FLOOD06381

• County: Otero
• Community: La Junta
• Stream Name: Arkansas River & Tributaries
• Basin: Arkansas
• Title: La Junta, Colorado Local Protection Project Phase I GDM Sediment Investigation
• Date: 9/1/1985
• Prepared For: US Army Corps of Engineers
• Prepared By: The Hydrologic Engineering Center
• Floodplain - Doc Type: Project

Evaluation of the program's performance for the Arkansas River sediment
study was based on both qualitative and quantitative measures. Qualitatively,
it was known that the river had been generally aggrading over the past several
decades and that deposits had been developing at the mouths of major
tributaries due to decreased main stem transport capacity resulting from the
irrigation diversions. These observed trends were all adequately reproduced
by the model without adjustment of any of the parameters.

The observed drift of the rating curve for the La Junta gage (#07123000)
was used for quantitative calibration. This measure was selected because it
is located in the area of interest and integrates, to a certain extent, the
behavior of a stream reach rather than just a single point or cross section.
Rating curves for the La Junta gage were obtained from the state Engineer" s
Office , Denver, Colorado (personal communication, Mr. Bud Walters) for the
period from 1950 to 1982. These rating curves, plotted at approximately ten
year intervals demonstrate, quantitatively, the aggradation taking place
within the study area (see Fig. 4.10). It appears that the river has aggraded
approximately two feet during this time period. Changes in channel roughness
could also effect the rating curve; however, the degree of roughness change
necessary to duplicate this magnitude of gage shift is physically unrealistic
(particularly at lower discharges).

The program was calibrated to reproduce the 1950 rating curve by adjusting
Manning n-values (Note: the cross section data in the La Junta sub-reach was
based on 1953 surveys.). This required use of an n value of 0.025 to 0.035
for the channel and 0.045 to 0.050 for the overbanks. The calculated initial
condition rating cut'Ve is also shown on Fig. 4.10

Movable boundary simulations were then made using the flow records (see
Sec. 4.3) developed for the 1951-1979 period. Rating cut'Ves at section 1198.1
(location of the gage) were calculated by HEC-6 at approximately ten year
periods and compared with the measured ratings. Initial calibration t'uns
indicated too little deposition. This was corrected by adjusting (decreasing)
the movable bed width. Final calibration results are also depicted on Fig.
4.10.

4.5 Simulation Results

The results of numerical simulations of 100 years of continuous flows
using HEC-6 are presented herein in three formats:

1) Graphs of calculated stream bed thalweg profiles at time zero, after
33 years. 67 years and 100 years. (Figures 4.11, 4.13-4.16, 4.21,
4.23)

2) Graphs of the calculated change in bed elevation at a cross section
versus time. (Figures 4.12, 4.17-4.20, 4.22, 4.24)

3) Table of the average change in bed elevation at the end of the
100-year period for the project sub reach (approximate river mile
1196.6 to 1200.5). (Table 4.4)

These displays were selected as those most pertinent to the evaluation of
the long-term performance of various flood control projects with respect to
the transport/deposition of sediment.

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