SUSTAIN from EPA...

Unfortunately, this system is integrated into a proprietary GIS (ESRI ArcGIS), but here it is:

SUSTAIN (System for Urban Stormwater Treatment and Analysis Integration) is a decision support system to facilitate selection and placement of Best Management Practices (BMPs) and Low Impact Development (LID) techniques at strategic locations in urban watersheds. It was developed to assist stormwater management professionals in developing implementation plans for flow and pollution control to protect source waters and meet water quality goals. From an understanding of the needs of the user community, SUSTAIN was designed for use by watershed and stormwater practitioners to develop, evaluated, and select optimal BMP combinations at various watershed scales on the basis of cost and effectiveness. SUSTAIN is a tool for answering the following questions:

* How effective are BMPs in reducing runoff and pollutant loadings?
* What are the most cost-effective solutions for meeting water quality and quality objectives?
* Where, what type, and how big should BMPs be?

http://www.epa.gov/ednnrmrl/models/sustain/index.html