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Kimley>»Horn <br />4. Hydrologic and Hydraulic Design Criteria <br />Hydrologic Design Criteria <br />A 100-year storm event was used to model the peak flow rates since the existing storm <br />drain systems are modeled using the 100-year storm as a design criterion. Soil type B <br />underlies the Site. The developed "DMA" areas are modeled at 90% impervious. AES <br />software was used to determine both the existing and proposed condition peak flow <br />rates. The site was modeled by adding area to the main line at the mainline TC rather <br />than modeling all connections as confluences. <br />Pipe Hydraulic Design Criteria <br />Hydraulic calculations were performed for the main storm drain pipes utilizing <br />Flowmaster, developed by Bentley. The software utilizes Manning's equation to <br />determine acceptable friction slopes for design. The downstream hydraulic grade lines <br />are based on hydraulic grades lines depicted on the City as-builts are low enough to <br />utilize Flowmaster estimate pipe sizes. An allowable friction slope of 0.3% was used to <br />keep the hydraulic grades below ground surface. Catch basins will be sized to <br />accommodate the 100-year storm (see appendix 5). Underground storm drains should <br />be sized to accommodate the 10-year storm event with a water surface below top of <br />curb as outlined in the Orange County Local Drainage Manual <br />Allowable Connection Flow Rates <br />The proposed flow rate at the existing confluence points will not exceed the existing <br />Q100 flow rate for each drainage area by more than 5%. This is consistent with the <br />hydromodification criteria listed in the Orange County Technical Guidance Document for <br />Water Quality Management Plans. <br />For individual projects, the future individual hydrology studies should demonstrate that <br />the flow rates do not exceed the flow rates in this master plan study by more than 5%. <br />The subarea peak flow rates should be used for this comparison, not the main line <br />added flow rates. <br />5. Conclusions <br />A comparison table of the allowable and proposed flow rates follows: <br />Drainage Area <br />100-Year Storm <br />Existing Condition <br />100-Year Storm <br />Proposed Condition <br />Difference <br />Discharge A <br />83 CFS <br />82 CFS <br />-1.2% <br />Discharge B <br />42 CFS <br />43 CFS <br />+2.4% <br />Page 8 of 14 <br />