FLOOD06271

• 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: 7/1/1999
• Prepared For: CWCB
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Project

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4. AL TERNA TIVES

4.1 CHANNEL IMPROVEMENT

Improvements to the Arkansas River channel to increase conveyance and reduce
maintenance were examined as one of several alternative. Improvements to each of the five
Problem Areas were modeled and are described below. The adopted configuration, after
numerous trials, consists of a relatively wide, uniform channel capable of carrying at least
3,000 cfs with a meandering 1,000 cfs stable channel within. A stable channel is one that
maintains its shape and profile over time without aggrading or degrading, though it may move
laterally. The 3,000 cfs minimum was the upper limit of capacity that could be gained without
extensive reconfiguration. The 1,000 cfs discharge parameter for the inner channel was chosen
based on a pattern of this as the approximate "effective discharge" of the problem areas using
the post-1981 discharge-duration data. This value represents a flow that has occurred
frequently enough to transport a substantial portion of the sediment load.

It should be noted that in order to maintain the gains of these improvements, some
conditions are implied. The sediment modeling performed used the post-198l hydrologic
regime. This hydrology included some flood releases. On average, flows of around 3,000 cfs
occurred about four days per year. In actuality they occurred only in certain years for longer
periods of time. In order to maintain sediment movement throughout the system, these higher
flows must continue. If runoff precludes achieving this value for four days of every year, it
should be targeted for 2 weeks every three years. This would require the cooperation of the
water users but is important to the maintenance of capacity. This recommendation does not
introduce the need for any additional water, but rather what has statistically occurred in the
past. Maintaining this may require some coordination and cooperation with water users but
will pay large dividends in terms of channel maintenance and conveyance capacity. The
second condition implied is no further encroachment on the floodplain. The impacts of
encroachment on river behavior should be apparent from the discussions above. The current
condition of the river is largely a product of the agricultural encroachment that has occurred in
the past. Some means of preventing encroachment, through agreement or easement would be
required.

Probtem Area 3. The improved channel for this sub-reach uses a 400 ft bottom-width
channel along the existing channel alignment with a slope of 0.00105 and 1:3 sideslopes, and a
more sinuous interior channel with a 120 ft bottom width on a slope of 0.000822. The
sides lopes of this interior channel were modeled at I :2.5 for the outside of the bend and 1 :10
for the inside. The "n" value for the large channel was estimated at 0.060 and the interior
channel was modeled using 0.038. This configuration will pass 3,000 cfs. A profile plot of
this area with the improved channel is shown in Plate 6 and a typical improved cross-section is
shown in Plate II.

The sediment yield calculated for this configuration was 15,500 tons/yr with a mean
daily transport of 43 tons/day. This represents a roughly four-fold increase in sediment
transport capacity for this subreach, which would greatly decrease required maintenance and

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