The Quantitative Analysis of Synchronized River Flood on the Global Scale Considering Multiple Flood Peaks

Flood is one of the most frequent and severe natural disasters all over the world. Among different types of floods, the compound flood has been considered as an alarming threat over the years due to climate change. River flood synchronization acts as a compound event which is mainly occurred at the downstream of the large river confluence zones. When multiple rivers become flooded at the same time, the resultant flood magnitude and flood duration at their confluences zone raise drastically. Only a few previous research addressed synchronized flood risk at a local scale, where simply yearly peak synchronization has been considered to avoid the complexity of detecting multiple peaks. However, several rivers occasionally show significant multiple peaks in a single year and sometimes yearly peak stays much below the flood limit.  Therefore, the existing technique either over-estimate or under-estimate synchronized flood risk, apparently, it is not applicable at the global scale.

The quantitative analysis of synchronized flood at a global scale considering multiple peaks is still a big challenge. Here, we have developed a flood return period-based new methodology to quantify synchronous flood precisely as well as to detect the background of the synchronous flood, either multiple peak synchronization or yearly peak synchronization. To find out the suitable confluence points on the global scale, we set two conditions. Firstly, the drainage area of contributing rivers is large enough to become different hydrological features and secondly, both rivers contribute a significant amount of discharge for the generation of flood at the respective confluence points. The next-generation global river routing model, CaMa-Flood, has been employed to compute discharge for return period-based analysis of selected rivers and confluence points. Finally, we check the contributing rivers return period discharge when the respective confluence point is at flooded condition, and if both rivers exceed corresponding 2-year return period discharge, those events are considered as synchronized floods.

We have found 53 confluence points on the global scale where catastrophic flood hazards may occur due to flood synchronization. The historical floods in 49 confluence points show different degrees of synchronization. Moreover, the Confluence zone flood at high latitude mostly affected by yearly peak synchronization may be due to the snowmelt domination. In contrast, the historical synchronized floods in tropical and sub-tropical regions affected by different levels of multiple peak synchronization where rainfall timing might be playing a significant role. This method emerges the physical mechanism of the historical catastrophic fluvial flood that took place where the large rivers have merged.