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

Abstract

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.