Discharge Characteristics of the Free Overfall: Use of Crest Section as a Control Provides Easy Means of Measuring Discharge

REFINEMENTS are constantly being introduced in the methods of measuring flowing water. In recent years, many attempts have been made to avoid the necessity of calibrating each individual measuring device, by utilizing the principle of critical depth for parallel flow. Unfortunately, in the critical-depth meters thus far devised, it has been impossible to prevent the apparent control section from shifting indeterminately upstream or downstream as the discharge varies, and so the need for experimentally determined coefficients has not yet been eliminated. In the present article Dr. Rouse points out the possibility of using the free overfall as a flow meter which needs no calibration. Although the flow at the overfall is not parallel, the crest section is that of true minimum energy and hence is the actual control section. Furthermore, the crest depth is a constant percentage of the computed critical depth for parallel flow. Dr. Rouse's analysis has been verified by experiment, and should provide a dependable and simple means of determining discharge at points of overfall.