Hydraulic Design of Drop Structures

Mohamed Abdel-Latif and Heather M. Ivory


The primary function of a drop structure is to carry the flow from surface sewer to deeper sewers, usually tunnels. As the flow falls, air intermixes resulting in a complex hydraulic two-phase flow, which creates many design issues. Proper hydraulic design of a drop structure should satisfy dissipation of energy created by transfer of potential energy to kinematics energy and minimize the amount of air entrained by falling flow that is transported to the main tunnel.

Vortex generating inlets have been developed with the objective of minimizing air entrainment and dissipate energy from the falling flow. The most common type of vortex generating inlet is the tangential inlet. Tangential inlet design is based on the research and physical models conducted in the early 80s. The physical models called for an optimized inlet configuration to ensure generation of flow vortex, being: slope of the bottom of inlet channel @ angle 27.5 ° and contraction of the inlet channel width at an angle of 16.8 °.

In this presentation the EPA:SWMM5 Model is utilized to examine the sensitivity of the inlet’s configuration on flow velocity and flow direction. Flow velocity and flow direction are the key parameters for vortex generation. The SWMM model was also utilized to examine the impact of flow variation from dry weather flow to wet weather flow on efficiency of vortex generation. Very small time step (0.001 second) was utilized in the SWMM model to account for inlet structure scale.

Permanent link: