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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The resulting sediment transport values for the chosen stable design discharges were also
used to generate tables of stable channel dimensions for the channel improvements. The
improved channel models were similarly used to generate new sediment transport rating curves.

Sediment yields for the existing conditions are not presented in this report. Annual yields
are typically calculated by integrating the sediment transport rating curve with the flow-duration
curve for the area. Because of the deterioration in channel capacity within the problem areas,
reliable sediment transport curves for the existing conditions could not be obtained. The limited
range of flows modeled hydraulically (up to 1,000 cfs) does not provide an adequate
representation of the river hydrology. Based on the discharge frequency peaks presented in Table
3.2, 1,000 cfs does not cover enough of a range of flows to provide meaningful yield
calculations.

Stable channel geometry "templates" were generated for the three problem areas. These
are conceptual floodway cross-sectional configurations designed to balance sediment inflow and
outflow. The target values for inflowing bed sediment load were estimated from the most
efficient channel segments with at least 1,000 cfs capacity and other data sources. The channel
configurations are compound. That is, they have more than one prism. The sediment transport
values for the chosen stable design discharges were used to generate tables of stable channel
dimensions for the channel improvements. A low-flow trapezoid was designed to convey the
current channel capacity, estimated to be about 800 cfs. A second trapezoid with a capacity of
2,500 cfs was included to carry higher flows. The 100 to 1,000 cfs sediment transport rating
curve was extended through extrapolation and integrated with the flow-duration curve for the
pre-dam condition from the Lower Arkansas River Planning Assistance to States Study (1999a).
This indicated an effective discharge of around 2,500 cfs. These two channel sizes were
combined together, with the low-flow (800 cfs) channel cut within the 2,500 cfs channel. Both of
the channels used 1 V:2.5H sideslopes. The widths varied with each problem area. The channel
template dimensions are discussed below under the specific Problem Areas. The resulting
compound channels were inset into a simple, lO-point cross-section, idealized as a 700-foot
wide, lO-foot deep floodplain. A 400-foot length segment of this idealized configuration for each
problem area was modeled in HEC-2 and the improved channel models were used to generate
new sediment transport rating curves and calculate sediment yield and average concentrations.
The results are listed in Table 3.7 and shown graphically in Figure 5. The variables listed in
Table 3.7 are reasonably uniform, with a generally increasing trend in the downstream direction.
This promotes transport through the study reach, transporting sediment to John Martin Reservoir,
and allows for some increase in load from tributaries.

Table 3.7. Improved channel template sedimentation results.

Sediment Variable Problem Area 3 Problem Area 2 Prob. Area 1
Mean Daily Load (ton/day) 2584 2673 2164
Mean Daily Concentration (mg/L) 1717 1776 1438
Annual Sediment Yield (ton) 942,902 975,619 789,624
Annual Sediment Yield (cu. yds.) 751,017 777,076 628,932

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