FLOOD05850 - Laserfiche WebLink

Home Browse Search

"I--n---------- , I ' II II II II II II I I I I I I 1-1 I I I I I I I 100-year floodplain limits were determined using state-of-art practices for the' development and presentation of tl:1e floodplain map (see maps 2, 3, and 4). 5. Stream and Channel Geomorphology: A comparison was made between the aerial photographs from 1977 and 1996 for the three gravel sites in question (see maps 5,6, and 7. The purpose of the comparison was to determine the degree of stream channel migration over time. Realizing that natural streams are dynamic in nature, one would expect a certain degree of cha.Jl!lel migration to OCCUIover time depending on geomorphologic conditions. In malyzing the photographic detail in maps 5,6, & 7, it appears that very little channel migration has taken place over the past twenty years. Consequently, it is the opinion of CWCB staff that the overall flood conveyance of the channel and the floodplain cross sections has not changed significantly since the time of the original floodplain study. 6. Impacts ofthe Gravel Pit Opel'lltions on the Designated 100- year Floodplain SITE NO.1 The operation is outside of the limits of the lOO-year floodplain delineation. Therefore, no specific comments aJ:e provided for this site. SITE NO.2 The CWCB has not received any engineering plans regarding the proposed operations for the gravel pit. Fralll preliminary information obtained from others, the CWCB staff finds that the hydralJlic control for the backwater computations lies just downstream from the proposed gravel operation. The proposed operation lies partially within the right overbank 100-year floodplain. The impacted floodplain is in a low velocity, low conveyance area; therefore, limited adverse impacts to the water surface elevations are expected. However, a large flood event may inundate the area causing major flood damage to the pit operations or capture of the gravel pits. Caution must be used when considering a levee system that may be placed on the river side of the gravel operation because it will transfer the floodwater conveyance area to the left overbank areas. The CWCB appreciates tile information provided by Mr. James Preston. Mr. Preston has stated a number of comerns regarding gravel pit operations in the river valley (see Appendix). I I ,~ I SITE NO.3 From previous discllssions and observations during the September 26, 2001 field inspection, this gravel pit operation is completely within the 100- year floodplain. No flood protection levee system was witnessed at the site. However, 'a huge pit excavation has taken place within the site. This excavation may provide the river channel an opportunity to relocate during a major flood event. Fortunately, the existing left and right overbank floodplain limits aJ:e adjacent to the bluff lines. Therefore, no impacts to the floodplain limits or to the adj acent lands outside of the floodplain would be expected. If the river does relocate during a major flood event, it is possible that downstream lands could be inundated even though they are not presently shown to be in the 100- year floodplain. 4