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1 <br /> ' be missing substantial portions of western subbasins of what we refer to as Bear <br /> Peak basin (1068 acres), David's Draw (450 acres) and areas along the west <br /> ' overflow floodplain between Highways 93 and 36 (126 acres). It appears that the <br /> Corps was having difficulty with handling the overflow areas, and only a portion of <br /> this area (at least 368 acres) was included with the tributaries downstream of <br /> ' Highway 36. There is no basin that appears to simulate the area above Marshall <br /> Reservoir (719 acres). This may have occurred because of unique operating and <br /> ' location considerations which effectively would trap all local rainfall. This needs to <br /> be verified. <br /> The tributary watershed below Marshall has a model area that is somewhat smaller <br /> than the detailed delineation markup of the City's 1993 maps (87 acres). There is <br /> no basin to simulate the prairie hillside east of South Boulder Creek upstream of <br /> ' Highway 36. <br /> We have identified 167 acres of area which potentially contributed to both Bear <br /> Canyon Creek and South Boulder Creek. <br /> The net effect is that the Corps' model is missing from 2203 acres [3.44 square <br /> tmiles] to 1485 acres [2.32 square miles] (without Upper Marshall) to 1318 acres <br /> [2.05 square miles] (without Upper Marshall and joint Bear Creek/South Boulder <br /> ' Basins). The missing 2.05 square mile area is significant because it is steep <br /> watershed located such that it will add significant flow to the discharge arriving at <br /> Highway 36 and potentially spilling a large fraction (1/3 to 1/2) to the West Valley <br /> ' Overflow below Highway 36. Note that the total drainage area we have measured <br /> amounts to 27,009 acres [42.2 square miles] below Gross. This is significantly <br /> larger than that reported previously. <br /> ' RAINFALL <br /> 1 The Corps' documentation received indicated that the 6 hour rainfall had been <br /> taken from the U.S. Department of Commerce NOAA rainfall atlas. However, the <br /> Corps' departed from NOAA and their results do not agree with current local <br /> ' procedures because they used 1 hour rainfall values based upon their examination <br /> of South Platte events rather than NOAA values, and they did not define rainfall <br /> increments for durations less than 1/2 hour, based on a statement that such <br /> increments were not of realistic probability, even though the data is supported by <br /> NOAA. While the review of South Platte records may have validity for regional <br /> ' major stream flows at their mouths, we do not agree with the discounting of the <br /> NOAA 1 hour values for studies associated with large tributaries and urban runoff. <br /> If the NOAA 1 hour rainfall values were used, it would increase peak 1 hour values <br /> ' on the order of 0.67 to 0.72 inches over that used by the Corps. This would result <br /> in significant increases in flow. <br /> ' III - 3 <br />