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<br />4. Desiqn for Scour at Abutments. <br /> <br />It is recommended that foundation depths for abutments be set by <br />AASHTO standards. Protection is afforded by riprap, designed <br />according to HEC 11 (22), and/or by guide banks (spur dikes), <br />designed per Appendix c. <br /> <br />The reason for this recommendation is the equations give the <br />worst case scour since there is no field verification. <br /> <br />LIVE-BED SCOUR AT ABUTMENTS <br /> <br />As a check on the potential depth of scour to aid in the design <br />of the foundation and placement of riprap or guide banks, <br />Froehlich's (26) LIVE-BED SCOUR equation given below can be <br />used. Appendix B presents an alternate design approach, using <br />material.contained in the original Interim Procedures for <br />Evaluating Scour at Bridges. <br /> <br />Froehlich (26) analyzed 170 live-bed scour measurements in <br />laboratory flumes to obtain the following equation: <br /> <br />y/y = 2.27 K1 <br />s . <br /> <br />K2 (a I /y.) 0.~3 <br /> <br />Fr 0.61 + 1 <br />. <br /> <br />(8) <br /> <br />Where: <br />K, = <br />K2 = <br /> <br />coefficient for abutment shape <br />coefficient tor angle of embankment to flow <br /> <br />K2 = (0/90)0.13 <br /> <br />0<900 if embankment points downstream <br />0>900 if embankment points upstream <br /> <br />at = the length of abutment projected normal. to flow <br /> <br />a' = A,jY. <br /> <br />A. = the flow area of the approach cross-section obstructed <br />by the embankment. <br /> <br />Fr. = <br />= <br />V. = <br />Q. = <br /> <br />Froude number of approach flow upstream of the <br />abutment. <br /> <br />v,/(gy.)O.s <br /> <br />Q,j A. <br /> <br />the flow obstructed by the abutment and approach <br />embankment. <br /> <br />47 <br />