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<br />320
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<br />BULLETIN OF THE ASSOCIATION OF ENGINEERING GEOLOGISTS
<br />
<br />in southwestern Colorado, two small tributaries to
<br />the Uncompahgre River were identified where con-
<br />ventional procedures of hydraulic backwater-pro-
<br />file analysis were unusable (A & S Consultants,
<br />1978, p. 12):
<br />
<br />" As the nature of flooding of Portland and Cas-
<br />cade Creeks was studied, it became increasingly
<br />apparent that the flooding did not follow patterns
<br />which could be evaluated by normal hydraulic
<br />methods, After evaluating other techniques
<br />which might be applied to rivers carrying high
<br />loads of silt or debris, a basic conclusion was
<br />reached-the floodplains of Portland and Cas-
<br />cade Creeks were variable, unpredictable, and
<br />could not be defined,"
<br />
<br />These basins appear to be typical of many small
<br />basins in the mountains of Colorado and the prob-
<br />lem identified in the flood-insurance study is not
<br />unique in this area, The accurate identification of
<br />a "flood plain" across a debris fan, using conven-
<br />tional hydraulic and hydrologic procedures is not
<br />possible (Magura and Wood, 1980). Channel block-
<br />age and debris-flow deposition result in continually
<br />changing channel patterns and locations of deposi-
<br />tion (Gundlach, 1977-78). The constantly shifting
<br />path of a debris flow across a debris fan makes iden-
<br />tification of a single flood plain impossible. In 1914
<br />a debris flow along Cornet Creek caused severe
<br />damage to the eastern parts of Telluride, Colo.,
<br />built on the debris fan, In 1969 another debris flow
<br />down the same stream damaged the western part of
<br />the town. Consequently, entire debris fans of small
<br />mountain streams could be classified as areas of
<br />hazard due to flooding, Not all flood-plain studies
<br />in mountain areas fail to recognize the debris flow
<br />problem. During the flood-insurance study of the
<br />town of Telluride, the debris-flow hazard from Cor-
<br />net Creek was correctly recognized (Federal Insur-
<br />ance Administration, 1978), However, standard
<br />procedures for dealing with flood hazards on debris
<br />fans and alluvial fans have been slow in developing
<br />(Magura and Wood, 1980; Gundlach, 1977-78;
<br />Dawdy, 1979).
<br />Inspection of channels following debris flows in-
<br />dicates that normal slope-area measurement of peak
<br />discharges would result in excessive water-dis-
<br />charge values, Some slope-area peak-discharge es-
<br />timates made in small mountain watersheds follow-
<br />ing debris flows can far exceed the value estimated
<br />for the 500-year flood in the Colorado Rocky Moun-
<br />tains, as shown in Figure 14, Four of the seven
<br />
<br />sites we investigated, originally reported as water-
<br />floods, far exceed the estimated 500-year flood
<br />(points, 3, 4, 5, and 7, Figure 14), Site I, Red Dirt
<br />Creek tributary near Toponas, Colo" for which we
<br />do not have a slope-area estimate, probably did as
<br />well. These flows probably were debris flows, for
<br />reasons explained in this paper. Downstream high-
<br />water marks and gaging-station records indicate
<br />that the water flow associated with these debris
<br />flows was only a small percentage of the total flow.
<br />Considered separately from their sediment loads,
<br />these water flows probably would plot well below
<br />the 500-year discharge estimate. Slope-area esti-
<br />mates incorrectly applied to debris flows produce
<br />excessive discharge values and imply rainfall and
<br />water discharges much greater than probably oc-
<br />curred, They also imply that large waterfloods are
<br />more common in mountain streams than is really the
<br />case, The resulting effect on flood-frequency stud-
<br />ies could lead to impractically large and costly de-
<br />signs for flood-control structures and overestimate
<br />the area at risk from water flooding, Thus, failure
<br />to identify debris flows can lead to a large error in
<br />the significance of the hazard from water flooding,
<br />Scientists and engineers need to be aware of this
<br />problem when investigating future flows in small
<br />mountain basins.
<br />Protective measures to reduce property damages
<br />from debris flows are different from mitigating mea-
<br />sures for waterfloods. For example, channelization
<br />for debris flows is ineffective because channels can
<br />quickly become blocked causing subsequent surges
<br />to flow in new directions. Channel improvements
<br />during the 1964 dry season in the Rio Reventado
<br />channel in Costa Rica proved unsuccessful. The
<br />first storm of the rainy season promptly filled the
<br />enlarged channel with mud and rock debris (Wal-
<br />dron, 1967). Reservoirs can become filled quickly
<br />and require extensive dredging to maintain design
<br />capacity, Protection from debris flows can include
<br />reinforcing uphill walls of structures, and construc-
<br />tion of arresting and breaking structures to deflect
<br />or trap the largest boulders moving in the flow
<br />(Mears, 1977; Hollingsworth and Kovacs, 1981).
<br />Another problem with widespread implications
<br />that involves the distinction between waterfloods
<br />and debris flows is the estimation of the associated
<br />rainfall. Using the debris flow along the East River
<br />tributary (Table ]) as an example, if the water-flood
<br />discharge was in the range of 108 to 134 m3/s (3,800-
<br />4,700 ft3/S) (determined by two indirect methods),
<br />then an unprecedented IO-minute rainfall intensity
<br />
<br />,
<br />
<br />"
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