Key points:- Pre-closure salinity intrusion into Haringvliet-Hollands Diep is known to have reached Biesbosch National Park at flood tide. Post-Delta21 salinity intrusion is projected to be less extensive than pre-closure. Maximum intrusion is estimated to reach the westernmost Moerdijk ports mainly due to diminished tidal flow at the estuary mouth.- Freshwater intake in the Haringvliet is projected to be compromised for the majority of the year whereas freshwater intake in Hollands Diep is compromised during prolonged drought.- Salinity outwash from the Haringvliet on ebb tide is projected to be poor under both drought and normal conditions due to widespread salinity diffusion in lateral and vertical directions on flood tide. The Delta21 framework, in which this study is positioned, aims at increased flood safety and ecological restoration of the Haringvliet, a former estuary in the Dutch southwestern delta that has been closed as part of the Delta works. To achieve this, Delta21 proposes to reopen the Haringvliet freshwater basin to tidal effects in an attempt to restore its estuarine character. Implementation of Delta21 introduces the risk of compromising agricultural and industrial activities around the Haringvliet-Hollands Diep by reintroducing salinity and tidal movement in the basin. The central problem lies in quantifying the extent and stability of the expected periodic salt intrusion post-Delta21. This has been done by projecting a geographical bandwidth of salinity intrusion patterns depending on the severity of SLR (2020-2100) and low-to-mean Rhine-Meuse discharges (in 2100 CE) using a numerical model. Mitigation strategies are then recommended based on these numerical results. The OSR-HV model (owner: Port of Rotterdam) is used to run predictive scenarios of salinity intrusion for the lower reaches of the Rhine-Meuse basin. OSR-HV runs in TRIWAQ (Rijkswaterstaat), which is 3D numerical modelling software that employs coupled hydrodynamics and constituent transport thereby resolving salinity transport. An upper-end critical scenario consists of a Rhine (Lobith) discharge averaging 1000 m3/s for 31 days combined with 85cm SLR in 2100. This resulted in an intrusion pattern reaching the westernmost port of Moerdijk in Hollands Diep. Results show that the basin geometry, possibly nudged by Coriolis deflection, initially causes a preferential path of salinity intrusion along the southern bank of the Haringvliet. Lateral and vertical mixing is extensive in the western part of Haringvliet which is thought to be a combined effect of weakened tidal flow at the estuary mouth and robust and erratic geometry of the basin. Further up-estuary, the historical flood-ebb tidal channel structures is the main transporter of salinity. The Haringvliet shows distinctly different estuarine behaviour compared to the neighbouring Rotterdam Waterways where stratification is more stable, causing less up-estuary diffusion. The relative robustness of the Haringvliet and mild freshwater forcing give rise to extensive 3D mixing which subsequently limits the maximum horizontal excursion of salinity. Salinity intrusion into the Old Meuse is observed to aggravate upon opening of the Haringvliet sluices due to flow reversal in Spui which connects Haringvliet with the Rotterdam Waterways. This effect can even result in salt intrusion from the Rotterdam Waterways via Old Meuse and Spui back into the Haringvliet, causing a secondary spike in salinity on ebb tide there. Finally, height-limitation of the Haringvliet sluice gates is somewhat effective in countering horizontal excursion of salinity but runaway diffusion in the Haringvliet results in similar salinity profiles compared to full opening of the sluice gates. A shipping channel that crosses the current Haringvliet front delta is included in Delta21. This deep feature promotes advection of salt through the Haringvliet sluices. Subsequent diffusion patterns cause for poor washout of salinity from the basin upon ebb tide. Limiting the depth of this channel is recommended if salt intrusion is to be reduced. Full opening of the Haringvliet sluices furthermore causes approx. 0.50m lowering of Mean Low Water (MLW) near Moerdijk which affects busy shipping routes between Rotterdam-Moerdijk-Scheldt. Partial reduction of the conveying area of the Haringvliet sluices may be used to suppress the tidal wave penetration into Haringvliet-Hollands Diep. Significant gain in ecological value is likely post-Delta21 due to addition of approx. 1900 ha of intertidal areas and a 40 km salinity gradient (excl. front delta). Ample recommendations on further research have been made in this exploratory study. It is recommended to further study the effects of Delta21 interventions on macro hydrodynamics of the Dutch coastal shelf. The interconnected nature of the region requires a larger modelling domain to prevent the occurrence of non-physical effects obtained from the current numerical schematization. Furthermore, hydrological relations and bathymetry were generated from 2020 data. It is therefore recommended to precede the assessment of salinity intrusion into the Haringvliet with numerical projections on change to these environmental factors. Lastly, significant gain in accuracy may be obtained from applying spatially varying temperature and wind to the domain to better replicate baroclinic flows and turbulent mixing.

Storm safety and durability of touristic destinations is of utmost importance in Cuba, considering high stakes regarding life and capital invested in the development of the touristic sector. This twomonth research focuses on one of the largest Caribbean beach tourism hotspots found in Varadero, Cuba.

Tourism along the Hicacos peninsula has been on the rise and will continue to do so. The Oasis beach hotel is capitalizing on this trend by building a larger new hotel. However, no characteristic flat white coral beach is present in front of it. Tourist demand is greatly driven by the presence of such a beach. Year-round erosion was verified to be partly caused by a blocking of longshore sediment transport.

The local harbour groyne at the eastern beach section was found to be responsible for this but may not be adapted, as it is a necessity for the harbour entrance. Demolition of existing hydraulic structures, construction of a groyne at the westward boundary and sand nourishing are proposed as a zero-solution to ensure sufficient beach width (40 metres) at the 800-metre coastal segment.
Durability is not achieved through this zero-solution as the average lifetime between maintenance nourishments is smaller than one year.

This result shows that successful development of the Oasis beach sector can only be achieved by considering cross-shore erosion processes as well. An important aspect of the cross-shore transport was expected to be the effect of tropical storms on Oasis beach. To ensure the safety of tourists in storm conditions, an assessment of the infrastructure was required.

A multidisciplinary path was chosen to ensure safety of inhabitants, tourists and capital, especially during hurricanes. Flood safety, coastal engineering, transport and infrastructure form the core pillars of the research. Two distinct events were simulated to determine the morphodynamical response of the Oasis beach, namely hurricanes Wilma (2005) and Irma (2017). Both affected the northern coast of Cuba greatly, but passed Varadero from opposite directions. A general hurricane scenario was tested in evacuation assessments of the entire peninsula, which has just one exit road. Furthermore, the capacity of the current evacuation scheme was projected on the expected population and tourists visiting the peninsula in 2048.

Proposed coastal solutions were the construction of a submerged breakwater along the entire length of the beach, an artificial reef or a combination of the two. Normal, cold front and hurricane conditions were imposed upon these structures using XBeach software to test their performance. The subsequent infrastructural safety of the hinterlands was modelled using Simio software.

Results yielded good performance of the artificial reef against structural erosion from both the normal north-eastern wave climate and the northern cold front waves. The submerged breakwater performed better in hurricane conditions though extension of both groynes was found necessary to keep sediments within the Oasis beach system.

A multi criteria analysis was used to determine the best coastal intervention given the project requirements. Initially, no financial motives were used to determine the optimal solution. The results of this analysis stated a preference for the installation of an artificial reef. Its added recreational value was not quantified and thus not discounted from the initial investment. In detailed design, cost estimations of all viable design alternatives showed that the rtificial reef was also a relatively affordable option.

The final recommended design that ensures beach durability thus consists of: demolition of weathered hydraulic structures, western groyne placement, initial nourishment, artificial reef deployment and maintenance nourishment for a 30-year lifetime. A modelling of the definitive design with, also including the vegetation of the dunes indicated the Oasis beach section to be flood safe.

The total cost of this intervention amounts to approximately $900,000 and takes 28 months to complete. If monitored well, the artificial reef will increase its coastal defence capacities and functionality is expected to exceed the 30-year lifetime.

Recommendations regarding storm safety also include the evacuation process of tourists during the extreme hurricane conditions, on which the coastal design was based. Various projections of the growth of the number of tourists on the Hicacos peninsula have been simulated in a Simio evacuation transport model. The model indicated that even for the largest projected growth of tourism in Varadero, the infrastructure suffices and using a 50:50 ratio between evacuees being transported in buses and cars, it is possible to evacuate the peninsula in twenty-four hours.

To be able to evacuate using this optimal ratio, an increase in car ownership in Cuba is required before 2048, as a shortage exists in the current situation. Therefore, the peninsula is still considered storm-safe, but monitoring of the actual increase in tourists is advised.