FLOOD01741

• Designation Number: 390
• County: Weld
• Community: Unincorporated Weld County
• Stream Name: Sheep Draw, Cache La Poudre River Basin
• Title: Floodplain Analysis - Special Study, Volume III, Sheep Draw, Cache La Poudre River Basin, Larimer-Weld Counties, Colorado
• Date: 10/1/1981
• Designation Date: 10/1/1981
• Floodplain - Doc Type: Floodplain Report/Masterplan

Table 1
FerceDt of Imperviouaness for Land Uaes
in the Sheep Draw Basin

fLOOD PROBLEMS

Lsnd !Jse

Impervious Area
(percenr)

Flood history is not readily available for Sheep Draw as the basin
hnsbeenajMrselydeveloped. Floodexpedenceinthearealndlcatea
that snowmelt flooding is uncommon and that Summer cloudbursta would be
the most likely sourCe of flooding. Flood damages are also possible in
the event of large amounts of runoff from tribotariea or small dralnage-
ways flowing into Sheep Draw. There are no flood control works 1n the
bssin. Small irrigation dams, irrigattoncanals, and roadcroaaing
structures provide some incIdental flood discharge reduction, as
discussed in the hydrologic analyais.

CCII.u'.ercts.l
Medium Density Residential
Agricultural

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The land use patterns were slightly altered to fit hydrologic sub-
areas in the hydrologic model. To depict urban land use 8S actually
used in the hydrologic model, areas with approximately 40 psrcent Or
more imperviousuess are indicated on plate 3. Any changes in these
lsnd use projections and future urbanlsation boundaries would change
the hydrologic and hydraulic data presented in this study.

ALTERNATIVE LAND USE CONDITIONS

The Sheep Drsw flood hazard was evaluated under future as well as
existing conditions since the Sheep Draw basin is subject to increasing
urban develop~nt. Changes in Imperviousness that would result from
land use chs.nges will affect the runoff potential. To reflect the
influence of changing development, three levela of urbanization were
considered. Bxisting and future urbanization are shown on plate 3.
Two of these urbaolsation levels, existing and projected, are ahown in
the report entitled Larimer-~eld Region Land Use Alternatives prepared
for the Larimer-Weld Regional Council of Govern~pnts in November 1977.
The existing urbanisation reflects no significant urban development.
Projected urbanization Is based upon year 2000 land use. ThIs level of
urbanization repreaents an intermediate level of development in that
11,,, <l""~Llbtr"."s I,..H or the Sh""p Dr..... n..~lLl b ur"""h"d. Tutal

HYDROLOGIC ANALYSIS

A hydrologic analysis was carried out to establish the peak dla-
charges for flood. of variou5 frequencie.. Ploods with lO-, 50-, 100-,
snd SOO-year recurrence intervals are presented in this report. A
detailed deacriptlon of the hydroiogic analyais is contained in Volume
II of this study.

Stream gaging records are not available for Sheep Draw. BFA's
SWMM ...as used to model rainfall - runoff chsracteristics of the baain.
Basin characteristics needed for the model were taken froa U.S. Geolog-
ical Survey (USGS) 7.5-Minute Quadrangle ~pping having s scale of
1:24 000 wIth a contOur Intervsl of 10 feet. Rainfall vslues for
,

urbanhation aSSUIIIS9 the entire baatn 18 urbanized.

I-hour sto~ eventa of vadous frequency "en. oOl..l"e,j f,,,,,, lh"
PrecIpitation-Frequency Atlaa of the Western United Statea, Atlas 2,
Volume III, Colorado, published by the National Oceanic and Atmospheric
AdminiatratIon (NOAA) in 1973. The SOO-year rainfall value ...5&
extrapolated from the lOO-year and more frequent eventS. Expected

The percent of imperviousness for the land areas WaB eatimated for
existing, projected urbanization. and total urbanization conditiona.
Table 1 lists the percent of imperviousness for esch land uae that Was
conaidered In the hydrologic model.

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