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<br />FHWA TECHNICAL ADVISORY T 5140.20, Chi. 1 <br />November 7, 1988 <br /> <br />7. Instruction for insp.cting .xisting count.rmeasur.s such as riprap, dit.s, <br />sills, .tc. <br /> <br />8. FortIS Ind proc.dur.s for doclllltnt1ng insp.ction results and 1nstruct\ons <br />r.glrding follow-up Ictions wh.n n.c.ssary. <br /> <br />9. Instillation of scour d.pth wlrning d.vic.s. <br /> <br />D. TYPES OF COUNTERMEASURES <br /> <br />An ov.rview of commonly us.d scour count.nDtlsur.s is provid.d below along with <br />r.f.r.nces for obtlihing d.sign proc.dur.s and crit.ril for their application to <br />I sp.cific site. S.l.ction of the appropriate counte,..asurtS 1s best <br />accomplish.d through a fi.ld and offic. .valultion of the conclitions at the <br />str.am crossing. <br /> <br />1. Riprl, It Pi.rs Ind Abutments. Th. FHWA is curr.ntly .valuating various <br />.quations for selection of riprap at bridge piers and abutltnts. For the <br />pr.sent, the FHWA r.commends us. of the d.sign approach in Chapter 6 of <br />Highways in the River Environment (6) for sizing riprap at abut.tnts. Th. <br />interim procedure pr.sented b.,ow is r.commended for d.signing riprap at <br />pi.rs: <br /> <br />o Determine the D.o size of the riprlp using the r.arranged Ishbash .qultion <br />to solve for stone diamet.r (in feet), for fr.sh wat.r: <br /> <br />D " .692 V. <br />.0 <br />(s-l) 2g <br /> <br />D.o " av.rlge stone diameter (ft) <br />V = velocity against stone (ft/s.c) <br />s = sp.cific gravity of riprap ..t.rial (no""'y 2.65) <br />g = 32.2 ft/sec. <br /> <br />To calculate V, first det.,.ine the v,'ocity of flow just upstr... of the <br />. <br />pi.r. This..y b. approximat.d by the v,'ocity in the contracted s.ction. <br />Then MUltiply.this vllu. by . factor of 1.5 to 2.0 to IPproxi..t. the <br />velocity of flow at the bas. of the pier. Pl.lse note thlt pr.,i.inary <br />res.lrch by FHWA indicat.s that I factor of about 1.5 .., b. I r.lSonable <br />design valu.. <br /> <br />wher.: <br /> <br />o Provide I ..t width that .xtends horizontally at least two ti..s the pi.r <br />width measured from the pi.r face. <br /> <br />o Plac. the ..t b.low str.ambed a depth .quival.nt to the .xpected scour. <br />The thickn.ss should b. thr.e stone di...ters or ~r.. <br /> <br />61 <br /> <br />'. <br />. <br />Ai" <br />rn"'c.' <br />'. <br />. <br />. <br />. <br />. <br />I <br />. <br />. <br />I <br />. <br />I <br />I <br />. <br />. <br />. <br />. <br />I <br />