Estimated impervious surface area and runoff

[cp_01]

Does anyone have any suggestions on how to estimate the area of impervious surface at the neighborhood level, and associated run-off (quick and dirty estimates with some level of validity)?

Thanks.

[Lee Nellis]

Check the tables in Dunne and Leopold's Water in Environmental Planning for some typicals. I believe you will find a reference there to a USGS study that actually did on the ground measurements. It is essential to eventually get some on the ground data in your community. I would also check aerial photos. If you have some at, say, 1:50, you can get a pretty good idea from them.

[H2OPlanner]

CP, Quick and Dirty??? The only way I can imagine to do an impervious surface/runoff analysis quick and dirty is to conduct a quick visual sample (say 5-10 percent of the residential structures) of the neighborhood to determine the following: a) structure densities (units per acre), B) floor area ratios (amount of building coverage per lot - estimated of course), c) some factor (add an additional percentage for each lot) for the interior streets and sidewalks, d) percent of structures with driveways, and e) is the neighborhood storm sewered or not. You can assume right off that between 80 and 90 percent of the rainfall hitting the buildings, driveways, sidewalks and streets heads directly into the storm sewers and using the old formula Q=CIA you can derive an estimate of the amount of runoff in CFS at any collection point. For commercial and industrial you can assume 90 percent of all rainfall hitting the entire acreage of the tract size (mostly roof and parking surfaces) will be runoff. Having all that good sample data allows you to just multiply runoff per tract times number of tracts (residential and commercial separated) and WA-LA!!! you've got a quick and dirty estimate of neighborhood runoff. That is very quick and dirty, but its validity would be based upon the quality of your sample data. Actually measuring a few of the lots of the sample would add to that validity. Lee's comment about aerial photographs is well taken, that would save a bundle of time. Good Luck

[Chris Alvarado]

Try also "The Importance of Imperviousness" in Watershed Protection Techniques,
Vol. 1, No. 3- Fall 1994.