Beach Changes Triggered by Ground Subsidence and Waves Generated by Easterly Trade Wind near Pekalongan on North Shore of Java Island

Abstract

The shoreline recession caused by ground subsidence due to the excessive pumping of underground water and that caused by wind waves incident from the easterly direction under the tropical monsoon weather was investigated, taking the Pekalongan area on Java Island as a study area. The study area was divided into nine subareas on both sides of the Comal River delta, and the shoreline changes in each subarea were investigated using satellite images. Field observation was also carried out on August 10, 2022, in the Pekalongan area, and the height of ground subsidence was measured together with the observation of the inundated area using a UAV. It was found that, in this area, the height of ground subsidence reached approximately 1.6 m, and the sinking area has been rapidly expanding. In this area, predominant waves are constantly incident from the easterly direction under the tropical trade wind condition, and in some cases, waves are obliquely incident at a significantly large angle relative to the direction normal to the shoreline. Along the east coast of the Comal River mouth, the mean wave incidence angle was as large as 22° clockwise, whereas on the west side, the angle increased up to 42°, causing marked shoreline changes due to high-angle wave instability.