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. Amazon Creek Enhancement(ACE) PE-HY File No.6512 <br /> . Project <br /> Channel Vegetation Type Manning's Roughness <br /> Pacific Willow (Poles) 0.050 <br /> Fascine Willow 0.100 <br /> Grass 0.035 <br /> Historic Channel bottom and grass 0.035 <br /> Table 1 —Manning's Roughness values used in this analysis. <br /> The HEC-2 model was run with the FEMA 100-year flood hydrologic dataset for pre- <br /> project conditions and for future project conditions (established riparian vegetation <br /> conditions). Results of modeling are listed in Table 2 and Figure 1. Generally,channel <br /> widening increases the conveyance of the reach. However, fully established vegetation <br /> offsets the increase in conveyance. Also, many channel sections remain in their historic <br /> configuration (i.e. bridge crossings). Thus,the overall conveyance of the project reach <br /> has not been increased. As one can see in Figure 1, FEMA 100-year flood profiles are <br /> nearly the same for before and after project conditions. Metaphorically, "a chain is only <br /> as strong as it's weakest link." In terms of channel conveyance, the weakest links in the <br /> project reach are located at bridge crossings. <br /> HEC-2 modeling suggests that as long as the historic portion of the channel cross-section <br /> maintains its historic conveyance properties, riparian vegetation growth is allowable to <br /> the full extent of excavated overbank areas. Critical locations within the channel must be <br /> maintained free of woody vegetation and debris. Critical locations include all bridge <br /> crossings and all channel constrictions such as the area at a pipe outfall near channel <br /> station 366+00 that was left in its historic condition. The project reach will require <br /> routine inspection and maintenance. Riparian vegetation growth must be limited to the <br /> overbank areas only. In no case can woody vegetation be allowed near bridge crossings. <br /> Areas upstream,under and downstream of bridge crossings must be maintained free of <br /> woody vegetation and debris for this analysis to be valid(see"Channel Maintenance"). <br /> b. Field Investigation <br /> A field evaluation of the constructed channel was made on July 29, 1998 by PE-HY staff. <br /> Planted vegetation within the constructed channel was beginning to grow and in some <br /> areas had become well established. It was noted that vegetation plantings of fascine <br /> Date: 08/26/98 Page 2 Channel Maintenance Analysis <br /> CENWP-PE-HY;MTK <br />