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<br />2. INTRODUCTION <br /> <br />The Arkansas River between Pueblo, Colorado, and John Martin Dam, a <br />distance of about 125 river miles, is an alluvial, sand-bed river. It meanders <br />between bluffs in a flood plain about one mile in width. During geologic time <br />the downstream (eastern) portion of this reach has been migrating southward due <br />to heavy sediment loads from northern tributaries. A local flood control <br />project has been studied for the town of La Junta, which is in the downstream <br />one-third of this reach. <br /> <br />At the request of the Albuquerque District, the Hydrologic Engineering <br />Center (HEC) undertook a study to thoroughly define all factors involved in the <br />production and movement of sediment within the Arkansas River Basin from the <br />vicinity of La Junta to Pueblo Dam. This reach of the river is approximately 80 <br />miles in length and involves an intervening drainage area of about 7500 square <br />miles. The future performance of various flood control alternatives with regard <br />to channel stability, sediment movement, and project maintenance was evaluated. <br />The alternatives considered were various channel and levee configurations. <br />Evaluations were based on both long-term (lOO-year period) and short-term <br />(single flood event) hydrologic scenarios. Specific work items are given in the <br />Scope of Work (Appendix B). The primary tool used in this study was the movable <br />boundary computer program HEC-6 entitled "Scour and Deposition in Rivers and <br />Reservoirs"(9). The hydrologic and sediment regimes of the study reach are <br />complex due to four tributaries and eleven major irrigation diversions. <br /> <br />The strategy used in this comprehensive study was to integrate both <br />qualitative and quantitative analyses to best identify critical factors <br />pertaining to the stream's behavior and, therefore, predict the stream's <br />response to various channel modification plans. A major study component was the <br />application of HEC-6 to simulate changes in the stream bed profile. It is <br />important to note that the process of assembling, interpreting and analyzing the <br />field data necessary to apply the model led to a comprehensive understanding of <br />the stream's behavior and contributed directly to the qualitative aspects of the <br />study. <br /> <br />A unique aspect of this study was the use of HEC-6 to simulate both the <br />long-term (100 years) and short-term (single flood event) performance of the <br />existing channel and the proposed flood control channel. The approach used was <br />to recognize the basin-wide scale of long-term stream behavior and localized <br />response of short period, high-flow events. Consequently, different data sets <br />were employed for the different aspects of the study. The long-term simulations <br />covered approximately 100 river miles and utilized historical observed water and <br />sediment discharges (Figure 2.1). The single-event simulations focused on the <br />stream channel in the vicinity of La Junta only, and utilized synthetic water <br />discharge hydrographs and sediment load curves. <br /> <br />3 <br />