Storm surges and high discharge

The low-lying Netherlands is at risk from multiple threats of sea level rise, storm surges and extreme river discharges. Should these threats occur simultaneously, a catastrophe will be at hand. Knowledge about the likelihood of simultaneous occurance or the so-called ‘joint probability’ of such threats is essential to provide guidance on legislation for dike heights, flood barrier design and water management in general.
In this study, we explore the simultaneous threats of North Sea storm surges and extreme Rhine river discharge for the current climate in a large 17-member global climate model ensemble. We use a simple approach, taking proxies of North-Northwesterly winds over the North Sea and multiple-day precipitation averaged over the Rhine for storm surge and discharge respectively, so that a sensitivity analysis is straight forward to apply. By investigating soft extremes, we circumvent the need to extrapolate the data and thereby permit the synoptic development of selected events to be inspected.
Our principle finding is that the probability of extreme surge conditions following extreme 20-day precipitation sums is around 3 times higher than that estimated from treating extreme surge and discharge probabilities as independent, as previously assumed.