STUDY OF LONGSHORE CURRENT IN THE MOUTH OF TUNTANG RIVER IN MORODEMAK VILLAGE, DEMAK REGENCY, INDONESIA AND ITS POSSIBLE EFFECT ON FORMING THE COASTAL MORPHOLOGY

The coastal morphology in Morodemak Village is unique since the erosion and sedimentation areas exist only in narrow area separated by the Tuntang River. Combining field measurement and the simulation based on the 10 years wind data, we investigated the characteristics of longshore current which may influence the coastal morphology of Morodemak Village. The distribution of bottom sediment in the Morodemak Village was mainly influenced by the ocean dynamics and it was not by the river input since there was no difference of sediment type between near and away from the river mouth. The characteristic of wind in Morodemak Village was influenced by the south east Asia monsoon system which blew north westerly during rainy season (December, January, February) and south easterly during dry season (Jun, July, August). Longshore current in Morodemak Village only flew south west wardly, occurred during dry and transition season. In the rainy season, longshore current was not generated since the wave breaker angle to the coastline was 0°. The river flow in the mouth of Tuntang River inhibited the speed of south west ward longshore current so that the speed of longshore current in the western side of river mouth was weaker than in the eastern side. The stronger (weaker) longshore current may scrap the coastal line (cause the suspended sediments easily to settle), enhanced the erosion (sedimentation) along the western coast of Morodemak Village.