Sota Nakajo

Estuarine Circulation Patterns in a Complex Geometry Estuary: Dinh An Estuary, Mekong River

Estuarine circulation (EC) is a natural phenomenon that contributes to the evolution of estuaries worldwide. This paper uses the Dinh An estuary, Mekong River as a case study to investigate patterns of EC in a complex cross-section estuary. An Acoustic Doppler Current Profiler (ADCP) measured a series of river discharge and current profiles at a transect near the mouth of the estuary between 15 and 30 September 2009. Tidal elevation was also recorded concurrently with the river discharge by using a leveling rod. The observed data reveal that the river-discharge phase lagged 6xa0h behind the tidal phase in the high-flow season (May to October). Four main types of EC were observed during the reversal time in the ebb tide with river discharges in a range of −5000xa0m3/s to 13,600xa0m3/s. There were two main flood tide patterns, which corresponded to the discharges from −6000xa0m3/s to 5000xa0m3/s. The classical EC type (seawater flows landward near the bed and river water flows seaward near the surface) appeared only in the deepest channel in the ebb tide as saline intrusion induced a density imbalance. The results suggest that tidal phase and bed geometry play important roles in forming EC patterns in estuaries. This paper provides essential information related to flow structure in complex bathymetry estuaries for further studies on saline intrusion, environmental processes, sediment dynamic and estuarine morphology.

Projection of decrease in Japanese beaches due to climate change using a geographic database

ABSTRACT This study models shoreline retreat due to sea level rise by using geographic data and applies the model to future projections of decreases in beach area for 806 beaches in Japan. The model uses a foreshore slope (angle) based on data from a digital elevation model, and influence of the present simplified method for estimation of the shoreline retreat is examined through comparisons with previous studies at typical locations. The proposed method gives a distance of shoreline retreat due to sea level rise similar to that predicted using the Bruun rule for minimal retreat less than 30 m, but the difference becomes substantial for more extensive decreases. The decrease in beach area is projected for different sea level rises based on four Representative Concentration Pathway (RCP) scenarios from the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. The decrease in beach area becomes more severe for the RCP8.5 scenario, and the proposed method predicts that a third of current sandy beaches in Japan will disappear. The extent of the decrease depends not only on the sea-level-rise scenario but also on the SLR projection model.

The Future Prediction of the One-Dimensional Topography of the Estuary Delta of the Shirakawa River

Predicting the long-term topographical change based on results of a time-slice experiment using a General Circulation Model (GCM) is difficult because actual change is a result of the continuum of events occurring in succession leading from the past to the future. We developed a one-dimensional topographical model of an estuary delta as the first step of assessing climate change effects. Three major effects, i.e. tidal flow, waves and sediment supply from the river, were included in this model. In order to estimate the sensitivity of these effects, simulations with virtual conditions were conducted. These simulations show equilibrium profiles that are close to the results of Roberts et al. [2000] “Predicting the profile of intertidal mudflats formed by cross-shore tidal currents,” Proc. Marine Sci. 3, 263–285.]. The simulation results were validated with observation data from the Shirakawa River delta. As a long-term prediction (about 37 years), the propagation of the rollover point was less than the actual data showed. The gradient of the subtidal zone was gentler than that observed. However, the short-term prediction (about 17 years) agrees with the observation data. These results show that old, unreliable, observation data used as a boundary condition significantly affects the reproducibility of the actual tidal flat profile. Finally, the effect of continuous Sea Level Rise (SLR) over 100 years from the present was investigated. As expected, the simulation results show a shift of the shoreline landward. The water depths in the intertidal and subtidal zones increase compared to a no-SLR condition. Therefore, the topset area grows as a consequence of SLR. Additionally, it was shown that future accumulation in the subtidal zone is reduced with SLR.