THE STUDY OF LAND SUBSIDENCE RATE AT COASTAL AREA OF SEMARANG CITY, INDONESIA

Semarang is a coastal city that has been suffered by rob flood for at least 25 years. One of main factors causes rob flood in Semarang is land subsidence. Land subsidence occurs in Semarang is a complex problem because the rate of land subsidence is different in places and changes in the development of time. Therefore it is necessary to monitor the land subsidence rate in Semarang. The aim of this research are to check the newest elevation of Bench Marks in coastal area of Semarang, to monitor land subsidence rate in coastal area of Semarang, To make elevation and land subsidence contour map in coastal area of Semarang. The result of this research shows that bench marks which are above MSL +100 m in height are stable enough. The closer to the sea, the faster land subsidence occurs. Soil type in coastal area of Semarang which is included young-soft soil type makes it still undergoes compression / consolidation. More over, coastal area of Semarang which is the center of the city activity such as Industry, buildings, reclamation, crowded transportation, and many artisian well has made land subsidence happens faste