Between July 12 and 15, 2021, extreme rainfall in Luxembourg, Belgium, Germany, and the Netherlands caused severe flooding. The total damage in the Netherlands is estimated to be between € 350 and € 600 million, with Valkenburg aan de Geul suffering the most damage. The economic and tourist heart of this city is located in the lowest part of the valley, therefore developing a flood reduction measure is challenging due to the limited space and restrictions in this location to protect the culture and nature.

In previous studies, different solutions were proposed for reducing the flood risk such as flood walls in Valkenburg; however, due to the locational difficulties, many options are difficult to implement except for the option of a flood bypass tunnel. A flood bypass tunnel has never been applied in the Netherlands, however; it is applied in mountainous countries. A flood bypass tunnel rapidly conveys floodwaters through densely populated areas. For Valkenburg, a flood bypass tunnel will reduce the flood risk locally while taking up little space and keeping the historic district intact.

Recent research did not look further into the opportunity of a flood bypass tunnel as a flood measure assuming it would be too expensive (Asselman & van Heeringen, 2023). However, different studies concluded that a flood bypass tunnel is a viable option (Van Dijk, 2022; Kallen et al., 2022; Leijser & Nijhof, 2022). Nonetheless, a flood bypass tunnel has never been applied in the Netherlands, and there is no prior research detailing the hydraulic and operational applicability of a flood bypass tunnel for Valkenburg aan de Geul. The main objective of this thesis is therefore to develop a hydraulic flood bypass tunnel design for Valkenburg aan de Geul which should operate to reduce the flood risk.

The objective is reached by combining the design approach for hydraulic structures with a Systems Engineering approach. First, the river system and its environment are analysed. Secondly, the basis of the design is determined. Thirdly, two alternative designs are developed from the two different types of reference projects: a passive and an active flood bypass tunnel.

The first design alternative is based on a passive flood bypass tunnel. It consists of a 2.4-meter-tall and 24.5-meter-wide Ogee weir, two tunnel tubes of 3.5 m diameter, and allows for a maximum discharge capacity of 55 m³/s. Once the tunnel is filled, the flow is pressurised. After a flood, the remaining water will be pumped out. A co-current channel is designed to prevent water from refilling the tunnel from the outlet side in a non-flooding situation.

The second design alternative is based on an active flood bypass tunnel. It consists of four vertical moving flat gates at the inlets and outlets, two tubes of 3.5 m diameter, and allows for a maximum discharge capacity of 58 m³/s. The water level will be controlled by the flat gates using a system for early automatic detection of flood hazards. The tunnel is always filled, thus pressurised. After a flood, the gates at the in- and outlets will close off the tunnel from its environment. The gates are tested twice a year, during which the system is flushed and refreshed.

The two design alternatives were evaluated according to six weighted criteria and ranked using a multi-criteria analysis in discussion with the municipality (gemeente Valkenburg aan de Geul) and the waterboard (waterschap Limburg), and a cost analysis was conducted. The first design alternative with the passive flood bypass tunnel was selected due to its high reliability and serviceability, which were highly valued, and low maintenance costs, compared to the second design alternative

The flood bypass tunnel reduces the flood risk based on discharge reduction from an estimated once every 19 years to once every 250 years in the current climate. This accounts for uncertainty due to climate change and ensures flood risk reduction in the future. The flood bypass is only active when a flood is impending; hence the water remains to flow through the Geul and does not interfere with the cultural heritage and tourism of Valkenburg aan de Geul. Due to the cost-efficient pipe jacking method, the total construction cost is approximately €40 million with an estimated yearly maintenance cost of €100 k.

Cement-Bentonite (CB) walls are low permeability vertical cut-off barriers that are mainly used to prevent groundwater flow and isolate contaminated areas. The hydraulic performance of the CB walls depends on the flow rate (discharge) through the wall. Therefore, the walls must achieve very low hydraulic conductivity. Poor in-situ hydraulic performance of these walls due to construction and post-construction defects may lead to surface settlements, groundwater contamination, and instability of the construction. This thesis investigates the range of in-situ hydraulic conductivity values of CB walls installed within the Netherlands. To achieve this, pump data for the various projects (Westerschelde Tunnel, A2 Motorway at Best, Motorway A4 Delft-Schiedam, Griftpark Utrecht, and Richard Hageman Akwadukt) were analyzed to determine the discharge through the walls. The in-situ hydraulic conductivity of the walls was calculated by Darcy’s law using groundwater level data and dimensions of the walls. The CB walls must be "keyed" into an underlying low permeable layer (aquitard), so that seepage of water through the aquitard is prevented. It was difficult to compute the in-situ hydraulic conductivity of the CB walls for Motorway A4 Delft-Schiedam and Griftpark Utrecht. The reason was that the walls in these projects were embedded in a permeable aquitard, which affected the total discharge value. The calculated in-situ hydraulic conductivity values were compared with the required hydraulic conductivity values and laboratory test results. In most of the cases, the in-situ hydraulic conductivity values of the walls were larger than the values acquired from the laboratory samples. The reason is that the estimation of the hydraulic conductivity of the CB walls from laboratory tests was based on small sample areas. This remains unreliable to evaluate the field performance of the CB walls. The average hydraulic conductivity of the CB walls at Richard Hageman Akwadukt was required to be less than or equal to 1·10-9 m/s. The calculated hydraulic conductivity values of the CB walls for the northern and southern polders were approximately 2.5·10-9 m/s and 2·10-8 m/s, respectively. This study also shows that the hydraulic performance of the CB walls is mainly affected by insufficient connections between the panels. Also, the impact of the defects on the discharge through the wall has been investigated by developing a threedimensional numerical groundwater flow model using MODFLOW. Simulations were performed for different types of defects, such as fully and partially penetrating windows, insufficient embedment, and connection between the panels of the CB walls. The results of the simulations indicated that fully penetrating windows have more impact on the discharge through the wall compared to partially penetrating windows. Partially penetrating windows with a hydraulic conductivity lower than 1·10-6 m/s are often insignificant. The size of a fully penetrating window also plays an important role. The difference in discharge (ΔQ) was calculated by subtracting the discharge through a perfect wall and imperfect wall (Qimperfect - Qperfect). The (ΔQ) through a CB wall with a window whose area is equal to 1 m2 can increase by a factor of 10 compared to a window whose area is equal to 0.1 m2. The location of the defects has no impact on the discharge through the wall. The simulations also show that the flow rate is unaffected by the embedment depth of the wall, but it must be ensured that the wall is in direct contact with the aquitard (low permeability soil layer). The vertical deviation of panels also affects the discharge through the wall. The MODFLOW results can be used to get an indication of the increase in discharge caused by various types of defects. From this, the possible defects for a particular project can be determined.