Drainage_and_Flooding_Report-dated_10_03_2008

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Mr. Ron Bottoms, City Manager k I o t z a s s o c i a t e s October 1, 2008 Page 18 of 31 Thus, two different methods were used to assess the capacity of major ditches and channels, the methods being selected to utilize the available data in a manner consistent with the level of detail about the ditch or channel. For major drainage ditches for which FEMA hydrologic and hydraulic models were available (with revision to reflect new information), the 10-, 50- and 100- year flood flows were simulated. Figures 2 through 16 show the flow profiles for various flood frequencies for the various modeled channels in the different major watersheds composing the study area. Note that the profiles sometimes combine separate FEMA models into a single model. The location and extent of the drainage ditches for which these flow profiles were determined are shown in Exhibit 11. Table 3 tabulates the models. When the model computed flood level rises above the bank elevation (compare the computed water surface profiles to the left and right bank elevation in these figures in Figures 2 through 16), channel capacity is at least locally deficient and im flood levels within bank for the frequency of interest. Potential measures to accomplish such lowering are discussed in section 5 below. Exhibits 16, 17 and 18 compare the locations where the computed flow depths exceed the channel banks for various flood frequencies to the locations of flooding reports (these reports have been discussed in section 3.2.1 above). It is seen that some areas with of out -of -bank conditions occur in the near vicinity of areas where flood reports are frequent, while in other situations out -of -bank locations do not closely correspond to the locations where flooding reports are numerous. Table 2 compares of the out -of -bank conditions to areas where flooding reports are numerous. Based on where out -of -bank conditions occur relative to flooding reports, an assessment can be made as to whether inadequate channel capacity is likely a significant source of reported flooding problems in a particular area. Table 2 provides such assessment: For a storm frequency of interest, it is concluded that the overflow from the major channel is or is not a likely significant contributor to the reported flooding problems. If the drainage ditch or channel is judged to be a significant source of reported flooding problems, channel improvements should be given high priority in the type of solution proposed to address flooding problems along or near the channel.