It is well known that estuarine systems are significantly affected by hydrodynamic conditions such as river discharge, storm surges, waves and tidal conditions. In addition to this, human interferences through developmental projects have the capability of disrupting the natural morphological processes occurring at estuaries. In West Africa, the goal to improve standards of living through large-scale dam construction, offshore ports and coastal erosion countermeasures has triggered alarming changes in the morphodynamics of estuarine systems. The estuaries at the Volta River mouth (Ghana) and “Bouche du Roi” inlet (Benin), located along the Bight of Benin coast, West Africa, were selected as two case study sites to examine their long-term morphodynamics and sandspit evolution. In this study, we primarily analyzed estuarine morphology using remotely sensed images acquired from 1984 to 2020. We further estimated the longshore sediment transport for this region using results from the image analysis and the depth of active sediment motion. Our results reveal that the longshore sediment transport rates for this region are in the magnitude of 10⁵–10⁶ m³/year. Comparative analysis with other estuaries and sandy coasts suggests that the longshore sediment transport along this coast has one of the largest rates estimated in the world.

On 11 March 2011, the Great East Japan Earthquake generated huge tsunami waves. Then, tsunami propagation occurred in rivers, resulting in further expansion of the flooded area. Full data sets of tsunami characteristics such as tsunami inland and river propagation distances as well as the river geometries from all rivers within the Tohoku District were compiled and analyzed. It was found that tsunami propagation distance in rivers was about 1.2 to 4.5 times that of the inland area. There was a good correlation between propagation distance in rivers and the river bed slope. Furthermore, new empirical formulae for calculating the damping coefficient of tsunami wave height and tsunami intrusion length were successfully derived based on the current comprehensive data sets covering a wide range of river geometry and bottom slope. Moreover, tsunami-induced flow discharge was evaluated by using measured water level variation.

The 2011 Great East Japan Earthquake and Tsunami are one of the most catastrophic natural disasters that took place in Japan. The disaster caused widespread destruction to the estuarine and coastal areas along the northeastern region of Japan. The Abukuma River mouth, among many estuaries in Miyagi Prefecture, suffered major damage from the tsunami in which the sand spit and the sand terrace were severely eroded. The present study aims to provide a better comprehension of the morphology of the Abukuma River mouth within a decade after the 2011 tsunami based on the collected datasets of imagery and offshore bathymetry surveys. The consecutive changes of the sand terrace and sand spit indicate their connection influencing the morphological recovery in the Abukuma River mouth. About a year after the tsunami, the sand terrace re-appeared by a flood event and became the sediment source to recover the sand spit under the intrusion process. When the sand spit almost recovered and achieved a stable configuration, the sediment from the sand terrace into the river was restricted. As a result, more sediment from the sand terrace was expected to be transported to the downdrift area under the dominant wave condition in the Abukuma River mouth.

The Nobiru Coast is situated on the southwest of the Ishinomaki Bay. The 2011 Great East Japan Tsunami severely devastated the Nobiru Coast and the adjacent Naruse River mouth. In this study, an investigation was conducted based on the available historic maps and images combined with in situ surveys that revealed the century-to-decade morphology change and sediment budget alteration in the Nobiru Coast. During the past two centuries, the longshore transport on the northeast coast and sediment supply from the Naruse River were the principal sediment supply onto the Nobiru Coast and the estimated annual net sediment input into the coast was 87,000 m³/y. Until several decades ago, the construction of the Ishinomaki Port and the erosion preventing constructions (breakwaters, headlands) along the Ohmagari Coast on the northeast areas caused a dramatic reduction of longshore transport to the Nobiru Coast. Hence, the net sediment input fell to 46,000 m³/y. After the tsunami, the sediment input was further reduced to 29,000 m³/y and this loss was closely related to the intruded sediment into the Naruse River. The outcomes of this study are highly valuable for the government authorities to manage the long-term coastal and riverine morphological changes after the 2011 tsunami.

The 2011 Great East Japan earthquake and tsunami caused significant damage along the coastal region in Miyagi Prefecture. In the Natori River of Miyagi Prefecture, the recovery process of the estuarine morphology has been observed since the tsunami arrived. In this study, detailed analyses of collected aerial photographs and beach topography in front of the river mouth were conducted to evaluate whether the river mouth has reached a new equilibrium. The shoreline analysis suggests that the river mouth has obtained an equilibrium state after 2014 in which the sediment volume can be preserved within the littoral system. In addition, the convergence process to the stable topography was detected by the first spatial and temporal eigenfunctions obtained from the empirical orthogonal function (EOF) analysis of the beach profiles. From the results, it can be concluded that the river mouth has obtained a new equilibrium of morphology after the tsunami. As the recurrence of the tsunami can be expected in the next several centuries, the findings of this study can be useful for long-term coastal and riverine management against future disasters in this river mouth, and other coastal regions that are prone to large-scale disasters in the near future.