Analysis of drainage-dependent compound flood hazard along the German North Sea coast

The controlled drainage of diked hinterlands via sluices and pumping stations is a critical component of flood risk management in low-elevation coastal zones (LECZs), where floods are shaped by the interaction of rainfall, tides, storm surges, and sea-level rise. Effective drainage operation requires consideration of complex flood and tidal dynamics as drainage capacity is primarily impacted by the hydraulic gradient between inland water and downstream marine or estuary systems. As downstream water level dynamics dictate periods of gravity-driven drainage and the efficiency of pump operations, drainage capacity varies over time and depends heavily on tidal and storm surge conditions. Reduced drainage capacity significantly increases hinterland flood hazard, highlighting the importance of concurring and compounding events for flood risk management in LECZ.To better understand the interaction of flood drivers in low-land drainage areas, we develop a statistical framework to describe the impact of seaside conditions on drainage capacity, focusing on gravity-driven drainage along the German North Sea coast. Using multi-decadal tidal gauge records, high-resolution digital elevation models, and site-specific inland control stages, we derive threshold-based drainage conditions at more than one hundred coastal catchment outlets. We define free-drainage periods as intervals with tidal water levels below the inland control stage and tidal low-water exceedance spells as periods during which consecutive tidal low waters remain above the control stage, preventing gravity-driven drainage processes completely. Based on these characteristics, we statistically analyse the impact of coastal water level conditions on drainage operation. Further, we link them to inland precipitation to analyse situations of increased compound flood hazard where rainfall coincides with reduced or precluded gravity-driven drainage using multivariate extreme value statistics.Using this approach, we (i) define drainage condition metrics consistently across coastal drainage systems, (ii) quantify the duration, frequency, and temporal trends of compound flood hazard and (iii) demonstrate implications for the water management in LECZ.