FLOOD06449

• County: Delta
• County: Montrose
• Community: Collbran, Cedaredge, Paonia
• Stream Name: Buzzard, Plateau, Grove Creeks
• Basin: Gunnison
• Title: Hydrology Report Unincorporated Areas of Delta and Montrose Counties
• 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

the length of record actually available for examination turned out
to be less than what was recorded in the U.S.G.S. Water Supply
Papers. Those gage stations with too few existing gage charts were
either abandoned or included for comparison purposes and less
weight was given to them in the overall analysis.

The procedure for selecting an overall skew coefficient for each
region involved several steps. In keeping with the statistical
concept that a large sample defines a more accurate overall
relationship, two overall skews from the Water Resources Council
Bulletin No. 17, Guidelines for Determinin Flood Flow Fre uenc ,
were applied to eac set 0 ata. e s ew coe lClents were
weighted using the Water Resources Council guidelines based on the
number of years of record. For the rainflood peaks, a "generalized
skew" taken from country-wide isolines of skew in the Bulletin
No. 17 map (Figure 14-1 in Bulletin No. 17) were applied with the
weighting. A second run using the "regional skew" or "average
skew" from the same map based on gaging station data wi thin each
one-degree quadrangle in the country was also made. The data was
plotted for each station using the Weibull plotting position along
with the two skew lines from the flood-flow-frequency analysis.
The skew line which fit each set of data best was selected
independently, and the type of skew which fit the majority of cases
best in each region was chosen as the overall type of skew for that
region. Tables 4 and 5 summarize these calculations. For the rain
induced peak the regional skew fit most data plots best in
Region I, and the generalized skew fit most plots best in Region
II.

Two trials were also made on the snowmelt peak data using the
"generalized skew" and the computed skew calculated from the
station data. The "regional" or "average" skew was not used as it
is based on rainfall data and the resulting bias was not considered
accurate for any snowmelt analysis. As might be expected, the
computed skew fit all the data plots best for the snowmelt plots.
For each region and event a weighted average based on the number of
years of record was figured and all the data sets were run again
with this weighted skew (see Table 4 for a tabulation of the
results of calculation).

The final skew run was plotted on Figures 6-33 (using the Computed
Flow rather than the Expected Probability) and a combined flow-
frequency curve was drawn using the individual rainfall and
snowmelt frequency curves. From these combined curves, drainage
area versus discharge curves were fitted visually for each region
and frequency, giving less weight to stations w\th less than 12
years of record (Figures 34-44). Figures 38 and 44 show a summation
of the four frequency curves for each region. The areas for each
drainage basin were measured at the endpoints of the study length.
If the flows at the endpoints found from the Regional Drainage Area
versus Discharge curves differed by more than 2:0%, intermediate

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