FLOOD09424

• County: Mineral
• Basin: San Juan/Dolores
• Title: Interdisiplinary Paleoflood Assessment of Questionable Extreme Rainstorm and Flooding and their Implications to Flood-Hazard Management
• Date: 2/1/1996
• Prepared For: Mineral County
• Floodplain - Doc Type: Flood Documentation Report

Creek near Trinidad. All these storm are commonly used to define flood-frequency relations, which
overestimate flood potential (Jarrett, 1993), and design of hydraulic structures located in floodplains.
Doesken (Colorado Climate Center, personal commun., 1996) indicated that the majority of the largest
point-rainfall values for the largest storms in Colorado raise questions as to their validity and accuracy. He
also found that the maximum values at many individual many gages are questionable. Doesken concurs
that a reassessment of the accuracy and validity of rainfall data.

Colorado extreme-rainfall and flood data are transposed and used in hydrometeorologic investigations
in other Rocky Mountain states. Similarly, extreme rainstorms that have occurred in other Rocky Mountain
states are used for design hydrology in Colorado. Thus, use of questionable extreme-rainfall data affects
design hydrology throughout the Rocky Mountain region. As a result, a reliable and hydro meteorologically
acceptable data base of extreme precipitation for Colorado and the Rocky Mountain region is being
developed.

A comprehensive compilation of extreme-precipitation and flood data for Colorado and an evaluation of
their reliability currently is being conducted by the Colorado Climate Center for the State Engineer.
Because extreme-value data has the greatest role in the design of hydraulic structures and for flood-plain
management, ascertaining its accuracy and validity is critical to any subsequent use of the data. This is
true whether the data is used to define enveloping CUlVes of maximum rainfalVfloods, for statistical
analyses, risk-based design, or in deterministic rainfalllhydrologic modeling. The interdisciplinary
paleoflood approach used to evaluate the reliability of the June 3, 1952 rainstorm in Cimarron can be used
to complement detailed meteorologic analyses of other extreme rainfall-runoff events (historic or future).

SUMMARY

Onsite paleoflood investigations along the Cimarron River and nearby streams and a
hydrometeorologic analyses were conducted to estimate the maximum paleoflood in the Cimarron River.
The evaluation was used to help assess the reliability of an extreme rainstorm of (5.25-inches in about
1.5 hours) that was recorded on June 3, 1952 at an official raingage (Cimarron 3SI:) located near
Cimarron. The maximum peak discharge was 1,840 cubic feet per second for the Cimarron River above
Squaw Creek at Cimarron and occurred on June 11, 1952, which resulted from snowmelt runoff. The
Cimarron River basin is located in central Colorado, which one of the drier areas of state.

The net peak rainfall-runoff in the Cimarron River was about 70 cubic feet per second on June 3, 1952,
which was less than 10 percent of antecedent-snowmelt runoff. A simplified rainfall-runoff relation was
developed for Colorado flood data. This relation indicates that had 5.25 inches of rain occurred in the
Cimarron River basin, the expected peak discharge would have been about 7,000 +/- 2,000 cubic feet
per second. Typically, short-duration rainstorms of an inch or less falling on other, similar basins in
Colorado have resulted in peak rainfall-runoff discharges of between of about 500 to 600 cubic feet per
second. Along the Cimarron River, the maximum paleoflood discharge ranges from 3,000 to 4,000 cubic
feet per second (drainage area is 220 square miles), which probably resulted from snowmelt during 1957
or 1984 after this gage was discontinued. The corresponding unit discharge is about 18 cubic feet per
second per square mile, which is typical for maximum unit discharge for snowmelt runoff in central Colorado
for similar sized basins.

Our interdisciplinary paleoflood investigation indicates that the June 3, 1952 rainfall amount at Cimarron
3SE probably was less than 1-inch and most likely only about 0.5-inch. This conclusion is supported by
an independent assessment of the meteorologic conditions and rainfall estimates of the June 3, 1952
rainstorm. Other Cimarron rainfall data during the 19505 also appear to be questionable. Several times,
the monthly precipitation at this raingage was the maximum for any precipitation station in Colorado, yet
little or no rainfall runoff occurred in the Cimarron River. That the Cimarron precipitation gage had greater
amounts than nearby gages including the Wolf Creek Pass gage is unlikely; Wolf Creek Pass is one of
the wettest regions in Colorado. This also demonstrates the inconsistency of the Cimarron precipitation
data. Analyses of hydroclimatic and paleoflood data indicate that the rainfall amount most likely was
incorrectly measured or reported. We believe that a reassessment of the Cimarron 3SE rainfall data needs
to be done.

There are many questionable extreme-rainfall data in Colorado. That such a large number of
questionable/erroneous rainfall (mostly overestimated) exist in the extreme rainfall-data base is
disconcerting. This is especially true because questionable/erroneous extreme rainstonn and flood data

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