Selection and Design of Channel Rehabilitation Methods
18.104.22.168 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 1991). A plan and
profile drawing of the improved structure is shown in Figure 6.11.
22.214.171.124 Alternative Construction Materials
While riprap and concrete may be the most commonly used construction materials for grade control
structures, there are many situations where cost or availability of materials may prompt the engineer to
consider other alternatives. Gabion grade control structures are often an effective alternative to the standard
riprap or concrete structures (Hanson et al., 1986). Guidance for the construction of gabion weirs is also
provided by the USACE (1974).
Another alternative to the conventional riprap or concrete structure which has gained popularity
in the southwestern U.S. is the use of soil cement grade control structures. These structures are constructed
of on site soil-sand in a mix with Portland Cement to form a high quality, erosion resistant mixture. Soil
cement grade control structures are most applicable when used as a series of small drops in lieu of a single
large-drop structure. Experience has indicated that a limiting drop height for these structures is on the order
of three feet. Design criteria for these structures is presented by Simons, Li, & Associates, Inc. (1982).
EFFECTIVENESS OF GRADE CONTROL STRUCTURES
Design considerations for improving the effectiveness of grade control structures include
determination of the type, location and spacing of structures along the stream, along with the elevation and
dimensions of structures. Siting grade control structures is often considered a simple optimization of
hydraulics and economics. However, hydraulics and economics alone are usually not sufficient to define
the optimum spacing for grade control structures. In practice, the hydraulic considerations must be
integrated with a host of other factors, which vary from site to site, to determine the final structure plan.
Each of these factors should be considered in determining the effectiveness of the structures.
One of the most important steps in the siting of a grade control structure or a series of structures
is the determination of the anticipated drop at the structure. This requires some knowledge