FLOOD08915

• County: Delta
• County: Montrose
• Community: Unincorporated Delta and Montrose Counties
• Stream Name: Surface Creek, San Miguel River
• Basin: Gunnison
• Title: Hydrology Report FIS
• Date: 4/1/1979
• Prepared For: FIA
• Prepared By: Meuer Serafini & Meuer, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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HYDROLOGY:
Montrose, Delta and Mesa Counties, Colorado

In Region II snowmelt also creates the highest and most sustained
flows. On the San Miguel River at Naturita, Colorado, the highest
recorded flow (7,100 cfs) occurred on April 15, 1942 and must have
been primarily snowmelt. For smaller drainage areas, thunderstorms
cause the most severe flooding.

4. Hydrology - The flood histories of Regions I and II indicate
that a synthetic storm hydrograph, predicting streamflow based on
rainfall alone, is not adequate. For these regions a flood-flow
frequency analysis based on peak separation of the mixed population
of snowmelt and rainfall runoff was used to calculate frequency
curves for the study areas. Because the data available for small
drainage basins in Region II is so limited, the Rational Method was
used to find flows for Tributary I to the San Miguel River at
Naturita which has a very small watershed (1.3 square miles).

For the flood-flow-frequency analysis of mixed flow populations it
is necessary to separate the peak flow caused by each of the events;
in this case, the rainfall and snowmelt runoff. When peak flows for
each event are found, flood-flow-frequency analysis is performed on
the two sets of data. The resulting curves are combined statis-
tically to give the maximum flow from both events. The procedures
of combining rainfall and snowmelt frequency curves are described
as follows:

(1) select a peak discharge

(2) read the probabilities for rainfall and snowmelt events
from the respective frequency curve at the selected peak
discharge

(3) combine probabilities by the following equation:

pet) = per) + pes) (p( r) x pes))
Where pet) = total probability of the selected peak discharge
per) = probability of the rain flood event at the selected
peak discharge

pes) = probability of the snowmelt flood event at the
selected peak discharge.

A definite methodology to isolate the quantity of rainfall runoff
from its snowmelt (or other) base has not been established.
Discussion continues as to whether the base flow should or should
not be subtracted from the total flow occurring during or after a
rainstorm. In a particular region equal rainflood peaks occurring
at different times of the summer will indicate different amounts of
rainfall runoff, depending on the amount of the snowmelt base.
Theoretically, then the snowmelt base should be subtracted out to

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