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<br />hourly recordi ng rai n gage Saturday ni ght at
<br />Allenspari< about 15 miles south southwest of the
<br />intense rain centers. Two residents in the Rist
<br />Canyon area about 15 mfles northeast of the Big
<br />Thompson intense rainfall centers reported
<br />Saturday night rafnfall totals of 6.B2 and
<br />7.26 inches measured by nonrecording rain gages.
<br />Each observer reported incremental reading times
<br />and accul1ll1ative rainfall. An hourly recording
<br />rain gage designated Fort Collins 9 NW about
<br />5 miles northeast of Rist Canyon reported a
<br />total of 4.7 inches from 2215 MDT 31 July to
<br />0115 MDT 1 August. A 3-hour depth-duration curve
<br />was drawn from data for Fort Collins 9 NW with
<br />accumulative rainfall amounts expressed as the
<br />ratio to total amount. Similar 3-hour depth-
<br />duration curves were drawn for thunderstorm rain-
<br />fall distribution data from Bureau of Reclamation
<br />(1973) and for the duration vs. precipitationl
<br />moisture curve by Clark (1971) discussed earlier.
<br />These curves were then replotted expressing
<br />
<br />.
<br />
<br />precipitation as percent of total and time as
<br />percent of 3-hour duration. These dimensionless
<br />depth-duration curves are shawn in Figure 3.
<br />After several trial hydrograph applications a
<br />composite curve (number 4 in Figure 3) was
<br />adopted that is approximately an average of
<br />curves 1 and 3. This adopted dimensionless
<br />depth-duration curve was assumed to be appli-
<br />cable to rainfall bursts of 2 to 4 hours
<br />duration.
<br />
<br />Average rainfall for the perfod
<br />1B30 MDT 31 July to 0300 MDT 1 August over each
<br />subbasin for which peak discharge estimates were
<br />made was measured from the isohyetal pattern in
<br />Figure 2. As the rainfall-runoff trials pro-
<br />gressed, it became evident that the isohyetal
<br />pattern in Figure 2 needed further adjustment
<br />with a shorter duration to more accurately
<br />portray the flash flood-producing rainfall. The
<br />4-hour time period between 1B30 and 2230 MDT
<br />31 July 1976 which corresponded to the reported
<br />
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<br />
<br />Figure 1. Basin Map of the Big Thompson River, Colorado, between the mouth of Big
<br />Thompson Canyon and Estes Park. The contributing area for the flash flood of
<br />31 July 1976 is bounded by the dashed line. The drainage aredS for which the
<br />Geological Survey made peak discharge estimates are identified by numbers.
<br />Additional subdivisions for this study are identified by letters or by numbers
<br />combined with letters.
<br />
<br />136
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