1. INTRODUCTION
In recent years, among the various types of coastal structures, concrete blocks have been widely used as one of the important components due to their effectiveness against wind wave attacks. The design method for wind waves is well established and concrete blocks work best. On the other hand, tsunami and long-period waves also cause problems in Japan. Numerous breakwaters were severely damaged in the 2011 Off the Pacific Coast of Tohoku Earthquake Tsunami. Many ports suffered from disturbance in cargo handling due to ship motion caused by long-period waves.
In this paper, we introduce our new methods using concrete blocks for tsunami protection and long-period wave absorption as countermeasures to such problems.
2. NEW TECHNOLOGY FOR TSUNAMI PROTECTION USING CONCRETE BLOCKS
(1) WIDENED PROTECTION COVERED WITH CONCRETE BLOCKS
Many composite breakwaters were seriously damaged in the 2011 Off the Pacific Coast of Tohoku Earthquake Tsunami. One of the causes of failure was a scouring of the rubble foundation on the harbor-side of breakwaters due to the tsunami overflow. This was a formerly inconceivable type of failure. One possible countermeasure is placement of a widened protection using additional rubble stones behind the breakwater to prevent the sliding of the caisson. Installing concrete blocks on the rubble mound on the harbor-side would also be required to prevent scouring around the rubble mound. We have developed a stability estimation method for concrete blocks against tsunami overflow. The method is based on a series of laboratory experiments conducted in a wide range of conditions.
(2) EXPERIMENTS
The experiments were carried out by changing the shape of the harbor-side rubble mound, harbor-side water level, the shape of concrete blocks and the mass of the concrete blocks. A steady over flow was generated by a submersible pump. The stability limits for the concrete blocks were examined by gradually increasing the overflow depth. The main results are as follows. 1) The stability of the concrete block is greatly influenced by impingement position of the overflow jet. 2) The stability of the concrete blocks increases as the harbor-side water level rises. 3) The failure modes of the concrete blocks are divided roughly into two modes. One is the overturning mode caused by the rotation of the block. The other is the sliding mode caused by the external force exceeding the frictional force. 4) The overflow depth of the stability limit was almost proportional to the nominal diameter of the block in the case of the overturning mode, while it was nearly independent of the size of the block in case of the sliding mode. 5) The holes in the concrete blocks enhance the stability due to the reduction of the uplift forces.
(3) STABILITY ESTIMATION METHOD
Empirical formulae for the stability estimation were derived based on the experimental results. The overflow depth of the stability limit corresponding to each failure mode can be obtained by the two formulae. These formulae involve the stability number of each concrete block determined through the experiments. According to this calculation method, it is possible to determine the mass of concrete blocks against the tsunami overflow. This calculation method has already been used for actual design.
3. NEW TECHNOLOGY FOR LONG-PERIOD WAVE ABSORPTION BY USING CONCRETE BLOCKS
(1) SUBMERGED MOUND TYPE WAVE ABSORBING STRUCTURE COVERED WITH CONCRETE BLOCKS
In many ports, it has been reported that long-period waves cause trouble in cargo handling. As a countermeasure to this, a wave absorbing mound installed on the harbor-side of the breakwater has been proposed. Because of the low wave absorbing performance of such conventional mound type structures, the required width to absorb the long-period waves becomes more than 30m. It is important to reduce the size of the structure to apply to various site conditions. The crown height of a conventional mound type structure is almost equal to that of the caisson. On the contrary, we propose a submerged mound type structure covered with concrete blocks. The basic concept of this proposed structure is to level the crest elevation to the water surface to establish high efficiency in energy dissipation on the surface of the crown of the concrete blocks.
(2) WAVE ABSORBING PERFORMANCE AND ITS MECHANISM
A series of hydraulic model experiments was carried out to evaluate the wave absorbing performance. Monochromatic waves with periods of 30 to 120s were used for the experiment. The reflection coefficient was obtained from the recorded water surface elevation. Throughout these experiments, it became clear that the reflection coefficient of the submerged type is smaller than that of the conventional type, independent of the wave period.
The wave absorbing mechanism was investigated using hydraulic model experiments and numerical analysis. It was concluded that the cause of effective energy dissipation of the submerged type is related to significant increase of the flow velocity around the concrete blocks due to the flow contraction onto the crest.
(3) STRUCTURE WIDTH ESTIMATION METHOD
We obtained the relationship between the reflection coefficient and width of structure from the hydraulic model experiments and provided a calculation chart. The appropriate width of the structure under the allowable value of reflection coefficient is determined by using the calculation chart. According to this calculation method, it is possible to design the submerged mound type wave absorbing structure. This method has already been used for actual design.