Stormwater Infiltration

In December 2009, the EPA–under Section 438 of the Energy Independence Act–published the Technical Guidance on Implementing the Stormwater Runoff Requirements. This document states that “Knowledge accumulated during the past 20 years has led stormwater experts to the conclusion that conventional approaches to control runoff are not fully adequate to protect the nation’s water resources” (National Research Council, 2008).

This document requires green infrastructure and low-impact development tools be used to maintain or restore pre-development hydrology. The key to this approach is stormwater infiltration.

So the answer to “why,” if asked 20 years ago, may have been the lack of awareness of infiltration utility and practice. But currently the answer seems to be the lack of understanding of soil properties and processes by regulators and practitioners.

Clay soils vary greatly in permeability and shrink-swell potential, even within specific mapped soil types. The second project described above, even though mapped under a single-soil series, displayed a significant variation in the pre-development field-infiltration tests.

To make things more confusing, presently there is no one method of soil analysis that accurately estimates the shrink-swell potential for all soils. This does not help the confidence level of designers and regulators regarding the application of permeable pavements. However, this is overcome by detailed site investigation.

All clay soils are not equal when it comes to the expansion or shrink-swell potential. Cecil soils, for example, are Kaolinitic, and contain elements of aluminum and iron that inhibit expansion. Of all the Piedmont clay soils, Cecil soils are in the lowest range of the Expansive Soil Index.

It should also be realized that permeable pavement types are not all equal. Segmental pavements do not have the “bridging” capacity of a rigid pavement, such as pervious concrete. For example, under a proposed parking lot, small, incidental confined pockets of less stable soil can be bridged with the larger surface area of rigid pavement.

New policies are being rewritten, and it will eventually be common practice to implement stormwater infiltration in a broader range of clay soils. Stormwater infiltration should be the first option considered as part of any planned stormwater system. Therefore, it is now incumbent upon the designer to have a greater understanding of these issues in order to reach the sustainability goals in stormwater design.

Christopher J. Estes is president of Estes Design Inc., an environmental design and consulting company specializing in stormwater management. Currently he is focusing his efforts on the restoration and protection of urban water quality. His most recent projects include modeling and monitoring of combined best-management practices such as bio-retention and pervious concrete. His current research is stormwater infiltration rates into clay soils.

Estes Design Inc., nationally recognized in stormwater research, is an environmental design and consulting company that specializes in low-impact stormwater, water-quality regulation, and stream restoration. Founded on research and 18 years of design-build experience, Estes Design Inc. has worked with a host of clients ranging from public and private entities and universities to federal regulators to produce research-based design strategies for sustainable development. For more information, call (704) 841-1779, or visit www.EstesDesign.com.

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