Under the Dutch Flood Protection Programme (HWBP), extensive reinforcement of primary flood defences is planned by 2050. However, progress is slow and costs are exceeding expectations. Current reinforcement strategies face significant challenges: inward reinforcement is constrained by spatial limitations, while construction-based solutions are costly and lack flexibility in light of climate policy uncertainty. Outward dike reinforcement (ODR) offers an alternative, but its application is currently restricted by regulations under the Rivierkundig Beoordelingskader (RBK). In particular, the ‘1-mm rule’ requires that any hydraulic impact be mitigated to within 1 mm, often necessitating substantial mitigation works that render ODR disproportionately complex and expensive, and thus rarely applied despite its promise.

This thesis investigates whether ODR can be considered a technically feasible and cost-effective alternative under a relaxed RBK framework, in which its hydraulic impact is absorbed by the flood defence system, eliminating the need for mitigation measures. In doing so, it aims to support engineering practice through preliminary design insights, while also identifying regulatory elements that may warrant reconsideration to facilitate broader applicability of ODR.

Four self-designed conceptual dike variants are created to assess the hydraulic impact, technical feasibility, and cost-effectiveness of ODR under varying conditions. The designs are developed at a preliminary level and represent inward, construction-based, and two outward variants with 5-metre (Tuimeldijk) and 20-metre expansions, applied to a hypothetical reinforcement of the southern Waal banks. The outward variants define a bandwidth for evaluating the effects of expansion, providing a framework for generalising ODR performance. The hydraulic impact is analysed using a simplified 1D compound channel model to identify influential parameters and general trends, complemented by detailed D-Hydro Suite simulations for site-specific effects. These results inform the feasibility assessment, which uses exceedance-based limit-state formulations of the main failure mechanisms to evaluate dike performance and potential functional lifetime reduction (FLR). Cost-effectiveness is assessed through short-term design comparison and long-term net present value (NPV), using a life cycle cost (LCC) analysis that incorporates adaptability and feasibility outcomes.

The expected water level difference (WLD) due to ODR in the Waal River ranges from 0 to 4 cm, depending on the application and floodplain characteristics. Two design graphs are presented, offering first-order estimates of WLD for 5-metre and 20-metre expansions. The relationship between expansion and WLD is observed to be approximately linear, enabling interpolation across the design graphs. Bottlenecks in the river system show the highest WLD sensitivity, as narrow and smooth floodplains amplify hydraulic impact. Continuous reinforcement results in higher WLD than discrete interventions, although short interventions are penalised by steep local water level gradients and cause larger WLD per unit of intervention length. Adaptation lengths required to absorb WLD effects span several tens of kilometres, but depend strongly on endpoint definition.

ODR is technically feasible considering the performance of affected dikes. Of the main failure mechanisms, only overflow and piping are influenced by the hydraulic impact of ODR, resulting in an estimated FLR of approximately two years if applied in the Waal. Dike stability is never compromised while WLD remains below 10 centimetres, and effects on overtopping can be neglected. FLR is primarily governed by the annual climate-change-induced increase in hydraulic load, the magnitude of WLD, dike subsidence (for overflow), and the additional robustness provided by the blanket layer or sheet pile (for piping). If these structural elements include a buffer equal to or half the WLD, respectively, FLR due to piping is unlikely. Two strategies are considered to address FLR: adding an asphalt layer to all affected dikes or accepting the reduced lifetime.

The cost-effectiveness of an optimised ODR design is comparable to, and under certain scenarios, proves more cost-effective than construction-based reinforcement over a 100-year horizon, while offering greater adaptability where inward reinforcement is either unfeasible or prohibitively expensive. This is particularly true when recycled soil is used and in light of uncertainties regarding spatial constraints on the inner slope, sheet pile prices, and functional lifetime. The optimised design should minimise outward extent and material use, in line with minimal designs such as the Tuimeldijk variant, while ensuring sufficient resistance against piping without requiring sheet piles. It should be applied over stretches exceeding 10 kilometres, as cost-effectiveness increases with length. Choosing FLR proves to be the more cost-effective strategy of the two, as uncertainties associated with FLR have limited impact on overall cost-effectiveness. In short-term cost comparisons, ODR is only cost-effective if the design closely resembles the Tuimeldijk variant or incorporates recycled soil. Otherwise, the excessive soil volumes make it less competitive than non-outward alternatives.

These findings demonstrate that ODR, where the flood defence system absorbs the hydraulic impact, can be a technically feasible and cost-effective alternative to conventional reinforcement strategies, provided that RBK regulations are relaxed. It is therefore recommended to integrate this variant of ODR as the final step in the current line of reasoning for riverward reinforcement upheld by the HWBP.
To enable practical application, it is proposed to relax the hydraulic limit in the RBK to 2 centimetres. This adjustment would allow a broader range of ODR configurations to be assessed using the presented design graphs, while maintaining cost-effectiveness and feasibility.
Additionally, the 1-mm rule should serve as a threshold for defining adaptation length, provided a clear backwater formulation is integrated into the RBK. Further research is required to confirm that other regulatory conditions within the RBK are upheld under this revised hydraulic limit. This would allow ODR to serve as an additional alternative in complex situations, helping to reduce project complexity today while preserving flexibility for future challenges.

The Mekong Delta in Vietnam, one of the world’s most fertile and ecologically rich deltas, faces environmental challenges that threaten its biodiversity and local communities. Historically, the delta’s extensive mangrove forests provided natural coastal protection, but in recent decades, agricultural expansion, aquaculture, and infrastructure developments have degraded these ecosystems.
This research focuses on Bac Liêu, a region acutely affected by these changes. With diminishing mangrove buffers, local vulnerabilities to environmental hazards have increased, putting pressure on sea defenses. In response, the Dutch government and Vietnamese partners have introduced the “Mekong Living Lab,” an initiative for in-field research that promotes mangrove restoration and sustainable coastal management.
Conducted by TU Delft students, this study contributes to the Living Lab’s goals by exploring the causes of mangrove decline in Bac Liêu. Combining interviews with local residents and field data on coastal profiles, this multidisciplinary approach seeks to safeguard the ecological and economic future of the Mekong Delta.
The study suggests an integrated approach within the Living Lab framework, emphasizing research, showcasing, and education to bridge hydraulic, ecological, and socio-economic perspectives. Priority recommendations include continuous cross-sectional measurements, sediment retention analysis, stakeholder engagement strategies, and further interdisciplinary studies on mangrove viability. These initiatives aim to align technical insights with stakeholder needs, advancing observation-driven solutions for Bac Liêu’s mangrove ecosystems.