Flood frequencies in the southeastern United States.

Regional flood frequency analysis is one of the most useful alternatives for determining the flood frequency relationship for an ungaged site or a site with a short record. In this study, a regionalized flood prediction technique, based on the concept of limited flood magnitude suggested by Malvick (1980), is modified and used in flood frequency analysis for the hurricane areas in the southeastern United States in order to study the characteristics of the maximum expected flood and flood magnitude of various return intervals in the region. The proposed method is suitable for making simple and reasonable estimates of the maximum expected flood and flood frequencies for a region and is usable for some engineering applications to hydraulic structure design, especially bridge hydraulic design. Goodness of fit of the proposed flood frequency distribution to observed flood frequency data was tested, and results indicate that flood frequencies of more than half the streamflow stations estimated using the suggested method adequately describe the observed flood frequency data. Simulation of flood data having the same record length as the observed data was made to test the possibility that deviation of the proposed flood frequency distribution from observed flood frequency data might be due to randomness. By examining the observed and simulated data, it is believable that the discrepancy between the observed and predicted flood frequency data is partly due to a chance of randomness. The maximum expected floods and flood frequencies for the sub-regions as a function of drainage basin area are compared. For the six states, Texas(eastern), Louisiana, Mississippi, Alabama, Georgia, and Florida, the maximum expected floods in eastern Texas are comparatively higher than the other states. Relatively, Florida has the lowest maximum expected floods. Maximum expected floods and flood frequencies of three 100-mile-wide strips parallel to the coast are found to be slightly different. The time of occurrence of the largest flood events and hurricanes is investigated and compared to see whether they are coincident. No apparent correlation of the largest floods and hurricanes was found