SSA Incorrectly Calculating External Inflow using Hydrodynamic Analysis Method

[qchalmersKACEH]

I am modeling an overflow discharge network from a large detention pond. The peak 100 year flow from the pond is 7.45 CFS, which I have added as an external inflow to a junction leaving the pond. This is just a test so I do not need to model the actual discharge structure from the pond.

When I run the model using Hydrodynamic method the peak flow in the pipe immediately downstream of this junction is much higher, peaking at 10.06 CFS during the first few minutes of the analysis.

This is clearly incorrect as the only flow entering this pipe is the constant external inflow of 7.45 CFS. This can be confirmed by creating a time series of the total inflow to the junction.

If I change the analysis method to Kinematic Wave or Steady State the flow in the pipe is calculated correctly as a constant 7.45 CFS:

Why does using Hydrodynamic method cause such an error in the maximum flow in the pipe?

 

[matt_anderson_pe]

Without knowing anything about your system, the rapid rise, and fall of the flow is simply water being stored in the structure, adding head in the structure as flow beginning to fill the downstream pipe in the first few minutes of the simulation until it reaches a uniform flow.

[bforr]

I had the same situation and Matt's answer is the same i got from a different responder.

[qchalmersKACEH]

Hi Matthew, thank you for the response.

The rise of flow is shown in the pipe directly downstream of the junction structure (link 113 on my figure in my original post). The volume in the junction structure (node 111) does not change over the model run and remains at zero, indicating that there is no storage occurring in the structure.

 Would you have time to take a look at the model itself? I can upload it here as a zip file.

[qchalmersKACEH · Accepted solution]

I found the issue. This model was originally set up by an EIT who set the initial water surface elevation of the outlet junction structure (111) to the rim to try to represent the overflow occurring from the pond into the structure. This is incorrect, as it implies that the structure is full at the start of the model run (it is not). Resetting the initial water surface elevation to the invert of the outlet pipe (113) corrects the model and the maximum flow in pipe 113 now correctly shows 7.45 cfs.

[matt_anderson_pe]

Ok - you had additional stored water in the manhole + the steady flow.   This is why you had extra peak flow in the downstream pipe.