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NORTHEASTERN AREA <br />Tc = 8.0 minutes <br />Q1 = CIA = 0.86 x 3.8 in/hr x 2.3 acres = 7.5 cfs <br />Q100 = 0.86 x 9.0 x 2.3 = 17.8 cfs <br />PROPOSED SITE DEVELOPMENT <br />The owner proposes to expand the northwestern building by approximately 72,200 <br />sq. ft., add a new railroad spur to the facility, curb and gutter the perimeter <br />of the site and pave the existing gravel and oil parking area to the east. The <br />only drainage area which will have an in reased flowrate will be the eastern one, <br />where the gravel and oil surface will be paved. The remaining drainage areas will <br />have essentially the same f lowrates after development - although the actual flow <br />route may change slightly due to the proposed curb and gutter. <br />The developed runoff rate f or.the 1-year and 100-year event using the rational <br />method is: <br />EASTERN AREA <br />Tc = 11.5 minutes <br />Q1 = CIA = 0.90 x 3.2 in/hr x 3.1 acres = 8.9 cfs <br />Q100 = 0.90 x 7.6 x 3.1 = 21.2 cfs <br />STORAGE REQUIREMENTS <br />To meet the watershed's requirements for surface water drainage the developed <br />runoff rates for the developed site can not exceed the undeveloped runoff rates <br />for the 1-year and 100-year events. The excess runoff must be temporarily stored <br />in a detention basin. As already noted, the only drainage area with an increased <br />flowrate is the eastern area. It is proposed to construct a small depression at <br />the southern edge of this drainage area to store the excess flowrate. The peak <br />discharges will be controlled by a two -staged outlet structure attached to the <br />skimmer. <br />The total runoff of the Eastern Area using the SCS method is: <br />Surface soils ranked as Urban Land (bituminous cover) <br />Curve Number = 98 <br />Precip. of 1-yr., 24 hr. = 2.35 inches, Runoff (CN=98) = 2.13 inches <br />Precip. of 100-y r., 24 hr. = 5.9 inches, Runoff (CN=98) = 5.67 inches <br />Volume of 1-yr Runoff = R1 = 2.35 inches / 12 in/ft x 3.1 acres = 0.61 acre ft. <br />Volume of 100-yr Runoff = R100 = 5.67 inches / 12 in/ft ti 3.1 acres = 1.46 acre ft. <br />Storage Req'd. for 1-yr = S1 = Rl (Q1 - Q1*) 2 = 0.61 ac. ft (8.9 - 7.4) 2 <br />Q1 8.9 <br />= 0.017 ac. ft. <br />Storage Req'd for 100-yr = S100 = R100(Q100 - Q100*)2 = 1.46 ac. ft. (21.2 - 17.9)2 <br />Q100 21.2 <br />= 0.035 ac. ft. <br />Required storage capacity of detention basin = 0.035 acre ft. <br />Q* = predeveloped Q <br />I <br />I <br />i <br />i <br />