FLOOD07093

• County: Otero
• County: Pueblo
• Community: Otero, Pueblo Counties
• Stream Name: Arkansas River
• Basin: Arkansas
• Title: Arkansas River Channel Capacity and Riparian Habitat Planning Study
• Date: 8/1/2001
• Prepared For: CWCB
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Project

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3.2.3 Hydraulic Modeling

Numerical hydraulic models were assembled for each of the problem areas to analyze
current conditions. The HEC-2, Water Surface Profiles program (US ACE 1990) was used for
hydraulic analysis. This program is applicable to steady, gradually varied flow conditions. It
was assumed that the controlling hydraulic conditions were represented within the boundaries of
the geometry provided, i.e., that conditions downstream ofthe areas would not substantially
influence the profiles within the computer models. The downstream boundary condition was
calculated using the Slope-Area method. Cross-section geometry was obtained from Digital
Terrain Modeling (DTM) provided by Kucera International, Inc., under contract to CWCB.
Reach lengths between cross-sections along the channel were generated when the cross-sections
were extracted. The overbank reach lengths were scaled from the orthophotos. The State
Highway 109 Bridge was modeled according to dimensions provided by the State. Manning's
"n" roughness values were assigned based on a combination of previous studies, field conditions,
sediment transport theory, and engineering judgement. The specific "n" values for the individual
problem areas are shown in Table 3-4. Contraction and expansion coefficients used were 0.1 and
0.3, respectively.

Table 3-4. Manning's roughness values.

Problem Area

Left Overbank

Right Overbank

0.030-0.070
0.030-0.070
0.035-0.070

Channel

1
2
3

0.030-0.090
0.030-0.070
0.035-0.070

0.025
0.025
0.025

Flows of 100, 200, 300, 500, 750, and 1,000 cfs were modeled to indicate channel capacities and
potential problem areas under current conditions. Most areas had a channel capacity of less than
1,0DOcfs.

There were numerous problems with the lack of topographic detail between cross-sections in
detennining the location of the channel banks as well as the overbank areas. Therefore, we
estimated bank locations using the "Edith2" program and measuring the approximate channel
widths on the orthophotos.

3.2.4 Sediment Analysis

Sedimentation was analyzed using Hydraulic Design Package for Channels (US ACE
1997), also known as SAM. This is an integrated system of programs developed to aid engineers
in analyses associated with designing, operating, and maintaining flood control channels and
stream restoration projects. SAM provides the computational capability to include erosion,
entrainment, transportation, and deposition of sediment in channels. The hydraulic, sediment
transport and sediment yield modules of the program were used in this study.

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