Geoscience Reference
In-Depth Information
Table 2 Values of
coefficients of
a
and
b
in (
1
)
Slope (
h
: '
)
a
b
1:1
0.238
0.526
2:1
0.169
0.654
3:1
0.208
0.647
3.5:1
1.736
0.279
when a stilling basin has not been provided at the downstream. The extent of such
scour hole may result in the instability of the spillway or even its failure. Predicting
the scour hole depth can help the engineers to design the spillway more safe.
Review of literature reveals that no such study, to the knowledge of the authors,
has been conducted. Therefore, it is the purpose of this study to conduct experi-
mental tests and to develop a relationship for predicting the maximum scour hole
depth, downstream of the gabion stepped spillways.
2 Theoretical Consideration
Consider a scour hole downstream of stepped spillway; for an equilibrium scour
hole, the particles on the bed are at incipient motion. Forces exerted on the parti-
cles are the drag and lift forces and the submerged weight of particles. From
the stability analysis of a particle at threshold condition one can obtain (Shafai-
Bajestan et al.
1995
):
d
s
D
50
¼
y
1
D
50
f
SN,
(3)
where
d
s
is the maximum scour hole depth,
D
50
is the median particle size of
sediment, and SN is the stability number defined as follows:
V
1
g
SN
¼
p
(4)
ð
G
s
Þ
D
50
1
In these equations,
y
1
and
V
1
are the flow depth and flow velocity just down-
stream of the spillway,
G
s
is the particle specific gravity and
g
is the acceleration of
gravity.
3 Experimental Setup
Equation
3
is a general relation for predicting the scour hole depth. To determine
the coefficients of this equation, a series of experimental tests have to be conducted.
The experimental tests were conducted in a flume 50-cm wide, 8-m long and 1.5-m
