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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Paleoflood data also were used to validate flood-frequency relations for an ungaged basin such as
Box Elder Creek developed using rainfall-runoff modeling. The 100-year flood for Box Elder
Creek ranges from about 5,700 ft3/S at Elizabeth to 7,900 ft3/s near 1-70 in Adams County.
Floods the magnitude of PMF values for Cherry and Box Elder Creeks plot far in excess of a
10,000-year event. Study results are consistent with flood-frequency analysis for other
streams in eastern Colorado. For example, flooding in Plum Creek in 1965 has been shown to
have a recurrence interval of at least 1,500 years (Osterkamp and Costa, 1987; Jarrett and
Costa, 1988) demonstrating the small frequency of the 1965 flood. Jarrett and Costa (1988)
summarize flood-frequency estimates for other extreme floods in eastern Colorado.

The maximum paleoflood of 74,000 ft3/S for Cherry Creek near Cherry Creek Reservoir is
about 11 percent of the PMF of 662,000 ft3/S. The maximum paleoflood of 8,700 ft3/s for
Box Elder Creek near Watkins is about 2 percent of the PMF of about 410,000 ft3/S. Several
recent paleoflood studies. primarily conducted by the U.S. Geological Survey and Bureau of
Reclamation (Jarrett and Costa, 1988; Jarrett, in review a, Levish and others, 1994; Ostenaa
and Levish, 1995), also demonstrate large differences between maximum paleoflood estimates
and PMF estimates in the Rocky Mountain region. Jarrett (in review a) found that maximum
paleofloods average about 7 percent of PMF values for streams at higher elevations in the Rocky
Mountains. Such large differences in estimates of maximum flooding demonstrate the need to
conduct additional hydrometeorologic research to reduce the uncertainty in design-flood
hydrology. Maximum rainfall and flooding in eastern Colorado are associated with areas of
high-topographic relief. Using extreme rainstorms and floods that have occurred in these areas
of high-topographic relief to assess maximum rainfall and flood potential in low-topographic
relief areas, which cover about 75 percent of the high plains of eastern Colorado, needs to be
assessed. Spatial variability of extreme rainfall affects not only peak discharge but also the
volume of flood runoff, which in turn affects f1ood-control storage requirements.

A statewide regional flood-frequency study for Colorado (Vaill, in review) is incorporating the
results of this paleoflood study and results from Thomas' preliminary flood-frequency
assessment (writ. commun., 1997) into the regional relations for eastern Colorado streams.
Results of the paleoflood study will help explain the spatial variability of flooding (to define
hydrometeorologic regions), and help identify additional explanatory variables used in the
regression models.

The important issues in flood hydrology related to these study 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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