Coastal erosion at Hai Hau beach in the Red River Delta, Vietnam

Many coastal areas in Vietnam suffer from erosion. One of the worst affected areas is the Hai Hau beach, located in the Red River delta in northern Vietnam. Over the last hundred years, vast parts of the beach have eroded at an average rate of approximately 25 m per year. Important agricultural land has vanished into the sea and families have been forced to abandon their houses because of the erosion is not explained but suggested reasons are for example dam construction, weak dikes and unfavourable hydrodynamic conditions The wave climate, and thus the sediment transport, in northernVietnam is highly dependent on the two main wind directions from the northeast and the south, because of the winter and ) summer monsoon, respectively . Vietnam is also.subject to sporadic typhoons that cause great damage when they strike the coast, especially in combination with storm surges. Based on a 20-year long wind record from 1976 to 1995, obtained from two islands outside the coast of northern Vietnam, the longshore transport rate over these years was determined by a one dimensional numerical wave model. The model consists of three independent parts: offshore waves, nearshore waves and -longshore sediment transport. This model indicates a southward-directed transport along the Hai Hau beach. In addition, sea maps and satellite photos from the 20th century were compared in order to find out the historic shoreline development generally. The main protection method that is generally used in Vietnam, as well as in Hai Hau district, is sea dikes. These dikes consist of an earth core covered by a revetment layer of cobbles. Unfortunately, these dikes are not strong enough to withstand the impact of harsh waves for any longer periods. Historically, a line of sea dikes has been destroyed at least once every decade. A couple of hundred meters behind the front sea dike typically lies another dike, ready to meet the waves when the first one breaches. Consequently, this defence arrangement is often recognized as a double dike system, but also as a retreat strategy since the erosion problem is not solved, just slowed down. The major cause of erosion is likely not due to human activities (such as dam buildings,or river cut-offs), instead because of the natural properties of Hai Hau coastal area, seasonally bounded longshore currents in combination with weak sea dikes favour erosion.KWP-collectio