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

about 3.5 hours (Follansbee and Spiegel, 1937) is the largest known rainstorm in Colorado (Hansen and
others, 1988) and pertlaps in the Rocky Mountain region. The Republican River basin in Colorado
sustained the most flood damage. According to historical accounts (Follansbee and Spiegel, 1937), flood
stages were the highest since at least white settlement of the area in about the mid-1800s and remains so
until the present (1996).

Jarrett and others (in review) reviewed published (Follansbee and Spiegel, 1937; Hansen and others,
1988) and unpublished (Bureau of Reclamation, rainstorm-data file) rainfall data and interpretations of the
May 1935 rainstorm. Their review identified that no official or unofficial raingage data was measured in the
areas of maximum rainfall. The maximum rainfall amount was based on several indirect observations of
rainfall in containers located on ranches and homes; now commonly referred to as rainfall-bucket surveys
(Lindsley and others, 1982). These containers mostly were irregular-shaped (less accurate for estimating
rainfall amounts) buckets or stock tanks. The 24-inch value was an estimate measured in one stock tank
and noted on the moming after the rain had stopped. The next larger rainfall-bucket estimates had values
of 13, 13, 12, 11, and 10 inches and were located within a mile or two of the 24-inch estimate. The
cooperative weather observer near Stratton, which is located about 40 miles southwest of Hale, reported
9 inches of rain in 2 hours during the late aftemoon of the 30th (U.S. Department of Agriculture, 1935).

Either the 1935 rainstorm was extremely localized or the 24-inch estimate is questionable. One factor
that could explain the large difference in rainfall amounts was the amount of antecedent moisture. May
1935 was the wettest month on record since April 1905 (U.S. Department of Agriculture, 1935). Five to
over 8 inches of rain were measured at official raingages (eg., Colorado Springs, Calhan, Byers,
Monument) in the two to three weeks before the 1935 rainstorm. These gages were located on the flanks
of the Palmer Divide and not near the areas of maximum orographically enhanced rainfall. For example,
Monument had 8.63 inches of total precipitation in May 1935, but only 0.35 on the 30th. Observed
precipitation on the Palmer Divide ranges from about a factor of 1.5 to 6 times greater at gages located on
the flanks of the Palmer Divide (Hansen and others, 1978; maps 16 to 20). It is very likely that greater
amounts of antecedent rainfall may have occurred in the areas of maximum 1935 rainfall where the
orographic effects were greatest. By multiplying the observed antecedent rainfall in non-Palmer Divide
areas by a conservative factor (ratio of 1.8 from these maps), 9 to 14 inches of antecedent rainfall may
have occurred in the areas near the centers of the May 30, 1935 rainstorm. It also seems very unlikely
that ranchers would have emptied their stock tanks or other containers after each day of antecedent
rainfall. This seems particularly true in the middle of the 19305 severe drought in eastem Colorado (Collins
and others, 1991). Thus, Jarrett and others (in review) believe the 24-inch value probably reflects a
rainfall total of several days or more, and that the May 30th rainfall may have been only 12 to 13 inches.
These reviewers also noted that there was no documentation as to the source of the 3.5-hour duration for
the 1935 storm. They believe that the 1935 rainstorm may have been longer. A more detailed reanalyses
of this storm appears warranted because it is the largest rainstorm believed to have occurred in the Rocky
Mountain region and is one of the principal design rainstorms used throughout the Rocky Mountain region.

Other flood specialists and meteorologists also have documented questionable/erroneous rainfalllflood
estimates in Colorado and in other the Rocky Mountain States. Pruess (in press) recently completed
interdisciplinary paleoflood research that assessed the accuracy of the 1911 Gladstone, Colorado. The
official rainfall amount for the October 4th and 5th rainstorm was 8.05 inches in 24 hours. Gladstone is
located at an elevation of about 11,000 feet near Silverton in the San Juan Mountains. The Gladstone
storm is the largest rainstorm reported at higher elevations in the Rocky Mountains and commonly is
transposed to other sites in the Rocky Mountains for design-flood hydrology. Pruess documented that
little out-of-bank flooding occurred in 1911 or any other year in the Gladstone area. He also compiled
historical newspaper accounts and photographs taken just before and after the 1911 rainstorm. The
newspaper accounts indicated that little flooding occurred in the Silverton area; Silverton newspaper
reporters criticized the state newspaper reports as being largely exaggerated. The photographs clearly
show that little out-of-bank flooding occurred, except locally from backwater at bridges. Pruess concluded
that the 1911 rainstorm maximum value is questionable and that there has been no substantially flooding
in the Silverton area. Hence, use and transposition of the 1911 rainstorm in hydrologic investigations also
may lead to erroneous and overestimated design hydrology.

Other notable extreme rainstorms in Colorado that we question (amount and/or areal extent) include,
but are not limited to, the rainstorms of: 1894 in Boulder, 1896 and 1938 in Bear Creek near Denver, 1938
in Missouri Canyon near Masonville, 1921 and 1974 at Steamboat Springs, 1965 in Plum Creek near
Denver, 1973 at Big Elk Meadows near Lyons, 1976 in Big Thompson River basin, and 1981 Frijole

10