Normal force on the bed was measured with a flat 15 × 15 cm steel load plate. Each rotation of the wheel produced a force profile in the form of a time series of force of the entire length of the flow from front to back. Multiple profiles were collected in each experimental run.
We measured basal normal force with a steel load plate attached at the plate’s center to a shear-compensating Interface Force Model SWP10-5K-B000 Precision Force Transducer. The transducer has a maximum load of 2273 kg (5000 lb) and a resting noise of roughly 0.2 kg. The plate and load cell assembly are attached to the drum and further stabilized by two large, steel rectangular tube bars on the outside of the drum. To water-seal the plate, a neoprene gasket was attached on the back side of the plate-bounding gap. The gap was filled with silicone to prevent lodging of sediment, which would restrict plate movement. Power and data were transferred to and fromthe plate through a slip ring on the hub of the wheel. Measurements were made using an analog-to-digital board at 200 kHz, then averaged to 1 ms, effectively sampling at 1 kHz. Load cell measurements were synchronized with laser scanner measurements through a trigger from a rotary encoder mounted in the hub of the wheel.