Multiple CN numbers for a catchment and multiple catchments with a single outlet node

  Applies To 
  Product(s): CivilStorm, SewerGEMS, PondPack
  Version(s): CONNECT Edition, V8i
  Area:  Modeling
  Original Author: Scott Kampa, Bentley Technical Support Group


Is it possible to have more than one CN value for a catchment, or to have more than one catchment go to a single outlet node?


If you have a catchment area with different land and drainage types, you will see the rainfall runoff act differently. Pavement will see less infiltration than grassy areas, etc. To make sure you have the most accurate results, you may want to split these areas up so that the runoff is correctly applied to an outflow node. Consider the following options:

Option 1 - Separate catchments

First, you can model the different land types with different catchments. Each catchment area will have a different CN value. For example the impervious area (such as pavement) could be modeled as a separate catchment with a CN of 98. If these catchment areas drain to the same outflow node, it is possible to assign the same outflow node to multiple catchments. (for example both catchments assigned to a single inlet). This option tends to provide more accurate results for smaller storms compared to using one of the two methods below (see this discussion for example).

Option 2 - Subareas / Composite CN

Second, you could consider using the "Multiple Subareas" feature (SewerGEMS and CivilStorm) or "Composite CN" (PondPack) to model the separate areas for a single catchment (the portion that is impervious vs. pervious for example). This calculates a weighted CN value which is then used for the runoff calculation of the catchment. Currently this method does not automatically split the calculations into two separate catchments (and rather just uses the weighted CN) which can result in a smaller amount of runoff volume compared to option 1 (separate catchments for pervious vs. impervious) for smaller storm events. Reference # 933250. Also see this discussion)

For SewerGEMS and CivilStorm, select "Multiple Subareas" for the "Area Defined By" field:

Then, click the "Subareas" field, click the ellipsis (...) button and configure the description, CN value, and the area for each land type in the catchment. The program will calculate composite CN value based on this input.

For PondPack, select "Composite CN" for the "CN Input Type" field:

A new attribute will become available: CN Area Collection. Click the ellipsis (...) button in this field to open the CN Area Collection dialog.

In this dialog, you can enter a description, the CN value, and the area for each land type in the catchment. The program will calculate composite CN value based on this input.

Option 3 - Manually adjust CN

The alternative to this is to model the different land types in the same catchment area, but use a composite CN value. In SewerGEMS and CivilStorm, you would simply modify the SCS CN value instead of using the subareas option. In PondPack, select "Simple CN" for the "CN Input Type" field in the catchment and adjust the SCS CN value.

Option 4 - EPA-SWMM Runoff Method

If you are not required to use the Unit Hydrograph method and have the data available to do so, consider using the EPA-SWMM Runoff Method (available in CivilStorm and SewerGEMS). With this, you can use the "Subarea Routing" feature to configure how pervious and impervious areas are simulated within the same catchment element. See: Assumptions behind Subarea Routing with the EPA-SWMM Runoff Method