Simulating past severe flood events to evaluate the effectiveness of nonstructural flood countermeasures in the upper Chao Phraya River Basin, Thailand

•After 2011 flood, none of recent studies have investigated nonstructural measures.•Nonstructural measures from Master Plan seem insufficient to overcome severe flood.•Nonstructural measures from Master Plan might prevent small- to medium scale flood.•A long-term perspective is needed when developing flood management strategies.

Floods devastate communities and result in large economic losses in the lower reaches of the Chao Phraya River Basin, Thailand. The aim of this study was to determine whether nonstructural flood countermeasures would help prevent flash floods in the upstream area of the Upper Chao Phraya River Basin, and control the volume of floodwaters reaching the downstream area.Numerical models were used to quantify the effects of nonstructural measures, namely land use regulation, reforestation, and retention areas, and to examine how efficient these proposed nonstructural approaches would have been during the severe flood events that occurred in 1995, 2006 and 2011. Three scenarios, reforestation, retention areas, and the combination of reforestation and retention areas, were developed for these nonstructural flood countermeasures, as outlined in the Thai Master Plan for Water Resources Management.Their effectiveness in the Upper Chao Phraya River Basin was quantitatively assessed by comparing the model results for the actual conditions with the scenario results. Results showed that the proposed nonstructural measures had considerable potential to reduce peak discharges and flood volumes in the Upper Chao Phraya River Basin. Integration of these proposed nonstructural flood countermeasures with the existing countermeasures in the Chao Phraya River Basin may be the most practical way to cope with the challenges of future flood disasters.