12.3.1 SIMPLE BED CONTROL STRUCTURES
Perhaps the simplest form of a grade control structure consists of dumping rock,
concrete rubble, or some other locally available non-erodible material across the channel to
form a hard point. These structures are often referred to as rock sills, or bed sills. These type
of structures are generally most effective in small stream applications and where the drop
heights are generally less than about 2 to 3 feet. A series of rock sills, each creating a head
loss of about two feet was used successfully on the Gering Drain in Nebraska (Stufft, 1965).
The design concept presented by Whitaker and Jaggi (1986) for stabilizing the streambed with
a series of rock sills is shown in Figure 12.3. The sills in Figure 12.3 are classic bed control
structures which are simply acting as hard points to resist the erosion of the streambed.
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 12.4a and 12.4b 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.
12.3.2 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 12.5a and 12.5b. 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 12.6a and 12.6b. 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 12.7.