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Flow generated from undeveloped area

I am trying to model a catchment (sand/undeveloped catchment) in SewerGEMS to generate the hydrograph and get the flow curve by using EPASWMM runoff method, and since the area is rural I used SCS CN = 61. I also used impervious % = 0, the other parameters are typical (as slope 1%, catchment width 50m, ,,,etc). I am getting flow=0 l/s.

is that because I am using impervious %=0? As I guess, if yes what is the approach to find out the flow by modeling with EPASWMM method?

I tried Unit Hydrograph for the same catchment area and used CN =61 and the program generate flow, I know both has different methodology, so how that can be done and find the pre-developed flow from that catchment by using EPASWMM runoff method, if I assume the catchment is totally grass or sand; of course will have a big infiltration but still will be runoff and the flow will not be zero.

Regards,

Mohamad

  • Hello Mohamad, 

    Do you get zero flow for the entire simulation? Did you check the graph? 

    I made changes to a sample model with EPA-SWMM runoff method impervious % = 0 however flow is generated for the catchment at a later time step. 

    Could you please attach required screenshots? 

    Regards,

    Sushma Choure

    Bentley Technical Suppport

  • Hello Sushma,

    please check the catchment CM-1 in the attached model.

    Regards,

    Mohamad

    Test Catchment.stswpkg.zip

  • If we change the value of Storage (Pervious Depression) mm to 6 or less than that catchment generates flow. 

    Looking for reference of exact values of these storages and their relation. 

    Regards,

    Sushma Choure

    Bentley Technical Suppport

  • Would you please elaborate on why 6 or less? and what is the theory behind that?

    Thank you,

    Mohamad

  • Hello Mohamad,

    The depression storage is the amount of rainfall that can be stored on a catchment because of ponding or surface wetting. In the original setup, that value was sufficiently high that a combination of infiltration and storage meant that all of the rainfall remained on the catchment and did not run off to the manhole. By entering a smaller depression storage, this allowed for some runoff.

    I would recommend reviewing the SWMM manual for more information. A search for "depression storage" may be a place to start. In the SWMM manual I found the following: 

    3.4.1 Surface Runoff

    The conceptual view of surface runoff used by SWMM is illustrated in Figure 3-7 below. Each subcatchment surface is treated as a nonlinear reservoir. Inflow comes from precipitation and any designated upstream subcatchments. There are several outflows, including infiltration, evaporation, and surface runoff. The capacity of this "reservoir" is the maximum depression storage, which is the maximum surface storage provided by ponding, surface wetting, and interception. Surface runoff per unit area, Q, occurs only when the depth of water in the "reservoir" exceeds the maximum depression storage, ds, in which case the outflow is given by Manning's equation. Depth of water over the subcatchment (d) is continuously updated with time by solving numerically a water balance equation over the subcatchment.

    Regards,

    Scott

    Answer Verified By: Mohamad Azzam