Agenda - 11-01-1993 - VIII-A - Laserfiche WebLink

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�- /- �. Excerpts: Watershed Management Plan: Lake Michie and Little River Watersheds Camp, Dresser and McKee, 1989 3.4 NONPOINT POLLUTION LOADING MODEL 3.4 .1 DELIVERY RATIO/TRAVEL TIME The nonpoint pollution loading factors shown in Table 3-2 represent measurements of loadings which have been discharged into a storm sewer, swale, or stream channel. For urban and agricultural land uses, sediment deposition during overland flow is already accounted for. Therefore, these loading factors represent discharges into the stream channels upstream of Lake Michie and Little River Reservoir. In large watersheds like Lake Michie and Little River, where maximum instream travel times are one day or greater, the storm event loadings discharged to streams are likely to be reduced (e.g. , sediment deposition) enroute to the reservoirs. Since large infrequent flood events can scour out stream beds and transport deposited pollutant loads downstream, some studies make the assumption that 100% of the nonpoint pollution loadings discharged into a stream will ultimately be delivered to the water supply reservoir. For the Lake Michie/Little River study,: we were requested to incorporate a pollutant delivery ratio into annual loading evaluations. - The delivery ratio method used in this study is an approximation of relative differences in pollutant delivery potential for various locations in the watershed. This pollutant delivery ratio method is consistent with the nonpoint pollution loading factors (Table 3-2) in that it only accounts for pollutant decay during instream transport to the water supply reservoir. Locational differences in the discharge of pollutants into streams are not accounted for, although land areas which adjoin a stream are likely to deliver a higher loading than land areas which are further removed from the streambank. Because there is insufficient monitoring data available in the literature on relative_differences in-pollutant loadings - for "onstream" and "offstream" sites, it is generally assumed that the available monitored loading factors (e.g. , Table 3-2) reflect an average of the two: In other words, within the single land use test watersheds which were monitored to develop these loading factors (e.g. , NVPDC, 1979; USEPA, 1983b) , it -is assumed that- the- areas adjoining the drainageway produced a -- 3-19