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<br />EVALUATING SCOUR AT BRIDGES <br /> <br />CHAPTER 1 <br /> <br />INTRODUCTION <br /> <br />A. PURPOSE <br /> <br />The purpose of this manual is to provide guidance in regard to: <br /> <br />1) designing new or replacement bridges to resist scour, <br />2) evaluating existing bridges for vulnerability to scour, <br />3) inspecting bridges for scour, <br />4) providing scour countermeasures, and <br />5) improving the state-of-practice of estimating scour at <br />bridges. <br /> <br />B. ORGANIZATION OF THIS CIRCULAR <br /> <br />These procedures contain the state-of-knowledge and practice for <br />dealing with scour at highway bridges. Chapter 1 gives the <br />background of the problem and the general state-of-knowledge of <br />scour. Basic concepts and definitions are presented in Chapter <br />2. Chapter 3 gives recommendations for desig~i~g bridges to <br />resist scour. Chapter 4 gives equations for calculating scour <br />depths at piers and abutments. Chapter 5 provides procedures for <br />conducting scour evaluation and analysis at existing bridges. <br />Chapter 6 presents guidelines for inspecting bridges for scour. <br />Chapter 7 gives a plan of action for installing countermeasures <br />to strengthen bridges that are considered vulnerable to scour, <br /> <br />In the appendices additional information <br />examples of what several states are doing <br />their scour problems is given, <br /> <br />on abutment scour and <br />to assess and evaluate <br /> <br />C. BACKGROUND <br /> <br />The most common cause of bridge failures stems from floods. The <br />scouring of bridge foundations is the most common cause of flood <br />damage to bridges. The hydraulic design of bridge waterways has <br />and is typically based on flood frequencies somewhat less than <br />those recommended for scour analysis in this publication. During <br />the Spring floods of 1987, 17 bridges in New York and New England <br />were damaged or destroyed by scour. In 1985, 73 bridges were <br />destroyed by floods in Pennsylvania, Virginia, and West Virginia. <br />A 1973 study for the FHWA of 383 bridge failures caused by <br />catastrophic floods showed that 25 percent involved pier damage <br />and 72 percent involved abutment damage (1). A second more <br /> <br />1 <br />