Daisuke Sugawara

IMPACT OF THE 2011 TOHOKU EARTHQUAKE AND TSUNAMI ON BEACH MORPHOLOGY ALONG THE NORTHERN SENDAI COAST

At 14:46 JST on March 11, 2011 a magnitude 9.0 earthquake (2011 Tohoku Earthquake and Tsunami) occurred off the Pacific Coast of Miyagi Prefecture. This study investigated the extensive changes in beach morphology due to the earthquake and tsunami along the 15 km Northern Sendai Coast using remotely sensed data. The remote sensing analysis on the beach topography and coastal forest demonstrated the following notable characteristics of beach morphological change: erosion of the northern barrier at the mouths of the Nanakitagawa and Natorigawa Rivers; erosion at an old river channel; scour landward of the seawalls in the longshore direction; erosion and deposition in beach areas with detached breakwaters; and deposition in coastal forest areas. Linkage of the deposition in the forest areas with the damage type of coastal forests was observed. The impact of the earthquake and tsunami on the beach morphology was serious; roughly 60% of the study area was degraded by 0.2–0.5 m in elevation mainly due to land subsidence, and a total of 0.4 km2 of beach area was eroded mainly due to erosion of the northern barrier at the mouths of the Nanakitagawa and Natorigawa Rivers. This study explores the geographical changes brought on by a tremendous earthquake and tsunami, which will help to elucidate the mechanisms of coastal forest destruction, beach erosion, and their interaction during tsunami events.

CHAPTER THREE – TSUNAMIS AND TSUNAMI SEDIMENTOLOGY

Abstract Tsunamis are one of the most catastrophic wave motions, which cover a large parts of the sea and behave intricately especially in coastal zones. Studies on tsunami sedimentology have revealed that tsunamis induce various types of sedimentation in marine, lacustrine and onshore environments. Tsunami deposits are sedimentological evidence of tsunami events. This contribution describes the nature of tsunamis and of tsunami sedimentation as an introduction to this volume. Hydrodynamic aspects are introduced to explain the propagation of tsunamis. The diversity of tsunami deposits is illustrated on the basis of literature data. Onshore tsunami sedimentation is discussed in particular. Tsunami sedimentation appears to depend on the hydrodynamic and hydraulic character of the tsunami. The distribution pattern, grain-size variation and many other sedimentological structures reflect the characters of the tsunami such as the height, current velocity and period. Therefore, tsunami sedimentation should be interpreted based on careful consideration of the characteristics of tsunamis. This may result in a reliable reconstruction of ancient tsunami events.

Field measurements and numerical modeling for the run-up heights and inundation distances of the 2011 Tohoku-oki tsunami at Sendai Plain, Japan

We conducted an urgent field survey at the Sendai Plain to measure the run-up heights and inundation distances of the 2011 Tohoku-oki tsunami. We used GPS measurements because of the remarkably long inundation distances (ca. 5.4 km). We established an accurate measurement scheme using the far electric reference points (about 350 km). Using this method, we quickly measured 69 run-up heights within 3 days. The tsunami run-up heights and inundation distances varied mainly according to the local topography, ranging from 9.6 m at 0.4 km to 0.2 m at 5.4 km, respectively. Furthermore, artificial structures and topography played an important role in constraining the inundation limit. Our observations are important for future analyses using aerial and satellite imagery and numerical modeling in the area because the maximum inundation area might be underestimated in the images as a result of the subtle traces of the tsunami inundation, which were difficult to identify in the field. However, results show that numerical modeling might not reproduce minor inundation beyond the highway without sufficiently high-resolution topographic data because data for the modeling are usually rough, and the highway, small channels, and street gutters, which played an important role in local inundation, are too small a resolution to be recognized in the model.

Near‐field tsunami inundation forecast using the parallel TUNAMI‐N2 model: Application to the 2011 Tohoku‐Oki earthquake combined with source inversions

To contribute to tsunami early warning systems, we investigated the currently achievable speed of tsunami inundation simulations on a parallel computer as well as the benefits of high-resolution and faster than real-time inundation predictions. We found that a 5 m resolution inundation simulation can be 75 times faster than real time, requiring only 1.5 min to overview the inundation situation in Sendai City for the 2011 Tohoku tsunami. We developed a novel parallel tsunami model based on the well-known TUNAMI-N2 model and achieved 9.17 tera Floating-point Operations Per Second (FLOPS) on 9469 CPU cores. The present model can accurately hindcast the observed inundated regions of the 2011 Tohoku tsunami using tsunami source estimations of the tFISH and tFISH/RAPiD algorithms, which can be instantly derived from real-time observation data. The present high-resolution predictions can provide clear images of imminent hazards/disasters and can provide guidance for appropriate evacuation actions.

Liquefaction as an important source of the A.D. 2011 Tohoku-oki tsunami deposits at Sendai Plain, Japan

This paper describes the topographic change and the recovery process, as well as the sediment sources, for tsunami deposits based on field surveys and analysis of digital elevation model data before and after the A.D. 2011 Tohoku-oki tsunami at the Sendai Plain, Japan. We found that the amount of sediment deposited on land was approximately four times greater than the eroded volume of sediment at the beach. Large amounts of the sediments deposited at the studied transect probably originated from liquefaction. This result suggests that the vented sediments might have been an important source of the tsunami deposits if liquefaction is generated by the strong ground motion of a near-field earthquake. In contrast, minor erosion was observed at the beach, and the beach berm was rebuilt within three months after the tsunami. Moreover, the erosional channel that had cut into the beach had been filled by sand within 13 days after the tsunami. Therefore, it is not expected that a sedimentary record of the tsunami will be preserved in the nearshore zone along the Sendai coast, although remnants of small scours on land might remain long after the tsunami.

DISTRIBUTION AND SIGNIFICANCE OF THE 2004 INDIAN OCEAN TSUNAMI DEPOSITS: INITIAL RESULTS FROM THAILAND AND SRI LANKA

Abstract The Indian Ocean tsunami of 26 December 2004 left tsunami deposits in the coastal areas of the surrounding countries. We conducted a preliminary investigation of the tsunami deposits on the coasts of Thailand and Sri Lanka 3 months after the event to clarify their distribution and significance. We observed abundant tsunami-transported reef blocks, megaripples and selective erosion and sedimentation in the backshore of Pakarang Cape in Thailand. Our data indicate eastward inundation of the west side of the cape by the tsunami. We also investigated sheet-like tsunami deposits at Bang Sak beach in Thailand and at Garanduwa in Sri Lanka, where the tsunami direction was recorded by bent grasses and trees. The thickness and grain size of the tsunami deposits are dependent on the local topography, but generally the deposits thin and fine landward. The deposits consist of silicate minerals, shell fragments and coral skeletal fragments transported from the beach or the offshore sea bottom by the tsunami waves. In the studied sites, the tsunami deposits are restricted to the lower half of the inundated zone, reflecting the low sediment transport energy of the waves.

A multivariate generalized linear tsunami fragility model for Kesennuma City based on maximum flow depths, velocities and debris impact, with evaluation of predictive accuracy

The recent losses caused by the unprecedented 2011 Great East Japan Tsunami disaster have stimulated further research efforts, notably in the mechanisms and probabilistic determination of tsunami-induced damage, in order to provide the necessary information for future risk assessment and mitigation. The stochastic approach typically adopts fragility functions, which express the probability that a building will reach or exceed a predefined damage level usually for one, sometimes several measures of tsunami intensity. However, improvements in the derivation of fragility functions are still needed in order to yield reliable predictions of tsunami damage to buildings. In particular, extensive disaggregated databases, as well as measures of tsunami intensity beyond the commonly used tsunami flow depth should be used to potentially capture variations in the data which have not been explained by previous models. This study proposes to derive fragility functions with additional intensity measures for the city of Kesennuma, which was extensively damaged during the 2011 tsunami and for which a large and disaggregated dataset of building damage is available. In addition to the surveyed tsunami flow depth, the numerically estimated flow velocities as well as a binary indicator of debris impact are included in the model and used simultaneously to estimate building damage probabilities. Following the recently proposed methodology for fragility estimation based on generalized linear models, which overcomes the shortcomings of classic linear regression in fragility analyses, ordinal regression is applied and the reliability of the model estimates is assessed using a proposed penalized accuracy measure, more suitable than the traditional classification error rate for ordinal models. In order to assess the predictive power of the model, penalized accuracy is estimated through a repeated tenfold cross-validation scheme. For the first time, multivariate tsunami fragility functions are derived and represented in the form of fragility surfaces. The results show that the model is able to predict tsunami damage with satisfactory predictive accuracy and that debris impact is a crucial factor in the determination of building collapse probabilities.

Impact of Tsunami Inundation on Soil Salinisation: Up to One Year After the 2011 Tohoku-Oki Tsunami

The long-term effect of tsunami inundation on soil salinisation was assessed following the 2011 Tohoku-oki tsunami in two areas on the Sendai Plain, near Sendai airport in the Miyagi Prefecture and Matsukawa-ura near Soma in the Fukushima Prefecture. Data gathered over four sampling seasons 2, 5, 9 and 11 months after the tsunami near Sendai airport show that the salt content generally decreased with time. Concentrations were nevertheless higher in February 2012 than in October 2011, probably due to capillary action and evaporation following long periods with little precipitation in the winter, while the lower concentrations in October were attributed to dilution due to intense rainfall prior to the sampling period. In February 2012, the area with chloride concentrations over the guidelines for the establishment of rice seedlings still extended for nearly 1 km between 2.45 and 3.33 km inland. Chloride concentrations also reached the guideline values at the land surface 1.71 km inland. This corresponded to the limit of the area deemed not suitable for rice production by local rice farmers. However, recent observations revealed that rice crops were not only halted in 2011 but also in 2012, probably due to high salinisation of soil and/or surface and groundwater. Our study shows that soil salinisation was still recorded to nearly 15 cm depth in areas with fine-grained organic-rich soil ~2.5 km from the shoreline 11 months after the tsunami, and that water-leachable ions were preferentially retained in organic-rich muddy sediment and soil, reflecting the long-term impact of tsunami inundation. In Matsukawa-ura, salt crusts still covered the area flooded by the tsunami in February 2012 and both the soil and muddy tsunami deposit were characterised by high chloride and sulphate concentrations. The latter might also lead to sulphide toxicity. Remediation measures have been implemented in certain areas, but further research needs to be carried out to test the effectiveness of the measures being used to allow rice production to resume.

Numerical Simulations of Large-Scale Sediment Transport Caused by the 2011 Tohoku Earthquake Tsunami in Hirota Bay, Southern Sanriku Coast

A numerical sediment transport model (STM) was used to investigate coastal geomorphic changes that resulted from the 2011 Tohoku earthquake tsunami in Rikuzentakata City and Hirota Bay on the southern Sanriku Coast of Japan. The simulation was verified using observed inundation processes and heights, measured topographic changes and sediment deposition. Aerial video footage recorded by the Iwate Prefectural Police was also used. The results show that the numerical model was able to predict the spatial distribution and volume of erosion and deposition in Hirota Bay, as well as sediment transport processes. The effects of sediment transport on tsuneimi inundation were also investigated. Numerical results revealed that the majority of the sand dunes were eroded by the first wave, especially during the strong return flow of the receding wave. Large flows and sand dune erosions can occur elsewhere if tsunamis inundate a plain with a limited shore-normal width. These events could cause large-scale morphological changes comparable to those that occurred in Rikuzentakata City.

Ecological Status of Sandy Beaches After Tsunami Events: Insights from Meiofauna Investigations After the 2011 Tohoku-oki Tsunami, Sendai Bay, Japan

Tsunami may strongly impact beach ecosystems. To assess its magnitude five beaches along the Sendai Bay, Japan, were studied 2 months after the 11th March 2011 Tohoku-oki tsunami with focus on their recovery and meiofauna assemblages within few weeks after the event. The beaches recovered and new meiofauna assemblages established, which were strongly correlated to sediment grain size. The new data and review of previous works suggest that for beach ecosystems tsunami plays a role of ecosystem disturbance, not a catastrophe.