FLOOD10359 ‎(2)‎

• County: Delta
• Community: Delta County
• Stream Name: Minnesota Creek
• Title: Supplemental 100 Year Flood Study for Minnesota Creek
• Date: 9/1/2002
• Prepared For: Delta County
• Prepared By: Buckhorn Geotech
• Floodplain - Doc Type: Floodplain Report/Masterplan

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feet (msl) at Mount Gunnison to approximately 6000 feet (msl) at the lower end of the
study, Vegetation and topography vary widely with elevation and landuse, The lower
river valley is primarily in agricultural land use with heavy vegetation in the river
corridor and on the channel sidebanks, The channel is deep and well defined in some
locations with lower banks and a smaller cross-section where it passes through fields and
has been manipulated to accommodate farming practices.

The U.S, Geological Survey published an open-file report (84-137) of stream statistics
that shows a stream gaging station on Minnesota Creek near Paonia with 11 years of data
(1936-1947), The station (number 09134000) is listed as having a basin area of 41.5
square miles, mean basin elevation of 8500 feet and mean annual precipitation of 30.4-
inches. Land use for this basin is shown at 40-percent forest, 10-percent agricultural, 49-
percent range and I-percent urban use, A second gage noted as Minnesota Creek at
Paonia (number 09134050) had reported flows from 1976-1979,

5.0 tOO-YEAR FLOOD FLOW

The 100-year flood flow given in the USGS Report for the gaging station on Minnesota
Creek near Paonia is 345-cfs based on 11 years of stream data, The gage noted as
Minnesota Creek at Paonia show a 100-year flood flow of912-cfs based on 3-years of
stream data, The 1983 FEMA study used 1375-cfs for the flood flow on Minnesota
Creek based on regional floodflow/frequency analysis, For the purpose of this study we
have used the FEMA 100-year flood flow of 1375-cfs, although an analysis was
performed with the 912-cfs (for comparison only) and generally showed computed water
surface elevations of approximately I-foot lower,

6.0 METHOD

The hydraulic analysis utilized the D,S, Army Corps of Engineers HECRAS model to
determine water profile and flow characteristics, A field survey was used to determine
geometric conditions for the creek, overbanks and culverts for input to the HEC-2 model.
Roughness factors (Manning's "n" values) were determined based on visual observation
and were estimated at 0.15 for heavily vegetated overbanks and 0,035 for the channel.

The flow regime for this section of the creek is mixed with sections of subcritical and
sections of critical flow, The slope area method was used to determine starting water
elevation at the upstream cross-section, The model interpolated cross-sections between
measured cross-sections where necessary due to changes in channel conditions. Three
culverts were modeled where the County Road (050 Road) crosses the creek,

The hydraulic analysis was made based on unobstructed flow, All elevations are
referenced to the NGVD Datum 1929,

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