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 Grade Stabilization
12.3.3. STRUCTURES WITH PRE-FORMED SCOUR HOLES
A significant feature that distinguishes the sloping riprap structure of Figure 12.7 from
the other structures discussed in Sections 5.1 and 5.2 is the preformed, rock protected scour
hole. A scour hole is a natural occurrence downstream of any drop whether it is a natural
overfall or a man-made structure. As mentioned in Section 5.1 a rock grade control
structure must have sufficient launching rock to protect against the vertical scour immediately
below the weir section. However, the lateral extent of the scour hole must also
be considered to ensure that it does not become so large that the structure is subject to being
flanked. With many simple grade control structures in small stream applications, very little,
if any attention is given to the design of a stilling basin or pre-formed scour hole, but rather,
the erosion is allowed to form the scour hole. However, at higher flow and drop situations,
a pre-formed scour hole protected with concrete, riprap, or some other erosion resistant
materials is usually warranted. This scour hole serves as a stilling basin for dissipating the
energy of the plunging flow. Sizing of the scour hole is a critical element in the design
process which is usually based on model studies or on experience with similar structures in
the area.
The stability of rock structures is often jeopardized at low tailwater conditions due
to the stability of the rock, which is often the limiting factor in determining the maximum drop
height of the structure. One way to ensure the stability of the rock is to design the structure
to operate in a submerged condition. This is the basis for design of the bed stabilizer shown
in Figure 12.8 (U.S. Army Corps of Engineers, 1970). These structures generally perform
satisfactorily as long as they are designed to operate at submerged conditions where the
tailwater (T') does not fall below 0.8 of the critical depth (Dc) at the crest section (Linder,
1963). Subsequent monitoring of the in place structures confirmed their successful
performance in the field (U.S. Army Corps of Engineers, 1981).
In many instances, the energy dissipation in a grade control structure is accomplished
by the plunging action of the flow into the riprap protected stilling basin. This is generally
satisfactory where the degree of submergence is relatively high due to small drop heights
and/or high tailwater conditions. However, at lower submergence conditions where drop
heights are large or tailwater is low, some additional means of dissipating the energy must be
provided. Little and Murphey (1982) observed that an undular hydraulic jump occurs when
the incoming Froude number is less than 1.7. Consequently, Little and Murphey developed
a grade control design that included an energy dissipating baffle to break up these undular
waves (Figure 12.9). This structure which is referred to as the ARS type low-drop structure
has been used successfully in North Mississippi for drop heights up to about six feet by both
the U.S. Army Corps of Engineers and the Soil Conservation Service (U.S. Army Corps of
Engineers, 1981). A recent modification to the ARS structure was developed following model
studies at Colorado State University (Johns et al., 1993; and Abt et al., 1994). The modified
ARS structure, presented Figure 12.10 retains the baffle plate but adopts a vertical drop at
the sheet pile rather than a sloping rock-fill section.
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