Calculating hydraulic grade line on storm drainage with two outfalls

[Bill R]

Hello, we are performing some analysis on a retrofit situation where additional flow is being added to an existing drainage system via some additional inlets and pipes to capture previously overland flow. Currently the network has a single outfall to a wet detention lake and our results are showing the hydraulic grade line exceeding the inlets, so no good. Since we cannot resize the pipes in the ground, another option we have is to connect the system to another lake. Our problem is that the software we currently use (ASAD by Hiteshew) does not support the second outfall. I downloaded a trial of Stormwater Studio by Hydrology Studio and that does not appear to either. Designing with two outfalls did not appear as a consideration in HEC-22, but I did only skim that section and have not done that analysis by hand before.

My thoughts are that the two outfalls may require the system to be run in multiple stages. Assume a Y-shape for the system, with the two upper branches of the Y leading to two outfalls and that there is an inlet accepting surface water at each junction. Each separate branch should be analyzed alone to the intersection of the two, then the HGL of each can be compared. I believe the lower of the two would control as it would represent the path of less resistance. Then a third analysis should be performed with that junction as the outfall and using the HGL of the lower branch as the "outfall". If the system had no branches and was a single run of pipes with an outfall at each end in a different lake (assume the lakes are linked via a separate and much larger pipe system to equalize them), again two systems should be created each with one outfall to generate that HGL. This time each analysis would have the entire network except for the opposite outfall pipe. Then, the intersection of the two HGLs would be the effective point to split the systems.

Hopefully I am on the right track with this design but am open to any insight that can be provided. Thank you!

 

[gbam]

So the hydraulics in a storm drain with a diversion will be similar to that in HECRAS split flow analyses; based on energy/momentum. I think you have a good plan; however, I would use EGL rather than HGL to account for velocities and the EGL should match upstream of the diversion. The flow rate at the diversion will have to be manually adjusted to account for tailwater considerations. Have you looked at EPA SWMM? I am not sure; but, SWMM may accommodate the diversion. If not, spreadsheet will work. I too have never had to model a diversion of a system under pressure. Note that if the systems are supercritical downstream the diversion may be straight forward and not influenced by tailwater. Hope that helps and I understood your approach.

 

[Bill R]

gbam: Good point about going with the EGL. I'm not sure if ASAD utilizes it or always goes with HGL, will have to explore that. I have actually downloaded SWMM recently but have not gotten around to installing and working with it. I like to have as much experience with different programs as possible so I can understand their capabilities. Whether or not our company will license multiple paid programs is another matter, so free ones are nice to have.

Currently we are simply splitting the system at the junction (which is possible in our retrofit situation to actually do) and the results have passed. The perspective my supervisor has is that the additional connection will be a further benefit. I'm taking this opportunity to understand everything conceptually to improve my design skills and to back up his idea or else be ready to prove it computationally. Thanks for your insight!