Compounding Effects of Riverine and Coastal Floods and Its Implications for Coastal Urban Flood Resilience

Globally more than 600 million people reside in the low elevation (< 10 meters elevation) coastal zone. The densely populated low-lying deltas are vulnerable to flooding primarily in two ways: (1) Due to extreme coastal water level (ECWL) because of either storm surges or heavy rain-induced river floods generated by a severe storm episode. (2) Co-occurrence or successive occurrence of ECWL and river floods as a result of storm-producing synoptic weather conditions leading to compound floods that causes a severe impact than when each of these extremes occurs in an isolation at different times. Most of the earlier assessments that analyzed compound floods, often do not consider the delay between rainfall and streamflow events. River runoff, which also includes subsurface groundwater recharge component, cannot be adequately described by extreme precipitation alone. While most of the literature is limited to analyzing joint dependence between variables considering only central dependence, challenges to flood hazard assessment include difficulty in delineating the severity of riverine floods, especially due to long upper tails of the variables that influence interdependencies between underlying drivers. Despite uncertainties, utilizing the rich database of northwestern Europe, here we assess compound flood severity and its trend by examining spatial interdependencies between annual maxima coastal water level (as an indicator of ECWL) and d-day lagged peak discharge within ±7 days of the occurrence of the ECWL event. Our analysis reveals a spatially coherent dependence pattern with strong positive dependence for gauges located between 52° and 60°N latitude, whereas a weak positive dependence across gauges in > 60°N latitude. Based on a newly proposed index, Compound Hazard Ratio (CHR) that compares the severity of compound floods with at-site design floods, our proof-of-principal analysis suggests nearly half of the stream gauges show amplifications in fluvial flood hazard during 2013/2014’s catastrophic winter storm Xaver that affected most of northern Europe. Furthermore, a multi-decadal (1889 – 2014) temporal evolution of compound flood reveals the existence of a flood-rich period between 1960s and 1980s, especially for the mid-latitude gauges (located within 47° to 60°N), which might be closely linked to the North Atlantic Oscillation (NAO) teleconnection pattern prevailing in the region. On the other hand, gauges at high-latitude (> 60°N) show decreasing to no trend in compound floods. The approach presented here can serve as a basis for developing coastal urban flood risk management portfolios aiding improved resilience and reduce vulnerability in the affected areas.