Laserfiche WebLink
<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I I <br />I <br />I <br />I <br />I <br />I <br /> <br />7 ln~truction for inspecting existing countermeasures such as riprap. dikes, <br />sills, etc. <br /> <br />B. Forms and procedures for doc~menting inspection results and instructions <br />regarding follow-up actions When necessary. <br /> <br />9. Installation of scour depth warning devices. <br /> <br />D . "TYPES OF COIJNl'ERl'lEASURES <br /> <br />An overview of commonly used scour countermeasures is provided below along with <br />references for obtaining design procedures and criteria for their application to <br />a specific site. Selection of the appropriate countermeasures is best <br />accomplished through a field and office evaluation of the conditions at the <br />stream crossing. <br /> <br />1. RiDra~ at Piers and Abutments. The FHWA is currently evaluating various <br />equat ons for selection of riprap at bridge piers and abutments. For the <br />present, the FHWA recommends use of the design approach in Chapter 6 of <br />Highways in the River Environment (6) for sizing riprap at abutments. The <br />interim procedure presented below is recORlended for designing riprap at <br />piers: <br /> <br />o Determine the D50 size of the riprap using the rearranged 1shbash equation <br />(See Breusers, Nicollet. Shen, 1977) to solve for stone diameter (in <br />feet), for fresh water: <br /> <br />O - 1.384 V. <br />50 - <br />(s-l) 2g <br /> <br />D50 = average stone diameter (ft) <br />V = velocity against stone (ft/sec) <br />s = specific gravity of riprap material (normally 2.65) <br />g = 32.2 ft/sec. <br /> <br />To calculate V, first determine the velocity of flow just upstre.. of the <br />pier. This may be approximated by the velocity in the contracted section. <br />Then multiply this value by a factor of 1.5 to 2.0 to approximate the <br />velocity of flow at the base of the pier. Please note that preliminary <br />research by FHWA indicates that a factor of about 1.5 may be a reasonable <br />design value. <br /> <br />where: <br /> <br />o Provide a mat width that extends horizontally at least two times the pier <br />width measured from the pier face. <br /> <br />o Place the mat below streambed a depth equivalent to the expected scour. <br />The thickness should be three stone di...ters or more. <br /> <br />61 <br />