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i <br />j <br />j <br /><•» « <br />1 <br />1 <br />j <br />j <br />1 <br />j <br />10. The acre feet of storage required to prevent the flood <br />level fron rising nore than h foot. <br />11. The percentage of each narsh needed to control <br />flooding fron a 24-hour, 100-year storn. <br />Storn water runoff storage for each narsh was conputed <br />by assuning that the water in the narsh could rise <br />one-half foot during a storn. This rise would cover <br />both the pernanent and internittent narshlands. The <br />potential storage was then converted into a percentage <br />of the marsh needed based on the relationships of <br />drainage area, sub-watershed area, and sub-watershed <br />storage required. Therefore, the percentage of the <br />narsh required for storage indicates the pro rata area <br />required compared with the entire sub-watershed. <br />Using the phorphorus removal rates, the gross assimila­ <br />tive capacity v^as comouted (as shown in Table 2) and <br />compared with the pollution load for each area. <br />This was then converted into a percentage of the marsh <br />required to balance the phosphorus load fron the <br />drainage area. The values shown are based on the <br />assumption that the stormwater is introduced to the <br />narsh by overland flow through grassy areas and is <br />prevented..frcLn short circuiting to the outlet (if there <br />is one). The outlet from any narsh should not be improved <br />without careful study to assure that the detention storage <br />tine is not affected under normal operation. <br />The potential duck pond areas were calculated by <br />subtracting the beneficial effect or sufficient pernanent <br />water area with emergent vegetation to just balance the <br />assimilation capacity of the remaining narsh with the <br />•anticipated pollution load. It tvas assumed that the <br />normal duck pond construction procedure would be to <br />excavate in areas presently covered with shallow water. <br />If nornallv dry areas are utilized for this purpose, <br />only one-fifth of the stated area can be excavated. <br />d <br />I <br />a <br />1 <br />TJ VI-6