Field Measurement of Boulder Flow Drag

Abstract

A field method for measuring the flow drag force on individual boulders on a riverbed is identified and tested. Drag is determined from the momentum deficit in the boulder wake, obtained by measuring the flow velocity in a traverse across the wake. A field test yielded a mean drag coefficient of 0.36, which is low compared with flume measurements of 1.1–1.5 for cubes. This may be the result of higher velocity water near the riverbed being sucked into and reenergizing the wake. Further tests are needed to define the conditions in which the method can be used and to indicate how well the assumptions behind the method can be accommodated in a natural flow environment.