I would like to show a very simple yet commonly mistaken procedure committed by many SSA users with tail water condition at the outfall of the system. Below is a screenshot of a profile plot with fixed water elevation at the outfall, however the profile plot shows a different HGL where one would expect the HGL to match the fixed water elevation i.e. 980 ft. for this model.
Figure 1. Profile Plot for tail water at the outfall @ 980 ft. using Kinematic wave routing.
The reason for the above profile plot reporting a Max HGL at outfall as 977.40 ft. is due to Kinematic wave routing used for the analysis which provides a non-linear reservoir formulation for channels and pipes, including translation and attenuation effects that assume the water surface is parallel to the invert slope. It does not account for backwater effects or flow reversals.
The key to analyzing any tail water, flow reversal or looped network is to analyze with Hydrodynamic routing method. It solves the complete St. Venant equations throughout the drainage network and includes modeling of backwater effects, flow reversal, surcharging, looped connections, pressure flow, tidal outfalls, and interconnected ponds.
Figure 2. Profile Plot for tail water at the outfall @ 980 ft. using Hydrodynamic routing.
I hope this helps. Stay tunned for more tips to come.