Grade Stabilization
12.3.4 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 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 12.11, (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 12.12. 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.
12.3.5 CHANNEL LININGS
Grade control can also be accomplished by lining the channel bed with a non-erodible
material. These structures are designed to ensure that the drop is accomplished over a
specified reach of the channel which has been lined with riprap or some other non-erodible
material. Rock riprap gradient control structures have been used by the U.S. Soil
Conservation Service for several years (U.S. Soil Conservation Service, 1976). These
structures are designed to flow in the subcritical regime with a constant specific energy at the
design discharge which is equal to the specific energy of flow immediately upstream of the
structure (Myers, 1982). Although these structures have generally been successful, there have
been some associated local scour problems. This precipitated a series of model studies at the
WES to correct these problems and to develop a design methodology for these structures
(Tate, 1988; and Tate, 1991). A plan and profile drawing of the improved structure is shown
in Figure 12.13.
12.3.6 ALTERNATIVE CONSTRUCTION MATERIALS
While riprap and concrete may be the most commonly used construction materials for
grade control structures, many situations where cost or availability of materials may prompt
the engineer to consider other alternatives. Gabion grade control structures are often
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