|
<br />and others, 1994). One significant advantage of conducting paleoflood investigations to obtain flood data
<br />is that it interprets real processes, rather than the current trend of conventional flood science of theoretical
<br />or statistical idealizations of flood attributes (Baker, 1994), He indicated that emphasis on idealizations of
<br />flood concepts, particularly the over reliance on computer models, nelllects the importance of
<br />understanding floods, which provide realistic estimates of catastrophic floods. Responsible engineering
<br />incorporates professional experience and utilization of all sources of data on flooding (Baker, 1994).
<br />
<br />Paleoflood hydrology now is being used to complement engineering hydrology because of
<br />improvements in interpretation, accuracy, and incorporation of paleoflood data into conventional f100d-
<br />frequency analysis (Stedinger and Cohn, 1986; Stedinger and Baker, 1987), It also provides a reliable,
<br />scientific, engineering, and regulatory acceptable data and method to estimate the probabilities of extreme
<br />floods to make a hydrologic assessment of dam safety (Jarrett and Costa, 1988; Jarrett, 1989; Baker,
<br />1994; Levish and others, 1994). A basic assumption of the Probable maximum precipitation and probable
<br />maximum flood (PMP/PMF) methodology is the utilization of storm transposition (World Meteorological
<br />Organization, 1986; Hansen, 1987; Hansen and others, 1988; Cudworth, 1989). The leading premise and
<br />justification for storm transposition is that extreme precipitation and flood data generally are not available
<br />for a basin, Paleoflood hydrology provides such estimates of extreme-flood data.
<br />
<br />One study that utilized paleoflood hydrology was for reevaluation of the emergency spillway for
<br />Olympus Dam located on the Big Thompson River in Estes Park, Colorado (Jarrett and Costa, 1988),
<br />The city of Estes Park is located immediately east of Rocky Mountain National Park at an elevation of
<br />7,500 feet. The spillway was designed in the 1950's for a flood of 22,000 ft3/S. Design requirements
<br />developed in the 1980's for spillways in the Rocky Mountains would have required a spillway modification
<br />to accommodate a flood of 84,000 ft3/s. The USGS research showed that streamflow upstream from
<br />Olympus Dam has not exceeded about 5,000 ft3/S in at least 10,000 years, except for the Lawn Lake
<br />Dam failure flood of 1982 (Jarrett and Costa, 1986, 1988). Thus, the USGS, in collaboration with the
<br />Bureau of Reclamation, was able to demonstrate that a spillway modification of about $10 million was
<br />unnecessary. The USGS methods have been applied to dams in other parts of the country. By using
<br />paleoflood studies, approximately $60 million was saved in unnecessary spillway modification for
<br />Bradbury Dam on the Santa Ynez River in Califomia (Levish and others, 1994). Modifications of the
<br />existing spillways of the 162 high-risk dams in Colorado are expected to cost over $200 million
<br />(Changnon and McKee, 1986), There are tens of thousands of dams throughout the Rocky Mountain
<br />region that would have required spillway modifications costing many tens of billions of dollars, Throughout
<br />the Rocky Mountain region (and other States and countries), dam-safety, decision-making and
<br />spillway-size criteria are being reevaluated. Other uses of paleoflood investigations include flood-plain
<br />management, design of highway structures in floodplains, hazardous-waste deanup, f100d-waming
<br />systems, ecological assessments, and related environmental studies (Jarrett and Costa, 1988; Jarrett,
<br />1989, 1991; Baker, 1994; Levish and others, 1994; Jarrett and Waythomas, in press; Jarrett and others, in
<br />review),
<br />
<br />ONSITE INVESTIGATIONS
<br />
<br />Onsite paleoflood investigations were done prior to review of available hydrodimatic data, and
<br />therefore provide an independent assessment of past flooding in the Cimarron area. On October 12th
<br />and 13th, 1995, Joe Capesius, a project member who has done extensive paleoflood and flood
<br />investigations in Colorado, conducted a reconnaissance of streams located in the Cimarron area (figure 1).
<br />He noted debris-flow deposits in a tributary to Squaw Creek, which is located about 3 miles west of
<br />Cimarron or about 6 miles west of the 1952 Cimarron precipitation gage, Squaw Creek enters the
<br />Cimarron River at Cimarron. Inaccessibility to private property preduded onsite inspection of the tributary
<br />basin. These deposits contain boulders as large as 12 to 15 feet in diameter, which suggest transport by
<br />a large event and probably debris flows. Description, identification, and criteria to differentiate between
<br />flood- and debris-flow deposits are provided by Costa and Jarrett (1981), Costa (1984,1988), Jarrett
<br />(1986), Waythomas and Jarrett (1994), and Jarrett and Waythomas (in press). Identification of deposit
<br />origin is needed is because if hydraulic methods used to estimate the discharge of waterfloods are used for
<br />debris flows, then estimated discharges will be overestimated (Costa and Jarrett, 1981),
<br />
<br />Because of the importance of the June 3, 1952 rainstorm and uncertainty in the dassification of tributary
<br />deposits (debris flow or water flood), The senior author visited the Cimarron area November 3 and 4. Joe
<br />is correct that the Squaw Creek tributary deposits resulted from multiple debris flows. Several sites near
<br />Cimarron were visited including: Big Blue Creek near Sapinero, Uttle Cimarron and Cimarron Rivers,
<br />
<br />3
<br />
|