Risk-based control of external salt water intrusion for the Rhine-Meuse Estuary

Abstract

The fresh water supply will be more under pressure, due to the predicted effects of climate change in the Netherlands. More frequent salt water intrusion during the summer semester is caused by the joint occurrence of low river discharges and the expected sea level rise. The control of external salinity is necessary to guarantee a sufficient water quality of the main water system and so protect the fresh water inlets from the intruding saline water. Consequently, regional water systems are able to take in fresh water of the main water system to control internal salinity, through counteracting salt seepage by means of salt ?ushing. The control of external salinity can be realized by the implementation of measures that interfere in the main water system; e.g. by optimizing the fresh water distribution. Whether a measure will be implemented depends on the decision-making process. This study is initiated, because of the arisen discussions about the pursued fresh water policy for the drought in 2003. Salinity risk management aims to assess the cost-effectiveness of measures that focus on the fresh water supply, by means of evaluating the costs and the bene?ts of a measure. This study investigates the possibilities of the implementation of a risk-based approach within the present Dutch fresh water policy, which is nowadays based on a deterministic approach. A salinity risk management model is developed that basically is composed of three phases that research the following questions; i.e. 1.) How does the system of external salt water intrusion in the Rhine-Meuse Estuary function for given scenarios? 2.) What is the frequency of occurrence of external salinity? Given that external salinity occurs, what are the consequences? What is the resulting salinity risk? 3.) Is the established risk acceptable? If not, which alternative measures are able to reduce the present risk level? The developed research model is examined in a case study for the risk evaluation of external salt water intrusion in the Hollandse IJssel, in particular the fresh water inlet of Gouda that provides fresh water to the control area of Rijnland. This study concludes that a risk-based approach is implementable in the Dutch fresh water policy, but extended research is necessary to obtain more reliable exceedance frequencies of a Chloride concentration. This study developed two probabilistic models; i.e. for tide-dominated locations and river-dominated locations. A third type probabilistic model should be developed for locations that are not tide- or river-dominated. Secondly, more precise statistical analysis should be conducted after the discharge variation in course of time for low river discharges. Besides, statistical research is recommended after the variation of the probability distributions thorough the summer semester of low river discharges, high sea water level set-ups and a precipitation de?cit.