Analysis of Topographic Changes on Nusa Dua and Tanjung Benoa Beaches on Bali Island, Indonesia

Nusa Dua and Tanjung Benoa Beaches on Bali Island are world-famous coastal resorts, and coral reefs are well developed along the coastline. Along these beaches, beach erosion has started from the south end of the coast owing to the blockage of longshore sand transport caused by the closure of the channel behind Nusa Besar Island, and the erosion zone has expanded northward. To consider measures against beach erosion, shoreline changes were investigated using the aerial photograph and satellite image taken in 1997 and 2020, respectively, together with the analysis of topographic changes of the coast using bathymetric survey data. It was found that on Tanjung Benoa Beach located in the northern part, beach erosion occurred over time because of the decrease in the rate of northward longshore sand transport. The rate of longshore sand transport across the south end of this beach was estimated to be 1400 m3/yr. Taking the simultaneous occurrence of sand accumulation and erosion into account, it was concluded that sand back pass would be a useful measure to inhibit the beach changes on this beach, instead of adopting hard measures. On the other hand, on the south Nusa Dua Beach, the foreshore area has increased over time because of the supply of sand from the coral reefs. On the basis of the bathymetric survey data, the rate of sand supply from the coral reefs was estimated to be 3.0 m3/m/yr.