New technical guidance from EPA came out last week. This particular piece applies to federal government facilities and puts more teeth to federal facility directives that have been out for a while. And while you might think that you don't deal with federal facilities, many municipalities and county offices actually do.
This guidance applies to "any building that is constructed, renovated, leased, or purchased in part or in whole for use by the Federal Government". It also specifies that the guidance applies to facilities with a footprint of greater than 5,000 square feet (sf) - this "includes all land areas that are disturbed as part of the project". This means it applies to the entire cleared construction area even if the structure is tiny or only remodeled.
Overall, about half the document is the "why" of LID. The rest is about implementing LID. A surprising amount of the implementation portion is dedicated to development of the design precipitation event, as discussed below.
The document provides two options for determining the volume of water to be retained on site. In the first option, the design storm (95% 24-hour event) must be fully retained on site. The document provides guidance on development of the 95% 24-hour event depth with several helpful examples.
In the second option, the post-construction hydrology must be equal to the pre-construction hydrology. It provides a site-specific approach to runoff control, rather than the first option which provides a more generalized approach.
Both options provide for a variety of methods to be used in converting rainfall to runoff. Direct Determination, TR-55, Rational Method, and SWMM are compared in the text.
The document provides some limited criteria regarding BMP design as well (Appendix A). It specifically addresses design of bioretention areas, with and without underdrains, using Horton's equation to assess infiltration and storage. Similar methods are used to assess green roofs and permeable pavers.
The main body of the document also notes cisterns and reuse as options for retention but addresses only infiltration BMPs in its analysis.
While I'm glad to see some guidance provided for federal LID development - there's still a lot to be desired in terms of design criteria. Given, federal facilities span the continent and the world, so its nearly impossible to provide criteria that would apply to every site. This document gives some basics for implementing three common LID techniques every stormwater engineer should have in their toolbox.
