Coastal Engineering Journal

The dynamic response of a vertical breakwater to breaking waves has been studied by the authors with prototype tests on two breakwaters in Genoa Voltri and one in Brindisi. The tests were interpreted by Mass Spring Dash-pot elements connected among them in order to represent the Array structure typical of a breakwater (MSDA model). The model is similar to the classical MSD one, except that it also considers the force exchanged by adjacent caissons through the foundation.

The objective of this note is to present an analysis of the caisson interaction and to study and compare the different responses to breaker impacts on average European and Japanese caisson breakwaters. These represent typical section shapes using two areas with scaling rules relating caisson dimensions to the incident design significant wave height. Inertia, foundation stiffness and added masses are quite different for the two types of structures and their effect is analyzed.

The dynamic response is presented in terms of a static equivalent response factor given as a function of the ratio between impact load duration and the main structure oscillation period, of the ratio between caisson length and width and of the fraction of the total mass represented by foundation added mass (the latter being the essential difference between European and Japanese design).

Although the modeling of the array structure is essential to interpret the oscillations measured in the prototype, and is obviously important to study the effects of a load that varies along the breakwater, the prediction of the classical approach is similar in terms of maximum load on the foundations whenever the applied forcing is longitudinally homogeneous.

Climate change is expected to lead to increases in both sea level and typhoon intensity, which could threaten the stability of breakwaters in the future. In this study, calculations using the SWAN model showed that a 10% potential increase in the future wind speed of typhoons resulting from the warming of surface sea temperatures can lead to a 21% increase in the significant wave heights generated by these winds. To understand the effect that this would have on breakwater stability, the expected sliding distances for the breakwaters at Shibushi Ports in Japan were estimated using a probabilistic design method. The results show that in the future the expected sliding distances may become five times greater than at present, due to a combination of increases in sea level and wave height.

A bow thruster mounted in the bow of a vessel is generally used to improve its maneuverability in ports. When a bow thruster is used near a berth, a jet generated from the bow thruster can cause sediment transport, resulting in local scouring of the seabed in front of quay walls. Countermeasures against such scouring are essential to ensure the stability of the quay walls. In recent years, new protective units called filter units have been used to protect seawalls and river dikes. However, it remains to be determined whether filter units covering the seabed in front of quay walls are effective in preventing jet-induced local scouring. In this study, jet-induced local scouring in front of a quay wall and its countermeasure using filter units are investigated using a three-dimensional two-way coupled fluid-sediment interaction model. The predictive capability of the three-dimensional coupled model is verified against experimental data in terms of average jet flow velocity, pore-water pressure, and the final seabed profile. The results show that the filter units covering the seabed prevent negative effective stress fluctuations on the surface layer of the seabed, and demonstrate the effectiveness of the countermeasure using the filter units.