This study assessed the drivers of snowmelt floods in relation to compound events. Compound events are when multiple drivers/hazards occur in the same geographic region/time scale, thus amplifying their impacts. The climate drivers considered in this study are temperature, precipitation, snow thickness, snow liquid water equivalent, wind speed, vapour pressure and soil moisture content. 107 different catchments across North America and Europe were investigated, from the years 1979-2019. Each annual maximum flood was sorted into a different flood typology. These typologies were rain-on-snow floods, snowmelt floods, long precipitation floods, and short precipitation floods. These four flood typologies are all split into another two categories, with a wet initial condition and a dry initial condition.
The results indicate that the considered catchments have snowmelt floods being the dominant flood type. The high elevation catchments had the dominant typologies being short precipitation floods and long precipitation floods. This initially surprised us, as higher elevations have colder weather and thus are expected to be more influenced by snowfall. However, these mountainous regions often experience large summer rainstorms, thus creating the maximum annual flood. Lower elevation catchments had snowmelt driven floods as the dominant typology. The wet initial conditions were also much more prevalent than the dry initial conditions, proving the importance of the soil moisture condition. This was confirmed through an investigation of the relative influence of the climate factors on the determination of the river discharge. Here, soil moisture had the largest relative influence.
Results were determined for the seasonality of the floods as the average day of the year the floods occur on. Based on the results of this, there are geographic regions, such as northeastern North America and central Sweden and Norway, which have a strong seasonality and the floods occur at roughly the same time every year. This can lead to spatially compounding events, where many hazards occur in one geographic region at the same time, thus amplifying their impacts.
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