Daily Topographic Change Patterns of Tidal Flats in Response to Anthropogenic Activities

Abstract

The natural geomorphology of muddy coastal zones can easily change in a short period of time, especially under the influences of coastal engineering. However, although short-term morphological change patterns can affect the establishment of salt marsh vegetation on bare tidal flats, these patterns are rarely studied. This work presents the results of an investigation of the daily variation pattern of tidal flats in response to anthropogenic activities by using microtopographic units (areas of local uplift) as indicators. The changes exhibited by these microtopographic units (microunits) were monitored by a coastal video system with two high-definition cameras for 29 days. The results show that microunits initially tended to form near the sluice gate, spread throughout the study area in the alongshore direction, and then gradually disappeared with increase in time. Finally, the geomorphology of the tidal flat recovered to the state before the scouring event in only a few days. A statistical analysis of the areas of individual microunits showed that they tended to become miniaturized and fragmented. The number of small microunits (area between 1 and 10 m2) increased significantly during the recovery period under the influence of the tidal current. This dynamic equilibrium was also observed through an analysis of one region of interest, thereby illustrating an iterative equilibrium by shifting between fragmentation and flattening during the recovery period. As a result, this work presents the first usage of a coastal video system to monitor tidal flats and gives several contributions to prove that microtopographic units can be regarded as useful indicators for studying the daily change patterns of tidal flats in response to anthropogenic activities, thereby providing an alternative method for analyzing the morphological changes corresponding to other short-term events, such as storms.