Scaling properties of estuarine beaches

Estuarine beaches near large rivers are dynamic systems constantly shaped by tides, waves, and fluvial sediment inputs. However, little research has been done on the intrinsic characteristics of these geomorphic systems. Using eleven high resolution bathymetries, our results show that human disturbance mingled with natural forcings have induced bathymetric changes in Nanhui beach in the Changjiang estuary, China. Isobaths display a fractal geometry, with a lower fractal dimension when tides smooth the bathymetry and a higher dimension when waves dominate. Rates of sediment accretion and erosion present a Gaussian distribution driven by tidal and wave action. Episodic extreme wave forcing or frequent land reclamation is responsible for the intermittent adjustment of the estuarine beach bathymetry. After these events the distribution of erosion and accretion becomes power-law, possibly indicating disequilibrium. The fractal dimension of isobaths and the distribution of erosion and deposition rates can therefore be used as metrics to determine the dominant processes in estuarine beaches and whether the system is close to equilibrium or not.