Hydraulic and hydrologic aspects of flood-plain planning

Abstract

The valid incentives compelling occupation of the flood plain, up to and even into the stream channel, undoubtedly have contributed greatly to the development of the country. But the result has been a heritage of flood disaster, suffering, and enormous costs.Flood destruction awakened a consciousness toward reduction and elimination of flood hazards, originally manifested in the protection of existing developments. More recently, increased information and realization of the problem have shown the impracticability of permitting developments that require costly flood protection. The idea of flood zoning, or flood-plain planning, has received greater impetus as a result of such realization.This study shows how hydraulic and hydrologic data concerning the flood regimen of a stream can be used in appraising its flood potential and the risk inherent in occupation of its flood plain. The approach involves the study of flood magnitudes as recorded or computed; flood frequencies based on the experience shown by many years of gaging-station record; use of existing or computed stage-discharge relations and flood profiles; and, where required, the preparation of flood-zone maps to show the areas inundated by floods of several magnitudes and frequencies.Methods are outlined that will enable the planner to delineate areas subject to inundation by floods of specific recurrence intervals for three conditions; (1) for the immediate vicinity of a gaging station, (2) for a gaged stream at a considerable distance from a gaging station, and (3) for an ungaged stream. Curves have been developed from which the average depth for a flood of specific frequency can be estimated on the basis of the average depth for bankfull flow; this simplified approach should be very useful in the initial stages of flood-plain planning.Brief discussions are included on various types of flood hazards, the effects of urbanization on flood runoff, and zoning considerations.