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5. ESTIMATE OF CHANNEL CHANGES FOR A SINGLE FLOOD EVENT 5.1 Development of Geometric Data for HEC-6 Single-Event Simulation 5.1.1 ExistinK Conditions A study reach for the La Junta project was established from river mile 1194.2 to 1201.0. Cross sections must be more closely spaced for the single-event analysis than for the long-term analysis in order to maintain numerical stability in the HEC-6 calculations. The additional geometric information was obtained from a 1976 survey map of the La Junta a"rea. For computational purposes, the study reach was extended upstream two miles to the Fort Lyon Canal Diversion. Cross sections were extended downstream 17 miles in order to tie into the stream gage at Las Animas. The resulting geometric data set contained 31 cross sections of which 20 were within the study reach. Figure 5.1 presents a diagram of the study area along with cross-section locations. 5.1.2 Proiect Conditions The HEC was provided with a sut'Vey map showing the project levee configurations proposed by the Albuquerque District. These data were obtained from the District's most up-to-date surveys taken in 1976. One levee location was proposed for the south bank in combination with one of two levee locations proposed for the north bank. These correspond to project configurations #3 and #4 in Table 4.2 and are also shown in Figure 5.1. The single-event analysis was originally to include an investigation of a l75-foot wide low-flow channel. It is suggested here that the presence or absence of such a channel with either levee configuration would have little effect on the overall sediment response of the river during a severe flood event. The low-flow channel would contain less than 5~ of the 212,000 cfs flowing during a standard project flood. Therefore, the low-flow channel was not included as part of the channel cross section for the single-event study. The existing geometry (from 1976 field data) was modified to include the proposed levee configurations. The levee heights were chosen so that over- topping would not occur at any cross section with levees. 5.2 Development of Sediment Data Keasured field data for the sediment load that accompanies the high flows of a single-event flood are generally quite rare. Extrapolation of load curve data can lead to serious errors. For example, it is possible to obtain a load in tons/day that corresponds to a sediment concentration greater than one million parts per million (ppm) (or solid rock) for the particular flow. To prevent this from occurring, an isoconcentration line for the maximum expected sediment concentration was plotted. This line was then used as an asymptote for the total load curve at higher flows. Procedures for developing bed material and inflowing sediment load characteristics for the Arkansas River are discussed in the following sections. 49