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4. ESTIMATE OF CHANNEL CHANGES OVER A 100-YEAR PERIOD 4.1 Development of Geometric Data for HEC-6 Long-Term Simulation 4.1.1 ExistinK Conditions The basic geometric information was developed from a set of cross sections provided by the Albuquerque District that cover the reach from Great Bend, Kansas to Pueblo, Colorado. The sections are dated 1940 and 1945. The 1945 sections were selected for use because bed material size distribution information was available for 1946. A study reach from Las Animas to Avondale was identified. This reach was selected to avoid the need to simulate behavior of John Martin Reservoir and to avoid having to develop separate sediment load information for both the Arkansas R. and Fountain Creek. Good flow records exist for both Las Animas and Avondale (USGS gages) as well as at two intermediate locations; La Junta and Nepesta (State of Colorado gages, reported in USGS water supply papers). All 1945-surveyed cross sections for this reach were digitized by hand in HEC-6 format. Where additional spatial resolution was necessary. supplemental data were obtained from 1953 cross-section survey information, also provided by the Albuquerque District. Two sections were provided upstream of Avondale for purposes of calculating an equilibrium inflowing load curve. The section at Avondale was not thought to be "typical." In general, these sections seem to be biased towards constric tions (bridges, diversion dams); therefore, where necessary. valley sections were used as repeated sections. In some locations it was necessary to repeat cross-sections for additional resolution or to provide unique entry points for all tributaries and diversions. See Table 4.1 for a description of the cross sections used. Plots of the cross sections are shown in Appendix C. The resulting geometric data set contained 42 cross sections. Channel lengths were based on section river mile and overbank lengths scaled from the river range location map that is part of the 1953 cross section data. 4.1.2 Proiect Conditions Originally the HEC was provided with HEC-2 data decks and output files for existing and project conditions for the project subreach in the vicinity of La Junta. A profile and plan of the preliminary project design were also provided. Locations of the existing conditio!'. cross sections were not shown on this plan, however. The existing condition sections were based on those used for the Special Flood Hazard Information report (Aug. 1977). As these sections did not conform in time to those being used as initial conditions for the 100-year simulation (1945-1953 sections) it was decided not to attempt to use these sections in the modeling effort. The approach used was to simulate project conditions by replacing the 1945-1953 cross sections in the La Junta subreach with the "project sections." 9