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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

By Robert H. Flynn

Thumbnail of and link to report PDF (5.45 MB)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 water-surface 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.

Revised November 18, 2011

First posted October 6, 2011

For additional information contact:
Director
U.S. Geological Survey
New Hampshire-Vermont Water Science Center
361 Commerce Way
Pembroke, NH 03275
(603)226-7800

http://nh.water.usgs.gov

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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.)



Contents

Abstract

Introduction

Purpose and Scope

Previous Investigations

Description of Study Reach

Geohydrology of the Suncook River

Characterizing Sediment Transport and the River Streambed

Channel Equilibrium

Stream Temperature

Bankfull Flow as Effective Discharge

Aggradation and Degradation

Methods of Analyses

Channel-Geometry Data

Contour Data

Streamflow and Temperature Data

Streambed and Sediment Sampling

Simulation of Flow and of Sediment Transport

Initial- and Boundary-Condition Data

Hydrologic Data

Hydraulic Data

Calibration of the HEC–RAS Hydraulic Model

HEC–RAS Sediment-Transport Function Selection

Limitations of the HEC–RAS Model

Markov Chain Analyses of Flow

Geophysical Data

Channel Morphology and Stream Processes

Lateral Migration of Streambed

Distribution of Sediment Grain Size

Bankfull Channel Dimensions

HEC–RAS Sediment-Transport Model

Flood-Generated Shear Stress and the Potential for Entrainment

Model and Field-Data Sediment-Transport Curves

Simulating Sediment Transport to Investigate the Impact of Future Sediment Loads on Flood Flows

Electrical Resistivity Data

Sediment Load and its Effect on Streambed and Flood Elevations

Summary

References Cited

Appendix 1. Particle-Size Gradation and Median Diameter (d50) of Streambed Sediment Samples Collected at 30 Locations in the Suncook River

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