Katsutoshi Tanimoto

Developing effective vegetation bioshield for tsunami protection

To elucidate the effectiveness and limitations of coastal vegetation for tsunami protection, the impact of vegetation structure on drag forces was analysed using the observed characteristics of reference tree species. The drag coefficient, including the vertical stand structures of trees, C d−all , and the vegetation thickness per unit area, dN u (d, reference diameter of trees; N u , number of trees per unit area), varies greatly with different species. Based on data analyses, dense Casuarina equisetifolia and Pandanus odoratissimus grown in beach sand were found to be especially effective in providing protection from tsunami damage due to their density and complex aerial root structure. The breaking moment of trees was investigated as a function of tree diameter. The breaking moment equation of P. odoratissimus explains well the damage caused to trees by the 2004 Indian Ocean tsunami and the 2006 Java tsunami. Numerical simulation indicates that inertia is the dominant force (99.1%) in comparison with the drag force when a wave front collides with a vegetation face, but while the water depth is very shallow and the total force is 6% of the maximum. Drag resistance is the dominant force in reducing both water depth and current velocity, but inertia resistance is active in reducing current velocity only in front of the vegetation. The breaking condition of trees can be discussed by the drag-force moment alone because when the total moment reaches the maximum, the contribution of the inertia moment is in the range of 0.1–0.3%. Considering the limitations of P. odoratissimus in reducing tsunami water depth and the other roles that coastal vegetation can play in mitigating tsunami-related damage, a forest with two layers in the vertical direction of P. odoratissimus and dense C. equisetifolia was found to be effective for increasing drag and trapping floating debris.

NEW STABILITY FORMULA FOR WAVE-DISSIPATING CONCRETE BLOCKS COVERING HORIZONTALLY COMPOSITE BREAKWATERS

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

Design and construction of caisson breakwaters: the Japanese experience

Abstract The history of breakwater construction in Japan is reviewed and present design methods of vertical breakwaters are discussed in terms of wave forces and breakwater stability. Current design techniques of vertical breakwaters in Japan are the culmination of many years of experiences and practical research in this field. The Goda formula, which accurately predicts the design wave forces under most conditions of vertical breakwaters, is an excellent aid in the design of the breakwaters. Under certain conditions, excessive impulsive forces due to the action of breaking waves are exerted on the upright section. Such conditions must be minimized in the design of vertical breakwaters, and the adoption of vertical breakwaters covered with wave dissipating concrete blocks or the wave dissipating caisson breakwaters offer potential solutions of this problem. The bearing capacity of rubble mound foundation and the stability of armor units are also discussed.

Tsunami mitigation by coastal vegetation considering the effect of tree breaking

Damage to vegetation by tsunami moment and reduction of potential tsunami force are discussed based on a numerical simulation. A numerical model based on two-dimensional nonlinear long-wave equations that include drag forces and turbulence-induced shear force due to the presence of vegetation was developed to estimate tree breaking. The numerical model was then applied to a coastal forest where two dominant tropical vegetation species, Pandanus odoratissimus and Casuarina equisetifolia, were considered. The threshold water depth for tree breaking increased with increasing forest width, and the analysis was consistent with the field investigation results that the critical tsunami water depth for breaking is around 80% of the tree height for P. odoratissimus. C. equisetifolia is stronger than P. odoratissimus against tsunami action, but P. odoratissimus can reduce a greater tsunami force than C. equisetifolia due to its complex of aerial root structures. Even if breakage occurs, P. odoratissimus still has high potential to reduce the tsunami force due to its dense aerial root structures. Previous numerical models that do not include the breaking phenomena may overestimate the vegetation effect for reducing tsunami force. The combination of P. odoratissimus and C. equisetifolia is recommended as a vegetation bioshield to protect coastal areas from tsunami hazards.

Tsunami current inundation of ground with coastal vegetation effects; an initial step towards a natural solution for tsunami amelioration

A densely grown coastal vegetation belt of Pandanus odoratissimus for reducing the tsunami energy was quantitatively analyzed by an enhanced one-dimensional numerical model that included variations of topography and tsunami characteristics. The drag and inertia forces were assumed as the total resistance generated by the vegetation. It was found that a relatively small period tsunami wave was more destructive than a relatively large period tsunami wave of the same height, although densely grown vegetation effectively reduced the tsunami energy in the case of the small period tsunami wave. A very mild ground slope was also more vulnerable to thrashing by tsunami waves than a relatively steep ground slope. Moreover, densely growing coastal vegetation on very mild ground slope dissipated tsunami energy more efficiently than the same vegetation on relatively steep ground slope.

NUMERICAL SIMULATION OF BREAKING WAVES BY LARGE EDDY SIMULATION AND VOF METHOD

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.

ESTIMATING THE SLIDING DISTANCE OF COMPOSITE BREAKWATERS DUE TO WAVE FORCES INCLUSIVE OF IMPULSIVE FORCES

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

Uplift Wave Forces Due to Compression of Enclosed Air Layer and Their Similitude Law

Uplift forces on the ceiling slab of a wave dissipating caisson are a typical example of the impact wave force due to compression of an entrapped air layer. A theoretical model is introduced in order to describe the mechanism of the impact force and the similitude law of the impact force is proposed. A calculation method of the uplift force is also presented. The theoretical model and the similitude law as well as the calculation method are well confirmed by model experiments and field observations.

A Two-Dimensional Multi-Scale Turbulence Model for Breaking Waves

In this paper, a two-dimensional multi-scale turbulence model is proposed to study breaking waves. The purpose of developing the two-dimensional multiscale turbulence model was to produce a relatively accurate model with moderate computer requirements. The free surface was tracked by the VOF technique, the log-law profile for the mean velocity was applied at the bottom. Compared to RANS models, the present model showed better agreement with experiments in terms of predicting breaking wave surface elevations, especially near the breaking point. The undertow predicted by the present model is reasonable. The SGS turbulent transport mechanism was also discussed in the paper. I n t r o d u c t i o n The present stage of Navier-Stokes equations based numerical modeling (MAC or VOF) of breaking waves in coastal and ocean engineering can be classified into three levels: (I) those that solve the Navier-Stokes equations directly, containing no turbulence models (Miyata, 1986), or include constant eddy viscosity in both space and time (Petit,et al.,1994); (II) those that solve the Reynolds Averaged Navier-Stokes (RANS) equations (Lemos, 1992; Lin and Liu, 1998a,1998b; Bradford, 2000); (III) and those that solve the space-filtered Navier-Stokes equations (Zhao and Tanimoto, 1998; Watanabe, 1999; Wijayaratna and Okayasu, 2000). 1Center for Applied Coastal Research, University of Delaware, Newark, DE. 19716 USA. email: zhao@coastal.udel.edu 2Department of Mechanical Engineering, Sydney University, Sydney 2006, Australia. email: armfield@mech.eng.usyd.edu.au ~Department of Civil Engineering, Saltama University, Urawa, Saltama 338-8570, Japan. email: tanimoto@post .saitama-u.ac.jp

PREDICTION OF SHORELINE CHANGE CONSIDERING CROSS-SHORE SEDIMENT TRANSPORT

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.