Today was our monthly webinar on using Civil 3D in production; specifically Hydraflow Storm Sewers and Civil 3D, presented by Steven Costa. A related and unrelated question came up about how to use Hydrographs to model the New Jersey water quality storm event (1.25″ precipitation over 2-hour duration). Instead of taking time during the presentation, I said we would post the answer here.  Read on to find out how to accomplish this….

The base question is really this -

How do I model a BMP event with an X-inch precipitation over a Y-hour duration?

This can be set up as a “Custom” storm and saved for later use. Simply divide each cumulative rainfall amount by the base precipitation amount. Then enter those values as a “Custom” design storm using a 5-minute time interval. The last value should then be 1.00. Save this storm (filename.cds) with any given name you like. Hydraflow will automatically load it each time you start the program.

Incidentally, the NJ Water Quality storm has been provided with the Hydrographs Extension. The storm is saved as NJWaterQuality.cds in “C:\Program Files\Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2008″. Then simply “Open” this file from the Custom Design Storm screen or from the SCS Runoff Hydrograph screen.

Important! Enter 1.25 for the precipitation value in the Event Manager screen under the 1-year event. (Be sure to activate the 1-year while you’re there.) When using the SCS method, select “Custom” as the storm distribution rather than Type II for example, and select a 5-minute time interval.