INTRODUCTION

The investigation of the performance of current meters in measuring the velocity of water in shallow depths, recently made by the Geological Survey at the National Hydraulic Laboratory, National Bureau of Standards, Washington, D. C., was arranged primarily for the purpose of determining coefficients to be applied as correction factors to velocities obtained by current meters when used under the adverse conditions of very shallow water. The scope of the investigation covered measurements of discharge in the 12-foot flume with standard-size current meters and with cup-type pygmy current meters that have a bucket wheel 2 inches in diameter. The investigation was limited to water between a minimum depth of 0.2 foot and a maximum depth of 1.5 feet. The velocity ranged from 0.1 foot to 1.5 feet per second. Coefficients for the 0.6-depth method and for the 0.2- and 0.8-depth method, where standard-size current meters were used, were determined for the entire range of velocities. For the 0.5-depth method where standard-size current meters were used and for all methods where pygmy meters were used coefficients were determined for the entire range of velocities except for those below 0.2 foot per second. The depths of water in which measurements were made by the various methods are shown by the depths for which coefficients are given in the diagrams.

The depth of water in the flume was regulated by needle gates a short distance below the place of measurement, the needle gates being adjusted so as to obtain the desired depth and velocity for a given discharge. An 8-inch or a 4-inch venturi meter calibrated in place was used for determining discharges less than 3.5 second-feet. Discharges greater than 3.5 second-feet were measured by a sharp-crested weir calibrated in place. The discharge as measured by the current meter was compared with the weir or venturi-meter discharge measurement to obtain the correction factor for the current-meter measurement. Conditions of beds of smooth concrete, %-inch gravel, and coarse gravel were investigated. The coarse gravel was run-of-bank gravel, all retained on a 1-inch screen but passing through a 5-inch screen.

Other phases of the investigation, such as studies of pulsations, vertical velocity curves, distribution of velocities near the side walls of the flume, and performance of current meters when used near the water surface and near the flume walls, were incidental to the main purpose of the investigation. The information obtained from these studies was used in analyzing and interpreting the results of the discharge measurements.

Posted April 5, 2010