|
<br />topographic maps (Reference 17), and rainfall distri ution values
<br />were taken from the National Oceanic and Atmospheric Administration
<br />Atlas for Colorado (Reference 18).
<br />
<br />Discharges for Fish Creek and Fall River were comput d based upon
<br />records of stream gages located on the two streams. The gage on
<br />Fish Creek has 30 years of record, and the Fall Rive gage has 9
<br />years of record. These records were analyzed using log-Pearson
<br />Type III distribution as recommended in u.s. Water R sources Council
<br />Bulletin 17 (Reference 9). These discharges were we'ghted with
<br />those obtained using regression equations from the C lorado Water
<br />Conservation Board's Technical Manual No.1 (Referen e 8).
<br />
<br />'"
<br />
<br />...
<br />
<br />Peak discharges for the 10-, 50-, and 100-year flood
<br />Cache La Poudre River were provided by the u.s. Army
<br />Omaha District, (Reference 5). The 500-year discharg
<br />by straight line extrapolation of the 10-, 50-, and
<br />provided by the u.s. Army Corps of Engineers.
<br />
<br />along the
<br />Corps of Engineers,
<br />s were estimated
<br />OO-year floods
<br />
<br />f^
<br />
<br />Peak discharge-drainage area relationships for Big Tompson River,
<br />North Fork Big Thompson River, Big Thompson River Tr'butaries 1, 2,
<br />3, and 4, Long Gulch, Rabbit Gulch, Quillan Gulch, W st Creek, Fox
<br />Creek, Dark Gulch, Cedar Creek, Devils Gulch, Bobcat Gulch, Buckhorn
<br />Creek, Redstone Creek, Fish Creek, Fall River, Dry G lch, Noel's
<br />Draw, Dickson Gulch, Miller Fork, Dry Creek, Box Eld r Creek, and
<br />Cache La Poudre River are shown in Table 1.
<br />
<br />3.2 Hydraulic Analyses
<br />
<br />Analyses of the hydraulic characteristics of streams in the community
<br />were carried out to provide estimates of the elevati ns of floods of
<br />the selected recurrence intervals along each stream tudied in the
<br />community.
<br />
<br />Cross sections for the Big Thompson River and its tr'butaries were
<br />taken from topographic maps at a scale of 1:2400,wit a contour
<br />interval of 2 feet (References 19, 20, and 21, respe tively). These
<br />tributaries include the North Fork Big Thompson Rive , Big Thompson
<br />River Tributaries 1, 2, 3, and 4, Long Gulch, Rabbit Gulch, Quillan
<br />Gulch, West Creek, Fox Creek, Dark Gulch, Cedar Cree , Devils
<br />Gulch, Bobcat Gulch, Dry Gulch, Noel's Draw, Dickson Gulch, and
<br />Miller Fork. Bridges and culverts were measured to etermine size
<br />and structural geometry.
<br />
<br />Roughness coefficients (Manning's "n") were estimate by field
<br />inspection. Channel values ranged from' 0.035 to 0.0 5 and overbank
<br />values ranged from 0.050 to 0.060.
<br />
<br />
<br />Water-surface elevations were computed using the u.s. Army Corps of
<br />Engineers' HEC-2 computer program (Reference 22). W ter-surface
<br />profiles for the lower portion of the Big Thompson R'ver (near
<br />
<br />12
<br />
|