Centennial to Multi-Decadal Morphology Change and Sediment Budget Alteration with Consideration of the Impacts of the 2011 Tohoku Earthquake Tsunami along the Nobiru Coast, Japan

Abstract

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.

Morphology Recovery and Convergence of Topographic Evolution in the Natori River Mouth after the 2011 Tohoku Tsunami

“…For example, in a big river mouth such as the Abukuma River mouth, also located in the Sendai Plain, due to the significant submarine sediment deposition created by the post-tsunami floods, the intrusion rate in the channel is very big, reaching 10 × 10 4 (m 3 /y) and the intrusion distance of sediment is up to 1 km from the mouth entrance [21]. While, in the Naruse River in Ishinomaki Bay, since it is a small river with almost the same catchment area and sediment discharge amount as the Natori River, the accumulation in the river is rather small at about 1.8 × 10 4 (m 3 /y) [20]. However, the impact of wave intrusion was significant since the intruded waves could even severely erode the bank of the river channel.…”

“…The wet-dry line or the high-water line (HWL) dividing the dark (wet) and light (dry) sand on the beach is defined as the shoreline position using the unsupervised K-means clustering algorithm. A detailed description and some applications of this cluster analysis are referred to in [20,34]. For a small number of images where the features of wave breaking bubbles are neutralized on the shore, a manual shoreline detection based on the edge gradient and direction between sand and water was implemented on each image to detect the shoreline.…”

“…Monecke et al [11] simulated the cross-shore profile change under the prevailing hydrodynamic conditions to investigate the recovery process, while Martínez et al [12] applied a more complicated sediment transport model (Delft3D) in conjunction with morphology and topography data analysis to discuss the erosive behavior of the microtidal Tubul-Raqui wetland in south-central Chile. In Japan, more thorough and multidisciplinary research of the coastal and riverine damage and recovery after the tsunami was conducted using the sufficient availability of both pre-and post-tsunami data [13][14][15][16][17][18][19][20][21].…”

“…Therefore, after the tsunami, the recovery process in these river mouths is expected to be different from that of natural ones. Hiep et al [20,21] have recently discussed the littoral sediment budget changes in two rivers in Sendai Bay (the Abukuma and Naruse Rivers) caused by the tsunami, taking into account hydrodynamic conditions and anthropogenic disruption.…”