Influence Mechanism of Geomorphological Evolution in a Tidal Lagoon with Rising Sea Level

A tidal lagoon system has multiple environmental, societal, and economic implications. To investigate the mechanism of influence of the geomorphological evolution of a tidal lagoon, the effect of critical erosion shear stress, critical deposition shear stress, sediment settling velocity, and initial bed elevation were assessed by applying the MIKE hydro- and morpho-dynamic model to a typical tidal lagoon, Qilihai Lagoon. According to the simulation results, without sediment supply, an increase of critical erosion, deposition shear stress, or sediment settling velocity gives rise to tidal networks with a stable terrain. Such an equilibrium state can be defined as when the change of net erosion has little variation, which can be achieved due to counter actions between the erosion and deposition effect. Moreover, the influence of the initial bed elevation depends on the lowest tidal level. When the initial bed elevation is below the lowest tidal level, the tidal networks tend to be fully developed. A Spearman correlation analysis indicated that the geomorphological evolution is more sensitive to critical erosion or deposition shear stress than sediment settling velocity and initial bed elevation. Exponential sea level rise contributes to more intensive erosion than the linear or the parabolic sea level rise in the long-term evolution of a tidal lagoon.

Velocity Components of Currents and Transport Mechanism in the Ie Meulee Waters, Weh Island, Indonesia

Weh Island has become an area of significance in which many developments are ongoing. Ie Meulee region was one of the developed areas where a new fishing port has been built which may impact to coastal instability. This study aims to determine the sea currents regime in the Ie Meulee waters and its relation to the transport mechanism. Field survey was conducted on March 2017 by which Tide Gauge and ADCP have installed within and without the port, respectively. Flow model was also simulated depicting current patterns for four extreme tidal conditions. The elliptical form of zonal and meridional components of current proves that the tidal current role takes place. The speed of tidal current ranged from 0-0.42 m/s and 0-0.85 m/s in the bottom and surface, respectively. The most prominent positive value of current components indicates that the transport mechanism is primarily surface ward during both ebb and flood tides. Reflection and diffraction are identified in the simulation testifying that the emergence of the fishing port has changed the hydrodynamic features. We also founded that the low-speed currents induced sediment settling within the jetty, so that it is essential to conduct regular dredging and bathymetry survey in the surrounding port.