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

Hydraulic Reference Manual (Army Corps 2001). However, this additional methodology still
defined close to a 1: 1 contraction ratio criterion. It should be noted that these ineffective flow
areas disappear once the flow overtops the structure.

The two flumes noted in Figure 2.2.2 were not explicitly modeled as structures in HEC-RAS, but
were represented in the cross-section geometries. A "lid" obstruction was placed on the
Commodore Mine flume to represent the steel rails and ties that are present. Levees have been
pushed up on much of Willow Creek downstream of XS 14 as well as portions of East Willow
Creek downstream of XS 11. These levees were represented in HEC- RAS so that water-surface
elevations had to exceed the levee elevation before flow could enter lower areas outside the
levees. The levees between XS 14 and XS 6 may fail before they are overtopped, and this should
be noted when considering results in this area. Several backwater areas, such as the pond near
XS 6.1 were defined as ineffective flow areas until bank elevations at the south end of the pond
are exceeded. Buildings were represented as flow obstructions within the model cross-sections.

The Manning's "n" coefficient represents the roughness of the channel in the hydraulic
calculations (using Manning's equation). Both the size and angularity of sediment within the
bed (grain roughness) as well as larger shapes and bedforms (form roughness) and vegetation
within the channel affect the coefficient value. The HEC-RAS Hydraulic Reference Manual
(Army Corps 2001) lists typical Manning's n values for "mountain streams" with a bed of
gravels, cobbles, and few boulders ranging from 0.03 to 0.05 with an average of 0.04. With a
bed of cobbles with large boulders it lists values ranging from 0.04 to 0.07 with an average of
0.05. These values are supported by other hydraulics texts (Chow, 1959). Table 2.2.5 lists main
channel Manning's n values that were used in the model following field evaluations of sediment
size and roughness conditions. It should be noted that values for a cross-section describe the
area from the cross-section to the next cross-section downstream.

Table 2.2.5. Manning's n values for model cross-sections (main channel)

Stream Reach Cross-Sections Manning's n Ave. Cobble Size (inch) Location Notes
Willow Creek 2.0 to 5.0 0.04 3 forebay to flume
6.0 to 8.5 0.045 4
9.0 to 10.0 0.04 3 upstream of timber drop
11.0 to 13.7 0.045 4
14.1 to 15.0 0.04 3 forebay to weir
15.0 to 22.0 0.05 5
West Willow Creek 22.4 to 25.0 0.05 5
26.0 to 26.4 0.06 10
26.6 to 28.0 0.05 6 upstream of culvert
29.0 to 37.3 0.06 8
38.2 to 38.4 0.05 6 upstream of flume
40.0 to 42.0 0.07 12 very steep slope
43 0.055 6 more gradual
44.0 to 45.0 0.06 8
East Willow Creek EO.4 to E6.0 0.05 5
E7.0 to E8.0 0.06 10 upstream of culvert
E9.0 to E13.0 0.05 4
E14.0 to E18.0 0.06 8

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