FLOOD10929

• County: Mineral
• Community: Creede
• Stream Name: Upper Willow Creek
• Basin: Rio Grande
• Title: Upper Willow Creek Watershed - FLood Control and Stream Stability Study
• Date: 10/1/2002
• Prepared For: Willow Creek Reclaimation Committee
• Prepared By: Agro Engineering
• Floodplain - Doc Type: Floodplain Report/Masterplan

2.2 Baseline Floodplain Analysis

A hydraulic model was used to determine water surface elevations resulting from flood events at
several return intervals in Willow Creek. These water surface elevations define the spatial extent
of flooding within the floodplain. The term baseline refers to the analysis of current conditions in
Willow Creek. The construction of measures to control flooding problems or improve other
stream functions may change the current floodplain and may need to be evaluated separately.

2.2.1 Past Floodplain Studies
In 1986, the CWCB studied water surface elevations for the 100-year flood for about 700 feet
upstream of the Army Corps flume entrance using HEC-2 (Mullen, 1986). A 100-year peak
flood flow of 1120 cfs was used for the study (slightly more than used in the current study).
Three cross-sections were surveyed upstream of the flume and used in the model. Given the
limited study length and available topography, the study analysis found that the 100-year flood
would be contained by the flume approach levees and enter the flume. The floodplain map from
the study is provided in the Appendix.

2.2.2 Hydrology
Prior to construction of the masonry flume through town in 1950, several high streamflow events
occurred in Willow Creek causing damages in Creede. The Army Corps estimated streamflows
for these events from high water indications and personal accounts. These estimates,
documented by Mullen (1986) are noted in Table 2.2.1

Table 2.2.1. Estimated peak flows for floods prior to gage installation (Mullen, 1986)

Year Estimated Peak Flow (cfs) Event Type
1911 1800 Rain
1921 1400 Snow
1927 1400 Rain on Snow
1941 1400 Snow
1948 1200 Snow
1949 1300 Snow

In 1951, the USGS installed a stream gage on Willow Creek at the entrance to the flume. From
1951 through 1982, the gage recorded annual flood peaks ranging from 66cfs to 430cfs (Yochum
and Hyde, 2002). Unfortunately, the gage was not operated between 1983 and 1998, but the
recorded range of peak flows is significantly smaller than the pre-gage, pre-flume, estimated
flood events.

Several sets of flood hydrology estimates using log-Pearson discharge-frequency methods have
been made for Willow Creek by the CWCB and Army Corps. Various combinations of historic
flow estimates and gage data were apparently used, leading to a wide range of flood return
estimates. The uncertainty in these diverse estimates encouraged the Natural Resources
Conservation Service (NRCS) and CWCB to perform a hydrologic analysis using a regional
watershed methodology. Seventeen watersheds of similar size and relatively close proximity in
the Rio Grande, Gunnison, and San Juan River watersheds were studied. A regression analysis
of watershed variables such as drainage area, percent forest, maximum and minimum elevation,

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