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<br />Floodwater elevations determined in any particular area are not <br /> <br />always indicative of the depths actually attained. Often floodwaters <br /> <br /> <br />become isolated and entrapped in overbank areas resulting in depths <br /> <br /> <br />greater than those sho\<D on the flood profiles. These illustrative <br /> <br /> <br />plates were prepared on the basis of existing watershed conditions. <br /> <br />Obstructions <br /> <br />Of major concern is maintenance of channel and over- <br /> <br />. " <br /> <br />bank capacity of a stream to convey flood waters without causing <br /> <br /> <br />serious flood damage. Reducing waterway openings by approach embank- <br /> <br /> <br />ments at bridges and changing channel anG overbank capacities will <br /> <br /> <br />impede floodflow and cause greater backwater depths with consequent <br /> <br /> <br />increase in flooded areas. Obstructive new developments and future <br /> <br /> <br />land filling in or near the stream floodway can be a major factor in <br /> <br /> <br />multiplying the adverse effects described. <br /> <br /> <br />Estimated future flood crests and pertinent eleva- <br /> <br /> <br />tions at bridges within the study area are listed in Table 4. For <br /> <br /> <br />study purposes it has been assumed that limited clogging can be <br /> <br /> <br />expected and all bridge structures would stand intact. Significant <br /> <br /> <br />changes in this premise, imposed by differing conditions of a future <br /> <br /> <br />flood, could alter the estimated flood crests and flood limits <br /> <br /> <br />depicted in the tables and related plates. <br /> <br />. <br /> <br />App. 4 <br />20 <br />