Riprap Design for Pipe Spillway Outlets 1.0 = TW/Do = 0.7

This study was conducted to develop criteria to determine the geometry of riprap-lined plunge pools downstream of pipe spillways with tailwater elevations in the range of 1.0 pipe diameter below to 0.7 pipe diameter above the outlet invert elevation. Dimensionless relationships were developed to determine the dimensions of plunge pool and riprap size required for stability using a laboratory-scaled physical model with the following test conditions: pipe diameter (Do; 0.0628 and 0.0884 m); tailwater elevations (TW/Do; 0.7, 0.5, 0.25, 0.0, and 1.0); riprap sizes (0.127 = D50/Do = 0.707); downstream channel bed elevations (Z/Do; 0.0, 0.5, and 1.0); and flows varying from 0.0 to 11.5 Q2/gDo5. Example design problems are presented to illustrate use of the recommended design procedure. The design equations will result in stable riprap-lined plunge pools downstream of pipe spillways at 1.0 = TW/Do = 0.7 over a wide range of flow conditions.