Tsunami Overflow Research Articles

ISPH Simulation of Scour Behind Seawall Due to Continuous Tsunami Overflow

Tsunami overflow may destroy the coast-protection structures through continuous scour behind them. In this paper, the incompressible smoothed particle hydrodynamics (ISPH) method [Gotoh, H., Khayyer, A., Ikari, H., Arikawa, T. & Shimosako, K. [2014] “On enhancement of incompressible SPH method for simulation of violent sloshing flows,” Appl. Ocean Res. 46, 104–115.] is modified for simulation of erosion process behind the seawall under continuous tsunami overflow. Concepts of numerical turbid water particle (TWP) and clear water particle (CWP) are proposed to deal with the sediment-entrained flow. Reasonable value of 1250 kg/m3 is suggested for the initial density of the TWPs based on studies of bottom sediment movement. Sediment particle is judged to initiate its movement if the exerting shear stress exceeds the critical shear stress (CSS) of incipient motion of sediment calculated from a prescribed formula. The numerical results are compared with the data of flume experiment [Arikawa, T., Ikeda, T., Kubota, K. [2014] “Experimental study on scour behind seawall due to tsunami overflow,” J. Jpn. Soc. Civil Eng., Ser. B2 (Coastal Eng.) 70 (2), I_926-I_930 (in Japanese).]. The predicted results of the maximum depth as well as the shape of the scour pit reasonably agree with the experimental data.

Coastal vulnerability analysis during tsunami-induced levee overflow and breaching by a high-resolution flood model

The coastal levee is a crucial defense element in low-lying coastal areas, but is vulnerable to overflow when water levels increase extremely. This study focuses on the failure of the concrete-armored levees caused by local scouring at the toe of their landward slope that was widely observed in the 2011 Tohoku tsunami in Japan. Using a high-resolution flood model with a shock-capturing property, we explore the actual breach processes and their impacts on the protected areas in one of the most devastated districts. The model is based on nonlinear shallow water equations and successfully reproduces detailed flow structures of the tsunami overflow at 2m resolution. However, because we neglect the non-hydrostatic pressure over steep levee slopes, the modeled overflow discharge is slightly underestimated. During simulation, supercritical flow developed on the land side of levee sections with slowly rising tailwater water level due to hinterland topographic features, despite the large overflow discharge. Topographic data analysis revealed large-scale scour holes or trenches at the locations of the supercritical flow development, suggesting that levee vulnerability can be identified by the local flow parameters based on the high-resolution model. The levee breach was modeled as a sudden removal of the corresponding levee section, and the consequences were investigated. The levee breach changed the spatial balance of the flood intensity, but did not significantly worsen the damage in this case, because the narrow low-lying areas were fully inundated even without breach.

Performance of rubble mound breakwaters under tsunami attack, a case study: Haydarpasa Port, Istanbul, Turkey

Ports are one of the most vulnerable coastal utilities in case of marine natural hazards such as tsunamis and need to be protected against their devastating effects. Thus, studying the effects of tsunamis on protective structures such as breakwaters is critical. The Sea of Marmara is a part of an active earthquake zone that has generated tsunamis in the history. In terms of population density, coastal utilization, and economic potential, Marmara coastline seems most vulnerable to marine hazards. The availability of natural stones allows for wide use of rubble mound breakwaters as coastal protective structures in Turkey. The stability of these types of structures under the attack of storm waves has already been studied. However, their stability and performance under the effect of long waves and tsunami attacks have not yet been studied experimentally. The present study is a case study focusing on Haydarpasa Port, located at the southern entrance of Istanbul Bosphorus Strait (North coast of the Sea of Marmara). It aims to investigate the performance level of the port in case of tsunami attack.Physical model experiments were conducted in the 105-m long wave flume in the Port and Airport Research Institute (PARI), Japan, with a Froude-type length scale of 1/30. The experiments conducted to test the stability of rubble mound breakwater were twofold: (i) solitary wave experiments and (ii) tsunami overflow experiments. The heights of incoming tsunami waves were selected from results of simulations were conducted in the same region (Oyo Int. Co., 2007; Ayca, 2012; Yalciner et al., 2014; Guler et al., 2014; Aytore, 2015). First, the incoming solitary wave heights were selected as 5, 7.5, and 10cm. Using the overflow heights obtained from solitary wave experiments, i.e., wave height at the top of crown wall when the solitary waves are overtopping the crown wall, tsunami overflow experiments were conducted ranging from an overflow height of 1.1cm to 4.6cm. Results of these experiments showed that Haydarpasa Breakwater, especially the crown wall of the breakwater, is not stable under a moderate tsunami attack. Therefore, an improved cross section was also tested under the same conditions, and the improvement proved successful.

Study on Local Scouring of Gravel Mound due to Tsunami Overflow

Many breakwaters were damaged due to tsunami actions in Tohoku earthquake in 2011. The local scouring of the gravel mound at the foot of the breakwater was reported as one of the typical failures. This local scouring on the gravel mound is assumed due to a large vortex formed by the tsunami overflow. The purpose of this study is to clarify the characteristics of local scouring under several tsunami overflow conditions. This study conducted hydraulic experiments and numerical simulation. The scale of the scouring mainly depends on the overflow condition, gravel diameter, and water depth above the mound. This study proposed some experimental equations that estimate the maximum scoured depth and the width of the scoured topography. Numerical simulation based on SPH, Smoothed Particle Hydrodynamics, was employed in this study to clarify the interaction between the large vortex flow on the gravel mound and the deformation of the mound.

Stability of Concrete Blocks Covering Protection Mounds of Fishing Port Breakwaters against Tsunami Overflow

Stability of Concrete Blocks Covering Protection Mounds of Fishing Port Breakwaters against Tsunami Overflow

津波の越流による海岸堤防背後の洗掘とその対策法

津波の越流による海岸堤防背後の洗掘とその対策法

STABILITY OF ARMOR UNITS COVERING RUBBLE MOUND OF COMPOSITE BREAKWATERS AGAINST A STEADY OVERFLOW OF TSUNAMI

This paper presents a simple and highly accurate stability estimation method for armor units covering breakwater rubble mounds against tsunami overflow. In this method, overflow depth is used to represent the external force. This enables an easier and more robust estimation of the required mass of the armor units than the conventional method based on flow velocity. This method takes into account the influence of the impingement position of the overflow jet and the influence of harbor-side water depth, both of which are important factors for armor stability. Numerical computation is also carried out aiming at the establishment of a stability analysis method for armor units. The validity of the computation method is confirmed by comparing the measured current field. The stability of armor units is investigated by computing the hydraulic force acting on each armor unit.

Mitigation of Disasters by Earthquakes, Tsunamis, and Rains by Means of Geosynthetic-Reinforced Soil Retaining Walls and Embankments

The number of relevant case histories on the use of geosynthetic-reinforced soil (GRS) structures, including retaining walls and embankments, and their performance against large earthquake loads and heavy rainfalls is still limited. Their performance against tsunami is not well known, either. In view of this, in the first part, while focusing on the case histories during and after the March 11, 2011 East Japan Great Earthquake Disaster, Japan, performances of GRS structures against earthquake load and tsunami overflow are briefly summarized. Then, their application to rehabilitation works of the damaged structures is reported. In the second part, the development of L-shaped geosynthetic drain (LGD) is introduced, together with the relevant case history of rainfall-induced damage. In the LGD system, the water collected at vertically mounted geosynthetic drain sheet is drained through a horizontally installed drain. By having such an arrangement properly applicable to the drainage system, it is verified through a series of model tests and numerical analyses that the GRS structure is protected against any water invasion. It is also demonstrated that the use of steel slag in the construction of GRS structure is promising.

Delayed Survey of the 2011 Tohoku Tsunami in the Former Exclusion Zone in Minami-Soma, Fukushima Prefecture

Post-tsunami field surveys in the Minami-Soma exclusion zone in the Fukushima Prefecture were delayed for 15 months after the 2011 Tohoku tsunami. The area was subject to access restrictions until June 2012 due to high radiation levels caused by the meltdown at the Fukushima Dai-ichi Nuclear Power Plant. The distribution of the measured tsunami heights is presented in combination with observed infrastructure damage. The enhanced tsunami heights in the areas along the shoreline are attributed to wave reflection, funneling and splash-up at cliffs and seawalls, as well as the increased flow resistance as the tsunami plowed through coastal pine-tree forests. Consequently, large tsunami heights exceeding 10 m were limited to areas within 500 m from the shoreline. Onshore profiles of the maximum inundation levels were dependent on inland topography: tsunami heights increased inland in steep V-shaped valleys, while decaying with inundation distance along flat coastal plains. Tsunami flood levels in the coastal plains are affected by the extent of seawall damage: coastal flood levels are higher behind completely destroyed seawalls than behind partially damaged coastal defenses. Remnant seawalls provided valuable lessons to be implemented in future designs of tenacious structures based on the Japanese concept of ‘nebari’ representing resiliency to endure tsunami overflow as the original design height is exceeded.

津波の越流に対する海岸堤防の対策法の有効性に関する数値解析

津波の越流に対する海岸堤防の対策法の有効性に関する数値解析

津波越流時における海岸堤防の洗掘に関する数値解析モデルの構築

津波越流時における海岸堤防の洗掘に関する数値解析モデルの構築

越流する津波に対する防波堤の安定性評価における空気相の影響に関する考察

越流する津波に対する防波堤の安定性評価における空気相の影響に関する考察

数値解析に基づくケーソン背後の津波越流の再現と被覆材の安定性の検討

Tsunami overflowing a breakwater is reproduced by a numerical model to evaluate the stability of armor units covering the harbor-side of the rubble mound. This model solves the overflow jet above the water surface and the harbor-side flow field separately to avoid entrainment of excessive bubbles. Using this model, the applicability of the Isbash formula for the stability of armor stones and concrete blocks against tsunami overflow is examined. The relationship between the Isbash number and the degree of movement of armor stones is clarified. In the case of concrete blocks, the Isbash number of the stability limitation varied with the thickness of the overflow jet. The Isbash formula by C.E.R.C. (1977) tends to overestimate the effect of slope angle, when applied to concrete blocks. A stability prediction method for armor blocks based on fluid forces acting on each armor block is presented as well.

津波越流量抑制のための多重防潮堤の設置条件の実験的検討

Although use of multi tsunami barriers is one of fundamental countermeasures for tsunami, reduction of tsunami overflow due to multi barriers has not been investigated. In the present study, we performed a series of model tests in the laboratory to evaluate the effects of multi tsunami barriers on reduction of tsunami overflow and fluid force to buildings in its hinterland. The test results indicated that tsunami overflow in two barriers with the height H was smaller than that in a single barrier with same height H. Furthermore the indices like overflow and fluid force in two barriers with relatively lower height are comparable to that in s single barrier with higher height, showing the fundamental performance of two barriers on reduction of tsunami overflow.

津波越流に対する鋼矢板壁構造の堤防補強効果に関する実験的検討

津波越流に対する鋼矢板壁構造の堤防補強効果に関する実験的検討

津波越流時の防波堤基礎の不安定化と腹付工の効果

津波越流時の防波堤基礎の不安定化と腹付工の効果

海岸堤防を越流する津波による洗掘の発生機構と被覆工の安定性に関する研究

Tsunami-induced local scouring at the landward toe of a coastal dike is investigated experimentally and numerically in terms of the process of the scouring, the response of the foundation, and the stability of armor blocks. From a comparison between experimental data and numerical results, the predictive capability of a numerical model taking into account fluid-sediment-seabed interactions is demonstrated in terms of water surface elevations and scour profiles. Furthermore, it is found that armor blocks below the top and berm of the landward slope of the dike receive large landward and upward force because of an increase in air pressure inside the dike and a decrease in water pressure induced by flow separation, suggesting that these blocks would be vulnerable against tsunami overflow.

津波越流時の海岸堤防の被覆工下面に作用する圧力に関する気液二相流解析

津波越流時の海岸堤防の被覆工下面に作用する圧力に関する気液二相流解析

防波堤の腹付け被覆ブロックの安定性

防波堤の腹付け被覆ブロックの安定性

東北地方太平洋沖地震津波で被災した胸壁の津波越流に関する数値計算による検証

東北地方太平洋沖地震津波で被災した胸壁の津波越流に関する数値計算による検証