EPA Technical Guidance for Federal Facilities Out

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.

Permeable Pavement for Oklahoma

Over the past few years I've fielded a number of questions about the use of permeable pavement in Oklahoma. Permeable pavements are an alternative to conventional concrete and asphalt paving that allow for infiltration of stormwater through the pavement and into a subsurface storage area or drain. Recently, there have been some significant installations, including Chicago's U.S. Cellular Field parking lot consisting of 6 acres of interlocking pavers. Its important to look into the advantages and limitations for its use on local sites.

Initially, I began by looking into the ratings for permeable pavement provided by the EPA. EPA showed permeable pavements to have a failure rate of about 75%1,2. That was not encouraging. I looked into the studies behind these findings and found that they were from 1992. Well, a lot has happened in the LID world since 1992, so what about more recent studies?

My inquiries took me to states like North Carolina and Georgia, where water quality regulation is intense, and to the EPA's LID Development Center in Portland, Oregon. As you may know, some advantage of permeable pavement include:

* The lifespan of a northern parking lot is typically 15 years for conventional pavements due to freeze/thaw stress, which is also common in Oklahoma. Porous asphalt or concrete parking lots can have a lifespan of more than 30 years because saturation during freeze/thaw is reduced3.

* Permeable pavement transforms areas that were a source of stormwater runoff to a system that can reduce or eliminate runoff that would have been generated from traditional paving. Studies show stormwater retention rates ranging from 25-100% depending upon storm intensity. http://www.thewaterchannel.tv/index.php?option=com_hwdvideoshare&task=viewvideo&Itemid=53&video_id=349

* Permeable pavement can reduce the amount of pollutants in stormwater runoff and can address suspended solids (high removal rates), acid rain (some types effective), phosphorous (moderate removal), nitrogen (moderate removal), and metals pollution (best in vegetated systems).

So what's not to like? There are some drawbacks and limitations to use including:

* Permeable pavement is not recommended, or in some states even allowed, where surface soils are silt or clay. That's most of northeastern Oklahoma. Generally, permeable pavement is only recommended in areas with sandy surface soil, like the western part of the State, or where stormwater has been pretreated.

* Even when properly sited, maintenance requirements are critical for the success of permeable pavement. Proper maintenance includes vacuum sweeping or high-pressure water washing at regular intervals and inspections after each storm. If you can't commit to maintenance, there are better options.

* Porous pavement has reduced strength compared to conventional materials and isn't appropriate for applications with high volumes, high speeds, or heavy loads.

So how might permeable pavement be used in Oklahoma? Walkways, driveways, alleyways, and overflow parking are some options. And consideration should be given to preventing sediment from reaching the pavement, especially during construction, which can trouble a pavement system before it's even in use.

Finally, we have a responsibility to put our best foot forward for LID as these topics are being introduced in local communities, to hit the ground with practices we know have positive reproducible results. Who wants to hear, "More of my tax dollars gone to waste." again? The future of LID is in our hands!

1 Schueler, T.R., P.A. Kumble and M.A. Heraty. 1992. A Current Assessment of Urban Best Management Practices. Metropolitan Washington Council of Governments, Washington, DC.

2 J. Galli. 1992. Analysis of Urban BMP Performance and Longevity In Prince George's County, Maryland. Prepared for Department of Environmental Resources, Prince George's County, MD.

3 Gunderson, J., Pervious Pavements: New Findings About Their Functionality and Performance in Cold Climates, Stormwater, September 2008.