Safety assessment of sea dikes in Vietnam; a case study in Namdinh Province

Abstract

Vietnam has more than 3200 km of coastline, primarily consisting of low-lying coastal areas which are protected by sea dikes, natural dunes and mountains. More than 165 km of coastline lies within the Red River Delta, a densely populated region which experiences substantial dynamic changes and destruction due to frequent intense impacts from the sea (typhoons, changes in sea level, currents, etc). This dynamic coastline is mainly protected by sea dike system which has been developed for almost hundred years. The NamDinh coast, constitutes part of this coastline, has total length of about 90 km long of low-lying coastal zone, which is and had to be protected by sea dikes. This coastline changes very often due to erosion and accretion processes. Since the beginning of last century, about more than 30 km of the coastline of Hai Hau district has been facing erosion with estimated erosion rates of about 10-20m per year causing damages to the local economy housing, and loss of land, etc. Furthermore, the sea dike system along NamDinh coast has been heavily damaged. There were many times of dike breach which caused serious flooding and losses. The situation of NamDinh coastline and sea dikes can be considered as a representative for coastal area in Northern part of Vietnam. In recent years there has been a number of studies aiming at understanding the situation of coastline changes and sea defences system in NamDinh. However, due to the lack of data and design tools the results of these studies, somehow, are still limited and the problem is still poorly understood. Therefore, study on natural processes of coastline changes and assessments of the safety of the sea defensive system are necessary. This study focuses on assessment of safety of Namdinh sea dikes. Firstly, the historical development of sea dike system in Namdinh province is analysed base on historical record and collected data. Based on that the possible causes of old-dike failures are carried out. Secondly, the study investigates all possible failure mechanisms and their causes of the existing dikes. Follows by, the safety assessment of the dikes is performed for possible failure modes in terms of hydraulic, structural and geotechnical related aspects. Finally, conclusions on safety of Namdinh sea dikes are stated and some recommendations (guidelines) of new sea dike design in Namdinh and Vietnam, in general, are given. Safety assessment in this study is based on both deterministic and probabilistic approaches. The latest Vietnamese codes and Dutch codes for design of sea dikes and revetments are the basic references for these analyses. Comparisons will be made to applying different design codes for design of sea dikes in Namdinh as well as in Vietnam. In general, analytical methods are applied in this study. However, for solving some specific related problems the advanced mathematic models are used as calculation tools, such as: Using CRESS for some hydraulic related problems; GEO-Slope and PLAXIS for geotechnical related ones; VaP and MathLab models for probabilistic calculations.