By Adam Regn Arvidson
The management of runoff from rain storms is regulated by state and federal governments. This is why nearly all major residential, retail or office developments have large ponds near their edges. These so-called “NURP ponds” are named for the Nationwide Urban Runoff Program (NURP), a study conducted by the U.S. Environmental Protection Agency (EPA) between 1979 and 1983. The study found that high levels of copper, lead, zinc and harmful bacteria are present in urban runoff, and that the erosion caused by that high-speed, high-volume runoff could be detrimental to ecosystems and downstream waterways.
In 1987, an amendment to the Federal Clean Water Act required local governments and industry to address urban runoff as a source of pollution. Though the specific design standards for stormwater management are controlled by state and local governments, the “wet pond” has become a widespread solution.
In recent years, however, many designers have begun to question the efficacy of these ponds at truly purifying water. Though they allow sediments to drop to the bottom of the pond, they don’t do as well at removing phosphorous, nitrogen and other nutrients. In addition, stormwater management systems based on wet ponds typically require significant investment in infrastructure-pipes, catch basins, riprap-and dedication of large tracts of land.
A new technique, most often called “low-impact development” (LID), seeks to return water to the ground as near to where it falls as possible. Infiltration (the soaking of water into the native soil) and evapotranspiration (the uptake and offgassing of water by plants) are LID’s two primary purification methods.
Numerous projects around the world now employ these techniques, and geotextile fabrics play important roles in many of them. This article looks at three common LID stormwater management elements and examines how geotextiles are employed in each.
Conventional thinking about water on roofs is to make sure that water gets off the roof as quickly as possible. Green roofs reverse that paradigm, trying to keep as much water on the roof as possible, using it to nourish plants installed there. This keeps water out of the local storm sewer system and local waterways. Geotextile membranes are commonly used as roof membranes, both on conventional and green roofs, but geotextiles also serve several other important functions on a green roof. Most importantly, they can constitute the all-important filter and drainage layers.
Depending on the type of green roof, plants typically grow in either soil or a planting medium consisting of rock mixed with organic material. Underneath this planting medium, and above the roof membrane and insulation, is the filter and drainage layer. The drainage layer typically looks like a dimpled egg crate, and it stores water either for use by the plants or for gradual release to the storm sewer system. The filter layer, just above, is a water-permeable fabric that keeps the planting medium from dropping into the drainage layer and clogging the works. For more detailed information on green roof geotextile applications, read the article for another CE Unit called “Specifying green roof fabrics” (Fabric Architecture, Jan. 1, 2009).
On the recently constructed Target Center green roof in Minneapolis, geotextiles also help wick water across the entire surface of this 133,000 sq. ft. roof and help prevent wind uplift. Read more about that project in this issue of Fabric Architecture.
Asphalt and concrete parking lots, driveways and roadways contribute a lot of water to the storm sewer system. Typically, water runs from these hard surfaces into storm drains and pipes, then into nearby waterways without any treatment whatsoever. Permeable pavement allows water to seep through the surface of the road or parking lot to an underground gravel storage area. The water can then, depending on local soil conditions, either seep into the surrounding soil or be released into another basin or waterway very slowly.
Almost any surface can serve as permeable pavement, as long as there is a way for water to get through. Permeable pavement is all about the base. There are typically several layers of different types of aggregate beneath the pavement, the main one being a volume of generally angular gravel with stones all the same size (open-graded aggregate). This allows for air spaces, which can be filled with water when it rains.
Because this open graded aggregate is almost always coarser than the pavement base above it and the soil beneath and around it, geotextile fabrics are often used to prevent infiltration of smaller particles into the water storage area. Were that to happen, according to Bruce K. Ferguson’s Porous Pavements, the most comprehensive resource on this type of design, both the water-holding capacity and the structural integrity of the aggregate base could be compromised. In some cases, such as where the native soils are contaminated or would become too plastic when wet, impermeable geotextile fabrics should be used to prevent water from the aggregate storage area from moving into the surrounding soil.
Erosion control with plants
In low-impact development, water still moves from place to place, though typically not in pipes underground. Though the water velocities are deliberately slow, there is still the potential for erosion in swales, shallow detention basins and receiving waters. In conventional stormwater management, erosion-prone areas are often clad in riprap, essentially armoring the slopes and channels with rock. Low-impact design relies on plants both to slow water and remove it from the system through evapotranspiration. While plants can provide erosion control, sometimes they are not enough.
An open-weave geotextile erosion control mat is a useful addition to a drainage swale or riverbank. Once the mat is placed on the land, plants can be seeded or plugged into it. The combination of rooting plants and erosion control mat makes for a stable slope. This method was employed along the Red River of the North in Grand Forks, N.D., after severe flooding prompted a redesign of the entire river frontage (see “Turning the Red River green,” Fabric Architecture, Sep/Oct 2008).
Geotextiles can even be used in near vertical walls surrounding stormwater basins or flanking channels. Layers of fabric are wrapped around compacted packets of native soil. These packets are constructed one on top of the next. They can even be seeded or plugged with plants, creating a nearly vertical green wall.
Low-impact development is an attempt to mimic historical hydrologic regimes, namely by slowing runoff and allowing water to infiltrate the soil or be drunk by plants near where it falls. Geotextile fabrics play an important role in many of the best management practices associated with LID.