Tsunami Damages of Nakoso Coast Due to the 2011 Tohoku Tsunami

Abstract

A post-2011 Tohoku earthquake tsunami survey was conducted at the Nakoso Coast, Fukushima Prefecture, Japan, on Mar 24-25 and Apr 1-2, 2011. Spatial distribution of the tsunami inundation height, together with the seawall height and damages, was investigated. We found a sharp contrast of tsunami damages within the target area; tsunami damage is the most serious in the north region (north of the Samegawa River) where seawalls with different heights played diverse roles for protection of local community. A relatively high seawall in this region kept intact after tsunami attack, and protected the lee side human properties; whereas, 71% of the low seawalls were broken during the tsunami and the disaster developed in the corresponding sheltered area. A sharp contrast at the boundary of these two types of seawalls was observed. Behind the high seawall, the tsunami intensity and velocity were significantly reduced and the inundation depth is only 0.6 m. Nevertheless, tsunami intensity was enhanced in the seawall-broken area with an inland inundation depth being 4.7 m, which ruins away the local houses. In the middle region between the Samegawa River and the Bindagawa River, the high seawall survived from the tsunami attack. We confirmed that the coastal pine tree forest, serving as a buffer zone, played a significant role in reducing the tsunami height approximately by 4 m within a cross-shore distance of 230 m. To the south of the Bindagawa River, the tsunami damage is the mildest due to the protection by the robust high seawall as well as high elevation of land topography.

Destruction Mechanism of Coastal Structures Due to the 2011 Tohoku Tsunami in the South of Fukushima

“…The location of the failure appeared to be dependent on the ground level on the land side. The collapse of seawalls at the joint is also reported in Nakoso by Sato et al (2012). The enforcement of the joint is considered to be another key point in strengthening the structure.…”

“…In the southern part of Fukushima Prefecture, on the other hand, although many seawalls were broken as observed in Nakoso in Figure 2, the contrast between broken and survived structures can be observed. Lessons learned from such observations will help us to design tenacious structures for tsunami overflow (e.g., Sato et al, 2012). Seawalls further south exhibited different performance in Ibaraki and Chiba Prefectures.…”

“…found that the distribution of coastal tsunami heights in Fukushima Prefecture was strongly affected by complex offshore bathymetry. Several follow-up surveys were conducted in Minami-Soma, north of the Fukushima Dai-ichi Power Plant (Sato et al, 2014) and in Nakoso and Naraha, south of the power plant (Sato et al, 2012;Sanuki et al, 2013).…”

Understanding flow characteristics from tsunami deposits at Odaka, Joban Coast, using a deep neural network (DNN) inverse model

“…Sanuki et al (2013) estimated the inundation flow velocity from video images near the mouth of the Kido River in the Minamisoma region to be distinctively higher than that in the Sendai Plain, which was 10-15 m s −1 in the river and coastal plain. Sato et al (2012a) and Sanuki et al (2013) conducted a numerical simulation of tsunami inundation and also estimated high-velocity flows in the area. Sato et al (2014) estimated a flow velocity higher than 11 m s −1 from the collapse of buildings.…”

Understanding flow characteristics from tsunami deposits at Odaka, Joban coast, using a DNN inverse model

“…Sanuki et al (2013) estimated the flow velocity to be 10-15 m/s in the river and coastal plain from video images. Sato et al (2012a) and Sanuki et al (2013) conducted a numerical simulation of tsunami inundation and also estimated high velocity flows in the area. Sato et al ( 2014) estimated a the flow velocity higher than 11 m/s from the collapse of buildings.…”