FLOOD10617 (2) - Laserfiche WebLink

Home Browse Search

place but the deck removed. Head losses at fully obstructed bridges were determdned by weir computations. Unobstructed bridge losses were computed by using the no~l bridge routine in HEC-2. Roughness coefficients (Manning's "nil) were estimated by field investigation using a paper by V.V. Golubtstov (Reference 48). The roughness values for the main channels ranged from 0.030 to 0.750, and for the overbanks from 0.060 to 0.100. Water-surface elevations for James Creek were started at normal depth. Water-surface elevations for Little James Creek and Unnamed Tributary to Little James Creek were started at their respective confluence elevations resulting from coincident discharges. Cross section data for Coal Creek and Rock Creek were taken from 1974 photography and mapping of the study area (Reference 19). All bridges and culverts were surveyed to obtain their elevation and size. The no~l openings of the bridges and culverts were used to determine flood data at road crossings. Typical Manning's "nil values for the channel and flOOdplain range from 0.085 to 0.120 (Reference 19). Starting water-surface elevations were determdned by the SCS (Reference 19). Using stream slope at a starting valley section, water-surface profiles for the 10-, 50-, 100-, and SOO-year floods were developed using the SCS WSP2 and TR20 computer programs (References 3S and 36, respectively). Water-surface elevations of floods of the selected recurrence intervals were computed through the use of the USACE HEC-2 water-surface profiles computer program (Reference 46). Starting water-surface elevations for Lefthand Creek, Dry Creek No.1, and Spring Gulch correspond to the computed water-surface elevations for the St. Vrain Creek at the confluence of the three streams. The flooding in Loamiller Basin is in the fo~ of sheet runoff, in which velocities are low, in depths less than 1.0 foot. The 50-, 100-, and SOa-year floods along Dry Creek No. 1 are not contained by the Dry Creek No. 1 channel. Excess flows for these frequencies will be directed north to St. Vrain Creek along most of the study reach. The water-surface profiles for this stream were therefore computed by no~l calculation methods using control sections at culverts to reflect this condition. Detailed cross section data for the St. Vrain Creek were obtained from the USACE and supplemented with additional cross sections taken from maps at a scale of 1:4,800, with a contour interval of 2 feet, also prepared by the USACE (Reference 6). Detailed cross sections for Lefthand Creek, Dry Creek No.1, and Spring Gulch were field surveyed in September 1975. The cross sections were located at close intervals above and below bridges and culverts in order to accurately compute backwater effects at these structures. USGS topographic mapping enlarged to a scale of 1:6,000, with a contour interval of 10 feet, was used to supplement field-survey data (Reference 49). 45