Coastal Engineering Journal

The economy and life in Vietnam is associated with the existence and development of large river deltas, i.e. the Mekong Delta and the Red River Delta. The latter is located in the northern part of Vietnam and apart from thousands of square kilometers of the area, it includes more than 165 km of coastline as well. In this area, the seashore and the adjacent densely populated, low-lying regions are often subject to intense impacts from the river (floods) and the sea (typhoons, changes in sea level, currents, etc.) and therefore undergo substantial and dynamic changes and destructions.

In the current study, one of the coastal segments that are most vulnerable to destruction in Vietnam is analyzed. A synthetic analysis of the available information was carried out first and then the modeling of wave climate, currents and sediment movement patterns in the nearshore zone was done. The formulation of a phenomenological model of sediment budget, adequate for the amount of available information, allowed for assessment of the existing sediment fluxes and their relations to morphodynamic processes taking place in the studied area.

The functional performance of a submerged and essentially horizontal plate for offshore wave control is summarized. The historical development of the concept is observed with comments on the extraordinary features of this simple type of structure. Analytical, numerical, and semi-empirical methods for evaluation of the reflectivity and transmissivity of and the hydrodynamic forces on the plate are comparatively examined. Known results on the effects of the length, the submergence, the porosity and the inclination of the plate are surveyed. The three-dimensional effects, the effects of wave irregularity, wave nonlinearity, fluid viscosity and current presence are also discussed.

With a numerical model of Northwestern Pacific region, the water movements associated with real time tides and actual storms were reproduced. Making use of reasonably representative model simulations together with data derived from the coastal observations can lead to the required estimates of extreme conditions for the purpose of coastal defense and design of offshore structure. Here, we used the model of 1/12° latitude by 1/12° longitude resolution. 75 typhoon and monsoon surge simulations were performed with specification of 8 boundary tides for real time tides. Typhoon and winter monsoon wind fields were computed from JWA/GPV data set applying Cardone's PMBL (Planetary Marine Boundary Layer) model over the whole region with embedding parameterized typhoon wind fields onto it. Present result is an updated version of papers presented earlier (Choi and Ko, 1999; Choi and Eum, 2000). Preliminary maps showing distribution of extreme sea levels of whole modeled area including coastal boundaries and surge induced extreme currents are presented in terms of recurrence intervals.

An extensive 3D model test program has been performed to study the effects of wave obliquity and multidirectionality on the stability of armour units on rubble mound breakwater. Four types of armour units — dolosse, Accropode, hollow-square and quarry stones were tested under five angles of wave attack (0°, 15°, 30°, 45° and 60°) for both long-crested waves and directional waves. Two wave steepnesses (s_op = 0.05 and 0.03) and three directional spreading (s = 10, 40 and ∞) were chosen for the test. A method is proposed to take into account the effects of wave obliquity and multidirectionality on the armour unit stability.