Why is low impact development so important? As a community grows, so does the amount of surface area covered by parking lots, roads, and rooftops. Precipitation cannot soak through these hard (impervious) surfaces. Traditional approaches carry precipitation offsite through a conventional "collect-and-convey" system of pipes, ditches, and storm drains. The volume of storm water (runoff) that is discharged to and transported by municipal storm-drain systems is one of the main causes of water quality issues in most urban areas. Reducing runoff, as well as decreasing the pollutants that are caught up in the runoff, is one of the most commonly applied LID principles today.
LID principles help protect water quality by mimicking nature. In nature, vegetation-covered soil soaks up precipitation. LID principles rely on structural controls to retain precipitation onsite or non-structural controls to manage site development. These principles include:
- Infiltration: Storm water infiltrates into the ground to reduce the volume of water flowing through the site. In some cases, this infiltration can contribute to groundwater recharge.
- Evapotranspiration: Plant transpiration and evaporation transfers precipitation into the atmosphere.
- Harvest: Controls collect and use storm water onsite rather than allowing it to migrate offsite.
- Reduction in Impervious Areas: Minimization of the area of roads, sidewalks, driveways, and parking lots that block water from passing through the surface to reduce the volume of storm water migrating offsite.
When precipitation falls on parking lots, roads, and rooftops where LID principles are not utilized, the water doesn't remain at its source. Instead, storm water flows quickly across these impervious surfaces, gathers pollutants, and passes through a series of ditches and storm drains that funnel it away from the site. Eventually, the untreated storm water ends up in local waterways.
At first glance, residential streets in the LID areas of Spanish Fork City, Utah only look slightly different than other streets in the city. But look a little closer, and the subtle changes become more obvious. City street right-of-ways in the LID areas are the same size, but planter strips have been extended to 11 feet in intersections, reducing the impervious surface to 23 feet at these intersections. Installation of eight-foot planter strips has narrowed the overall width of streets to 29 feet. Roads in commercial areas are individually designed using the same principles that are being applied in residential areas.
Series of "milk crate" storm chambers
The biggest difference lies underground. A series of storm chambers resembling four-sided milk crates have been installed about 18 inches below the surface in the planter strips next to the sidewalks. These storm chambers provide infiltration of storm water into the surrounding soil without disturbing the space above. Although storm chambers may need to be cleaned eventually, they should not need to be replaced.
In addition to the benefits of reducing impervious surfaces such as pavement and retaining precipitation on site, the city has seen safety benefits: Spanish Fork has recorded fewer accidents at intersections in LID areas.
Spanish Fork City also found that its transition to LID saves money. In fact, city engineers, commercial developers, and home builders in Spanish Fork see LID practices as a cost-effective way to manage storm water and ensure that the city meets municipal storm water regulations.
Storm water runoff is commonly transported through municipal separate storm sewer systems (MS4s). The recently renewed General Permit for Discharges from Small MS4s requires regulated MS4s to manage rainfall onsite and prevent the offsite discharge of the precipitation from all rainfall events less than or equal to the 90th percentile rainfall event. Cities are finding that they can meet regulatory requirements by utilizing LID tools that are designed, constructed, and maintained to manage storm water through infiltration, evapotranspiration, harvest (collection), or reuse.
Low impact development represents a paradigm shift in the way urban areas manage storm water and provides an alternative storm water strategy that simulates natural water movement and reduces storm water transport offsite. Reduced water pollution, cost savings, and safety benefits make LID a win-win for municipalities.