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r <br />1 <br />i « <br />J <br />1 <br />1 w <br />1 <br />1 <br />■J <br />.1 <br />i <br />The Point Runoff Hydrographs were multiplied by <br />the specific acreages and combined to generate <br />a single point hydrograph for the sub-watershed <br />area. The slope, distance, and flowing comfigura <br />tion were considered in calculating the time of <br />concentration to collect the runoff at a common <br />point to form the Runoff Hydrograph. <br />An equation for the storage area for each sub­ <br />watershed was developed to relate the storage <br />avaialble at elevations above normal water level. <br />The outflow equations were derived for the dis­ <br />charge from the storage area to relate outflow <br />rate to elevation above normal. The runoff was <br />then put into the storage area and the rise in <br />water level and outflow were calculated for each <br />time increment. This produced the Outflow Hydro­ <br />graph which was then routed through the channel <br />to the next sub-watershed storage area with the <br />timing and flow dependent on the slope and con­ <br />figuration of the channel. <br />A summary of the information generated is given <br />in Table 1, Simulated Storm Water Runoff Hydrologic <br />Data. _ _ _ _ _ _ _ <br />B. Water Quality Considerations <br />The following method was used to analyze storm water <br />runoff quality utilizing natural means for filtering <br />and improving the quality of the water. <br />One means for evaluating water quality is the quantity <br />of algae in the lake. The exact quantity of algae <br />present in a lake is a complex function of many environ­ <br />mental parameters, but the size of the population is <br />closely related to the rate at which nutrients are <br />supplied. Nutrients are supplied at varying rates, <br />and the single nutrient supplied at the slowest rate <br />relative to need exerts a controlling effect on algae <br />growth. <br />The Minnesota Pollution Control Agency study titled <br />"A Program for Preserving the Quality of Lake ilinnetonka <br />determined that phosphorus was the limiting nutrient <br />in Lake Minnetonka. <br />Pollution loads to the marshes and ponds were determined <br />based on phosphorus as the limiting nutrient. These <br />loads took into consideration tributary acreage and <br />potential development on the <br />Maximum phosphorus loading comes f’ <br />residential.development and is 3.46 Ibs/acre/year. <br />VI-4