Geoscience Reference
In-Depth Information
Experimental Study on Gabion Stepped Spillway
Mahmood Shafai-Bejestan and Gh. Kazemi-Nasaban
1
Introduction
Rocks in their natural form are the most abundant and economical material in
hydraulic engineering practice. Rocks have been used in dam construction, river
engineering works, river intakes, etc. The size of rock in these practices depends on
the hydraulic conditions, such as flow velocity, shear stress, hydrostatic, and
hydrodynamic pressures. When these conditions are high, the required size of
stone will be large and impractical. In such a case, the alternative is to tie the stones
together by some means. Gabions are hexagonal mesh boxes filled with small-size
stones. The advantages of gabions are (a) their stability, (b) low cost, (c) flexibility,
and (d) porosity (Chinnarasri
2003
). The gabions retain the advantages of rock fill
in the flexibility and permeability, so that water pressure is minimized. The wire
mesh container can be used for this purpose. Nowadays, the gabion is used for
hydraulic engineering works such as revetments, channel linings, weirs, groins, and
energy dissipation structures (Stephenson
1979a
,
b
). One application of gabion is
for constructing the stepped spillways. In this structure, the kinetic energy of flow is
dissipated, as water flows downstream. The flow through the rock mixes with the
flow over the crest, resulting in energy dissipation by jet impingement as well as
due to friction loss through the rock fill. In addition, the energy is dissipated as flow
cascades from one step to another. Three types of flow occur in gabion stepped
spillway: (a) “napped flow,” when flow cascades from one step to another; (b)
“skimming flow,” when the water fully flows through the steps; and (c) “pooled
flow,” when a step is provided at the end of each step (Sorenson
1985
). Many
empirical equations have been developed to distinguish three types of flow. Table
1
shows some of these formulas. In these equations,
y
c
is the critical depth, approxi-
mately equal to the flow depth at the crest or can be calculated as
y
c
¼
p
.
Symbols
h
and
l
are the height and length of each step, respectively. Figure
1
shows
the definition of variables.
q
2
3
g
