Percent impervious

Question

Hi, 
 Do we need to define the "percentage impervious (%)" attribute to account for the runoff coefficient? 
 Or SewerGEMS would automatically pick up the net/scaled flow by using the Curve Number?

Sushma Choure · Answer

Percent Impervious field is applicable to EPA-SWMM method only, you don't need to consider that for unit hydrograph runoff method. 
 The proper hiding of the percent impervious field was implemented in a patch for version 10.03.04.53. If your organization has a SELECT subscription, Technical Support can provide you with a link to the patch for your IT/Helpdesk to install. However, this will only simply hide the field to reduce confusion. This change will also be included in the next new version release. 
 Percent impervious field - which implies percent of land area which is impervious , please search for Percent impervious in help documentation. File > Help >SewerGEMS Help. 
 Assumptions behind Subarea Routing with the EPA-SWMM Runoff Method

Jesse Dringoli · Answer

In response to Karl's points: although the field called "Percent Impervious" is only applicable to the EPA-SWMM runoff method, one can still account for impervious area with other runoff methods. If there is a need to use the Unit Hydrograph Method with the SCS Loss method and a portion of the catchment area is impervious, there are a few options to account for the impervious area: 
 
 Model it as two separate catchments both with the same Outflow Element, one configured to model the pervious area and corresponding CN and the other to model the impervious area (for example with a SCS CN of 98). 
 Use the "Subareas" option for specify the component parts (the portion that is impervious vs. pervious) and have it calculate the resulting composite CN. 
 Adjust the catchment's SCS CN to account for impervious area 
 
 See: Multiple CN numbers for a catchment and multiple catchments with a single outlet node 
 However as seen in the article Sushma provided, the routing of the impervious and pervious areas can be more complex. The engineer may need to decide on an appropriate and/or conservative approach or consider using the EPA-SWMM runoff method with the " Subarea Routing" options. 
 Karl Dauber are you saying that you find method 1 to provide more accurate runoff volume for smaller storms? 
 Note: the term "runoff coefficient" could also refer to the separate "Runoff Coefficient" Loss Method which is available for some runoff methods like Time-Area and is different than the Rational C or SCS CN. However from the screenshot provided, it appears that the SCS CN loss method is being used.

Jesse Dringoli · Answer

I can confirm that subarea option that we currently offer for the Unit Hydrograph/SCS method does not automatically split the calculations into two separate catchments (and rather just uses the weighted CN). 
 I have updated the Wiki article previously mentioned and added a note about this: Multiple CN numbers for a catchment and multiple catchments with a single outlet node 
 I have passed your feedback along to product management for consideration in a future release (reference # 933250)