A submersible device for measuring drag forces on aquatic plants and other organisms

This paper describes a device, the submersible drag gauge (SDG), that can be used for drag measurements in studies of flow-biota interactions at low (setup A) and high (setup B) load applications. The device is designed so that solely drag forces acting on a test object in a boundary layer flow are measured. The SDG can measure drag on organism(s), such as macrophtyes, crayfish or mussels, as well as a combination of such organisms and sediments assembled in a variety of arrangements in both laboratory and field conditions. Laboratory calibrations over the range of 0 to 4 N showed that setup A registered forces as low as 0.02 N, whereas setup B often did not register forces up to c. 0.15 N. The relative standard error of drag measurements was < 3% for forces > 0.2 N (setup A) or < 4% for forces > 1 N (setup B). Using a rigid cylinder, and natural and artificial (plastic) Egeria densa as examples, showed how the SDG can serve in studies of flow-biota interactions such as drag-velocity relations, assessments of drag coefficients, or the frequency spectrum of macrophyte drag.