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3) The proposed storm sewer replacement could be installed as perforated pipe to enhance <br />infiltration with a control weir in the downstream structure. Based on hydraulic modeling <br />results, approximately 11,000 cubic feet of additional storage is needed. Several options were <br />considered to achieve this volume in the 280' of pipe planned to be replaced in options 2 and <br />4 down stream of Main Street. <br />Modeling was performed to test the sensitivity of a 48" diameter perforated pipe and <br />control weir. In order to maximize the potential infiltration volume, the weir in the <br />control structure would have to be set at the top of the 48" pipe. This does well for <br />the smaller events (less than 10-year) but does not mitigate peak flows for the larger <br />events (10-year and larger). <br />b. To mitigate the larger storm events, A 60" perforated pipe would be needed with <br />additional open graded rock storage 2.5' below and 2' on each side of the pipe with a <br />void capacity of 40%. <br />Table 3: Summary of Peak Discharges at Morton Ave Storm Sewer Outfall. <br />Rain <br />Event <br />Discharge Rate <br />Ex (cfs) <br />Option <br />1 <br />Options 2 <br />Option 3 <br />Prop <br />(cfs) <br />Diff <br />(cfs) <br />% <br />Increase <br />Prop <br />(cfs) <br />Diff <br />(cfs) <br />% <br />Increase <br />Prop <br />(cfs) <br />Diff <br />(cfs) <br />% <br />Increase <br />2-Year <br />3.0 <br />3.0 <br />0 <br />0% <br />2.5 <br />-0.5 <br />-16.7% <br />2.0 <br />-1.0 <br />-33.3% <br />10-Year <br />13.3 <br />13.3 <br />0 <br />0% <br />12.3 <br />-1.0 <br />-7.5% <br />1 12.8 <br />-0.5 <br />-3.8% <br />25-Year <br />20.3 <br />20.3 <br />0 <br />0% <br />18.9 <br />-1.4 <br />-6.9% <br />20.2 <br />-0.1 <br />-0.5% <br />50-Year <br />26.9 <br />26.9 <br />0 <br />0% <br />23.8 <br />-3.1 <br />-11.5% <br />26.9 <br />-0.0 <br />-0% <br />100-Year <br />1 33.9 <br />1 33.9 <br />0 <br />0% <br />27.8 <br />-6.1 <br />-18.0% <br />33.8 <br />-0.1 <br />-0.3% <br />V. Results <br />During this evaluation, several solutions to flooding were modeled and analyzed to minimize the <br />flooding potential. These generally include storm pipe size increases and adding catch basins and <br />manhole structures to maximize the system's capacity to reduce surface discharge. The sources of <br />flooding are multifaceted including potential pipe bottlenecks, reduced inlet capacity, and high <br />overland discharges. The study resulted in the following results. <br />1) The 10-year storm is a good target for improvements. It is an industry standard to provide <br />enough pipe capacity to manage a 10-year storm, and surface ponding should be minimized <br />(not always eliminated) to reduce the potential for hydroplaning. Based on this hydraulic <br />assessment, the current system meets the 10-year design standards for pipe capacity such <br />that the depth in the gutter stays below the top of curb (i.e. less than 0.5'). This depth results <br />in inundation across most of Morton Ave. Adding inlet and pipe capacity helps protect <br />Morton Ave to more frequently observed high intensity, short duration storms that have <br />return probabilities of 25-, 50-, and 100-year events. <br />2) The 12" bottleneck under Main St is not a preferred design. This has been a maintenance <br />issue, limits the upstream capacity of the system and causes the upstream pipe to surcharge. <br />At a minimum, this pipe should be upsized to match the upstream capacity and eliminate the <br />maintenance issue. The higher sloped 12" pipe downstream of Main Street could be replaced <br />to increase system capacity above the 10-year design. <br />Prepared by: Bolton & Menk, Inc. Results <br />Morton Avenue Drainage Study I OR11.127809 Page 6 <br />