 2010 

flood study of the reach (Flynn, 2010). Using HEC–RAS, 

Flynn (2010) modeled flood events of a magnitude that are 

Figure 2. Preavulsion aerial imagery showing the new and abandoned channels resulting from the 

May 2006 Suncook River avulsion, Epsom, New Hampshire.

Base map from New Hampshire Department of Transportation,

2006, 2005 1-foot color aerial photos, southeastern New Hampshire

Round Pond

 Bear 

Island

71°22'30" 71°22' 71°21'30" 71°21'

43°13'30"

43°13'

43°12'30"

S uncook River

EXPLANATION

Abandoned, preavulsion channel

New, postavulsion channel

28

4

Suncook Rive r

250 500 METERS

2,000 FEET

0

0 1,000

Little Su ncookRi ver

Leighton Brook

Production well
Introduction 5

expected to be equaled or exceeded once in the Suncook 

River (on average) during any 2-, 5-,10-, 25-, 50-, 100-, or 

500-year period. These flood events, also referred to as the 

2-, 5-,10-, 25-, 50-, 100-, and 500-year recurrence-interval 

floods, have 50-, 20-, 10-, 4-, 2-, 1-, and 0.2-percent 

probabilities, respectively, of being equaled or exceeded 

during any year. Previous flood studies were completed for 

the towns of Epsom, Pembroke, and Allenstown. A Flood 

Insurance Study (FIS) (January 1978) and a Flood Insurance 

Rate Map (FIRM) (July 3, 1978) were prepared for the 

town of Epsom (U.S. Department of Housing and Urban 

Development, Federal Insurance Administration, 1978b). FISs 

(October 1978) and FIRMs (April 2, 1979) were also prepared 

for the towns of Pembroke and Allenstown (U.S. Department 

of Housing and Urban Development, Federal Insurance 

Administration, 1978a and c).

Description of Study Reach

Originating south of Lake Winnipesaukee in Gilford, 

N.H., the Suncook River flows through the western portions 

of the towns of Epsom and Allenstown. State Route 28 

(fig. 3B) runs parallel to the Suncook River and connects the 

towns of Chichester, Epsom, Pembroke, and Allenstown. The 

Suncook River is approximately 39 mi long and flows in a 

southerly direction with a drainage area of 154 square miles 

(mi2) at the USGS Suncook River streamgage 1089500 in 

North Chichester. At its confluence with the Merrimack River 

in the village of Suncook (in Allenstown and Pembroke), the 

Suncook River has a total drainage area of 256 mi2. 

The prevailing winds in Epsom, Pembroke, and 

Allenstown are from the northwest and result in cold, dry air 

in the winter and cool, dry air in the summer. Strong winds 

from the south occur in July and August, and winds from 

the east typically coincide with summer and winter storms 

(National Climate Data Center, 2008). Precipitation falls on 

an average of 1 day out of 3 during the year; the frequency 

is slightly higher for April and May and slightly lower for 

August to October. Snow cover typically lasts from midDecember until the last week of March, although bare ground 

is not a rarity in the winter. Rain, sleet, or freezing rain may 

also occur during the winter (National Climate Data 

Center, 2008). 

The average daily maximum and minimum temperatures 

for July in Concord, N.H., are 82.2 and 57.2°F, respectively. 

The average daily maximum and minimum temperatures for 

January in Concord are 31.1 and 10.7°F, respectively. Average 

annual precipitation in Concord for 1979–2008 (rain, snow, 

and sleet) is 37.9 in. Average annual snowfall in Concord for 

1979–2008 is 63.8 in. (National Climate Data Center, 2008). 

The topography of Epsom, Pembroke, and Allenstown 

is hilly and characterized by stratified and unstratified material transported and deposited by retreating glacial ice. The 

flood plains adjacent to the rivers are composed of alluvial 

silt overlying glacial outwash. The predominant soil group 

is composed primarily of well-drained sandy loam in glacial 

till. Kames, terraces, deltas, and outwash plains are glacial 

landforms common to this area. Elevations range from 192 ft 

(National Geodetic Vertical Datum of 1929) (NGVD 29)) at 

the confluence of the Suncook and Merrimack Rivers to 

1,413 ft (NGVD 29) at the summit of Fort Mountain in 

Epsom. Wetland areas throughout the three towns serve as the 

headwaters for many smaller streams.

In general, the flood plains along the Suncook and Little 

Suncook Rivers (fig. 2) are sparsely developed with the exception of areas near the intersection of the Suncook River and 

U.S. Route 4, the intersection of the Suncook River and U.S. 

Route 3, downstream through the village of Suncook, and near 

Epsom along the Little Suncook River. The Suncook River 

study area contains a mixed landuse of forested, agricultural 

and residential. Analysis of aerial photography from 1953 

to 2003 showed an increase in the forested area surrounding 

the Suncook River (VHB, 2008). The population of the four 

towns in the study area increased between 2000 and 2009 with 

the greatest percent increase in population occurring in the 

northern study-area towns of Chichester and Epsom, where the 

population is smaller (table 1).

The Suncook River has no major flood-control structures. 

The Pittsfield Mill Dams in Pittsfield, N.H., Buck Street Dams 

in Pembroke, N.H., and Webster, Pembroke, and China Mill 

Dams in the village of Suncook are all run-of-river dams and 

do not act as flood-control dams. The Webster, Pembroke, and 

China Mill Dams are used for hydroelectric-power generation. 

These dams do little to attenuate major flood peaks.

The new channel through the former sand pit created 

by the breach of May 15, 2006, is approximately 0.42 mi 

in length. The abandoned primary (eastern) channel is 

approximately 0.87 mi in length, and the abandoned western 

channel is approximately 1.14 mi in length, giving a total 

length for the abandoned channel (fig. 2) of 2.01 mi. The new 

channel from north of the Suncook River’s former confluence 

with the abandoned channel (upstream of the avulsion), 

through the former sand pit, south into the east channel, and 

to the downstream location of the former confluence with 

the abandoned west channel (southern end of Bear Island) 

is 1.02 mi (5,385 ft) in length. As a result of the reduction in 

total stream length from 2.01 to 1.02 mi, the river’s average 

gradient has increased by approximately 40 percent between 

the upstream and the downstream locations of the abandoned 

channels. Based on 2008 thalwegs (deepest location in the 

channel) of the cross-section surveys (cross sections CM to 

DE) (fig. 4C), the streambed gradient is 14.0 feet per mile 

(ft/mi), whereas the average gradient of the abandoned east 

and west channels between these same locations was 

10.0 ft/mi before the avulsion (U.S. Department of Housing 

and Urban Development, Federal Insurance Administration, 

1978b). Because the channel is no longer split around Bear 

Island and is now shorter and steeper, the average velocity of 

the river has increased, which in turn will increase the river’s 

capability to erode both vertically and laterally.
6 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Figure 3. Overview of the Suncook River study reach in Epsom, Pembroke, and Allenstown, New Hampshire.

Merrimack River

Soucook River

cnuS oo k R vi

e r

Limit of study

Limit of study

Route 28

bridge

Short Falls

Road

Bridge

Round Pond

Route 4 bridge

Bear Island

Route 3 bridge

Buck Street

Dams

4

28

3

4

28

3

93

93

U.S. Geological Survey

Suncook River streamgage,

at North Chichester, N.H.

(01089500)

Figure 4A

Pembroke

Allenstown

Chichester

Epsom

Figure 4B

 Figure 4C

Figure 4D

Short Falls Road
Introduction 7

Geohydrology of the Suncook River

The study reach of the Suncook River has numerous 

oxbows in the flood plain that are remnants of the river’s postglacial meanderings. Located in the Upper Merrimack River 

Basin, the study area is underlain by bedrock associated with 

the Central New Hampshire Anticlinorium (Lyons and others, 

1986), a type of geological fold that is convex upward (anticline) with minor folds (anticline and syncline) superimposed. 

The anticlinorium trends in a north-northeast to south-southwest direction and contains metamorphic rocks of Devonian 

and Silurian age, including gneiss, schist, and quartzite. These 

metamorphic rocks were intruded by granite, granodiorite, 

syenite, and monzonite of Devonian age (Lyons and others, 

1986). A fault line (Pinnacle Fault) extends northeast to southwest through Pittsfield and into Suncook and trends southwest 

into Massachusetts (Stekl and Flanagan, 1997). 

The Suncook River flows through an area of fine- to 

coarse-grained stratified-drift deposits (Stekl and Flanagan, 

1997). Four stratified-drift aquifers were identified along the 

study reach—Upper Suncook River Aquifer, Suncook and 

Round Pond Aquifer, Bear Brook Aquifer, and the Lower 

Suncook River Aquifer (Stekl and Flanagan, 1997). These 

aquifers are potentially significant sources of groundwater 

based on the following criteria: generally composed of 

wellsorted sand and gravel; saturated thickness (volume of 

aquifer in which pore spaces are completely filled with water) 

of sand and gravel equal to or greater than 40 ft; and surface 

areas equal to or greater than 0.2 mi2. 

The Upper Suncook River Aquifer (fig. 4D), with 

a surface area of approximately 1.1 mi2, lies primarily 

in the northwestern part of Epsom. It is composed of a 

heterogeneous mix of coarse sand and gravel deposits that 

are underlain and intermixed with fine-grained lake-bottom 

deposits. Discontinuous beds of clay and silt are present 

throughout the aquifer, and the streambanks contain a 

highly varied stratigraphy of alluvium, lake-bottom clay, 

and stratified-drift deposits. The saturated thickness of the 

stratified drift is approximately 40 ft throughout much of 

the aquifer and approximately 60 ft in the center (Stekl and 

Flanagan, 1997). 

On the upstream opening of the Route 4 bridge over 

the Suncook River on the northeast overbank of the western, 

primary channel, a lithologic log of well ESW10 (unnamed 

owner) (figs. 4C and D), which is at longitude 71°21′25′′ W 

and latitude 43°13′37′′ N, indicates bedrock at altitude 305 ft 

(+5 ft, NGVD 29). According to the well log, the material into 

which the well was placed in 1990 is predominantly fine to 

coarse sand (Stekl and Flanagan, 1997). Auger-drilling refusal 

was found at 35 ft below the land surface (Stekl and Flanagan, 

1997), possibly indicating the top of a consolidated layer. 

Cross-section surveys (Flynn, 2010) determined that the altitude of the thalweg of the Suncook River at the approximate 

location of well ESW10 is 329.8 ft (NGVD 29), indicating a 

sand layer of approximate thickness 24.8 ft (+5 ft) beneath the 

channel at this location. A headcut has formed on the Suncook 

River (fig. 4C, approximate location at cross section DG) as 

a consequence of the avulsion that created a new shorter but 

steeper channel downstream of the avulsion. A headcut is an 

abrupt downward change in the elevation of the streambed. 

Water flowing over the headcut causes erosion on the face of 

the overfall area, gradually moving the headcut upstream. 

The Suncook River and Round Pond Aquifer (fig. 4C), 

which has a surface area of approximately 1.5 mi2, lies south 

of the Upper Suncook River Aquifer, trending northeast to 

southwest, beginning east of the southern tip of Bear Island 

and extending 1.3 mi to the southwest. This aquifer is bounded 

to the north and south by relatively impermeable very fine 

sand, silt, and clay lake-bottom deposits. A deep bedrock 

trough extending south of Bear Island contains a layer of as 

much as 65 ft of medium to coarse sand and gravel deposits 

overlying a thick layer of fine sand and silt deposits, with finegrained deposits typically found outside of the channel. The 

saturated thickness of the material in the bedrock trough is 

greater than 120 ft in places (Stekl and Flanagan, 1997). 

Near the former sand pit, a study of stratified-drift 

aquifers in the upper Merrimack River Basin (Stekl and 

Flanagan, 1997) included a lithologic log of well ESW6 

(fig. 4C), which is located at longitude 71°21′48′′ W and 

latitude 43°12′58′′ N on the eastern overbank of the Suncook 

River. According to the well log, the material into which the 

well was placed in 1989 is predominantly fine sand with some 

Table 1. Population information for the towns in the Suncook River study area.

[mi2, square miles; Data from New Hampshire Employment Security, Economic and Labor Market Information Bureau]

Town Land area 

(mi2)

Inland water area 

(mi2)

Population 

(2009)

Population per square 

mile of land area

Percent in population increase 

(2000–2009)

Allenstown 20.5 0.1 4,957 247 2

Chichester 21.2 0.1 2,583 120 14

Epsom 34.5 0.1 4,609 135 14

Pembroke 22.6 0.2 7,344 325 6
8 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Figures 4A–D. Aerial imagery showing locations and saturated thicknesses of four stratified-drift aquifers (Stekl and others, 1997), location of sediment-transport-model cross sections, and locations of 30 streambed-material collection sites along the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire.

Limit of study

Limit of detailed study

Limit of detailed study

Limit of detailed study

AI AJ

A

AH

B

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S

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AD

AG

C R

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Y T

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AQAP AT

AS

80

60

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Lower

Suncook River

Aquifer SRM-360 SRM-350

SRM-340

SRM-330

SRM-320

SRM-310

SRM-300

SRM-280

71°27' 71°26'

43°09'30"43°08'30"

43°07'30"

Route 3

Route 28

bridge

71°26' 71°25' 71°24'

EXPLANATION

Saturated thickness of stratified-drift aquifer, in feetCross section where bed-sediment data were collectedStreambed-material collection site and number

SRM-320

Saturated thickness of stratified-drift aquifers

mapped by Stekl and Flanagan, 1997

Base map is from New Hampshire Department of Transportation,2006, 2005 1-foot color aerial imagery—southeastern N.H., imageryacquired at an elevation of approximately 9,800 feet above meansea level on May 11, 2005

250 500 METERS

2,000 FEET 0

0

1,000

Buck Street

Dams

China Mill

Dam

Main Street

Webster Dam

Pembroke Dam

A
Introduction 

9

Figures 4A–D. Aerial imagery showing locations and saturated thicknesses of four stratified-drift aquifers (Stekl and others, 1997), location of sediment-transport-model cross sections, and locations of 30 streambed-material collection sites along the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire.—Continued

20 40

20

60

40

40 60

40 20 60

20

EXPLANATION

SRM-250

SRM-280

SRM-270

SRM-260

SRM-250

SRM-240

Bear Brook Aquifer

Limit of detailed study

mi Li t of detailed study

Route 28

bridge

43°09'30"

43°10'30"

43°11'30"

71°25' 71°24' 71°23' 71°22'

Saturated thickness of stratified-drift aquifer, in feetCross section where bed-sediment data were collectedStreambed-material collection site and number

BI

BG

BR

AY

BQ

AW

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BF

BD

BH

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AX

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BJ

AV

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AR AU AQ

AP AT

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BI

BG

BR

AY

BQ

AW

BC

BA

BF

BD

BH

BP

AX

BB

BJ

AV

BE

BL

BM

BT

BK

BS

BN

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AN

AZ

AR AU AQ

AP AT

AS

Buck Street Dams

250 500 METERS

2,000 FEET 0

0

1,000

Base map is from New Hampshire Department of Transportation,2006, 2005 1-foot color aerial imagery—southeastern N.H., imageryacquired at an elevation of approximately 9,800 feet above meansea level on May 11, 2005

Saturated thickness of stratified-drift aquifers

mapped by Stekl and Flanagan, 1997

B
10 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

100

80

60

40

20

120

40

20

20

43°13'30"43°12'30"43°11'30"

71°24' 71°23' 71°22' 71°21'

Round Pond

Short Falls Road

Bear

Island

State Route 28

Route 4

ed study il ta e d of t mi Li

Saturated thickness of stratified-drift aquifers

mapped by Stekl and Flanagan, 1997

CF CI CE

CD

CG

CH

CC

CB

DI

BR

DJ

CA

DN

BV

DL

BW

BQ

BZ

BP

BX

CK

BT

CZ

BU

BS

CL

DE

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DF

DC

CW CS

CQ

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CR

DH

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CN

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CM

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CF CI CE

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CH

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Base map is from New Hampshire Department of Transportation,2006, 2005 1-foot color aerial imagery—southeastern N.H., imageryacquired at an elevation of approximately 9,800 feet above meansea level on May 11, 2005

SRM-240

SRM-238

SRM-232 *

SRM-234

SRM-230

SRM-220

SRM-210

SRM-200

SRM-140

SRM-130 ESW 6

SRM-110

SRM-120

SRM-100

SRM-70

SRM-80

ESW 10

Suncook River and Round Pond Aquifer

EXPLANATION

SRM-320

Well and number ESW 6

Saturated thickness of stratified-drift aquifer, in feetCross section where bed-sediment data were collectedStreambed-material collection site and number

250 500 METERS

2,000 FEET 0

0

1,000

Figures 4A–D. Aerial imagery showing locations and saturated thicknesses of four stratified-drift aquifers (Stekl and others, 1997), location of sediment-transport-model cross sections, and locations of 30 streambed-material collection sites along the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire.—Continued
Introduction 11 Figures 4A–D. Aerial imagery showing locations and saturated thicknesses of four stratified-drift aquifers (Stekl and others, 1997), location of sediment-transport-model cross sections, and locations of 30 streambed-material collection sites along the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire.—Continued

20

60

40 20

D

Base map is from New Hampshire Department of Transportation,2006, 2005 1-foot color aerial imagery—southeastern N.H., imageryacquired at an elevation of approximately 9,800 feet above meansea level on May 11, 2005

DW

DV

DU

DT

DS

DR

DQ

DO

DN

DM

DL

DK DJ

DI DH

DG

DW

DV

DU

DT

DS

DR

DQ

DO

DN

DM

DL

DK DJ

DI DH

DG 43°13'30"

43°14'30"43°15'30"

71°24' 71°23' 71°22' 71°21'

Saturated thickness of stratified-drift aquifers

mapped by Stekl and Flanagan, 1997

Upper Suncook River Aquifer

ESW 10

Route 4

Limit of study

State Route 28

SRM-1

SRM-10

SRM-15

SRM-70

SRM-80

EXPLANATION

SRM-10

Well and number ESW 10

Saturated thickness of stratified-drift aquifer, in feetCross section where bed-sediment data were collectedStreambed-material collection site and number

250 500 METERS

2,000 FEET 0

0

1,000
12 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

coarse material (sand and gravel) between 7 and 15 ft below 

the top of the well. Refusal was found at 39 ft below the land 

surface (Stekl and Flanagan, 1997), possibly indicating the top 

of a consolidated layer. A geohydrologic section interpreted 

from seismic-refraction data as extending approximately 

530 ft northwest from well ESW6 indicates bedrock at 

265 ft (+5 ft, NGVD 29) at well ESW6, with a bedrock 

elevation of 285 ft (+5 ft, NGVD 29) 530 ft northwest of well 

ESW6. Cross-section surveys (Flynn, 2010) determined that 

the thalweg of the Suncook River, at the approximate location 

of ESW6, is at an altitude of approximately 301.5 ft (NGVD 

29), indicating a sand layer extending approximately 36.5 ft 

(+5 ft) beneath the channel at this location.

The Bear Brook Aquifer (fig. 4B), which has a surface 

area of approximately 1.7 mi2, lies south of the Suncook River 

and Round Pond Aquifer. It is at the confluence of Bear Brook 

and the Suncook River and trends northeast to southwest. The 

sediments in the bedrock channel are primarily coarse sand 

and gravel deposits. The stratified-drift sediments are predominantly sand, silt, and clay lake-bottom deposits. Saturated 

thickness of the stratified drift is 65 ft at the Suncook River, 

with a maximum of 81 ft and a minimum of 19 ft east of the 

Suncook River (Stekl and Flanagan, 1997).

The Lower Suncook River Aquifer (fig. 4A), which has 

a surface area of approximately 0.9 mi2, lies south of the Bear 

Brook Aquifer. It is in east Pembroke and trends east-northeast 

to west-southwest. The aquifer contains coarse-grained deltaic 

deposits overlying fine-grained lake-bottom deposits. Near 

Route 28 east of the Suncook River, the aquifer consists 

primarily of coarse sand and gravel deposits with a saturated 

thickness of less than 20 ft. The average saturated thickness of 

the aquifer is 33 ft (Stekl and Flanagan, 1997).

Characterizing Sediment Transport and the 

River Streambed

Sediment transport is defined as the conveyance of sediment by flowing water that is initiated when the threshold for 

movement has been exceeded. Although sediment transport 

is closely tied to known factors, including water flow, energy, 

and sediment source, understanding the movement of river 

sediment can be difficult because sediment transport has a 

transient nature. However, with a proper sampling and assessment program, sediment transport can be quantified. Sediment 

transport may vary significantly from the initial flush of a 

storm to the time of peak flow in the watershed with respect to 

total sediment load (suspended load plus bedload) and distribution of particle size. 

Sediment in streams can adversely affect water quality and aquatic life and habitats, and it may present a hazard 

to downstream infrastructure and residents. Suspended fine 

sediments reduce the clarity of the water and possibly also biologic productivity, but the suspended sediments can also carry 

nutrients that may increase biological productivity. Coarse 

suspended sediments can bury stream habitats and can diminish the overall abundance and diversity of species.

A catastrophic event, such as the avulsion that occurred 

on the Suncook River in May 2006, significantly changes the 

slope and shape of the sediment-discharge curve. This curve 

can be used to estimate sediment concentration when data on 

water discharge, but not on sediment, are available. Because 

the Suncook River is still adjusting to attain equilibrium, the 

sediment-transport curve will continue to change. Some of 

the other parameters that can affect the slope and shape of 

sediment-discharge curves are seasons, timing of sediment 

peaks versus flow-discharge peaks, and extreme high-water or 

sediment-load events (Glysson, 1987). Although this relation 

is referred to as a sediment-discharge curve, the phrase is not 

descriptive, as it implies a cause-and-effect relation between 

discharge and sediment and a specific value of sediment 

concentration for each discrete value of streamflow (Glysson, 

1987). Colby (1964) states that “at a cross section of a stream, 

the sediment discharge may be considered to depend on depth, 

width, velocity, energy gradient, temperature, and turbulence 

of the flowing water; on size, density, shape, and cohesiveness of particles in the banks and beds at the cross-section and 

upstream channels; and on the geology, meteorology, topography, soils, subsoils, and vegetal cover of the drainage area.” 

In alluvial streams, such as the Suncook River, sedimentdischarge curves typically exhibit a wide scattering of the data, 

a circumstance that should not be interpreted simply as sample 

error. The range in scatter between high and low data is more 

accurately attributed to aggradation, degradation, changing 

bedforms in the stream channel, or sediment-transport rates 

affected by variation in water temperature (Thomas, 1977). In 

addition, for a given flow rate on either side of the flood peak, 

the quantity of suspended material over the course of a flood 

peak completes a hysteresis loop, being considerably larger 

at the rising water stage is than at the falling stage (American 

Society of Civil Engineers (ASCE, 2008)).

Hydrologic, geologic, geographic, and biologic factors affect the sediment load of a stream. Season, snowmelt, 

duration and intensity of rainstorms, watershed use, vegetation cover, watershed field slope, soil types, and human and 

animal activities further determine the amount of sediment 

entering and transported by the stream (ASCE, 2008). Seasons 

can have a significant effect on the natural level of sediment 

induction into a stream. During the winter, the ground may 

be frozen, and precipitation may be in the form of snow. The 

absence of the impact of raindrops to loosen the soil and frozen ground holding the soil together are factors in transporting 

less sediment. During the summer, the impact of raindrops 

from high-intensity storms results in high concentrations and 

transport loads of sediment. Areas along the river that are used 

for agricultural purpose can further contribute to sediment 

loads. The fields are bare in winter and spring but, as the crops 

emerge, plant growth protects the soil from erosion. In these 

cases, sediment concentrations and loads for a given discharge 

may be low in the winter (frozen ground) and in summer 
Introduction 13

(rooted crops) but high in the spring (before planting and 

growth of crops) and in fall (after harvest) (Glysson, 1987).

The driving force for sediment transport is the shear 

stress (τ 0, eqn. 1) exerted on the channel boundary by flowing 

water, which is a measure of the stream’s ability to entrain bed 

material. The mean boundary shear stress in a channel can be 

expressed as

τ0 = γDS, (1)

where

τ0 is the mean boundary shear stress, in pounds 

per square foot;

γ is the specific weight of water, in pounds per 

cubic foot;

D is the mean flow depth, in feet; and

S is the energy gradient or slope of the water 

surface, in foot per foot.

Boundary shear stress (τ0) is dependent on flow depth, 

which is influenced by river discharge. In alluvial rivers, 

deposition (which occurs where shear stress is decreasing) and 

erosion (which occurs where shear stress is increasing) follow 

shear stress. Whether aggradation or deposition occurs also 

depends on the concentration of particles in suspension in the 

reach (Berenbrock and others, 2005).

Sediment movement (entrainment) in a stream is in part 

a function of the boundary shear stress (τ0) that is created by 

flowing water acting on sediment particles. The entrainment 

potential is estimated from the relation between a floodgenerated shear stress and the critical shear stress (τc

), that is, 

the shear stress at which sediment particles begin to move. 

Critical shear stress has been related to characteristics of 

sediment size. Many studies have used the Shields (1936) 

equation (eqn. 2) to estimate the critical shear stress for 

entrainment of the median grain size of sediment (referred to 

as the d50):

τc = τ*

c

 (γs – γ) d50, (2)

where

τc is the critical shear stress, in pounds per 

square foot;

τ*

c is the dimensionless critical shear stress or 

Shields parameter;

γs is the specific weight of sediment (assumed to 

be 2.65 times the specific weight of water);

γ is the specific weight of water; and

d50 is the median sediment particle size.

The mean boundary shear stress is related to the flow 

depth and water-surface slope, and so it also varies with 

increasing or decreasing discharge. In comparing the boundary 

shear stress (τ0) for a particular discharge with critical shear 

stress (τc), it is possible to evaluate the sediment entrainment 

potential for that discharge with respect to the streambed. 

Streamflows that generate a boundary shear in excess of the 

critical shear will initiate the movement of bed material. Over 

a period of time, and if the excess shear stress is great enough, 

it will result in bedload transport and, possibly, in channel 

adjustments leading to channel degradation. When the shear 

stress is less than critical shear stress, channel aggradation will 

likely occur, and when shear stress equals critical shear stress, 

channel equilibrium will likely occur.

Streambed material is defined as the mixture of sediment of which the bed is composed (Edwards and Glysson, 

1999). It is sediment in the streambed that is at rest but that 

may resuspend and move as coarse suspended sediment or as 

bedload. The mechanisms by which it moves are various and 

complex. Bed material may include grain sizes that travel both 

as bedload and as suspended load (ASCE, 2008). Bed-material 

data were collected for creating sediment-gradation curves in 

the model to describe the variation of particle sizes in the sediment mixture. Sediment-gradation curves indicate the percentage of sediment passing through a particular sieve mesh size.

Total sediment discharge is defined as the sum of the 

suspended-sediment discharge and the bedload-sediment 

discharge. Finer material is typically carried in suspension, 

while larger particles tend to roll, slide, or bounce along the 

streambed. Suspended sediment is defined as that part of the 

total sediment load that is carried in the water column. It has 

also been described as sediment that is carried in suspension 

in the flow of a stream for appreciable lengths of time, being 

kept in this state by the upward components of flow turbulence or by Brownian motion (ASTM D 4411–03, 2004, and 

Thomas, 1977). Bedload sediment is defined as that part of the 

total sediment that is carried along the bottom by the tractive 

force of the moving water (Federal Interagency Sedimentation 

Project (FISP), 1941) intermittently to almost continuously in 

contact with the streambed by rolling, sliding, and bouncing 

(USEPA, 2008b; Thomas, 1977). It is sometimes part of the 

flow a few diameters above the bed. Bedload usually contributes only a small portion of total sediment discharge (Thomas, 

1977)—5 to 15 percent of a stream’s total sediment load 

(USACE, 1993). 

Wash load is defined as sediment carried by, and remaining near the top of, flow in the water column, moving with the 

mean velocity of the stream, and commonly considered to be 

the silt and clay fraction of the bed sediment (ASCE, 2008). 

For fine sediments that are nearly uniformly distributed over 

the depth of flow and that make up the wash load, the concentrations of sediment in the lower-to-middle parts of flow that 

move at a large fraction of the mean streamflow velocity (the 

suspended load) are essentially equal to the concentrations of 

total sediment discharge. 

Channel Equilibrium

The geomorphic role of a river is to transport flow and 

sediment without aggrading or degrading the channel while 

maintaining its dimension, pattern, and profile. Instability of 

the stream channel is often associated with an excess load and 
14 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

(or) size of delivered sediment that is beyond the carrying 

capacity of the river, thus leading to aggradation, a condition 

that is occurring downstream of the Suncook River avulsion. 

If streambanks are unstable—a condition that can be seen 

in reaches both upstream and downstream of the Suncook 

River avulsion—the ratio of width to depth will increase, 

and slope and pattern of the river will change. Shear stress 

is proportional to the square of streamflow velocity and 

is defined as the force that is applied by a fluid parallel or 

tangential to the face of a material: stream power is defined as 

the rate of energy dissipation at a given point in a river system 

and is a function of streamflow, water density, acceleration 

due to gravity, and channel slope. Stream power (a product 

of specific weight of water, discharge, and energy slope) is 

inherently linked to sediment-transport competency—the 

ability of the stream to perform geomorphic work—channel 

stability, and sensitivity of the channel to high-magnitude 

flood events; it is also linked to flood-plain formation and 

to channel planform (the contour of the channel as viewed 

from above) (Reinfelds and others, 2003). The corresponding 

reduction in shear stress and stream power due to the change 

in the distribution of energy that results from a higher ratio 

of width to depth will lead to a decrease in the river’s depth, 

velocity, and slope. For example, unless unstable Suncook 

River streambanks are stabilized, these channel and hydraulic 

changes will ultimately decrease the river’s sediment 

competency and capacity for transporting and will lead to 

excess deposition of sediment (USEPA, 2008c).

Channels are formed, maintained, and altered by the 

water and sediment that they carry. For channels such as that 

of the Suncook River, with noncohesive sediments, Lane’s 

relationship (1955) can be used to qualitatively predict channel 

conditions that will be erosive. Lane’s stream-balance equation is useful for making qualitative predictions concerning 

impacts on a channel due to changes in runoff or in sediment loads. Quantitative predictions, however, require more 

complex calculations. Lane showed that channel equilibrium 

involved the interplay of four factors: sediment discharge 

(Qs), median bed-sediment particle size (d50), streamflow 

(Qw), and stream slope (S), where Qs * d50 is proportional 

to Qw * S. In the case of the reach below the avulsion—in 

particular, in the reach from upstream to downstream of Bear 

Island—streamflow (Qw) remains the same while the median 

sediment particle size (d50) of the streambed decreased in the 

new channel (as compared to that in the abandoned channel), 

and the channel slope (S) has increased dramatically. Accordingly, sediment discharge (Qs) must increase. The resulting 

headcut, which “mines” the channel, supplies the sediment 

load as required for equilibrium. Various mechanisms may 

reduce or arrest the magnitude of the headcut degradation. A 

rock outcrop across the stream or development of an armor 

layer will stabilize the bed. Often, the hydraulic conditions 

downstream from the degrading reach will change because 

of a downstream aggradation condition, and these hydraulic 

changes will extend upstream. 

A streambed can be in equilibrium because the material is being removed as rapidly as it is deposited (Thomas, 

1977). Channel equilibrium occurs when all four of the 

variables detailed by Lane 1955) are in balance. If one variable changes, then one or more of the other variables must 

increase or decrease proportionally to maintain equilibrium. 

For example, if the channel slope is increased in a reach and 

streamflow remains the same (as is the case for the Suncook 

River avulsion reach), either the sediment load or the median 

bed-sediment particle size must also increase. The stream will 

also seek a new equilibrium by tending to erode more of its 

banks and its bed, resulting in transporting larger particle sizes 

and a greater sediment load. If excess energy remains after 

the load is moved, channel adjustment occurs as the stream 

picks up more load through erosion of its banks or through 

scouring of the streambed. Channel adjustment is occurring in 

the reach upstream of the avulsion, where degradation of the 

streambed has resulted in a headcut that is moving upstream 

toward the Route 4 bridge. In time, the erosion of bed sediments decreases as the slope is reduced through degradation of 

the streambed. 

Although the new channel below the avulsion consists 

primarily of sand and silt, the reaches of the abandoned channel contain more coarse sediment. As the channel actively 

adjusts toward equilibrium, it will continue to adjust until 

(a) sufficient conveyance of sediment is reduced and (b) erosive forces on the banks decrease sufficiently such that a new 

flood-plain terrace develops. This evolution could take 

many decades. 

Stream Temperature

Stream-temperature data are important in sediment transport because a change of a few degrees in water temperature 

will affect viscosity, which in turn can create a large difference in sediment-transport capacity. Cold water has a higher 

viscosity than warm water, resulting in an increased ability to 

transport sediment. In addition, colder water temperatures can 

cause the streambed to change from dunes to plane bedform. 

The fall velocity of sediment also decreases as stream temperature decreases; a decrease in water temperature will therefore 

cause the concentration of suspended sediment to tend toward 

uniformity. According to the Rouse equation (Rouse, 1937), 

the concentration of suspended sediment will tend to become 

more uniform over the depth of a flow as the fall velocity of 

the sediment decreases (ASCE, 2006). Assuming that the sediment concentration near the streambed, and the velocity and 

bed shear stress, do not change with stream temperature, the 

Rouse equation shows that the discharge of suspended sediment will tend to increase as stream temperature falls (ASCE, 

2006). Studies have shown that the Manning’s roughness coefficient, n, will decrease and that velocities will increase (Colby 

and others, 1965; ASCE, 2006) as discharge of the suspended 

sediment increases with decreasing stream temperature. 
Methods of Analyses 15

Bankfull Flow as Effective Discharge

The flow that fills the active channel and begins to spread 

onto the flood plain represents the break between channel 

processes and flood-plain processes. This flow is assumed to 

equal the dominant or effective discharge, which is defined as 

the discharge that transports the largest fraction of the sediment load during a specific period of time. Although extreme 

fluvial events can heavily erode and enlarge stream channels, 

steeply incising them, more modest flow regimes transport the 

greatest quantity of sediment material over time because modest flows occur with higher frequency (Wolman and Miller, 

1960). The effective discharge incorporates the principle that 

the channel-forming discharge is a function of both the magnitude of the event and the frequency of occurrence (Wolman 

and Miller, 1960). 

For stable alluvial streams such as the Suncook River 

prior to the avulsion, the effective discharge has been shown to 

be highly correlated with bankfull discharge (USEPA, 2008a). 

Bankfull discharge is the streamflow that is associated with 

a momentary maximum flow; it is found to govern channel 

shape and form. Bankfull stage is identified from geomorphic features in the field. The effective or bankfull discharge 

represents the single flow that is responsible for transporting 

the most sediment during a period of time in which channel 

maintenance is the most effective, resulting in the average 

morphological characteristics of the channel. However, a 

range of flows on either side of the effective discharge also 

carry a significant amount of the total annual sediment load. 

Aggradation and Degradation

Aggradation and degradation generally refer to trends 

in the profile of the streambed. An aggrading reach is 

one that is depositing more material than it is removing 

(or eroding). This condition persists when the sediment 

load exceeds the transport capacity of the channel-flow 

hydraulics; it is then said to be “depositional,” elevating 

the streambed and potentially the flood plain, with the bed 

profile tending to become steeper (Thomas, 1977). The ratio 

of stream width to depth increases, and channel capacity 

decreases correspondingly. Overbank flows tend to occur 

more frequently with low recurrence-interval floods. Adverse 

consequences associated with aggradation include channel 

avulsion(s) along with change in the evolution of stream 

type. The supply of sediment and its adverse effects can be 

high during these adjustments of the channel—fish habitat 

may decline, stream temperatures may become elevated, and 

biological function may decline (USEPA, 2008e). In streams 

having fine sediment with noncohesive bed and banks, such 

as the Suncook River, a tendency to meander is associated 

with the aggrading reach. Because flow passing through an 

aggrading reach causes hydraulic sorting of sediment,with 

fine sediment dominating in the flow leaving the downstream 

end of the reach, the aggrading reach thereafter provides 

a temporary stopping place for sediment material moving 

downstream. A shift in the downstream streambed control 

point due to a degradation condition may stop aggradation 

(Thomas, 1977).

A degrading reach is defined as one that projects a net 

lowering of bed elevations, with the bed profile tending to 

become flatter over time (Thomas, 1977). This condition 

persists when the capacity of the river to transport sediment 

exceeds sediment supply; the river then picks up material 

stored in the channel and banks, resulting in a degradational 

trend or lateral erosion. The erosion of the streambed 

lowers the stage for a given flow, increases bank height, 

and causes the flood plain to be abandoned. As can be seen 

in the Suncook River study reach upstream of the avulsion, 

degradation of the stream reach as a result of excess shear 

stress due to changes in flow regime (USEPA, 2008d) can 

cause a headward advancement of the headcut. Although the 

degradation upstream of the avulsion is a result of the steeper 

channel and of excess shear stress following the avulsion, 

causes of degradation of a streambed reach are complex and 

can be attributed to many sources. 

Over time, the streambed material will coarsen (sediment-size distribution will shift to larger particles) in the 

Suncook River. Vanoni (ASCE, 2006) stated that, because the 

bed-sediment load of streams is finer than the bed sediment 

from which it derives, we may conclude that the streambed 

will coarsen as the stream degrades. Conversely, the fact that 

beds of degrading streams do coarsen may be taken as evidence that the load is finer grained than the bed sediment. The 

prolonged degradation and coarsening of the bed sediment can 

lead to armoring of the bed and a drastic reduction in the rate 

of degradation and sediment discharge (ASCE, 2006).

Degradation is the dominant process for adjustment in the 

Suncook River channel at and above the avulsion site. Severe 

degradation of greater than 14 ft occurred at the location of 

the May 2006 avulsion of the Suncook River, and an active 

headcut moved upstream to an area above the confluence of 

the river with the Little Suncook River (fig. 2). Downstream 

of the former sand pit, aggradation is the dominant process of 

channel adjustment. Major aggradation has occurred above 

and below Round Pond, in the area adjacent to the production 

well (fig. 2) and, in particular, in the reach between the former 

sand pit and the Buck Street Dams (figs. 4A and B).

Methods of Analyses

In order to characterize flow and sediment transport in 

the Suncook River following the May 2006 avulsion, the study 

collected cross-section elevation and streambed-sediment 

data in the channel and on the overbanks throughout the study 

reach (fig. 4). These data were then used to construct a onedimensional HEC–RAS sediment-transport model (USACE, 

2008a) that estimated sediment transport, degradation, and 

aggradation along the river. A quasi-unsteady-state flow model 

was then run for the simulated WY 2010. The resultant cross 
16 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

sections were imported into a steady-state flow model, which 

was then run to determine the effects of the altered channel 

bed on flood elevations for the 50-, 10-, 2-, 1-, and 0.2-percent 

annual exceedence-probability floods.

Channel-Geometry Data

River cross sections were obtained from field surveys 

conducted during the summer of 2008. Valley and belowwater portions of the cross-section data were obtained from 

field measurements (using Acoustic Doppler Current Profiler 

(ADCP) and a surveying total station with level loop) that 

were referenced to the NGVD 29. Additional valley crosssection data were obtained using contour-interval data created 

from aerial photogrammetry collected in May 2007 (Eastern 

Topographics, 2007) and in April 2008 (Eastern Topographics, 

2008). All bridges, dams, and culverts in the study reach were 

field-surveyed to obtain elevation data and structural geometry. Bridge, dam, and culvert cross-section data were collected at close intervals upstream and downstream of bridges 

and dams in order to compute the potential backwater effects 

of these structures. Over 150 cross sections (of channel and of 

structure) were surveyed in 2008 to define cross-section geometry for a flood study (Flynn, 2010) and for this sedimenttransport study. 

Field-surveyed cross sections and flagged flood highwater marks from the April 2007 flood (Flynn, 2008) that had 

been used in calibrating the Suncook steady-state flow model 

(Flynn, 2010) were referenced to NGVD 29, in feet, using 

closed level-loop surveying methods from either a known 

reference datum or from a reference datum established using 

Leica System 1200 Global Positioning System (GPS) with 

3-millimeter (mm) (0.01-ft) horizontal accuracy and 10-mm 

(0.03-ft) vertical accuracy in static mode (Leica Geosystems, 

2008). Between the Short Falls Road Bridge and the Buck 

Street Dams, cross-section and profile data were also collected 

using a GPS in conjunction with an ADCP based on techniques described by Muller and Wagner (2009). 

Contour Data

Where needed, additional valley cross-section data 

were obtained using 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). These data were referenced to the horizontal North 

American Datum of 1983 (NAD 83) and vertical NGVD 

29. The 1-ft contour-interval data were created for the reach 

of the Suncook River from Route 4 to approximately onefourth of a mile downstream of the Short Falls Road Bridge. 

These contour-interval data extend 660 ft to either side of 

the Suncook River reach. The 4-ft contour-interval data were 

created for the reach of the Suncook River from the Short 

Falls Road Bridge to the confluence with the Merrimack River. 

These contour-interval data extend 1,000 ft to either side of 

the Suncook River reach downstream of the Short Falls 

Road Bridge.

Streamflow and Temperature Data

Streamflow and temperature data are input parameters 

to the quasi-unsteady-state HEC–RAS (USACE, 2008a) 

sediment-transport model. In HEC–RAS, the only quantity 

directly related to temperature is viscosity, and its effect 

is seen only on the sediment-transport calculations within 

the model. Sediment quantities, such as sediment-transport 

capacity and particle-fall velocity, are indirectly related to 

temperature through viscosity; stream characteristics such as 

roughness or backwater effects, however, are independent of 

temperature. Daily mean streamflow and temperature data for 

WY 2008 and 2009 were obtained from the Suncook River 

streamgage and entered into the model. Daily mean streamflow data were adjusted for locations downstream of the 

streamgage, whereas daily mean temperature data were not 

adjusted for downstream locations.

Streambed and Sediment Sampling

Sediment-transport models require data on stream-channel geometry, flow, and water temperature as well as information on the distribution of particle size as input parameters. 

Data on stream-channel geometry and on streambed (bedmaterial) sediments were collected during WY 2008–2009 for 

model input, while total sediment load (suspended sediment 

plus bedload sediment) data was collected during the same 

period for model calibration. Sediment samples of streambed 

and streambank material were collected at 30 locations in the 

study reach (fig. 3 and appendix 1). Suspended- and bedloadsediment data and instantaneous streamflow data were collected during November 2007–May 2008 for flows between 

65 and 2,410 ft3/s at four bridge locations: Route 3 bridge 

in Allenstown (Suncook) and Pembroke; Route 28 bridge in 

Allenstown and Pembroke; Short Falls Road Bridge in Epsom; 

and Route 4 bridge in Epsom (fig. 3). 

The study used the equal-discharge-increment (EDI) 

sampling method (Edwards and Glysson, 1999) to collect 

suspended- and bedload-sediment samples. Because this 

method requires knowledge of the distribution of streamflow 

in the cross section, the study obtained streamflow data in the 

field from discharge measurements made immediately prior 

to selecting sampling verticals. Using the EDI method, the 

study obtained samples from the centroids of equal-discharge 

increments, where the mean concentration is the average of the 

several sampling verticals. Three samplers were used—

US DH–48, US DH–95, and US D–95 (Edwards and Glysson, 

1999). The US DH–48 is a lightweight, aluminum, hand-held 

depth-integrating sampler used in wadeable stream reaches. 

The US DH–95 sampler is a plastic-coated, low-lead bronzecast, hand-line sampler that can be used in streams up to 15 ft 

deep and in velocities ranging from 1.7 to 7.4 feet per second 
Methods of Analyses 17

(ft/s). The US D–95 sampler is a plastic-coated, bronze-cast, 

suspension-system (for example, reel-and-crane) sampler 

that can also be used in stream depths up to 15 ft and stream 

velocities ranging from 1.5 to 7.5 ft/s (Davis, 2005).

Bedload-sediment samples were collected at the same 

four sampling sites and at the same time as the suspendedsediment samples (fig. 3) using US BLH–84 and US BL–84 

samplers (Edwards and Glysson, 1999). The US BLH–84 

sampler—a lightweight, aluminum, hand-held bedload 

sampler used in wadeable streams—has a nozzle width of 

0.25 ft (76.2 mm). The sampler consists of an expanding 

nozzle, a sample bag, and a wading-rod assembly. The 

US BL–84 sampler is a stainless steel and aluminum cablesuspended bedload sampler used to collect samples from 

stream reaches that cannot be waded. The sampler consists of 

an expanding nozzle mounted in a frame and a sampler bag 

(Davis, 2005).

Because bedload samplers typically protrude above the 

bedload transport zone, they inevitably trap some suspended 

sediment. When bedload is transported over dunes, sediment 

carried over the separation zone in suspension may be trapped 

by a bedload sampler that is placed in or near the trough. This 

trapped sediment does not contribute to the downstream movement of the dune, and it is consistently finer in gradation and 

better sorted than at other locations over the dune. However, it 

causes distinctive temporal variability in the particle-size distribution in the bedload and may complicate estimating rates 

of bedload transport (Thomas, 1977). This effect was apparent at the collection site at the Short Falls Road Bridge during 

flows greater than the annual mean discharge.

Samples of suspended and bedload sediment and of 

bed material were analyzed by the USGS Kentucky Water 

Science Center (WSC) Sediment Laboratory for sediment 

concentration and for gradation of particle size. The Kentucky 

WSC Sediment Laboratory analyzed sediment ranging 

in size from 0.0625 mm (0.0025 in.) to 64 mm (2.5 in.). 

Sediment bed-material sizes measured using the US SAH–97 

(gravelometer) ranged from 2 mm (0.08 in.) to more than 

180 mm (7.09 in.). The field-collected total sediment 

(suspended-sediment plus bedload-sediment) discharge 

was compared with the model-determined total sediment 

discharge (based, in part, on bed-material gradation data) 

to aid in calibration and verification of the quasi-unsteadystate sediment-transport model. All resulting sediment data 

are stored in the USGS National Water Information System 

(NWIS, 2011) database.

To create sediment-gradation curves, the study determined the distribution of data on particle size from samples 

of streambed material collected in 2008. Selection of cross 

sections for bed-material sampling was determined on the 

basis of accessibility, of obvious visual change in bed material 

from upstream conditions, and on whether cross sections had 

constricted or nonconstricted areas. Bed material was obtained 

with grab samples using a US BMH–53, a shovel and (or) a 

US SAH–97 (Edwards and Glysson, 1999). The US BMH–53 

is a hand-held piston-type tubular sampler that is used to 

collect a sample of material from the bed of a shallow stream. 

The US SAH–97 is an aluminum, hand-held analyzer that is 

used to grade or measure gravel- to cobble-sized bed sediments in the field. The US SAH–97 is also known generically 

as a gravelometer; it has 14 square openings ranging in size 

from 2 mm (0.08 in.) to 180 mm (7.09 in.) (Davis, 2005).

For substrate with material coarser than the mediumgravel limitation of the samplers, a pebble count was 

conducted using the US SAH–97 to measure surface particle 

size up to and greater than 180 mm (7.09 in.). A minimum of 

100 particles of streambed material were manually collected 

from the cross section and measured (Wolman, 1954) to 

determine the distribution of particle size. Particles smaller 

than 2 mm in diameter were labeled “<2mm.” 

Simulation of Flow and of Sediment Transport

HEC–RAS contains four one-dimensional components 

for analysis of rivers—computations of water-surface profiles 

of steady flow; simulation of unsteady flow; computations 

for movable-boundary transport of sediment; and analysis of 

water quality. HEC–RAS, version 4.0.0 (USACE, 2008a) was 

used in this study to model flow and sediment transport. It is 

a one-dimensional sediment-transport model that incorporates 

the sediment-transport functionality of HEC–6 (USACE, 

1993) and is designed to calculate profiles of the water and 

sediment-bed surfaces by computing the interaction between 

sediment material in the streambed and the sediment mixture 

in the flowing water. The functionality of the sedimenttransport model simulates the capability of a stream to 

transport sediment, given the yield from upstream sources. 

This computation of transport includes both bedload and 

suspended sediment.

HEC–RAS performs computations of routing of mobile 

bed sediment using data from quasi-unsteady-state flow (histogram) series. The model also calculates hydraulic parameters 

required for sediment processes and uses various available 

methods to calculate sediment-transport capacity. In conjunction with sorting and armoring algorithms, the model solves 

the sediment-continuity equation to determine the volume 

of deposition or erosion. Additionally, functions of temporal 

entrainment and deposition of sediment have been adapted to 

HEC–RAS (Brunner and Gibson, 2005).

The sediment-transport component of the one-dimensional HEC–RAS continuous-simulation model uses a series 

of steady flows to represent the flow hydrograph; it is designed 

to simulate and predict changes in a river profile that result 

from aggradation or degradation of sand, silt, and clay. The 

HEC–RAS model incorporates flow hydraulics, sediment 

transport, channel roughness, and related changes in boundary 

geometry into the sediment-transport simulation.

In its sediment-transport function, the model partitions a 

continuous-flow record into a series of steady flows of variable 

discharges and duration. For each flow, it calculates a watersurface profile, thereby providing values of energy slope, 
18 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

velocity, depth, and other parameters of flow for each cross 

section. It then computes potential sediment-transport rates. 

These rates, combined with the duration of the flow, permit 

a volumetric accounting of sediment within each reach. The 

amount of scour or deposition at each section is then computed 

and the cross section adjusted accordingly. The computations 

proceed to the next flow in the sequence, and the cycle is 

repeated, beginning with the updated geometry. Using grainsize fraction, the sediment calculations are performed, thereby 

allowing the simulation of hydraulic sorting and armoring. 

The model output includes water-surface elevations; changes 

in elevation due to erosion or deposition; changes in streambed material, sediment concentrations, and sediment load; and 

bed shear stresses (USACE, 2008c). The potential uses for the 

output of the model are to evaluate erosion and deposition at 

the stream-reach scale, to estimate changes in water-surface 

elevations, and to assess the long-term stability of the river.

The Suncook River HEC–RAS sediment-transport 

simulation involved two steps. The first step simulated a 

steady-state fixed bed; the second step simulated a quasiunsteady-state moveable bed. The fixed-bed simulation 

involved hydraulic computation, which included determining 

water-surface profiles and flow velocities at each cross section 

along the study reach. The water-surface profile was calculated 

from downstream to upstream using the standard step-backwater method to solve the one-dimensional energy equation 

(USACE, 2008c). 

Initial- and Boundary-Condition Data

The HEC–RAS quasi-unsteady-state sediment-transport 

model uses four categories of input data: three for initial 

conditions (hydrologic (with stream temperature), hydraulic, 

and sediment), and one for boundary conditions. The model 

requires the user to supply an upstream boundary condition 

for either an inflow sediment-load series, a rating curve, or an 

equilibrium load. The equilibrium-load option, which is based 

on sediment-transport capacity, was selected for this study.

Hydrologic Data

The HEC–RAS quasi-unsteady-state sediment-transport 

model simulation of changes in the channel bed requires the 

analysis of complete hydrographs; the analysis of a singleflowvalue, such as a flood peak, is thus not adequate for moveablebed model computation. Accordingly, a hydrograph was 

developed using daily mean streamflow values as determined 

at the Suncook River streamgage. 

For forecasting the future streambed profile, the daily 

mean flow and temperature data for WY 2009 (October 1, 

2008 through September 30, 2009) were used for simulating streamflow for both WY 2009 and 2010 (October 1, 

2009 through September 30, 2010) at the Suncook River 

streamgage. The daily mean streamflow data were adjusted 

at locations of change in lateral-boundary flow (Flynn, 2010) 

downstream of the streamgage using the following drainagearea adjustment ratio (eqn. 3): 

Q/Qg = (A/Ag)n, (3)

where

Q is the streamflow at the ungaged site, in cubic 

feet per second;

Qg is the streamflow at the USGS streamgage, in 

cubic feet per second;

A is the drainage area at the ungaged site, in 

square miles;

Ag is the drainage area at the USGS streamgage, 

in square miles; and

n is the drainage-area adjustment value.

Hydraulic Data

Analyses of hydraulic characteristics were carried out to 

provide estimates of the water-surface elevations of floods at 

the 50-, 20-, 10-, 4-, 2-, 1-, and 0.2-percent annual exceedence 

probabilities (recurrence intervals of 2, 5, 10, 25, 50, 100, and 

500 years; respectively), and of the elevation of the April 2007 

flood for the Suncook River (Flynn, 2010). 

Starting elevations of water surface for the Suncook 

River were based on normal depth analysis, the depth at which 

flow is steady and hydraulic characteristics are uniform. The 

computational procedure for the HEC–RAS model is based on 

the solution of the one-dimensional energy equation (USACE, 

2008b). Energy losses are caused by and evaluated through 

analyses of friction contraction, and expansion. The momentum equation is used in situations where the water surface is 

rapidly varied, such as in calculations of mixed-flow regimes 

(for example, hydraulic jumps) and of the hydraulics of 

bridges; it is also used in evaluating profiles at river confluences (stream junctions).

Manning’s n roughness coefficients (Barnes, 1967; 

Arcement and Schneider, 1989), used in the hydraulic computations, were assigned on the basis of engineering judgment 

and field observations of the stream and flood-plain areas. 

Manning’s n values for the Suncook River study reach ranged 

from 0.02 to 0.09 in the channel and from 0.02 to 0.12 on 

the channel overbanks. The hydraulic analyses for this study 

were based on an assumption of unobstructed flow. Therefore, 

the flood and streambed elevations shown in the profiles are 

considered valid if hydraulic structures remain unobstructed, 

operate properly, and do not fail. After calibration and verification, the steady-state flow-model-geometry data were imported 

into the quasi-unsteady-state sediment-transport model.

Calibration of the HEC–RAS Hydraulic Model

Calibrating the sediment model involved three steps. 

The steps include calibrating the steady-state Suncook River 

flow model to the flood of April 2007 (Flynn, 2008; 2010), 
Methods of Analyses 19

importing the channel geometry data into the quasi-unsteadystate sediment-transport model, and calibrating the quasiunsteady-state flow model (using daily mean flow, watertemperature, and sediment data with various transport, sorting, 

and fall-velocity methods) to closely represent the change in 

the channel bedform that was observed in the field for the 

400-day period between collection of cross-section data.

The Suncook River HEC–RAS steady-state flood 

model (Flynn, 2010) was calibrated using 22 high-water 

marks that were flagged and surveyed following the flood of 

April 16–18, 2007 (Flynn, 2008) by comparing the fieldsurveyed high-water-mark flood elevations (Flynn, 2008; 

2010) with the water-surface elevations as simulated by the 

HEC–RAS model. The high-water marks typically consisted 

of debris lines, wash lines, or mud/silt lines left behind by the 

peak water surfaces during the flood. 

In the second step, the hydraulic parameters determined 

in the steady-state simulation, along with sediment-gradation 

(variation of particle size in a sediment mixture) data and daily 

mean flow and temperature data for the calibration period, 

were imported into the quasi-unsteady-state sediment-transport model to simulate movable-bed evolution. The daily mean 

flows used in the model for simulation at locations of lateralboundary flow change (such as tributaries) were obtained 

from streamflow data with a drainage-area adjustment for the 

Suncook River streamgage (Flynn, 2008). 

The greatest fluctuation in streambed elevation in the 

study reach has been the streambed reach of the Suncook 

River between the avulsion at the former sand pit and the 

Buck Street Dams (fig. 3), where moving sediment dunes 

have formed. For this reason, calibrating the sediment quasiunsteady-state model focused on assuring agreement between 

two sets of field-collected data on cross-section geometry—the 

Suncook River channel cross-section data along a 4.2-mi reach 

of the Suncook between the Buck Street Dams and Short Falls 

Road Bridge. On May 8, 2008, channel-bed profiles and cross 

sections of the 4.2-mi reach were determined using an ADCP 

and a surveying total station, along with differential leveling 

techniques. On June 11, 2009, the channel-bed profiles and 

cross sections were again measured, and this information was 

used in calibrating the sediment-transport model. This 400-day 

period was used for calibrating and verifying the model using 

daily mean flow and temperature data.

HEC–RAS Sediment-Transport Function 

Selection

Sediment-transport equations are used to compare sediment loads and energy. Predictions of channel changes are 

based on rates of erosion or deposition when future changes in 

the flow regime are expected. Erosion or deposition rates can 

be obtained from sediment-discharge formulas, principles of 

fluvial morphology, regime theory, or other methods that consider the forces exerted on the stream boundaries. However, 

streams continually adjust their dimensions to accommodate 

change in discharge and in sediment-transport regimes. The 

thresholds of sediment movement and transport will therefore 

vary temporally and spatially as the stream adjusts to variation 

in slope, bed material, and discharge.

The HEC–RAS sediment-transport model incorporates 

empirical-transport equations, and for each cross section, it 

computes the inflow-sediment load, thalweg profiles, gradation of material in the active layer, and the transport capacity. 

Transport equations that can be used in HEC–RAS include 

those given by Toffaleti (1968), Ackers and White (1973), 

Engelund-Hansen (1967), Laursen (1958) and Copeland 

(1989), Meyer-Peter Muller (1948), Yang (1973), and 

Wilcock (2001).

The Laursen (Copeland) transport function, in conjunction with the Exner 5 sorting method and the Report 12 

(Interagency Committee on Water Resources, 1957) Fall 

Velocity Method were used for the forecast model. Exner 5 is 

a three-layer algorithm to compute bed-sorting mechanisms by 

separating the active layer into two sublayers and simulating 

coarsening of the streambed. After each timestep, the sediment cover layer is evaluated; if the bed is partially or fully 

armored, the amount of sediment material available to satisfy 

excess capacity can be limited (Gibson and others, 1996). 

According to USEPA, sediment models using streampower equations are preferred because they include velocity 

and shear stress as components that are often associated with 

channel disturbance (USEPA, 2008e). The Laursen (1958) 

sediment-transport function is a total-load function of excess 

shear and the ratio of the shear velocity to the fall velocity. 

The Laursen equation was based on his original work with 

sand-bed streams and flumes. The flume experiments used 

median particle sizes ranging from 0.0004 to 0.1606 in. (0.011 

to 4.08 mm). Laursen supplemented the flume data with field 

observations of sediment discharge using median particle sizes 

ranging from 0.003 to 0.027 in. (0.08 to 0.7 mm). Later work 

by Copeland (1989) generalized the equation for gravel transport so that the equation could be used for beds that included 

gravels up to a median size of 29 mm (coarse gravel). A 

distinctive feature of the Laursen (Copeland) equation is that 

it was developed for the range of sediment material extending 

to silt. None of the other functions currently included in HEC–

RAS was developed for silt-sized particles. To use the other 

HEC–RAS functions for determining silt sediment potentials 

would have required extrapolations, thus compounding the 

standard uncertainty associated with computing transport 

capacity. Recent work at Colorado State University has determined that the Laursen equation outperforms other transport 

equations in the silt range (USACE, 2008c).

Limitations of the HEC–RAS Model

The HEC–RAS one-dimensional flow, sediment-transport, and sediment-degradation- and -aggradation model used 

in this study has a number of important limitations. 
20 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

1. The model is unable to simulate the development of 

meanders or to specify a lateral distribution of sediment 

load across a cross section. In one-dimensional modeling, the solution of the sediment-continuity equation 

provides a change in the simulated cross-sectional area. 

That change in area is equally allocated to each coordinate point across the cross section. But the shape of the 

cross section cannot be addressed with a one-dimensional 

sediment-transport model (ASCE, 2008). Therefore, the 

simulation’s calculation of how sediment transport is 

allocated is only an approximation. In one-dimensional 

mobile-bed modeling, the heterogeneous processes of 

sediment entrainment, deposition, and movement are 

described for the cross section as a whole in the form of 

bulk channel properties. These properties include average velocity, overall discharge, average depth, bulk shear 

stress, and average composition of bed material in the 

section (ASCE, 2008).

2. Bedforms throughout the modeled reach are not simulated 

but can be approximated indirectly using Mannings n values as a function of the river elevation or river discharge.

3. In comparing measured and simulated suspended-sediment discharges, reconciling differing scales can become 

a problem. Although the samples of suspended and 

bedload sediment represent a time-averaged sample at a 

particular cross section, it is not uncommon to see nonmonotonic patterns in the measured data, thus suggesting 

the existence of large-scale variability (large eddies, dune 

migration) that a numerical model cannot fully resolve 

(ASCE, 2008). For this study, however, the output data 

from the model were calibrated as closely as possible by 

comparing modeled sediment loads and streambed elevations with field-collected data for these parameters. 

4. Distribution of sediment grain sizes may be representative 

for the location and time at which these data were taken, 

but size distributions collected at a particular location may 

not be representative of the general area of the river at 

the spatial or temporal scale that the model can resolve. 

Samples of bed material may also not be representative 

of a quasi-equilibrium bed condition for the width of the 

river for hundreds of feet of the river’s length but instead 

more closely represent the portion of the dune from which 

they were taken or the finer material moving through the 

entire river system (ASCE, 2008).

Markov Chain Analyses of Flow

To enable the study to predict future sediment discharge 

for WY 2010, a hydrograph was developed to represent future 

flows. Analysis using Markov Chain Theory was chosen for 

the determination of future flows because it is applicable to 

processes that can be formulated in terms of discrete values 

as well as for continuous processes that can be discretized 

for computational convenience (Salas, 1993). At each step, 

the system may change its state or remain in the same state, 

according to a certain probability distribution. The changes of 

state are called “transitions,” and the probabilities associated 

with various changes in state are called “transition probabilities.” This study used Markov Chain Theory to determine 

the probability of occurrence of flows greater than and less 

than the annual peak flow and the annual mean flow (sum 

of all daily mean flows divided by the number of days in the 

year) based on historical data from the USGS Suncook River 

streamgage to aid in determining an appropriate simulated 

mean daily flow hydrograph for WY 2010. The initial data for 

the Markov Chain Theory analysis were drawn from the years 

during which the streamgage was operative only intermittently 

(1919–20, 1922–27, and 1929–77 with peak-flow data collected only during 1971–77).

A frequently used surrogate for the channel-forming 

discharge in empirical regression equations is the annual 

mean flow (Leopold and Maddock, 1953; Federal Interagency 

Stream Restoration Working Group (FISRWG), 1998). The 

annual mean flow is the equivalent of the discharge that would 

yield the same volume of water in a water year as the sum of 

all measured discharges (FISRWG, 1998). Leopold (1994) 

found a consistent proportion between annual mean flow and 

bankfull discharge within the same region—the ratio of bankfull discharge to annual mean discharge was found to 

be approximately 8.3 for 13 streamgages in the eastern 

United States. Assuming bankfull flow is equal to the 

1.8-year recurrence-interval flood based on the historical 

record at the streamgage, a value of 5.5 was found to be the 

ratio of bankfull discharge to annual mean flow for WY 2009 

at the Suncook River streamgage.

Because the annual mean flow is a surrogate for the 

channel-forming discharge (Leopold and Maddock, 1953; 

FISRWG, 1998), Markov Chain Theory analysis was used to 

assess data on the annual mean flow for the Suncook River 

streamgage during the record gap before the streamgage was 

reactivated in November 2007. For the 47 water years of 

record before 1971, the average of the annual mean flows at 

the Suncook streamgage is 237 ft3/s; when incorporating the 

annual mean flow for WY 2008 and 2009, the average of the 

annual mean flow for the 49 water years of record is 243 ft3/s. 

The maximum annual mean flow for the historical Suncook 

streamgage data (before the 2007 reactivation) was 346 ft3/s 

in 1953, and the minimum annual mean flow was 96 ft3/s 

in 1965. 

The Suncook streamgage was not active during October 1 through November 9, 2007. For the partial WY 2008 

beginning November 9, 2007 through September 30, 2008, 

the annual mean flow was 418 ft3/s, and for the full WY 2009 

(October 1, 2008 through September 30, 2009), the annual 

mean flow was 375 ft3/s. The annual mean flows for these two 

water years were 20.8 and 8.4 percent greater, respectively, 

than the maximum annual daily mean flow of 346 ft3/s that 

occurred in WY 1953.
Channel Morphology and Stream Processes 21

These annual mean flows for WY 2008 and 2009 are 

76.4 and 58.2 percent greater, respectively, than the average 

annual mean flow (237 ft3/s) for the water years before the 

streamgage was reactivated in November 2007. The study 

used the historical Suncook River streamgage maximum 

annual mean flow value of 346 ft3/s as a benchmark to 

estimate the probability of an annual mean flow greater 

than the pre-1971 annual mean flow of 237 ft3/s but less 

than the annual mean flow values for WY 2008 and 2009. 

Using Markov Chain Theory to analyze the period of record 

(1919–70) for annual mean flow, the study determined that the 

probability of an annual mean flow greater than 346 ft3/s in 

any given year is 0.02; conversely, the probability of annual 

mean flow less than 346 ft3/s in any given year is 0.98. The 

WY 2009 annual mean flow was 375 ft3/s.

The annual peak-flow data at the Suncook River 

streamgage were also analyzed using Markov Chain Theory 

for water years (October 1–September 30) other than 2007. 

For WY 2006, the peak flow was determined to be 7,600 ft3/s 

(Olson, 2007). For WY 2007, the peak flow was determined to 

be 10,600 ft3/s (Flynn, 2008). For WY 2008, the peak flow was 

determined to be 3,430 ft3/s. Using Markov Chain Theory to 

analyze the streamgage record from 1919–77, it was determined that the probability, in any given year, of a peak flow 

greater than that of WY 2008 (the lowest among the annual 

peaks of WY 2006, 2007, and 2008) at 3,430 ft3/s, would have 

been 0.24. The annual peak flow for WY 2009 was 2,500 ft3/s; 

it occurred on November 26, 2008. Using the streamgage 

record from 1919–77, the study determined that the probability, in any given year, of a peak annual flow greater than 

the provisional annual peak for WY 2009 of 2,500 ft3/s would 

be only 0.36. On the basis of these analyses of annual peak 

flow since the Suncook River streamgage was reactivated and 

of annual mean flow data for partial WY 2008 and 2009, and 

because the probability of an annual mean flow being greater 

than 346 ft3/s in any year before 1971 is 0.02, it was determined that a hydrograph using the mean daily flow data for 

WY 2009 would best represent the mean daily flow regime for 

WY 2010 for predicting sediment transport. 

Geophysical Data

Geophysical data-collection activities, using direct-current (DC) electrical resistivity methods, were conducted during summer 2008 in the area of the sand pit to determine the 

depth of the surficial sediment layer. The electrical resistivity 

of the subsurface materials can be used to characterize lithologic materials. A consolidated layer, such as bedrock, would 

have a higher resistivity response than gravel, which would 

have a higher resistivity response than sand. Apparent resistivity is calculated from electrical resistivity values and geometric factors such as electrode spacing and the arrangement of 

current and potential electrodes in relation to each other. Variation in apparent resistivity indicates the existence of zones 

of contrasting resistivity, which under ambient water-quality 

conditions specifies differing lithologic materials. Data are 

inverted to convert apparent resistivity values into estimates of 

values at specific depths.

Channel Morphology and Stream 

Processes

The total sediment loads collected at the four bridge 

locations along the Suncook River were primarily suspended 

sediment. Analysis of sediment occurrence and transport indicated that the median grain size (d50) of all sediment sample 

locations in the Suncook River typically corresponded to 

coarse gravel (d50 of 16–32 mm) above the avulsion and coarse 

sand (d50 of 0.5 to 1 mm) below the avulsion. Degradation is 

the dominant process for channel adjustment in the Suncook 

River at and above the avulsion site. Severe degradation of 

greater than 14 ft has occurred at the location of the May 2006 

avulsion of the Suncook River, and an active headcut has 

moved upstream to an area above the river’s confluence with 

the Little Suncook River (figs. 2, 4C). The elevation of the 

thalweg of the headcut that is advancing upstream at cross section DG is 302.7 ft (NGVD 29). Cross section DG is approximately 1,000 ft downstream of the upstream face of the Route 

4 bridge (figs. 4D and C). Potentially, a consolidated layer, 

such as shown in cross section DG, could halt the advancement of the headcut before it reaches the Route 4 bridge if this 

resistive layer is bedrock (elevation of 305 ft (+5 ft), NGVD 

29) and if it extends downstream of the Route 4 bridge at an 

elevation greater than 302.7 ft (NGVD 29), the elevation of 

the headcut thalweg. Downstream of the avulsion and former sand pit, aggradation is the dominant process of channel 

adjustment. Significant aggradation has occurred above and 

below Round Pond in the area adjacent to the production well 

(fig. 2) and, in particular, in the reach between the former sand 

pit and the Buck Street Dams (figs. 4A and B).

Lateral Migration of Streambed

Mass failure of steep, cohesive banks is a function of 

bank height, bank angle, and soil properties. Downstream of 

the confluence area and upstream of the large meander bends 

(fig. 4C, cross sections CB to CI), a small vegetated berm is 

keeping the channel in its present (2010) location. A small 

flood-bypass channel on the west side of the river at the first 

of the large meander bends splits the flow and may eventually 

cause the river to completely bypass the first meander bend 

(fig. 4C, cross sections CG to CI). If allowed to evolve naturally over time, the form and location of the channel will likely 

change between Round Pond and the large meander bends. 

These changes (2010) are occurring in the form of an avulsion at Round Pond and at the first of the large meander bends 

where flow now occurs through the flood-bypass channel west 

of the river. Lateral migration of the river into the large field 
22 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

area east of the river and (or) formation of multiple channels 

may also occur. In addition, sediment will continue to fill in 

the channel, with the most significant aggradation occurring 

between the sand pit area and the Buck Street Dams.

An historical analysis of the width of the meander 

belt and the river’s migration rates by VHB, Inc. (written 

commun., 2007) determined that, in spite of its sinuosity, 

the preavulsion Suncook River had been stable, its lateral 

migration rate remaining at 4 cm per year during the 50-year 

period 1953–2003. However, the new channel through the area 

of the former sand pit is unstable and will actively continue 

to adjust. Despite its instability since the May 2006 avulsion, 

there has been little lateral migration in the new channel. The 

top of banks were delineated in figure 5A (1:2,500) based on 

aerial photography from a flight on May 12, 2007 (Eastern 

Topographics, 2007). In figure 5B (1:2,500), the delineation 

of the top of bank on May 12, 2007 has been superimposed 

over an aerial photograph taken on April 18, 2008 (Eastern 

Topographics, 2008). Figures 5A and B show that, while the 

new avulsion channel reach is continuing to transport and 

deposit sediment, and bank erosion is occurring, there was 

little lateral migration of the new channel through the sand pit 

during the 11-month interval between May 2007 and 

April 2008. 

Distribution of Sediment Grain Size

Streambed sediment-size gradation was determined for 

30 locations in the study reach (fig. 4), with four locations 

corresponding to the bridges where samples were collected for 

the determination of total sediment load (suspended and bedload). The sediment grain size (appendix 1, figs. 6A–C) and 

median grain size (d50) (appendix 1, fig. 7) of the streambed 

material collected at the sampling locations ranged from less 

than 0.0625 mm (0.0025 in.) (silt/clay) to more than 180 mm 

(more than 7.1 in.) (large cobbles). The d50 for the stream overbank bed material ranged from less than 0.0625 mm (less than 

0.0025 in.) (silt/clay) to 58.8 mm (2.3 in.) (very coarse gravel) 

(appendix 1). Data for the d50 indicated a smaller median 

grain size for the overbanks than for the stream, with the 

exception of one bed-material sampling location (SRM–280). 

The median grain size (d50) for all of the sediment-sampling 

locations upstream of the avulsion was an average of 

45.7 mm (very coarse gravel) for the streambed and an average of 18.3 mm (coarse gravel) for the streambed and banks. 

The d50 downstream of the avulsion for all of the sedimentsampling locations was an average of 3.66 mm (very fine 

gravel) for the streambed and an average of 0.56 mm (coarse 

sand) for the streambed and banks. 

At Route 4, the median grain size of the streambed is 

small cobble (d50 grain size of 109 mm) downstream of the 

bridge and coarse gravel upstream of the bridge (d50 grain size 

of 26 mm); at Short Falls, median grain size is coarse sand 

(d50 grain size of 0.69 mm); at Route 28, median grain size is 

coarse silt (d50 grain size less than 0.0625 mm); and at 

Route 3, median grain size is medium sand (d50 grain size 

of 0.38 mm). At the downstream face of the Route 4 bridge 

(appendix 1, SRM–80 site), 6 percent of the sediment in the 

streambed is finer than medium sand (less than 0.25 mm), 

18 percent is finer than very coarse sand (less than 1 mm), 

19 percent is finer than fine gravel (less than 4 mm), and 

100 percent is finer than small boulder (less than 256 mm). 

At the downstream face of the Short Falls Road Bridge 

(appendix 1, SRM–238 site), 4 percent of the sediment in the 

streambed is finer than medium sand (less than 0.25 mm), 

75 percent is finer than very coarse sand (less than 1 mm), 

and 100 percent is finer than medium gravel (less than 8 mm). 

At the downstream face of the Route 28 bridge (above the 

Buck Street Dams) (appendix 1, SRM–280 site), 54 percent 

of the sediment in the streambed is finer than very fine sand 

(less than 0.063 mm) and 100 percent is finer than very fine 

gravel (less than 2 mm). At the downstream face of the Route 

3 bridge (appendix 1, Route 3 site), 20 percent of the sediment 

in the streambed is finer than medium sand (less than 

0.25 mm), 97 percent is finer than very coarse sand (less than 

1 mm), 99 percent is finer than very fine gravel (less than 

2 mm), and 100 percent is finer than medium gravel (less than 

8 mm). 

The particle-size gradation data and curves (figs. 6A–C, 

appendix 1) show that the streambed material is more coarse 

upstream of the avulsion, transitioning downstream from a 

median grain size of small cobble at the Route 4 bridge to 

coarse sand at the Short Falls Road Bridge to coarse silt at the 

Route 28 bridge. Downstream of the Route 28 bridge and the 

Buck Street Dams, the median grain size is larger—a medium 

sand. This gradation confirms that most of the finer sand and 

silt are falling out of suspension in the reach between the Short 

Falls Road Bridge and the Buck Street Dams, just below the 

Route 28 bridge. 

After the May 2006 avulsion, the sediment load in the 

Suncook River increased substantially but, as the river moves 

toward equilibrium, it is anticipated that the erosion of bed 

sediments will gradually decrease in the new channel, with 

bed degradation reducing the channel slope (Lane, 1955) near 

the avulsion channel. The expected result is that the median 

particle size of the bed sediment will gradually increase. On 

the overbanks between the avulsion and the Buck Street Dams 

and in the channel from cross section CN to the Buck Street 

Dams (figs. 4B to C), however, much of the fine silt and sand 

that was deposited during the May 2006 and April 2007 floods 

has been retained. This overbank material washes into the 

river during precipitation events and is carried and deposited 

by flow in the channel and on the overbanks.
Channel Morphology and Stream Processes 23

Figure 5A–B. A, May 2007 and 

B, April 2008 aerial imagery of 

the avulsion and former sand pit 

showing the May 2007 channel 

location and geophysical-survey 

cross sections (lines 1–4) and 

model cross sections CR through 

DD (1:25,000).

Line 2

Line 1

Line 3

Line 4

Line 2

Line 1

Line 3

Line 4

CZ

CY

CX

DC

CW

CS

DB

DA

DD

CV

CT

CR

CU

DE

CZ

CY

CX

DC

CW

CS

DB

DA

DD

CV

CT

CR

CU

DE

CZ

CY

CX

DC

CW

CS

DB

DA

DD

CV

CT

CR

CU

DE

CZ

CY

CX

DC

CW

CS

DB

DA

DD

CV

CT

CR

CU

DE

0 250 500 FEET

0 50 100 METERS

0 250 500 FEET

0 50 100 METERS

Base map is from aerial photogrammetry collected by Eastern Topographic, in May 2007,

and May 2005, New Hampshire Department of Transportation tif imagery.

Model cross section

EXPLANATION

Channel location

Base map is from aerial photogrammetry collected by Eastern Topographic, in May 2007,

and May 2005, New Hampshire Department of Transportation tif imagery.

Model cross section

EXPLANATION

Channel location

Cross section where bed-sediment data were collected

71°21'30"

43°13'

Cross section where bed-sediment data were collected

71°21'30"

43°13'

A

B
24 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

0

20

40

60

80

100

SRM1 at XS DW

SRM10 at XS DR

SRM15 at XS DQ

SRM70 at XS DJ

SRM80 at XS DH

SRM100 at XS DD

0

20

40

60

80

100

SRM110 at XS DB

SRM120 at XS CY

SRM130 at XS CW

SRM140 at XS CU

SRM200 at XS CQ

SRM210 at XS CO

SRM220 at XS CN

SRM230 at XS CM

SRM232 at XS CI

SRM234 at XS CA

PERCENTAGE OF PARTICLES FINER THAN INDICATED SIZE

10-2.00 10-1.00 100.00 101.00 102.00 103.00

GRAIN-SIZE DIAMETER, IN MILLIMETERS

0

20

40

60

80

100

SRM238 at XS BV

SRM240 at XS BS

SRM250 at XS BM

SRM270 at XS AX

SRM280 at XS AU

SRM300 at XS AM

SRM310 at XS AI

SRM320 at XS AF

SRM330 at XS AD

SRM340 at XS AC

SRM350 at XS Z

SRM360 at XS O

A

B

C

Sediment reference material (SRM) number at cross section (XS)—

Location of cross section shown in figures 4C−D

EXPLANATION

Location of cross section shown in figure 4C

Location of cross section shown in figures 4A−C

Figure 6A–C. A, Streambed-sediment grain-size distribution at six cross sections on the Suncook River upstream of the May 2006 

avulsion, B, streambed-sediment grain-size distribution at ten cross sections on the Suncook River between the May 2006 avulsion and 

Short Falls Road Bridge, and C, streambed-sediment grain-size distribution at ten cross sections on the Suncook River downstream of 

the Short Falls Road Bridge.
Channel Morphology and Stream Processes 25

Bankfull Channel Dimensions

Bankfull dimensions were not identifiable in the new 

channel of the Suncook River because the channel is still 

actively adjusting. Wolman and Leopold (1957) suggested 

that the channel-forming discharge has a recurrence interval 

of between 1 and 2 years. However, there is no universally 

accepted definition of bankfull discharge or stage that can be 

consistently applied and that also integrates all the processes 

that create the bankfull dimensions of the river. The present 

study assumes that bankfull discharge is the flow having a 

recurrence interval of 1.8 years, which corresponds to a flow 

of 2,060 ft3/s at the Suncook River streamgage. The variability 

of the bankfull-flow mean depth in the channel reach (fig. 8A 

and table 2) and the variability of the bankfull-flow top width 

(fig. 8B and table 2) of the channel reach, along with figures 6 

and 7, indicate the Suncook River geomorphic structure.

HEC–RAS Sediment-Transport Model

The HEC–RAS steady-state flow model used to create the 

quasi-unsteady-state sediment-transport model was developed 

and calibrated using stream-channel cross-section geometry 

data and April 2007 high-water floodmark data (Flynn, 2010). 

A comparison of the HEC–RAS model calibration of the April 

2007 flood to 13 surveyed high-water marks rated as “good” 

yielded an average elevation difference of 0.14 ft (approximately 1.7 in.) and a standard deviation of 0.55 (Flynn, 2010). 

Following verification of the steady-state flow model, a 

400-day quasi-unsteady-state calibration model was validated 

for sediment transport by comparing field-collected total load 

(suspended and bedload) data (table 3) with the HEC–RAS 

simulation results for total sediment load determined using 

field-collected streambed-material data and the Laursen 

(Copeland) sediment-transport equation. In the model simulations, the Laursen (Copeland) equation, in conjunction with 

the Exner 5 sorting method and the Report 12 (Interagency 

Committee on Water Resources, 1957) fall velocity method, 

were found to most closely simulate the streambed conditions 

for the 400-day calibration period and the field-collected total 

sediment loads. 

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000

CUMULATIVE DISTANCE UPSTREAM OF CONFLUENCE WITH MERRIMACK RIVER, IN FEET

0

50

100

150

Channel

Left bank

Right bank

MEDIAN SEDIMENT GRAIN SIZE (D50), IN MILLIMETERS

200

Grain size (d50), in millimeters, at cross section

Figure 7. Median sediment 

grain sizes (d50) for the Suncook 

River in Epsom, Pembroke, and 

Allenstown, New Hampshire.
26 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000

CUMULATIVE DISTANCE UPSTREAM OF CONFLUENCE WITH MERRIMACK RIVER, IN FEET

0

500

1,000

1,500

2,000

2,500

BANKFULL-FLOW TOP WIDTH, IN FEET

Top width at cross section

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000

0

5

10

15

20

Mean depth at cross section

CUMULATIVE DISTANCE UPSTREAM OF CONFLUENCE WITH MERRIMACK RIVER, IN FEET

BANKFULL-FLOW MEAN DEPTH, IN FEET

25 A

B

Figure 8. A, Bankfull-flow mean depth and B, bankfull-flow top width of the Suncook River in 

Epsom, Pembroke, and Allenstown, New Hampshire.
Channel Morphology and Stream Processes 27

Suncook 

River cross 

section

Cumulative 

distance upstream 

(ft)

Bankfull flow 

top width 

(ft)

Bankfull flow 

depth 

(ft)

A 0 241.71 2.27

B 477 242.14 2.28

C 804 307.24 3.50

D 1,681 220.12 1.90

1,864 119.32 4.48

1,883 122.48 4.74

E 2,043 104.86 3.33

F 2,193 147.63 2.50

2,337 178.07 2.20

2,363 241.81 13.79

G 2,507 191.60 11.06

H 3,079 80.19 10.24

3,093 107.92 23.05

3,124 43.47 11.45

3,176 43.48 11.64

I 3,401 73.01 6.87

J 3,467 95.38 3.40

K 3,880 149.03 4.53

3,889 164.29 4.32

3,898 140.84 4.36

3,907 140.06 4.29

3,914 139.17 4.18

3,916 141.13 4.29

3,925 140.33 4.18

L 3,934 139.42 4.07

M 4,433 131.13 2.51

N 4,507 131.36 2.51

4,509 113.49 3.63

4,525 335.92 13.38

O 4,640 223.98 10.53

P 4,886 318.31 7.86

Q 5,126 409.32 9.87

R 5,283 474.47 11.00

S 5,575 909.94 7.52

T 5,860 256.59 7.10

U 6,045 186.98 6.88

6,057 148.09 7.42

6,109 148.14 7.43

V 6,172 149.62 10.13

W 6,935 144.86 4.46

X 6,945 144.93 4.50

Suncook 

River cross 

section

Cumulative 

distance upstream 

(ft)

Bankfull flow 

top width 

(ft)

Bankfull flow 

depth 

(ft)

Y 7,981 116.44 8.10

Z 9,331 131.77 10.63

AA 10,614 130.42 10.03

AB 11,542 159.05 8.80

AC 12,282 143.40 10.14

AD 15,482 147.18 8.47

AE 17,581 127.06 10.39

AF 19,123 128.07 12.12

AG 22,592 433.85 3.12

AH 23,529 435.20 2.40

AI 24,763 416.23 2.80

AJ 25,387 514.59 2.21

AK 26,696 118.66 9.99

AL 27,674 119.61 9.00

AM 28,253 144.39 6.71

AN 28,443 205.77 5.42

AO 28,766 215.08 3.30

AP 28,851 171.70 4.65

28,852 134.03 2.67

28,866 217.69 7.24

29,011 188.52 7.15

29,038 198.10 6.83

AQ 29,063 202.39 7.97

AR 29,088 215.40 7.69

AS 29,216 179.40 8.69

29,233 177.47 9.82

29,278 177.49 9.82

29,285 153.80 10.21

AT 29,347 145.28 10.86

AU 29,600 140.97 9.96

AV 30,715 306.09 4.87

AW 31,747 200.35 7.53

AX 33,142 344.68 4.35

AY 34,227 515.28 4.02

AZ 34,927 125.35 10.08

BA 35,563 270.16 5.80

BB 36,078 125.34 10.63

BC 36,613 143.96 11.52

BD 37,933 392.19 3.17

BE 38,426 193.70 6.66

BF 39,121 234.26 7.59

Table 2. Bankfull-flow mean depth and top width of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire.

[ft, feet]
28 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Suncook 

River cross 

section

Cumulative 

distance upstream 

(ft)

Bankfull flow 

top width 

(ft)

Bankfull flow 

depth 

(ft)

BG 39,861 224.96 6.01

BH 40,506 588.58 1.68

BI 41,066 679.52 1.76

BJ 41,899 517.94 2.47

BK 43,019 326.55 3.54

BL 43,741 298.78 5.93

BM 44,821 128.83 8.51

BN 45,161 130.57 8.41

BO 45,724 149.62 10.96

BP 46,356 122.09 8.59

BQ 46,731 171.29 8.86

BR 47,375 384.82 3.97

BS 48,053 140.17 9.17

BT 48,853 144.98 8.06

BU 50,158 135.74 8.74

BV 50,624 312.19 4.96

BW 50,829 698.44 8.86

50,989 501.02 8.13

BX 51,081 237.77 7.00

51,134 81.95 7.29

51,175 82.76 7.44

51,239 82.37 6.59

51,254 91.95 6.14

BY 51,305 91.46 6.03

BZ 51,975 129.05 6.05

CA 52,463 866.26 1.80

CB 53,600 526.54 1.27

CC 54,415 853.95 0.73

CD 55,869 2,368.69 1.24

CE 56,345 1,883.13 0.98

CF 56,880 2,575.37 1.54

CG 57,636 1,233.32 1.43

CH 58,327 1,268.26 2.09

CI 58,367 1,290.38 2.57

CJ 59,099 1,004.93 2.03

CK 59,429 566.26 2.00

CL 59,964 260.18 2.52

CM 60,520 862.65 1.43

CN 61,160 196.06 2.37

Suncook 

River cross 

section

Cumulative 

distance upstream 

(ft)

Bankfull flow 

top width 

(ft)

Bankfull flow 

depth 

(ft)

CO 61,868 131.91 4.47

CP 62,547 120.04 5.22

CQ 63,035 88.23 6.52

CR 63,475 179.94 3.77

CS 63,795 237.69 4.56

CT 64,132 123.86 5.51

CU 64,400 127.09 5.08

CV 64,632 182.67 4.67

CW 64,893 169.08 3.78

CX 65,150 138.16 4.41

CY 65,883 140.18 4.67

CZ 65,893 128.78 4.77

DA 66,029 144.11 5.14

DB 66,140 174.89 4.32

DC 66,345 142.41 4.92

DD 66,627 110.65 2.59

DE 66,929 138.82 3.29

DF 67,651 128.58 7.49

DG 68,733 87.87 11.60

DH 69,552 77.84 2.92

69,665 50.78 3.89

69,726 61.64 4.77

DI 69,783 119.14 8.13

69,998 85.62 4.48

DJ 70,008 102.13 4.62

DK 70,501 307.31 4.20

DL 70,813 108.01 5.20

DM 71,389 455.23 2.17

DN 71,617 348.61 2.99

DO 72,166 101.25 6.67

DP 74,513 91.50 7.71

DQ 74,551 128.64 6.89

DR 75,309 206.49 3.88

DS 76,283 198.65 4.32

DT 78,389 203.03 4.36

DU 79,631 99.01 5.70

DV 81,372 186.00 5.01

DW 83,919 188.19 4.37

Table 2. Bankfull-flow mean depth and top width of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire.

—Continued

[ft, feet]
Channel Morphology and Stream Processes 29

Table 3. Field-collected total sediment loads and estimates of total sediment loads from sediment-transport curves.[ft3/s, cubic feet per second; mi2, square miles]

Route 4 bridge Short Falls Road Bridge Route 28 bridge Route 3 bridge

Date Flow (ft3/s) Sediment discharge (tons/day) Date Flow (ft3/s) Sediment discharge (tons/day) Date Flow (ft3/s) Sediment discharge (tons/day) Date Flow (ft3/s) Sediment discharge (tons/day)

5/30/2008 65 0.18 5/30/2008 84 0.68 5/30/2008 95 0.51 5/30/2008 101 0.55

11/26/2007 134 0.36 11/26/2007 195 1.05 11/26/2007 216 1.17 11/27/2007 528 5.702/28/2008 481 26.0 2/29/2008 632 152 1/11/2008 781 33.7 4/18/2008 1,330 14.41/10/2008 519 481 1/10/2008 688 881 4/18/2008 1,250 20.3 4/29/2008 1,370 88.83/11/2008 1,010 259 4/18/2008 1,090 129 4/29/2008 1,290 90.6 3/11/2008 1,690 50.24/17/2008 1,010 2,610 4/29/2008 1,130 458 4/3/2008 1,960 132 4/3/2008 2,000 6862/14/2008 1,020 151 2/14/2008 1,320 1,780 3/10/2008 2,150 226 4/30/2008 2,410 2024/29/2008 1,100 205 4/3/2008 1,700 868 4/30/2008 2,270 2574/2/2008 1,370 274 3/10/2008 1,900 4,3904/30/2008 1,820 206 4/30/2008 1,990 1,080

Cross section

Estimates of total sediment load from sediment-transport curves using field-collected total (suspended and bedload) 

sediment data from table 2

Location Drainage area (mi2) Bankfull discharge, (ft3/s) 1Qb load (tons/day) WY 2009 average mean daily flow, (ft3/s) 2Qa load (tons/day)

DH Route 4 bridge 161.2 2,150 2,000 390 100

DS Short Falls Road Bridge 210.1 2,750 2,000 501 150DS Route 28 bridge 240.1 3,120 500 568 2DS Route 3 bridge 255.2 3,300 750 601 4

Cross section

Estimates of sediment load from sediment-transport curves using HEC-RAS output with bed material gradation data

Location Drainage area (mi2) Bankfull discharge, (ft3/s) 1Qb load (tons/day) WY 2009 average mean daily flow, (ft3/s) 2Qa load (tons/day)

DH Route 4 bridge 161.2 2,150 2,000 390 150

DS Short Falls Road Bridge 210.1 2,750 1,000 501 150DS Route 28 bridge 240.1 3,120 500 568 0.01DS Route 3 bridge 255.2 3,300 750 601 2

1,2 Qa (WY 2009 annual mean discharge) and Qb (bankfull discharge) loads estimated from sediment transport curves (figs. 9–12).
30 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Streambed thalweg elevations as determined by the 

Laursen (Copeland) sediment-transport equation and the 

streambed thalweg elevations as surveyed after the 400-day 

period for the repeated survey of the Suncook River HEC–

RAS model-calibration reach between the Buck Street Dams 

and the Short Falls Road Bridge are shown in table 4 and 

figure 8. Measured change compared to simulated change in 

streambed thalweg elevation at cross-section locations for the 

same calibration reach of the Suncook River are are shown in 

figure 9. In general, the simulated and measured streambed 

thalweg-elevation change for the calibration reach were in 

agreement. Cross section AZ is an exception, however, with a 

measured streambed thalweg elevation increase of 0.23 ft and 

a simulated decrease of 3.43 ft. This location is at the confluence of Bear Brook with the Suncook River and indicates 

that, given additional flow from Bear Brook, a large amount 

of sediment would be transported from this location. The next 

downstream cross section at AY, however, had a much better 

agreement between measured and simulated streambed thalweg elevations with a measured streambed-elevation increase 

of 0.36 ft and a simulated streambed-elevation change of 

0.00 ft. It is important to note that a limitation of the onedimensional HEC–RAS model is that the model is unable to 

simulate a lateral distribution of sediment load along a cross 

section, and the simulation’s calculation of how sediment 

transport is allocated is only an approximation.

Kendall’s Tau rank correlation coefficient is a statistic used to measure the correlation between two measured 

quantities. Values of Kendall’s Tau range between -1 and +1, 

with a positive correlation indicating that the ranks of both 

variables increase together. A Kendall’s Tau rank correlation 

coefficient of 0.4 was determined for the statistical agreement 

between the measured and simulated changes in streambedelevation data for the cross sections in the calibration reach. 

This indicates a moderate positive correlation, albeit not 

statistically significant, between the two datasets. The root 

mean square error (RMSE) is the square root of the variance 

of the residuals and is a good measure of the absolute fit of 

the model-predicted values to the observed data. Lower values 

of RMSE indicate a better fit. The RMSE for the two datasets 

was determined to be 1.2 ft. 

Flood-Generated Shear Stress and the Potential 

for Entrainment

For fine sand and finer sizes of sediment (less than 

0.25 mm), moving particles will go directly into suspension 

following initiation of sediment motion. The corresponding 

critical shear stress, which equates applied forces in the 

direction of flow to resisting forces, is low, so that material 

of these sizes are frequently mobilized if cohesive effects are 

not large (ASCE, 2008). Fine to coarse sand (0.25 to 1 mm) 

moves initially as bedload, with particles beginning to go into 

suspension at high shear stresses. Medium sand begins to 

move at boundary shear stresses of between 0.004 to 

0.006 lb/ft2 (0.0002 to 0.0003 kilopascals (kPa) or 2 to 

3 dynes per square centimeter (dy/cm2)), and incipient 

suspension begins at boundary shear stress of 0.02 to 

0.06 lb/ft2 (0.001 to 0.003 kPa or 10 to 30 dy/cm2). Shear 

stresses of such magnitude can be reached during moderate 

river flows (ASCE, 2008). For fine gravel (up to 8 mm), the 

critical shear stress is 0.094 lb/ft2 (0.0045 kPa or 45 dy/cm2), 

and particle movement is possible at shear stress greater than 

1.15 lb/ft2 (0.055 kPa or 550 dy/cm2).

For a particle Reynolds number greater than 

approximately 200, the dimensionless critical shear stress, 

or Shields parameter, becomes independent of the particle 

Reynolds number. Shields’ studies showed that in gravel-bed 

channels of homogeneous sediment sizes and turbulent flow, 

the value of dimensionless critical shear stress is 0.06. Typical 

values of the constant Shields parameter vary between 0.03 

and 0.06 for incipient motion. Based on the median (d50) 

sediment sizes in appendix 1, with most larger than 0.17 mm, 

a value of 0.04 (for fine gravel) was used in the analysis but 

was compared to the more conservative Shields parameter 

value of 0.06 (table 5). The critical shear stress at each of the 

four bridge locations in this study was less than the average 

shear stress for the d50 (see table 5). Based on mean daily 

shear stress from mean daily flow for the model period, 

sediment grains of the median (d50) size would be in motion. 

Information by specific site includes

• At Route 3 (fig. 3A), given the d50 of 0.384 mm 

(medium sand), the critical shear stress is less than the 

streambed shear stress on 98 percent of the simulation 

days, thus indicating that the d50 is in motion for 

98 percent of the simulation days. 

• At Route 28 (fig. 3B), given the d50 of less than 

0.0625 mm (coarse silt to very fine sand), the critical 

shear stress is less than the streambed shear stress on 

73 percent of the simulation days, thus indicating 

that the d50 is in motion for 73 percent of the simulation days. 

• At Short Falls Road (fig. 3C), given the d50 of 

0.6875 mm (coarse sand), the critical shear stress is 

less than the streambed shear stress on every day of 

the model period, thus indicating that the median (d50) 

sediment size was in motion for 100 percent of the 

simulation days. 

• At Route 4 (fig. 3D), given the d50 of 26 mm (coarse 

gravel), the critical shear stress is less than the streambed shear stress on 95 percent of the simulation days, 

thus indicating that the d50 is in motion for 95 percent 

of the simulation days.
Channel Morphology and Stream Processes 31

Table 4. Streambed thalweg elevations for the Suncook River, New Hampshire, study reach, surveyed 

(2008, 2009) and modeled (2009) using the Hydrologic Engineering Center River Analysis System (HEC–RAS) 

during a 400-day calibration, May 8, 2008–June 11, 2009.

[Location of cross sections AS–BT are shown in figure 3. ft, feet; NGVD 29, National Geodetic Vertical Datum of 1929; --, no data]

Suncook River 

cross section

Cumulative 

distance upstream1

(ft)

Minimum channel elevations

Surveyed Modeled June 11, 2009 

difference

May 8, 2008

(ft, NGVD 29)

June 11, 2009

(ft, NGVD 29)

June 11, 2009

(ft, NGVD 29)

Surveyed vs. 

modeled

(ft, NGVD 29)

AS 29,216 280.40 280.08 279.41 -0.67

-- 29,285 280.93 280.50 281.20 0.70

AT 29,347 279.21 280.25 279.36 -0.89

AU 29,600 279.10 279.24 279.11 -0.13

AV 30,715 279.32 280.08 278.74 -1.34

AW 31,747 276.97 277.01 276.96 -0.05

AX 33,142 276.47 278.04 276.46 -1.58

AY 34,227 279.64 280.00 279.64 -0.36

AZ 34,927 277.09 277.32 273.66 -3.66

BA 35,563 278.13 278.24 277.59 -0.65

BB 36,078 278.99 279.69 279.43 -0.26

BC 36,613 275.28 276.34 275.57 -0.77

BD 37,933 280.62 280.27 280.69 0.42

BE 38,426 279.70 279.89 279.82 -0.07

BF 39,121 277.56 279.13 279.09 -0.04

BG 39,861 277.90 278.32 279.20 0.88

BH 40,506 284.98 284.44 284.56 0.12

BI 41,066 285.20 284.12 285.15 1.03

BJ 41,899 278.65 276.65 278.78 2.13

BK 43,019 284.04 283.34 284.49 1.15

BL 43,741 279.70 282.71 280.45 -2.26

BM 44,821 285.93 285.24 285.76 0.52

BN 45,161 286.48 284.70 286.03 1.33

BO 45,724 278.21 280.04 279.03 -1.01

BP 46,356 286.15 286.19 285.89 -0.30

BQ 46,731 280.10 282.06 281.15 -0.91

BR 47,375 286.45 286.04 287.32 1.28

BS 48,053 283.65 286.32 286.75 0.43

BT 48,853 282.55 286.77 284.41 -2.36

1

 Feet upstream from confluence with Merrimack River (1 mile = 5,280 ft).
32 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

30,000 35,000 40,000 45,000 50,000

RIVER DISTANCE UPSTREAM FROM CONFLUENCE WITH MERRIMACK RIVER, IN FEET

270

280

290

300

Surveyed June 11, 2009 thalweg elevation

Model-predicted June 11, 2009 thalweg elevation

ELEVATION, IN FEET ABOVE NGVD 29

Buck Street Dams

Route 28

Short Falls Road Bridge

Figure 9. Suncook River streambed thalweg elevation between the Buck Street Dams and the Short Falls Road Bridge as 

surveyed on June 11, 2009, and as model-predicted using the Laursen (Copeland) sediment-transport function.

Table 5. Daily mean shear stress at four total sediment-load collection locations, Suncook River, Epsom to Allenstown, 

New Hampshire, during May 2008–end of water year 2009.

[%, percent; Shear stress values in pounds per square foot. Grain sizes in millimeters. d50, median grain size; d75, 75th-percentile grain size. TC, critical 

shear stress]

Bridge Route 3 Route 28 Short Falls Road Route 4

% days shear stress greater than 0.004–0.006 

(medium sand incipient motion)

97–99 25–32 100 100

% days shear stress greater than 0.02–06

(medium sand in suspension)

76–90 0.3–4 100 100

Channel mean shear stress 0.1875 0.0048 0.9517 1.259

Channel minimum shear stress 0.0021 0.00003 0.2199 0.1139

Channel maximum shear stress 1.47 0.08027 3.3451 5.2531

d50 0.384 <0.0625 0.6875 26

d50 classification name Medium sand Coarse silt to 

very fine sand

Coarse sand Coarse gravel

Shields (parameter = 0.06) critical shear stress 0.008 0.0012 0.0142 0.5269

Shields (parameter = 0.06) percentage of days d50 in motion 96 66 100 87

Shields (parameter = 0.04) critical shear stress 0.0053 0.0008 0.0095 0.3513

Shields (parameter = 0.04) percentage of days d50 in motion 98 73 100 95

d75 0.5 0.125 1 121

d75 classification name Medium to 

coarse sand

Very fine to 

fine sand

Coarse to very 

coarse sand

Small cobbles

Lane’s (TC) critical shear stress 0.0079 0.002 0.0158 1.905

Lane’s (TC) percentage of days d75 in motion 96 51 100 13
Channel Morphology and Stream Processes 33

At the Route 28 bridge during the study simulation 

period, the modeled daily mean cross-section shear stress 

ranged from 0.00003 to 0.08 lb/ft2, with an average daily mean 

shear stress of 0.005 lb/ft2. On 68 percent of the days, the daily 

mean cross-section shear stress was less than 0.004 (medium 

sand not moving, but fine sand moving); on 75 percent of 

the days, it was less than 0.006 (coarse sand not moving, but 

medium and fine sand moving). On 96 percent of the days, the 

daily mean cross-section shear stress was less than 0.02, and 

on 99.7 percent of the days the daily mean cross-section shear 

stress was less than 0.06 (table 5). On 25 to 32 percent of the 

days, the sediment smaller than or equal in size to medium 

sand was in motion. On approximately 0.3 to 4 percent of the 

days, the sediment smaller than or equal in size to medium 

sand was in suspension. On 73 percent of the simulation days, 

sediment smaller than or equal in size to very fine sand was in 

motion (table 5). These results are based on daily mean flows. 

The shear stress analysis results for the Route 28 bridge 

are in contrast to those for the next upstream bridge at Short 

Falls Road, where sediment smaller than or equal in size to 

medium sand was in suspension and sediment smaller than or 

equal in size to coarse sand was in motion for 100 percent of 

the study simulation days. Much of the sediment passing the 

Short Falls Road Bridge had dropped out of suspension before 

reaching the Route 28 bridge. The analysis at the Route 28 

bridge shows that for only 25 to 32 percent of the simulation 

days, sediment smaller than or equal in size to medium sand 

was in motion. This indicates that the reach between Short 

Falls Road Bridge and the downstream Route 28 bridge is 

retaining much of its coarser sediment load based on the 

streambed shear stress and the reach median grain sizes (d50) 

of a very fine gravel for all sediment-sampling locations below 

the avulsion and of a coarse sand at the Short Falls Road 

Bridge. Shear stress is a function of the energy gradient (or 

slope of the water surface) and hydraulic radius (approximated 

by the mean depth). An increase in either or both of these 

parameters will result in greater shear stress and an increased 

ability of the water in the channel to move sediment. 

At the Short Falls Road and Route 4 bridges during the 

model period, the average daily mean cross-section shear 

stresses were 0.95 and 1.26 lb/ft2, respectively (table 5). The 

daily mean cross-section shear stress analysis at Route 4 

indicates that sediment sizes smaller than or equal in size to 

coarse gravel were in motion for 95 percent of the simulation 

days, and medium sand was in suspension for 100 percent of 

the simulation days. 

Although the Shields method has been widely used to 

determine initiation of motion, it does not account for the 

shielding of smaller particles by large particles. Because of 

this, a comparison was made to Lane’s (1953) method 

(table 5). Lane observed that smaller particles were 

consistently shielded by larger ones, and that critical shear 

stress was related to the diameter of the particle. Lane felt that 

initiation of motion was better represented by a particle size in 

which only 25 percent of the particles were larger by weight 

(d75). Results for the bridge sampling locations were similar to 

the more conservative Shields parameter value (0.06) results 

with the exception of the Route 4 bridge location. Critical 

shear stress calculated using Lane’s method was 3.6 times 

greater than that calculated with a Shields parameter value 

of 0.06, with the d75 (121 mm) being in motion for only 

13 percent of the study-period days. This d75 sediment size 

falls within the range of small cobbles (64 to 128 mm) (ASCE, 

2006) and indicates that sediment sizes in the cobble and 

boulder ranges are generally not in motion in this location.

The critical shear stress associated with sediment entrainment has been described as being, at best, a minimum estimate of the critical discharge because only a small area of the 

surface of the entire streambed or only a few particles of the 

d50 size might be entrained by the critical discharge (Lisle and 

others, 1993). The complete mobilization of a size fraction, 

such as d50, has been observed as occurring at roughly twice 

the shear necessary to initiate movement of individual particles of that size fraction (Wilcock and McArdell, 1993). The 

distribution of streambed shear stress for the 50- and 1-percent 

annual exceedence probability (2- and 100-year recurrenceintervals, respectively) floods and the critical streambed shear 

stress based on the d50 particle size were plotted (figs. 10A–B) 

to show areas where, for either the 50- or 1-percent annual 

exceedence probability flood, little to no sediment is moving 

(as defined by: τ0 less than τc), some sediment is moving (as 

defined by: τ0 greater than τc and τ0 less than 2 τc), and much 

sediment is moving (as defined by: τ0 greater than 2 τc); 

where τ0 is the mean boundary shear stress and τc is the critical 

shear stress.

MEASURED STREAMBED ELEVATION CHANGE, IN FEET

SIMULATED STREAMBED ELEVATION

CHANGE, IN FEET

-4 -3 -2 -1 4

-2

0

2

4

Suncook River measured vs. simulated streambed thalweg change

-4

0 1 2 3 5

1:1

Figure 10. Measured vs. simulated streambed thalweg-elevation 

changes for the Suncook River from the Buck Street Dams to the 

Short Falls Road Bridge, Epsom and Pembroke, New Hampshire, 

June 11, 2009.
34 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Figures 11A and B show that, in general, there is much 

(as defined by: τ0 greater than 2 τc) sediment moving for 

recurrence-interval floods greater than the 50-percent annual 

exceedence probability flood (based on d50). In figure 11A 

(table 6), the reach (2,315 ft in length) from approximately 

90 ft downstream of the Buck Street Dams to cross section 

AK has some (as defined by: τ0 greater than τc and τ0 less than 

2 τc) sediment moving for the 100-year flood, and little to no 

(as defined by: τ0 less than τc) sediment moving for the 2-year 

flood. In figure 11B (table 6), the reach (1,091 ft in length) 

between cross sections CK and CM (located adjacent to Round 

Pond) has some to no sediment moving for the 2-year flood. 

The reach (4,651 ft in length) between cross sections CM and 

CD (downstream of the flood chute at the large meander bends 

to adjacent to Round Pond) has some (as defined by: τ0 greater 

than τc and τ0 less than 2 τc) sediment moving for the 100-year 

flood (flow is in channel and on overbanks). The reach 

(2,797 ft in length) between cross sections DE and CT (new 

reach through the former sand pit) has some (as defined by: τ0 

greater than τc and τ0 less than 2 τc) sediment moving for the 

2-year flood. The 584-foot reach between cross sections DE 

and DC (upstream of the avulsion) has some (as defined by: τ0 

greater than τc and τ0 less than 2 τc) sediment movement for the 

100-year flood. 

The variability in potential for entrainment is important 

because sediment entrained at one cross section may be unable 

to move at a cross section farther downstream, resulting in 

deposition and possibly aggradation over a period of time. The 

potential for aggradation in the Suncook River is greatest in 

the reach downstream of the former sand pit—from adjacent 

to Round Pond to downstream of the flood chute, at the large 

meander bends (cross sections CA to CM), and from downstream of the Short Falls Road bridge to approximately 

3,800 ft upstream of the Route 28 bridge.

Model and Field-Data Sediment-Transport 

Curves

The HEC–RAS sediment-transport model requires analyses of data on grain size for use with empirical equations in 

order to estimate total sediment transport, among other parameters. Total-sediment-transport curves (figs. 12A–D) were 

developed from total-sediment-load data (suspended load plus 

bedload) collected in the field at four bridge locations along 

the Suncook River (table 3); and from model-simulated total 

sediment loads based on field-collected streambed-gradation 

data. These curves show mean daily streamflow plotted against 

mean daily total-sediment load; they were created using fieldmeasured flow data, field-collected total-sediment data, and 

model output of total-sediment load (based on mean daily 

flow data from May 8, 2008 through the end of WY 2009). 

The field-collected streambed-gradation data were input into 

the model and generated sediment-load output estimates that 

were compared with field-collected total-sediment load data to 

calibrate and to verify the sediment-transport model. 

Scatterplots of logarithms of these data tend to plot as 

linear patterns. The sediment loads (table 3) at the Route 3 

bridge (fig. 12A) and Route 28 bridge (fig. 12B) are similar for 

the mean daily flow. Although the Route 4 bridge (fig. 12D) 

has a greater sediment discharge for a given flow than the 

Route 3 and Route 28 bridges, the data for the Route 4 bridge 

sediment-discharge curve are similar in trend to the data from 

both downstream sites. The Short Falls Road Bridge has a 

much greater sediment discharge for a given flow than any 

of the other three sites. The largest source of sediment for the 

study reach is the area of the sand pit and Leighton Brook 

(fig. 2) downstream of the avulsion, and most of this sediment load is being deposited in the reach between the sand-pit 

area and the Route 28/Buck Street Dams, a reach distance of 

approximately 7.0 mi. 

These sediment-transport curves include the values of 

the bankfull discharge (Qb), which has been approximated 

as a 1.8-year recurrence-interval discharge, and of the annual 

mean discharge (Qa). Both these values have been adjusted for 

drainage area for each of the four locations at which data 

were collected.

Differences between observed and predicted sediment 

discharge can be seen for flows below 600 to 800 ft3/s in the 

sediment-transport curves (figs. 12A–D) for the four total-load 

collection sites. To aid in validating the sediment-transport 

model, estimates of the field-collected total sediment load 

were compared with estimates of the model-output total 

sediment load, which were determined using the bed-material 

gradation data. Estimated sediment discharge (in tons per day) 

for the field-collected total sediment load deviates from that 

for the modeled bed-material dataset for flows below 

600 to 800 ft3/s. Downstream of the Short Falls Road Bridge, 

at Route 28 and Route 3, field-collected total sediment-load 

data yield higher estimates of sediment discharge for flows 

less than 600 to 800 ft3/s than does the sediment-transport 

model using bed-material gradation data. At the Short Falls 

Road Bridge and upstream at Route 4, however, field-collected 

data on total sediment load yielded lower estimates of 

sediment discharge for flows less than 600 to 800 ft3/s than the 

sediment-transport model using bed-material gradation data.

The sediment-transport curve determined at the Route 28 

bridge (fig. 11B) indicates that, based on sediment-gradation 

values obtained in the field and from simulation results, the 

volume of sediment transport (at flows less than 400 ft3/s) 

is either too little for the model to determine, or sediment 

is settling out of suspension (less than 0.01 tons per day). 

Field-collected total-sediment loads at discharges less than 

400 ft3/s are likely greater than model-predicted total-sediment 

loads due to the large volume of fine silt and sediment on 

the overbanks that washes into the river during precipitation 

events. These sediment data were subsequently field-collected 

as suspended sediment. In the sediment-transport curve for 

Route 28, the sediment-transport model deviates from the 

field data at flows less than approximately 800 ft3/s in that the 

model predicts a much lower sediment discharge for flows 

less than 800 ft3/s. It is possible that the sediment data entered 
Channel Morphology and Stream Processes 35

0 10,000 20,000 30,000 40,000

DISTANCE UPSTREAM FROM CONFLUENCE WITH MERRIMACK RIVER, IN FEET

0

2

4

6

SHEAR STRESS, IN POUNDS PER SQUARE FOOT

50-percent annual exceedence flood shear stress (TO)

10-percent annual exceedence flood shear stress (TO)

1-percent annual exceedence flood shear stress (TO)

Critical shear stress (TC)

2 times critical shear stress (2TC)

A

40,000 50,000 60,000 70,000 80,000

0

2

4

6

DISTANCE UPSTREAM FROM CONFLUENCE WITH MERRIMACK RIVER, IN FEET

SHEAR STRESS, IN POUNDS PER SQUARE FOOT

B

50-percent annual exceedence flood shear stress (TO)

10-percent annual exceedence flood shear stress (TO)

1-percent annual exceedence flood shear stress (TO)

Critical shear stress (TC)

2 times critical shear stress (2TC)

Figure 11B. Distribution 

of streambed shear 

stress in the Suncook 

River for the 50-, 10- 

and 1-percent annual 

exceedence probability 

flood, and of critical 

streambed shear stress 

based on the d50 (cross 

sections BL to DW).

Figure 11A. Distribution 

of streambed shear 

stress in the Suncook 

River for the 50-, 10- 

and 1-percent annual 

exceedence probability 

floods, and of critical 

streambed shear stress 

based on the d50 (cross 

sections A to BL).
36 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Suncook 

River 

cross 

section

Cumulative 

distance 

upstream 

(ft)

Channel shear stress of 

annual exceedenceprobability flood 

(lb/ft2)

Critical 

shear 

stress 

(lb/ft2 50 )

percent

10 

percent

1 

percent

A 0 1.16 1.15 1.38 0.0055

B 477 0.73 0.78 1.29 0.0055

C 804 0.16 0.24 0.43 0.0055

D 1,681 2.93 3.79 4.52 0.0055

1,864 0.93 1.75 4.4 0.0055

1,883 3.64 5.03 6.96 0.0055

E 2,043 0.37 0.6 0.76 0.0055

F 2,193 3.58 3.45 2.85 0.0055

2,337 3.01 3.08 2.05 0.0055

2,363 2.6 3.49 4.58 0.0055

G 2,507 0.12 0.2 0.37 0.0055

H 3,079 0.09 0.2 0.44 0.0055

3,093 0.17 0.48 1.29 0.0055

3,124 0.02 0.06 0.18 0.0055

3,176 0.52 1.43 3.35 0.0055

I 3,401 0.52 1.43 3.34 0.0055

J 3,467 0.51 1.37 3.15 0.0055

K 3,880 0.37 0.66 0.93 0.0055

3,889 4.38 4.35 2.17 0.0055

3,898 0.43 0.67 0.84 0.0055

3,907 0.21 0.37 0.57 0.0055

3,914 0.37 0.59 0.77 0.0055

3,916 0.45 0.69 0.85 0.0055

3,925 2.8 3.63 3.11 0.0055

L 3,934 0.17 0.41 0.82 0.0055

M 4,433 0.19 0.46 0.89 0.0055

N 4,507 0.17 0.43 0.82 0.0055

4,509 3.35 4.21 5.37 0.0055

4,525 3.34 4.19 5.39 0.0055

O 4,640 2.08 2.65 3.43 0.0055

P 4,886 0.11 0.25 0.49 0.0055

Q 5,126 0.12 0.31 0.71 0.0055

R 5,283 0.12 0.25 0.44 0.0055

S 5,575 0.1 0.17 0.28 0.0055

T 5,860 0.09 0.14 0.21 0.0055

U 6,045 0.08 0.07 0.08 0.0055

6,057 0.11 0.26 0.53 0.0055

6,109 0.13 0.32 0.7 0.0052

V 6,172 0.19 0.5 1.2 0.0052

W 6,935 0.58 1.22 1.99 0.0052

Table 6. Distribution of streambed shear stress in the Suncook River for the 50-, 10- and 1-percent annual exceedence 

probability flood.

[ft, feet; lb/ft2, pounds per square feet]

Suncook 

River 

cross 

section

Cumulative 

distance 

upstream 

(ft)

Channel shear stress of 

annual exceedenceprobability flood 

(lb/ft2)

Critical 

shear 

stress 

(lb/ft2 50 )

percent

10 

percent

1 

percent

X 6,945 0.28 0.61 0.65 0.0052

Y 7,981 0.07 0.18 0.29 0.0052

Z 9,331 2.28 2.78 2.17 0.0083

AA 10,614 0.18 0.42 0.76 0.0083

AB 11,542 0.11 0.22 0.29 0.0083

AC 12,282 0.08 0.23 0.4 0.0105

AD 15,482 0.07 0.13 0.23 0.0105

AE 17,581 0.13 0.24 0.13 0.0115

AF 19,123 0.13 0.31 0.45 0.0115

AG 22,592 0.08 0.14 0.27 0.0132

AH 23,529 0.18 0.17 0.03 0.0054

AI 24,763 0.25 0.41 0.04 0.0054

AJ 25,387 0.21 0.25 0.06 0.0054

AK 26,696 0.31 0.31 0.17 0.0054

AL 27,674 0.24 0.39 0.84 0.2604

AM 28,253 0.24 0.33 0.56 0.2604

AN 28,443 0.19 0.28 0.51 0.2604

AO 28,766 1.34 0.87 1.09 0.2604

AP 28,851 1.74 1.54 1.85 0.2604

28,852 1.45 1.74 2.33 0.2604

28,866 0.13 0.32 0.76 0.2604

29,011 0.15 0.41 1.11 0.2604

29,038 0.08 0.2 0.33 0.0008

AQ 29,063 0.03 0.08 0.24 0.0008

AR 29,088 0.03 0.07 0.12 0.0008

AS 29,216 0.02 0.07 0.14 0.0008

29,233 0.02 0.07 0.14 0.0008

29,278 0.02 0.07 0.15 0.0008

29,285 0.03 0.08 0.17 0.0008

AT 29,347 0.03 0.08 0.11 0.0008

AU 29,600 0.03 0.09 0.19 0.0008

AV 30,715 0.04 0.11 0.22 0.0008

AW 31,747 0.03 0.09 0.14 0.0008

AX 33,142 0.03 0.08 0.14 0.0008

AY 34,227 0.05 0.11 0.18 0.0019

AZ 34,927 0.08 0.21 0.41 0.0019

BA 35,563 0.04 0.11 0.21 0.0019

BB 36,078 0.15 0.34 0.54 0.0019

BC 36,613 0.09 0.22 0.42 0.0019

BD 37,933 0.18 0.29 0.36 0.0019
Channel Morphology and Stream Processes 37

Suncook 

River 

cross 

section

Cumulative 

distance 

upstream 

(ft)

Channel shear stress of 

annual exceedenceprobability flood 

(lb/ft2)

Critical 

shear 

stress 

(lb/ft2 50 )

percent

10 

percent

1 

percent

BE 38,426 0.14 0.29 0.43 0.0019

BF 39,121 0.12 0.2 0.24 0.0019

BG 39,861 0.12 0.25 0.4 0.0019

BH 40,506 0.19 0.25 0.26 0.0019

BI 41,066 0.14 0.16 0.17 0.0041

BJ 41,899 0.12 0.2 0.27 0.0062

BK 43,019 0.23 0.39 0.58 0.0062

BL 43,741 0.13 0.25 0.43 0.0062

BM 44,821 0.19 0.38 0.68 0.0062

BN 45,161 0.15 0.36 0.51 0.0062

BO 45,724 0.09 0.18 0.31 0.0062

BP 46,356 0.23 0.42 0.74 0.0062

BQ 46,731 0.15 0.25 0.39 0.0031

BR 47,375 0.16 0.17 0.18 0.0031

BS 48,053 0.25 0.39 0.55 0.0031

BT 48,853 0.19 0.3 0.46 0.0031

BU 50,158 0.27 0.4 0.58 0.0093

BV 50,624 0.05 0.07 0.1 0.0093

BW 50,829 0.14 0.09 0.1 0.0093

50,989 0.03 0.08 0.15 0.0093

BX 51,081 0.41 0.37 0.46 0.0093

51,134 0.25 0.51 0.98 0.0093

51,175 0.37 0.61 1.03 0.0095

51,239 0.32 0.64 1.3 0.0095

51,254 0.6 1.01 1.71 0.0095

BY 51,305 0.34 0.54 0.88 0.0095

BZ 51,975 0.18 0.2 0.23 0.0095

CA 52,463 0.12 0.08 0.08 0.0095

CB 53,600 0.22 0.09 0.06 0.0095

CC 54,415 0.18 0.04 0.02 0.0095

CD 55,869 0.08 0.02 0.01 0.0172

CE 56,345 0.19 0.04 0.02 0.0172

CF 56,880 0.06 0.02 0.01 0.0172

CG 57,636 0.2 0.05 0.02 0.0172

CH 58,327 0.13 0.06 0.03 0.0172

CI 58,367 0.23 0.1 0.03 0.0172

CJ 59,099 0.29 0.17 0.05 0.0172

CK 59,429 0.27 0.26 0.1 0.2162

CL 59,964 0.54 0.31 0.05 0.2162

CM 60,520 0.27 0.31 0.1 0.2162

Suncook 

River 

cross 

section

Cumulative 

distance 

upstream 

(ft)

Channel shear stress of 

annual exceedenceprobability flood 

(lb/ft2)

Critical 

shear 

stress 

(lb/ft2 50 )

percent

10 

percent

1 

percent

CN 61,160 1.23 3.07 0.92 0.0180

CO 61,868 0.37 0.71 1.2 0.0158

CP 62,547 0.88 1.38 2.28 0.0158

CQ 63,035 0.3 0.66 1.28 0.0115

CR 63,475 0.59 0.78 1.27 0.0115

CS 63,795 0.14 0.19 0.26 0.0115

CT 64,132 0.38 0.62 1.26 0.2027

CU 64,400 0.4 0.62 1.21 0.2027

CV 64,632 0.56 0.57 0.86 0.2027

CW 64,893 0.45 0.6 0.8 0.1588

CX 65,150 0.62 0.76 0.97 0.1588

CY 65,883 0.34 0.54 0.96 0.2635

CZ 65,893 0.34 0.58 1.09 0.2635

DA 66,029 0.76 0.84 1.15 0.2635

DB 66,140 0.53 0.68 0.93 0.3423

DC 66,345 2.62 2.54 3.5 0.3423

DD 66,627 1.45 2.49 4.08 2.2403

DE 66,929 5.59 5.61 6.48 2.2403

DF 67,651 0.71 1.29 2.38 2.2403

DG 68,733 0.22 0.51 1.05 2.2403

DH 69,552 4.43 2.63 1.97 1.4728

69,665 3.63 6.47 5.54 1.4728

69,726 2.04 3.09 3.78 0.3513

DI 69,783 0.1 0.22 0.37 0.3513

69,998 0.73 1.49 1.6 0.3513

DJ 70,008 0.55 0.84 0.69 0.3513

DK 70,501 0.07 0.11 0.13 0.3513

DL 70,813 0.41 0.54 0.24 0.3513

DM 71,389 0.16 0.16 0.15 0.3513

DN 71,617 0.34 0.33 0.3 0.3513

DO 72,166 0.12 0.21 0.24 0.3513

DP 74,513 0.16 0.33 0.64 0.0419

DQ 74,551 0.28 0.37 0.57 0.0419

DR 75,309 0.14 0.25 0.5 0.0419

DS 76,283 0.25 0.4 0.68 0.0090

DT 78,389 0.28 0.36 0.32 0.0090

DU 79,631 0.34 0.51 0.5 0.0251

DV 81,372 0.25 0.27 0.3 0.0251

DW 83,919 0.24 0.24 0.33 0.0251

Table 6. Distribution of streambed shear stress in the Suncook River for the 50-, 10- and 1-percent annual exceedence 

probability flood.—Continued

[ft, feet; lb/ft2, pounds per square feet]
38 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

DAILY MEAN DISCHARGE, IN CUBIC FEET PER SECOND

105

104

103

102

101

100

10-1

10-2

10-3

10-4

10-5

104 103 10 3 4 5 6 7 8 2 2 3 4 5 6 7 8 2 3 4 5 6 7 8 Qa = 568 ft3/s Qb = 3,120 ft3/s

U.S. Geological Survey field-collected total load

Laursen (Copeland) sediment-transport function, mean daily sediment discharge

DAILY MEAN SEDIMENT DISCHARGE, IN TONS PER DAY

Qa = 601 ft3/s

Qb = 3,300 ft3/s

106

105

104

103

102

101

100

10-1

10-2

10-3

10-4

10-5

10-6

104 103 10 4 5 6 7 8 2 2 3 4 5 6 7 8 2 3 4 5 6 7 8

Qa

Qb

Water year 2009 annual mean flow, in cubic feet per second (ft3/s)

Bankfull discharge, in cubic feet per second (ft3

/s)

U.S. Geological Survey field-collected total load

Laursen (Copeland) sediment-transport function, mean daily sediment discharge

Qa

Qb

Water year 2009 annual mean flow, in cubic feet per second (ft3

/s)

Bankfull discharge, in cubic feet per second (ft3

/s)

A Route 3 bridge

B Route 28 bridge

Figure 12A–B. Sediment-transport curve for Suncook River at A, Route 3 bridge, and B, Route 28 

bridge.
Channel Morphology and Stream Processes 39

Figure 12C–D. Sediment-transport curve for Suncook River at C, Short Falls Road Bridge, and 

D, Route 4 bridge.

C Short Falls Road bridge

D Route 4 bridge

Qa = 501 cfs

Qb = 2,750 cfs

104

103

102

101

100

10-1

10 2 3 4 5 6 7 8 2 10 2 3 4 5 6 7 8 3 2 3 4 5 6

U.S. Geological Survey field-collected total load

Laursen (Copeland) sediment-transport function, mean daily sediment discharge

Qa

Qb

Water year 2009 annual mean flow, in cubic feet per second (ft3/s)

Bankfull discharge, in cubic feet per second (ft3

/s)

Qa = 390 cfs

Qb = 2,150 cfs

MEAN DAILY DISCHARGE, IN CUBIC FEET PER SECOND

MEAN DAILY SEDIMENT DISCHARGE, IN TONS PER DAY 105

104

103

102

101

100

10-1

10-2

10-3

103 2 3 10 2 3 4 5 6 7 8 2 101 2 3 4 5 6 7 8

U.S. Geological Survey field-collected total load

Laursen (Copeland) sediment-transport function, mean daily sediment discharge

Qa

Qb

Water year 2009 annual mean flow, in cubic feet per second (ft3/s)

Bankfull discharge, in cubic feet per second (ft3

/s)
40 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

into the model are based on gradation data for bed material 

that is coarser than that of the suspended and bedload field 

data, and that this difference in grain size, in turn, is due to 

the extensive aggradation in the reach from upstream of the 

Route 28 bridge to the Short Falls Road Bridge, both in the 

channel and on the overbanks. Bed material at the upstream 

and downstream faces of the Route 28 bridge does not consist 

of a high proportion of fine sand, silt and clay sediment; 

the lower weight density of the sediment and higher flow 

velocities at the bridge ensure that the fine sediment remains 

in suspension and is carried downstream of the bridge. This 

fine sediment in suspension was sampled on days that field 

data were collected. Much of the sediment generated by the 

avulsion appears to have settled between the Short Falls Road 

Bridge and the east and west Buck Street Dams, and some has 

passed over the dams. To a lesser degree, the same deviation 

in the field-collected total-load data in relation to the modeled 

bed-material data for sediment discharge at lower flows at 

the Route 28 bridge (fig. 12B) can also be seen at the Route 3 

bridge (fig. 12A) downstream of the Route 28 bridge.

In the sediment-transport curve for the Short Falls Road 

Bridge (fig. 12C), the HEC–RAS sediment-transport model 

(using bed-material gradation) predicts a higher sediment 

discharge than the field-collected total-load data for flows less 

than 600 ft3/s. An explanation may be that the sediment data 

entered into the model are based on gradation data for bed 

material that is finer than the suspended and bedload material, possibly indicating a later change to a coarser gradation 

upstream of the Short Falls Road Bridge. At the downstream 

face of the Short Falls Road Bridge, the river widens abruptly. 

An abrupt decrease in flow velocities associated with this 

widening is a result of the increased cross-sectional flow area. 

Finer sediment therefore falls out of suspension downstream 

of the Short Falls Road Bridge, thereby strongly influencing 

the gradation of bed material. In addition, the field-collected 

data on bed material for the reach upstream of the Short Falls 

Road Bridge may not reflect the volume of fine silt and clay 

sediment on the overbanks and in the stream that is found in 

the reach downstream of the Short Falls Road Bridge. To a 

lesser degree, the same discrepancy between low flows and 

sediment discharge at the Short Falls Road Bridge (fig. 12C) 

can be seen at the Route 4 bridge upstream of the Short Falls 

Road Bridge (fig. 12D). At the Route 4 bridge, however, the 

modeled data on bed material show a finer gradation with 

higher sediment discharge for a given flow than do the fieldcollected data on total load, perhaps because the channel has 

much more cobble and gravel than the channel at the Short 

Falls Road Bridge downstream. These anomalies add to the 

uncertainties that field-collection activities provide for accurately representing suspended sediment and bedload sediment 

near the streambed. 

Simulating Sediment Transport to Investigate the 

Impact of Future Sediment Loads on Flood Flows

The sediment model using the Laursen (Copeland) 

sediment-transport equation was used to determine sediment 

transport and streambed elevations for the end of WY 2010. 

Using channel geometry, mean daily flow and water-temperature data from WY 2009, the quasi-unsteady-state sedimenttransport model was run for WY 2010. The resultant cross 

sections were imported into a steady-state-flow model, which 

was then run to determine the effect of the altered channel bed 

on elevations for the 50-, 10-, 2-, 1-, and 0.2-percent annual 

exceedence probability floods. Although the HEC–RAS model 

uses data from the simulated quasi-unsteady flow to determine 

specific values for the streambed elevation, these values represent predicted averages for each stream reach. 

Suncook River streambed-thalweg elevations from 

May 2008, Flood Insurance Studies (surveyed in 1976), 

and predicted by the model for end of WY 2010, are shown 

in figures 13A–D. These thalweg elevations have been 

estimated between surveyed cross sections by straight-line 

interpolation. In general, it can be seen that the reach below 

the avulsion has aggraded, while the reach above the avulsion 

has degraded. The depths of aggradation and degradation can 

be approximated from these figures by comparing the lowest 

stream cross-section elevations (thalweg) from the 1978 Flood 

Insurance Study and the 2008 USGS surveyed elevations with 

the elevations predicted for the end of WY 2010, although 

other areas of the cross section may have a larger or smaller 

level of aggradation or degradation.

Changes in elevation (at each cross section) of the 

streambed thalwegs for the selected floods predicted by the 

model for the end of WY 2010 as compared with the streambed thalwegs surveyed in May 2008, and predicted by the 

steady-state modeled water surface using the May 2008 data 

(Flynn, 2010), are shown in table 7 (in back of report). The 

change in streambed thalweg (table 8) at all of the crosssection locations was a mean degradation of 0.46 ft (median 

degradation of 0.57 ft). There was greater mean degradation (channel erosion) above the avulsion as compared to 

that below the avulsion for the model simulation period. The 

model simulation shows that for all cross-section locations, 

overall degradation resulted in the greatest effect for the higher 

annual exceedence probability (lower recurrence interval) 

floods (table 8) with a decreased mean flood elevation for the 

50-percent annual exceedence probability (2-year recurrence 

interval) flood of 0.39 ft (median decrease of 0.01 ft).

The change in the streambed thalweg for the crosssection locations between the Short Falls Road Bridge and 

the Buck Street Dams for the May 2008 to the end of 

WY 2010 simulation period was a mean aggradation (net 

raising of streambed) of 0.88 ft (median aggradation of 0.17 ft) 

(table 8). The simulation-period results show that this channel 

reach is continuing to fill with sediment, with the greatest 

effect of the sediment deposition on water-surface elevation 

occurring for the higher annual exceedence probability 
Channel Morphology and Stream Processes 41

Figure 13A–B. Thalweg elevations of the Suncook River streambed from data published in May 2008 and in 

1978 Flood Insurance Studies and from model predictions for end of water year 2010 in A, cross sections A 

to AI, and B, cross sections AJ to BT.

0 5,000 10,000 15,000 20,000

160

200

240

280

320

May 2008 thalweg and cross-section label

Predicted October 2010 thalweg

1978 Flood Insurance Study thalweg and

cross-section label A

B

C

D

E

F G H

I

ABC

D

E

F

G

H

I

J

K

L

M

N

O P RST

U V

W

X Y Z AA AB AC AD AE AF AG AH AI

China Mill Dam

Pembroke Dam

Webster Dam Spillway

Webster Dam

Route 3 

25,000

Q

F

Y

25,000 30,000 35,000 40,000 45,000

260

270

280

290

May 2008 thalweg and cross-section label

Predicted October 2010 thalweg

1978 Flood Insurance Study thalweg and

cross-section label

J

K

L

M

N O

P

A

B

C

D

E

AJ AK

AL

AM

AN

AO

AP

AQ AR

AS

AT

AU AV

AW AX

AY

AZ

BA

BB

BC

BD BE

BF BG

BH BI

BJ

BK

BL

BM BN

BO

BP

BQ

BR

BS

BT

CUMULATIVE RIVER DISTANCE UPSTREAM FROM CONFLUENCE WITH MERRIMACK RIVER, IN FEET

ELEVATION, IN FEET ABOVE NGVD 29

Buck Street Dams

Route 28

50,000

300

O

BA

A

B
42 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Figure 13C–D. Thalweg elevations of the Suncook River streambed from data published in May 2008 and in 

1978 Flood Insurance Studies and from model predictions for end of water year 2010 in C, cross sections BU 

to DQ, and D, cross sections DR to DW.

75,000 79,000 83,000 87,000

310

320

330

340

May 2008 thalweg and cross-section label

Predicted October 2010 thalweg

1978 Flood Insurance Study thalweg and

cross-section label

AB

DR

DS

DT DU

DV

DWUpstream Corporate Limit 

Epsom, N.H.

CUMULATIVE RIVER DISTANCE UPSTREAM FROM CONFLUENCE WITH MERRIMACK RIVER, IN FEET

350

AB

DU

50,000 55,000 60,000 65,000 70,000

270

280

290

300

310

320

May 2008 thalweg and cross-section label

Predicted October 2010 thalweg

1978 Flood Insurance Study thalweg and

cross-section label

F

G

H

I

J

K

L

M

N

O

P

V

W

X Z AA

BU

BV

BW

BX

BY

BZCA CB

CC

CD

CE CF

CG

CH

CI

CJCKCL

CM

CN

COCPCQCRCSCT

CU

CVCW

CX

CY

CZDA

DB

DC

DD

DE

DF

DG

DH

DI

DJ

DKDLDM

DN

DO DPDQ

ELEVATION, IN FEET ABOVE NGVD 29

Abandoned channel location between "P" and "V"

Short Falls Road

Avulsion location

Route 4 

75,000

330

Y

M

CG

C

D
Sediment Load and its Effect on Streambed and Flood Elevations 43

(lower recurrence interval) floods. For the 50-percent annual 

exceedence probability (2-year recurrence interval) flood, the 

simulated mean aggradation of 0.88 ft (median aggradation of 

0.17 ft) resulted in an increased mean flood elevation of 0.20 ft 

(median increase of 0.14 ft). 

The mean change to the water-surface elevations at all 

the cross sections (table 8) was a lowering by 0.39 ft (median 

of 0.01 ft) of the 50-percent annual exceedence probability 

(2-year recurrence interval) flood, 0.28 ft (median of 0.03 ft) 

for the 10-percent annual exceedence probability (10-year 

recurrence interval) flood, 0.19 ft (median of 0.06 ft) for the 

2-percent annual exceedence probability (50-year recurrence 

interval) flood, 0.18 ft (median of 0.14 ft) for the 1-percent 

annual exceedence probability (100-year recurrence interval) 

flood, and 0.16 ft (median of 0.14 ft) for the 0.2-percent 

annual exceedence probability (500-year recurrence 

interval) flood.

Electrical Resistivity Data

Preliminary USGS geophysical data obtained during 

summer 2008 using electrical resistivity methods indicate a 

near-surface resistive (consolidated) layer, possibly bedrock or 

till, along the northern and southern banks of the new channel 

in the area of the sand pit (figs. 5A–B, 14A–B). The nearsurface resistive layer (figs. 14A–B) on the north bank of the 

new channel in the sand pit is at an elevation higher than the 

river, extending to an elevation 3 to 6 ft (0.9 to 1.8 m) below 

the right-bank terrace of the sand pit (approximately 

8 ft (2.4 m) above the river thalweg), as shown in geophysical transect line 4 (fig. 14B). At cross section CU, the channel 

elevation is 303.38 ft (NGVD 29), whereas the elevation of 

the top of the near-surface resistive layer on the north bank in 

the sand pit is approximately 311 ft (NGVD 29). The 2-year 

recurrence-interval flood, which is approximately the bankfull, 

or channel-forming, discharge, is at an elevation of 

310.8 ft at cross section CU. If the near-surface resistive layer 

is bedrock, it may serve to constrain lateral migration of the 

new channel into the former sand pit during bankfull flow. 

That constraint may avert the greater spatial and temporal 

influx downstream of sediment that could occur if the river 

were to migrate laterally into the sand-pit area. The extent of, 

and the degree to which, the near-surface resistive layer will 

constrain lateral migration of the river during flows greater 

than bankfull events remains unclear because geophysical data 

are limited.

Sediment Load and its Effect on 

Streambed and Flood Elevations

Simulation results and field-collected sediment data 

indicate that, for the daily mean flows downstream of the 

avulsion channel during the study field-collection period, 

approximately 70 to 500 tons of sediment per day were 

moving downstream past the Short Falls Road Bridge, 

approximately 0.02 to 50 tons of sediment per day were 

moving past the Route 28 bridge, and approximately 1.0 to 

200 tons per day were moving past the Route 3 bridge. An 

analysis of the HEC–RAS quasi-unsteady-state flow model 

(table 9, fig. 15) showed that, for WY 2009 (October 1, 

2008 through September 30, 2009), the mean daily sediment 

discharge (for mean daily flows) for all cross sections in 

WY 2009 was 165 tons per day, with a standard deviation of 

Table 8. Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted changes in water-surface elevation and in 

thalweg elevation above and below the avulsion and between Short Falls Road and Buck Street Dams from 2008 through the end of 

water year 2010.

[XS, cross section; ft, feet; %, percent]

All XS 

mean 

change 

(ft)

All XS 

median 

change 

(ft)

Above 

avulsion 

mean 

change 

(ft)

Above 

avulsion 

median 

change 

(ft)

Below 

avulsion 

mean 

change 

(ft)

Below 

avulsion 

median 

change 

(ft)

Short Falls 

Road to Buck 

Street Dams 

mean change 

(ft)

Short Falls 

Road to Buck 

Street Dams 

median change 

(ft)

Thalweg -0.46 -0.57 -0.49 -0.39 -0.45 -0.62 0.88 0.17

Annual exceedance 

probability flood (%)

50 -0.39 -0.01 -0.41 -0.10 -0.38 -0.01 0.20 0.14

10 -0.28 -0.03 -0.30 -0.17 -0.28 -0.02 0.17 0.16

2 -0.19 -0.06 -0.30 -0.22 -0.17 -0.02 0.12 0.13

1 -0.18 -0.14 -0.25 -0.15 -0.17 -0.10 0.04 0.05

0.2 -0.16 -0.14 -0.16 0.01 -0.16 -0.18 0.06 0.06
44 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

ELEVATION, IN FEET

105100

959085807570

Sand, siltand gravel

Resistive material

(possibly bedrock

or till)

110

6560

0

24

48 72

96

120

144 North

South

ELEVATION, IN FEET

105100

959085807570

0 40 80

120 160

Sand, siltand gravel

Resistive material

(possibly bedrock or till)

West

East

50 98 192 376 738 1,446 2,835 5,556

RESISTIVITY, IN OHM METERS

A

B

EXPLANATION

Line 2

Line 4

Figure 14A–B. Electrical resistivity geophysical survey transect for A, line 2, and B, line 4.
Sediment Load and its Effect on Streambed and Flood Elevations 45

Suncook 

River cross 

section

Cumulative 

distance 

upstream 

(ft)

WY 2001 Laursen 

(Copeland) 

function mean of 

daily sediment 

discharges 

(tons/day)

WY 2001 Laursen 

(Copeland) 

function median 

of daily sediment 

discharges 

(tons/day)

A 0 184.92 21.30

B 477 184.92 21.30

C 804 184.92 21.30

D 1,681 184.89 21.23

1,864 184.89 21.23

1,883 184.89 21.23

E 2,043 184.89 21.23

F 2,193 184.89 21.23

2,337 184.89 21.23

2,363 184.89 21.23

G 2,507 184.41 20.02

H 3,079 203.26 19.11

3,093 202.54 19.30

3,124 202.66 20.08

3,176 202.66 20.08

I 3,401 202.66 20.08

J 3,467 202.66 20.08

K 3,880 202.66 20.08

3,889 202.74 20.08

3,898 202.75 20.08

3,907 202.75 20.08

3,914 202.75 20.08

3,916 202.75 20.08

3,925 203.22 20.07

L 3,934 203.15 20.18

M 4,433 203.15 20.18

N 4,507 203.15 20.18

4,509 203.15 20.18

4,525 203.15 20.18

O 4,640 208.42 20.34

P 4,886 216.15 29.77

Q 5,126 225.69 39.25

R 5,283 226.33 46.61

S 5,575 217.91 46.95

T 5,860 204.83 28.16

U 6,045 203.95 30.06

Route 3 6,057 205.14 41.93

6,109 205.16 41.94

Suncook 

River cross 

section

Cumulative 

distance 

upstream 

(ft)

WY 2001 Laursen 

(Copeland) 

function mean of 

daily sediment 

discharges 

(tons/day)

WY 2001 Laursen 

(Copeland) 

function median 

of daily sediment 

discharges 

(tons/day)

V 6,172 205.14 42.03

W 6,935 204.84 42.03

X 6,945 204.84 42.03

Y 7,981 204.84 42.03

Z 9,331 204.91 42.03

AA 10,614 203.14 44.46

AB 11,542 193.11 37.89

AC 12,282 192.78 36.50

AD 15,482 138.55 23.80

AE 17,581 138.44 26.22

AF 19,123 129.11 27.15

AG 22,592 116.92 12.02

AH 23,529 53.77 1.20

AI 24,763 54.49 1.31

AJ 25,387 53.70 1.29

AK 26,696 53.29 1.19

AL 27,674 55.19 1.95

AM 28,253 53.38 1.21

AN 28,443 53.34 1.19

AO 28,766 53.35 1.19

AP 28,851 53.34 1.19

28,866 53.34 1.19

29,011 53.35 1.19

29,038 53.33 1.19

AQ 29,063 53.33 1.19

AR 29,088 53.33 1.19

AS 29,216 53.33 1.19

29,233 53.34 1.19

Route 28 29,278 53.36 1.19

29,285 53.39 1.19

AT 29,347 53.54 1.20

AU 29,600 53.94 1.22

AV 30,715 54.04 1.24

AW 31,747 54.07 1.19

AX 33,142 54.07 1.18

AY 34,227 55.57 0.98

AZ 34,927 42.01 0.23

BA 35,563 42.18 0.23

Table 9. Suncook River mean and median of the daily sediment discharges for water year 2009 using the Laursen (Copeland) function.

[ft, feet; WY, water year]
46 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Suncook 

River cross 

section

Cumulative 

distance 

upstream 

(ft)

WY 2001 Laursen 

(Copeland) 

function mean of 

daily sediment 

discharges 

(tons/day)

WY 2001 Laursen 

(Copeland) 

function median 

of daily sediment 

discharges 

(tons/day)

BB 36,078 38.98 0.23

BC 36,613 43.49 0.43

BD 37,933 47.91 0.55

BE 38,426 49.40 0.53

BF 39,121 51.23 0.48

BG 39,861 73.49 1.11

BH 40,506 82.86 2.96

BI 41,066 80.65 2.86

BJ 41,899 79.79 2.35

BK 43,019 81.75 2.51

BL 43,741 83.80 3.65

BM 44,821 88.31 5.74

BN 45,161 87.14 5.15

BO 45,724 83.73 4.14

BP 46,356 87.97 6.95

BQ 46,731 88.89 7.82

BR 47,375 94.16 11.35

BS 48,053 101.46 15.60

BT 48,853 110.62 24.49

BU 50,158 114.73 32.34

BV 50,624 138.70 66.36

BW 50,829 138.41 65.96

50,989 188.08 100.57

BX 51,081 260.56 166.64

51,134 260.22 162.81

Short Falls 

Road

51,175 260.28 163.21

51,239 260.08 167.64

51,254 260.25 167.30

BY 51,305 260.00 164.99

BZ 51,975 258.48 163.25

CA 52,463 254.54 158.37

CB 53,600 250.28 146.94

CC 54,415 243.88 122.96

CD 55,869 252.11 135.34

CE 56,345 247.99 144.21

CF 56,880 248.58 140.02

Suncook 

River cross 

section

Cumulative 

distance 

upstream 

(ft)

WY 2001 Laursen 

(Copeland) 

function mean of 

daily sediment 

discharges 

(tons/day)

WY 2001 Laursen 

(Copeland) 

function median 

of daily sediment 

discharges 

(tons/day)

CG 57,636 241.36 134.63

CH 58,327 238.97 121.70

CI 58,367 234.49 117.12

CJ 59,099 232.48 102.56

CK 59,429 232.61 102.15

CL 59,964 233.03 101.50

CM 60,520 233.84 100.05

CN 61,160 227.55 96.94

CO 61,868 240.04 108.56

CP 62,547 227.52 116.07

CQ 63,035 223.87 117.27

CR 63,475 223.46 116.28

CS 63,795 217.96 116.25

CT 64,132 217.16 115.30

CU 64,400 210.75 102.00

CV 64,632 212.03 102.08

CW 64,893 210.11 98.30

CX 65,150 210.37 97.93

CY 65,883 203.70 99.25

CZ 65,893 207.00 99.99

DA 66,029 204.61 94.80

DB 66,140 210.33 95.42

DC 66,345 199.23 90.60

DD 66,627 205.25 90.65

DE 66,929 202.96 90.65

DF 67,651 196.85 90.30

DG 68,733 199.07 92.12

DH 69,552 195.58 91.19

Route 4 69,665 195.58 91.19

69,726 195.63 91.19

DI 69,783 195.61 91.19

69,998 195.59 91.12

DJ 70,008 195.43 91.41

DK 70,501 195.18 91.03

DL 70,813 194.44 93.16

DM 71,389 194.42 93.18

DN 71,617 192.30 90.23

Table 9. Suncook River mean and median of daily sediment discharges for water year 2009 using the Laursen (Copeland) function.

—Continued

[ft, feet; WY, water year]
Sediment Load and its Effect on Streambed and Flood Elevations 47

70 tons per day, resulting in a coefficient of variation of 0.42. 

For the Suncook River reach downstream of the Short Falls 

Road Bridge (table 9, fig. 15; between cross sections BW and 

AD), the mean daily sediment discharge (for mean daily flows) 

in WY 2009 was 72 tons per day, with a standard deviation of 

29 tons per day, resulting in a coefficient of variation of 0.40 

(low variance). This reach is aggrading, retaining much of the 

sediment from the former sand pit upstream. The maximum 

daily mean sediment discharge for all daily mean flows in 

WY 2009 was 261 tons per day in the reach downstream of the 

Short Falls Road Bridge (table 9, fig. 4C; downstream of cross 

section BX). The minimum daily mean sediment discharge 

for WY 2009 was 39 tons per day and occurred at the reach 

upstream of the confluence of the Little Suncook River (figs. 2 

and 4C; downstream of cross section BB).

In WY 2009, the maximum median of daily sediment 

discharges (for mean daily flows) for all cross sections was 

168 tons per day, and occurred in the reach downstream of 

the Short Falls Road Bridge (figs. 2 and 4C; downstream of 

cross section BX). The minimum median of daily sediment 

discharges (for mean daily flows) for all cross sections for 

WY 2009 was 0.23 tons per day, and occurred at the reach 

upstream of the confluence of the Little Suncook River (figs. 2 

and 4C; downstream of cross section BB). The average of the 

medians of daily sediment discharges (for mean daily flows) 

for all cross sections in WY 2009 was 53 tons per day, with a 

standard deviation of 50 tons per day, resulting in a coefficient 

of variation of 0.95. Distributions of the coefficient of variation that are less than 1 are considered to be low variance. For 

the Suncook River reach downstream of the Short Falls Road 

Bridge (figs. 2 and 4A–C; between cross sections BW and 

AD), the average of the medians of daily sediment discharges 

(for mean daily flows) in WY 2009 was 7 tons per day, with a 

standard deviation of 14 tons per day, resulting in a coefficient 

of variation of 1.94 (high variance).

The sediment model using the Laursen (Copeland) transport function was used to determine sediment transport in 

WY 2010. Using daily mean flow and water temperature data 

from WY 2009, the quasi-unsteady-state sediment-transport 

model was run for a simulated water year ending September 

30, 2010. The overall trend in the simulation is that sediment 

is being deposited (aggrading) downstream of the avulsion—

in particular, in the channel reach between the Short Falls 

Road Bridge and the Buck Street Dams—and sediment is 

being eroded from (degrading) the channel reach above 

the avulsion. 

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000

CUMULATIVE DISTANCE UPSTREAM FROM

CONFLUENCE WITH MERRIMACK RIVER, IN FEET

0

50

100

150

200

250

Mean sediment discharge

Median sediment discharge

ANNUAL MEAN AND MEDIAN OF DAILY

SEDIMENT DISCHARGE, IN TONS PER DAY

Route 3 

Route 28

Short Falls

Road

Route 4

300

Figure 15. Suncook River mean and median of the daily sediment discharges for water year 

2009 using the Laursen (Copeland) function.
48 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

The change in streambed thalweg for all of the crosssection locations from its elevation in May 2008 to its 

simulated elevation at the end of WY 2010 was a mean 

degradation of 0.46 ft (median degradation of 0.57 ft). There 

was greater mean degradation (channel erosion) above the 

avulsion as compared to that below the avulsion for the model 

simulation period. The results show that for all of the crosssection locations, the overall degradation conditions resulted 

in the greatest impact for the higher annual exceedence 

probability (lower recurrence interval) floods with a decreased 

mean flood elevation for the 50-percent annual exceedence 

probability (2-year recurrence interval) of 0.39 ft (median 

decrease of 0.01 ft).

The change in the streambed thalweg for the crosssection locations between the Short Falls Road Bridge and the 

Buck Street Dams for the simulation period from May 2008 to 

the end of WY 2010 was a mean increase of 0.88 ft (median 

aggradation of 0.17 ft). The results show that this channel 

reach is continuing to fill with sediment with the greatest 

effect of the sediment deposition on water-surface elevation 

for the higher annual exceedence probability (lower recurrence interval) floods. The mean changes to the water-surface 

elevations for the cross sections between the Short Falls Road 

Bridge and the Buck Street Dams were an increase of 

0.20 ft (median of 0.14 ft) for the 50-percent annual 

exceedence probability (2-year recurrence interval) flood, 

0.17 ft (median of 0.16 ft) for the 10-percent annual 

exceedence probability (10-year recurrence interval) flood, 

0.12 ft (median of 0.13 ft) for the 2-percent annual exceedence 

probability (50-year recurrence interval) flood, 0.04 ft (median 

of 0.05 ft) for the 1-percent annual exceedence probability 

(100-year recurrence interval) flood, and 0.06 ft (median of 

0.06 ft) for the 0.2-percent annual exceedence probability 

(500-year recurrence interval) flood. The simulation results 

show that the increased aggradation in the reach between the 

Short Falls Road Bridge and the Buck Street Dams will yield 

an increase in observed flood elevations for the more frequently occurring lower recurrence interval floods.

The mean change to the water-surface elevations at all the 

cross sections was a lowering by 0.39 ft (median of 0.01 ft) for 

the 50-percent annual exceedence probability (2-year recurrence interval) flood, 0.28 ft (median of 0.03 ft) for the 10-percent annual exceedence probability (10-year recurrence interval) flood, 0.19 ft (median of 0.06 ft) for the 2-percent annual 

exceedence probability (50-year recurrence interval) flood, 

0.18 ft (median of 0.14 ft) for the 1-percent annual exceedence 

probability (100-year recurrence interval) flood, and 0.16 ft 

(median of 0.14 ft) for the 0.2-percent annual exceedence 

probability (500-year recurrence interval) flood. The overall 

decrease in the selected annual exceedence flood elevations at 

the model cross-section locations reflects the mean degradation of 0.46 ft for these cross sections. As indicated previously 

for the reach between the Short Falls Road Bridge and the 

Buck Street Dams, subreaches can have an overall increase in 

the selected annual exceedence probability floods.

Summary

During May 13–16, 2006, rainfall in excess of 8.8 inches 

flooded central and southern New Hampshire. A breach in the 

bank of the Suncook River in Epsom, N.H., on May 15, 2006, 

caused the river to follow a new path. The Suncook River 

drains several lakes south of Lake Winnepsaukee, and, it flows 

approximately 30 miles to the Merrimack River at a location 

between Concord and Manchester, N.H. Before the May flood, 

the Suncook River flowed southward around Bear Island in 

two stream reaches that joined west of Round Pond. After 

May 16, 2006, there was no flow in the western reach or in the 

northeastern reach of the Suncook River around Bear Island. 

Instead, the Suncook River created a path through a gravel 

pit, resulting in a new channel upstream of Bear Island and 

two new dams at the north end of Bear Island, with the new 

channel rejoining the old channel to the east of Bear Island. 

During April 16–18, 2007, more than 7 inches of precipitation 

resulted in another flood event. Both floods transported large 

quantities of gravel, sand, silt, and clay downstream. In 

order to assess and predict the effects of the sedimentand the 

subsequent flooding on the river and flood plain, a study by 

the U.S. Geological Survey characterizing sediment transport 

in the Suncook River was undertaken in cooperation with 

the Federal Emergency Management Agency and the New 

Hampshire Department of Environmental Services.

For many rivers, the relation between suspended 

sediment and explanatory variables such as discharge is not 

constant over time, making accurate forecasts of changes in 

streambeds difficult. Although several factors affect sediment 

discharge, transport and discharge of sediment are closely tied 

to flow. Knowledge of flow data is critical in predicting sediment transport and the resultant effect of streambed and total 

sediment load on recurrence-interval floods. An understanding 

of sediment-transport characteristics is important for applications that include flow-regime change, channel-restoration 

efforts, and predictions of the effects of landuse (Wilcock, 

2001). In order to establish river equilibrium for river-corridor 

protection and restoration, knowledge of the relation between 

flow and sediment transport and the of the ability of the existing channel to transport sediment is critical. The measurement 

of sediment load and particle size moving through a reach at 

various flows for development of sediment-rating curves is 

an important tool for river-corridor protection and restoration 

design. Once the conditions required for sediment transport 

are understood, a determination of the channel dimensions, 

pattern, and profile required for sufficient transport of the 

expected sediment supply can be made (Vermont Agency of 

Natural Resources, 2009).

As a result of the avulsion, the new channel through 

the gravel pit is 1.0 mile (mi) long, whereas the abandoned 

channel is 2.0 mi long. It was estimated that 150,000 cubic 

yards of sediment were introduced into the Suncook River, 

with sheets of silt and sand up to 5 feet (ft) thick, deposited 

downstream of the avulsion channel. Downcutting of the 

stream continues in the avulsion area and upstream because 
Summary 49

of the increased gradient and the convergence of flow, both of 

which increase the rate of erosion of the bank in the avulsion 

area and upstream.

A one-dimensional sedimentation model (HEC–RAS, 

version 4.0.0) of the Suncook River was developed for 

the reach from its confluence with the Merrimack River in 

Allenstown to the upstream limit of Epsom (15.9 mi) to study 

the effect of the streambank avulsion on sediment transport. 

The study was undertaken and the sedimentation model was 

created to aid State and local authorities in predicting sediment 

levels, riverbed stability, and sediment movement under 

various flow scenarios, and to assist future restoration efforts 

by furthering the understanding of the effect of sediment 

transport on the environment. 

Developing and verifying the model required obtaining 

data on the profile of and cross sections in the channel and on 

the overbanks, on bed sediments, on inflowing concentrations 

of sediment, and on daily discharges. Verification of sedimentation patterns consisted of comparing the observed changes in 

cross sections of the channel with computed changes in these 

cross sections for a 400-day period during 2008 and 2009. 

Field data were collected at bridges, dams, and 148 channel cross sections. Cross sections were surveyed with a total 

station using closed level-loop surveying techniques from 

either a known reference datum or from a reference datum 

established using a Global Positioning System (GPS) with a 

horizontal accuracy of 3 mm (0.01 ft) and a vertical accuracy 

of 10 mm (0.03 ft) in static mode. Additional cross-section and 

profile data were collected using a GPS in conjunction with an 

Acoustic Doppler Current Profiler.

The sediment model using the Laursen (Copeland) transport function was used to determine sediment transport and 

streambed elevations for the end of water year (WY) 2010. 

Using channel geometry and daily mean flow and watertemperature data from WY 2009, the quasi-unsteady-state 

sediment-transport model was run for WY 2010. The resultant 

cross sections were imported into a steady-state flow model, 

which was then run to determine the impact of the altered 

channel bed on flood elevations for the 50-, 10-, 2-, 1-, and 

0.2-percent annual exceedence probability floods. The predicted change in streambed thalweg at all of the model crosssection locations between May 2008 and the end of WY 2010 

was a mean degradation of 0.46 ft (median degradation of 

0.57 ft). The model simulation shows that for all cross-section 

locations, overall degradation resulted in the greatest impact 

for the higher annual exceedence probability (lower recurrence 

interval) floods with a decreased mean flood elevation for the 

50-percent annual exceedence probability (2-year recurrence 

interval) flood of 0.39 ft (median decrease of 0.01 ft).

The change in the streambed thalweg for the crosssection locations between the Short Falls Road Bridge and the 

Buck Street Dams for the simulation period from May 2008 

to the end of WY 2010 was a mean aggradation (deposition 

of sediment) of 0.88 ft (median aggradation of 0.17 ft). The 

results show that this channel reach is continuing to fill with 

sediment, with the greatest effect of the sediment deposition 

on water-surface elevations for the higher annual exceedence 

probability (lower recurrence interval) floods. For the 50-percent annual exceedence probability (2-year recurrence interval) flood, the simulated mean aggradation of 0.88 ft (median 

aggradation of 0.17 ft) resulted in an increased mean flood 

elevation of 0.20 ft (median increase of 0.14 ft).

The mean change to the water-surface elevations at all 

the cross sections was a lowering by 0.39 ft (median of 

0.01 ft) for the 50-percent annual exceedence probability 

(2-year recurrence interval) flood, 0.28 ft (median of 0.03 ft) 

for the 10-percent annual exceedence probability (10-year 

recurrence interval) flood, 0.19 ft (median of 0.06 ft) for the 

2-percent annual exceedence probability (50-year recurrence 

interval) flood, 0.18 ft (median of 0.14 ft) for the 1-percent 

annual exceedence probability (100-year recurrence interval) 

flood and 0.16 ft (median of 0.14 ft) for the 0.2-percent annual 

exceedence probability (500-year recurrence interval) flood.

Simulation results and field-collected sediment data 

indicate that, for the average daily mean flow during the study 

period and downstream of the avulsion channel, approximately 

70 to 500 tons of sediment per day were moving downstream 

past the Short Falls Road Bridge; that approximately 0.02 to 

50 tons of sediment per day were moving past the Route 28 

bridge; and that approximately 1.0 to 200 tons per day were 

moving past the Route 3 bridge. The sediment-transport rate 

for the average daily mean flow at the Route 28 bridge was 

lower than the sediment-transport rate at the downstream 

Route 3 bridge and at the upstream Short Falls Road Bridge 

during the study period; this difference reflects the large quantity of sediment that is still moving downstream from the sand 

pit and the reach above Short Falls and that is settling out in 

the river downstream of Short Falls and upstream of Route 28. 

A much lower rate of sediment transport, likely a result of the 

large quantity of sediment that was deposited during the 2006 

and 2007 peak-flow events, is occurring at Route 28 as compared to Route 3. The sediment remains in the channel and on 

the flood plain in this reach and is still moving downstream in 

the reach downstream of Route 28. 

Data from the model and from measured cross sections 

and field-collected sediment verify that much of the sediment 

is being deposited (aggrading) downstream of the avulsion 

—in particular, in the channel reach between the Short Falls 

Road Bridge and the Buck Street Dams—and sediment is 

being eroded from (degrading) the channel reach above the 

avulsion and in Leighton Brook. 

The shear stress analysis results for the Route 28 bridge 

are in contrast to those of the next upstream bridge at Short 

Falls Road, where sediment smaller than or equal in size to a 

medium sand was in motion and suspension for 100 percent 

of the study simulation days. The analysis for the Route 28 

bridge shows that for only 25 to 32 percent of the simulation 

days, sediment smaller than or equal in size to a medium sand 

was in motion; in addition, the analysis indicates that the reach 

between Short Falls Road Bridge and the downstream Route 

28 bridge is retaining much of its coarser sediment load given 

the streambed shear stress and the median grain size (d50) of a 
50 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

very fine gravel for all sediment-sampling locations below the 

avulsion and of a coarse sand at the Short Falls Road Bridge. 

Shear stress is a function of the energy gradient (or slope of 

the water surface) and hydraulic radius (approximated by the 

mean depth). An increase in either or both of these parameters 

will result in greater shear stress and an increased ability of the 

channel to move sediment. 

Aerial photographs taken in 2007 and 11 months later 

in 2008 in the area of the new channel through the sand pit 

show that the new channel reach is continuing to transport and 

deposit sediment and that there has been some bank erosion, 

but that there is little channel migration in the area of the sand 

pit. Preliminary geophysical data indicate that a potential 

near-surface consolidated layer may constrain the new channel 

from laterally migrating into the sand pit area.

The driving force for sediment transport is the shear 

stress (τ0) that flowing water exerts on the channel boundary. 

Sediment movement, or entrainment, in a stream is in part an 

effect of this boundary shear stress that flowing water creates 

by acting on sediment particles. The potential for entrainment 

is estimated from the relation between a flood-generated 

shear stress and the critical shear stress (τc), the shear stress 

at which sediment particles begin to move. Critical shear 

stress has been related to characteristics of sediment size. A 

shear stress analysis was undertaken to determine potential 

areas of erosion and deposition. The distribution of streambed 

shear stress for the 50- and 1-percent annual exceedence 

probability floods (2- and 100-year recurrence interval floods, 

respectively) and the critical streambed shear stress based on 

the d50 particle size were determined in order to define the 

areas for which little to no sediment is predicted as moving 

(as defined by: τ0 less than τc), some sediment is predicted 

as moving (as defined by: τ0 greater than τc and τ0 less than 

2 τc), and much sediment is predicted as moving (as defined 

by: τ0 greater than 2 τc) for either the 50- or 1-percent annual 

exceedence probability (2- or 100-year recurrence-interval, 

respectively) floods. Based on the median grain size (d50) 

and shear stress analysis, the study found that, in general, 

for floods greater than the 50-percent annual exceedence 

probability (2-year recurrence interval) flood, the shear stress 

in the river is greater than the critical shear stress in much of 

the river study reach. The expected result is that streambed 

sediment movement and erosion will happen even at high 

exceedence probability events in the streambed, pending 

stream equilibrium through stream-stabilization measures 

or over time as described by Lane’s stream-balance relation. 

The potential for aggradation in the Suncook River is greatest 

in the reach downstream of the avulsion and, specifically, in 

the reaches downstream of the former sand pit from adjacent 

to Round Pond to downstream of the flood chute at the 

large meander bends (cross sections CA to CM) and from 

downstream of the Short Falls Road Bridge to approximately 

3,800 ft upstream of the Route 28 bridge.

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54 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

DW 50-year 2,228 326.19 337.39 326.19 337.75 0 0.36

10-year 4,680 340.88 341.04 0.16

2-year 7,994 343.19 343.28 0.09

1-year 9,819 344.17 344.25 0.08

0.2-year 15,290 346.97 347.03 0.06

DV 50-year 2,228 324.51 336.47 325.5 336.55 0.99 0.08

10-year 4,680 340.08 340.11 0.03

2-year 7,994 342.4 342.41 0.01

1-year 9,819 343.36 343.38 0.02

0.2-year 15,290 346.23 346.25 0.02

DU 50-year 2,228 321.38 335.51 319.98 335.54 -1.4 0.03

10-year 4,680 339.12 339.1 -0.02

2-year 7,994 341.62 341.58 -0.04

1-year 9,819 342.62 342.59 -0.03

0.2-year 15,290 345.64 345.63 -0.01

DT 50-year 2,228 323.63 334.77 324.48 334.84 0.85 0.07

10-year 4,680 338.44 338.43 -0.01

2-year 7,994 341.09 341.05 -0.04

1-year 9,819 342.12 342.1 -0.02

0.2-year 15,290 345.27 345.28 0.01

DS 50-year 2,228 322.43 333.78 322.94 333.6 0.51 -0.18

10-year 4,680 337.3 337.07 -0.23

2-year 7,994 339.79 339.58 -0.21

1-year 9,819 340.77 340.6 -0.17

0.2-year 15,290 344.09 344.03 -0.06

DR 50-year 2,228 322.18 333.4 320.79 333.29 -1.39 -0.11

10-year 4,680 336.88 336.7 -0.18

2-year 7,994 339.26 339.1 -0.16

1-year 9,819 340.16 340.04 -0.12

0.2-year 15,290 343.46 343.46 0

DQ 50-year 2,228 320.25 333.16 322.55 332.88 2.3 -0.28

10-year 4,680 336.59 336.25 -0.34

2-year 7,994 338.89 338.56 -0.33

1-year 9,819 339.76 339.49 -0.27

0.2-year 15,290 343.15 343.1 -0.05

DP 50-year 2,228 320.25 333.08 318.75 332.91 -1.5 -0.17

10-year 4,680 336.44 336.23 -0.21

2-year 7,994 338.67 338.47 -0.2

1-year 9,819 339.51 339.36 -0.15

0.2-year 15,290 342.99 342.98 -0.01
Table 7 55

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

DO 50-year 2,228 322.38 332.14 322.1 332.16 -0.28 0.02

10-year 4,680 335.23 335.16 -0.07

2-year 7,994 337.62 337.44 -0.18

1-year 9,819 338.47 338.35 -0.12

0.2-year 15,290 342.57 342.59 0.02

DN 50-year 2,228 319.6 331.98 322.07 331.84 2.47 -0.14

10-year 4,680 335.11 334.95 -0.16

2-year 7,994 337.52 337.3 -0.22

1-year 9,819 338.38 338.24 -0.14

0.2-year 15,290 342.54 342.55 0.01

DM 50-year 2,228 320.78 331.88 319.89 331.78 -0.89 -0.1

10-year 4,680 335.07 334.92 -0.15

2-year 7,994 337.51 337.28 -0.23

1-year 9,819 338.36 338.21 -0.15

0.2-year 15,290 342.53 342.54 0.01

DL 50-year 2,228 321.91 331.33 321.46 331.33 -0.45 0

10-year 4,680 334.65 334.47 -0.18

2-year 7,994 337.34 337.1 -0.24

1-year 9,819 338.21 338.06 -0.15

0.2-year 15,290 342.46 342.48 0.02

DK 50-year 2,228 321.28 331.34 320.92 331.34 -0.36 0

10-year 4,680 334.62 334.46 -0.16

2-year 7,994 337.24 336.99 -0.25

1-year 9,819 338.1 337.95 -0.15

0.2-year 15,290 342.42 342.43 0.01

DJ 50-year 2,228 323.77 330.89 323.22 330.96 -0.55 0.07

10-year 4,680 334.03 333.91 -0.12

2-year 7,994 336.77 336.43 -0.34

1-year 9,819 337.77 337.58 -0.19

0.2-year 15,290 342.35 342.37 0.02

DI 50-year 2,228 321.52 329.82 317.47 331 -4.05 1.18

10-year 4,680 332.55 333.91 1.36

2-year 7,994 334.91 336.23 1.32

1-year 9,819 335.88 337.37 1.49

0.2-year 15,290 341.14 342.33 1.19

DH 50-year 2,320 320.98 326.12 317.99 323.62 -2.99 -2.5

10-year 4,873 328.13 327.72 -0.41

2-year 8,325 332.17 331.55 -0.62

1-year 10,225 333.83 333.2 -0.63

0.2-year 15,922 337.23 336.72 -0.51
56 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

DG 50-year 2,320 302.68 324.42 302.26 323.43 -0.42 -0.99

10-year 4,873 327.83 327.04 -0.79

2-year 8,325 331.22 330.52 -0.7

1-year 10,225 332.75 332.06 -0.69

0.2-year 15,922 336.35 335.72 -0.63

DF 50-year 2,854 313.22 323.8 313.74 322.52 0.52 -1.28

10-year 5,994 326.66 325.44 -1.22

2-year 10,239 329.46 328.26 -1.2

1-year 12,576 330.75 329.51 -1.24

0.2-year 19,583 333.97 332.68 -1.29

DE 50-year 2,854 316.36 321.76 313.4 317.93 -2.96 -3.83

10-year 5,994 323.3 320.5 -2.8

2-year 10,239 325.06 322.99 -2.07

1-year 12,576 325.97 324.09 -1.88

0.2-year 19,583 328.28 326.71 -1.57

DD 50-year 2,854 311.42 315.35 308.44 315.59 -2.98 0.24

10-year 5,994 317.92 318.1 0.18

2-year 10,239 320.48 320.63 0.15

1-year 12,576 321.54 321.72 0.18

0.2-year 19,583 323.96 324.25 0.29

DC 50-year 2,854 306.08 314.01 307.09 313.28 1.01 -0.73

10-year 5,994 316.94 316.3 -0.64

2-year 10,239 319.6 318.94 -0.66

1-year 12,576 320.66 320.03 -0.63

0.2-year 19,583 322.95 322.37 -0.58

DB 50-year 2,854 304.28 313.53 305.02 312.8 0.74 -0.73

10-year 5,994 316.6 315.96 -0.64

2-year 10,239 319.37 318.78 -0.59

1-year 12,576 320.46 319.93 -0.53

0.2-year 19,583 322.83 322.43 -0.4

DA 50-year 2,854 306.36 313.36 306.89 312.32 0.53 -1.04

10-year 5,994 316.33 315.45 -0.88

2-year 10,239 319.02 318.21 -0.81

1-year 12,576 320.11 319.32 -0.79

0.2-year 19,583 322.61 321.98 -0.63

CZ 50-year 2,854 306.19 313.07 303.93 312.25 -2.26 -0.82

10-year 5,994 315.98 315.35 -0.63

2-year 10,239 318.49 318.03 -0.46

1-year 12,576 319.46 319.09 -0.37

0.2-year 19,583 321.89 321.55 -0.34
Table 7 57

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

CY 50-year 2,854 306.19 313.08 304.1 312.24 -2.09 -0.84

10-year 5,994 316.01 315.36 -0.65

2-year 10,239 318.57 318.07 -0.5

1-year 12,576 319.56 319.14 -0.42

0.2-year 19,583 322.02 321.64 -0.38

CX 50-year 2,854 303.58 311.79 302.59 311.03 -0.99 -0.76

10-year 5,994 314.85 314.2 -0.65

2-year 10,239 317.45 316.91 -0.54

1-year 12,576 318.52 318.06 -0.46

0.2-year 19,583 321.07 320.66 -0.41

CW 50-year 2,854 304.21 311.35 302.65 310.67 -1.56 -0.68

10-year 5,994 314.55 313.91 -0.64

2-year 10,239 317.22 316.69 -0.53

1-year 12,576 318.27 317.83 -0.44

0.2-year 19,583 320.8 320.39 -0.41

CV 50-year 2,854 303.48 311.17 302.37 310.28 -1.11 -0.89

10-year 5,994 314.4 313.67 -0.73

2-year 10,239 317.01 316.4 -0.61

1-year 12,576 318.02 317.49 -0.53

0.2-year 19,583 320.3 319.78 -0.52

CU 50-year 2,854 303.38 310.8 301.28 310 -2.1 -0.8

10-year 5,994 313.93 313.31 -0.62

2-year 10,239 316.3 315.84 -0.46

1-year 12,576 317.13 316.79 -0.34

0.2-year 19,583 318.72 318.52 -0.2

CT 50-year 2,854 302.17 310.49 300.01 309.71 -2.16 -0.78

10-year 5,994 313.59 312.98 -0.61

2-year 10,239 315.83 315.4 -0.43

1-year 12,576 316.58 316.27 -0.31

0.2-year 19,583 317.99 317.84 -0.15

CS 50-year 2,854 301.23 310.33 298.61 309.61 -2.62 -0.72

10-year 5,994 313.53 312.94 -0.59

2-year 10,239 315.93 315.49 -0.44

1-year 12,576 316.81 316.48 -0.33

0.2-year 19,583 318.59 318.4 -0.19

CR 50-year 2,854 300.55 309.85 299.64 309.07 -0.91 -0.78

10-year 5,994 312.92 312.27 -0.65

2-year 10,239 315.11 314.55 -0.56

1-year 12,576 315.98 315.49 -0.49

0.2-year 19,583 317.87 317.6 -0.27
58 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

CQ 50-year 2,854 299.49 309.2 296.72 308.77 -2.77 -0.43

10-year 5,994 311.98 311.76 -0.22

2-year 10,239 313.89 313.83 -0.06

1-year 12,576 314.7 314.69 -0.01

0.2-year 19,583 317.03 316.84 -0.19

CP 50-year 2,854 299.17 308.58 299.92 307.93 0.75 -0.65

10-year 5,994 311.2 310.6 -0.6

2-year 10,239 312.77 312.32 -0.45

1-year 12,576 313.45 313.09 -0.36

0.2-year 19,583 316.16 315.93 -0.23

CO 50-year 2,854 299.16 307.55 297.47 307.17 -1.69 -0.38

10-year 5,994 310.09 309.56 -0.53

2-year 10,239 311.27 310.94 -0.33

1-year 12,576 311.82 311.75 -0.07

0.2-year 19,583 315.41 315.24 -0.17

CN 50-year 2,854 298.38 305.06 298.66 305.68 0.28 0.62

10-year 5,994 306.02 306.66 0.64

2-year 10,239 308.68 309.06 0.38

1-year 12,576 310.58 310.73 0.15

0.2-year 19,583 315.1 314.94 -0.16

CM 50-year 2,854 299.31 304.51 297.89 304.92 -1.42 0.41

10-year 5,994 305.88 306.22 0.34

2-year 10,239 308.79 308.98 0.19

1-year 12,576 310.59 310.69 0.1

0.2-year 19,583 315.13 314.96 -0.17

CL 50-year 2,854 296.75 303.61 296.48 304.19 -0.27 0.58

10-year 5,994 305.38 305.8 0.42

2-year 10,239 308.72 308.91 0.19

1-year 12,576 310.55 310.66 0.11

0.2-year 19,583 315.12 314.94 -0.18

CK 50-year 2,854 296.2 302.75 297.1 303.38 0.9 0.63

10-year 5,994 304.87 305.25 0.38

2-year 10,239 308.59 308.77 0.18

1-year 12,576 310.47 310.56 0.09

0.2-year 19,583 315.08 314.89 -0.19

CJ 50-year 2,854 296.17 302.54 295.92 303.02 -0.25 0.48

10-year 5,994 304.8 305.13 0.33

2-year 10,239 308.58 308.76 0.18

1-year 12,576 310.47 310.56 0.09

0.2-year 19,583 315.08 314.9 -0.18
Table 7 59

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

CI 50-year 2,854 296.89 302.33 295.26 302.46 -1.63 0.13

10-year 5,994 304.68 304.96 0.28

2-year 10,239 308.55 308.72 0.17

1-year 12,576 310.45 310.53 0.08

0.2-year 19,583 315.07 314.88 -0.19

CH 50-year 2,854 296.89 302.29 294.26 302.44 -2.63 0.15

10-year 5,994 304.66 304.93 0.27

2-year 10,239 308.54 308.71 0.17

1-year 12,576 310.44 310.53 0.09

0.2-year 19,583 315.06 314.87 -0.19

CG 50-year 2,854 295.63 301.99 294.68 302.05 -0.95 0.06

10-year 5,994 304.59 304.86 0.27

2-year 10,239 308.52 308.69 0.17

1-year 12,576 310.43 310.51 0.08

0.2-year 19,583 315.05 314.87 -0.18

CF 50-year 2,854 294.58 301.88 291.88 301.98 -2.7 0.1

10-year 5,994 304.57 304.85 0.28

2-year 10,239 308.52 308.68 0.16

1-year 12,576 310.43 310.51 0.08

0.2-year 19,583 315.05 314.87 -0.18

CE 50-year 2,854 294.28 301.59 294.57 301.74 0.29 0.15

10-year 5,994 304.53 304.81 0.28

2-year 10,239 308.51 308.67 0.16

1-year 12,576 310.42 310.5 0.08

0.2-year 19,583 315.04 314.86 -0.18

CD 50-year 2,854 292.55 301.51 292.91 301.64 0.36 0.13

10-year 5,994 304.51 304.79 0.28

2-year 10,239 308.5 308.66 0.16

1-year 12,576 310.41 310.49 0.08

0.2-year 19,583 315.04 314.85 -0.19

CC 50-year 2,854 293.58 300.59 293.01 301.03 -0.57 0.44

10-year 5,994 304.41 304.71 0.3

2-year 10,239 308.46 308.63 0.17

1-year 12,576 310.38 310.47 0.09

0.2-year 19,583 315.02 314.83 -0.19

CB 50-year 2,854 289.59 300.01 289.79 300.52 0.2 0.51

10-year 5,994 304.27 304.59 0.32

2-year 10,239 308.39 308.56 0.17

1-year 12,576 310.32 310.41 0.09

0.2-year 19,583 314.97 314.78 -0.19
60 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

CA 50-year 2,854 289.73 299.64 290.2 300.17 0.47 0.53

10-year 5,994 304.13 304.45 0.32

2-year 10,239 308.29 308.46 0.17

1-year 12,576 310.22 310.31 0.09

0.2-year 19,583 314.88 314.68 -0.2

BZ 50-year 2,854 289.81 299.39 289.84 299.92 0.03 0.53

10-year 5,994 303.84 304.17 0.33

2-year 10,239 308.01 308.18 0.17

1-year 12,576 309.95 310.02 0.07

0.2-year 19,583 314.56 314.34 -0.22

BY 50-year 2,854 290.47 298.72 288.96 299.56 -1.51 0.84

10-year 5,994 303.22 303.74 0.52

2-year 10,239 307.3 307.61 0.31

1-year 12,576 309.19 309.4 0.21

0.2-year 19,583 313.69 313.56 -0.13

BX 50-year 2,976 287.32 298.4 288.67 299.15 1.35 0.75

10-year 6,252 302.71 303.29 0.58

2-year 10,679 306.52 306.98 0.46

1-year 13,117 308.28 308.66 0.38

0.2-year 20,425 311.95 312.24 0.29

BW 50-year 2,976 271.36 298.43 288.07 299.22 16.71 0.79

10-year 6,252 302.76 303.4 0.64

2-year 10,679 306.6 307.12 0.52

1-year 13,117 308.38 308.8 0.42

0.2-year 20,425 312.09 312.44 0.35

BV 50-year 2,976 286.8 298.37 284.86 299.22 -1.94 0.85

10-year 6,252 302.69 303.37 0.68

2-year 10,679 306.53 307.08 0.55

1-year 13,117 308.31 308.77 0.46

0.2-year 20,425 312.02 312.4 0.38

BU 50-year 2,976 285.85 298.22 287.99 299.03 2.14 0.81

10-year 6,252 302.47 303.09 0.62

2-year 10,679 306.22 306.7 0.48

1-year 13,117 307.96 308.36 0.4

0.2-year 20,425 311.58 311.91 0.33

BT 50-year 2,976 282.55 297.77 283.89 298.39 1.34 0.62

10-year 6,252 301.9 302.4 0.5

2-year 10,679 305.61 306.02 0.41

1-year 13,117 307.37 307.69 0.32

0.2-year 20,425 310.94 311.22 0.28
Table 7 61

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

BS 50-year 2,976 283.65 297.54 286.69 297.98 3.04 0.44

10-year 6,252 301.59 301.94 0.35

2-year 10,679 305.27 305.54 0.27

1-year 13,117 307.03 307.23 0.2

0.2-year 20,425 310.54 310.7 0.16

BR 50-year 2,976 286.45 297.37 287.6 297.73 1.15 0.36

10-year 6,252 301.46 301.8 0.34

2-year 10,679 305.21 305.49 0.28

1-year 13,117 307 307.21 0.21

0.2-year 20,425 310.57 310.75 0.18

BQ 50-year 2,976 280.1 297.24 282.29 297.52 2.19 0.28

10-year 6,252 301.26 301.52 0.26

2-year 10,679 304.93 305.15 0.22

1-year 13,117 306.7 306.86 0.16

0.2-year 20,425 310.26 310.41 0.15

BP 50-year 2,976 286.15 297.07 286.72 297.33 0.57 0.26

10-year 6,252 300.97 301.23 0.26

2-year 10,679 304.51 304.72 0.21

1-year 13,117 306.24 306.39 0.15

0.2-year 20,425 309.76 309.92 0.16

BO 50-year 2,976 278.21 297 280.21 297.21 2 0.21

10-year 6,252 300.87 301.07 0.2

2-year 10,679 304.42 304.58 0.16

1-year 13,117 306.16 306.26 0.1

0.2-year 20,425 309.61 309.71 0.1

BN 50-year 2,976 286.48 296.84 285.86 297.06 -0.62 0.22

10-year 6,252 300.56 300.81 0.25

2-year 10,679 304.07 304.29 0.22

1-year 13,117 305.87 306.02 0.15

0.2-year 20,425 309.39 309.51 0.12

BM 50-year 2,976 285.93 296.71 285.82 296.94 -0.11 0.23

10-year 6,252 300.34 300.6 0.26

2-year 10,679 303.72 303.95 0.23

1-year 13,117 305.48 305.64 0.16

0.2-year 20,425 308.99 309.13 0.14

BL 50-year 2,976 279.7 296.5 280.87 296.69 1.17 0.19

10-year 6,252 300.06 300.26 0.2

2-year 10,679 303.39 303.55 0.16

1-year 13,117 305.17 305.26 0.09

0.2-year 20,425 308.68 308.77 0.09
62 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

BK 50-year 2,976 284.04 296.28 284.59 296.44 0.55 0.16

10-year 6,252 299.73 299.9 0.17

2-year 10,679 303 303.14 0.14

1-year 13,117 304.78 304.85 0.07

0.2-year 20,425 308.22 308.3 0.08

BJ 50-year 2,976 278.65 296.01 278.85 296.15 0.2 0.14

10-year 6,252 299.39 299.55 0.16

2-year 10,679 302.67 302.8 0.13

1-year 13,117 304.48 304.54 0.06

0.2-year 20,425 307.9 307.96 0.06

BI 50-year 2,976 285.2 295.79 285.1 295.94 -0.1 0.15

10-year 6,252 299.2 299.38 0.18

2-year 10,679 302.52 302.67 0.15

1-year 13,117 304.36 304.43 0.07

0.2-year 20,425 307.8 307.86 0.06

BH 50-year 2,976 284.98 295.53 284.68 295.72 -0.3 0.19

10-year 6,252 298.94 299.15 0.21

2-year 10,679 302.31 302.47 0.16

1-year 13,117 304.18 304.25 0.07

0.2-year 20,425 307.6 307.67 0.07

BG 50-year 2,976 277.9 295.41 278.91 295.58 1.01 0.17

10-year 6,252 298.72 298.9 0.18

2-year 10,679 301.99 302.12 0.13

1-year 13,117 303.85 303.9 0.05

0.2-year 20,425 307.38 307.45 0.07

BF 50-year 2,976 277.56 295.34 280.35 295.42 2.79 0.08

10-year 6,252 298.62 298.7 0.08

2-year 10,679 301.89 301.94 0.05

1-year 13,117 303.76 303.75 -0.01

0.2-year 20,425 307.23 307.25 0.02

BE 50-year 2,976 279.7 295.18 279.85 295.23 0.15 0.05

10-year 6,252 298.33 298.38 0.05

2-year 10,679 301.53 301.56 0.03

1-year 13,117 303.41 303.37 -0.04

0.2-year 20,425 306.8 306.79 -0.01

BD 50-year 2,976 280.62 295.03 280.79 295.07 0.17 0.04

10-year 6,252 298.14 298.18 0.04

2-year 10,679 301.36 301.38 0.02

1-year 13,117 303.28 303.22 -0.06

0.2-year 20,425 306.67 306.66 -0.01
Table 7 63

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

BC 50-year 2,976 275.28 294.85 276.45 294.82 1.17 -0.03

10-year 6,252 297.77 297.73 -0.04

2-year 10,679 300.83 300.74 -0.09

1-year 13,117 302.73 302.57 -0.16

0.2-year 20,425 306.06 305.97 -0.09

BB 50-year 2,976 278.99 294.75 280.15 294.68 1.16 -0.07

10-year 6,252 297.55 297.43 -0.12

2-year 10,679 300.55 300.39 -0.16

1-year 13,117 302.47 302.25 -0.22

0.2-year 20,425 305.77 305.64 -0.13

BA 50-year 3,099 278.13 294.7 277.17 294.66 -0.96 -0.04

10-year 6,509 297.46 297.38 -0.08

2-year 11,118 300.41 300.3 -0.11

1-year 13,656 302.33 302.16 -0.17

0.2-year 21,265 305.61 305.51 -0.1

AZ 50-year 3,325 277.09 294.58 276.12 294.55 -0.97 -0.03

10-year 6,985 297.13 297.1 -0.03

2-year 11,931 299.92 299.86 -0.06

1-year 14,654 301.85 301.71 -0.14

0.2-year 22,820 304.99 304.93 -0.06

AY 50-year 3,325 279.64 294.54 278.67 294.53 -0.97 -0.01

10-year 6,985 297.11 297.09 -0.02

2-year 11,931 299.97 299.91 -0.06

1-year 14,654 301.93 301.79 -0.14

0.2-year 22,820 305.11 305.05 -0.06

AX 50-year 3,325 276.47 294.46 276.45 294.45 -0.02 -0.01

10-year 6,985 296.89 296.86 -0.03

2-year 11,931 299.58 299.52 -0.06

1-year 14,654 301.55 301.39 -0.16

0.2-year 22,820 304.58 304.51 -0.07

AW 50-year 3,325 276.97 294.39 276.93 294.37 -0.04 -0.02

10-year 6,985 296.7 296.67 -0.03

2-year 11,931 299.37 299.31 -0.06

1-year 14,654 301.39 301.22 -0.17

0.2-year 22,820 304.42 304.35 -0.07

AV 50-year 3,325 279.32 294.31 279.14 294.3 -0.18 -0.01

10-year 6,985 296.51 296.49 -0.02

2-year 11,931 298.99 298.93 -0.06

1-year 14,654 300.93 300.77 -0.16

0.2-year 22,820 303.57 303.51 -0.06
64 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

AU 50-year 3,325 279.1 294.27 279.1 294.25 0 -0.02

10-year 6,985 296.4 296.38 -0.02

2-year 11,931 298.81 298.74 -0.07

1-year 14,654 300.78 300.61 -0.17

0.2-year 22,820 303.46 303.39 -0.07

AT 50-year 3,325 279.21 294.27 279.37 294.26 0.16 -0.01

10-year 6,985 296.42 296.39 -0.03

2-year 11,931 298.9 298.83 -0.07

1-year 14,654 300.95 300.78 -0.17

0.2-year 22,820 303.8 303.73 -0.07

AS 50-year 3,374 280.4 294.25 279.41 294.25 -0.99 0

10-year 7,087 296.37 296.38 0.01

2-year 12,106 298.73 298.71 -0.02

1-year 14,870 300.63 300.5 -0.13

0.2-year 23,155 303.05 303.03 -0.02

AR 50-year 3,374 283.13 294.24 282.15 294.24 -0.98 0

10-year 7,087 296.37 296.38 0.01

2-year 12,106 298.78 298.75 -0.03

1-year 14,870 300.73 300.58 -0.15

0.2-year 23,155 303.28 303.24 -0.04

AQ 50-year 3,374 283.13 294.24 282.13 294.24 0

10-year 7,087 296.36 296.37 0.01

2-year 12,106 298.76 298.74 -0.02

1-year 14,870 300.71 300.57 -0.14

0.2-year 23,155 303.25 303.21 -0.04

AP 50-year 3,374 279.83 288 278.97 287.73 -0.86 -0.27

10-year 7,087 291.59 291.11 -0.48

2-year 12,106 294.55 294.47 -0.08

1-year 14,870 295.82 295.56 -0.26

0.2-year 23,155 299.12 298.89 -0.23

AO 50-year 3,374 281.21 287.39 280.36 287.24 -0.85 -0.15

10-year 7,087 291.31 290.8 -0.51

2-year 12,106 294.34 294.26 -0.08

1-year 14,870 295.65 295.35 -0.3

0.2-year 23,155 299.01 298.74 -0.27

AN 50-year 3,374 279.58 286.2 278.61 285.1 -0.97 -1.1

10-year 7,087 290.76 289.98 -0.78

2-year 12,106 293.83 293.69 -0.14

1-year 14,870 295.13 294.74 -0.39

0.2-year 23,155 298.54 298.2 -0.34
Table 7 65

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

AM 50-year 3,374 277.21 286 274.37 285.03 -2.84 -0.97

10-year 7,087 290.5 289.82 -0.68

2-year 12,106 293.44 293.43 -0.01

1-year 14,870 294.71 294.42 -0.29

0.2-year 23,155 298.11 297.83 -0.28

AL 50-year 3,374 275.27 285.78 275.64 284.69 0.37 -1.09

10-year 7,087 290.25 289.45 -0.8

2-year 12,106 293.11 293.02 -0.09

1-year 14,870 294.31 293.9 -0.41

0.2-year 23,155 297.6 297.21 -0.39

AK 50-year 3,374 273.13 285.51 273.43 284.24 0.3 -1.27

10-year 7,087 289.89 288.97 -0.92

2-year 12,106 292.47 292.33 -0.14

1-year 14,870 293.63 293.06 -0.57

0.2-year 23,155 297.08 296.48 -0.6

AJ 50-year 3,374 273.31 285.08 273.5 283.48 0.19 -1.6

10-year 7,087 289.51 288.38 -1.13

2-year 12,106 292.32 291.88 -0.44

1-year 14,870 293.67 293 -0.67

0.2-year 23,155 297.27 296.75 -0.52

AI 50-year 3,374 272.96 284.86 271.45 283.2 -1.51 -1.66

10-year 7,087 289.32 288.14 -1.18

2-year 12,106 292.28 291.66 -0.62

1-year 14,870 293.64 292.96 -0.68

0.2-year 23,155 297.26 296.73 -0.53

AH 50-year 3,374 270.36 284.37 269.08 282.65 -1.28 -1.72

10-year 7,087 288.68 287.37 -1.31

2-year 12,106 291.89 291.05 -0.84

1-year 14,870 293.59 292.9 -0.69

0.2-year 23,155 297.23 296.7 -0.53

AG 50-year 3,374 271.35 284 268.53 282.34 -2.82 -1.66

10-year 7,087 288.56 287.21 -1.35

2-year 12,106 291.96 290.97 -0.99

1-year 14,870 293.56 292.87 -0.69

0.2-year 23,155 297.21 296.68 -0.53

AF 50-year 3,374 268.58 283.52 268.54 281.71 -0.04 -1.81

10-year 7,087 287.93 286.43 -1.5

2-year 12,106 291.41 290.3 -1.11

1-year 14,870 292.99 292.21 -0.78

0.2-year 23,155 296.5 295.91 -0.59
66 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

AE 50-year 3,374 267.7 283.32 267.13 281.4 -0.57 -1.92

10-year 7,087 287.58 285.97 -1.61

2-year 12,106 291.01 289.78 -1.23

1-year 14,870 292.58 291.73 -0.85

0.2-year 23,155 296.06 295.42 -0.64

AD 50-year 3,374 266.9 282.96 266.52 280.85 -0.38 -2.11

10-year 7,087 287.08 285.1 -1.98

2-year 12,106 290.74 289.27 -1.47

1-year 14,870 292.39 291.46 -0.93

0.2-year 23,155 295.99 295.32 -0.67

AC 50-year 3,374 269.33 282.57 266.98 280.4 -2.35 -2.17

10-year 7,087 286.49 284.45 -2.04

2-year 12,106 290.02 288.47 -1.55

1-year 14,870 291.64 290.7 -0.94

0.2-year 23,155 295.21 294.51 -0.7

AB 50-year 3,374 270.97 282.46 269.07 280.29 -1.9 -2.17

10-year 7,087 286.3 284.26 -2.04

2-year 12,106 289.8 288.25 -1.55

1-year 14,870 291.42 290.5 -0.92

0.2-year 23,155 294.94 294.28 -0.66

AA 50-year 3,374 270.63 282.3 270.12 280.03 -0.51 -2.27

10-year 7,087 285.95 283.79 -2.16

2-year 12,106 289.3 287.58 -1.72

1-year 14,870 291.05 289.85 -1.2

0.2-year 23,155 294.58 293.86 -0.72

Z 50-year 3,374 267.96 282.14 267.38 279.74 -0.58 -2.4

10-year 7,087 285.67 283.37 -2.3

2-year 12,106 289 287.1 -1.9

1-year 14,870 290.76 289.51 -1.25

0.2-year 23,155 294.26 293.49 -0.77

Y 50-year 3,374 270.26 281.73 267.27 279.31 -2.99 -2.42

10-year 7,087 284.85 282.61 -2.24

2-year 12,106 287.76 285.92 -1.84

1-year 14,870 289.45 288.37 -1.08

0.2-year 23,155 292.7 292.12 -0.58

X 50-year 3,374 274.18 279.88 271.21 277.68 -2.97 -2.2

10-year 7,087 281.75 280.32 -1.43

2-year 12,106 283.7 283.14 -0.56

1-year 14,870 286.8 286.64 -0.16

0.2-year 23,155 291.69 291.23 -0.46
Table 7 67

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

W 50-year 3,374 274.18 279.14 271.19 277.49 -2.99 -1.65

10-year 7,087 280.82 280.17 -0.65

2-year 12,106 282.99 283.02 0.03

1-year 14,870 286.71 286.6 -0.11

0.2-year 23,155 290.41 290.45 0.04

V 50-year 3,374 268.84 277.54 265.9 277.72 -2.94 0.18

10-year 7,087 279.96 280.23 0.27

2-year 12,106 282.62 282.95 0.33

1-year 14,870 286.44 286.51 0.07

0.2-year 23,155 290.08 290.32 0.24

U 50-year 3,573 270.65 276.77 267.7 277.19 -2.95 0.42

10-year 7,506 278.51 279.12 0.61

2-year 12,820 280.28 281.1 0.82

1-year 15,747 281.11 282.03 0.92

0.2-year 24,521 282.85 284.18 1.33

T 50-year 3,573 268.32 276.75 267.51 277.15 -0.81 0.4

10-year 7,506 278.6 279.07 0.47

2-year 12,820 280.55 281.07 0.52

1-year 15,747 281.49 282.02 0.53

0.2-year 24,521 283.68 284.28 0.6

S 50-year 3,573 269.45 276.85 270.4 277.07 0.95 0.22

10-year 7,506 278.88 279.13 0.25

2-year 12,820 281.05 281.3 0.25

1-year 15,747 282.1 282.34 0.24

0.2-year 24,521 284.6 284.85 0.25

R 50-year 3,573 263.79 276.83 268.04 276.92 4.25 0.09

10-year 7,506 278.84 278.94 0.1

2-year 12,820 280.98 281.07 0.09

1-year 15,747 282.01 282.08 0.07

0.2-year 24,521 284.45 284.5 0.05

Q 50-year 3,573 265.07 276.81 268.85 276.85 3.78 0.04

10-year 7,506 278.79 278.83 0.04

2-year 12,820 280.89 280.91 0.02

1-year 15,747 281.9 281.91 0.01

0.2-year 24,521 284.28 284.29 0.01

P 50-year 3,573 265.02 276.68 265.68 276.72 0.66 0.04

10-year 7,506 278.51 278.59 0.08

2-year 12,820 280.46 280.59 0.13

1-year 15,747 281.41 281.55 0.14

0.2-year 24,521 283.61 283.78 0.17
68 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

O 50-year 3,573 263.45 276.66 265.65 276.63 2.2 -0.03

10-year 7,506 278.41 278.32 -0.09

2-year 12,820 280.21 280.05 -0.16

1-year 15,747 281.07 280.88 -0.19

0.2-year 24,521 282.93 282.6 -0.33

N 50-year 3,573 259.63 265.25 258.64 264.36 -0.99 -0.89

10-year 7,506 267.43 266.7 -0.73

2-year 12,820 269.59 268.94 -0.65

1-year 15,747 270.61 269.97 -0.64

0.2-year 24,521 273.09 272.57 -0.52

M 50-year 3,573 258.46 264.08 257.47 263.18 -0.99 -0.9

10-year 7,506 266.27 265.52 -0.75

2-year 12,820 268.42 267.77 -0.65

1-year 15,747 269.44 268.82 -0.62

0.2-year 24,521 273.05 272.58 -0.47

L 50-year 3,573 251.54 257.82 248.6 259.73 -2.94 1.91

10-year 7,506 260.59 262.03 1.44

2-year 12,820 263.58 264.27 0.69

1-year 15,747 265.5 265.53 0.03

0.2-year 24,521 274.59 274.25 -0.34

K 50-year 3,573 250.58 257.59 247.7 256.02 -2.88 -1.57

10-year 7,506 260.26 259.24 -1.02

2-year 12,820 263.19 262.48 -0.71

1-year 15,747 265.16 264.53 -0.63

0.2-year 24,521 274.38 274 -0.38

J 50-year 3,573 242.2 254.09 239.2 252.28 -3 -1.81

10-year 7,506 256.47 255.98 -0.49

2-year 12,820 261.1 260.39 -0.71

1-year 15,747 263.7 262.97 -0.73

0.2-year 24,521 274.04 273.61 -0.43

I 50-year 3,573 242.1 251.35 239.1 251.03 -3 -0.32

10-year 7,506 256.21 255.87 -0.34

2-year 12,820 261.07 260.59 -0.48

1-year 15,747 263.51 262.97 -0.54

0.2-year 24,521 273.85 273.46 -0.39

H 50-year 3,573 219.73 230.61 218.77 230.94 -0.96 0.33

10-year 7,506 233.26 233.67 0.41

2-year 12,820 235.84 236.23 0.39

1-year 15,747 236.61 237.03 0.42

0.2-year 24,521 237.91 238.54 0.63
Table 7 69

Table 7. Comparison of Hydrologic Engineering Center–River Analysis System (HEC–RAS)-predicted flood elevations for 2008 and the 

end of water year 2010 for the 50-, 10-, 2-, 1-, and 0.2-percent annual exceedence probability floods and streambed elevations at each 

cross section.—Continued

[Location of cross sections A–DW are shown in figure 4. WY, water year; %, percent; ft3/s, cubic feet per second; ft, feet; NGVD 29, National Geodetic Vertical 

Datum of 1929]

Cross

section

Annual 

exceedance 

probability 

flood

(%)

Flow

(ft3/s)

WY 2008 End WY 2010 2008–2010 change

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

Minimum 

channel 

elevation

(ft, NGVD 29)

Watersurface 

elevation

(ft, NGVD 29)

G 50-year 3,573 214.79 230.67 218.85 230.87 4.06 0.2

10-year 7,506 233.46 233.62 0.16

2-year 12,820 236.3 236.29 -0.01

1-year 15,747 237.27 237.18 -0.09

0.2-year 24,521 239.38 239.09 -0.29

F 50-year 3,573 204.12 207.87 201.12 204.86 -3 -3.01

10-year 7,506 209.56 207.39 -2.17

2-year 12,820 211.36 211.15 -0.21

1-year 15,747 212.46 213.07 0.61

0.2-year 24,521 220.28 220.43 0.15

E 50-year 3,573 198.43 204.23 195.43 202.05 -3 -2.18

10-year 7,506 206.5 205.85 -0.65

2-year 12,820 209.69 210.04 0.35

1-year 15,747 211.77 212.06 0.29

0.2-year 24,521 220.33 220.33 0

D 50-year 3,573 196.62 200.32 193.62 197.44 -3 -2.88

10-year 7,506 201.59 199.37 -2.22

2-year 12,820 202.98 201.25 -1.73

1-year 15,747 203.68 201.93 -1.75

0.2-year 24,521 205.49 203.77 -1.72

C 50-year 3,573 184 189.81 181.06 187.22 -2.94 *

10-year 7,506 Backwater 195.1 189.56 *

2-year 12,820 Backwater 197 191.49 *

1-year 15,747 Backwater 198.3 192.32 *

0.2-year 24,521 Backwater 205 194.48 *

B 50-year 3,573 183.54 188.82 180.54 186.28 -3 *

10-year 7,506 Backwater 195.1 188.61 *

2-year 12,820 Backwater 197 190.25 *

1-year 15,747 Backwater 198.3 190.84 *

0.2-year 24,521 Backwater 205 192.31 *

A 50-year 3,573 181.57 186.44 178.57 183.64 -3 *

10-year 7,506 Backwater 195.1 186.42 *

2-year 12,820 Backwater 197 188.31 *

1-year 15,747 Backwater 198.3 188.89 *

0.2-year 24,521 Backwater 205 190.35 *

*

Water-surface elevation for cross-sections A,B,C not reported due to backwater effects from Merrimack River.
70 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Appendix 1. Particle-size gradation and median diameter (d50) of streambed sediment samples collected at 30 locations in the 

[coord dms, coordinates in degrees, minutes, seconds; ID, identification number; N, north; W, west; mm, millimeters; %, percent; <, less than; m, meters; 

Site ID

(coord dms) Old ID Latitude (N)

Longitude 

(W)

d50

(mm)

d50 sediment 

class name

Particle size (mm) 

% <63m % <125m % <250m % <500m % <1mm % <2mm 

431505071222401 SRM-1L 0.161 Fine sand 10 37 82 96 98 99 

431505071222401 SRM-1C 43 15′ 05.3′′ -71 22′ 24.5′′ 1.857 Very coarse sand 0 1 5 22 38 52 

431505071222401 SRM-1R 0.163 Fine sand 10 34 86 99 100

431421071214101 SRM-10L 0.215 Fine sand 3 19 62 92 98 100

431421071214101 SRM-10C 43 14′ 21.4′′ -71 21′ 40.6′′ 0.667 Coarse sand 4 10 24 40 70 91 

431421071214101 SRM-10R 0.151 Fine sand 6 38 96 100

431419071213801 SRM-15L 0.166 Fine sand 6 31 89 100

431419071213801 SRM-15C 43 14′ 19.2′′ -71 21′ 38.1′′ 3.1 Very fine gravel 0 1 3 8 20 37 

431419071213801 SRM-15R 0.385 Medium sand 6 18 43 56 60 64 

431340071212801 SRM-70L 0.14 Fine sand 13 45 87 97 99 100

431340071212801 SRM-70C 43 13′ 39.7′′ -71 21′ 27.5′′ 26 Coarse gravel 0 0 3 19 30 36 

431340071212801 SRM-70R 0.406 Medium sand 7 20 45 53 54 55 

431335071212601 SRM-80L 0.212 Fine sand 8 27 60 88 95 99 

431335071212601 SRM-80C 43 13′ 34.7′′ -71 21′ 25.8′′ 109 Small cobbles 0 1 6 14 18 19 

431335071212601 SRM-80R 4 Fine gravel 6 16 31 42 45 48 

431313071212901 SRM-100L 1.067 Very coarse sand 2 5 16 30 49 64 

431313071212901 SRM-100C 43 13′ 13.0′′ -71 21′ 29.4′′ >180 Large cobbles 0 0 0 0 1 1 

431313071212901 SRM-100R 58.818 Very coarse gravel 0 1 3 6 9 11 

431309071213101 SRM-110L 0.615 Coarse sand 3 6 19 44 70 83 

431309071213101 SRM-110C 43 13′ 08.9′′ -71 21′ 30.9′′ 25.333 Coarse gravel 2 5 8 14 19 22 

431309071213101 SRM-110R 0.203 Fine sand 9 22 67 96 97 97 

431306071213001 SRM-120L 6.154 Fine gravel 0 1 3 8 20 27 

431306071213001 SRM-120C 43 13′ 06.3′′ -71 21′ 29.9′′ 19.5 Coarse gravel 0 0 2 7 11 14 

431306071213001 SRM-120R <0.063 Silt 88 97 100

431302071213501 SRM-130L 0.478 Medium sand 1 5 29 52 64 70 

431302071213501 SRM-130C 43 13′ 01.5′′ -71 21′ 35.2′′ 11.75 Medium gravel 1 3 12 19 22 23 

431302071213501 SRM-130R 0.714 Coarse sand 5 13 26 41 62 73 

431301071214201 SRM-140L 0.482 Medium sand 17 19 24 52 75 89 

431301071214201 SRM-140C 43 13′ 01.4′′ -71 21′ 41.6′′ 15 Very coarse gravel 0 0 1 2 2 2 

431301071214201 SRM-140R 0.067 Very fine sand 49 63 69 83 97 100

431254071215201 SRM-200L 0.245 Fine sand 1 5 52 93 100

431254071215201 SRM-200C 43 12′ 53.6′′ -71 21′ 52.3′′ 0.85 Coarse sand 2 8 19 36 56 71 

431254071215201 SRM-200R 0.154 Fine sand 13 40 83 100

431246071220201 SRM-210L 0.226 Fine sand 6 20 57 95 100

431246071220201 SRM-210C 43 12′ 45.7′′ -71 22′ 02.4′′ 1.1667 Very coarse sand 2 5 12 27 47 65 

431246071220201 SRM-210R 0.153 Fine sand 10 38 91 100

431237071221101 SRM-220L 0.197 Fine sand 4 19 73 98 100

431237071221101 SRM-220C 43 12′ 37.4′′ -71 22′ 11.0′′ 1.333 Very coarse sand 1 3 15 32 44 62 

431237071221101 SRM-220R 0.192 Fine sand 4 20 76 99 100

431236071221301 SRM-230L 4.291 Fine gravel 2 7 20 33 44 46 

431236071221301 SRM-230C 43 12′ 35.6′′ -71 22′ 12.6′′ 16 Medium gravel 0 0 4 12 24 29 

431236071221301 SRM-230R 0.148 Fine sand 10 40 95 100
Appendix 1 71

Suncook River.—Continued

>, greater than; L, left bank; C, channel; R, right bank]

Particle size (mm)

% <2.8mm % <4mm % <5.6mm % <8mm % <11mm % <16mm % <22mm % <32mm % <45mm % <64mm % <90mm % <128mm % <180mm % >180mm

99 100

63 77 88 100

98 100

48 56 69 77 82 83 83 85 87 93 97 98 99 100

71 78 100

36 38 38 41 42 45 48 53 56 59 66 77 81 100

55 55 55 55 55 55 55 55 55 55 55 64 73 100

100

19 19 20 20 23 24 27 29 32 37 45 55 67 100

50 100

65 65 68 72 81 85 91 100

3 4 5 6 8 13 16 18 20 21 24 29 44 100

11 11 11 16 16 32 42 42 42 53 53 58 63 100

83 89 94 94 94 94 100

22 23 25 29 34 42 48 54 59 68 76 82 88 100

97 97 97 97 97 97 97 97 97 97 97 97 97 100

33 33 47 60 67 87 100

14 15 17 20 25 43 55 69 85 91 99 100

70 70 72 74 81 88 93 98 98 100

24 25 31 38 47 67 83 92 98 100

75 81 85 88 90 92 94 96 100

91 91 100

2 4 8 18 34 54 71 89 98 100

71 71 75 78 82 84 86 86 88 90 92 94 96 100

66 71 81 87 92 97 98 99 100

62 62 66 69 79 87 94 98 100

46 48 59 70 83 91 96 100

30 31 32 35 38 50 66 81 95 99 100
72 Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood

Appendix 1. Particle-size gradation and median diameter (d50) of streambed sediment samples collected at 30 locations in the 

[coord dms, coordinates in degrees, minutes, seconds; ID, identification number; N, north; W, west; mm, millimeters; %, percent; <, less than; m, meters; 

Site ID

(coord dms) Old ID Latitude (N)

Longitude 

(W)

d50

(mm)

d50 sediment 

class name

Particle size (mm) 

% <63m % <125m % <250m % <500m % <1mm % <2mm 

431215071221601 SRM-232L 0.394 Medium sand 4 9 31 64 82 88 

431215071221601 SRM-232C 43 12′ 15.3′′ -71 22′ 16.5′′ 1.273 Very coarse sand 0 0 6 29 47 58 

431215071221601 SRM-232R 0.104 Very fine sand 21 65 98 100

431221071224401 SRM-234L 0.132 Fine sand 12 48 85 94 100

431221071224401 SRM-234C 43 12′ 21.0′′ -71 22′ 43.9′′ 0.704 Coarse sand 0 0 3 30 79 96 

431221071224401 SRM-234R 0.125 Fine sand 15 50 93 100

431202071225601 SRM-238L 0.206 Fine sand 6 26 63 78 89 97 

431202071225601 SRM-238C 43 12′ 01.6′′ -71 22′ 56.2′′ 0.6875 Coarse sand 0 0 4 35 75 95 

431202071225601 SRM-238R 0.291 Medium sand 8 17 44 81 97 100

431142071225101 SRM-240L 25

431142071225101 SRM-240C 43 11′ 41.6′′ -71 22′ 50.9′′ 0.230 Fine sand 1 7 58 84 92 96 

431142071225101 SRM-240R 0.107 Very fine sand 21 62 94 98 100

431121071225401 SRM-250L 0.095 Very fine sand 22 75 97 99 100

431121071225401 SRM-250C 43 11′ 21.4′′ -71 22′ 54.2′′ 0.460 Medium sand 1 3 13 57 92 99 

431121071225401 SRM-250R 0.094 Very fine sand 27 72 95 98 99 100

431031071232001 SRM-260L 25

431031071232001 SRM-260C 43 10′ 31.2′′ -71 23′ 20.2′′ NA 30

431031071232001 SRM-260R 35

431008071240201 SRM-270L 0.172 Fine sand 14 32 80 99 100

431008071240201 SRM-270C 43 10′ 08.5′′ -71 24′ 01.7′′ NA 40

431008071240201 SRM-270R 0.141 Fine sand 23 45 83 98 99 99 

430938071241901 SRM-280L 0.114 Very fine sand 21 56 89 95 98 100

430938071241901 SRM-280C 43 09′ 37.5′′ -71 24′ 18.6′′ <0.063 Coarse silt 54 100

430938071241901 SRM-280R 0.172 Fine sand 10 32 80 99 99 100

430929071242601 SRM-300L 0.176 Find sand 20 33 75 99 100

430929071242601 SRM-300C 43 09′ 29.0′′ -71 24′ 26.4′′ 19.273 Coarse gravel 1 2 4 10 14 16 

430929071242601 SRM-300R 0.156 Find sand 15 40 80 98 99 100

430856071244701 SRM-310L <0.063 Silt 57

430856071244701 SRM-310C 43 08′ 56.2′′ -71 24′ 47.2′′ 0.398 Medium sand 5 11 24 68 89 92 

430856071244701 SRM-310R 0.164 Find sand 9 32 89 99 100

430858071252501 SRM-320L 0.09 Very fine sand 35 69 94 98 99 100

430858071252501 SRM-320C 43 08′ 57.6′′ -71 25′ 24.8′′ 0.979 Coarse sand 1 3 10 27 51 70 

430858071252501 SRM-320R 0.123 Very fine sand 23 51 85 96 99 100

430835071253401 SRM-330L 0.226 Find sand 7 16 58 97 100

430835071253401 SRM-330C 43 08′ 35.0′′ -71 25′ 33.6′′ 0.85 Coarse sand 0 1 1 15 65 91 

430835071253401 SRM-330R 0.168 Find sand 12 33 82 98 100

430813071255201 SRM-340L 36

430813071255201 SRM-340C 43 08′ 12.9′′ -71 25′ 52.2′′ 0.774 Coarse sand 2 5 20 33 64 89 

430813071255201 SRM-340R 0.14 Find sand 21 46 79 96 99 100

430747071262101 SRM-350L 23

430747071262101 SRM-350C 43 07′ 47.4′′ -71 26′ 20.8′′ 0.616 Coarse sand 0 0 1 37 93 100

430747071262101 SRM-350R 0.226 Find sand 9 17 58 96 99 100

1089985 at Route 3 0.384 Medium sand 1 3 20 76 97 99 

430746071265901 SRM-360C 43 07′ 46.5′′ -71 26′ 58.9′′ 0.409 Medium sand 7 11 20 67 91 99 
Appendix 1 73

Suncook River.—Continued

>, greater than; L, left bank; C, channel; R, right bank]

Particle size (mm)

% <2.8mm % <4mm % <5.6mm % <8mm % <11mm % <16mm % <22mm % <32mm % <45mm % <64mm % <90mm % <128mm % <180mm % >180mm

90 91 92 100

65 76 91 100

99 100

99 100

99 100

98 100

100

100

16 20 23 29 39 44 55 73 78 86 93 95 97 100

93 93 100

85 94 100

99 100

92 93 93 100

99 100

100
Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Director

U.S. Geological Survey

New Hampshire-Vermont Water Science Center

331 Commerce Way, Suite 2

Pembroke, NH 03275

dc_nh@usgs.gov

or visit our Web site at:

http://nh.water.usgs.gov
P

rinted on recycled paper

Flynn—Transport of Sediment by the Suncook River in Epsom, Pembroke, and Allenstown, N.H., after the May 2006 Flood—Scientific Investigations Report 2011–5088