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Dec-04-O1 O2:23P ~ ~ Mr. Jeff Schwarz I]ecember 4, 2001 Pegc 2 an aerial base map with topography superimposed. Une can see contours, which irregularly define a depression adjacent to the flood control levee. '!'his Reclamation Plan mapping was based ort the Gingery and Associates, 1977 mapping (this mapping was also utilized for the flood insurance study). Point 3 Part A Geomorphic ArrnlysLs Un ()ctobcr 1 U, 2001, 1 prepared and submitted to you a forensic analysis of the bank failure and stream capture by the pre-existing Bull Scep 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 rhat such a feature was an engineered structure. In the case of the May 5, 2001 failure, the actual area that failed was a "rebuilt stream bank". This seine location had Tailed in the pass 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 pus( 1972 aerial photos. To the best of my knowledge, (1) this reconstructed bank was not engineered and (2) the "builders" of thls "bank" di(I nut prepare any "as-builts" or perfnr+tt ;toy qualify control during construction. It is my understtnding that the material used in this bank fill was a combination oC unclassified (silts, sands, gravels and clays) materials durivetl from the Macl»tosh properly, It is also my understanding that broken eoiterote 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 su+ndards and therefore should not be referred to as "a flood control levee". P.O3 My letter of Uctobcr 10"' pn:scnted a historical analysis of the fluvial system (sec 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 sontetinte after 1973 and cnding around 19$3. Gravel mining also occurred near the Bull Sccp Slough's outlet to the Snutlt Platte River (i.e., the Hazeltine Pit). This is documented in several aerial photos of 1960s vintage, In•channel mining significantly affected sediment transport and channel morphology. !n my IulWr of October 10`x', 1 also noted that the "artificial grade control" known as the Branfier Diversion Dam influenced the cltartnel morphology of our reach of interest. 1 have included ITigure 2.2 to illustrate the qualitative impacts of "in cltanitel mining" and the presence of a grade control such as the Brantner Diversion Ditch. !n suittmaty , upstreairt "in•ehannel" mining will Trap sediment and will release "hungry water". Tltis "hungry" water will be erosive and will result in channel bed degradation and/or bank erosion. Baitk erosion typically results in an increase in the channel's tendency to meander or Increase in sinuosity. Since the cltanttel grade is controlled at the Brantner, bed degradation is minimal. 'Therefore, the net response to "in-channel mininb' 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 lime in the reach of interest. 'I'hc hank line analysis illustrates a decrease in channel slope through an increase in channel length. The erosional processes which arc 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 dnwnstrcam bend, /ts the bar continues to grow, the flow often becomes more focused resulting in a more direct line of attack on the outer bank of the next dnwnstrcam bend.