Exploring compounding rainfall events and potential adaptation measures in complex delta systems

Climate change is projected to increase the frequency and intensity of extreme climate events such as precipitation events and floods. Furthermore, compound events - multiple (extreme) events or hazards that occur simultaneously - are likely to become more prevalent on a warmer planet. Adaptation plans developed for single hazards may (unknowingly) ignore compound events, leading to an adaptation gap and potentially negative effects for other hazardous situations. However, uncertainties in changes of climate extremes, their concurrence, and the complexity of their interactions, render adaptation planning fundamentally more difficult for compound events than for individual events. This research explores adaptation strategies for uncertain compound events in the Netherlands, a low-lying urbanized deltaic system that faces the threat of compound events driven by extreme rainfall, storm surges, or high river discharges. We focus on compound events where extreme rainfall is the primary hazard driver (e.g., two rainfall events or a rainstorm and a storm surge) and aim to create a better understanding of the effects of past and future compound events in the Dutch water system. Our study provides a first insight into the effect of adaptation measures at a regional scale using archetype areas that schematically represent the Dutch delta system.  

We create a compound event database to develop storylines narrating past events and illustrating how adaptation measures were used to protect the Netherlands against high water levels and inundation. The database identifies areas that are especially vulnerable to compound events. We then stylize these areas into archetypes in a hydrologic model, and use the model to explore different adaptation measures, such as drainage, pumps, and storage capacity, under various compound event scenarios. We utilize an exploratory modeling method for decision-scaling based stress testing using the hydrologic model specified for the different archetypal areas in the Netherlands to determine the nature of (future) compound events that may generate water system vulnerability.  

Past events recorded in the database show that the east and west of the Netherlands may respond differently to similar compound events due to differences in hydrological setting, water management (free draining in the east and mainly man-made controlled systems in the west), and the dependency of drainage on downstream conditions, which in turn are affected by storm surges and sea level. Our first model results for the western archetype region show the importance of feedback between the different delta components in the development of flood risks, as e.g., high water levels downstream may affect adaptation requirements and limits upstream. They also confirm that the compounding effect of rainfall events may disproportionally amplify the problem, for example, while the system can deal with an individual rainfall event, a subsequent event can substantially increase inundation.