Three types of micro-bubble aerators were set on a constructed tidal flat in the innermost part of Tokyo Bay for A.D. 2003-2005, to reduce the baneful effect of the surrounding hypoxic seawater on benthic animals and to preserve innate functions of tidal-flat ecosystems. The improvement in dissolved oxygen (DO) was observed; especially, DO was increased by the aeration system when the water-mass remained on the constructed flat during the falling tide and was enclosed by a stone wall. This suggests that enclosures like containment boom can enhance the effect of aerators. Although survival of cultured bivalves were improved by the aerators, the effect on benthic animals has not clearly been confirmed on the constructed flat. Benthic animals suffered from not only hypoxic seawater but environmental conditions of sediment and freshet. A long-term monitoring is needed for evaluating the effect of aerator on benthic animals.
Yatsu Higata is one of the most significant remaining tidal flats in Japan. In recent years, Yatsu Higata has witnessed an extraordinary growth of Ulva sp. algae. This paper explores the possible origins of the Ulva sp. in Tokyo Bay and describes field studies intended to explore the nutrient dynamics in Yatsu Higata and Tokyo Bay. Study results show that the Ulva sp. flourishes in Yatsu Higata due to a ready supply of nutrient runoff supplied by the Yatsu River. The water of Yatsu Higata contains high levels of nitrate nitrogen, which is considered to strongly affect the growth of Ulva sp. The ability of Yatsu Higata to purify seawater is reduced in the summer and winter due to the presence of Ulva sp.
The Ministry of Land, Infrastructure and Transport and Aichi Prefecture are jointly involved in various environmental rehabilitation and creation projects inside Mikawa Bay. New tidal flats and shallow waters have been created and capping has been conducted through the utilization of high quality dredged sand arose from the development of Nakayama waterway. Results from the monitoring survey in 7 years showed improvement in sediment quality and disappearance of oxygen depleted water column in the tidal flat. Also diversity of bivalves and other benthic organisms have increased in the tidal flat area, suggesting an improvement of the habitat environment. In addition, Bivalves play an important role in water purification function. When the increased purified nitrogen in water, owing to recovery of bivalves in the created tidal flat, is multiplied by the total project area (620ha), the amount of purified nitrogen becomes 815kgN/day. It is equal to approximately 2% of the whole nitrogen load to Mikawa Bay.
The Sone tidal flat in Kitakyushu city has a dry flat of about 517ha during ebb tides. It is one of the nicest spots for the wild bird observation through the four seasons. It is also famous as the breeding ground of Horseshoe crab (Tachypleus tridentatus). A construction of an artificial island for a new airport about 3km offshore started in 1994, which was opened in March, 2006. A fishing port is under construction in the center of the Sone tidal flat. Therefore, there is a possibility that natural environments of the tidal flat may change. The aim of this study is to understand characteristics of the natural environments of the Sone tidal flat through field investigations of ground elevation, water quality, bottom sediment, and benthos. These investigation results are compared with the past findings. The results of the study indicate that the natural environments of the Sone tidal flat and its neighbor have not significantly changed at the present time. However, there is concern that a slight deterioration of bottom sediments and diversity of benthos has been detected. Detailed regular monitoring studys will be necessary.
Numerical simulation was performed on the relation between the interannual variation of the Tsushima warm current along the southwest coast of Hokkaido and the concentration of nitrogen, the production of a kelp Laminaria religiosa. The downward tendency of northward volume transport along the southwest coast was maintained at a rate of year 0.036SV in 1986 and afterwards. Reduction of northward flux falls the surface water temperature of the coast, and making nitrate concentration increase was suggested. According to simulation, the year yields of the kelp in the southwest coast increased about 32 times, when northward flux decreased to 60% of the present condition.
A new procedure for collecting seeds of eelgrass (Zostera marina) was developed to minimize the damage to an original eelgrass bed. An appropriate treatment by fresh water to control the sprouting time was introduced in the seedling production, which not only improves the germination rate but also reduces the total time for seedling production. These procedures were tested in a field. Sixty shoots of Z. marina raised in tank were transplanted to sea area and increased to 1700 shoots in 18 months. This newly developed technique will be of enormous help in the speedy restoration of eelgrass bed while preserving the genetic diversity of Z. marina.
Field experiments to estimate chlorophyll a of benthic algae habited at tidal flat were carried out by using portable fluorometer in the period from February to September, 2005. Chlorophyll a values obtained by fluorometer with in vivo analysis and by chemical analysis of DMF reagent indicate approximately in linear. The fluorometer method was introduced to measure a distribution of benthic algae quantity in some different conditions. For example, the survey results indicated that quantity of benthic algae was related to the habited ground elevation. It is ensured by the field test that this portable fluorometer enables much easily to evaluate a distribution of benthic algae.
Demolition work of the Sea Fort No. 3 and dredging work in the area of Naka-no-se channel are ongoing to improve navigability of Tokyo Bay Waterway. Crushed concrete materials produced by the demolition works and dredged sand of the channel are utilized respectively as the artificial fishing reef and the sea bottom capping materials to confine the nutrition of bottom sediment. Field surveys are carried out to evaluate the effects of above two measures. The fish inhabitation effects around the fishing reef are confirmed significantly by underwater observation. And the sea bottom capping effects are verified by nutrient elution reduction from the sediment and inhabiting benthos.
The effect of a new type of micro bubble aeration system for improving seawater quality in an enclosed bay was examined. The design of the system is as follows: The water inlet is set at the desired depth. The micro bubble nozzle is contained in a vertical pipe running from the surface to the same depth as the water inlet. Although normal bubbles rise in the pipe, it was expected that the water with micro bubbles would sink to the pipe outlet and then disperse widely at that depth. From the results of the field observation, it is found that the system can supply high DO water to the desired depth. The effects are estimated by the numerical model based on the field observation.
Sandminig for commercial construction aggregate and for landfilling projects has left borrow pits on the bottom in several coastal regions in Japan. The nation-wide survey revealed that the subaqueous borrow pits can be classified geomorphologically into a local depression type and a flatten-out type. As the local depression type of the borrow pits, for example, often cause severe deterioration of water quality such as formation of anoxia and blue tide, recontouring (raising the bottom) is expected to be effective to restore damaged ecosystems. In order to promote such geomorphological restoration effectively and to minimize negative effect, we summarized an impact-response flow diagram originated from such actions, displaying causal chains after placing materials into borrow pits. We also propose several research tasks on this matter; e. g., predictions of dispersion processes of particles and oxygen depleted water mass, fate and effects of fine chemicals contained or accumulated in the borrow pits, and propagation of restoration effects toward the whole basin-wide environment.
The structures, along the shore around big cities, are asked for the function of disaster prevention, utility, and the environmental consideration. As an environmental element, the tidepool is added to seawalls in the intertidal zone. In order to check and proof the function of a tidepool, we built an experimental structure at Ichikawa city, in Tokyo Bay. In the tidepool, it is checked that many benthos and fry of a fish had appeared. The tidepool keeps water, enables more species to live, and increases diversity, compared with the case of setting free water type structure. We carry on various kinds of experiments in this structure. For example, marine sessile organisms recruitment to the quality of the materials, degradation of material, etc.
Species diversity in a terrace-type tidal flat with a tidepool was investigated in a constructed tidal flat at Han-nan 2nd section, Osaka. The tidal flat has been featured by two sub class of tidal flat; a terrace-type and a slope-type. A tidepool is formed in the terrace-type tidal flat during low water. Species of benthos in the tidepool was almost same as at mean monthly-lowest water level (L.W.L) in the slope-type of tidal flat. In addition, Shannon-Wiener diversity index of macrobenthos in the terrace-type tidal flat was almost same as one in the slope-type tidal flat. The tidepool on the terrace-type tidal flat seems to have a same role of L.W.L area of the slope-type tidal flat, and enhances environmental diversity and species diversity to the tidal flat.
Although caisson walls are sometimes collapsed by the actions of impulsive pressures induced by breaking waves, these huge pressures are generally not considered in the present design procedures for reinforced concrete slabs of caisson walls. The present paper proposes new design procedures which take the impulsive pressures into account for the design of the caisson wall. The limit state design method is employed for an ultimate state and an initial cracking state of the caisson wall. The actual wave pressure acting on the wall is estimated under the consideration of inner filling resistant force and wave pressure reduction due to the dynamic response of a caisson on a rubble mound. The validity of the proposed design procedures is confirmed by the fact that they can clearly distinguish damaged and undamaged caisson walls.
The drift force can be calculated either by near-field method or by far-field method. When a structure includes corners or edges, calculated values of the drift force by using the Green's function method are often different by the methods. In this study, the cause of the difference has been examined, and found that 1) the rapid change in the velocity distributions close to the corners or edges seems to be the main reason why the near field method gives inaccurate drift force, 2) a fine meshing close to the corners or edges is an efficient method for improving the accuracy, and 3) when a fine meshing is difficult, introduction of a smooth corner would be an efficient method to obtain a better approximate value.
A strictly combined BEM-VOF model has been developed by making the best use of both method's merits. To carry out stable and accurate computations, the water surfaces obtained from BEM and VOF are smoothly connected in the transmission domain at each time step by using weighted means, and the velocity potential and its normal derivative in BEM and the value of the function of the volume of fluid in VOF are also corrected. The validity of the model was confirmed by comparing numerical results with theoretical values for Stokes' 5th order wave propagation through three domains of BEM-VOF-BEM as well as two domains of BEM-VOF. Wave propagation for a random waves through two domains was also checked. The model makes it practically possible to calculate wave-structure interactions under random waves by reducing the computational time and minimizing the numerical dissipation which is inevitable especially in single VOF calculations.
The main purpose of this study is to develop a two-dimensional numerical wave flume based on a VOF method in order to compute nonlinear interaction between wave and a moving structure. The numerical algorithm developed here is verified by comparing the numerical results with the theoretical ones on the movement distance of bottom-seated and bottom-detached structures in water. The numerical results, furthermore, reveal that the interaction of wave and a moving flap-type structure causes the difference of water levels between the offshore and onshore sides of the structure and has a strong effect on water surface elevation and water particle velocity field around the structure.
A flap-type gate for protecting against storm surge and tsunami is the mobile barrier hinged along a sea-bottom axis. Present paper describes the motion characteristics of the gate rising to the water surface or sinking to the bottom. Extensive experiments are performed with the scale model of the rising or sinking gate in a water tank. The effects of the air chamber volume in the gate and the side wall beside the gate on the motions are investigated. Further, a simple mathematical model is derived for the dynamics of rising motion of the gate. The rising motion predicted numerically using the mathematical model compares well with experiment motion, if the coefficients for the equation of the motion in the mathematical model can be suitably determined.
The North breakwater of Otaru Harbour, which was designed and constructed by Japanese engineers in Meiji Period, was the first modern breakwater facing the open sea in Japan. It has not yet been evaluated in the history of harbour construction throughout the world. The North breakwater in Otaru City is well known to have been constructed by sloping block system which was one of the most mechanical construction methods in those days. The present study addresses that Colombo breakwater was the complete form of breakwater constructed by sloping block system and the model of the North breakwater. To evaluate the North breakwater, we compared the technical level of the North breakwater with that of the Colombo breakwater by some essential factors of harbour construction technology in the 19th century in the world. The study results reveals that the former was superior to the latter in terms of three engineering factors and that the highest technology was adopted in the construction of the North breakwater of Otaru Harbour.
The stability of a breakwater during execution should be checked as well as that after completion. However, rational specific design methodology has not been established so far as to the stability during execution. We discuss the stability of caisson-type breakwaters covered with wave dissipating blocks during execution. We conducted a series of reliability analyses and showed that the design method using the design wave of ten years return period and partial factors of the caisson-type composite breakwater gave the conservative result from the viewpoint of LCC minimization.
It is common to cover a vertical part of a composite breakwater with wave-dissipating blocks when the breakwater is constructed in deepwater areas. However in these areas, the construction cost becomes quite expensive due to its volume of the structure. Therefore, we attempt to develop a new type of breakwater without wave-dissipating blocks in order to reduce the cost. In this paper, two-dimensional hydraulic model experiments are carried out to clarify the hydraulic characteristics of the reflected waves and transmitted waves, using the new type of breakwater. Two types of the structure are applied: “Sloping top caisson breakwater with sloped slit wall” and “Sloping top caisson breakwater with vertical slit wall”. The angle of the sloping caisson and slit wall is changed in several ways. The result of both experiments on reflected waves and transmitted waves is satisfied the chosen criteria.
In recent years, there have been an increasing number of projects where construction of a “sloping top caisson breakwater”is contemplated with a view toward a reduction in cost involved in implementation of such projects. Using five different shapes of a superstructure work of a “sloping top caisson breakwater”, this study was conducted to elucidate the wave dissipating work characteristic, transmitted wave characteristic and stability characteristic of rear mound covering stone on the basis of hydraulic model tests. The basic data obtained is expected to be greatly helpful in future breakwater design.
Methods improving the gently sloping revetment to enhance coastal use were investigated through field observations along ten coasts in Chiba Prefecture. The shapes of the revetment such as slope and height of step were measured. For use of the gently sloping revetment as steps, the height less than 20cm is preferred, whereas for the use as the chair step, height larger than 20cm is preferred. Uneven surface of the revetment in the longshore direction is not recommended for use. A method attaching a wooden chair to the revetment for the use of the steps is proposed to avoid strong heat conduction from the concrete surface in summer.
Some partial factors have been proposed to introduce the level-one reliability-based design method for port facilities. However, any particular partial factor for Sand-Compaction-Pile (SCP) method has not been proposed. In order to establish the partial factors for circular arc slip failure mode of port facilities including SCP improved ground, reliability indices against that failure mode were checked. In this analysis, the probability distribution of friction coefficient of SCP on the basis of SPT-N records is identified. As the result of the analysis, it is confirmed that the safety level by the present design method is higher than that of the minimum expected total cost, and that some parameters, such as SCP-friction coefficient, has a significant impact on stability. On the basis of the sensitivity factors of parameters, the partial factors designed for facilities including SCP improved ground are proposed.
This paper describes a numerical analysis of pore water pressure responses in seabed. The numerical method is developed by introducing a constitutive equation for soils and Biot's consolidation theory into the finite element method. The constitutive equation is based on Fukutake model. Fukutake model can consider dilatancy behaviour of soils under cyclic loads by employing R-O model and Bowl model, and it can simulate soil behaviours under seismic loads. However, it cannot consider the effect of axial stress variations on the non-liner stress-strain relation of soils and the model does not satisfy the principle of frame indifference. This model is not suitable for the seabed analysis under ocean wave action, because the direction of the principal stresses varies in the seabed. Then the Fukutake model is modified and the new model can consider the stress condition of seabed under ocean wave action. The finite element model is examined by comparing with observation data in the foundation soils under a breakwater.
To evaluate the applicability of high holding anchors for the tout mooring, we have carried out some studies on two kinds of anchors. One of anchors is a pile type anchor, which is called free fall anchor and it is driven into seabed soil by dropping. The other one is a plate type anchor, namely vertical loaded anchor, which is penetrated into sea bed through dragging. First we conducted installation analyses on the free fall and penetrating simulations, as well as the dragging and penetrating ones, to obtain their penetration depth and inclination. Then, structural and soil analyses for both anchors were carried out by using the beam column theory and FEM with nonlinear soil springs, respectively. The paper presents the results of the simulations, the models for structure and soil analyses to examine stresses and deformations and the calculated holding present capacities.
The 2ndphase construction work of KANSAI International Airport are being performed even farther offshore than in the 1stphase where the water depth is approximately 19.5m and reclamation area is about 545ha. To reclaim land over such a vast area and complete the island airport in a short period of time, maximum use is being made of the experience accumulated during the 1stphase construction. To prevent uneven settlement, many magnetictransmission type water pressure gauge and settlement plates have been installed to measure the amount of settlement during construction work and consideration of the predicted settlement during and after construction should be reflected in the design of the airport island. This report outlines management for settlement in the 2ndphase construction work.
Topography change in the vicinity of submerged breakwaters is investigated in two-dimensional experiments. Under incident waves with shorter wave period, the sediment under the submerged breakwater was transported offshore, whereas the sediment in front of the submerged breakwater was transported onshore and was deposited in the submerged breakwater body under incident waves with longer wave period. The authors evaluate cross-shore sediment transport rate through the submerged breakwater body from the measured topography change. The authors show the possibility that the sediment behind the submerged breakwater is transported offshore by constructing the submerged breakwater.
It has become known that a curtain walled type dissipater is effective for reducing reflected waves from a vertical wall for comparatively long waves. It consists of a vertical barrier in front and a vertical wall on the back. The dissipation mechanism of reflected waves is to enhance the vortex flow under the front curtain wall by the pistonmode wave resonance in the water chamber. However it may be concerned that the stability of rubbles in front of the dissipater due to generation of strong vortex flows. In this study, to protect the rubbles in front of the dissipater from wave induced vortex flows, the stability of amour concrete units is examined experimentally. Special attentions are paid on the behavior of vortex flows from the lower edge of the front curtain wall. For the practical design of a cover layer in front of the dissipater, the conventional stability formula is applied to find out the adequate stability number for this dissipater.
Stability of an Filter Unit (FU) on sloping breakwater has been investigated in relation with wave forces acting on it. Laboratory experiments have been conducted to measure the stability and pore pressures. Numerical simulation also computes the pore pressure of sloping breakwater. It is found that the uplift force due to difference of pore pressures on upper and lower surfaces of an FU acts on the FU installed near the shoreline and it affects stability significantly. Numerical analysis is confirmed to reproduce well the features of pore pressure in the FU layer as well as velocity field above the sloping breakwater. It is also revealed that the Hudson coefficient of FU is larger than that of armor rubble, which indicates that an FU is suitable in practical use as far as stability is concerned.
The purpose of this paper is to estimate the stability of an unburied offshore pipeline under severe storm condition. Pore pressure accumulation and pipeline movement during cyclic loading on pipeline are numerically investigated from the view points of the stiffness of seabed and the fine grained soil content in seabed. Both drag and lift forces are numerically obtained for 100 years return period storm condition using the Fourier decomposition method. Pipeline movement strongly depends on the seabed characteristics as well as the cyclic loadings estimated from storm condition. When the stiffness of seabed and the fine grained soil content are large, not only the pipeline movement but also the pore pressure response in seabed beneath the pipeline increases with the number of cycles.
Local scour around coastal structures may lead instability of them. This study investigates experimentally the local scour in front of an impermeable submerged breakwater with vertical front face. Experimental results show that the scour pattern in front of the submerged breakwater is similar to that in front of a vertical breakwater, although there is a little difference in locations of the scour and deposition. Also, the maximum scour depth normalized by the incident wave height is found to decrease exponentially with relative water depth to the wavelength. The scour depth depends much on the reflection coefficient. This study proposes the estimation method of the scour depth considering the effect of wave reflection and it is demonstrated that the proposed method gives good estimations of the scour depth for various reflection coefficient.
For the design of beaches and beach nourishment, and coastal structures constructed in a surf zone or near a shoreline, the following knowledge was acquired from analyses of field data. 1) A beach slope required for the calculation of overtopping rate can be obtained with sufficient accuracy using the formula of Dean, Yamamoto et al. or Sunamura. A beach face slope can be obtained with sufficient accuracy using the formula of Yamamoto, Uda or Kriebel. Moreover, a median sediment grain diameter can be found with comparatively good accuracy due to the formula of Yamamoto et al. or Sunamura. 2) A scouring amount required for the foot protection design of the structure in the surf zone can be got with comparatively good accuracy using Yokoyama et al. 's diagram. 3) Sufficient consideration not only for wave pressure and a beach change but also the influence of a flow is required for the stable design of the structure near the shoreline.
Ofunato Bay is highly enclosed because of a tsunami breakwater. Therefore, water quality problems, such as eutrophication and ocean anoxic, have been caused in the inside of the bay. In order to prevent eutrophication, it is a very important subject to calculate correctly the amount loads of the nutrients from a river. We tried to calculate the amount loads of nutrients from the continuation data of turbidity. We led the L-L equation that presumed the load of the amount of the nutrient from impurity. The phytoplankton in the Ofunato bay shows the tendency to increase due to the flood that happens in June in the ordinary year from the observation result.
It is necessary to grasp the behavior of the river water which gives pollution load, because of the design of the water quality improvement policy of Tokyo Bay where the deterioration advances. So this paper examined the flow characteristic of river water around the head of the Tokyo Bay in the flood using numerical simulation and satellite image. First, the pattern of advective diffusion of the river water was caught from LANDSAT image, and second it was simulated numerically by MEL3D which is a three-dimensional model. As the result, flow pattern of the surface layer which has appeared in the satellite image was recreated well with MEL3D. Finally, the relationship between hydrologic and meteorologic condition and behavior of river water was examined by the calculate condition systematically changing, it was shown that the flow of surface which is generated by the wind around the head of Tokyo Bay influenced the advective diffusion of the river water.
Poor understanding in relation to their aggregation hampers prediction of the suspension of the estuarine sediment. In this study, in situ experiment of the re-suspension of the estuarine sediment was conducted in an urban estuary, the Tsurumi River, Japan. The experiment was consists of the forceful suspension of sediments by the water jet pump and measurement of the turbulence, OBS and particle diameter. As a result of the experiment, It is clarified that surface layer of the sediment is easy to suspension and their clay particles were aggregated over 40%. It is concluded that low concentration of SS in the surface mud and aggregation of the clay particles resulted to high erodability estuarine fine sediment.
Modern submarine environment at Orido-Komagoe nearshore zone off Miho Peninsula, Shizuoka, Japan, is studied on the basis of grain size analysis on sediments from this area. This study clarifies distribution pattern of bottom materials and modern sedimentary environment, and also suggests seaward transport of beach sediments. Continuous deepening of the storm wave base during last twenty years has resulted in significant changes of the bottom material in the area and is thought to be caused by intensive mass transportation of beach and shallow water sediments seaward.
Coastal currents in Ise Bay are closely related to river inflow as well as meteorological disturbances. Particularly, large floods have a great effect on coastal currents and density structures. It is, therefore, of importance to examine flow and density structures at large floods. This study is aimed at discussing the flow and density structures in Ise Bay at large floods from a perspective of numerical analysis. A hindcasting based on an Atmosphere-Ocean-Wave coupled model is performed using the observation data including river discharge at large floods. The validity of the coupled model is verified by comparing between the observation and computation results of coastal currents. Furthermore, the estuarine circulation and density distribution in Ise Bay at large floods are examined based on the numerical results.
This paper describes density intrusion which influences water quality in Ohfunato Bay. Field experiments were carried out to directly measure the change in the DO concentration and velocity due to density intrusion which appears over a sill around the bay mouth. As suggested in the previous studies, it is found from the field experiments that density intrusion occurred during the flood tide and the DO concentration on the seabed is greatly influenced by the density intrusion since the high DO concentration water can be brought into the deep water region by the density intrusion. A non-hydrostatic three-dimensional LES model was used to analyze the effect of the density intrusion. The computation was verified from the comparison with the observed velocity. It is revealed from the computation that the front of the density intrusion can rapidly propagate into the seabed around the centre of the bay and the density intrusion may change the DO concentration on the seabed in a few hours.
Ise Bay and Mikawa Bay are typical semi-enclosed bays. For this reason in both bays oxygen-deficient water mass is generated mainly during summer and has caused heavy damage to the sea area environment. It is important to reduce the generation of the oxygen-deficient water mass to improve the sea area environment of both bays. In this research, numerical modeling of oxygen-deficient water mass is added to an Atmosphere-Ocean-Wave coupled model that can estimate coastal currents appropriately. Then, predictions of this model are compared and show excellent agreement with field observations.
This study describes typical mass transports in spring, summer, autumn and winter in Tokyo Bay by using a non-hydrostatic model, MEL3D. Some typical salinity and water temperature distributions in each season obtained from field observation were used in the numerical computation. Furthermore, spatial distributions of wind over Tokyo Bay evaluated using nine meteorological observation stations were applied in the computation as boundary conditions. To investigate mass transport, neutral particle tracking simulations were carried out by putting particles along near the coastal line by about 4km. As the results, especially in summer, it was demonstrated that the characteristic of the neutral particle transport in Tokyo Bay is strongly affected by the vertical current of circulation around the western coast in the head of Tokyo Bay.
The energy of the tidal and sea current is one of the potential renewable energies as well as offshore wind ones because of high density of water to air and more steadiness of flow to winds. We demonstrate their distribution and volume in the surrounding sea of Japan from the viewpoint of the utilization. This paper also presents the results of the study of the volume of energy recoverable from them by using the hydro turbine generation system.
The tidal and sea current turbines are one of potential utilization of renewable energies as well as offshore wind ones. Although they possibly produce electricity efficiently because of high density of water to air and more steadiness of flow to winds, hydro turbine generation system is less employed than wind one. We are studying an application of Straight Wing Vertical Axis Hydro Turbine Generation Systems in the tidal current and waves. This paper presents the results of hydraulic model tests using 2-dimensional channel.
Characteristics of resonant phenomena in narrow gaps of very large floating structure composed of a number of rectangular modules are investigated experimentally and theoretically. Especially, this study focus on the case of three modules with two gaps in order to make clear the difference with the case of one gap in which the authors have already had the experimental confirmation of occurrence of fluid resonance in the gap. Appearance conditions of fluid resonance are derived using the conservation of energy in the gaps and around two modules on the side of incident wave. In the derivation, first, two natural frequencies in the gaps are led as the frequencies described by the gravity acceleration and the geometrical conditions of floating structure including a water depth. Then, the appearance conditions are obtained substituting a dispersion relation of incident wave into the natural frequencies as kl1, 2tanh kh=1 (k is the incident wave number, l1, 2: l1, l, 2are the geometrical conditions, h is the water depth). The validity of the appearance conditions are verified by the comparison with the experimental results
In order to reduce comparatively long waves, like swells, a floating breakwater with a water chamber is newly proposed. The influence of piston mode wave resonance in the chamber on the floating body dynamics and wave energy dissipation is extensively examined. We have also examined the performance of a water-chamber type breakwater that is fixed at the water surface. It is intended to know the basic performance of the breakwater without body motions by waves. It is confirmed that the influence of body motions by waves is very strong on the performance of the breakwater. It is also seen that the piston mode wave motion in the chamber affects body dynamics and the dissipation of reflected waves.
The stability of immersed tunnel element that is under constructing at SHINWAKATO is investigated. While the tunnel element is kept afloat in temporary places, it is completed by placing High-fluidity concrete. Draft of the element is changed by increasing the weight, so the slack of the mooring lines appears. IF the slack appears, it interferes with the stability of the floating element. We proposed the mooring reaction strain model considering the slack by using the observations of waves, winds, and strain of the mooring lines, checked the validity of the model. Next, we studied the necessity and the timing to adjust mooring lines through the mooring analysis considering the slack. The result of study, we judged that the mooring lines are adjusted during under construction.
A floating wind turbine platform concept is developing for accessing the higher winds out at sea, and converting wind energy efficiently to hydrogen, methane and electricity. This paper describes the experimental results on dynamic responses of a base floating structure. The box-girder type floating structure mounted two 5MW wind turbines is proposed as a concept. Polyester mooring rope and vertical loading anchor are applied in mooring system. Tank tests were carried out. Response amplitude operators of motions and its mooring forces were measured in NMRI basin. And other, the motion responses and mooring forces are calculated by the potential theory with the aim of verification. Fin stabilizers are installed under the keel of hull girders for reduction of motion.
A new system was developed to estimate wind velocity distribution around the offshore wind-power generators, by assuming the wind energy conservation equation. Based on the long-term wind and power-generation observed data at the offshore and on-land stations, applicability of the tri-dimensional numerical wind simulation based on the k-ε model was confirmed. Interactions among the wind-power generators were estimated by using the numerical simulation, and example of the estimated power generation rate distribution around the existing generators was introduced. It was concluded that wind power generators reduce the wind velocity and wind-power potential of the surrounding sea-surface in the large area more than 5 times of the wind generator's diameter.
The offshore wind turbines (OWTs) are getting larger in scale in order to efficiently generate the electricity. Therefore the natural period of OWT becomes longer due to heavier weight of a nacelle and higher supporting towers for OWTs, resulting in possible fatigue damage. As we estimate that the diameter of the conventional tower to be more than seven meters, the tower having large rigidity such as a truss may be alternatively required. Therefore we made a comparison of required weights and rigidities through structural analyses of the cylindrical and truss towers for 5000kW OWTs. We also conducted a 1/100 scale model test on the both towers to obtain the wind loads acting on a rotor and the tower and to examine the differences in their downstream flow. The paper describes the ascendancy of the truss over the cylindrical tower.
This thesis reports about the case study of fatigue analysis based on the observed oceanographic data. Presumed wind power generation facility for analysis consists of the tower which has a 2400kW generator, and mono-pile foundation constructed on the bottom of the sea depth 10m outside the breakwater. Fatigue analysis was executed by zero-upcrossing method and rain-flow method. It was concluded that a fatigue life of offshore wind power generation facility could be analyzed by those methods, and it was proved that the facility with the mono-pile foundation has a long enough fatigue life even when it couldn't be repaired with the inspection prescribed by DNV.
Numerical simulation using the contour line change model was carried out to predict the difference in movement of fine and coarse materials in beach nourishment. Coarse materials were stable, while accumulating at the shoreline with negligible offshore discharge, whereas fine materials such as the grain size of 0.1mm were easily washed away. On the basis of these results, field experiment on stabilization of sandy beach using coarse materials was carried out at the Jinkoji coast facing the Pacific Ocean. Stability of nourished coarse sand was confirmed.