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Design Criteria for Low Drop Grade Control Structures
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/ARSUserFiles/60600505/images/NSL Front.jpg Experiments at the NSL

Rapid migration of head-cuts through streams in northern Mississippi necessitated the development of an economical grade stabilization structure to halt the channel bed incision occurring in these tributaries. For reasons of cost and site-dewatering problems, reinforced concrete structures with Saint Anthony Falls type stilling basins were not suitable for many sites. Simple rock sills had little success and, more often than not, failed due to undercutting and large scour hole development.

Model studies were conducted at the National Sedimentation Laboratory on ways to modify these simple structures to avoid the aforementioned problems and still develop a design for an economical grade control structure. Early models were run for specific sites and a pattern of general behavior emerged. In situations where a relatively low drop in the streambed elevation, H, was less than the critical flow depth, Yc, for a given design discharge, Q, the tests produced an undular hydraulic jump downstream of the structures.

A higher relative drop did not produce this type jump but caused more damage to the streambed on site. A "low drop" was therefore defined for the purpose of this study as one with a relative drop height of H/Yc<=1; conversely, a "high drop" was defined as one where H/Yc>1. Modeling efforts were then concentrated on the "low drops" which could be made of rip-rap rather than reinforced concrete.

A steel sheet-pile cutoff at the upstream invert elevation, along with filter cloth beneath the rip-rap solved the undercutting problem. A preformed rock-lined scour hole below the sheet pile solved the on-site damage, and a baffle plate or pier in the middle of the scour hole changed the undular jump to a full hydraulic jump. Design criteria were then determined in the form of dimensionless equations to proportion and hydraulically design a stilling basin (scour hole) and the energy dissipator for these structures. The resulting structure design dissipates 80% of the energy gained in the drop in bed elevation at the stilling basin site. The structure design has been widely accepted and used in the DEC watersheds and other sites around the United States.

Point of Contact:

Dr. Carlos V. Alonso, Retired