Temporal Compensation of Erosion and Deposition Under a Two-Day Hydrograph in a Field-Scale Sand-Bed Flume

Riverbeds undergo alternating periods of erosion and sediment deposition under unsteady flow. These processes largely compensate over time, making overall changes appear small in conventional before-and-after riverbed surveys. We quantified this temporal compensation in a 180 m field-scale sand-bed flume subjected to a 2-day hydrograph. Continuous measurements from two fixed-point echosounders (450 kHz, 1 Hz) were combined with bracketing terrestrial laser scanning (TLS) surveys. The time series resolved stage-scale reversals (−22 mm overprinted by +28 mm), cancelled roughly 90% of the summed absolute stage-net changes at the downstream station (net-to-gross ratio 0.08 over the logged intervals; 90% interval 0.01–0.17 under measurement and stage-boundary uncertainty), and doubled fluctuation intensity at ES1 at the same nominal discharge over the antecedent bed, with falling-limb deposition delivering more than one-third of the final day’s aggradation in 1.5 h. The TLS DEM of difference (DoD) mapped a deposition front reaching +302 mm with spatial sorting; station rankings agreed, but residuals of 20–38 mm impede validation of absolute accuracy. Virtual resampling showed the retained gross change falling below 50% of its 5 min reference at intervals of tens of minutes to a few hours, with strong start-time dependence. These findings highlight the importance of monitoring frequency and timing and demonstrate that small net changes do not necessarily indicate a stable riverbed.