FLOOD09736

• County: Statewide
• Basin: Statewide
• Title: Estimating Flood Flows from Mixed Populations in Colorado
• Date: 1/1/1978
• Prepared For: State of Colorado
• Prepared By: CWCB
• Floodplain - Doc Type: Educational/Technical/Reference Information

The Animas River al so rises on the southern slopes of the San Juan
Mountains at an elevation of about 12,000 feet MSL, From its headwaters, the
river flows through narrow canyons and wide valleys generally in a southerly
direction and joins the San Juan River at Farmington, New Mexico. Above
Silverton, Colorado, the river has an average fall of 300 feet per mile; below
Silverton, it averages 100 feet per mile extending as far as the head of the
Animas Valley north of Durango, Colorado, Through the Animas Valley, the
fall approximates 16 feet per mile, From Durango to the mouth, the average
fall is about 22 feet per mile.
The climate in the basin is arid to semiarid except in the high head-
waters where precipitation is moderately heavy. The mean annual temperature
is about 450 F, and the humidity is generally quite low allowing for rapid
evaporation. Severe floods are most often caused by rainstorms occurring
during the period from July through October. Flood potential, also exists
from the rapid melting of heavy snowpacks in the late spring,
Twenty-two U.S. Geological Survey stream flow gauging stations located in
the basin were selected for the analysis (Table 3). Thirteen of the 22 sta-
tions are located in the Upper San Juan subbasin, and the other nine stations
are located in the Animas subbasin. These gauging stations were selected so
that peak flows are not significantly affected by diversions and regulations,
and the lengths of record are at least ten years. The annual peak flows of
rain floods and snowmelt floods, respectively, were determined from the
recorded stream flow charts with the aid of precipitation and temperature data
at each gauging station, Table 3 lists all the gauging stations, along with
their drainage areas, mean watershed elevations, and periods of record. The
drainage areas range from 41,2 to 1,990 square miles. The mean watershed ele-
vations range from 6,960 feet MSL to 11,800 feet MSL, The average length of
record is approximately 30 years,.
For each gauging station, the sample mean, standard deviation, and the
coefficient of variation and skew were computed from the annual flood series
for each type of flood event, and are tabulated in Table 4. The sample means
of the station means and variances with weights equal to their lengths of
record were also computed for each subbasin, Paired t-tests were performed
on the two subbasin means and variance for each type of flood event to deter-
mine dependence of the mean and variance in the subbasin. It was found that
the statistical parameter estimates are independent of subbasins at the level
of significance of 0.05 for both rain and snowmelt events; therefore, the
entire basin may be treated as a region.
Regression of the sample means, coefficients of variation and skews on
the drainage areas and mean watershed elevations were then carried out for all
stations in the region, Subsequently, sequential F-tests with the level of
significance of 0.01 were performed to test the significance of each indepen-
dent variable to be included in the regression equations. Final regression
thus obtained are as follows:

XR = 0.664 log A + 1.421

(4)

2
r = 0.48

se = 0,377

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