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Dec-04-01 02e23P • • P.03 Mr. Jeff Schwan necember 4, 2001 Page 2 art aerial hase map with topography superimposed. Une can see corttnurs, which irregularly define a dcpn:ssion adjacent to the flood control Icvet:. "I"his Reclamation Plan mapping was based on the Gingery and Associates, 1977 mapping (this mapping was also utilized for tltc flood insurance study). Point 3 Part A Geomorphic Analysts Un Oetobcr IU, 2001, I prepared acid submitted to you a forensic analysis of the bank failure and stream capture by the pre-existing Rull Seep Slough. "1'um Schreiner mentions a failure of a (lewd control levee in his letter of November 21, 2001. Specific:+lly, the implication of a flood control levee is that such a fcatttrc was an engineered structure. Itt the case of the May 5, 2001 failure, the actual area that tailed was a "rebuilt stream bank". Tltis sartte location ha+l Tailed in the past and this bank failure resulted in the Bull Seep Slough. '1'hc Bull Seep Slutrglt was a depression and often contained standing caster in many, if not all of the post 1972 aerial photos. To the best of my krtnwledge, (1) this reconstructed bank was trot engineered and (2) the "builders" of this "bank" dirt not prepare any "as-builts" nr 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 de;rivetl from the Macltttosh properly. !t is also my understanding that broken cottcrcu: anJ construction debris was hart of the construction fill. Based on the nature of this fill, it is unlikely that the material placement would meet modern construction skmdards and therefore should not be referred to as "a flood control levee". My letter of October 10'" presented :+ historical analysis of the tluvial system (sec Figure 2.1). Phis figure suggests that rho reach where the 2001 bank failure occurred is a very Aynamic stream reach. Review of F'ig+tre 23 illustrates that in-channel gravel mining occurred upstream of the Bull Seep Slough, heginrting sometime after 1973 and coding around 1983. Gravel mining .Zlso occurred Weer the Rull Sccp Slough's outlet to the Snutlt Platte River (i.c., the Hazeltine Pit). This is docurttertted in several serial photos of 19605 vintage. In-channel mining significantly affected sedirnertt transport and channel morphology. In my letter of Uctober 10`x', 1 also noted that the "artificial grade control" known as the Branfier Diversion Dam influenced the channel rnnrphology of our reach of interest, ! 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. !n sutttntety ,upstream "in-channel" mining will Irap setiimenl and will release "hungry water". This "hungry" water will be erosive and will result in channel bed degradation and/or bank erosion. Bartk erosion typically results in an increase in the channel's tendency to meander or increase in Sirtuosily, Since the cbanttel grade is controlled at the Brarttncr, bed degradation is minimal. 'Therefore, the net response to "irt-chartncl 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. '1'hc bank line analysis illustrates a decrease in cltennel slope through an increase in channel length. The erosional processes which arc associated with these changes in chermel morphology result in the development of bars. As the cltennel 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 nlicn becomes more focused resulting in a mare direct line of atts+:k on the outer bank of the next dnwnstrcam bend,