DESIGN OF RIPRAP FOR PROTECTION AGAINST. SCOUR ...

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l~':"'2C":;'§;{k)!"i;,,:dX'6B'~k;X~J/%#iiMtiiiW;i;l1~li.~1:i1t{l~'i;';';~Jf.~ >:~ ~# s: .. ., .". : / .,' e: ":.: f:- "':'.": : .. .:, ,.. .'0' (88) DESIGN OF RIPRAP FOR PROTECTION AGAINST SCOUR AROUND BRIDGE PIER VOL 16.(No, 1) -v ± 50 %. Data collected in no scour runs of present study are also plotted in the same figure. It is observed that the 75% of data collected in present study fall in the error band of ± 50 %. Thickness of riprap provided should also be economical. Blench (1957) suggested that thickness of riprap for bed protection may be three times the largest size of the stone in a mixture (DI~. Figure 4 shows the comparison between TlDloo observed and computed. If Eq. (11) is used to compute the thickness of riprap for Worman's and present data. thickness of riprapcomputed isfound to be orderone to three times D 100; however. it is four to five times that of D IOO in case of data collected by Chiew, Thickness of riprap layer using Worman and present data are found appropriate. however, it is slightly higher in case Chiew's data. In Worman's data the flow conditions, the characteristics of riprap, bed material and thickness of riprap used were pertaining to the observed "no scour" condition. In Chiew's data, the flow conditions given were intended for intact andfailed conditions of riprap. Average of flow data corresponding to intact and failed condition has been used in the present analysis to verify the Eq. (11). Theflow conditions pertaining to condition ofriprap described as intact. observed in his experiments may not be corresponding to the true Uno scour" condition. This could be possible reason for computed thickness of riprap found higher than observed. Line of Agreement. 1000 '.' tOO ',' .., -- ~ I •• ... to r I- , , , , , , , , .' , , Line of Agreement , , , , , , , , , , , , , , , , , , ~' _;)1 l"- I"'/Cl " B-' A, i-" ,6' •• ,.0 •• ,.111 0 1 10 100 1000 AG. 3 THICKNESS OF RIPRAP (PRESENT METHOD) ISH JOURNAL OF HYDRAUUC ENGINEERING. VOL. 16, 2010. NO, 1
'. 1) ne or xi Ie d It . ., f VOL. 16, (No.1) DESIGN OF RIPRAP FOR PROTECTION AGAINST SCOUR AROUND BRIDGE PIER Incipient Scour of Bed and Riprap Material The value of UfU c near the circular bridge pier in present study varied from 0.3 to 0.65 and authors recommends it as 0.438. When bed material around the circular bridge pier starts just moving, the value of UfU c is 0.438. Assuming the 'to alP, and corresponding fluctuations in the shear/velocity around bridge pier, authors found that maximum value of instantaneous shear stress near the bridge pier is about 12 'to. Size of the riprap can be calculated using Eq. (9) with (Jg of riprap between 2 to 3. 1· ,," 10010- , , , , , +lS t>~p , .' , , , "I
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