What about the 18,000 m3 river discharge?
The Rhine is currently designed to safely discharge to the sea a water volume of 15,000 m3/s at Lobith. The Room for the River measures will boost this discharge capacity by 1,000 cubic metres a second, to 16,000 m3/s.
Where does the additional water come from?
According to the most recent national and international studies and climate scenarios, the Rhine river discharge will increase in winter. This is primarily due to the additional precipitation and the rising temperature (higher temperatures mean less water is retained in the form of snow). As a result, the Rhine will tend to convert into more of a rain river in the future. If we translate the climate scenarios into discharge levels, the volume of water transported by the Rhine in the direction of the Netherlands will range from 17,000 m3/s to 22,000 m3/s or more by 2100. The “Rheinblick 2050” study, conducted by the International Commission for the Hydrology of the Rhine Basin, also calculates an increase in the discharge at Lobith. This calculation confirms the points of departure presented in the Dutch National Water Plan (2009) with respect to future Rhine discharges, which underpin the Rivers Delta Programme.
Whether the above discharges will actually reach the Netherlands depends on the extent to which the dykes in the German section of the Rhine basin overtop. Overtopping of dykes in Germany will tap water from the discharge wave, resulting in a diminishing downstream discharge, including in the Netherlands. This is referred to as “topping off the discharge wave”. Controlled overtopping also reduces downstream discharges. The degree to which the discharge wave is topped off depends on the scope of the flood protection measures implemented in Germany (dykes, dams, water storage). In the Action Plan on Floods of the International Commission for the Protection of the Rhine, Germany indicated its intention to complete its dyke improvement programme and implement the scheduled river widening projects in the near future. Another fact to be taken into consideration is that the German governments will take emergency measures (sandbags) in the event of extreme river discharges in order to prevent or reduce imminent floods.
Based on these insights into increasing discharges, topping off, and the impact of German measures in this regard, the Delta Programme has made allowances for the maximum discharge to reach the Netherlands to increase by 2,000 m3/s to a level of 18,000 m3/s in the long run. This maximum discharge is not, by definition, the discharge level used to underpin the design of particular dyke improvement or river widening projects. This so-called design level depends on the design horizon taken into consideration, the climate scenario, the standard in force, and the failure mechanism in place. The Minister of Infrastructure and the Environment explained this to the House of Representatives on 30 November 2015 in an Appendix to the “Water Memorandum”.
Best available knowledge
The above insights have recently been endorsed by an international panel of experts (see http://dtvirt35.deltares.nl/products/30888) and in the so-called GRADE study (Generator of Rainfall and Discharge Extremes, see http://dtvirt35.deltares.nl/products/30870). According to the Flood Risk Management Expertise Network, the GRADE methodology is currently the “best available knowledge”. The legislator has therefore incorporated the 18,000 m3/s in its key planning decision Room for the River (2006), as normative for 2100. In his proposal for DP2015, the Delta Programme Commissioner has followed suit. The adaptive approach entails that measures are adjusted if new research indicates the need to do so. The Delta Programme is updated annually.