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Jeffrey W. Schwarz Massey, Semeno[T, Schwarz, and Bailey Mobile Premix Concrete. Inc. October 10, 2001 Page 6 Bank erosion and channel avulsion of the magnitude described above is a predictable characteristic of this stream reach. In neither of the above cases was the bank erasion related to activities associated with MPC's off channel mining and reclamation activities. A single event, such as the failure of an artificial or rebuilt, non-engineered embankment due solely to natural hydraulic processes is a highly probable and predictable occurrence. A comparative bank line analysis for the period ranging from 1977 to 1998 was completed (reference Figures 2.4 and 2.5). The purpose of bank line analysis was to identify historical channel trends to identify future adjustments in planfotm and/or channel geometry. Historical channel patterns and movements, and an understanding of the factors which influence these movements, ofren provide information on the current and future channel stability of a river system. It is important to understand the past behavior of a river to predict the future propensity for channel change. Field reconnaissance and data collection along channel reaches upstream of, downstream of, and adjacent to the mine permit area were essential elements to understanding the factors which resulted in the May 5, 2001 bank failure. During the period between 1977 to 1998, the river adjusted in a lateral manner. As discussed previously, channel slope appears to have decreased by over 50%. Near the upstream limits of the reach of interest, in-channel mining occurred during a period ranging from 1975 to 1982 (reference Figure 2.3). Although the depth of the mine pit is not known, the width of the mine pit extended across the active channel width of the South Platte River. As such, all flow and sediment, generated from upstream sources were captured by the in-channel mine pit. Where the river's flow exited the downstream portion of the pit and reentered the natural channel it was sediment poor or considered "hungry water" (reference Figure 2.2). This "hungry water" is not in equilibrium with its sediment load; therefore the river must generate sediment through the processes of bed and bank erosion. Because the Brantner Diversion controls any further bed degradation, the sediment deficit is overcome by increased bank erosion and further channel meandering. During lower flows, the capacity to carry this sediment is reduced by the increased channel length and deposition of bed material along channel sand and gravel bars ensues. The comparative bank line analysis supports this summary of changes. From 1977 to 1998 the river channel increased its sinuosity and a more pronounced alternating bar sequence developed. At a left bank location immediately upstream of the May 5, 2001 "right bank" failure a large and pronounced gravel bar developed. Over the course of the 20 year period of record the angle of stream flow attack on the right bank "non-engineered" structure increased and the bank was subjected to more severe hydraulic forces. As flood stage rose on May 5, 2001, the flood flows continued to scour below the stream bank and "the concrete blocks began to slide". The presence of an existing, though relatively small, breach may have accelerated the rate of failure of this stream bank (reference the January 18, 2001 photos). In conclusion the relocation of the Bull Seep Drain Ditch did not contribute to the bank failure. Natural and progressive bank erosion processes were the principal mechanism of failure. Once the bank did fail, the existing Bull Seep Slough captured the flood waters and passed them to a location below the Brantner Diversion Structure.