تغییر ساحل به علت افزایش سطح دریا و فروپاشی زمین ناشی از افزایش موج انتقال از طریق موج شکن آب زیرزمینی Shoreline change caused by the increase in wave transmission over a submerged breakwater due to sea level rise and land subsidence

Many sandy beaches in the world globally are eroding because of various natural and anthropogenic causes including reduced sediment supply from rivers and unbalanced longshore sediment transport rates around coastal structures. Countermeasures against beach erosion are classified into soft solution, which includes beach nourishment and sand bypassing, and hard solution, which includes groins, detached breakwaters, and submerged breakwaters (Komar, 1998). Unlike detached breakwaters, submerged breakwaters do not interfere with the view of the horizon from the shore; therefore, submerged breakwaters are sometimes constructed as countermeasures against beach erosion where the availability of sediments for nourishment is limited and tourism is prevalent. However, because the crests of submerged breakwaters are below sea level, projected sea level rise (SLR) and land subsidence will lead to decreased wave energy dissipation by submerged breakwaters, causing instability in the sandy beaches. Thus, sandy beaches behind submerged breakwaters are vulnerable to SLR and land subsidence. Moreover, whether a sandy beach is protected with coastal structures or not, according to the Bruun Rule (Bruun, 1962), the shoreline of the beach is expected to retreat owing to the seaward sediment transport caused by the upward and shoreward shift of the equilibrium beach profile due to relative SLR. Using long-term data of shoreline position and sea level along the East Coast of the USA, Zhang et al. (2004) showed that the rate of shoreline retreat is highly correlated with that of SLR. List et al. (1997), on the other hand, reported that along the Louisiana coasts, USA, no correlation was found between the amount of shoreline retreat estimated by the Bruun Rule and that of SLR. This suggests that the SLR-induced shoreline change is caused by the mechanism assumed in the Bruun Rule as well as other mechanisms including sediment transport to/from dunes and offshore regions as suggested by Stive (2004); Davidson-Arnott (2005) and others. To predict shoreline changes caused by SLR, Karambas (2003) calculated the amount of shoreline retreat induced by several values of SLR ranging from 0.25 to 1.0 m using a process-based one-dimensional model, which predicts beach profile change by estimating the crossshore variation of cross-shore sediment transport rate and was validated against experimental data. Cowell et al. (2006) estimated the probabilities of the amount of future shoreline changes on the Manly and Mission beaches in Australia using a profile translation model. Ranasinghe et al. (2012) developed a shoreline prediction model that calculates the dune erosion caused by wave run-up and stochastically predicted the amount of shoreline change by 2100 on Narrabeen Beach in Australia using storm time series that were probabilistically produced. Future shoreline changes from 2008 to 2095 on the Hasaki coast in Japan facing the Pacific Ocean were estimated by Banno and Kuriyama (2014) using their shoreline prediction model and considering SLR and wave climate change under two scenarios. While a number of studies investigated the impact of SLR on natural beaches, research on the effect of SLR on beaches protected by coastal structures including submerged breakwaters is rare. Yet even when coastal structures successfully protect the beaches behind them as expected, these beaches that were subjected to erosion in the past may still be at risk of erosion. Hence, examining shoreline changes caused by relative SLR on a sandy beach protected by submerged breakwaters can provide important information for beach conservation strategies. Moreover, the investigation results may also be helpful for preserving healthy beaches behind coral reefs, which form natural submerged breakwaters. The objective of this study is to predict the future shoreline change caused by SLR and land subsidence along the Niigata West coast in Japan, which is protected by submerged breakwaters and is now experiencing land subsidence. First, the influences of sea level change and land subsidence on the shoreline change during the 10-year period from 2001 to 2011 is investigated. Then, the future shoreline change during the 100-year period from 2011 to 2111 is predicted using a shoreline prediction model.