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I I I I I I I I I I I I I I I I I I I be missing substantial portions of western subbasins of what we refer to as Bear Peak basin (1068 acres). David's Draw (450 acres) and areas along the west overflow floodplain between Highways 93 and 36 (126 acres). It appears that the Corps was having difficulty with handling the overflow areas, and only a portion of this area (at least 368 acres) was included with the tributaries downstream of Highway 36. There is no basin that appears to simulate the area above Marshall Reservoir (719 acres). This may have occurred because of unique operating and location considerations which effectively would trap all local rainfall. This needs to be verified. The tributary watershed below Marshall has a model area that is somewhat smaller than the detailed delineation markup of the City's 1993 maps (87 acres). There is no basin to simulate the prairie hillside east of South Boulder Creek upstream of Highway 36. We have identified 1 67 acres of area which potentially contributed to both Bear Canyon Creek and South Boulder Creek. The net effect is that the Corps' model is missing from 2203 acres [3.44 square miles] to 1485 acres [2.32 square miles] (without Upper Marshall) to 1318 acres [2.05 square miles] (without Upper Marshall and joint Bear Creek/South Boulder Basins). The missing 2.05 square mile area is significant because it is steep watershed located such that it will add significant flow to the discharge arriving at Highway 36 and potentially spilling a large fraction (1/3 to 1/2) to the West Valley Overflow below Highway 36. Note that the total drainage area we have measured amounts to 27,009 acres [42.2 square miles] below Gross. This is significantly larger than that reported previously. RAINFAll The Corps' documentation received indicated that the 6 hour rainfall had been taken from the U.S. Department of Commerce NOAA rainfall atlas. However, the Corps' departed from NOAA and their results do not agree with current local procedures because they used 1 hour rainfall values based upon their examination of South Platte events rather than NOAA values, and they did not define rainfall increments for durations less than Y:z hour, based on a statement that such increments were not of realistic probability, even though the data is supported by NOAA. While the review of South Platte records may have validity for regional major stream flows at their mouths, we do not agree with the discounting of the NOAA 1 hour values for studies associated with large tributaries and urban runoff. If the NOAA 1 hour rainfall values were used, it would increase peak 1 hour values on the order of 0.67 to 0.72 inches over that used by the Corps. This would result in significant increases in flow. 111-3