FLOOD04125

• County: Statewide
• Basin: Statewide
• Title: Interim Procedures for Evaluating Scour at Bridges
• Date: 9/1/1988
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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INTERIM PROCEDURES FOR EVALUATING SCOUR AT BRIIMiES
CHAPTER 1
INTRODUCTION

A. PURPOSE

To provide guidance in regard to (1) designing new or replacement bridges to
resist scour; (2) evaluating existing bridges for vulnerability to scour; (3)
providing scour countermeasures; (4) improving the state-of-practice of
estimating scour at bridges and (5) assist the development of HEC-18, Scour at
8ridges (1).

8. ORGANIZATION OF THIS CIRCULAR

These procedures contain the state-of-knowledge and practice for dealing with
scour at highway bridges. Chapter 1 gives the background of the problea and
the general state-of-know1edge of scour. Basic concepts and definitions are
presented in Chapter 2. Chapter 3 gives recommendations for designing bridges
to resist scour. Chapter 4 gives equations for calculating scour depths at
piers and abutments. Chapter 5 provides procedures for conducting scour
analysis and evaluation at existing bridges. Chapter 6 presents guidelines for
inspecting bridges for scour. Chapter 7 gives a plan of action for installing
countermeasures to strengthen bridges that are considered vulnerable to scour.

Tables and forms are provided to help in assembling the needed data and make
calculations.

C. BACKGROUND

The most common cause of bridge failures stems from floods, and scouring of
bridge foundations is the most common cause of damage to bridges during floods.
During the Spring floods of 1987, 17 bridges in New York and New England were
damaged or destroyed by scour. In 1985, 73 bridges were destroyed by floods in
Pennsylvania, Virginia, and West Virginia. A 1973 study for the FHWA of 383
bridge failures caused by catastrophic floods showed that 25 percent involved
pier damage and 72 percent involved abutment damage (2). A second more
extensive study done in 1978 (3) indicated local scour at bridge piers to be a
problem about equal to abutment scour problems. A number of case histories on
the causes and consequences of scour at major bridges are presented in
Transportation Research Number 950 (4).

D. OBJECTIVES OF A BRIDGE SCOUR EVALUATION PROGRAM

The need to minimize future flood damage to the nation's bridges requires that
additional attention be devoted to developing and implementing improved
procedures for designing and inspecting bridges for scour. (See National
Bridge Inspection Standards, 23 CFR 650 Subpart C.) Approximately 86 percent
of the 577,000 bridges in the National Bridge Inventory are built over
waterways. Statistically,we can expect thousands of these bridges to

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