Trends in Compound Flooding in Northwestern Europe During 1901–2014

We analyze trends in compound flooding resulting from high coastal water levels (HCWLs) and peak river discharge over northwestern Europe during 1901–2014. Compound peak discharge associated with 37 stream gauges with at least 70 years of record availability near the North and Baltic Sea coasts is used. Compound flooding is assessed using a newly developed index, compound hazard ratio, that compares the severity of river flooding associated with HCWL with the at‐site, T ‐year (a flood with 1/ T chance of being exceeded in any given year) fluvial peak discharge. Our findings suggest a spatially coherent pattern in the dependence between HCWL and river peaks and in compound flood magnitudes and frequency. For higher return levels, we find upward trends in compound hazard ratio frequency at midlatitudes (gauges from 47°N to 60°N) and downward trends along the high latitude (>60°N) regions of northwestern Europe.