Hood Inlet for Closed Conduit Spillways: Fid-Bau Portal

Hood Inlet for Closed Conduit Spillways

Blaisdell, Fred W.

The hood inlet for closed conduit spillways is formed by cutting a circular pipe at an angle and placing the pipe so that the crown projects beyond the invert. This forms a hood over the pipe entrance. The use of the hood inlet causes the pipe to fill with little or no submergence of the inlet crown. This filling occurs even though the pipe is laid on a slope that is hydraulically steep. Smooth pipes having slopes greater than about 1 ;5%, and corrugated pipes having slopes greater than about 8% are hydraulically steep. Therefore, many highway culverts and other drainage structures fall into this steep category and will not fill unless the entrance receives special attention. The capacity and performance of the spillway for variations of the hood inlet length, the conduit slope, the wall thickness, and the approach conditions are described. The great effect of vortices on the spillway capacity is shown and anti-vortex devices are developed. Scour in the vicinity of the hood inlet is determined for various sizes of stone and equations for the scour-hole dimensions are presented.