The effects of a storm surge event on salt intrusion: Insights from the Rhine-Meuse Delta

The Rhine-Meuse Delta is a low-lying delta in the Netherlands that is subject to both significant salt intrusion events and storm surges. Typically, these events do not co-occur. Salt intrusion occurs in the summer months (June, July, August) during droughts, while the storm season runs from October till April. The increased sea water level during a storm surge can temporarily cause increased salt intrusion. Because of the short time scales and higher discharges, storm surges generally do not cause problems with freshwater availability, in contrast to summer droughts. However, on December 5th 2013, a storm surge event caused a significant amount of salt water to intrude into the Rhine-Meuse Delta. This was followed by weeks of increased salinity in parts of the delta. We simulated the event using a 3D numerical model of the Rhine-Meuse Delta, to capture the dynamics of the system and to improve our understanding of salt intrusion during storm surges. The Rhine-Meuse Delta is a mixed wave tide dominated delta with two main branches. One of the branches, the New Waterway, has an open connection with the North Sea and is the main outlet of the River Rhine. The other branch, the Haringvliet, is closed off at low water by the Haringvliet Gates. The Haringvliet Gates were built as a storm surge barrier to protect against storm surges after the 1953 flood. In addition, they are routinely used to maintain stable water levels in the river branches for shipping and fresh water supply. During the 2013 storm surge event, the Haringvliet Gates were closed. However, sea water entered the Rhine-Meuse Delta via the Rotterdam Waterway, which effectively acted as a backdoor for salt intrusion into the Haringvliet. Consequently the Haringvliet basin stored the saltwater and delivered it to connected parts of the system in the weeks following. We will refer to this phenomenon as basin salt reflux. Uniquely our model captured the salinity dynamics of the system during the 2013 event. Both the modelled salinity peaks during the storm, as well as the higher salinities after the storm agreed with available observations. We present new insights on the dynamics of a salt intrusion event triggered by a storm surge in a semi-enclosed coastal system. With climate change and sea level rise, these events will most likely occur more frequently in the future. This highlights the importance of fundamental understanding and advanced 3D modelling of salt intrusion in complex deltas during storm surges.