• LID Education •

Why Consider LID?

LID has many practical and economical benefits for communities looking to improve the quality of their stormwater runoff. According to the EPA report Low Impact Development, A Literature Review, October 2000 "In general LID measures are more cost effective and lower in maintenance than conventional, structural stormwater controls. Not all sites are suitable for LID. Considerations such as soil permeablility, depth of water table and slope must be considered, in addition to other factors. Further, the use of LID may not completely replace the need for conventional stormwater controls."

The following information is taken from Low Impact Development, A Literature Review.

LID Pollutant Removal

Several studies have been conducted to analyze the effectiveness of various LID practices based on hydrology and pollutant removal capabilities. Bioretention areas, grass swales, permeable pavements and vegetated roof tops were the most common practices studied.

In general bioretention areas were found to be effective in reducing runoff volume and in treating the first flush (first ½ inch) of stormwater. Results from three different studies indicate that removal efficiencies were quite good for both metals and nutrients. Removal rates for metals were more consistent than for nutrients. Removal rates for metals ranged from 70–97% for lead, 43–97% for copper and 64–98% for zinc. Nutrient removal was more variable and ranged from 0–87% for phosphorus, 37–80% for Total Kjeldahl Nitrogen, 0–92% for ammonium and for nitrate 0–26%. Effluent volumes were lower than influent volumes.

The effectiveness of grass swales was also quite good for both pollutant removal and runoff volume reduction. In general, performance of swales is dependant on not only channel length, but also longitudinal slope and the use of check dams to slow flows and allow for greater infiltration. Further, the removal of metals was found to be directly related to the removal rate of total suspended solids, and the removal rate of metals was greater than removal of nutrients.

Reduction of impervious surfaces can greatly reduce the volume of runoff generated by rainfall. Several methods can be employed to reduce total impervious surface area. Permeable pavements and vegetated rooftops are two methods to accomplish this goal. Vegetated rooftops have been used extensively in Germany for more than 25 years and results show up to 50% reduction in annual runoff in temperate climates.

Permeable pavements can also reduce impervious surfaces. However, they are more expensive to construct than traditional asphalt pavements. Costs of these systems may be off set by the reduction of traditional curb and gutter systems to convey stormwater. Benefits of these alternate pavement types include better infiltration, ground water recharge, reduction in runoff volume and treatment of stormwater for pollutants.

Maintenance issues for LID can be more complicated than for conventional stormwater controls if the LID measures reside on private property. Generally, bioretention facilities require replacement of dead or diseased vegetation, remulching as needed, and replacement of soils after 5–10 years. Grass swales require periodic mowing and removal of sediments. Maintenance of permeable pavements requires annual high-powered vacuuming or scouring of the area to remove sediments.

Who is doing LID?

Many communities are turning to LID to assist with stormwater management. Conventional solutions to handling stormwater runoff are not always compatible with community interests, or local, state, and federal water quality regulations. Prince George's County, Maryland has pioneered many stormwater management practices and protective policies. The Puget Sound Action Team has taken a leading role in promoting LID in the Pacific Northwest

The benefits of LID can be large while the accompanying costs may be lower than expected. Many studies and reports have been completed that show benefits of LID along with reasonable costs. More information may be found in the Resource section.

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