Hybrid dunes are a coastal flood defense structure that combines the sandy and wave- dissipating capacity of natural dunes with the robustness of hard structures. Their increasing application in coastal environments highlights the need for a better understanding of how these systems respond to storm forcing. This study investigates the morphodynamic behavior of hybrid dunes under storm conditions through a large-scale field experiment at the Sand Engine (NL), aiming to improve predictive insight into erosion mechanisms and structural performance. High-resolution LiDAR, pressure sensors, and acoustic instruments were deployed across four engineered dune configurations, ranging from a fully sandy dune to hybrid setups with embedded sea dike and seawall cores, to monitor morphological changes and hydrodynamic forcing during five consecutive storms.
The main research question is: What are the dominant erosion mechanisms and structural influences of embedded hard elements in hybrid dunes during consecutive storm events?
The findings show that hybrid dunes transition through three consistent erosion phases: (1) initial sand-dominated retreat characterized by notching, slumping, and offshore sediment transport, (2) reduced erosion once hard elements become partially exposed, and (3) structure-dominated stability after full exposure, during which landward retreat halts. Core geometry strongly influenced behavior: The Dike-in-dune (S1) supported smoother energy dissipation and profile adjustment, while the wall-in-dune (S4) halted retreat more abruptly but caused localized vertical beach erosion. In contrast, the Sandy dune (S2) continued to retreat by over 7 m, with cumulative erosion volumes exceeding 11 m3 /m. Erosion volumes correlated significantly with 20 min time-averaged waterlevel, measured 80 m seaward of the dune foot. Field observation identified the total water level as the dominant driver of erosion. Once exposed, hard structures reduced wave run-up and changed energy distribution, influencing sediment mobility. Oblique wave conditions likely disrupted longshore sediment supply, especially toward unarmored sections, amplifying erosion near structural transitions. These interactions underscore the need to jointly assess cross- and longshore processes when evaluating hybrid dune behavior. Key design factors include sand cover thickness, core geometry, and especially transition zone reinforcement. Structural flanking and undermining were critical failure mechanisms, as seen in the collapse of the Dike-in-Dune and Dike sections during the fourth storm. These results highlight the importance of integrated design strategies addressing not only structural shape, but also lateral sediment continuity and toe stability.
Hybrid dunes shift erosion from retreat-driven to structure-constrained behavior, offering localized resistance under storm conditions. However, their long-term performance depends on robust, systemscale design, balancing sand volume, structural exposure, and sediment pathways to deliver adaptable and resilient coastal protection.

The research question addressed in this study is ”To what extent does the presence of a cloud forest canopy impact the Mestelá River catchment and how will this affect the various involved stakeholders?”. The study aims to investigate the importance of cloud forests in the Mestelá River catchment, Alta Verapaz, Guatemala, related to water security and the social impact of cloud forest conservation and management. The research methods used in this study were a combination of quantitative and qualitative methods.

Cloud forests play a vital role in regulating water flow in catchments. The Mestelá River catchment, where the NGO Community Cloud Forest Conservation (CCFC) is situated, is the focus of this research. The project’s primary aim was to establish a long-term canopy setup, ensuring future data collection. The project’s scope encompasses a range of methodologies, including the installation of a long-term measurement station in the canopy, computation of the Mestelá River discharge, the development of a rating curve, and the utilisation of a FLEX-Topo model to simulate the hydrological cycle in the catchment. Additionally, a stakeholder management analysis was conducted to understand the complex impact of cloud forests (conservation) on various stakeholders.

The study did not explicitly formulate any hypotheses, but the findings provide evidence for the impact of cloud forest canopies on river catchments and discharge. The study also has limitations, including the small sample size and the lack of long-term data. However, the study provides valuable insights into the importance of cloud forest ecosystems for water security and the social impact of cloud forest conservation and management. The stakeholder analysis reveals that for CCFC two methods of advocacy can be used. Whilst the CCFC is effective in bottom-up engagement with the community, in addition, a strip for small children was constructed. For top-down advocacy, using the FLEX-Topo
model for visualising water security in combination with cloud forest protection holds promise.

The implications of this work are substantial for cloud forest conservation and associated ecosystems. The findings offer valuable insights for developing effective conservation strategies that consider the canopy’s impact on the catchment and its stakeholders. It is important to note that the FLEX-Topo model is currently conceptual and requires further refinement and detail for the Mestelá River catchment. Nevertheless, this study contributes significantly to the understanding of cloud forest ecosystems and offers practical and theoretical applications for future research and conservation efforts.