Coastal Engineering Journal

The 2011 Tohoku Tsunami was observed around Japan by NOWPHAS equipment. These devices include GPS buoys which measure water surface levels by real-time kinematic GPS technology and which are moored at sites 10 km to 20 km offshore where water depths range from 100 m to 400 m, seabed wave gauges at depths between 20 m and 60 m, and coastal tide gauges. A GPS buoy moored offshore of Kamaishi Port recorded the first tsunami crest, which rose in two steps and exceeded 6 m approximately 30 min after the earthquake. Data from some sets comprising GPS buoys, nearby seabed wave gauges, and coastal tide gauges determined the extent of the tsunami propagation and amplification on the coast. Resonance caused by the tsunami continued a day or more after the initial tsunami arrival in some stations. A Doppler-type wave directional meter located in the Tokyo Bay entrance acquired data on the vertically uniform distribution of the tsunami current traveling in and out the bay. The tsunami was detected at almost all of the tide gauges around Japan, even in the semi-closed Ariake and Yatsushiro Bays located on the East China Sea coast of the Kyushu region.

The present study concentrates on the empirical formulas of Goda [1975, 2009]. The formulas were proposed for computing the transformation of three common representative wave heights (i.e. H_1/3, H_max and H_m0) on plane beaches. The objectives of the present study are to verify the Goda formulas for computing the transformation of H_1/3, H_max and H_m0 on unbarred beaches and to extend the formulas for computing the transformation of H_m, H_rms and H_1/10. Laboratory data from small-scale and large-scale wave flumes with unbarred beach conditions are used to verify the formulas. The verification shows that the formulas give very good predictions of H_1/3 and H_m0, but give fair prediction of H_max. The formulas are rewritten in the form of a general formula. The general form of Goda formulas is recalibrated and extended to compute other representative wave heights (i.e. H_m, H_rms and H_1/10). The general formula gives very good predictions of H_m, H_rms, H_1/3, H_1/10, H_max and H_m0.

The morphology of the Eastern Scheldt inlet in the southwestern Netherlands has been changing for the past 25 years in response to the construction of the Eastern Scheldt storm-surge barrier in 1986. As a result of the barrier, there has been a decrease in tidal amplitudes, tidal volumes, and average flow velocities, and there is hardly any sediment exchange through the barrier. Bathymetrical measurements of the ebb-tidal delta show multiple effects: (1) An overall decrease in sediment volume, (2) a decrease in morphological activity, (3) erosion of the shoals and sedimentation in most channels, (4) northward reorientation of channels and shoals, and (5) an increase in wave-driven features. Results from a process-based model show that the erosion is related to the wave action, and the reorientation is related to the interaction between cross-shore and alongshore tide. The steady erosive trend, combined with the decline of morphological activity, points toward a system dominated by relatively small and mostly negative bed-level changes. This system is still far from any kind of equilibrium, and is still adapting itself to the new hydraulic forcing regime, even though sediment transport capacities have decreased.

The authors propose a new method to estimate the tilting failure of caisson breakwaters, using a Monte-Carlo simulation that can quantitatively and stochastically estimate failure probability. An index, referred to as "Expected Frequency Exceeding of a Critical Load (EFEC)" that expresses the potential tilting failure probability of caisson breakwaters is proposed and adopted in this model. Application of the model to 40 breakwaters in Japan reveals that the threshold value of EFEC that specifies whether a caisson breakwater is safe or unsafe could lie in the range from 0.02 to 0.04. The authors also found that unstable caisson geometry, showing a large value in the aspect ratio of the caisson, can provide additional indication of when tilting failure is likely to occur. It is expected that the present method can provide a practical way for engineers to judge the possibility of tilting failure in caisson breakwaters.