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<br />West Creek, Fox Creek, Dark Gulch, Cedar Creek, Devils Gulch, Bobcat Gulch, Dry <br />Gulch, Noel's Draw, Dickson Gulch, and Miller Fork. Bridges and culverts were measured <br />to determine size and structural geometry. <br /> <br />Roughness coefficients (Manning's "n") were estimated by field inspection. Channel values <br />ranged from 0.035 to 0.045, and overbank values ranged from 0.050 to 0.060. Water- <br />surface elevations were computed using the US ACE HEC-2 computer program (Reference <br />22). <br /> <br />Cross sections for the backwater analysis of the Big Thompson River in Loveland were <br />obtained from photogrammetric topographic maps. Below-water sections were obtained <br />by field surveys. Bridges and culverts were surveyed to obtain elevation data and <br />structural geometry. <br /> <br />Cross sections for the backwater analysis of Dry Creek - BTR were obtained from the <br />topographic mapping. The two cross sections at the upstream end of the study reach <br />required supplemental station-elevation data at their far ends. The supplemental data were <br />obtained from an earlier study. <br /> <br />Water-surface elevations on both the Big Thompson River in Loveland and Dry Creek - <br />BTR for floods of the selected recurrence interval were computed using the USACE HEC- <br />RAS version 3.1.2 step-backwater computer program. Starting water-surface elevations <br />for the Big Thompson River were taken from the 1977 study of the Big Thompson River <br />prepared by the U.S. Army Corp of Engineers. Starting water-surface elevations for Dry <br />Creek - BTR were specified as normal depth with a friction slope of 0.005 feet/foot. <br /> <br />Channel and overbank roughness factors (Manning's "n" values) used in the hydraulic <br />computations were chosen by engineering judgment and were based on field observations <br />of the stream and floodplain areas. Surveyed high water marks and anecdotal evidence of <br />flood stage from flood that occurred on the Big Thompson River between April 30th and <br />May 1 st of 1999 were used to approximately calibrate the "n" value in the main channel of <br />the Big Thompson River. Horizontal variation was applied to the "n" values in the <br />overbanks and main channels throughout the study reaches. Overbank "n" values varied <br />from 0.013 (wet ponds) to 0.20 (dense vegetation). At some cross sections the overbank <br />"n" values were artificially set higher than 0.20 in order to maintain reasonable continuity <br />of flow in the overbanks between adjacent cross sections. Main channel bank "n" values <br />ranged from 0.013 (bike trails) to 0.08, and main channel bed "n" values ranged from 0.03 <br />to 0.035. <br /> <br />Cross-sections for the Big Thompson River in the area near the Town of Johnstown were <br />taken from existing topography from Weiland, Inc. Bridge information and road geometry <br />were based on survey information, field inspections, and construction plans. <br /> <br />Water-surface elevations for the Big Thompson River around Johnstown were computed <br />using the USACE HEC-RAS computer program. Starting water-surface elevations were <br />taken from the Weld County FIS and the Weld County Floodplain Information Report. <br /> <br />Manning's "n" roughness coefficients were estimated based on field inspection and <br /> <br />22 <br />