FLOOD06897

• County: Larimer
• Community: Wellington
• Stream Name: Boxelder Creek
• Basin: South Platte
• Title: Boxelder Creek Watershed Hydrologic Analyses
• Date: 12/1/1998
• Prepared For: FEMA
• Prepared By: UDFCD
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Attachment 3

.v~1
RECEIVED
heD~me2 1998

RegIonal Paleoflood Investigation to Assess Topographic Effects of
DivIde on flooding In Eastern Colorado

Robert D. Jarrell, U.S. Geological Survey, P.O. Box 25046, MS 412, Denver, CPlor.ld~~f.C.D.

ABSTRACT

There is substantial uncertainty and controversy in estimating flood magnitude and frequency,
particularly those of extreme floods, in the Rocky Mountain region. A regional paleoflood study
was conducted to assist dam-safety officials and flood-plain managers in assessing the risk of
large floods in eastern Colorado, particularly with respect to topographic effects on the spatial
variability of rainstorms and flooding. Regional envelope curves encompassing maximum gaged
and historical floods were developed for Palmer Divide streams. Paleoflood data for 134 sites
on streams draining from the Palmer Divide were used to document the largest floods that have
occurred during the last few hundred to several thousands of years and complement the regional
envelope curve of maximum flooding.

The approach was applied to Cherry and Box Elder Creeks, which drain northerly from the
Palmer Divide. Maximum paleofloods from rainfall in Cherry Creek range from about 30,000
ft3ls near Franktown to about ao,ooo ft3/S near Cherry Creek Reservoir. Maximum
paleofloods in Box Elder Creek range from about 9,000 ft3/s near Elizabeth to about a,700
ft3/s near Watkins (1-70). Maximum floods in Cherry and Box Elder Creek basins are
substantially smaller than floods on other streams draining from the Palmer Divide. Smaller
floods in these basins result from the topographic relief of the Palmer Divide and localized
areas of smaller topographic relief (less than about 300 ft in about 3 miles or less) that
substantially affects the spatial distribution of rainfall and maximum flooding in upslope and
lee-slope areas of the Divide. Study results demonstrate that the high plains of eastern
Colorado are not hydrologically homogeneous as has been long thought. The high plains needs to
be divided into at least two hydrologically homogeneous regions: basins having topographic
relief less than about 300 ft and more than 300 ft in less than about 3 miles distances. The
elongated shape and very high infiltration in sandy basins such as Box Elder Creek basin also
contribute to smaller maximum flooding. Flood-frequency relations for Cherry Creek, which
incorporate paleoflood data, indicate the 10,OOO-year flood ranges from about 44,000 ft3/S
(near Franktown) to about 75,000 ft3/S (near Melvin). Floods the magnitude of PMF values
for Cherry and Box Elder Creeks plot far in excess of a 10,000-year event. The maximum
paleoflood of about 75,000 ft3/S for Cherry Creek near Cherry Creek Reservoir is about 11
percent of the PMF of 662,000 ft3/s. Paleoflood data also were used to validate f1ood-
frequency relations for an ungaged basin such as Box Elder Creek. Important issues related to
these research results are: (1) large differences in flood hydrology for eastern Colorado using
different methods; (2) the major effects of topography, including local areas of relatively
small relief, and basin elongation on flood hydrology in eastern Colorado; and (3) the need to
improve the understanding of flood hydrometeorology in eastern Colorado, thereby reducing the
uncertainty of estimates of the magnitude and frequency of flooding. Future research in flood
hydrometeorology is needed to address these issues.

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