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Mr. Jeff Schwarz December 4, 2001 Page 2 an aerial base map vvi[It topography superimposed. One can see contours, which irregularly define a depression adjacent to the Flood control levee. This Reclamation Plan mapping was based on the Gingery and Associates, 1977 mapping ([his mapping was also utilized for [he flood insurance study). Point 3 Part A Geomorphic Analysis On October 10, 2001, [prepared and submitted to you a forensic analysis of the bank failure and stream capture by the pre-existing Bull Seep Slough. Tom Schreiner mentions a failure of a Flood control levee in his letter of November 21, 2001. Specifically, the implication of a flood control levee is that such a feature was an engineered structure. In the case of the May ~, 2001 failure, the actual area that failed was a'`rebuilt stream bank". This same location had tailed in the past and this bank failure resulted in the Bull Seep Slough. The Bull Seep Slough was a depression and often contained standing water in many, if not all of the post 1972 aerial photos. To the best of my knowledge, (1) this reconstructed bank was not engineered and (2) [he "builders" of this "bank" did not prepare any "as-builts" or perform any quality control during construction. It is my understanding that the material used in this bank fill was a combination of unclassified (silts, sands, gravels and clays) materials derived from the Macintosh property. It is also my understanding that broken concrete and construction debris was part of the construction fill. Based on the nature of this fill, it is unlikely that the material placement would meet modem construction standards attd therefore should not be referred to as "a flood control levee". My letter of October 10'" presented a historical analysis of [he fluvial system (see Figure 2.1). This figure suggests that the reach where the 2001 bank failure occurred is a very dynamic stream reach. Review of Figure 2.3 illustrates that in-channel gravel mining occurred upstream of the Bull Seep Slough, beginning sometime after 1973 and ending around 1983. Gravel mining also occurred near the Bull Seep Slough's outlet to the South Platte River (i.e., the Hazeltine Pit). This is documented in several aerial photos of 1960s vintage. In-channel mining significantl}• affected sediment transport and channel morphology. In my letter of October 10'h, [also noted that the "artificial grade control" known as the 8ranmer Diversion Dam influenced the channel morphology of our reach of interest. 1 have included Figure 2.2 to illustrate the qualitative impacts of "in channel mining" and the presence of a grade control such as the Brantner Diversion Ditch. In summary ,upstream "in-channel" mining will trap sediment and will release "hungro water". This "hungry' water will be erosive and will result in channel bed degradation and/or bank erosion. Bank erosion typically results in an increase in the channel's tendency to meander or increase in sinuosity. Since the channel grade is controlled at the Brantner, bed degradation is minimal. Therefore, the net response to "in-channel mining" is bank erosion or an increase in the channel's sinuosity. Figures 2.4 and 2.5 provide an historical analysis of the changes that have occurred through time in the reach of interest. The bank line analysis illustrates a decrease in channel slope through an increase in channel length. The erosional processes which are associated with these changes in channel morphology result in the development of bars. As the channel becomes more sinuous, the material eroded from the outside of a bend is deposited along the inside of the next downstream bend. As the bar continues to grow, the flow often becomes more tbcused resulting in a more direct line of attack on the outer bank of the next downstream bend.