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

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

Problem

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?

Solution

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. There are a couple of ways to do this.

Option 1 - Separate catchments

First, you can model the different land types with different catchments. Each catchment area will have a different CN value. If these catchment areas drain to the same outflow node, it is possible to assign the same outflow node to multiple catchments.

Option 2 - Subareas

Second, you could consider using the subarea feature to model the separate area for a single catchment.

Option 3 - Composite CN

The alternative to this is to model the different land types in the same catchment area, but use a composite CN value. To set this up, open the catchment properties and find the attribute "CN Area Type". Set this field to "Composite CN".

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.

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