Open Channel

Question

Dear All, 
 
 Attached is the profile of a channel section with conduit in between. As it is seen that at the upstream of the conduit there is a steep rise in the hydraulic profile. Kindly explain the consequences and the measures which can be taken to modify it.

Scott Kampa · Answer

Hello, 
 The following link has some information that may be useful: Hydraulic grade discontinuity at steep pipes or across nodes (Frontwater analysis) . In cases of steep slopes, the program will switch to a frontwater analysis, which can cause the sort of discontinuity you are seeing here. 
 If that isn't useful, a copy of the model files will be necessary. 
 Regards, 
 Scott

Jesse Dringoli · Answer

HSN, 
 Based on the screenshot, I assume you are using the GVF-Rational or GVF-Convex solver. With these solvers, overflow is not supported, so when the hydraulic grade is above the rim of an unbolted manhole during the backwater analysis, it resets the HGL to the rim elevation and proceeds in the upstream directly. 
 The profile screenshot shows that the conduits are likely undersized, causing surcharging. As you continue upstream, the HGL continues to rise, and when it encounters what is probably a manhole at station ~85 m, since the HGL is above the rim, it gets reset down to the rim elevation, which is why you see the sudden jump (it is actually a drop, when moving in a downstream to upstream direction). This is explained in more detail in the below article from our wiki. 
 Why is the HGL reset to rim elevation for flooded structures? 
 If you need to account for the impact of overflow, you will need to use the Explicit (SWMM) or Implicit dynamic solvers in SewerGEMS/CivilStorm. If you want to design the system so as not to flood, you may need to increase the size of those pipes.