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2 r , TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS . means with peak discharge measured by current meter or other direct means, the agreement, in general, has supported confidence in the reliabil- ity of the auxiliary methods. During the floods of May-June 1948 in the Columbia River basin, comparative studies using the slope-area method were made for 22 locations, where the discharge was known. One computation showed a differ- ence of 25 percent between the known and the computed discharges. Of the other 21 measure- ments, the maximum divergence was 15.6 per- cent; the average divergence was 6.7 percent. This study shed some light on the nature of conditions which lead to large inaccuracies, and, together with succeeding investigations, should help to avoid unfavorable conditions and thereby increase the accuracy of indirect methods in the future. Since 1953, when the most recent method ~as adopted for computing discharge through con- tractions, a program of field verification of the method has been carried on, with favorable results. Surveys have been obtained to date at 22 sites where discharges were known. Of these, about 80 percent gave results within 10 percent of known discharges; all were within 20 percent. Other verification studies have confirmed the reliability of computations over dams and through culverts. The Columbia River basin studies have been made the basis of a reference library of verified values of Manning's n, obtained by starting with the known peak discharge and computing the value of n. Color stereophotographs of the slope-area reaches were taken so that channel conditions corresponding to the computed n values could be identified. This so-called veri- fication program is continuing with the object of expanding the range of illustrated roughness conditions. Indirect measurements make use of the energy equation for computing discharge. The spe- eific equations differ for different types of flow, such as open-channel flow, flow over dams, and flow throngh culverts. However, all the meth- ods involve these general factors: 1. Physical characteristics of the channel: dimensions and conformation of channel within reach used, and boundary condi- tions. 2. Water-surface elevations at time of peak stage to define the upper limit of the cross- sectional areas and the difference in eleva- tion between two significant points. 3. Hydraulic factors based on physical char- acteristics, water-surface elevations, and discharge, such as roughness coefficients and discharge coefficients. Acknowledgments Many engineers in the Geological Survey COIl- tributed to the development of the methods described in this report. The original develop- ment of field techniques which Hollister John- SOil began was continued by Tate Dalrymple, M. A. Benson, R. H. Tice, H. H. Barnes, Jr., G. L. Bodhaine, Harry Hulsing, H. F. Matthai, W. P. Somers, R. E. Oltman, and many others. Many of the methods are based on extensive laboratory investigations by the Survey con- ducted by R. W. Carter, H. J. Tracy, Jacob Davidian, D. B. Simons, and E. V. Richardson. Professor C. E. Kind~vater, Georgia Institute of Technology, played a major role in the lab- oratory investigations while serving as a con- sultant to the Survey. . Collection of Field Data The data required for computation of dis- charge by indirect methods are obtained in a field survey of a reach of channel. The survey includes the elevation and location of high- water marks corresponding to the peak stage, cross sections of the channel along the reach, selection of a roughness coefficient, and descrip- tion of the geometry of dams, culverts, or bridges if this type of measurement is to be made. The selection of a suitable site is prob- ably the most important element in the applica- tion of the indirect method of discharge meas- urement. Selection of Site A thorough reconnaissance of the flood area is necessary for selection of sites at which de- . ' I I