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I I I I I I I I I I I I I I I I I I I Table 111-3 CORPS' MODEL PARAMETERS AND EFFECTS Typical Problem Effect on Flow Estimate Overland flow lengths Too long Under estimates flow Overland flow roughness Too low Over estimates flow Infiltration Initial rates too low Over estimates flow Main channel area lengths Too short Over estimates flow Depression storage in pervious Too low Over estimates flow area Percentage impervious Too low Under estimates flow Main channel depth before Too deep Over estimates flow overflow section used Channel roughness Not studied in detail Channel and overbank Not simulated Over estimates flow complete section and volume data for floodplain storage routing Rainfall 1 hour and shorter Values used are low and do Significant under estimation of increments not correspond to NOAA, flow UDFCD, or Boulder practice Area Missing basin areas at critical Significant under estimation of location above Highway 36 flow REVIEW OF CORPS' SWMM HYDROLOGY CALIBRATION We do not consider the adjusting SWMM model and other channel routing parameters to fit a statistical analysis of an interrupted 28 year record to be a calibration. A true calibration would be done by simulating an actual documented rainfall and runoff event, such as the 1938 or 1969 event, where an entire hydrograph was simulated and compared with measured flows indicated at gauges and highwater marks. At best, the Corps comparison to the gaging data at the mouth can only be taken as a reasonableness check at that point, and cannot be used to imply that estimations in the tributaries and streams in Boulder are correct. We believe that flood flows in Boulder tributaries will probably be higher than predicted in the Corps' model, and that significant routing effects will take place along the main streams, which is not adequately simulated. III - 8