Model Study for Non-Standard Principal Spillway

Abstract

Model tests were conducted on a non-standard principal spillway to evaluate the performance of the proposed drop inlet and impact basin and to evaluate the riprap size and placement downstream of the impact basin. An undistorted 1:24 model scale was used. The prototype pipe was 2.13 m (84 in.) in diameter. The outlet structure was a USBR Type VI impact basin. The design discharge was 43.9 m/s (1550 cfs). The spillway was a 1 3, two-stage, covered drop inlet with baffle trash racks and four low-stage orifices that occupied half the length of each side. The proposed design performed poorly with flow in the orifices. The trash rack was modified, performed very well, and was recommended for design. With the drop inlet in a berm [with the berm 0.610 m (2.0 ft) below the orifice invert elevations] the entrance loss coefficients were 0.28 and 0.26, respectively, with and without orifices. Locating the drop inlet in a berm will require riprap around the drop inlet entrance to prevent erosion of the berm. The outlet impact basin performed very well. There was minor scour over a considerable area downstream of the basin, but the riprap design is adequate.