DESIGN OF RIPRAP FOR PROTECTION AGAINST. SCOUR ...

.....
. . ..
..... -.. - - ..'~ , .•................•
VOL. 16,(No.1) DESIGN OF RIPRAP FOR PROTECTION AGAINST
SCOUR AROUND BRIDGE PIER
From this table, it is evident that Worman's method over predicts the size by about
5 to 8 times, and Chiew's method gives 2 times larger size than that of observed in the
experiments, where as in case of data collected in the present study it is 1.6 times
(average) larger in comparison with observed size of rip rap.
ANALYSIS OF RIPRAP TmCKNESS
For studying the effectiveness of riprap in reduction of scour, the parameter C. =
C a /C b has been calculated. Here C b is the value of constant obtained ill Kothyari et al.
(1992) equation for scour in clear water studies, for non-uniform base material (Eq.
(10».. .
(87)
(10)
C is value of C when riprap was used and some scour was observed.
~
The ratio of C/C b called C. takes in to account the effect ofU, Y, opening ratio a
and /1.Ys on scour and hence, it should be function of D,= dsJDso and T. = T/3cr. Dso
related to rip rap layer and bed material size only. Thickness of riprap layer can be non
dimensionalised by maximum size of the riprap material, which can be expressed as
3. Dso. Anew term therefore, introduced and expressed as T. =T/3cr a D so ' where T
is the thickness of the riprap layer, cr is the standard deviation of riprap mixture
a .
given by ~D84 /D 16 and Dso is median size ofriprap mixture. lfthe sizesin riprap are
distributed normally, 99.73 percent values will be within the range ofD so± 3c •. Hence,
3cra Dso is as good as maximum size of the riprap mixture when D100 is not known.
The experimental data having eight ranges of D. starting from 0.045 and 0.78
were plotted as C/C b Vs T. for respective range ofDiand the equation between them
is obtained as
C a _ C = 0.5 D~·98 T.- 2 . 50
C b - • (ll)
By assuming that at a value of C. as low as 0.05, riprap around the bridge pier will
be stable. Hence, this equation can be solved for with this value for determining the
thickness of riprap. Data collected by Worman (1989) and Chiew (1995) are used for
the comparison of the thickness of riprap layer computed using Eq. (10). Figure 3
shows the thickness of riprap layer used by these investigators in their experiments
for zero scour condition and thickness computed using Eq. (11). The plot shows 86%
of Worman's data points and 68 % ofChiew's data points fall within the error band of
. '\
ISH JOURNAL OF HYDR,AULIC ENGlNEERING, VOL. 16,2010, NO. 1_
':.'
-
'-"'.-
,:.: :.:
.:: .
;;.....