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<br />I' <br />I~, <br /> <br />., <br /> <br />I <br />I <br />1 <br />1 <br />I <br />1 <br />I <br />1 <br />I <br />I <br />1 <br />I <br />1 <br />1 <br />I <br />I <br />I <br /> <br />FHWA TECHNICAL AD~ISORY T 5140.20 <br />September 16, 1988 <br /> <br />\ t} Hydraulic stlldies should be prepared for bridges <br />over waterways in accordance with Article 1.3.2 of <br />the Standard Specifications for Highway Bridges of <br />the American Association of State Highway and <br />Transportation Officials (AASHTO) and the floodplain <br />regulation of the FHWA as set forth in 23 CrR 650, <br />Subpart A. <br /> <br />(2) Hydraulic studies should include estimates of scour <br />at bridge piers and abutments. Bridge foundations <br />should be desicmed to withstand the effects of scour <br />without failing for the worst conditions resulting <br />from floods equal to or less than the 100 year flood <br />in accordance with the interim procedures in <br />Chapters 3 and 4 of the Attachment. Bridge <br />foundations should be checked to ensure that they <br />will not fail due to scour reSUlting from the <br />occurrence of a superflood on the order of <br />magnitude of a 500 year flood. (see Attachment, <br />Chapter 3.) <br /> <br />(3) The geotechnical analysis of bridge foundations <br />should be performed on the basis that all stream bed <br />material in the scour prism above the total scour <br />line for the design flood (for scour) has been <br />removed and is not available for bearing or lateral <br />support. In addition, the ratio of ultimate to <br />applied loads should be greater than 1.0 for <br />conditions of scour for the superflood. (See <br />Attachment, Chapter 3.) <br /> <br />(4) Data on scour at bridge piers and abutments should <br />be collected and analyzed in order to improve <br />existing procedures for estimating scour. (See <br />Attachment, Chapter 1.) <br /> <br />c. Existing Bridqes. All existing bridges over tidal and <br />non-tidal waterways with scourable beds should be <br />evaluated for the risk of failure from scour during the <br />occurrence of a superflood on the order of magnitude of a <br />500 year flood. (See Attachment, Chapter 5.) <br /> <br />( 1) An initial screening process should be developed to <br />identify bridges most likely to be susceptible to <br />scour damage and to establish a priority list for <br />evaluation. (See Attachment, Chapter 5.) <br /> <br />3 <br />