Defining CN Area Collections for Catchments

Bentley StormCAD lets you define infiltration for a catchment based on CN (SCS Curve Number) and Area data that you specify. You define and save this data in a CN Area collection.

In Bentley StormCAD , the sub-basin runoff is defined solely by the CNinput for each watershed. The CN Area Collection dialog box automatically computes weighted CN values as a function of soil hydrologic class and cover characteristics, based on the CN Engineering Library entries that you select when you create a CN Area collection.

Note: The USDA has classified its soil types into four hydrologic soil groups. For a complete description of the CN values for various land uses and cover characteristics for each soil classification, see "The Runoff Curve Number"-1228.

To define a CN Area collection for a catchment:

Click a catchment in your model to display the Property Editor, or right-click a catchment and select Properties from the shortcut menu.
In the Runoff section of the Property Editor, select Unit Hydrograph as the Runoff Method. The Loss Method field becomes available.
Select SCS CN as the Loss Method. the SCS CN field becomes available.
Click the Ellipses (...) button next to the SCS CN field. The CN Area Collection dialog box appears.
Type a description for the collection in the Description field, or click the Ellipses (...) button to display the CN Libraries in the Engineering Libraries.
Click the plus signs to expand the list of items in the CN Libraries until you find the CN Value for the soil hydrologic class and cover characteristics that you want to use.
Click Select to close the Engineering Libraries dialog box and add the CN Value to the table in the CN Area Collection dialog box.
Bentley StormCAD automatically fills in the values for SCS CN and Area. Type values for Percent Connected Impervious Area and Percent Unconnected Impervious Area. You can change the SCS CN value by clicking the Ellipses (...) button next to the SCS CN field, then selecting a different CN Value from the CN Engineering Libraries.
Repeat Steps 5 - 8 for each item you want to add to the CN Area collection.
Click OK.

CN Area Collection Dialog Box

This dialog box lets you define infiltration based on Cn and Area data. The dialog box contains the Cn-Area table and the following buttons:


	New	Creates a new row in the cn-area table.
	Delete	Deletes the currently highlighted row from the cn-area table.
	Report	Opens a print preview window containing a report that details the input data for this dialog box.

The table contains the following columns:

Column	Description
Description	Lets you enter a description for the catchment.
CN	Lets you define the Cn value for the catchment.
Area	Lets you define the area for the catchment.
Percent Connected Impervious Area	This field allows you to enter the percent connected impervious area. This value represents the percentage of the area (for the current line of data) that contains directly connected impervious cover. Precipitation that falls on these types of impervious areas flow to the outfall point without ever flowing over pervious cover areas. An adjustment to the Cn for this line of data is made based on the % connected impervious areas (0% yields no adjustment). An example sequence of directly connected impervious areas are rainfall drops onto a concrete driveway, driveway drains to a paved gutter, gutter drains into a storm sewer, and storm sewer into a detention pond.
Percent Unconnected Impervious Area	Enter the percent unconnected impervious area. This value represents the percentage of the area (for the current line of data) that contains unconnected impervious cover. Precipitation falling on these types of impervious areas eventually flows over pervious areas before reaching the outfall. An adjustment to the Cn for this line of data is made based on the % unconnected impervious areas (0% yields no adjustment). An example sequence of directly unconnected impervious areas are rainfall drops onto a paved tennis court at a grassed park, water sheet flowing across a paved tennis court, water flowing off the tennis court onto a grass field in a park (there are no inlet drains on the court), and some of the water then being absorbed into the soil.