This page contains a Flash digital edition of a book. 03.2012

The main idea of good LID is to keep the stormwater on the surface and let it infiltrate as close to the source as soon as possible. With soils that don’t infiltrate well, consider green roofs, bioretention and other techniques of LID that can store a volume of water. That overflow can be naturally discharged over time to ground water, or taken up through plant evapotranspira- tion or overland flow.


ter at peak level over many more hours or even days, soils and natural barriers can’t handle the stress. That erosive energy cre- ates excessively wide stormwater channels and disrupts the natural fl ood plain.

Cascading Trouble But those outward erosion signs only hint- ed at the deeper ecological damage. Aquatic habitats and sensitive wetlands were se- verely compromised. In part, this is be- cause erosion often has a cascading eff ect. One problem leads to others. For example, stream erosion often causes formerly bur- ied sewer pipes to be exposed, resulting in pipe breaks that send sewers overfl owing into the stream. Also, because of stream bank ero-

sion, trees fall at the top of the bank into the channel during a fl ood event. As they


Looking for planning tools to help create an LID plan for your next project. Check out these free resources.

> City/County/State/Federal Maps > National Flood Insurance Program (NFIP) Floodplain Maps

> Aerial Photographs > National Wetlands Inventory (NWI) Maps > Topography/Soils (U.S. Geological Survey/ Soil Conservation Service / Natural Re- sources Conservation Service - USGS/SCS/ NRCS)

> Local Tax/Plat Maps > Seismic Maps > Hazard Maps > Coastal Zone Management Maps

Source: The Practice of Low Impact Development, PATH

hurtle downstream, they bang into exposed pipes and manholes, causing more breaks. This process not only causes raw sewage overfl ows—it also upsets the wastewater treatment process downstream, and robs the sewage collection system of capacity.

How Does Nature Do It? As the description above suggests, changes had to be made in how development han- dles stormwater. So LID changed the dis- cussion, beginning with the premise that water management has to closely replicate natural hydrologic systems. For example, in an undeveloped site,

plants, soil features, topography and solar energy combine to address fl ood events. Water is purifi ed, redirected and evapo- rated—not simply trapped in pipes and underground detention basins. It also has a chance to replenish aquifers this way. With LID, for example, instead of one

large pond which discharges stormwater, the plan includes multiple micro-facilities on the landscape, and multiple points of en- try for runoff . These include porous pave- ment and water pervious pavers that allow water to pass through them, into the soil. Green roofs are another option, still fairly uncommon in the U.S., they’re rapidly gain- ing acceptance in Asia and parts of Europe. Arguably, of even more importance is the growing science of bioretention—using plants and natural landscape features to slow, fi lter, absorb and otherwise manage storm events. One key to successful bioretention is

that the developer and aff ected homeown- ers adjust their pre-conceived ideas about their local landscape. Too often, lawns and other managed (i.e. fertilized, mowed and watered) landscape features are used in

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