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Hydraulic and hydrologic aspects of flood-plain planning

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.

Pennsylvania↗

Floods in the Skagit River basin, Washington

According to Indian tradition, floods of unusually great magnitude harassed the Skagit River basin about 1815 and 1856. The heights of these floods were not recorded at the time; so they are called historical floods. Since the arrival of white men about 1863, a number of large and damaging floods have been witnessed and recorded. Data concerning and verifying the early floods, including those of 1815 and 1856, were collected prior to 1923 by James E. Stewart. He talked with many of the early settlers in the valley who had listened to Indians tell about the terrible floods. Some of these settlers had referenced the maximum stages of floods they had witnessed by cutting notches at or measuring to high-water marks on trees. In order to verify flood stages Stewart spent many weeks finding and levelling to high-water marks such as drift deposits, sand layers in coves, and silt in the bark of certain types of trees. Gaging stations have been in operation at various locations on the Skagit River and its tributaries since 1909, so recorded peak stages are available at certain sites for floods occurring since that date. All peak discharge data available for both historical and recorded floods have been listed in this report. The types of floods as to winter and summer, the duration of peaks, and the effect of reservoirs are discussed. In 1899 Sterling Dam was constructed at the head of Gages Slough near Sedro Woolley. This was the beginning of major diking in the lower reaches of the Skagit River. Maps included in the report show the location of most of the dike failures that have occurred during the last 73 years and the area probably inundated by major floods. The damage resulting from certain floods is briefly discussed. The report is concluded with a brief discussion of the U.S. Geological Survey method of computing flood-frequency curves as applied to the Skagit River basin. The treatment of single-station records and a means of combining these records for expressing regional significance are exemplified. Historical data are used in the development of both the single-station and the regional curves.

Washington↗