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1 <br /> Table III-3 <br /> CORPS' MODEL PARAMETERS AND EFFECTS <br /> Typical Problem Effect on Flow Estimate <br /> Overland flow lengths Too long Under estimates flow <br /> Overland flow roughness Too low Over estimates flow <br /> ' Infiltration Initial rates too low Over estimates flow <br /> Main channel area lengths Too short Over estimates flow <br /> Depression storage in pervious Too low Over estimates flow <br /> area <br /> Percentage impervious Too low Under estimates flow <br /> Main channel depth before Too deep Over estimates flow <br /> ' overflow section used <br /> Channel roughness Not studied in detail <br /> ' Channel and overbank Not simulated Over estimates flow <br /> complete section and volume <br /> data for floodplain storage <br /> routing <br /> Rainfall 1 hour and shorter Values used are low and do Significant under estimation of <br /> increments not correspond to NOAA, flow <br /> UDFCD, or Boulder practice <br /> Area Missing basin areas at critical Significant under estimation of <br /> location above Highway 36 flow <br /> REVIEW OF CORPS' SWMM HYDROLOGY CALIBRATION <br /> ' We do not consider the adjusting SWMM model and other channel routing <br /> parameters to fit a statistical analysis of an interrupted 28 year record to be a <br /> calibration. <br /> A true calibration would be done by simulating an actual documented rainfall and <br /> runoff event, such as the 1938 or 1969 event, where an entire hydrograph was <br /> simulated and compared with measured flows indicated at gauges and highwater <br /> marks. At best, the Corps comparison to the gaging data at the mouth can only be <br /> ' taken as a reasonableness check at that point, and cannot be used to imply that <br /> estimations in the tributaries and streams in Boulder are correct. We believe that <br /> flood flows in Boulder tributaries will probably be higher than predicted in the <br /> Corps' model, and that significant routing effects will take place along the main <br /> streams, which is not adequately simulated. <br /> III - 8 <br />