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. . . . . . . . . . . . . . . i. , . . . . . . . . '. . . . ,. '. , . . . . . . . . . . . . .. . . I. . . . . I. . . . . LA STUDY AREA DESCRIPTION Community Description The Town of Granby is located in the central portion of Grand County, Colorado. Grand County is bordered by Jackson and Larimer Counties to the north, Boulder and Gilpin Counties to the east, Clear Creek and Summit Counties to the south and Routt County to the west. The population of Granby is approximately 1,000 full-time residents while the population of Grand County is approximately 10,519. The Town of Granby is located on the west side of the Continental Divide at an elevation of 7,935 ft. It lies near the intersection of Colorado State Highways 34 and 40 approximately 10 miles southwest of Lake Granby and the southern boundary of Rocky Mountain National Park. Watershed Description The Town of Granby is located in the Fraser Valley along either side of the Fraser River. The Fraser River is a tributary of the Colorado River located approximately 1.5 miles west of town along Highway 40. The August 15, 1975 FHBM includes approximate floodplain mapping for three flooding sources with relatively small drainage areas. The western most floodplain has been realigned due to development and is modified as shown herein. The two eastern-most floodplains are non-existent due to development. A portion of these floodplains have been modified as shown herein. The flooding sources are tributary Smith Creek northwest of town, which is subsequently tributary to the Colorado River just upstream of its confluence with the Fraser River. Their drainage areas are comprised of that area within and north of the Town between Highways 34 and 40. Most of the watersheds are within the urbanized areas of the town. A portion of the northern watershed is agricultural. The Town primarily sits on the south face of a small bluff. Consequently, much of the Town is "terraced" on the bluff resulting in steep topography changes from north to south. F1oodin!! Problems and F1oodin!! Historv Floods in the subject watersheds normally occur during the late summer and early autumn and are caused primarily by intense localized cloudbursts or thunderstonns. Typically, these floods have high peak discharges < with low volumes over short durations and there is usually a very short window of opportunity for flood warning and evacuation once a major stonns hits the area. Less damaging floods along more major rivers can also result from rapid melting of mountain snowpack in the spring and early summer months. A combination of rainfall and snowmelt can sometimes prove to be the worst flooding scenario for Colorado's mountain streams. Flooding within the County and Town is not well documented. The most significant recent stonn event occurred on July 19, 2000, when approximately 3.5 inches of rain fell in one hour over the Town, which is significantly higher than the 100-year precipitation event. Damage was primarily limited to flooded basements and erosion of the terraced banks. Previous Studies A "General Drainage Report for Granby High School and Middle School Campus" was completed in March, 2001. This report developed hydrology and proposed improvements for the drainage in and immediately downstream of the campus. At the outfall of the campus basin, the 2