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Design Considerations for Siting Grade Control Structures

PURPOSE: The purpose of this Coastal and Hydraulics Engineering Technical Note (CHETN) is to provide guidance and highlight possible areas of concern that may require consideration before siting grade control structures. INTRODUCTION: In the widest sense, the term grade control can be applied to any alteration in the watershed which provides stability to the streambed. By far the most common method of establishing grade control is the construction of in-channel grade control structures. There are two basic types of grade control structures. One type can be referred to as a bed control structure as it is designed to provide a hard point in the streambed that is capable of resisting the erosive forces of the degradational zone. The second type can be referred to as a hydraulic control structure as it is designed to function by reducing the energy slope along the degradational zone to the point that the stream is no longer capable of scouring the bed. The distinction between the operating processes of these two types is important whenever grade control structures are considered. Design considerations for siting 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, these factors alone are usually not sufficient to define the optimum siting conditions for grade control structures. In practice, hydraulic considerations must be integrated with a host of other factors, which vary from site to site, to determine the final structure plan. Some of the more important factors to be considered when siting grade control structures are discussed in the following paragraphs. HYDRAULIC CONSIDERATIONS: 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 of the ultimate channel morphology, both upstream and downstream of the structure, which involves assessment of sediment transport and channel morphologic processes. The hydraulic siting of grade control structures is a critical element of the design process, particularly when a series of structures is planned. The design of each structure is based on the anticipated tailwater or downstream bed elevation which, in turn, is a function of the next structure downstream. Heede and Mulich (1973) suggested that the optimum spacing of structures is such that the upstream structure does not interfere with the deposition zone of the next downstream structure. Mussetter (1982) showed that the optimum spacing should be the length of the deposition above the structure, which is a function of the deposition slope (Figure 1). Figure 1 also illustrates the recommendations of Johnson and Minaker (1944) that the most desirable spacing can be determined by extending a line from the top of the first