EPASWMM with Unit Hydrograph

Mohamad Azzam said:

1- Can run the model using the Explicit (SWMM) engine while I am using Runoff Method "Unit Hydrograph"? or should I use the Implicit solver?

Yes. Currently SewerGEMS can compute runoff hydrographs independent from the solver. Internally the computed runoff hydrograph is inserted into the SWMM model via inflow hydrograph.

Mohamad Azzam said:

2- Regarding this Wiki post "https://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/43834/results-different-between-unit-hydrograph-and-epa-swmm-runoff-methods"

It is true that the Unit Hydrograph and EPA-SWMM Runoff methods are two entirely different methodologies, and the results will not match, however, I did one model in both ways and when I compare the results the difference was very big. the SWMM gives higher values in spite I am using the same coefficient CN. which method is more recommended when coming to reality? Any calibration or study had been done previously for such comparison? 

Yes, the methodologies and data input are different. So, even if the same storm, area and CN are used, there are many other parameters that are different between the solvers, such as the Characteristic width. I tested this in a sample model (UH vs. SWMM with the same area and CN) and it seems to have a big impact on the attenuation:

EPA-SWMM Characteristic width = 50 ft:

EPA-SWMM Characteristic width = 200 ft:

Determining which method is most appropriate to use will depend on your local standards and engineering judgement. You may want to ask your reviewing agency and refer to any local standards. Other forum members may also be able to contribute anecdotes or refer you to other resources.

Thank you Jesse for the clarification.

Jesse Dringoli said:

Internally the computed runoff hydrograph is inserted into the SWMM model via inflow hydrograph

does that mean the solver will be used to calculate the hydraulics for the pipes?

I have noticed that when using percent Impervious 0% or bigger value with UH the flow does not change? please elaborate why

and what is the best solver to be used with Unit Hydrograph?

Is the Unit Hydrograph used in the SewerGEMS the same "SCS Dimensionless UH"? and does it consider the initial abstraction?

Regards,

Mohamad

Mohamad, we believe we have located the source of the confusion. This is actually related to a known issue that was fixed in the latest cumulative patch set for version 10.03.04.53. I will contact you privately regarding how to get the latest patch.

I checked the impervious% for the unit hydrograph in PCSWMM, that parameter is also there and when I asked about it here is the reply from CHI water:

"To answer your question, PCSWMM allows using imperviousness % with SCS unit hydrograph approach (similar to HEC-HMS). If you use composite CN, then you should change the imp% to zero. If you enter imp%, CN should only represent pervious areas (generally much smaller). This is the same case with SWMM CN runoff method (non-linear reservoir routing) too.

In SWMM runoff, CN is only used to compute infiltration and excess rainfall is routed using non-linear reservoir routing (therefore, two approaches may not give the same runoff results). To get similar results, you have to assign the same depression storage as initial abstraction in the SCS unit hydrograph approach. Hope this clarifies your question.".

here is the screenshot:

Also, seems the initial abstraction is a factor when using SCS losses with EPASWMM. Scott mentioned that only used with Horton.

The test is also carried out with InfoSWMM and the initial abstraction is a factor when using SCS losses. moreover, the impervious% exists with the same methodology used in the PCSWMM since both program engine is SWMM.

This is for your consideration and research

Regards,

Mohamad Azzam

Hi Mohamad,

If I understand correctly, you are wondering why the %Impervious and Initial Abstraction fields are not available when using the SCS Unit Hydrograph runoff method and SCS CN loss method.

When using the unit hydrograph method and SCS CN loss method, you can choose "Multiple Subareas" from the "Area Defined by" field to define the percent connected and unconnected impervious area for each subarea represented by the catchment. See more here: Percent Connected Impervious Areas and Percent Unconnected Impervious Areas

For the initial abstraction, this is calculated automatically based on the CN and precipitation as seen here: How is Initial Abstraction considered with the SCS Unit Hydrograph method?

Hello Jesse,

is it possible in further release of SewerGEMS to report the Initial Abstraction in the results? that will be great.

Thank you,

Mohamad

Hello Mohamad,

Feature request 711441 has been filed for this. It will be considered for a future release of SewerGEMS.

Regards,

Scott

Hello Mohamad,

Feature request 711441 has been filed for this. It will be considered for a future release of SewerGEMS.

Regards,

Scott