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AMAZON CHANNEL MAINTENANCE STUDY HYDRAULIC ANALYSIS As stated earlier, a hydraulic analysis of Amazon Channel was conducted as part of the study efforts. The COE's HEC-2 hydraulic program was used for the analysis. The current Amazon Channel input file was obtained which included the upcoming construction needed to relocate the split floodway around Spectra Physics. The model was executed and the simulated 100-year water surface elevations were verified against those currently published as part of the City's Flood Insurance Study (FIS). The first hydraulic exercise was to simulate the estimated 1993 winter conditions in the lower channel assuming no maintenance this summer and compare these 100-year water surface elevations to those in the FIS and those simulated assuming the lower channel is maintained this summer. Figure 1 presents these 100-year flood comparisons. Note that the 1993 winter conditions are higher than the published FIS values throughout the study reach. However, the water surface differences are relatively minor and average around 02 feet. If the lower channel was maintained this summer, the 100-ycar flood elevations would be significantly lower throughout this reach. One of the study objectives was to use existing topographical information to estimate the increase in the aerial extent of flooding that could result in the unmaintained condition (i.e. 1993 winter conditions) when compared to the FIS. The poor topographical information available for the lower channel floodplain and the relatively small elevations differences between the two conditions made this technique of estimation essentially impossible to use. However, output from the hydraulic model was used to estimate the increased flood risk to be approximately 18.4 acres from the SPRR to Bertleson Road. It is impossible to say whether any additional buildings would be flooded without good topographical mapping; however, it does not appear likely to be the case. The average increase in the floodplain width throughout the affected reach is estimated to be 120 feet. Although this may seem like a significant increase, it translates to loss than throe tenths of an inch at the scale of the floodplain maps published by FEMA (Reference 3). The second hydraulic exercise was to look at the differences between 1987 and 1992 conditions over a much longer reach and compare these to the FIS values. Figure 2 presents these results. The reach chosen was from the diversion to Madison Street, a total channel distance of approximately 6.6 miles. As expected, Figure 2 shows only slight differences in the lower channel reach. However, the differences in portions of the upper reach are quite significant. The recent maintenance (i.e. 1990 and . 1991) from West 11th Avenue to Chambers Street appears to have had some dramatic effects on lowering the risk of flooding throughout the upper reach of the Amazon Channel. The third and final modeling exercise involved the simulation of various assumed 'n" values held constant throughout the entire study reach and their comparison to the FIS. The "n" values were varied from 0.025 to 0.10, which reflects the range from just maintained' under existing practices to no maintenance ever.' The results of these simulations are presented in Figures 3 and 4. The results dearly show that channel maintenance serves an important flood control objective, and without sorne form of maintenance the risk of flooding would dramatically increase throughout the entire study reach. The results also show that the existing FIS has basically assumed that the Amazon Chalotel will be well maintained. Without some form of periodic maintenance, the flooding risk along the Amazon Channel will be much greater than that currently shown on the City's floodplain maps. 3 I