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Attachment 1 <br />Mid -Basin Injection Welis Project at Centennial Park <br />December 1, 2015 <br />Worksheet C: Capture Efficiency Method for Volume- Based, Constant Drawdown BMPs <br />Step T: Determine the design capture storm depth used forcalculaffng volume <br />1 <br />Enter design capture storm depth from Figure Ill, 1, d (inched <br />d= <br />0.75 <br />inches <br />Enter calculated drawdown time of the proposed BMP <br />2 <br />based on equotion provided in applicable BMP Fact Sheet, <br />T= <br />7 <br />hours <br />7- (hours) <br />Using Figure 111.2, determine the 'fraction of design capture <br />3 <br />storm depth" of which the BMP drowdown time (1} line <br />X, = <br />0.43 <br />achieves 80% capture efficiency, X, <br />4 <br />Enter the effect depth of provided HSCs upstream, a-C <br />(Inches) (Worksheet A) <br />dnsc= <br />0 <br />inches <br />5 <br />Enter capture efficiency corresponding to citisc, Yy <br />(Worksheet A) <br />Yr= <br />0 <br />Using Figure 111.2, determine the fraction of "design capture <br />6 <br />storm depth" at which the drnwdown time (T) achieves the <br />Xz= <br />0 <br />equivalent of the upstream capture efficiency(YZ), Xz <br />7 <br />Calculate the fraction of design volume [hot must be <br />{radian= <br />0.43 <br />provided by BMP, fraction =Xi - Xz <br />8 <br />Calculate the resultant design capture storm depth (inches), <br />fraction x d <br />dt n " "— <br />0.32 <br />inches <br />Sfep 2; Calculate the DCV <br />I <br />Enter Project area tributary to BMP (s), A (acres) <br />A= <br />0, 12 <br />acres <br />2 <br />Enter Project Imperviousness, imp (unitless) <br />impm <br />0,61 <br />3 <br />Calculate runoff coefficient, C= (0,75 gimp) + 0.15 <br />C= <br />0.6075 <br />Calculate runoff volume ,Vm,,s„ = xAx43560x <br />4 <br />(1 /i2)} <br />Ve�g,= <br />84,1 <br />"fl <br />supporNirtg Calculations <br />Describe system, <br />Permeable Concrete Sidewalk with 6 -Inch rock reservoir, which provides an effective depth of 0, 175 <br />fl. Therefore: <br />BMP Surface Area Needed = 84.1 cu -ft / 0. t 75 ft = 480.8 square feet. <br />Provide drawdown time calculations per applicable BMP Pact Sheet: <br />Drowdown time is based on 6 -Inch oggregafe base depth for retention of the DCV in the permeable <br />concrete sidewalk section. At 35% porosity, fhe effective depth is 2,1 inches or 0. f 75 feet, which will <br />drawdown In 7 hours with a design Infllfro4on rate of 0.3 inches /hour. <br />((0.5 ft " 0.35)'12 in.) / 0.3 In /hr —7 hours. <br />Orange County Water District 13 <br />City of Santa ,Ana <br />25C -390 <br />