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VIII-5 Numerous levees follow along short reaches of the river bank and criss-cross the fl oodpl ain. These levees are efforts of individual land owners and are not integrated into a flood control scheme. Only one levee protects a small part of the floodplain from submergence by the 1DO-year flood. EROSION PROBLEM The South Platte River is actively eroding its bed and banks wherever it is free to do so with the absence of controlled drops and adequate bank rip- rap. Three-quarters of a mile wide on the average, the floodplain is used pri- marily for agriculture. There are several gravel pits, the Adams County Fairgrounds, a landing strip and few buildings. The floodplain land away from the river is lower than along the riverbanks which causes drainage problems. Between Henderson and Brighton, the land at the riverbank is, on the average, 2.5 feet higher than in the back area. On the right or east side of the river between the East 104th Street bridge and Henderson some of the floodplain slopes away from the river, some has no lateral slope, and the remainder has been disturbed by mining operations. On the other side, the floodpl ain slopes towards the river in general. All tributaries flow downstream on the floodplain before emptying into the South Platte River. An inventory of conditions and structures in the river channel is presented in Table VIII-2. The inventory is based upon a field inspection of the entire forty mile study reach. Estimates of bed erosion have been made by Stevens (1983) by comparing historical 1963 data with 1983 river bed condi- tions. Chatfield to Oxford The river between the Ken Caryl sewer line crossing immediately downstream from C-470 from 1963 to 1983 has degraded a maximum of about 6 feet down- stream from the sewer crossing. The vertical downcutting diminishes rela- tively proportionately with distance to a point 1/2 mile upstream of the Columbine Dam, a distance of nearly two miles. Except for the Columbine Dam, erosion upstream would have been in excess of 6 feet. Sed i ment at ion The subject of sedimentation covers the erosion, entrainment, transporta- tion, deposition, and compaction of sediment. These are natural processes at work on the South Platte River. This bed erosion has been accompanied with bank erosion on both sides of the river. Trees have fallen into the channel and a gravel pit has been breached. The forces which affect sedimentation are water depth and velocity, wind, gravity, and ice thrust. All four affect the shape and character of the river. Fluvial sediment relates to water while aeolian sediment relates to wind. Immediately upstream of the Columbine Dam the river bed has aggraded approx- imately 3 to 4 feet due to the flatter hydraulic slope. Downstream of the Columbine Dam the 1963 to 1983 vertical erosion of the river bed is estimated to be approximately 6 feet, but it is reduced to 1 or 2 feet over a distance of the next 1 1/2 miles. Understanding the fundamentals of the sedimentation problem is necessary for master planning the South Platte River through the Denver Metropolitan area. However, the complexities involved, even on a short reach-by-reach basis, means that the preliminary engineering of a specific structure or segment of a river must be site specific and needs to deal with detail that is beyond the scope of a 40 mile river master plan. In the following pages the ero- sion problem of the South Platte River is quantitatively described. From Bowles Avenue to the Union Street Diversion Dam the bed erosion has averaged about 1 to 2 feet over the 20-year 1963 to 1983 period. The Union Street Dam has acted as a downstream control to stop further downcutting. At Oxford Avenue the bed has eroded about 3 feet.