Coastal Dikes Research Articles

高波による海岸堤防前面下端からの裏込め材吸出し量算定式の提案

高波による海岸堤防前面下端からの裏込め材吸出し量算定式の提案

東北地方太平洋沖地震津波による仙台平野南部での海岸堤防被災・洗掘に関する調査

Field survey on coastal dike damage and scouring induced by the Great East Japan Earthquake Tsunami was conducted in the Southern Part of Sendai Plain. Erosion rate of dike cross section was estimated by using topographical data obtained by airborne laser scanner. The erosion rate tends to increase with relative height of coastal dike from the landside ground. Tsunami overtopping caused scouring at the landward toe of coastal dike, being a trigger of dike failure. Cross-sectional shape of the ground induced by scouring and thickness of tsunami deposit was estimated by portable dynamic cone penetration test and trench survey.

津波の越流時に海岸堤防の法面に作用する圧力特性に関する実験的研究

津波の越流時に海岸堤防の法面に作用する圧力特性に関する実験的研究

東北地方太平洋沖地震津波による海岸堤防の被災分析

東北地方太平洋沖地震津波による海岸堤防の被災分析

Downfall of Tokyo due to Devastating Compound Disaster

Compound disasters are defined as double- or triplepunch disasters. As such, they cause more serious cumulative damage than individual disasters occurring independently. The independent occurrence of Tokyo metropolitan inland earthquakes is expected to kill 11,000 and cause ¥112 trillion in damage. An earthquake in Tokyo would also destroy river levees, coastal dikes, and disaster measure facilities such as water gates and locks due to liquefaction. Following such a earthquake, river flooding by the Tone and Arakawa rivers or storm surge overflow around Tokyo bay could easily occur along with strong typhoons. An Edo period (1603-1868) compound disaster involved the 1855 Ansei Edo earthquake and the 1856 Ansei Edo storm surge. With global warming progressively worsening, huge floods and storm surges are increasingly likely to occur independently. The risk that they will occur as part of a compound disaster is also increasing. Catastrophic disasters are characterized by being super-wide in area damage, compound in combining disasters, and prolonged in recovery. With the vast sea-level or low areas in Tokyo, long-term submergence due to inundation will be unavoidable. The most difficult problem, however, will be how to evacuate over 1 million people.

Laboratory Investigation of the Seepage Control Measures under Coastal Dikes

In the present study, the effects of seepage control measures under coastal dikes have been experimentally investigated. For the purpose of the study a laboratory model hydraulic flume 8.65 m long, 1 m deep, and 1 m wide was employed. The coastal dike was assumed as impermeable and made of a stainless-steel sheet of trapezoidal shape. The experiments were carried out employing five upstream and one downstream water levels. Besides, four different depths of sheet piles and four different lengths of impermeable blanket were used as seepage control measures. Results of the experiments were recorded at various points at the downstream foundation of the dike. The results indicated that the integrated application of two of the seepage control measures had more significant effect on reduction of uplift pressure. The results of the current study and two old empirical methods, that is Bligh and Lane, indicated that the Lane method underestimates the magnitude of uplift pressure and risk of piping in comparison with the Bligh’s method. Finally, the optimal values for impermeable blanket length and cut-off depth for minimizing the uplift pressure were determined.

WAVES OVERTOPPING A WIDE-CRESTED DIKE

If a coastal dike has a crest width of more than say a few meters, then wave overtopping will be significantly reduced due to the crest width, because kinetic energy can be dissipated on the crest and water on the crest can flow back towards the seaside. In coastal towns built on dune belts often a wide-crested coastal dike was constructed as both coastal protection and sea promenade. However, quantifying the effect of such a wide crest on wave overtopping is not straightforward because no empirical overtopping formulas are given in literature for this kind of configuration. Therefore physical scale model research was carried out. Also a simplified theoretical model concept was used based on an energy balance formulation of a single overtopping wave. Combining the scale model measurements with the theoretical model, a semi-empirical formula was developed and calibrated.

Engineering Performances of Suiken Dike Certified as a Heritage Site by Wakayama Prefecture

This paper describes the engineering performances of Suiken Dike certified as a heritage site by Wakayama Prefecture. The dike was constructed by using excellent construction techniques in Edo period. Especially, the main part of the dike is covered with many rectangular solid stones like a stone wall of a Japanese castle. The soil part is attached with the stone part. However, a functional role of this composite structure is unknown. We carried out the FEM analyses to study interactive effects between these two parts against wave forces as a coastal dike. A Tsunami wave force and a violent wave force due to a typhoon were used as an external force. It was clarified that the soil part effectively functioned to restrain the stone part rotating to the back side.

Numerical Simulation Model for Storm Surge Inundation Incorporating Accurate Estimation of Transition Process from Overtopping to Overflow

A numerical simulation model for storm surge inundation has been developed. Inundation simulation method induced by transition process from overtopping to overflow is incorporated into this model. Convenient estimation method for this transition process is based on the equations produced by Goda(1975a). Several parameters in the Goda's equations are revaluated by numerical wave simulations of the transition process from overtopping to overflow. These simulations are performed by using VOF method (CADMAS-SURF). According to these simulation results accurate flow rates over the coastal dikes are estimated at the time of storm surge. With this simulation model high storm surges generated by large typhoons around the coasts of Japan can be reproduced. This paper presents the outline of the simulation model for storm surge inundation.

Perturbation of regional ocean tides due to coastal dikes

The tidal regime modeling system for ocean tides in the seas bordering the Korean Peninsula is designed to cover an area that is broad in scope and size, yet provide a high degree of resolution in coastal development areas, including the Saemangeum area in the eastern Yellow Sea and the Ariake Sea in Japan, where serious environmental problems have occurred after the completion of interior tidal dikes. With this simulation system, we have estimated the changes in tidal regime due to barriers at Saemangeum and Isahaya Bay in the Ariake Sea. Some results in terms of perturbations in tidal elevations due to the construction of coastal dikes are presented and discussed.

Design parameters for coastal dikes

This paper presents results of the assessment of the design parameters leading to the definition of the crest level of a coastal dike along the German North Sea. Procedures to estimate the design water level have been proposed, distinguishing between comparative and single value procedures. The transformation of the wave characteristics from deep water towards the shallow foreshore was achieved through the application of a spectral wave model. To improve the wave parameter estimations, the existing model was nested to a grid with a higher resolution closer to the coast. The estimation of the wave run-up followed the Dutch procedure with some adjustments to the local wave characteristics and dike geometry. The computed maximum crest level of 8.4 m is below the crest height of the existing dike, which is 8.8 m. However a proposal for a more economical design should be carefully evaluated, paying attention to the uncertainties encountered in this research. The general recommendation is to enhance the reliability of the hindcasted wave parameters through calibration and validation of the wave model and to include in the design process an investigation of the effect of the medium term morphological developments.

A Century of Countermeasures Against Storm Surges and Tsunamis in Japan

The countermeasures against storm surges and tsunamis in Japan are briefly reviewed covering roughly the last century. In spite of 22,000 deaths resulting from the Meiji Great Sanriku Tsunami just before the 20th century, neither central government nor local governments took effective countermeasures. The first positive countermeasures were taken by the central and local governments after the Showa Great Sanriku Tsunami and the Muroto Typhoon in early 1930s. The Seashore Act was enacted in 1956. After the 1959 Ise Bay Typhoon and the 1960 Chilean Tsunami, it has been the general practice to construct coastal dikes 5-6 m high as defense countermeasures. Tsunamis exceeding this height are met by combining structures, tsunami-resistant town development and defense systems. Quantitative tsunami forecasting announced by the Japan Meteorological Agency is currently state-of-the-art globally in terms of swiftness, preciseness and details.

Experimental Study on Bore acting on a Coastal Dike

The properties of tsunami wave pressures acting on a coastal dike were studied in a tsunami experimental tank. The results obtained in this experimental study were as follows; 1) the greatest impulsive pressure occurs when a crest of bore-type tsunami just overtops at an upright coastal dike. 2) The greatest continuous pressure of tsunami generally exceeds the value estimated by Tanimoto's formula (1984). 3) An impulsive pressure of tsunami is not so much affected by sea-bottom slope. 4) In addition, an installation of breakwater in the front of a coastal dike is very effective for decrease of wave pressure.

Tsunami Damage Mitigation Effects of “Hard” Measures in the Hinterland

Coastal dike should be constructed and maintained to prevent damage in the hinterland caused by the design tsunami, but can not always be constructed because ofland availability etc. Besides this, larger tsunamis may overtop thecoastal dikeand run up on the hinterland. In such cases, “hard” measures in the hinterlandas well as measures prompting resident's evacuation should be considered. The measures, such as forest zones, elevating land and secondary dikes, can mitigate damage to houses and humans through tsunami runup control in the hinterland. To realize the measures, we evaluated tsunami damage mitigation effects ofthe “hard” measures in the hinterland.

築後４０年以上経過した築堤式海岸堤防のひび割れに関する現地調査

Lifecycle management of coastal dikes is required for making their life span as longer as possible. The purpose of this study is to investigate cracks, unevenness and compressive strength of concrete armored coastal dikes constructed more than 40 years ago along Mikawa Bay, by means of field measurement with 30m tape, 30cm steel rule with mm scale, level and Shumitt hammer. The field measurement shows that crackings are classified into seven patterns, and that the probability of crack occurrence on the concrete armored crown is about 0.4, which is higher than those of the seaward and landside slopes of the coastal dikes. The averaged crack-propagating speed per year after construction is found out to be less than 0.32cm/y on the armored crown and less than 0.17cm/y on the armored seaward and landward slopes.

An Effective Tsunami Damage Estimation Method Considering Dike Height

In Japan, the central and prefectural governments have conducted tsunami damagee stimation assuming non-failure of coastal dikes during earthquake and tsunamis. The maximum flood depth, from which damages to houses and humans were estimated, would vary with dike subsidence caused by earthquake motion, tsunami-induced dike failure, and topography in the hinterland. Many cases of tsunami simulation considering dike subsidence etc. are necessary for practical damage estimation, but cost much time and expense. Therefore, we developed an effective tsunami damage estimation method that does not need complicated tsunami simulations.

内湾を対象としたリアルタイム波浪予測システムの開発

The Flood Fighting Law stipulates that prefectural governors shall issue a flood fighting warning of storm surges on coasts previously recognized as being at high risk of storm surges. However, forecasts of storm surges and high waves before a typhoon approaches are insufficient information to issue a flood fighting warning because the forecasts do not indicate that the wave runup exceeds coastal dikes and wave forecasts are imprecise. Also, wave forecasts are not very precise near coasts because wave models do not consider wave transformation in shallow water such as shoaling and refraction. Therefore we are developing a real-time wave forecast system for inland seas to ensure proper flood fighting warnings

Impacts of climate change on coastal flood risk in England and Wales: 2030–2100

Coastal flood risk is a function of the probability of coastal flooding and the consequential damage. Scenarios of potential changes in coastal flood risk due to changes in climate, society and the economy over the twenty-first century have been analysed using a national-scale quantified flood risk analysis methodology. If it is assumed that there will be no adaptation to increasing coastal flood risk, the expected annual damage in England and Wales due to coastal flooding is predicted to increase from the current 0.5 billion pounds to between 1.0 pound and 13.5 billion pounds, depending on the scenario of climate and socio-economic change. The proportion of national flood risk that is attributable to coastal flooding is projected to increase from roughly 50% to between 60 and 70%. Scenarios of adaptation to increasing risk, by construction of coastal dikes or retreat from coastal floodplains, are analysed. These adaptations are shown to be able to reduce coastal flood risk to between 0.2 pounds and 0.8 billion pounds. The capital cost of the associated coastal engineering works is estimated to be between 12 pounds and 40 billion pounds. Non-structural measures to reduce risk can make a major contribution to reducing the cost and environmental impact of engineering measures.

沿岸防災施設の老朽化と被災

Superannuation of infrastructures has attracted attention to introduction of asset management to infrastructures. This paper summarizes present state of shore protection facilities, a method to evaluate deterioration of coastal dikes, and nationwide urgent check of coastal dikes and revetments, and discusses some points related to asset management of shore protection facilities.

地球温暖化による海面上昇に対応するための海岸保全対策のあり方

Japanese coasts have experienced extensive damage caused by storm surges and high waves which made new high records. The impacts of climate change and sea-level rise are suggested to increase in this century. To address these impacts, as protection by hard structures such as coastal dike and seawall has limited effects, integrated policy for coastal conservation should be introduced, combining hard and soft options. Soft options include land use change/regulation, early warning system and disaster prevention scheme. Trends of national population and financial conditions are another factors for the future policy. This paper examines coastal conservation and management policy against sea-level rise caused by global warming through review of existing policy framework in foreign countries as well as in Japan, options of integrated policy, questionnaire survey of coastal residents' opinion, and quantitative assessment of those options. Through these studies, future direction of coastal management is proposed taking into account the effects of climate change and sea-level rise.