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Hydraulics Engineering


Debris Control Structures Evaluation and Countermeasures
Hydraulic Engineering Circular No. 9

Chapter 1 - Introduction

1.1 Purpose

The purpose of this Federal Highway Administration (FHWA) Hydraulic Engineering Circular (HEC) is to provide information on debris accumulation, guidelines for analyzing impacts associated with debris accumulation, and design guidelines for selecting debris control countermeasures. The design guidelines are based on countermeasures that have been implemented by federal, State, and local transportation agencies at culvert and bridge structures.

1.2 Background

Debris accumulation at culvert and bridge structures openings is a widespread problem. The accumulation of debris at inlets of highway culverts and bridge structures is a frequent cause of unsatisfactory performance and malfunction. This accumulation may result in erosion at culvert entrances, overtopping and failure of roadway embankments and damage to adjacent properties, increased local scour at piers and/or abutments, and the formation of pressure flow scour. Consideration of debris accumulations and the need for debris-control structures should be an essential part of the design of all drainage structures.

Structural and non-structural measures have been used effectively to prevent or reduce the size of debris accumulations at bridges and culverts. Structural measures can include features that: (a) intercept debris at or upstream of a structure inlet; (b) deflect debris near the inlet; or (c) orient the debris to facilitate passage of the debris through the structure. Non-structural measures include management of the upstream watershed and maintenance. This document provides measures for both culvert and bridge structures. The measures available for culverts are based on the information included in earlier editions of this manual. Selection of a certain debris countermeasure depends upon the size, quantity, and type of debris, the potential hazard to life and property, the costs involved, and the maintenance proposed.

1.3 Document Organization

This HEC is organized to provide the following:

Summarize the various types of debris and the problems associated with debris accumulation at culvert and bridge structures (CHAPTER 2)

Provide a general procedure for estimating the volume of floating debris upstream of a bridge site, the potential for the debris to accumulate on a bridge structure, and the potential maximum size of the debris accumulation (CHAPTER 3).

Provide general guidelines for analyzing and modeling debris accumulations on a bridge structure to determine the impacts the debris would have on the water surface profile through the bridge structure and the hydraulic loading on the structure (CHAPTER 4).

Summarize (describe) the various types of debris countermeasures available for culvert and bridge structures (CHAPTER 5).

Provide general criteria for selection of debris countermeasures for culvert and bridge structures, and provide design guidelines for structural countermeasures for which guidelines are available (CHAPTER 6).

Provide general information on maintenance practices (CHAPTER 7).

Provide references and source materials that provide additional, more comprehensive information on debris and debris issues. The references are grouped alphabetically, by author.

Provide a synopsis of a survey of State Department of Transportation and American Association of State Highway Transportation Officials (AASHTO) debris issues and mitigation practices. The survey was conducted as part of the effort to update this document.

1.4 Dual System of Units

This edition of HEC-9 uses dual units, SI metric and English. The "English" system of units as used throughout this manual refers to U.S. Customary (CU) units. An explanation of the metric (SI) unit of measurement is provided in Appendix A. This appendix also contains conversion factors, physical properties of water in the SI and CU units, sediment particle size grade scale, and some common equivalent hydraulic units.

This edition uses the meter (m) or foot (ft) for the unit of length; kilogram (kg) or slug for the unit of mass; Newton (N) or pound (lb) for the unit of weight/force; Pascal (Pa, N/m2) or pounds per square foot (lb/ft2) for the units of pressure; degree Centigrade (°C) or Fahrenheit (°F) for the unit of temperature. The unit of time is the same in both systems. The value of some of the common engineering terms used in this reference in SI units and their equivalent English Units are given in Table 1.1.

Term SI Units English Units
Length 1 m 3.28 ft
Volume 1 m3 35.31 ft3
Discharge 1 m3/s 35.31 ft3/s
Acceleration of Gravity 9.81 m/s2 32.2 ft/s2
Unit Weight of Water 9810 N/m3 62.4 lb/ft3
Density of Water 1000 kg/m3 1.94 slugs/ft3
Density of Quartz 2647 kg/m3 5.14 slugs/ft3
Specific Gravity of Quartz 2.65 2.65
Specific Gravity of Water 1 1
Temperature °C = 5/9 (°F - 32) °F

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Updated: 09/22/2014

United States Department of Transportation - Federal Highway Administration