Stormwater Infiltration

Across the board, all seven standard water-quality indices–total phosphorus, soluble phosphorus, total nitrogen, nitrate, copper, zinc, and total suspended sediment–are removed to a higher degree through infiltration than any other single stormwater BMP; in fact, in most cases, they are significantly higher.

Municipal Planning

Stormwater infiltration is now becoming an integral part of municipal stormwater planning. Infiltration BMPs–such as bio-infiltration, infiltration trenches, and dry wells–have been used extensively with great success.

The logical and most versatile approach is targeting the composition of the pavements. The degree of retention and infiltration that naturally existed prior to development can be reached, and in most cases exceeded, using properly designed permeable paving systems.

Currently, most stormwater policies and regulations require minimum infiltration rates of the underlying soils in order to implement and receive credit for stormwater infiltration practices. These policies can prohibit significant achievable water-quality and -quantity gains.

For example, soils that have clay content with infiltration rates below 0.5 inches per hour may be regarded as inappropriate for stormwater infiltration. In most cases this is a lost opportunity in regions that desperately need infiltration to regain minimum lost hydrologic process and function.

Lower stormwater infiltration rates are critically beneficial to water quality and quantity goals. For example, infiltration rates as low as 0.01 inches per hour infiltrate 0.72 inches in 3 days and 1.2 inches in 5 days. Soils with moderate proportions of clay content have a higher capacity to store and clean water.

Cecil Soil Comparison

Out of 20 metro areas in the U.S. experiencing the most development in the last 20 years, 11 with the greatest land-conversion rate are in the Southeast. Six of these are located in the Piedmont plateau.

Piedmont soils are typically clay-like with the dominant soil type being the Cecil soil series. Over 10-million acres of Cecil soils are currently mapped in the Piedmont region, spanning from Alabama to Maryland.

In this context, the importance of understanding the infiltration properties of clay soils becomes blatantly apparent.

An example of two projects constructed in the Charlotte metro area of North Carolina underlain by Cecil soils have demonstrated after 4 to 6 years of monitoring, that proper design and consideration for these soils can result in successful capture and infiltration of more than 95 percent of the respective annual precipitation events.

The first pervious concrete parking lot in the Charlotte metro area was designed and monitored by Estes Design Inc. to capture, at a minimum, the 3.12-inches, 2-year rain event. The project was later monitored for an additional 2 years by the University of North Carolina at Charlotte.

The study concluded that precipitation events up to and greater than the 10-year-return interval storm had been captured and infiltrated during the study period from 2007 through 2009. The recorded average-infiltration rate corrected for volume is just 0.036 inches per hour. Draw-down rate for the 18-inch-depth stone reservoir, uncorrected for volume of the stone, was 0.105 inches per hour.

A second project designed as a low-impact, multi-family development incorporated bio-infiltration into the stormwater design. This project was constructed in 2007 with monitoring of infiltration rates beginning in February 2008 until current.

The monitoring data for this site includes 275 storm events, ranging from 0.1 to 3.85 inches. Recorded infiltration rates for the two BMPs range from 0.26 to 0.97 inches per hour for BMP 1 and 0.15 to 0.31 inches per hour for BMP 2. Variations in infiltration rates are due to the depth of head consistent with Darcy’s Law. Both BMPs have continued to function without any recorded overflows.

Although the important habitat and the function of evapo-transpiration of a forested area is lost to the degree that it previously existed, both of these projects demonstrate the capacity to infiltrate much more water per surface area than ever existed prior to development.

In the cases of these two projects, the first was a redevelopment project, and the second was an agricultural pasture. In Mecklenburg County, North Carolina, where these two projects are located, 40 percent of the county’s forest has been lost since 1990.

Searching For Answers

So why are we limiting ourselves in the use of stormwater-infiltration practices? In the U.S., urban infiltration has been around since the 1930s. The state of Maryland has been vigorously incorporating infiltration into its stormwater infrastructure since the 1980s. Landscape architect Bruce Ferguson of the University of Georgia published the first comprehensive book on stormwater infiltration in 1994.

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