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<br />_' I00•year floodplain limits were determined using state-of--art practices for the <br />development and presentation of the floodplain map (see maps 2, 3, and 4). <br />5. Stream and Channel Geomorphology: A comparison was trade between the aeriah <br />photographs from 1977 and 1996 for the three gravel sites in question (sec maps 5,6, and <br />7. The purpose of the comparison 4wa$ t0 determine the degree of stream chattttel <br />migration over time. Realizins that natural streams are dynamic in nature, one .would <br />expect a certain degree of cham~el migration to occur over time depending on <br />geomorphologic conditions. In analyzing the photographic detail in maps 5,6, & 7, it <br />appears that very little chatutel migration has taken place over the past riwenty years. <br />Consequently, it is the opinion of CWC$ staff that the overall flood conveyance of the <br />- channel and the floodplain cross sections has not changed significantly since the time of <br />" the original floodplain study. <br />G. Impacts of the Gravel pit Operations on the I)csignated 100-year Floodplain <br />SITE NO. 1 The operation is outside of the limits of the 100-year floodplain delineation. <br />Therefore, no specific comments are provided for this site. <br />51TE NO.2 The CWCB has not received any engineering plans regarding the proposed <br />operations for tltc gravel pit, frotu preliminary information obtained from others, the <br />CWCB staff finds that the hydraulic control for the backwater computations lies just <br />downstream fjom the proposed gravel operation. The proposed operation lies partially <br />.within the right overhank 100-year floodplain. The impacted floodplain is in a low <br />velocity, low conveyance area; therefore, limited adverse impacts to the water surface <br />elevations are expected. However, a lame flood event may inundate the area causing <br />major flood damage to the pit operations or capture of the gravel pits. Caution must be <br />used when considering a levee s}stem that may be placed on the river side of the gravel <br />operation because it will transfer the floodwater conveyance area to the Icft overbank <br />areas. The CWCB appreciates the information provided by Dlr. James Preston. Ivlr. <br />Preston has stated a number of concerns regarding gravel pit operations in the river valley <br />(see Appendix). <br />SITE NO.3 Ftom previous discussions and observations during the Scptentber 26, 2001 <br />' field inspection, this gravel pit operation is completely within the i00- year floodplain. <br />No flood protection levee system was witnessed at the site. However, a huge, pit <br />excavation has taken place within the site. This excavation may provide the rivet channel <br />an opportunity to relocate during a major flood event. Fortunately, the existing left and <br />right overbank floodplain limits are adjacent to the bluff lines- Therefore, no impactr to <br />the floodplain limits or to the adjacent lands outside of the floodplain would be expected. <br />If the river does relocate during a major flood event, it is possible that downstream lands <br />could be inundated even though they arc not presently shown to be in the 100- yeaz <br />- floodplain. <br /> <br />..,.. <br />4 ' <br />E51-d Bl0/5l5 d 556-! 955E998f5E s37a(10s3a' 1Vir~lYN 100~'i08d 9111 E55Z-1'0-]35 <br />