Riprap Design Downstream of Submerged Pipe Outlets

Abstract

Submerged pipe spillway outlet flow conditions can damage the downstream end of the spillway and the downstream channel. This study was conducted to determined the size and horizontal placement of riprap to stabilize the downstream channel geometry below pipe spillways with submerged flow conditions. Dimensionless relationships were developed to estimate the riprap size and its design size variation with distance from the outlet in the downstream channel using a laboratory scaled physical model with the following test conditions: pipe diameter (Do; 0.0628 and 0.0884 m); three submergence levels (TW/Do; 0.7, 1.0, 2.0, and 3.0); five plunge depths (Z/Do; –2.5, –1.75, –1.0, –0.5, and 0.0); and flows varying from 0 to 7.2 Q2/gDo5. The resulting equations were tested and validated with additional laboratory studies with the following conditions: Do of 0.0884 and 0.0628 m; W/Do of 4.0, 8.0, and 11.3; Q2/gDo5 of 1.04, 1.13, 1.64, 1.99, 2.21, 5.37, and 3.18; TW/Do of 0.8, 1.0, 1.5, and 2.0; and Z/Do of –2.5, –1.75, –1.0, –0.5, and 0.0 for selected cases. These validation tests resulted in minor changes in the design equations over the range of the variables. An example design problem is presented to illustrate the use of the recommended design method. The design equations and method will result in stable riprap without scour for submerged pipe outlet flow conditions over a wide range of flow conditions.