Prepared in cooperation with the New Hampshire Department of Transportation

Flood Study of Warren Brook in Alstead and Cold River in Alstead, Langdon, and Walpole, New Hampshire, 2005

By Robert H. Flynn

 

Open-File Report 2006–1313

 

AVAILABLE ONLINE ONLY

 

The body of the report is available in PDF Format ( 4,728 KB)

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Abstract

This report presents water-surface elevations and profiles as determined using the U.S. Army Corps of Engineers (USACE) one-dimensional Hydrologic Engineering Center River Analysis System, also known as HEC-RAS. Steady flow water-surface profiles were developed for two stream reaches: the Cold River from its confluence with the Connecticut River in Walpole, through Alstead to the McDermott Bridge in Langdon, NH, and Warren Brook from its confluence with the Cold River to Warren Lake in Alstead, NH. Flood events of a magnitude, which are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence interval), were modeled using HEC-RAS as these flood events are recognized as being significant for flood-plain management, determination of flood insurance rates, and design of structures such as bridges and culverts. These flood events are referred to as the 10-, 50-, 100-, and 500-year floods and have a 10-, 2-, 1-, and 0.2-percent chance, respectively, of being equaled or exceeded during any year. The recurrence intervals represent the long-term average between floods of a specific magnitude. The risk of experiencing rare floods at short intervals or within the same year increases when periods greater than one year are considered. The analyses in this study reflect the flooding potentials based on conditions existing in the communities of Walpole, Alstead and Langdon at the time of completion of this study.

TABLE OF CONTENTS

Abstract

Introduction

Purpose and Scope

Authority and Background

Coordination

Previous Studies

Flood Recovery Study Methods

Description of the Study Area

Principal Flood Problems Along Warren Brook and Cold River

Flood-Protection Measures Affecting Warren Brook and Cold River

Engineering Methods Used to Determine Flood Data

Hydrologic Analyses

Hydraulic Analyses

Flood-Plain Management Applications

Summary

References Cited

Appendix 1: Cold River Flood Profiles

Appendix 2: Warren Brook Flood Profiles

Appendix 3: Elevation Reference Marks

Figures

1–5. Maps showing—

1. Extent of the flood insurance study of Cold River and Warren Brook in Alstead, Langdon, and Walpole, New Hampshire
2. Location of cross sections along Cold River in Walpole and Langdon, New Hampshire
3. Location of cross sections along Cold River in Alstead and Langdon, New Hampshire
4. Location of cross sections along Warren Brook in Alstead, New Hampshire
5. Location of cross sections along Warren Brook in Alstead, New Hampshire

Tables

1. Summary of the drainage area-peak relations for streams in New Hampshire, studied by detailed methods
2. Manning’s “n” values for Cold River and Warren Brook
3. Cross sections and flood data for Cold River in Alstead, Langdon, and Walpole, New Hampshire
4. Cross sections and flood data for Warren Brook in Alstead, New Hampshire

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For further information, contact:

 

Keith W. Robinson, Director
U.S. Geological Survey
New Hampshire - Vermont Water Science Center
361 Commerce Way
Pembroke , NH 03275

dc_nh@usgs.gov

(603) 226-7807

or visit our Web site at: http://nh.water.usgs.gov

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