Technology Matrix Example: New Development Commercial

In this example, assume the site is a 10-acre site whose pre-construction impervious area is less than 20 percent. The site is located in a “zero increase” watershed. Investigation of existing site resources indicates that existing soils are HSG D. The site zoning allows up to 75% impervious coverage, and the user plans a 2-acre building on the property. Land discturbance exceeds more than 1 acre, and water quality BMPs are required. 15-year 20-minute differential runoff calculations show the additional undetained runoff is >2 cfs. Flood detention and channel protection are also required.

Using the matrix, the developer’s engineer proposes to use bioretention in the parking lot islands and adjacent to the building to meet water quality and runoff volume reduction requirements. A separate above-ground flood detention basin is proposed on the site for the additional storage needed to meet channel protection and flood detention requirements. The size of the flood detention basin is reduced based on the volume reduction provided by the bioretention cell. The approach is accepted by MSD.

BMP Technology	Performance/Function
	Water Quality Treatment	Runoff Volume Reduction	Channel Protection Storage	Peak Flood Detention Storage
Bioretention	X	X
Dry Detention Basin			X	X

An alternative approach that is encouraged and accepted by MSD would be to use more green infrastructure to accomplish more volume reduction and storage, and then demonstrating that the site’s pre- and post-construction runoff are the same in a simulated 100-year 24-hour storm. This could potentially be done by using pervious pavement to provide additional storage and infiltration of parking lot runoff, increasing the storage of bioretention cells, and adding a rainwater harvesting system for the roof. The additional investment in these BMPs would eliminate the need to construct the detention basin, allowing the land set aside for the basin to be used for other purposes. Rainwater captured from the roof could be used for irrigation, reducing costs associated with potable water.