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

4.5.1 ExistinK Conditions

Anticipated trends in deposition within the project subreach for
"existing" conditions (i.e., no modifications to the river channel shape,
alignment, roughness or hydrologic/sedimentary regime within the 100-year
study period) are shown in Figures 4.11 and 4.12. Note, the predicted
aggradation at the La Junta gage site (river mile 1198.1) is about 5.5 feet.
This compares favorably with the observed aggradation (inferred from the
rating curve shift, Figure 4.10) of approximately two feet over a 3l-year
period. The average computed bed change in the project subreach was +3.4 feet
after 100 years of flow (deposition).

4.5.2 Proiect Conditions

Results of the simulations for the various project conditions (refer to
Table 4.2) are shown in Figures 4.13 to 4.16 (profiles) and Figures 4.17 to
4.20 (bed elevation changes at the gage section). The average bed elevation
changes within the project subreach are show in Table 4.4.

4.5.3 Ft. Lyon Canal Analysis

During the course of this investigation, the role of the operation of the
Ft. Lyon Canal, particularly the sand sluicing operation, became of interest
to the District. Note that the sediment concentration in the diverted water
was assumed to be 75~ that of the ambient river water concentration (refer to
Section 4.2.4); this, presumably, accounts for the sand sluicing operation.
An attempt was made to evaluate the sensitivity of the calculated bed changes
in the Arkansas River in the vicinity of La Junta to operation of the Ft. Lyon
Canal by perfot'ming the following simulations:

1) A simulation similar to the "existing condition" simulation with the
concentration of sediment in the Ft. Lyon diversion equal to the ambient
concentration of sediment in the river. This reflects the situation of zero
sand sluicing. The results of this simulation are shown on Figures 4.21 and
4.22. This condition is shown on Table 4.4 as "FLlOO".

Table 4.4
Average Predicted Bed Elevation Changes
Within the Project Subreach After 100 Years

Project
Configuration

Average Bed
Elevation Change
(ft)

1
2
3
4
"Existing"
FLlOO'"
NOFL'"

+2.2
+2.4
+2.0
+2.1
+3.4
+3.2
+3.8

'" See Section 4.5.3

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