Untreated hexamethylenetetramine (HMT) was discharged into the Tone River in the central area of Japan, and the risk management plan in the watershed area has been strengthened because HMT is the precursor of formaldehyde (FA) regulated by Japanese water supply law. The release of HMT could occur not only in steady but also in unsteady environmental conditions. In this context, no quantitative environmental risk assessments have dealt with the combined events of FA precursor outflow and natural disasters, such as a massive tsunami attack. In this study, we estimated the time course changes of HMT concentrations at the near water treatment plant (WTP) intakes due to tsunami run-up in the river after HMT discharge from facilities along the river during a massive tsunami attack, then potential ecological and health impacts were estimated. This method has a strong analytical ability to reveal the relationship between wave source and inland water run-up in consideration of 3D density flow. For ecological risk, the half maximal effective concentration (EC₅₀) was employed. We found that HMT concentration would not reach a level of concern even after the accident. For potential health risk in terms of the possible maximum inlet concentration of HMT at the WTP, the FA formation amount was 5.3 × 10⁻² mg/L, which was below the water quality standard.

Tsunami run-up can transport released substances toward river drinking-water intakes. This study evaluates transport to the Kunijima intake in the Yodo River using a coupled two-dimensional horizontal–three-dimensional hydrodynamic model with tank-release, dissolved-substance transport, and density calculations. Twenty-three cases examine salinity, substance density, release location, release volume, and river-flow and operating conditions under one Nankai Trough tsunami scenario. Previously published density and release-location simulations are extended through intake-concentration analysis. A formaldehyde-based reference threshold of 0.08 mg/L provides a common comparison benchmark rather than a generic safety criterion. The substance did not reach the intake in the freshwater-like downstream-salinity case during the 36 h simulation, whereas the highest-salinity case exceeded the threshold for approximately 30 h. The density response was non-monotonic, with 1100 kg/m³ producing the highest peak among tested densities. Increasing release volume mainly increased concentration magnitude; the higher-flow condition delayed arrival and reduced the peak. Density diagnostics support salinity-related stratification, while analytical advection tests demonstrate resolution-dependent numerical spreading. The results support scenario-based interpretation of intake concentration responses, but absolute concentrations and exceedance durations remain conditional estimates without direct validation of the complete transport application.

東日本大震災以来，日本では原子力発電所の津波防災の強化が求められている．より安全側な津波対策のため，地すべり等に起因する非地震性津波の考慮が必要である．まず，米山らの2DH3DHybrid手法（以下Hybrid手法）を用いて既往実験の再現計算を行った．Hybrid手法は，時刻歴波形を3D解析と同様の精度で再現し，計算時間を大幅に短縮して解析可能であるため，地すべり津波解析に対する有効性が示された．続いて，津波波高に特に影響を与える地すべりパラメータの選定を目的として同手法による数値実験を行った．その結果，発生水深の浅い地すべりであるほどより規模の大きな津波を引き起こすこと，地すべり塊体積が津波規模に特に大きな影響を与えること，が考えられる．