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[HGM04-P02] Changes in coastline in Hamadori, Fukushima Prefecture between 1960's to present
Keywords:dam, beach erosion, sediment supply
Since World War II, forest conservation and flood control projects were promoted in Japan, leading to the rapid development of river basins and coast. While these engineering projects greatly reduced disaster risks, problems such as degradation of riverbed and coastal erosion have become apparent throughout the country. The main causes of coastal erosion are the construction of dams in mountainous areas and coastal structures. Shoreline change caused by the coast has already been studied extensively. It has long been pointed out that dam construction can significantly decrease the sediment supply to coasts. However, quantitative studies of shoreline change caused by dam construction are still limited. The Hamadori region of Fukushima Prefecture has been affected by the 2011 Tohoku-Oki Earthquake. In addition, several rivers in this region have dams in their upstream areas. However, detailed study about shoreline changes in this region has not been conducted, and the exact causes of shoreline change have not been identified. We studied shoreline changes between 1960s to the present and examined the impacts of coastal structure, dam, and the 2011 Tohoku-Oki Earthquake on the observed shoreline changes.
Beaches and sea cliffs alternate in the northern part of the Hamadori region, whereas beaches are formed extensively in the southern part. The predominant wave direction is from east to east-northeast, and littoral transport is directed northward. In this study, six beaches near Mano River, Niida River, Ota River, Ukedo River, Kitasako River, and Ohisa River were studied. Changes in shoreline position and beach area were investigated using aerial photographs. The amount of change in sediment supply from mountainous areas due to dam construction was estimated using standard deviation of elevation in 1km2 square, and the year of construction of coastal structures was estimated using aerial photographs. Based on these results, the factors influencing shoreline changes were considered.
Until 2012, the shoreline retreated on most beaches; however we also found beaches where the shoreline advanced significantly. The area change generally follows the same trend as the shoreline change. These changes were strongly related to the construction of coastal structures. In the southern sections of fishing ports, the shoreline advanced, whereas the shoreline retreated in the northern sections. In the area where offshore breakwaters were constructed, the shoreline mainly advanced. Furthermore, the tsunami caused by the 2011 Tohoku-Oki Earthquake caused significant shoreline retreat in most beaches. In particular, the shorelines near Ukedo River and Mano River were greatly retreated immediately after the earthquake. The shorelines of these two beaches had recovered to the pre-earthquake level by the present. In addition, offshore breakwaters tended to promote beach recovery after the tsunami. In this study, it was also confirmed that the shoreline advanced more rapidly after the earthquake in sections with offshore breakwaters than those without them.
At the four beaches where dams were constructed, we estimated that sediment supply to the beach reduced by 15–96% because of the dams. However, shoreline retreat after dam construction occurred only at Ukedo River, and the amount of retreat was small. At other beaches, shoreline retreat after dam construction was not observed. According to Kawata et al. (1998), sedimentation in dams is classified into three stages. Stage I, the initial stage, corresponds to the period immediately after dam construction. In this stage, although the amount of sedimentation is large, downstream riverbed sediments are transported to the beach, compensating for the reduced sediment supply from upstream areas. Therefore, it is possible that the transportation of riverbed sediments during Stage I buffered the shoreline change due to dam construction. In addition, the contribution of sediment supply from forests in the Mano River basin ranges from 16% to 67% and has been below 20% since 2017 (Thomas et al., 2024). Therefore, it is possible that the sediment supply from the upstream area of the river has a limited effect on the beach we studied. For the quick beach recovery after the 2011 earthquake, it may be related to the Fukushima Daiichi Nuclear Power Station Accident. After the accident, the contaminated agricultural soil was removed from Mano River basin. Thus, the temporary exposure of soil in farmland due to decontamination work may have contributed to the rapid shoreline advance after the earthquake.
Beaches and sea cliffs alternate in the northern part of the Hamadori region, whereas beaches are formed extensively in the southern part. The predominant wave direction is from east to east-northeast, and littoral transport is directed northward. In this study, six beaches near Mano River, Niida River, Ota River, Ukedo River, Kitasako River, and Ohisa River were studied. Changes in shoreline position and beach area were investigated using aerial photographs. The amount of change in sediment supply from mountainous areas due to dam construction was estimated using standard deviation of elevation in 1km2 square, and the year of construction of coastal structures was estimated using aerial photographs. Based on these results, the factors influencing shoreline changes were considered.
Until 2012, the shoreline retreated on most beaches; however we also found beaches where the shoreline advanced significantly. The area change generally follows the same trend as the shoreline change. These changes were strongly related to the construction of coastal structures. In the southern sections of fishing ports, the shoreline advanced, whereas the shoreline retreated in the northern sections. In the area where offshore breakwaters were constructed, the shoreline mainly advanced. Furthermore, the tsunami caused by the 2011 Tohoku-Oki Earthquake caused significant shoreline retreat in most beaches. In particular, the shorelines near Ukedo River and Mano River were greatly retreated immediately after the earthquake. The shorelines of these two beaches had recovered to the pre-earthquake level by the present. In addition, offshore breakwaters tended to promote beach recovery after the tsunami. In this study, it was also confirmed that the shoreline advanced more rapidly after the earthquake in sections with offshore breakwaters than those without them.
At the four beaches where dams were constructed, we estimated that sediment supply to the beach reduced by 15–96% because of the dams. However, shoreline retreat after dam construction occurred only at Ukedo River, and the amount of retreat was small. At other beaches, shoreline retreat after dam construction was not observed. According to Kawata et al. (1998), sedimentation in dams is classified into three stages. Stage I, the initial stage, corresponds to the period immediately after dam construction. In this stage, although the amount of sedimentation is large, downstream riverbed sediments are transported to the beach, compensating for the reduced sediment supply from upstream areas. Therefore, it is possible that the transportation of riverbed sediments during Stage I buffered the shoreline change due to dam construction. In addition, the contribution of sediment supply from forests in the Mano River basin ranges from 16% to 67% and has been below 20% since 2017 (Thomas et al., 2024). Therefore, it is possible that the sediment supply from the upstream area of the river has a limited effect on the beach we studied. For the quick beach recovery after the 2011 earthquake, it may be related to the Fukushima Daiichi Nuclear Power Station Accident. After the accident, the contaminated agricultural soil was removed from Mano River basin. Thus, the temporary exposure of soil in farmland due to decontamination work may have contributed to the rapid shoreline advance after the earthquake.