Thursday, February 2, 2012

Low Impact Development and Why it Matters

It has been called green infrastructure, conservation design, sustainable storm water design, natural stormwater management, and rain management but Low Impact Development, LID, seems to be the term that has taken hold in the United States for the site level actions and strategies. LID is a strategy of stormwater management emphasizing conservation and natural features combined with small scale stormwater controls to mimic as closely as possible the natural hydraulic properties of a site. The idea is to move water slowly through open conveyance systems and use distributed stormwater retention in open unpaved areas to allow infiltration of rain water into the earth. This reduces the quantity and velocity of stormwater as it leaves a site reducing the damage that uncontrolled stormwater runoff can cause when we change the amount of impervious surfaces a site has by building roads, sidewalks, playgrounds, and structures and compacting soil.

Traditional development practices cover large areas of the ground with impervious surfaces such as roads, driveways, sidewalks and buildings. These paved and impervious surfaces prevent rainwater from infiltrating into the ground, causing it to run off site at velocities and volumes that are much higher than would naturally occur. The collective force of such rainwater scours streams erodes stream banks resulting in large quantities of sediment and other pollutants entering streams, rivers, estuaries and bays every time it rains or snow melts. The US EPA believes that sediment and nutrient pollutions contained in runoff from urban areas is the largest source of water quality impairments to estuaries (areas near the coast where seawater mixes with freshwater) in the United States and has turned its water quality focus on these areas starting with the Chesapeake Bay Watershed and moving forward with the Gulf Coast estuaries.

Groundwater is recharged from rain and sources of surface infiltration. In many areas where development has occurred, we pump the groundwater for drinking water supplies (both public and private) and create barriers to rain infiltration by paving significant portion of the urban and suburban landscape as well as allowing if not encouraging storm water to leave a site as quickly as possible reducing the time that rainwater has to infiltrate the remaining soil and percolate into the subsurface. If we do not allow adequate rain water infiltration we will deplete the groundwater aquifers as we continue to pump water from wells. The U.S. Geological Survey’s (USGS) Groundwater Resources Program has found that the volume of groundwater stored in the earth is decreasing in many regions of the United States, and if this continues we could deplete our groundwater. We are running a groundwater deficit in many parts of our country, though we have adequate rainfall. LID can help by increasing water infiltration and reducing runoff.

In addition to the problems caused by stormwater and non point source runoff, many older cities (including many of the largest cities in the United States), have combined sewage and storm water systems which results in the storm water runoff overflowing the combined sewer system during storm events and diluted, but nonetheless raw sewage being released to rivers and estuaries. This is an ongoing problem in Baltimore and at Blue Planes in Washington DC as well as other cities throughout the nation. In the late 20th century, most cities that attempted to reduce sewer overflows did so by separating combined sewers, expanding treatment capacity, expanding storage within the sewer system, or by replacing broken or decaying pipes. San Francisco and many other cities have taken all of these steps, but still have much more that needs to be done. It is unfortunate that more of the stimulus dollars were not spent to repair expand and improve the waste water treatment facilities in our oldest cities instead of pursuing $54 billion in direct loans and loan guarantees to green energy companies. Repairs and improvements to our waste water treatment systems would have served our nation for several generations rather than been wasted on unproven technology or enriching favored entrepreneurs.

Managing rain water and snow melt is at the heart of LID. Rain water and storm water management under LID is landscape based and not particularly new. At the larger regional or watershed scale, green infrastructure is the interconnected network of preserved or restored natural lands and waters that provide essential environmental functions. Large-scale green infrastructure may include habitat corridors and water resource protection. At the community and neighborhood scale, green infrastructure incorporates planning and design approaches such as compact, mixed-use development, parking reduction strategies and urban forestry that reduces impervious surfaces and creates walkable, attractive communities.

At the site scale, green infrastructure is LID and mimics natural systems by utilizing permeable surfaces to absorb storm water back into the ground (infiltration), using trees and other natural vegetation to convert it to water vapor (evapotranspiration) and using rain barrels or cisterns to capture and reuse storm water. These natural processes manage storm water runoff in a way that maintains or restores the site’s natural hydrology, allowing groundwater to recharge. Site-level green infrastructure is LID, and can include rain gardens, porous pavements, green roofs, infiltration planters, trees and tree boxes and rainwater harvesting for non-potable uses such as toilet flushing and landscape irrigation. LID not only reduces the velocity and quantity of runoff protecting our streams, rivers, lakes and estuaries, it is essential to allow the recharge of groundwater.

The difficulty with LID is compliance and maintenance. Federal Clean Water Act requirements, such as the Combined Sewer Overflow (CSO) Control Policy and National Pollutant Discharge Elimination System (NPDES) permit program, do not allow for deviance from traditional control strategies. EPA guidance which encourages LID and green infrastructure to manage storm water is inconsistent with permit requirements under NPDES that call for more conventional methods of stormwater management.
NPDES regulations require development and implementation of a municipal separate storm sewer system (MS4) program to address post-construction runoff from newly developed and redeveloped areas. Investments in stormwater management and wastewater treatment plants are driven by compliance with regulations, which do not allow local policy makers to implement watershed-based or decentralized LID infrastructure solutions that may not yet have the data necessary to demonstrate performance and receive regulatory credit under a permit. Within the Chesapeake Bay Watershed the Chesapeake Bay Model provides credit under the Watershed Implementation Plans for LID retrofits, but not all practices are credited appropriately (both because of the amount of time needed for these practices to show long-term performance, as well as limitations in historic data collection). LID is by its nature a distributed design involving, rain gardens, porous pavements, green roofs, planters and rainwater harvesting require ongoing maintenance of the plants, replanting after severe winters or prolonged droughts, weeding, and clearing of porous pavements. There does not yet exist a method of ensuring that these features are maintained appropriately to continue functioning over time and that any repairs or replacements are done with LID in mind.

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