The flooding of the River Geul in July 2021 was an eyeopener for the society, the government and many others involved. Besides the major structural damage as a consequence of the flooding, the emotional damage was and still is massive. Therefore, the need for measures to reduce the impact of these extreme events is bigger than ever. Many people had to temporarily leave their house and companies had to shut down. One thing became clear: the impact of comparable or even worse flooding has to be lowered, and one way to achieve that is by implementing measures in the River Geul which reduce the flood risk. The main goal of this research is to find out the impact that certain measures have on the consequences of flooding of the River Geul. Therefore, the main research question is: What is the impact of hydraulic or hydrological measures on the consequences of extreme future flooding of the River Geul? An important aspect is to consider the multidimensional impact of each measure. This can be either social, economic, or ecological impact due to flooding and the implementation of the measure itself.

The evaluation of the impact of measures is done by using different methods. First of all, an existing SOBEK model is used to simulate two extreme events: July 2021 (recurrence interval ≈ 1/100 - 1/1000 per year) and a stress test (recurrence interval ≈ 1/10.000 per year) which simulates 60% larger discharge and precipitation amounts than July 2021. Based on the bottlenecks in the River Geul, which are either bridges, weirs, culverts or orifices, measures are defined and simulated in SOBEK. A distinction is made between possible measures down- and upstream of Valkenburg and in Valkenburg, dependent on the expected effect of the measure. All the measures are simulated for both July 2021 and the stress test. The measures that significantly reduce the flooding depth and local water level nearby urban areas, which follows from the longitudinal cross-sections of the maximum simulated water level, are rated as effective. The effective measures are also evaluated with a damage victim module (SSM2017) to calculate the reduction of damage and victims, but no cost-benefit analysis is performed. Finally, expert judgement is used to estimate the costs of the effective measures. Social impact and the impact on Natura 2000 area is also examined here.

Eleven measures were considered, and four of them showed effective results and therefore their impact is compared. The effective measures are: widen the river downstream Valkenburg, bypass through Valkenburg performed as a tunnel (in-situ or drilled) or channel, remove obstacles in the River Geul along Valkenburg and enlarge the current basins in the River Geul. For the flooding of July 2021, the optimization of current basins in the River Geul scored by far the worst on damage reduction and costs. River widening and removing obstacles both reduce the amount of damage by 6 percent, while the costs are at least a factor of four lower compared to the bypass (dependent on the type of bypass). However, the bypass reduces the damage by at least 34 percent. The most expensive bypass is the drilled tunnel, but it comes with less social impact, because barely any buildings have to be removed in the city centre of Valkenburg (assuming that the subsoil in Valkenburg is suitable for a drilled tunnel). For the stress test, the damage reduction of the effective measures was not significant compared to July 2021. Thus, for extremer events the measures have no longer a considerable impact. This is not an argument to not implement measures, because the probability that a similar event as the stress test recurs is at least a factor of ten lower compared to July 2021.

Depending on the preferences of decision makers the choice for a measure can be made. Based on this research and looking at the flooding of July 2021, a bypass in Valkenburg reduces the damage along the River Geul the most with approximately one-third. For a bypass performed as a drilled tunnel the social impact is limited as maximum five buildings have to be removed but a cost estimate of 112 - 207 million euros is associated with this choice. River widening downstream Valkenburg or removing obstacles in Valkenburg comes with lower costs of 4 - 23 million euros in combination with no social impact but this results in a damage reduction of only 6 percent.