FLOOD00714 - Laserfiche WebLink

Home Browse Search

be flowing full, such that any tributary runoff would flow across unimpeded, remaining within the same drainage basin. A conserva- tive procedure, the function of the ditch as a storm runoff convey- ance facility, was ignored. 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of the flooding from the sourceS studied were carried out to provide estimates of the eleva- tions of floods of the selected recurrence intervals. For the Uncompahgre River, the water-surface elevations of floods of the selected recurrence intervals were computed through use of the U.S. Army Corps of Engineers HEC-2 step-backwater computer program (Reference 9). The water-surface elevations on Montrose Arroyo were calculated as follows. Upstream of East Niagara Road, the 10-, 50-, 100-, and 500-year flood elevations were calculated using the U.S. Army Ccrps of Engineers HEC-2 step-backwater computer program (Reference 9). Downstream of East Niagara Road, the 10-year flood elevations were calculated using the HEC-2 computer program. No 50-year flood elevations were calculated. The culverts in this area are insuffi- cient to pass the 100-year flood. Therefore, 100-year water-surface elevations were calculated in the following manner. Proceeding downstream, calculations were made based on culvert rating curves and existing topography to determine the amount of flow lost from the channel at each insufficient culvert. Downstream of the culvert crossings, the discharge in the channel was reduced by the amount of flow lost from the channel. This procedure was repeated at each inadequate culvert crossing until the flow left in the channel had been reduced to a point where it could be handled by the remain- ing culverts. Using these reduced channel flows, flood elevations were determined from the rating curves at each crossing structure. Between crossings, the elevations were interpolated based on channel rating data (Reference 3). No 500-year flood elevations were deter- mined because the 500-year elevations would be nearly the same as the 100-year flood elevations. At insufficient culvert crossings on Montrose Arroyo, significant amounts of flow are diverted, resulting in shallow flooding through the streets. This type of flow diversion occurs at the 12th Street, 9th Street, 5th Street, 4th Street, 3rd Street, and 2nd Street culvert crossings. Between North First Street and Main Street, the first culvert is sufficient to pass the 100-year flood. However, the culvert under North First Street is not sufficient. Therefore, the 100-year flow backs up against North First Street, resulting in the isolated flooding between North First Street and Main Street. 12