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{if`-~,
<br />On the positive side, it may he possible for a
<br />community to define the impervious cover/stream qual-
<br />ity relationship in a short time and at relatively low
<br />cost. A suggested protocol for conducting a watershed
<br />monitoring study is presented in Table 4. The protocol
<br />emphasizes comparative sampling of a large popula-
<br />tion ofurban subwatersheds of different increments of
<br />imperviousness (perhaps 20 to 50).
<br />A rapid sampling program collects consistent data
<br />on hydrologic, morphologic, water quality, habitat and
<br />biodiversity variables within each subwatershed. For
<br />comparison purposes, series of undeveloped. and un-
<br />disturbed reference streams are also monitored. The
<br />sampling data are then statistically and graphically
<br />analyzed to determine the presence ofimperviousness/
<br />stream quality relationships.
<br />The protocol can be readily adapted to examine
<br />how BMPs can shift the stream quality/impcrvious-
<br />ness relationship. This. is done by adjusting the sam-
<br />plingprotocol to select two groups of ,
<br />study subwatersheds-those that arc Site desig
<br />effectively served by BMPs and those f t
<br />that arc not.
<br />. ~'iG(10.'1
<br />Hers can use a wide
<br />range o echniques to minimize
<br />impervious cover by 10 to 50%.
<br />Minimizing impervious cover ' ~ - '<~''~+
<br />Reducing impervious cover can be an effective
<br />element of the overall BMP system for a development
<br />site. As noted earlier, imperviousness need not be a
<br />fixed quantity. A site designer can utilize a wide range
<br />of techniques to minimize impervious cover at devel-
<br />opment site (Table 5) that collectively .;an reduce
<br />imperviousnessby lOto50%. (See Technical Notes 38
<br />and 39 iii thrs issue.)
<br />Table 4: Proposed protocol for defining functional relationships between watershed
<br />imperviousness and stream quality
<br />> General study design
<br />A systematic evaluation of stream quality for a population of 20 to 50 small subwatersheds that have
<br />different levels ofwatershed imperviousness. Selected field measurements are collected to represent key
<br />hydrological, morphological, water quality, habitat and biodiversity variables within each defined
<br />subwatershed. The population of subwatershed data is then statistically analyzed to define functional
<br />relationships between stream quality and imperviousness.
<br />Defining reference streams
<br />Up to 5 non-urban streams in same geo-hydrologicai region, preferablyfuliyforested, or atleast full riparian
<br />forest coverage along same length. Free of confounding NPS sources, imperviousness less than 5%,
<br />natural channel and good habitat structure.
<br />Basic subwatershed Variables
<br />Watershed area, standard definition and method to calculate imperviousness, presence/absence of
<br />BMPs.
<br />Selecting subwatersheds
<br />Drainage areas from 100 to 500 acres, known level of imperviousness and age, free of confounding
<br />sources (active construction, mining, agriculture, or point sources). Select three random non-overlapping
<br />reaches (100 feet) for summer and winter sampling of selected variables in each of five key variables
<br />groups:
<br />1. Hydrology variables: summerdryweatherflow, wetted perimeter, cross-sectional area of stream, peak
<br />annual storm flow (if gaged).
<br />2. Channel morphology variables: channel alteration, height, angle and extent of bank erosion, substrate
<br />embeddedness, sediment deposition, substrate quality.
<br />3. Water qualityvarlables: summerwatertemperature,turbidity, total dissolved solids, substrate fouling
<br />index, EP toxicity test, wet weather bacteria, wet weather hydrocarbon.
<br />4. Habitat Variables: pool-riffle ratio, pool frequency, depth and substrate, habitat complexity, instream
<br />cover, riffle substrate quality, riparian vegetative cover, riffle embeddeness
<br />5. Ecological Variabies:fish diversity, macroinvertebratedlversity, index of biological integrity, EPA Rapid
<br />eioasessment Protocol, fish barriers, leaf pack processing rate.
<br />--1y~ ~~ ~a~l~_: ~J~S;~i~iT ate, i~~ SWI,?~-;[ ~~ L; III,Y~~° ~ 109
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