Hi,
I am working on a SewerGEMS model and am having trouble understanding how the Hydraulic Grade Line within catchments is calculated. I will include screenshots of the model below. When I run the model and pass a storm event through the storm pipe network, there seems to be less surcharging in the manhole than I would expect. The furthest downstream pipe has a full flow capacity of 67.59cfs and the max flow in the pipe is 106.51cfs. When I do just a quick "sanity check" on this pipe flow I treat it as a quick culvert in Culvertmaster and look at the head. When I run the pipe as a culvert at 106.51cfs the head built up on the pipe is roughly 2 feet. However, when you look at the SewerGEMS profile for this pipe run, the surcharge into the structure is not nearly 2 feet. Maybe my way of performing a sanity check is incorrect, but this is a pretty large discrepancy. Is there something going on in the implicit solver that I do not understand? Any help on this problem will be greatly appreciated! Thanks!
Hi Joshua,
The difference you see may be due to the difference between normal depth / steady state conditions and gradually varied flow / unsteady conditions.
If you are using the Implicit or Explicit (SWMM) solver, or the GVF-Convex (SewerCAD) solver set to EPS, the simulation is considering the element of time, where there is not a single steady flow but rather a hydrograph. Typically you would use a runoff method such as the Unit Hydrograph Method to convert storm event rainfall to catchment runoff, where the peak of the runoff hydrograph will occur for a brief moment during the simulation. So, that peak of 106 cfs that you see may only occur for a short period of time, with smaller flow before and after it. Because of this and storage effects in the pipe network, the peak flow may not be occurring for long enough to produce additional surcharging.
Next, CulvertMaster and the "full capacity" figure seen in the pipe properties are based on normal depth, steady flow conditions, whereas the Implicit, Explicit or EPS GVF-Convex Solver simulate a gradually varied flow. If the pipe is not long enough, it may not have reached normal depth conditions. If you were to extend the pipe longer (and if the 106 cfs held steady for some time), you may notice that the pipe upstream side would eventually become surcharged (but you do not see this in profile view in the model if the pipe is not long enough for that condition to be reached and your downstream outfall is probably set as free outfall). This is explained in more detail in the following article: Why does the profile for the system appear to be at odds with the results for the capacity?
Lastly, CulvertMaster uses the HDS-5 standard for calculations and would be accounting for inlet control or outlet control conditions where restriction from the headwall may influence the headwater, whereas in SewerGEMS it uses the dynamic equations, and headwall restriction is not assumed unless you place the headwall element (or a conduit control structure).
Generally speaking if you are using the Implicit or Explicit solver in SewerGEMS and see questionable or unexpected results, you'll want to review the user notification list and address any problems (as they could influence the hydraulics), and carefully review model data input and assumed methodologies (like the catchment runoff method). Also, check the Continuity Error in the Calculation Summary as a high number here indicates instability and mass balance error that will need to be addressed (potentially with adjustment to the advanced calculation options) before results can be trusted. You can find further guidance in these articles:
Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver
Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.