Development of New Stilling Basin Models for Non-Circular Pipe Outlets

Pipe outlet flow under a high head of water emerges as a straight, concentrated jet. Hydraulic structures known as stilling basins are essential for reducing the energy of high-velocity flow to prevent erosion of the natural bed downstream and riverbanks. These transition structures dissipate the excess energy of high-velocity flow at the downstream face of the conduit outlet, ensuring the stability of the downstream channel bed and banks. This paper reports on experimental investigations leading to the development of new stilling basin designs for non-circular pipe outlets using various appurtenances. The USBR VI stilling basin model was studied and modified to enhance its effectiveness and cost-efficiency. The new models were tested with a rectangular pipe outlet measuring 10.8 cm by 6.3 cm, across Froude numbers of 1.85, 2.85, and 3.85, in comparison to the USBR VI model. Innovations include the insertion of an inverted T-shaped splitter block while maintaining other basin geometry configurations. Scour patterns were measured for each test, and performance was evaluated using a performance index (PI). After twelve test runs, results indicate that, for a given Froude number range, inserting the splitter block with consistent impact wall and end sill improves stilling basin performance and reduces its length by 29% compared to the USBR VI model for similar flow conditions.