November 15, 2001
Controlling stormwater the natural way
By CURTIS HINMAN
Washington State University
The transition from a forested landscape to a built environment increases impervious surfaces such as roads, parking areas, sidewalk, rooftops and landscaping. Native vegetation and the upper soil layers that evaporate, transpire, store or infiltrate up to 50 percent of our precipitation are typically removed.
Water quality is impaired as stormwater flowing from impervious surfaces collects oil, grease and other pollutants and is discharged to streams, lakes and wetlands.
The quantity and timing of stormwater flows also change dramatically.
Overland flows, shallow sub-surface flows and associated stream discharges increase significantly in the wet winter periods; and stream base flows, maintained by groundwater, decrease during the summer months. Groundwater (about 40 percent of domestic supplies) can be depleted if not recharged during wet periods.
Current stormwater practices efficiently collect and convey precipitation from residential and commercial development to control ponds.
Stormwater control design and maintenance standards often do not protect streams and wetlands from increased water volume discharged from urbanized landscapes. The change in the quantity and timing of stormwater runoff can significantly alter stream channel form. As a result, aquatic habitat and the ability of fish, insects and other stream life to survive are degraded.
The LID approach
Low impact development is a new land development and stormwater management strategy. LID emphasizes protection and use of on-site natural features integrated with engineered, small-scale stormwater controls at the parcel and subdivision scale to manage stormwater and maintain or restore pre-development watershed hydrologic functions.
Pre-development or natural hydrologic function is the pre-disturbance balance among the overland flow, infiltration, storage, groundwater recharge and evapotranspiration characteristics of the forested landscape predominant in the Puget Sound region.
Low impact development strategies focus on evaporating, transpiring and infiltrating stormwater on site through native soils, vegetation and bioengineering applications, rather than conveying stormwater — at increased volumes — through large structural systems to streams and wetlands.
The goal of low impact development is to generate no measurable impacts to streams from commercial and residential development and to meet new and more restrictive stormwater management standards to protect water quality and salmon listed under the Endangered Species Act.
Managing stormwater with LID strategies requires a comprehensive assessment of the project site to select the appropriate stormwater controls and integrated designs that maximize infiltration and minimize surface runoff.
Some of the essential strategies include:
Much of our focus with LID is to more effectively manage stormwater. Low impact development can and should address other livability issues such as:
Low impact development focuses on utilizing our most effective stormwater management tools: native vegetation, soils and topography. Appropriately applied, LID can reduce infrastructure needs (e.g. curb and gutters, excessive pavement, and large stormwater ponds).
Initial studies indicate that LID projects can be constructed for the same or below current design standard costs.
Where soil conditions, topography and zoning densities are particularly difficult for stormwater management, LID costs may increase over standard stormwater controls given the higher standard of protection. However, new state stormwater regulations will soon be implemented in Western Washington and will require significantly larger stormwater ponds. As a result, costs for LID and standard management practices will likely equalize on the difficult sites.
The next phase for integrating LID practices into the Puget Sound region is to design and construct well-conceived pilot projects to demonstrate and further monitor these new strategies. Our challenge is to inspire creativity and cooperation among diverse interests for the complex task of implementing low impact development practices that protect water quality and aquatic habitat, and provide livable communities.
Curtis Hinman works for Washington State University as extension faculty and Puget Sound Water Quality field agent.
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