Coastal Sedimentation, Erosion and Management on the North Coast of Kosrae, Federated States of Micronesia

Abstract

Kosrae, the Federated States of Micronesia, is a high volcanic island surrounded by a fringing reef. Along the north coast mangroves occupy the western part and sand beaches occur on the central and eastern sections. The development of erosion problems along the north coast can be traced to unforeseen ecological changes and coastal system adjustments resulting from sand extraction and reclamation projects. For the fill needed to build roads on the east coast and an airfield on the northwest coast, borrow pits, islets and an artificial channel were excavated on the reef flat. These excavations have caused severe erosion along the north coast. The shoreline has retreated 7 to 15 m, and reef flat, mangrove and beach sediments have become coarser. These shoreline changes have been caused by ecological changes that reduced the sand supply and by trapping of longshore sediment within the borrow pits and artificial channel. Continued removal of channel sediments southwestward into Okat Bay is caused by currents in the channel, which are not controlled by tide but by waves arriving at the channel entrance, forcing water forward the southwest under most conditions. Future stability of the shoreline along the north coast can be improved by filling the borrow pits on the reef flat  building a dike between the east corner of the airfield and the mangrove, and filling part of the artificial channel north of the dike.