Detention pond Modelling with Pond outlet Structure.

Question

Good Evening. 
 I am modelling a Detention pond with POS and the downstream network leading to outfall. During the EPS, i am experiencing the HGL downstream of POS is fluctuating above the Finished grade and Pond elevation while the HGL in pond is below the pond top. I am not able to understand what is that driving the HGL far above the FGL in gravity system. 
 Attached is model file and network file. 
 Appreciate feeedback comments and suggestion. 
 Sudhakar

DP_20HA_02A_NEWVERSION.stsw

Sushma Choure · Answer

Hello Sudhakar, 
 If you look at the catchment graph of say catchment CA-1, you will observe that flow after 12.00hrs is indeed peak flow originated from the storm data, which just passes through the pond into the downstream system. 
 
 The reason why HGL rises above the manhole HGL at peak hours is because depth / rise ratio is already 100 % in those pipes resulting in flooding and HGL rising above rim elevation. as seen in profiles.

Jesse Dringoli · Answer

Sudhakar, 
 The "capacity" figure for a pipe is a reference value that is technically only valid in normal depth conditions as explained here , but in the model, pipes are often not flowing at normal depth and can be influenced by more complex effects like tailwater/backwater. 
 For the issue with the pond HGL compared to the HGL in the downstream outlet pipe - the reason is because the pond outlet structures were not configured correctly. The tailwater range was set to Free Outfall, which means it was not considering downstream tailwater effects. Change it to "interconnected ponds", and select "automatic" for the "downstream element ID". 
 
 I have updated this article to mention this tip. 
 Note that you may experience some unstable results when using interconnected ponds with the Implicit solver. Consider switching to the Explicit solver or adjust the advanced calculation options per this article .