PRB Articles

Stormwater Infiltration

Landscape architects are a special breed.

All of us began our pre-career existence intrigued by our natural surroundings. As an infant, my first word was “Look.”

Most of us began the elementary-school years with an inherent attraction to art, spending many class periods doodling elaborate sketches in the margins of notebooks. But our interest in art was matched only by the desire to understand how things work. So we tended to fall between the categories of artist and scientist.

Personally, I am pulled more towards science, which may explain why my second word was “Why?”

It is important to understand how the basic concepts of our natural environment work in order to incorporate and mimic them into the planned and built environment.

Landscape architects are at the forefront of shaping and protecting natural resources through the art and science of natural and built environments. In the coming decades, population growth and development will tax natural resources at an unprecedented rate.

At the top of this list is water. By the year 2030, it is estimated 47 percent of the world's population will face severe water shortages.

Forested watersheds sustain the highest-quality water sources throughout the world. In North America, the majority of municipalities rely on forested watersheds for adequate quantities of high-quality water for human use. This is especially true where populations are growing rapidly in the eastern and western United States.

A conservative estimate is that 50 to 75 percent of the U.S. population relies on undeveloped forested watersheds to produce adequate water supplies. Conversely, the growth of municipalities relies on an extensive pavement infrastructure, causing the loss of forest resources and the associated water-quality and -quantity functions.

Understanding the functions of forest hydrology--including evapo-transpiration, interception and infiltration--is critical in attempting to mitigate the detrimental effect of growth and development on water resources.

Capturing Stormwater Runoff

In mature forested watersheds, surface runoff is rarely observed. In North America, studies have estimated that on average approximately 30 percent of the incoming precipitation is intercepted by the forest canopy and never reaches the soil. This leaves the remaining precipitation to be infiltrated into the soil strata.

Stormwater infiltration is therefore the key process for water utilization by terrestrial ecosystems, of which we are a part.

Stormwater infiltration is not only the key to water storage (quantity), but also water quality. Years of studies have shown that the most effective pollutant-removal efficiencies are obtained through infiltration Best Management Practices (BMPs).

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.

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 .

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