Feasibility study of a new harbour on the island of St. Eustatius
St. Eustatius is a small island in the Caribbean that belongs to the Kingdom of the Netherlands. Its existing harbour is due for upgrades and repairs on existing structures have been discussed in addition to the possibility of constructing a new harbour. The island is rugged, and the landward side of the coast is surrounded by cliffs except for a small stretch of coast where the hinterland is quite flat.
The island is not selfsufficient and all cargo comes in via the existing harbour. These cargo flows coming from the harbour are transported through the touristic centre of the island making for some unpleasant and dangerous situations. Additionally, the harbour is mostly used by large cargo vessels and leaves no room for leisure ships. The government of the island sees the potential to attract more tourists, but is also aware that the current state of the facilities could be improved. The objective of this Master’s thesis is to explore the feasibility of constructing a new harbour on the west coast of St. Eustatius and to find a suitable layout and a conceptual design for a new harbour. This will be done following the regular design approach in which the design steps are conceptualisation, verification, evaluation and selection. The location of the new harbour has already been determined in earlier research and is shown in Figure 1. The location is chosen where the hinterland is quite flat.
The conceptualisation starts with gathering relevant information about the requirements and boundary conditions of the project. The future cargo volumes and future vessel dimensions are estimated on which the dimensions of the required facilities of the harbour are based. These cargo volumes are determined with growth of population, welfare and the tourism sector in mind. Environmental conditions and hydrodynamic conditions of the proposed project location are also gathered. Data includes information of topography, bathymetry, soil characteristics, archaeological remains, environmental and aviation regulations and also data about waves, wind, currents and water levels. Here, it is found that the offshore wave heights during hurricane conditions are quite significant but break on the foreshore and subsequently lose energy. On the contrary, the daily wave climate is relatively calm. It is confirmed by the gathered data that the location of the new harbour has potential for construction since the hinterland is relatively flat and is located out of the zones of important nature parks. However, it is expected that significant amounts of historic remains are found in the area which need to be removed before construction. For future design steps it is advised to gather more accurate data.
The government of St. Eustatius is the client and has some clear wishes and demands. First and foremost, the harbour should be built in phases. The first phase would account for the current and future cargo volumes to the island. The second phase would be the expansion of the harbour to function as a transshipment hub and develop as a competitor to the congested harbour of St. Maarten. This way, the funding of the expansion can partly come from the operational profits of the harbour. The necessary buildings and facilities, including their dimensions, have been found in conversations with the client and through calculations. The required wet areas of the port and the elevation levels were found based on the dimensions of the design vessel. The road leading to the new harbour will be constructed on the already existing dirt road which needs upgrading.
In the development of the harbour layouts three alternatives have been developed. These are developed with the small scale of the harbour in mind which lead to the choice of discarding a northern breakwater. This choice is justified since the data showed that the waves during daily conditions almost solely come from the south. The turning circle has been moved out of the harbour basin too to save space which is justified based on the limited amounts of movement. The alternatives differ in their orientation of the berth and the usage of the breakwater as a berth. The conceptualisation step is finished by discussing the modifications on these alternatives. A shift of the harbour landwards, placement of the storage on land or water and a combination of both is considered.
The verification of these layout alternatives is done by analysing the possibility of constructing a breakwater, the impact on the morphology of the island and studying the wave penetration of the harbour basin of every alternative.
The breakwater of the different alternatives is situated at different water depths which makes for the design under different conditions. For the design of the breakwater two types, rubble mound and caisson, are considered, that have (dis)advantages under certain circumstances. It needs to be verified that a design for both types of breakwater can be designed for the different water depths. It turns out that the waves at the deeper location are too large to be able to construct a rubble mound breakwater there. In some alternatives, the use of a caisson breakwater could not be verified as a result of the small water depth. This is a result of the economic disadvantage of a caisson breakwater at such a shallow depth. These designs are subsequently dropped from the analysis.
The harbour should not have a negative influence on the morphology of the island and the sediment transport should not have a negative impact on the construction and operation of the harbour. The west coast of the island experiences very minor sediment transport in the northern direction as a result of lack of sediment availability in the surf zone. It is found that the construction of the harbour does influence the coastline in a positive way as the breakwater is able to capture the limited available sediment. The harbour would be an excellent contribution in combination with beach nourishment since the historic structures on the coastline are once again protected.
The last topic of the verification step treats the wave penetration into the harbour basin. Here, the wave direction has been divided into three main wave directions. The wave penetration has been determined on a basis of refraction and diffraction diagrams and the wave height at the berths has been calculated. The downtime of every alternative per wave direction showed to be on the high side after the comparison with allowable wave heights. The designs have been altered by means of an elongation of the breakwater to create calmer conditions and lower the downtime to be under the 1.1%. This was not possible in one alternative where the safety of navigation would be diminished and this alternative has been discarded. The harbour oscillations as a result of resonance of long ocean waves is also investigated. Here, the measured waves in the existing harbour are investigated and compared to the natural periods of the alternative layouts. Even though the natural periods of the harbour concepts are situated further from the measured period of the waves in the existing harbour, there still is a chance resonance will occur. More investigation into this subject is advised in more detailed design steps.
The final step in this analysis is the evaluation and selection of the most suitable alternative. This has been done by a multi criteria analysis in which weighted scores have been determined. The ratio between the costs and the weighted score of every alternative would result in a most suitable alternative which can be seen in Figure 3. This design is partly dug in into the land and is executed with a rubble mound breakwater. It might be possible to gather the required material from the island but it is highly likely that these need to be imported.
The price of this design, excluding the construction of the harbour buildings and the road, is estimated to be in the order of $35 million (±50%) and is very much influenced by the costs of the breakwater. The crosssection of the breakwater of the final design can be found in Figure 4.
This Master’s thesis is a first step for the exploration into the possibilities regarding the construction of a new harbour on the island of St. Eustatius. Therefore, the availability of data has not always been guaranteed and resulted in the need to estimate, or assume, necessary information. It is advised, in following design steps, to gather more accurate data by performing measurements and reallife experiments. The level of accuracy of the data should match the advance of the detail of design. Specifically, the structure of the subsoil and the wave behaviour should be investigated more thoroughly. Scale model tests and onsite measurements are part of this additional gathering of information and verification. Conclusions from further research might alter the design.