DESIGN OF RIPRAP FOR PROTECTION AGAINST. SCOUR ...

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VOL. 16, (No. I) DESIGN OF RIPRAP FOR PROTECTION AGAINST
SCOUR AROUND BRIDGE PIER
.\ TABLE-3
UlU c AND CORRESPONDING SHEAR AT PIER
S.N Name of the Investigator UIU 't p = M 't oc
c
Shear Stress by velocity Measurement
I Nicollet (1977) 0.42 5
Circular PierRounded nosed Pier 0.50-0.65 3
2 Lee Jong (1973)
Circular pier without 0.40 6.25
attachment Round nosed and 0.50 4.00
Rectangular
3 Bressure and Roudkivi (1977) 0.5 4
4 Chiew (1995) 0.3 11
5 Melville (1999) 0.34 8
6 Dey (l993) 0.475 4
7 Present 0.438 5
Shear Stress by Preston Measurement
8 Hjorth (1975) - 12
9 Melville (1975) - 3.5
10 Darghi (1987) - 3.5
Studies of Einstein and E1. Sarnni (l949), Gessler (1967) and Little Mayer (l972)
have shown that the lift as well as shear at the bed fluctuates in turbulent flow, and
follows Gaussian distribution as an approximation with standard deviation «J') in
dimensionless form varying from 0.45 to 0.57. Therefore, the maximum shear stress
near the pier can be 'tpmax = (Tp + 3 x 0.45Tp) = 2.35'tp. Here, value of o assumed is
0.45. Therefore, riprap layer can be disturbed when 'tpmax-2.35 (5'[) = 12 'to'
Patel and Raga Raju (1999) in their analysis of critical shear stress of non uniform
material have recommended the use of characteristic size of bed material D a' which
is given by D g x o g instead ofD so to for account non- uniformity of sediments. Here,
D g isgeometric median size of riprap and og geometric standard deviation. Therefore,
to prevent scour around the bridge pier , the size of the riprap material D can be
g
calculated as,
D = 12'[0
a flys t. co
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ISH JOURNAL OF HYDRAULIC ENGINEERING, VOL. 16,2010, NO.1
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