FLOOD04884

• County: Moffat
• Community: Craig
• Stream Name: Yampa River, Fortification Creek
• Basin: Yampa/White
• Title: Flood Hazard Information Hydrology
• Date: 9/1/1976
• Prepared For: State of Colorado
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Th.~ curve on chart 5 was prepared from this data and used to determine
the mean peak snowmelt flow at the ungaged locatio,ns. Suff:ld.ent streamflow
dal:a is available for both the Yampa River at Crai,g (index pcd.nt 12)
and Fortification Creek (index point 1) to define the mean pE""k flow
at these points. Adopted standard deviations and skel's for the
snowmelt portions of the frequency curves are listed in the preceeding
table.

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The rainflood portion of the flow frequency curves for thE! Yampa
River (index points 11 and 12) are graphica.l curve,s that werE' drawn by
pl"tting the computed SPF (see paragrap.h 7) at all exceedenc," interval
of 500 years. This exceedence interval is consistent with a plotting
of the SPS storm amount on the Craig l-.day precipi.tation frequency
curve, chart 6.

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Therainflood portions of the flow-.fre.quency curves at thE! other
index points were prepared from the synthetic cloudburst flood
computations discussed in paragraph 7. Cloudburst SPF' s werE' plotted
at an exceedence interval of 500 years to be consistent with a plotting
of the SPS 3-hour storm amount on the Craig 3-hour preci.pitaU.on
frequency curve, chart 7. The cloudburst and snowmelt frequE,ncy curves
were combined statistically to determin.~ the final flow-frequE!ncy curves"

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Typical flow-frequency curves are a.hown. on ebB.rts 8, 9, 10.

7. Synthetic rainflood analysis -

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a. Selection of types of floods- A ge,neral rain storm WllIE' selected
for use in the computation of a SPF on the Yampa River since the 1921
and 1974 floods resulted from snowmelt llugmented substantially by
ra:lnfall. This is consistent with earlier SPF cOllllputations for the Yampa
River at Steamboat Springs. A 3-hour dura.tion cloudburst thunderstorm
was selected for the computation of rainfloods on Fortification Creek,
Pille Ridge Gulch, Cedar Mountain Gulch and the other small tl'J.butaries
around Craig. A cloudburst is more cri.tical than a general ndn type
storm on these smaller areas.

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b. Storm amounts - A standard project general rain storm
(72 hour duration) equivalent to 35 percent of PMF' was selected for the
Yampa River basin. PMP amounts were fUJeni.shed by the Hydrometeorlogical
Branch of the National Weather Service for the mon.ths of June and
August. Average SPS precipitation over thE' Ya.mpa River basin above
Craig for the June and August storms was 5.60 and 4.87 inches,. respectiv"ly.
A check of the reasonableness of th"se SPS amounts was made b)r comparing
24 hour duration SPS point amounts at Craig with t.he histor'ic:al daily
precipitation amounts at Craig. This c.cJmparison. shown on Chart 6,
indicates the amounts are reasonable.

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