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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estimates developed for areas east of the 105th meridian (Schreiner and Riedel, 1978), which
is about the longitude of Denver, another PMF of 524,000 ft3/S was estimated (Corps of
Engineers, written commun., 1997). Flood-frequency relations for Box Elder Creek have been
of concern to the Urban Drainage and Flood Control District (UDFCD), Federal Emergency
Management Agency (FEMA), and other water-resources agencies. Flood-frequency relations
estimated with different methods in eastern Colorado have large differences. For example, the
100-year flood for Box Elder Creek near Watkins at 1-70 ranges from about 7,900 ft3/s (U.S.
Army Corps of Engineers, 1990) to 26,000 ft3/s (FEMA, 1993). The larger estimate was
based on regional flood-frequency methods developed by McCain and Jarrett (1976), which
have been shown to overestimate flood-frequency relations in eastern Colorado (Livingston and
Minges, 1987; Will Thomas, Michael Baker, Jr., Inc., written commun., 1997). Differences
in flood estimates such as for Cherry and Box Elder Creeks demonstrate the importance of
reducing the uncertainty in estimating the magnitude and frequency of flooding in eastern
Colorado. Perhaps the greatest cause of these uncertainties is the lack of data on extreme
flooding in eastern Colorado.

METHODOLOGY

Paleoflood Investigations

Paleoflood hydrology is the study of past or ancient floods (Baker, 1987). Floods leave
distinctive deposits and landforms in and along stream channels, as well as botanic evidence
(Jarrett, 1990, 1991; Hupp, 1988). Slack-water deposits of sand-sized particles, flood
scars on trees, accumulation of woody-flood debris, erosion scars, and bouldery flood-bar
deposits commonly used as indicators of past flood levels are called paleostage indicators
(PSis). When flows are large enough, streambed and bank materials are mobilized,
transported, and deposited as PSis. The types of river sites where flow competence decreases
and flood deposits commonly are found and studied include: (1) locations of rapid energy
dissipation, where flood sediments would be deposited, such as tributary junctions, reaches of
decreased channel gradient, abrupt channel expansions, or reaches of increased flow depth; (2)
ponded areas upstream from channel contractions; and (3) locations along the sides of valleys
in wide, expanding reaches where fine-grained sediments or slack-water deposits would likely
be deposited. Paleoflood data are particularly useful in providing probable upper limits of the
largest floods that have occurred in a river basin (Jarrett and Costa, 1988; Jarrett, 1990, in
review a; Enzel and others, 1993; Jarrett and Waythomas, in press).

Paleoflood discharge was determined from estimates of flood width and depth corresponding to
the height of PSis and channel slope for each cross section obtained during onsite visits to
streams. The slope-conveyance method (Barnes and Davidian, 1978) was used to estimate
paleoflood discharge. Flow-resistance coefficients and velocity were estimated from analysis of
data for Colorado rivers (Jarrett, 1985). Analysis of the differences in PSis and highwater
marks (HWMs) of extreme floods in the past 3 years for 122 sites in 80 streams of the
western United States indicated that the elevation of the top of flood-deposited sediments (PSis)
generally are within +/-0.2 ft of flood HWM elevations (Jarrett, in review b). Therefore, use
of the top of flood-deposited sediments as PSis for streams in this study provides a reliable
estimate of the maximum paleoflood depth that is then used to reconstruct the discharge of
paleofloods.

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