Selection and Design of Channel Rehabilitation Methods
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 6.6 (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 6.7). This structure, 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; Abt et al., 1994). The modified ARS structure, presented Figure 6.8 retains the baffle plate but
adopts a vertical drop at the sheet pile rather than a sloping rock-fill section.
126.96.36.199 Concrete Drop Structures
In many situations where the discharges and/or drop heights are large, grade control structures are
normally constructed of concrete. There are many different designs for concrete grade control structures.
The two discussed herein are the California Institute of Technology (CIT) and the St. Anthony Falls (SAF)
structures. Both of these structures were utilized on the Gering Drain project in Nebraska, where the
decision to use one or the other was based on the flow and channel conditions (Stufft, 1965). Where the
discharges were large and the channel depth was relatively shallow, the CIT type of drop structure was
utilized. The CIT structure is generally applicable to low-drop situations where the ratio of the drop height
to critical depth is less than one; however, for the Gering Drain project this ratio was extended up to 1.2.
The original design of this structure was based on criteria developed by Vanoni and Pollack (1959). The
structure was then modified by model studies at the WES in Vicksburg, Mississippi, and is shown
in Figure 6.9, (Murphy, 1967). Where the channel was relatively deep and the discharges smaller, the
SAF drop structure was used. This design was developed from model studies at the SAF Hydraulic
Laboratory for the U.S. Soil Conservation Service (Blaisdell, 1948). This structure is shown in Figure
6.10. The SAF structure is capable of functioning in flow situations where the drop height to critical depth
ratio is greater than one and can provide effective energy dissipation within a Froude number range of 1.7
to 17. Both the CIT and the SAF drop structures have performed satisfactorily on the Gering Drain for
over 25 years.