Improving Iowa’s Impaired Waterbodies through Best Management Practices: Part 2
Updated: Dec 5, 2018
Best Management Practices (BMP’s) used for storm water management include rain gardens, bioswales, vegetative roofs, native prairies, rainwater catchment, permeable pavement and more. Combining BMP’s with current storm water management infrastructure can help to lessen water loads during large rain events. This could help mitigate the effect on current infrastructure and streams, creeks, and waterways that storm water is released into. Design and implementation of these practices require research, data collection, and several calculations, but properly designed and installed BMP’s can offer many benefits.
A rain garden is a garden that has an inward slope filled with native plants in order to capture and infiltrate storm water runoff. They can be used to manage storm water for both smaller sites, such as problem areas in a homeowner’s yard, or larger sites, such as parking lots or around large buildings. When sizing a rain garden, the calculations may differ based off of the location for which it is being designed. The Iowa Rain Garden Design and Installation Manual contains specific design standards for storm water management practices in the state. When determining the area needed for a rain garden, the depth of the rain garden and the percolation rate of the site’s soil should be taken into account. The depth of the garden should allow collected water to infiltrate the soil within a 24-hour time period; if it takes longer than that, plant growth may be impeded. The Web Soil Survey, made by the USDA, can be accessed for free online and allows the user to search soil surveys of selected areas in the nation. The surface area of the impervious site that contributes to water runoff plays a role in determining needed area for a rain garden, as well. This should be calculated and considered when designing a rain garden, so pooling or overflow will not occur. When soils have low percolation rates (0.5- 1 inch per hour), an enhanced rain garden can be installed. This type of garden has a trench, typically eight to 12 inches wide, dug horizontally down the center of the rain garden to the frostline (three feet deep). A layer of washed, 1 inch rock 1- 2 inches thick is laid, an under drain is set on top of this, and then 8- 10 inches of 1 inch washed rock is placed around and on top of the drain. A layer of washed chip stone is set over the rock, extending up 14 and 3/8 inches. An enhanced rain garden allows for downsizing the surface area of the garden while maintaining its ability to collect and disperse storm water effectively. This is because enhanced rain gardens use underdrains beneath the garden’s surface, allowing excess water to be drained to another area (ISP, 2009). A design for an enhanced rain garden is pictured below.
A bioswale is similar to a rain garden in design; it is a garden with a depression angling inward, but unlike a rain garden, a bioswale has a slope running parallel with the garden. The outer edges of a bioswale slope towards a specified soil mix area in the middle, creating a slope for storm water to be captured and dispersed. This allows water to infiltrate the area of the bioswale, and excess water is conveyed to a desired area, such as a prairie garden or storm drain. Bioswales are typically long and narrow, and should be installed in areas with a slope between 1-6%, with 3% being optimal. The bottom layer of a bioswale is a layer of about 0.6 inches of washed rock. Above the washed rock is a one-half foot layer of gravel, and above that is a one-half foot layer of the specified soil mix described in the rain garden design. Check dams- Dams within the bioswale perpendicular to the length of the garden- should be installed in order to slow conveyance of water. An engineer or landscape architect may be needed to accurately determine the number and spacing of check dams when designing a bioswale. Once a bioswale has been installed, a diversion should be created so that the garden may establish before it is required to capture large amounts of water (Yocum, 2007).
A vegetative roof, also called a “green roof”, is a contained plant installation on top of a roof used to capture storm water. There are two types- Intensive and Extensive. Intensive vegetative roofs are deeper than extensive and often merge with on-structure landscapes, such as plazas. Extensive vegetative roofs are simpler, and designed to be six inches or shallower. Extensive green roofs commonly have a three to four-inch layer of soil mix with a layer of filter fabric and a moisture retention or drainage panel below. Underneath of the panel, a layer of insulation is needed, along with a root barrier and a protection course and capillary break. A waterproofing membrane should be between the roof and the bottom layer of the garden to protect from leaks. Sedums are a popular plant choice for simple, shallow green roofs because they are low-maintenance and do well in these kinds of plantings. There are many aspects to consider when designing and installing a green roof, such as the bearing weight of the roof, the snow load, accessibility and local fire codes. Experts may need to assist with green roof design to ensure structural safety. Properly installed vegetative roofs offer several benefits, including protection and prolonged service live of the roofing materials, reducing urban heat-island effects, and conserving energy (Miller, 2015).
A native prairie garden uses plants that are hardy for and native to the area and grow large root systems. These gardens can be used to capture and infiltrate water in areas that are not suitable for rain gardens, such as areas with poor soil quality. A mixture of grass, flower and forb perennials- Plants that grow for more than two years, are best to use in a prairie garden. Flowers suitable for southern Iowa prairie gardens include black eyed susans, varieties of milkweed, coneflowers, and spiderwort. Grass varieties that do well in Iowa include big bluestem and varieties of sedge. Additional benefits of native prairie gardens include soil restoration and increasing habitat area for pollinators.
When designing, installing and maintaining BMP’s for storm water management, extensive research and data collection for the specified site should be used to ensure success. Combining BMP’s with current storm water management could help to mitigate the effects that storm water has on water bodies, while helping to improve water quality in the area.