The Dutch Rhine-Meuse delta is expected to require many dike reinforcements on a short and long term, due to (accelerated) sea level rise. Average damage in unembanked areas will increase too, and a costly replacement of the Maeslantkering is expected after 2070. Delta21 is a project that aims to address these challenges related to flood protection, while also providing hydraulic energy storage and opportunities for valuable nature development.
Delta21 consists primarily of a large (salt)water storage lake attached to the Tweede Maasvlakte. It is connected to the North Sea with a pump-turbine station. On the southern side, a closable storm surge barrier spans the remaining gap to the island of Goeree Overflakkee. Under normal circumstances the lake functions as a hydraulic battery, using the pumps and turbines to store or generate electricity as needed. However, when water levels threaten to exceed NAP + 2.5 m at Dordrecht, the barrier closes and an upstream spillway into the lake is opened. To determine the viability of the project in the context of flood protection, the question arises: “To what extent is Delta21 capable of providing a cost-effective alternative to the current flood protection policy in the Rhine-Meuse delta?”
To answer this question, a one-dimensional hydraulic model is constructed with a detailed and accurate schemati-zation of Delta21. Results are processed into exceedance frequencies for a system with and without Delta21, to obtain reductions in water levels at normative frequencies. Furthermore, a sensitivity analysis is performed to de-scribe how various designs and configurations affect the magnitude of these reductions. Other than Delta21, no system-changing interventions are included in the scope of study.
Three strategies are formulated in which Delta21 potentially reduces costs, and evaluated using the model’s results:
1 Extending the lifetime of the Europoortkering before it needs to be replaced.
2 Obviate dike reinforcements.
3 Reduce the average yearly damage of unembanked areas.
Approach 1 is shown to be ineffective. Delta21 does not achieve changes in the Europoortkering’s closure frequency or failure probability per closure. Neither does Delta21 effectively mitigate a failure in the region where water level exceedance frequencies are dominated by the Europoortkering’s failure probability.
Approach 2 is far more successful, accounting for 96% of the total cost reductions. Reductions in water levels at normative frequencies are translated to reduced failure probabilities of dike segments using fragility curves, which potentially yields a positive reassessment. Delta21 achieves this and obviates dike reinforcement for 41 km by 2050, and 150 km more by 2100. These lengths comprise respectively 33% and 60% of the total considered dike lengths that would need reinforcement. The net present value of obviated reinforcement costs is €752 million, with a 90% confidence interval of [€220 million, €2,821 million]. This large interval is due to large uncertainties in reinforcement cost estimations. Delta21 most effectively obviates reinforcements in trajectories with stricter norms and closer proximity to the storage dominated region. Water levels at normative frequencies are very sensitive to the opera-tional control (i.e. closure criterion) of Delta21. An additional nominal cost reduction of ca. €278 million can be achieved when a criterion of NAP + 2.5 m at Dordrecht is maintained in the future. The sensitivity is far weaker to varying dimensions or capacities of Delta21, which are generally unnecessarily large during illustrative conditions.
Approach 3 provides the additional 4% contribution to total cost reductions. The only considered area is Dordrecht, due to its unique high economic value in low lying unembanked locations. Damage profiles are integrated with changes exceedance frequency curves to obtain the average yearly damage. This is reduced by approximately €53,000 per year now, and grows to €1.36 million per year in the future climate scenario (2100). The decrease in relation to the current system is about 42%, but the absolute value rises greatly with sea level rise. Implementation of lower closure criteria variations can yield an additional +20% now, and +10% in the future scenario.
The total net present value of cost-reductions by Delta21 is €783 million, with a 90% confidence interval of [€251 million, €2,852 million]. This covers about 20% of the total construction costs of €3.7 billion, and is insufficient to make Delta21 viable alone. However, not all costs are related to flood protection exclusively. If the components required for energy storage are viable on their own, merely the additional costs of the spillway and storm surge barrier have to be included in the cost-benefit analysis. Operational costs, which heavily depend on the operational control, must also be added. Smaller design dimensions or pump capacity of Delta21 has been shown to be just as effective, and would further cut costs. Additional savings beyond 2100 are plausible, but outside of this research’s scope. Furthermore, less people displacement or flooding of unembanked areas may achieve additional societal value, but is difficult to quantify. More research will have to indicate whether attributing only specific costs, including additional value sources, and estimating reinforcement cost more accurately ultimately lead to Delta21 being fea-sible in the context of flood protection.

In the Rhine-Meuse Delta over 100 scour holes are located. Many of these scour holes have dynamic behaviour and are growing in size and depth. A scour hole can lead to an increased probability of flooding. One of the potential hazards of the presence of scour holes is the occurrence of flow slides on the foreshore next to scour holes. Depending on the distance between the dike and the river, a flow slide can affect the water-retaining function of a dike. Flow slide is an indirect failure mechanism and can be used as a scenario in the safety assessment of direct failure mechanisms.

In the research, a data-driven method has been developed for the risk assessment of scour holes. The method is elaborated for flooding due to the failure mechanism overtopping. The development of a scour hole is coupled with the safety assessment regarding overtopping for the dike next to a scour hole. In the method, the development of a scour hole and the future dimensions are predicted in a probabilistic way by extrapolation of historically measured bed level data. With the predicted future scour hole dimensions, the probability of occurrence of a flow slide and the probability of several post flow slide profiles is calculated with the methods given in the Wettelijk Beoordelingsinstrumentarium (WBI). Based on the post flow slide profiles, the probability of exceeding the critical overtopping discharge is determined. Combining the probability of exceedance of the critical overtopping discharge with the consequences of a flood gives the flood risk due to the presence of a scour hole.

The developed data-driven method is applied to one case study, a scour hole in the river Spui. For this particular scour hole, the flood risk is assessed. A prediction is made for the future dimensions of the scour hole. The flood risk is determined with the probability of occurrence of flow slide, the probability of exceedance of the critical overtopping discharge and the expected consequences of a flood. The assessed flood risk is evaluated and risk mitigation measures are proposed. The economic feasibility of risk mitigation measures is analysed with a cost-benefit analysis.

In 2015, the Dutch government initiated the ‘longitudinal training dams pilot project’. With the help of longitudinal dams, a two-channel system is created in which the river is divided into a main and a side channel. A combination of an inlet and several openings between the dams allows water and sediment to be exchanged between the two channels. This is expected to influence the discharge (and sediment) distribution between the main and side channel. Understanding the hydraulic conditions and the impact of the regulatory inlet and opening crest heights is expected to improve the control strategy and future design of river interventions using the concept of longitudinal training dams. A one-dimensional (Matlab) and a two-dimensional (Delft3D-Flow) model are created for this research. Both schematize the two channels with dams and openings. The influence of several mechanisms, factors and dimensional design parameters on the discharge distribution is assessed. Additionally, using Delft3D-Flow, the detailed flow patterns around the inlet and openings are evaluated and the schematizing of the inlet as part of the local bed topography or by using subgrid weirs is compared. From the several model runs, the discharge distribution shows to be mainly controlled by differences in longitudinal water level slopes in both channels, caused by the combination of too short inlet and opening lengths. It is not influenced significantly by the placement of the longitudinal training dams in a river bend. For increased crest heights the flow conditions change from submerged to free-flowing. In submerged conditions the velocity magnitudes and flow angles atop the inlet and opening crests are shown to increase in downstream direction while for free-flowing conditions they remain nearly constant. The free-flowing conditions are well described by empirical relationships, while for submerged conditions an additional angle submergence coefficient is suggested to parameterize flow angles. After some modifications, which will be included in the new Delft3D-Flow software release, schematization as part of the bed topography and subgrid weirs both predict similar discharges (< 7% relative difference). If interest is only given to water levels and discharge distributions, the use of subgrid weirs is recommended in Delft3D-Flow. For models assessing other river functions requiring local flow patterns or sediment transport, schematization as part of the bed topography is essential. However, by including the suggested parameterization for flow angles, the errors in flow angles for subgrid weir can be reduced significantly. From this research, the adjustable inlet and opening crest heights do not show a large influence on the discharge distribution (5-10% difference). It mostly impacts the tipping point for submerged to free-flow conditions. In submerged conditions, the inlet crest height does reduce the specific discharge in the side channel, limits the transverse velocities in the main channel, and greatly influence the flow angles atop the crest. Besides the crest heights, the channel width and inlet length should also be included as adjustable parameters in the design or control strategy of new systems of longitudinal training dams.

The Dutch coast is affected by coastal erosion. In order to limit erosion and prevent inland migration of dunes, sand nourishments have been adopted as a common practice for the maintenance of the coast. The evaluation of the applied coastal erosion policy is achieved by the use of coastal indicators. Increased number of nourishments affects the coastal profiles and coastal indicators are used to describe those changes along the coastal zone. The present work intends to improve the definition of one of the indicators (dune foot position) by proposing a new methodology and to develop a Bayesian network in order to assess the effectiveness of nourishments on the coastal system. Coastal indicators, such as the Momentary Coastline and the dune foot position, are used in the Bayesian network in order to quantify those effects.

Available field measurements of the entire Dutch coastline (JARKUS morphometric database) are used in order to develop the methodology of the dune foot position detection. Taking into account the geometry of the profiles, the dune foot position can be detected by calculating the first and second derivatives of the measured points along the profiles. Comparison with visual, in-situ observations, constitute the validation method of the proposed methodology. Root mean square errors, with respect to visual observations, are used to compare the performance of two methods for the dune foot position detection; the proposed methodology and the current dune foot definition, which detects the dune foot at the most seaward crossing of the profile with the level of +3 m NAP. By looking at the performance of the methodologies at different areas, the results are diverse. However, the proposed methodology shows a general improvement of the detection. Moreover, the methodology is believed to be generic and applicable to other dune systems around the world, since it is purely based on the morphology of the profile.

A Bayesian modelling approach is used to assess the effectiveness of nourishments on coastal indicators for the Holland coast. The selection of the input and output parameters, the design of the network and the optimization of the structure are the main steps for the development of the model. Once the optimum configuration is chosen and the model is trained, the Bayesian network gives the possibility to investigate the relations between variables, by constraining the input nodes on a specific range of values and assessing changes on the indicators. By applying those constraints it can be found that nourishments have positive effects on the coastal indicators, since larger seaward displacement is observed compared to cases in which no nourishment has been implemented. Moreover, initial erosive trends of the dune foot position are constrained by the implementation of nourishments. In addition, the effects of nourishments at different time scales are investigated. To this end, time horizons of 1, 5 and 10 years after a nourishment implementation are chosen. Positive effects of nourishments are present at all the considered time horizons, with beach nourishment to have an immediate effect on the indicators, whereas shoreface nourishments reach a stronger effect 10 years after the implementation.
Finally, the Bayesian inference can be used as a decision support tool for coastal managers, since its user-friendly environment can lead to easy interpretation and use for coastal management purposes. The required sand volume can be estimated in order to achieve a specific magnitude of seaward displacement of the indicators or in order to preserve the coastline at the current state. The estimations concern large spatial scales.