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1998-01-22_AgendaPacket
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1998-01-22_AgendaPacket
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Grass Lake WMO
Commission/Committee - Document Type
Agenda/Packet
Commission/Committee - Meeting Date
1/22/1998
Commission/Committee - Meeting Type
Regular
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many shclliish bed closures (now that most point <br />sources have hccn controlled), no systematic attempt <br />has yet been made to relate watershed intpcrviousness <br />to the extent of shellfish bed closures. <br />Taylor"' exami ned the effect of watershed develop- <br />ment on 19 freshwater wetlands in King County, <br />Washington, and concluded that the additional storm- <br />watcr contributed to greater annual water level fluctua- <br />tions (WLF). When the annual WLFexceedcd about 8 <br />inches, the richness of both the wetland plant and <br />amphibian community dropped sharply. This increase <br />in WLF began to occur consistently when upstream <br />watersheds exceeded 10 to 15% imperviousness. <br />Implications at the Watershed Level <br />Tl~e many independent lines of research reviewed <br />here converge toward. a common conclusion- that it <br />is extremely difficult to maintain predevelopment <br />stream quality when watershed development exceeds <br />10 to 15% impervious cover. What implications might <br />this apparent threshold have For watershed planning'? <br />Should low density or high density development be <br />encouraged? <br />At first glance, it would seem appropriate to limit <br />watershed development to no more than 10% total <br />impervious cover. While this approach may be wise for <br />an individual "sensitive" watershed, it is probably not <br />practical as a uniform standard. Only low density <br />development would be feasible under a ten percent <br />zoning scenario, perhaps one-acre lot residential zon- <br />ing, with a few widely scattered commercial clusters. <br />At the regional scale, development would thus be <br />spread over a much wider geographic area than it <br />would otherwise have been. At the same time, addi- <br />tionalimpervious area (in the form of roads) would be <br />needed to link the community together. <br />Paradoxically, the best way to minimize the cre- <br />ation ofadditional impervious area at the regional scale <br />is [o concentrate it in high density clusters or centers. <br />The corresponding impervious cover in these clusters <br />is expected to be very high (25% to 100%), making it <br />virtually impossible to maintain predevelopmentstream <br />quality. A watershed manager must then confront the <br />fact that to save one stream's quality it may be neces- <br />sary to degrade another. <br />A second troubling implication of the impervious/ <br />stream quality relationships involves the large ex- <br />panses of urban areas that have already been densely <br />developed. Will it be possible to fully restore stream <br />quality in watersheds with high impervious cover? <br />Some early watershed restoration work does suggests <br />that biological diversity in urban streams can. be par- <br />daily [estUled, but Unly alter extenJ'1Ve StUrrnwateC <br />retrofit and habitat structures are installed. For ex- <br />ample, Gsh and macroinvcrtcbratc diversity has hccn <br />partially restored in one tributary of Sligo Creek, <br />Maryland." In other urban watersheds, however, com- <br />prehensive watershed restoration may <br />not be feasible, due to a lack of space, <br />feasible sites, or funding. ParadoxiC <br />A proposed scheme far classifying <br />ccrban stream quality potential <br />' - ~u I'~J <br />ally, the best way to <br />minimize the creation of addi- <br />tional impervious area at the <br />regional scale is to concentrate <br />it in high density clusters. <br />The thresholds provide areason- ,;,~ <br />able foundation for classifying the <br />potential stream quality in a watershed based on the <br />ultimate amount of impervious cover. One such scheme <br />is outlined in Table 3. It divides urban streams into <br />three management categories based on the general <br />relationships between impervious cover and stream <br />quality: <br />1. Stressed streams (1 to 10% impervious <br />cover) <br />2. Impacted streams (1 1 to 25% impervious <br />cover) <br />3. Degraded streams (26 to 100% impervious <br />cover) <br />The resource objective and management strategies <br />in each stream category differ to reflect the potential <br />stream quality that can be achieved. The most protec- <br />tive category are "stressed streams" in which strict <br />zoning, site impervious restrictions, stream buffers and <br />BMPs are applied to maintain predevelopment stream <br />quality. "Impacted. streams" are above the threshold <br />and can be expected to experience some degradation <br />after development (i.e., less stable channels and some <br />loss of diversity). The key resource objective for these <br />streams is to mitigate these impacu to the greatest <br />extent possible, using effective BMPs. <br />The last category, degraded streams, recognizes <br />thatpredevelopmentchannel stabiIityand biodiversity <br />cannot be fully maintained, even when BMPs or retro- <br />fits are fully applied. The primary resource objective <br />shifts [o protect. downstream water quality by remov- <br />ing urban pollutants. Efforts to protect or restore bio- <br />logical diversity in degraded streams are not aban- <br />doned; insome priority subwatersheds intensive stream <br />restoration techniques are employed to attempt to <br />,partially restore some aspects of stream quality. In <br />other subwatersheds, however, new development (and <br />impervious cover) is encouraged to take place so as to <br />protect stressed and impacted streams. <br />Watershed-based zocting <br />Watershed-based zoning is based on the premise <br />that impervious cover is a superior measure to gauge <br />~_ r ~, a i...',... a.... <br />iuc iiitpacisut grvwiu, Cviupareu tv pvpuia~i~u uc+~- <br />sity, dwelling units or other factors. The key steps in <br />,,~~1 <br />__~~-~V ~Il~t;: ^ '^ ai t~~t!!(~~ +,-1 1X~F~~L~j~, rt_ `s"`,: ~ *-~(1~~ %t _ 107 <br />
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