Time lag between reduction of sediment supply and coastal erosion

The disruption of sediment supply from river to coast by dam development has been a topic of global concern. In Japan, among the top 30 dams in terms of height, 14 dams were constructed prior to 1970 and another 6 dams before 1980. However, the coastline erosion did not surface up as a grievous problem until the 1980s. According to the River Bureau of Japan, the overall erosion rate along the coastline of Japan was 0.72×106m2/yr prior to 1980, but sharply increased to 1.6×106m2/yr since 1980. Therefore, there was a time lag between the disruption of sediment supply by dam and beach erosion. This paper presents a case study on what may have delayed the response of beach to the effect of dam construction.