Storm Surge and Extreme River Discharge: A Compound Event Analysis Using Ensemble Impact Modeling

Abstract

Many winter deep low-pressure systems passing over Western Europe have the potential to induce significant storm surge levels along the coast of the North Sea. The accompanying frontal systems lead to large rainfall amounts, which can result in river discharges exceeding critical thresholds. The risk of disruptive societal impact increases strongly if river runoff and storm-surge peak occur near-simultaneously. For the Rhine catchment and the Dutch coastal area, existing studies suggest that no such relation is present at time lags shorter than 6 days. Here we re-investigate the possibility of finding near-simultaneous storm surge and extreme river discharge using an extended data set derived from a storm surge model (WAQUA/DCSMv5) and two hydrological riverdischarge models (SPHY and HBV96) forced with conditions from a high-resolution (0.11 • /12 km) regional climate model (RACMO2) in ensemble mode (16 × 50 years). We find that the probability for finding a co-occurrence of extreme river discharge at Lobith and storm surge conditions at Hoek van Holland are up to four times higher (than random chance) for a broad range of time lags (−2 to 10 days, depending on exact threshold). This highlights that the hazard of a co-occurrence of high river discharge and coastal water levels cannot be neglected in a robust risk assessment.

Measuring compound flood potential from river discharge and storm surge extremes at the global scale

“…We assess the compound flood potential between riverine and coastal flood drivers using simulated daily river discharge and maximum daily storm surge as proxy variables, respectively. The latter is a common choice for studying compound flood hazard analysis in deltas and estuaries (Khanal et al, 2019;Klerk et al, 2015;Svensson and Jones, 2002;Ward et al, 2018). The research involves the following steps, each of which is described in the following subsections:…”

“…Drivers of maximum storm surge heights are particularly complex, and are influenced by external factors such as local bathymetry and the geometry of the coastline (Bloemendaal et al, 2019). Third, in large catchments, there may also be a lag of several days for river flood waves to reach the basin outlet (Allen et al, 2018), such that the riverine and coastal flood annual maxima do not interact (Kew et al, 2013;Klerk et al, 2015;Ward et al, 2018).…”

Pan-European hydrodynamic models and their ability to identify compound floods

“…Many studies have analysed compound flood hazard in Europe using various observational data sets and spatial scales, including the co-occurrence of storm surges with extreme precipitation (Bengtsson 2016), river discharges (Bevacqua et al 2017;Ward et al 2018;Ganguli and Merz 2019a, b;Hendry et al 2019), precipitation and discharges Jones 2002, 2004) and waves (Wahl et al 2012;Gouldby et al 2014). Some studies also used hydrodynamic models to derive drivers over larger areas (Petroliagkis 2018;Couasnon et al 2019;Khanal et al 2019). Given that climate change is expected to increase the level of hazard in many parts of the continent through higher sea levels (Vousdoukas et al 2016a), river discharges (Alfieri et al 2015) and extreme precipitation (Lehtonen et al 2014), there is a strong need to model compound floods and produce future projections.…”

The prediction of floods in Venice: methods, models and uncertainty

“…In the Adriatic Sea, distinction could be made between basin-wide seiches (characterized by periods that are close to the inertial period and therefore considerably influenced by the Coriolis force) and local seiches (having periods that are much smaller than the inertial period and thus marginally influenced by the rotation of the Earth). 205 Investigation of Adriatic-wide seiches was pioneered by Kesslitz (1910), who inspected de-tided sea-level records in time domain, by Vercelli (1941), who performed filtering of tide-gauge records, and by Bozzi Zadro and Porettti (1971), who subjected sea-level time series through spectral analysis. These analyses, and their numerous sequels, showed that the amplitudes of Adriatic-wide seiches could reach 50 cm and that the periods of two basic modes are close to 21.2 h and 10.9 h. The periods were verified by applying numerical modeling to the Adriatic Sea, one-dimensional (Sterneck, 1919) and two-210 dimensional (Accerboni and Manca, 1973) with nodal line imposed in the Otranto Strait, and two-dimensional (Schwab and Rao, 1983) with the Adriatic being considered as a part of a wider Mediterranean Sea.…”