HGL Elevation at Pipe Exit of USBR Type VI Impact Basin

Abstract

Model tests on a Soil Conservation Service principal spillway with a U.S. Bureau of Reclamation Type VI impact basin for the outlet structure provided an opportunity to establish a relationship to predict the hydraulic grade line (HGL) elevation at the pipe exit as a function of the tailwater (TW) elevation. The relationship is \IHGL\N/\ID\N = 0.85 + 0.76(\ITW\N/\ID\N); \ITW\N/\ID\N > 0.65 or \IHGL\N/\ID\N > = 1.31, where \IHGL\N is the HGL elevation relative to the pipe-exit invert; \ID\N is the pipe diameter; and \ITW\N is the tailwater elevation downstream of the basin relative to the pipe-exit invert. This relationship permits accurate determination of the head for flow computations. The equation is only valid at design discharge for impact basins designed according to \ITechnical Release No. 49,\N U.S. Dept. of Agriculture, Soil Conservation Service. Because the results showed that a \ITW\N/\ID\N = 0.65 had no adverse effect on the HGL elevation, a minimum \ITW\N/\ID\N = 0.65 is recommended for design because the tailwater reduces water surface roughness and bed erosion.