DESIGN OF RIPRAP FOR PROTECTION AGAINST. SCOUR ...

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VOL. 16, (No.1) DESIGN OF RIPRAP FOR PROTECfION AGAINST
(83)
SCOUR AROUND BRIDGE PIER
.,
1. Incipient Scour of bed material around the pier
2. Incipient scour of riprap of different size and thickness
3. Scour with riprap protection.
Over 150 runs were conducted using circular pier of 50 mrn diameter under clear
water condition. Table 1 gives standard deviation, size of bed and riprap material
tested in the experiments. The circular ring with engraved marking in nun was used to
lay riprap of appropriate thickness and flush with original bed level, Bed material of
predetermined thickness was removed from scoured area, weighed and riprap of same
weight was then slowly added to the scour hole and levelled to the undisturbed bed
level.
In addition to the data collected in the present study, data collected by Knight
(1975), Dey (1995), Chiew (1995), Melville (1997),ha:ve also been used for
determining DIU c for incipient scour and those by Worman (1989) and Chiew (1995)
for size and thickness of riprap layer. Worman (1989) had used three bed materials of
median diameter 0.17,0.36 and 0.78 mm and five ripraps of size varying from 8 to 48
mm. Depth of the flow varied from 300 to 400 mm whereas Chiew (1995) used bed
material of mean size 0.96 mm and three riprap of size 2.60, 4 and 4.85 mm.
TABLE-l
CHARACTERISTICS OF BED AND RIPRAP MATERIAL
(pRESENT STUDy)
ANALYSIS OF DATA
Bed Material Riprap Material
d so (mm) 0'& Dso(mm) O'g
0.20 2.45 1 1.37
0.27 2.69 2. 1.90 .
0.36 1.28 3 2.36
0.40 2.53 4 2.50
0.50 2.63 5 2.66
0.68 2.73 8 1.58
Limiting values ofUfUcfor Incipient Scour
The critical velocity at which sediment of a given size will just move in an
un~bstructed uniform flow was obtained by combining Shields' and Yalin-Karman
relationship with Karman-Prandl's equation of Diu. in hydro-dynamically smooth
and rough channels. Analyzing the generated data, following type of equation was
obtained.
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ISH JOURNAL OF HYDRAULIC ENGINEERING, VOL 16,2010, NO. I
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