Shigehiro Fujino

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.

THICKNESS AND GRAIN-SIZE DISTRIBUTION OF INDIAN OCEAN TSUNAMI DEPOSITS AT KHAO LAK AND PHRA THONG ISLAND, SOUTH-WESTERN THAILAND

Abstract The landward changes in grain size and thickness of the 26 December 2004 Indian Ocean tsunami deposit on Phra Thong Island and in the Khao Lak Area, south-western Thailand, are summarized in this chapter. The maximum heights of the tsunami on Phra Thong Island and Khao Lak were 7 and 10 m, respectively. The tsunami produced 1- to 4-m-high erosional scarps at the shoreline in these areas. The tsunami deposit is a sheet of sand ranging from a few centimetres to 22-cm thick over a distance of 1500 m inland. It thins near its distal margin, but the thickness is strongly affected by local topography and lacks a consistent trend. The deposit is composed of medium to very fine sand, and fines inland, reflecting loss of energy. It is graded or massive. Sets of graded units result from multiple waves.

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.

Variations in the 2004 Indian Ocean tsunami deposits thickness and their preservation potential, southwestern Thailand

Field surveys were conducted in March 2005 and December 2008 in southwestern Thailand to investigate local variation in the thickness and preservation potential of onshore deposits formed by the 26 December 2004 Indian Ocean tsunami. The 2008 survey results revealed that the thickness of deposits varies by a few centimeters in pits located less than 10 m apart because of local undulation of the topography and possible bioturbation. At 13 among all 24 sites, the difference in thickness during 2005 and 2008 surveys was within the range of local variation. In fact, very thin tsunami deposits with 1 cm thickness in the 2005 survey were well preserved during the 2008 survey. Furthermore, tsunami deposits near the maximum inundation limit were found in the 2008 survey, with thicknesses that are consistent with those reported from the 2005 survey. At no site was a tsunami deposit eliminated completely. Based on these observations, we infer that the tsunami deposit thickness is well preserved, even in a tropical climate with heavy rains such as that of Thailand.

Erosion and sedimentation during the September 2015 flooding of the Kinu River, central Japan

Erosional and sedimentary features associated with flooding have been documented in both modern and past cases. However, only a few studies have demonstrated the relationship between these features and the corresponding hydraulic conditions that produced them, making it difficult to evaluate the magnitude of paleo-flooding. This study describes the characteristics associated with inundation depth and flow direction, as well as the erosional and sedimentary features resulting from the disastrous flooding of the Kinu River, central Japan, in September 2015. Water levels rose rapidly due to heavy rainfall that eventually overtopped, and subsequently breached, a levee in Joso City, causing destructive flooding on the surrounding floodplain. Distinctive erosional features are found next to the breached levee, while depositional features, such as a sandy crevasse-splay deposit are found further away from the breach. The deposit can be divided into three units based on sedimentary facies. The vertical and lateral changes of these sedimentary facies may be the result of temporal and spatial changes associated with flow during the single flooding event. These observations and quantitative data provide information that can be used to reveal the paleohydrology of flood deposits in the stratigraphic record, leading to improved mitigation of future flooding disasters.