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<br />The SC should identify those areas where depths averaging more than 1 foot could <br />occur and then should undertake a more detailed analysis of these areas. In the <br />unlikely occurrence of sheet runoff with an average depth of more than 3 feet, <br />the SC should contact the Regional PO for guidance. The SC should select the <br />specific methods to be used in the detailed analysis; however I normal depth <br />calculations are usually used, with effective flow areas established using <br />available topographic information, historical information, and engineering <br />judgment, Losses through ground infiltration normally should not be considered, <br /> <br />The SC should determine the lOO-year flood discharge at the head of a sheet flow <br />area by an appropriate method. In the absence of a p~rmanent rnanmade channel <br />or large-scale topographic features to restrict its flow, this discharge should <br />be routed uniformly across the entire area susceptible to sheet flow, Cross <br />section and slope information must be obtained to determine average flood depths <br />across the area. Whenever small-interval contour mapping exists, cross sections <br />should be developed directly from those maps; otherwise, a limited number of <br />cross sections should be taken across the area to determine average flood depths. <br />Cross sections should be maintained perpendicular to flow over the surface. <br /> <br />In urban areas I sheet runoff is affected by buildings, sewer and drainage <br />systems, and street design. In many cases, storm sewer and street systems are <br />intended to carry the total discharges of only relatively frequent floods, Less <br />frequent floods, including the lOO-year flood, will often result in shallow <br />flooding as the capacity of designed drainage networks is exceeded. Such <br />problems, if amenable to detailed study at all, would be exceedingly costly to <br />analyze. Because such areas are already developed, improved .drainage systems <br />may be the only short-term solution to the problem. Analysis of local drainage <br />problems is considered beyond the scope of Frs preparation. Therefore, the SC <br />should rely on. historic data and the reports of local engineers and residents <br />to identify such areas, and use field reconnaissance and engineering judgment <br />to delineate them. <br /> <br />The procedures outlined in this Appendix will be adequate to determine areas <br />susceptible to sheet flow flooding, but they may not indicate the severity of <br />the possible local hazard, Any available information, including reports of local <br />residents, historical data, and especially photographs of past floods, should <br />be included in the Frs Report to document the possible velocity, depth, debris, <br />and shifting channel hazards that may exist. <br /> <br />A2-S <br />