Long-term outage of drinking water supply after earthquakes has large adverse impacts on the lives of residents and cities’ water supply systems. Priority assessment is required to determine the effective order of preparing the measures against these hazards. Previous studies have insufficiently focused on the effects of seismic resistance of core pipelines, thereby affecting all the other branched pipelines that supply the water to each residential area. In our research, we aimed to propose the appropriate order of core pipelines derived from the center of the water treatment plants in Osaka, Japan against the Nankai Trough earthquake. Solutions that cause less damage have been needed because natural disaster prediction has had a lot of uncertainty. We proposed indices that can evaluate an appropriate renewal plan that considers the damage of core pipelines of water and the renewal cost for core pipeline networks based on the scenario analysis and the Monte Carlo simulation. The amount of water distribution of core pipelines in each area is the top priority when renewing them in terms of cost-effectiveness and expected rate of water outage under the disaster.

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.