This paper describes the development of the generalized design rules for a new stilling basin for use with the straight drop spillway. This generalized stilling basin design was developed because experience in the field had shown that there was no satisfactory stilling basin for the straight drop spillway. However, limited field experience indicates that this new design will adequately protect the downstream channel from scour.
Water falling over the spillway crest falls onto a flat apron. The nappe is broken up by floor blocks, which also prevent damaging scour of the downstream channel banks. Scour of the downstream channel bed is prevented by an end still. Flaring wingwalls, triangular in elevation, prevent erosion of the dam fill. For proper operation of the stilling basin, the contraction of the flow at the ends of the spillway opening must be partially suppressed.
The stilling basin can be used for a wide range of discharge, head on the crest, crest length, height of drop, and downstream tailwater level.
An important finding is that the stilling basin length computed for the minimum tailwater levels. Dangerous scour of the downstream channel may occur if the nappe is supported sufficiently by high tailwater so that it lands beyond the end of the stilling basin. A method of computing the stilling basin length for all tailwater levels is presented.
The design rules developed as a result of the laboratory tests were carefully checked an verified. An example shows how these rules are applied to the design of field structure.
United States Department of Agriculture; Agricultural Research Service; Soil and Water Conservation Research Branch; Minnesota Agricultural Experiment Station
Donnelly, Charles A.; Blaisdell, Fred W..
Straight Drop Spillway Stilling Basin.
St. Anthony Falls Hydraulic Laboratory.
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