RIPRAP DESIGN DOWNSTREAM OF SUBMERGED PIPE OUTLETS

Submerged pipe spillway outlet flow conditions can damage the downstre am end of the spillway and the downstream channel. This study was cond ucted to determined the size and horizontal placement of riprap to sta bilize the downstream channel geometry below pipe spillways with subme rged flow conditions. Dimensionless relationships were developed to es timate the riprap size and its design size variation with distance fro m the outlet in the downstream channel using a laboratory scaled physi cal model with the following test conditions: pipe diameter (D(o); 0.0 628 and 0.0884 m); three submergence levels (TW/D(o); 0.7, 1.0, 2.0, a nd 3.0); five plunge depths (Z/D(o); -2.5, -1.75, -1.0, -0.5, and 0.0) ; and flows varying from 0 to 7.2 Q2/gD(o)5. The resulting equations w ere tested and validated with additional laboratory studies with the f ollowing conditions: D(o) of 0.0884 and 0.0628 m; W/D(o) of 4.0, 8.0, and 11.3; Q2/gD(o)5 of 1.04, 1.13, 1.64, 1.99 2.21, 5.37, and 3.18; TW /D(o) of 0.8, 1.0, 1.5, and 2.0; and Z/D(o) 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 ex ample design problem is presented to illustrate the use of the recomme nded design method. The design equations and method will result in sta ble riprap without scour for submerged pipe outlet flow conditions ove r a wide range of flow conditions.