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<br />1 . EXECUTIVE SUMMARY <br /> <br />A comprehensive study was perfonned to identify and quantify potential <br />impacts of a proposed flood control project on the Arkansas River in the <br />vicinity of La Junta, Colorado (see Figure 1.1). The project being considered <br />for reduction of flood threat to La Junta consists of various levee and <br />channelization components. The Arkansas River in this area and for many miles <br />upstream and downstream is an alluvial stream with a bed composed of fine sand. <br />The stream bed in the vicinity of La Junta has been aggrading slowly during the <br />past several decades. The hydrologic and geomorpholigic regimes of the river in <br />this reach have been substantially altered by a series of major irrigation <br />diversions. <br /> <br />The study objectives were to identify the existing stream regime and <br />detennine how this might be impacted by the construction of various project <br />configurations. Of specific interest were the anticipated future maintenance <br />requirements for the project, and possible adverse impacts of the local <br />channelization works on upstream and downstream reaches. <br /> <br />The study was perfonned using a variety of technical methods. These <br />methods ranged from simple hand calculations of stream bed stability based upon <br />bed shear stress criteria to complex numerical modeling of the interaction <br />between bed scour and deposition, sediment transport, and channel geometry. The <br />reason for using an array of techniques was that conclusions would be better <br />supported if indicated independently by a variety of techniques. The state of <br />the science of predicting impacts of any given activity on the behavior of an <br />alluvial stream is such that substantial engineering judgement must be applied <br />to interpret the results of these complex computations. <br /> <br />A major aspect of the study involved use of the computer program HEC-6 <br />"Scour and Deposi tion in Rivers and Reservoirs." This numerical model was used <br />to estimate project perfonnance for both long-tenn (100 years of historical <br />flows) and short tenn (design stonn) hydrologic scenarios. An array of <br />levee/channel configurations was simulated in addition to the no-project <br />(existing) condition. Project impacts were defined by comparison of the <br />predicted stream bed evolution for the project condition with that for the <br />existing condition. <br /> <br />In general, it was concluded that all of the projects considered would <br />decrease the trend of deposition within the project reach. This is due to the <br />narrowing and straightening of the river which increases the sediment transport <br />capacity. A potential problem that was identified was that the design channels <br />may be unstable even during low flows and require bank protection. Another <br />problem is that, because of the increased transport capacity within the project <br />reach, sediment loads are increased downstream of the project. It appears that <br />the downstream reach has insufficient capacity to convey these loads resulting <br />in substantial increased. deposition over existing conditions in the downstream <br />reach due to the project. These conclusions were supported by both the long <br />tenn and single event analyses. <br /> <br />1 <br />