FLOOD09740

• County: Boulder
• Community: Boulder
• Stream Name: Sunshine Creek
• Basin: South Platte
• Title: Major Drainageway Planning Volume 1, Part A
• Date: 9/1/1969
• Prepared For: Boulder County
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Mr. Andy Briscoe
3 September 1969

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Hydrology

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A hydrologic analysis was undertaken to detemine the size and shape
of the IOO-year stom runoff hydrograph. The cri teri a presented in the
Denver Regional Council of Governments' Urban Storn [lrainage Criteria
Manual were utilized. The rainfall for the Sunshine Gulch drainage
basin is presented in Figure I. The Colorado ~rban ,'ydrograph Pro-
cedure "laS used \dth the CltlP <'rograr". The resulting computer print-
out is sh",m in Figure 2. Coefficients suitable for tnis rural type
basin were uti 1 ized.

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The resulting hydrograph shovln in tabular forn in Fiuure 2,.las then
plotted on graph paper and modifications 'Iere ;lade to account for the
numerous upstream ponding areas caused by undersized road culvert5.
and the fact that un~lanned storm runoff detention Iii I J occur Juring
the iOO-year sturm runoff.

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The resulting flood hydrograph, for design pur~oses, I.'as detemine,;
to have a volume of 160 acre-feet and a peak discharge of 1,000 cfs.

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An investigation of the sedir,lent probler.1s \,,!;ich !.ligl:t re~ult fro!.l
serious erosion in the upstream basin during a "iljOt. stom, indi:ateJ
that the nunlerous ponding areas vlould hold back rluch of the sediment
and boulders "Ihich often are carried dO"1n oy the flond ,.,ilters. Itu",ever,
deposition must be considered in any dO'..m strCJ~ channel rlar~njng and
for this reason the I'aster Plan is based upon design flO\.'s of ',isher-
than-usual velocities so that the sediClent car, be c~rried dO'.", ;nco
C001der Creek to a significant degree.

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Flood Plain

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/\ f;eld instrument survey ViaS performed on SUllshine Gulcil tu defir:c
cross sections at critical locations and to Jeterr,ine the aCLual slope
of the existing channel. The cross sections clearly indicated the
basic problem existing on Sunshine Gulch, i.e., the alluviur:: \"/hich
has been bui It up at the mouth of Sunshine Gulch has cO'lpletely el ini-
nated any semblance of a natural flood plain. The topography is such
tha t once "a te rs I eave the chanjf I they f I (J" to 10,.'- I Y i n9 U rounds
lying primari Iy to the east.

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The Farmers Ditch area has been si Ited and the original capacity of
the overshoot reduced sufficiently to Ciluse the lOa-year flood to
flO\/ into the ditch and to continue dovln the ditch to the first re-
striction which \~ould then cause it to overtop, flooding those areas

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