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. . . GENERAL FIELD AND OFFICE PROCEDURES FOR INDIRECT DISCHARGE MEASUREMENTS By M. A. Benson and Tate Dalrymple Abstract The discharge of streams Is usually measured by the current-meter method. During flood periods, how- ever, it is frequently impossible or impractical to measure the discharges by this method when they occur. Consequently, many peak discharges must be determined after the passage of the flood by indirect methods, such as slope-area, contracted-opening, :flow- over-dam, and flow.through-CUlvert, rather than by direct current-meter measurement. Indirect methods of determining peak discharge are based on hydraulic equations which relate the dis- charge to the -water-'surface profile and the geometry of the channel. A field survey is made after the flood to determine the location and elevation of high-water marks and the characteristics of the channel. Detailed descriptions of the general procedures used in collecting the field data and in computing the dis- charge are given in this report. Each of the methods requires special procedures described in subsequent chapters. Introduction The discharge of streams is usually measured by means of a current meter. Techniques of making current-meter measurements are stand- ardized and well known. During floods, how- ever, it is frequently impossible or imprac- tical to measure the peak discharges when they occur, because of conditions beyond control. Roads may be impassable; structures from which current-meter measurements might have been made may be nonexistent, not suitably located, or destroyed; knowledge of the flood rise may not be available sufficiently in advance to permit reaching the site near the time of the peak; the peak may be so sharp that a satisfac- tory current-meter measurement could not be made even with an engineer present at the time; the flow of debris or ice may be such as to pre- vent use of a current meter j or limitations of personnel might make it impossible to obtain direct measurements of high-stage discharge at numerous locations during a short flood period. Consequently, many peak discharges must be determined after the passage of the flood by indirect methods, such as slope-area, contracted- opening, f1ow-over-dam, f1ow-through-culvert, . critical-depth, or others, rather than by direct current-meter measurement. A knowledge of peak discharges or volumes of flood runoff is extremely important for the design of flood-control works or other struc- tures along river channels. The discharges as obtained from stage-discharge relation curves at gaging stations are used generally without question of accuracy. Because the upper por- tions of many such rating curves are necessarily defined by indirect measurements, it is impor- tant that the methods used in these measure- ments should be based on the proper data and should make use of the best procedures known, in order that the highest possible accuracy be obtained. This manual describes the general field and office procedures for making indirect measure- ments as done by the Geological Survey, Water Resources Division. The methods are the re- sult of integrated experience over a period of years, of past investjgations, and of recent re- search, in both the field and in the laboratory, designed to improve the general knowledge and accuracy of such methods. Practices peculiar to each method will be found in four subsequent chapters, A2-A5, of Book 3, Techniques of W ater- Resources Investigations. In order to evaluate the accuracy of indirect methods, comparisons have been made at every opportunity. Where it has been possible to compare peak discharge computed by indirect 1