BEHAVIOUR OF PUMPING PITS IN HARBOUR ENTRANCE ON AN EXPOSED SANDY COAST.

Abstract

In the summer of 2010 a ferry harbour was opened at the south coast of Iceland connecting the Vestmannaeyjar islands to the mainland. The harbour was constructed on a sandy beach in a dynamic area open to the Atlantic Ocean causing massive longshore sediment transport across the navigational channel to the harbour. Sedimentation in the harbour entrance has turned out to be more than expected causing the harbour to be closed for navigation for large part of the winter time. The volcanic eruption of the Eyjafjalljokull glacier started four months before the opening of the harbour in April 2010. The floods that followed from the river Markarfljot, caused large amount of fine sediments, estimated as 2 million m3 of sand and 20 million m3 of mud, to be drifted down through the delta of the river located only 2 km east of the harbour. In the following years the deposition of fine sediments caused navigation problems in the harbour as the necessary navigation depth could not be obtained without heavy dredging. The last two years however the main settlement of sand has been taking place in a limited area between the two breakwater roundheads. In the winter time, it has turned out to be difficult to dredge in the harbour entrance. This is due to the high ambient wave conditions during the winter time where Hs is larger than 2 m 80% of the time. The limiting wave height for operating dredging vessels in the entrance is less than 2 m. The aim of this study is to assess the possibility of keeping the necessary navigation depth across the harbour entrance by placing a series of static pumps under the seabed in the harbour mouth. Physical model tests were carried out focusing on that problem. First focusing on the effect of a single pump located under the seabed and then on a combined effect of several pumps.