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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areas investigated, the profile ofthis reach is fairly well-behaved, A profile plot of this area is
shown in Plate 3,
The planform of the river exhibits two different forms in this area. Downstream of the
irrigation diversion the channel shows a pronounced meandering channel pattern form with
sandy point bars, Upstream of the diversion the river channel is much less sinuous until the
upper end of the reach, with fewer and smaller point bars and more braiding, This is consistent
with a change in the sediment transport regime and is due to the diversion of a larger
proportion of water than sediment. No attempt to quantify this effect was made because of
inadequate data.

The "Bristol Bridge" particle size distribution was used for this sub-reach for sediment
transport calculations, The sediment yield for this problem area under existing conditions was
30,200 tons/yr with a mean daily load of 83 tons/day (averages of below and above diversion),

Problem Area 6, Hydraulic modeling of Problem Area 6 indicated that the capacity
within the active channel banks is approximately 1,500 cfs, This is the value above which flow
begins to significantly inundate the overbank areas adjacent to the river channeL Potentially
damaging flows begin above approximately 2,500 cfs at some cross,sections, The next
potentially damaging flow occurs at approximately 3,500 cfs, upstream of a severe constriction
caused by agricultural encroachment. The 3,000 cfs operational discharge was modeled to
indicate potential problem areas under current conditions, The flow area extents for this
discharge were connected to generate pseudo-floodplains, as described previously, and are
shown on Sheet 4.

Examination of the profile indicated by the survey shows varying slopes between
adjacent cross-sections, though not to the degree of area 4, The reach-length weighted average
slope for the problem area is 0.00129, but the individual values range from a relatively steep
0,00615 to flat. There are no adverse slopes within this reach, Compared to other areas
investigated, the profile of this reach is reasonably well-behaved, A profile plot of this area is
shown in Plate 4.

The planform of the river reveals extensive encroachment along the reach, with one
area near the downstream end where this is particularly extreme (shown in Figure 7). In this
area, a circle, irrigation plot, coupled with its appurtenant levee and one on the other side of the
river, has restricted most flows (i,e" well above the channel capacity) to a width of only 200
feet. This severely reduces the area available for overbank flows and eliminates channel
sinuosity, Without extensive bank stabilization and maintenance, this area can be expected to
be very problematic, As mentioned above, this constriction also causes lowered capacity
upstream with its backwater effect. A projection of an average slope through the reach shows
that the channel bottom is overly deep in the area of this constriction, Again, this is likely the
result of the actual channel work performed (probable over, excavation) and the channel's
response to it.

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