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't -.are ;`~ . i-~;r Table 3: A possible scheme for classifying and managing for headwater urban streams based on ultimate imperviousness Urban Stream Stressed Impacted Degraded Classification (0-10% Imperv.) (11-25% /mperv.) (26-100% Imperv.) Channel stability Stable Unstable Highly Unstable Wafer quality _. .. Good -,:.... Fair Fair-P:oor Stream biodiversity Good-Excellent Fair•Good Poor ,._ aesdurce objective _ ,. Protect biodrversity Maintain critical eie- Minimize downstream. ,'; and channel sfabtl~ty ments of stream quality pollutant loads Water quality Sediment and Nutrient and Control bacteria objectives temperature metal loads BMP~Selecfion ~~ Secondary enutronmental Removal efficiency Removal efficiency Factors impacts -- . Land Use Controls Watershed-wide imp. Site imp. cover limits Additional infill and cover limits (ICLs), (ICLs) redevelopment , site ICLs .. en^oura ed ~ g ; Moniforrrr and ~ g GLS monifortn df im g p. ~ - - Same as "Stressed _ .:... P,.Ilutant road enforcement:- cover btomonttoring modeling Development rights Transferred out None Transferred in Ripanan buffers ,; 'Ntdest buffer network Average bufferw~Gth - Greenways watershed-based zoning are as follows. First, a com- munity undertakes a comprehensive physical, chemi- cal and biological monitoring program to asses the current quality oC its entire inventory of streams. The data are used to identify the most sensitive stream systems and to refine impervious/stream quality rela- tionships. Next, existing imperviousness is measured and mapped at the subwatershed level. Projections of future impervious cover due to forecasted growth are also made at this time. The third step involves designating the futurestream quality for each subwatershed based on some adapta- tion of the urban stream classification scheme pre- sentedearlier. The existing land use master plan is then modified to ensure that future growth (and impervious cover) is consistent with the designated stream classi- tication for each subwatershed. The final step in the watershed-based zoning pro- cess involves the adoption of specific resource objec- tives for each stream and subwatershed. Specific poli- cies and practices on impervious cover limits, BMPs, and buffers are then instituted to meet the stream resource objective, and these practices directly applied to future development projects.. Watershed-based zoning should provide managers with greaterconfidence that resource protection objec- tives can be met in future development. It also forces loeal governments to make hard choices ghoul which strcams will be fully protected and which will become at least partially degraded. Some environmentalists and regulators will be justifiably concerned about the streams whose quality is explicitly sacrificed under this scheme. The explicit. stream quality decisions which are at the heart of watershed-based zoning*, however, are preferable to the ~,ni nfonned and random "non-decisions" that are made every day under tl;e present zoning system. A cautiorrat}~ nole While the research on impervious coverand stream quality is compelling, it is doubtful whether itcan serve as the sole foundation for legally defensible zoning and regulatory actions at the current time. One key reason is that the research has not been standardized. Different investigators, for example, have used different metl;- ods to define and. measure imperviousness. Second, researchers have employed a wide number of tecl;- niques to measure stream quality characteristics that are not always comparable with each other. Third, most of the studies have been confined to few ecoregions in the country. Little research has been conducted iii the Northeast, Southeast, Midwest, and semi-arid Western regions. Lastly, none of the studies has yet examined the effect of widespread application of BMPs on imperviouscover/stream quality relationships. Until studies determine how much BMPs can "cheat" the impervious cover/stream quality relationship, it can be argued that structural practices alone can compensate for imperviousness effects. 108 ~j±,;f~r3~ - 4>> i~~~~~r.'3r~-~~~Ta~CL~~ a ~vt~7E> >~„1~ ~~ ~~ , ~ 1n--`~~i:~_~ o ;,