Role of rainfall-induced runoff discharge and human disturbance on the morphodynamics and sedimentation in the semienclosed macrotidal flats (Shinsi tidal flats, Korea)

Abstract Waves and tidal currents are considered the main processes that govern the tidal-flat morphodynamics; however, meteorological control such as rainfalls remains less constrained. To quantify the tidal-flat morphodynamics over hundreds of thousand square meters, unmanned aerial vehicle (UAV) surveys augmented with sedimentological and hydrodynamic data were seasonally conducted over four years in the semienclosed Shinsi tidal flats, Korea. Tidal-flat morphology is spatially variable on a seasonal timescale due to changes in relative dominance between tide, wave, and rainfall-induced runoff discharge, leading to non-seasonal tidal-flat morphodynamics. Despite the sheltered location from offshore waves, tidal flats received sustained wave activity to cause the landward migration of shelly ridges and net offshore sediment transport. The rapid growth of tidal channel networks and incipient rill channels during summer signifies the impact of rainfall-induced runoff discharge, accentuated by the impermeable nature of catchment area and embayment morphology. Flood tidal dominance with mud supply, promoted by the dike adjacent to the flats, leads to the rapid burial of channels and accretion of tidal flats during periods with a subdued wave and rainfall activity. The tidal flats have evolved with an increased topographic relief in response to the dual influence of waves and rainfall-induced runoff discharge. During the study period, the tidal flats became steepened with net sediment export in the lower intertidal zone, suggesting a reduced sediment supply presumably caused by dike construction. The Shinsi tidal flats showcase how human disturbance dampens the seasonality in the tide-flat morphodynamics and sedimentation under a strongly seasonal wave climate.