07-22-1985 Council Packet - Laserfiche WebLink

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Quality Analysis Runoff Analysis Use Nutrients Generated Acres Present/Planned Present/Planned (lbs./year) Single Same 10 10 25 Family Hernes 2 acre Otx n Single 12 57.6 40 Space Family 1 acre Open Commercial 3 1 13.3 10 Space Present Watershed U Nutrient Loading 10 Ibs/year = 125 acres x ,4 Ibs/acre/year) 12 Ibs/year = (40 acres x ,3 Ibs/acre/year) 3 Ibs/year = (10 acres x .3 Ibs/acre/year) Total = 25 Ibs/year Planned Watershed Use Nutrient Loading 10 Ibs/year = (25 acres x ,4 Ibs/acre/year) 57.6 Itrs /year = (40 acres x .144 Ibs/acre/year) 13.3 ILs/year = (10 acres x 1.33 Ibs/acre/year) Tota' ) Ibs/year 'te ass,milativr capacity of the marsh is 4 pounds of nutrients her marsh acre per multiplied by 25 acres which equals 100 Ibs/year. ' Since the assimilative capacity of the marsh (100 Ibs/year) is greater than the planned loading (80.9 Ibs/year), the marsh may filed to some extent based on the quality analysis. The maximum acreage of fill may be determined by balancing the lost assimilative cap city per acre of fill (4 lbs/acre/year) plus the additional loading due to the use of the area (1.44 Ibs/acre/ year) with the surplus assimilative :apacity (100--80.9). The resulting equation is: F illable Acres = �1_00-�80.9j It>s/year (E ► 1.44) Ibs/acre/year Fillable Acres - 3.5 acres The Minimum Wetland Preservation Area for the assimilation of nutrient load is equal to the area of the marsh minus the area that can be filled (25 acres -- 3.5 acres) or 21.5 acres. Use Runoff (acre-feet) Acres Soil Type Present/Planned Present/Planned 25 Moderately Sloping Family Sandy Loam 2 acres Open Single 6 8 40 Flat Sand - Loam Space Family 1 acre Open Commercial 2.25 4 10 Flat Cray Space Present Watershed Use Runoff 5 acre-feet = (.4 x .5 feet rain x 25 acres) 6 acre-feet = (.3 x .5 feet rain x 40 acres) . 2.25 acre-feet = (.45 x .5 feet rain x 10 acres) r Total = 13.25 acre feet Planned Watershed Use .Runoff R=CPA 5 acre-feet = (A x .5 feet rain x 25 acres) 8 acre-feet = (.4 x .5 feet rain x 40 acres) 4 acre-feet = (.8 x .5 feet rain x 10 acres) Total = 17 acre-feet " Increased runoff due to the planned change in land use is: Increased runoff = 17 a.:re-feet - 13.25 acre-feet Increased runoff = 3.75 Assuming the marsh normally has water in it, the volume increase due to a storm is equal to the magnitude of the storm times the area of the marsh (C=1,0). Acre -Feet of Marsh Runoff (.5 feet of rain x 25 acres) 12.5 acre-feet • r , The Minimum Wetland Preservation Area to store stormwater runoff should be large enough to store the runoff due to the planned change in land use plus the marsh runoff (17 acre-feet 12.5 acre-feet) or 29.5 acre feet (approximately 1.2 feet rise in water level) of the marsh without causing flood damage around the marsh. The District generally advises 2' of freeboard between the 100 year pond level and the lowest basement entrance e'eva tiun, or that elevation where structutal dami(4e would occur.