Under the background of urbanization and climate change, flood disasters are getting more and more serious. Floods tend to shift from single site to multi-sites and from urban area to suburb area, however, the mechanism of flood formation and transformation at different scales under changing environment is still unclear. In this study, we proposed a process-based flood event identification and scale division method. We then detected the characteristics, changes and driving mechanisms of local, regional and basin-scale floods at the Taihu Plain region during the past 50 years (1971-2020). The results indicate that floods in the study area showed distinct scales, with regional floods dominated (69.34%), and extraordinary floods were at basin sclae or regional scale. Floods in the study area showed a significant increase trend, mainly due to the increase of regional floods (accounting for 80.8%) in 1971-2020. Flood characteristics at different scales are significantly correlated with rainfall, tidal level, water level of the Taihu Lake, and upstream water amount. The antecedent conditions of water level were the main driving factor of local and regional floods (contributing 53.2% and 54.98%, respectively). Total rainfall played a dominant role in the rise of water level for local, regional and basin-scale floods, with the relative contributions of 64.91%, 65.01% and 78.72%, respectively. The main driving factors of flood characteristics at different scales and their influencing degree also have some spatial differences. The results of this study show that the spatial scale of floods has changed significantly under the background of changing environment in the Taihu Plain region, which would provide a feasible idea and reference for the identification of floods and mechanism analysis of its changes at different scales in this region or a similar estuary deltas area. This can help to ensure the regional flood control safety under the changing environment.

Keywords： floods; scale features; influencing factors; random forest; Taihu Plain region