Hydraulic and hydrologic aspects of flood-plain planning

Abstract

The valid incentives compelling occupation of the flood plain, up to and eve n 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 knowledge of the problem has shown the impracticability of permitting development that requires costly flood protect/on. The idea of flood zoning, or flood-plain planning, has received greater impetus as a result of this 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 based1 on experience shown by many years of gaging-station record; use of existing or computed stagedischarge 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. The planner can delineate areas subject to inundation by floods o* specific recurrence intervals for three conditions: (a) for the immediate vicinity of a gaging station; (b) for a gaged stream at a considerable distance from a gaging station; and (c) for an ungaged stream. The average depth for a flood of specific frequency can be estimated on the basis of simple measurements of area of drainage basin, width of channel, and slope of streambed. This simplified approach should be 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. INTRODUCTION The frequent occurrence of floods throughout the country, and the growing consciousness of the great damage and sufferng resulting from these floods, have aroused a widespread effort to reduce flood destruction. Failure to recognize that the natural function of a flood plain is to carry away excess water in time of flood, often has led to rapid and haphazard development on flood plains with a consequent increase in flood hazards. 2 FLOOD-PLAIN PLANNING It is economically infeasible and often physically impossible to provide adequate flood-control measures for every locality subject to flood damages. Hence, corrective and preventive measures must be taken in order to adjust man's activities on flood plains to the regimen of streams. Such measures, generally known as flood-plain zoning or planning, can help solve or ease many flood problems. Fundamental to effective flood-plain planning is the recognition of the flood potential of streams and the hazards involved in flood-plain occupation. Where necessary restrictions are imposed on communities in their flood-plain development, a marked reduction in flood damage is possible. Basic data on the regimen of the streams, particularly the magnitude of floods to be expected, the frequency of their occurrence, and the areas they will overflow, are essential to floodplain planning. The report was initiated through a cooperative agreement between the U.S. Geological Survey, Water Resources Division, and the Commonwealth of Pennsylvania, Department of Forest? and Waters. It was prepared under the direction of J. J. Molloy, district engineer, Surface Water Branch, U.S. Geological Survey, Harrisburg, Pa. The studies were made, and the report written, by S. W. Wiitala and K. R. Jetter, hydraulic engineers, U.S. Geological Survey, and by A. J. Sommerville, hydraulic engineer, Pennsylvania Department of Forests and Waters. The provision in the Federal Flood Insurance Act of 1956, Public Law 1016, that involved consideration of flood-plain planning or zoning as a requisite for participation in the benefits of the Act, was the incentive for this study. Recognition of the need for such studies by the organizations involved in or proposing flood-plain planning in the Commonwealth provided further support. Many of the data used in this report were collected over many years by the U.S. Geological Survey in cooperation with the Corps of Engineers, Department of the Army, and other Federal agencies, the Pennsylvania Department of Forests and Waters, and various public utilities. The data for the studies in phase III, pa.rt 2, and supplementary field data for all phases were collected by tbe U.S. Geological Survey and the Pennsylvania Department of Forests and Waters. All public and private authorities involved in flood-plain planning activities that were contacted for this report, cooperated by furnishing data on the flood situation within their service areas. Additional data were obtained from local industries and organizations, State, county, and local government officials, and interested individuals. HYDRAULIC AND HYDROLOGIC ASPECTS 3