The rise in the sea level is not discernible in trends of extreme water levels, so a study by Wageningen University, Utrecht University and Nanjing University reveals. When it comes to extreme water levels in the Dutch delta, the effects of sea level rise were found to be negligible compared to the impact of human interventions.
Worldwide, river deltas are exposed to a rise in sea level and subsidence as a result of reduced sediment supply, ground water pumping and the extraction of minerals. In the media, this tends to be immediately linked to an increased risk of flooding. However, the influence of water levels in deltas as a result of river discharge, tidal movement and the average sea level is not a straightforward equation but the result of a complex interaction of mutually influencing physical processes.
Nynke Vellinga (Institute for Marine and Atmospheric Research, Utrecht University) and Ton Hoitink (chair group Hydrology and Quantitative Water Management, Wageningen University), in partnership with Hohai University of Nanjing (China), recently published an analysis of 70 years of water level figures from 13 measurement stations in the northern part of the Rhine-Meuse delta. They studied the variation in these tidal waters over time and space in relation to water management works compared to the rise in sea level. In the magazine Coastal Engineering, they demonstrate that the average water levels in the delta rise in tandem with the average sea level. However, the trends in extreme water levels vary strongly from one measurement station to another.
"In the periods between the years in which major civil engineering works were carried out, such as the construction of the Haringvliet locks and the connection of the Hartel canal (at Europoort, near Rotterdam) to the canal network, the extreme water levels actually fell," says Ton Hoitink. "This may be connected to the gradual deepening of the waterways. However, those gradual trends are small compared to the abrupt changes caused by human interventions. When it comes to extreme high and low water levels, the effects of human interventions on the tides are more important than sea level rise. As such, policy should focus more on the consequences of water management works on the tides and the resulting effect on flooding risks in deltas."