Some jurisdictions may have stormwater standards that are similar to the LEED requirements. For example, Portland, Oregon's Title 17, Public Improvements, Chapter 17, 17.38.030 Section C, states that the quantity and flow rate of stormwater leaving the site after development shall be equal to or less than the quantity and flow rate of stormwater leaving the site before development, as much as is practicable.

Research historical climate records to understand the frequency, intensity, and duration of the design storm event. A longer record of daily rainfall events (rather than monthly rainfall averages) will result in more accurate sizing of components like cisterns.

We recommend that the civil engineer conduct a cost-benefit analysis of stormwater-reduction strategies, including cisterns, porous pavement, rain gardens, parking garages (instead of parking lots), detention ponds, green roofs, sand filters, or detention tanks.

Develop a project-wide water budget and a landscape irrigation water budget. This helps teams decide if reusing rainwater may be appropriate and where to use it—typically either in irrigation or toilet flushing.

The Rational Method is the most common for determining peak discharge rate and runoff volume. It requires the runoff coefficient for each surface type, the total area for each surface type, and the total project area. Runoff rate and volume are directly proportional to landscape or hardscape porosity or perviousness. Undeveloped land has little imperviousness, while previously developed land will have more. However, many materials that seem to be impervious do not necessarily have 100% imperviousness.

Determine the imperviousness of the existing site. The Rational Method (see Resources for more information) is most commonly used to determine the weighted runoff coefficient. Then multiply by 100 to get the percent imperviousness. The imperviousness of the site determines which compliance path the project must take.

Research local regulations on stormwater reduction requirements, as well as regulations on the collection, storage, and reuse of rainwater. (See Resources for examples.)

A site visit and tests are integral to understanding the natural hydrology, site topography, and soil infiltration rates.

The owner and civil engineer determine the feasibility and rough costs of appropriate stormwater management techniques. Identifying cost tradeoffs for complementary strategies is a crucial component of the decision process. For example, a rooftop runoff collection system may be more cost-effective when combined with a graywater collection and reuse system. Fully explore the potential for LID strategies such as rainwater cisterns, green roofs, and bioswales.

Most municipalities require stormwater documentation. In these cases, the documentation for LEED requirements should not represent a significant soft-cost premium.