Selection and Design of Channel Rehabilitation Methods
Construction of bed sills is sometimes accomplished by simply placing the rock along the streambed
to act as a hard point to resist the erosive forces of the degradational zone. In other situations, a trench may
be excavated across the streambed and then filled with rock. A critical component in the design of these
structures is ensuring that there is sufficient volume of non-erodible material to resist the general bed
degradation, as well as the local scour at the structure. This is illustrated in Figures 6.2a and 6.2b which
shows a riprap grade control structure designed to resist both the general bed degradation of the
approaching knickpoint as well as any local scour that may be generated at the structure. In this instance,
the riprap section must have sufficient mass to launch with an acceptable thickness to the anticipated scour
hole depth.
6.2.1.2 Structures with Water Cutoff
One problem often encountered with the above structures is the displacement of rock (or rubble,
etc.) due to the seepage flow around and beneath the structure. This is particularly a problem when the
bed of the channel is composed primarily of pervious material. This problem can be eliminated by
constructing a water barrier at the structure. One type of water barrier consists of simply placing a trench
of impervious clay fill upstream of the weir crest. This type of water barrier is illustrated in Figures 6.3a
and 6.3b. One problem with this type of barrier is its longevity due to susceptibility to erosion. This
problem can be avoided by using concrete or sheet piling for the cutoff wall. The conceptual design of a
riprap grade control structure with a sheet pile cutoff wall is shown in Figures 6.4a and 6.4b. In the case
of the sloping riprap drop structures used by the Denver Urban Drainage and Flood Control District, an
impervious clay fill is used in conjunction with a lateral cutoff wall (McLaughlin Water Engineers, Ltd.,
1986). This design is illustrated in Figure 6.5.
6.2.1.3 Structures with Pre-formed Scour Holes
A significant feature that distinguishes the sloping riprap structure of Figure 6.5 from the other
structures 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. A rock grade control structure must
have sufficient launching rock to protect against the vertical scour immediately downstream of 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
208
Previous Page
