Tsunami-Resilient Coastal Reconstruction in Japan After the 2011 M9 Earthquake and Tsunami: Field Surveys and Numerical Modelling

Abstract For the first time, this study presents a critical assessment of Japan’s coastal reconstruction following the March 2011 M 9.0 earthquake and tsunami, integrating numerical modelling with field surveys. Extensive surveys were conducted at 20 coastal sites, comparing post-event conditions in 2011 with the reconstructed coastlines observed 12 years later in May 2023. Field observations are combined with numerical simulations of tsunami amplitudes, flow velocity, and hydrodynamic loading to evaluate the design, performance, and current condition of coastal defence structures. Detailed factor of safety analysis is provided for one site. Both structural and non-structural measures were implemented. Structural defences were dominated by earth-fill dikes with concrete linings, present at 13 of 18 protected sites (72%), while concrete seawalls were observed at 5 sites (28%). The representative post-2011 defence is a concrete-lined dike with an average height of 9.4 m. Defence heights, mostly 7.2–11.8 m, generally align with the maximum simulated coastal tsunami amplitudes rather than extreme runup values, which can reach up to 40 m in narrow valleys. The normalised tsunami defence height (i.e., defence height divided by the maximum simulated coastal tsunami amplitude) ranged from 0.5 to 1.3. For locations with a normalised defence height below 1.0, overtopping may occur during an M9 event (L2 tsunami). This is consistent with Japan’s post-2011 strategy, which designs seawalls for M8-class earthquake-generated tsunamis (L1 tsunamis). Concrete seawall stability under tsunami loading is highly sensitive to water velocity, with higher velocities significantly reducing safety factors. Two common maintenance issues were identified: (i) vegetation growth undermining concrete slabs on dike slopes, and (ii) unfilled structural joints allowing water infiltration beneath slabs. These results are of global significance, as they can inform and guide tsunami-resilient reconstruction strategies in tsunami-prone regions worldwide.