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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Examination of the profile indicated by the topography shows little in way of varying
slopes between adjacent cross-sections. The reach-length weighted average slope for the
problem area is 0.0011, which is steeper than both areas 1 and 3. Aerial photography indicates
numerous old river scroUs within the cropland, and shows a large meander bend truncated by
encroachment. There is a high possibility that the inaccurate mapping information causes this
slope, which is represented by the water surface and would change with either better mapping or
field surveys. There were no photos provided from CWCB for Area 2. Area 2 has a much
steeper slope which explains the large number of areas of overbank sediment deposits. The large
sediment deposits are also high in this area due to the inflows from Horse Creek and Adobe
Creek. Salt Cedar is limited to dense growth not causing encroachment to the channel
conveyance.

Problem Area 2 is also likely aggrading, since the PZF plots of both the Arkansas River
at La Junta and the Arkansas River at Las Animas show aggradation for the period after 1974
(see Figure 6). The effective average channel width is approximately 135 feet.

The channel improvement template for this problem area uses a 70-foot bottom width for
the low-flow channel, with a slight meander that results in a channel slope of 0.00101. The 2,500
cfs channel has a 125-foot bottom width and foUows the vaUey slope, estimated to be
approximately 0.00103.

Problem Area 3. Hydraulic modeling of Problem Area 3 indicated that the capacity
within the active channel banks is approximately 1,000 cfs. This is the value above which flow
begins to significantly inundate the overbank areas adjacent to the river channel. Most cross-
sections within the model indicates flows in excess of 1,000 cfs would overbank into this very
dense vegetated area adding to the already very high water table. Most of the high water surface
is due to the water surface elevation of John Martin Reservoir. The flood plain in this area is
very wide and meanders much more than Areas 1 and 2.

Examination of the profile indicated by the topographic maps shows similar slope to that
of Area 2, however the flow decreases rapidly due to backwater effects of John Martin
Reservoir. The reach-length weighted average slope for the problem area is 0.001. Aerial
photography indicates numerous old river scroUs within the cropland, and shows a large meander
bend truncated by encroachment. There is a high possibility that the inaccurate mapping
information causes this slope, which is represented by the water surface and would change with
either better mapping or field surveys. There were no photos provided from CWCB for Area 3.

The channel exhibits a similar change in channel conveyance. There is an increase in
dense overbank vegetational growth of about 90%. The overbank area in Area 3 widely covered
with ponds and standing water due to a wide flood plain and the high water table. Problem Area
3 is suspected to be aggrading, although this can not be stated with certainty at this point. This is
consistent with the apparent downstream movement of an aggradational trend observed in the
degradation rangelines. Area 3 has about the same slope as Area 2. Salt Cedar is limited to
dense growth, causing some encroachment to the channel conveyance.

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