Is there a necessity to adapt the harbour constructions in the harbour of Brouwershaven, or to secure them against the reduced tide in the Grevelingen lake?

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After the big flood in 1953 the Grevelingendam and the Brouwersdam were built as a part of the ‘Deltawerken’. By constructing these dams the Grevelingen was separated from the North Sea, which created the largest salt water lake in Europe. Several decades later it was discovered that during hot summers the deeper areas of the lake were leaking oxygen. This leads to a massive mortality of the fauna and flora living in these depths. Since this area is spreading to the shallow areas it was decided by Rijkswaterstaat to bring back a reduced tide into the Grevelingen lake.
The idea is to bring this reduced tide back by constructing a sluice caisson or tidal power plant into the Brouwersdam. This tidal range was determined in a way that the fauna and flora on the islands could remain. Another problem that arises with this reduced tide is that it is unknown what the consequences are for the harbours around the Grevelingen lake and their structures. Brouwershaven specifically gets its income from the harbour and its tourism. This made the Gemeente Schouwen-Duiveland ask to investigate the consequences of a potential reduced tide in its harbour. This led to the following research question:’ Is there a necessity to adapt the harbour constructions in the harbour of Brouwershaven, or to secure them against the reduced tide in the Grevelingen lake?’.
This research was started by investigating the different boundary conditions such as:
• Wind 1,54 m/s Southwest
• Occurring water levels +0,7 m NAP and -0,5 m NAP
• Not exploded explosives Not taken into account
• Soil structure Exists mainly of clay and peat, with a thick sand layer at -16 m NAP
• Profile of the harbour bottom Design level of the harbour bottom at -2,75 m NAP
• Shipping Limiting factors: ship draught of 2 m and length of 14 m
• Flow rate through the guard lock In case of tidal power plant: 0,154 m/s In case of sluice caisson: 0,0719 m/s
The new part of the harbour was designed after the closure of the Grevelingen. This is why the option was to check the stability of the structure in this part of harbour. At the end of the calculation it turned out that there was no danger for the structures to become unstable by the reduced tide. However, there is a statistical probability that the scaffoldings as well as the quay wall will be flooded once in a hundred years. The bigger problem that was found was the accessibility of the harbour. The harbour is now only accessible for ships with a draught of 2 m at a water depth of 2,5 m. Which at a lower water level would cause problems to safely enter and manoeuvre in the harbour.
In the search for a solution a brainstorm session was held with the construction company ‘Aquavia’. With the help of a multi criteria analysis (MCA) it was found that the best solutions were:
• Construction a new harbour in front of the guard lock
• Creating a new function for the existing harbour and shifting the harbour function to a new location in front of the guard lock
• Demolition of the sills in the guard lock and dredging the harbour to a deeper level
In consultation with ‘Gemeente Schouwen-Duiveland’ it was decided to design the first and the last bullet in more detail.
The first variant that was dealt with was that of the demolition of the sills in the guard lock and the dredging of the harbour. The idea here was to lower the bottom of the harbour and the guard lock to at least a level of -2,75 m NAP, which produces a volume of 5143 m3¬ of material such as silt to be dredged away. Which includes the possibility of:
• Finding not exploded explosives
• The quay walls of the oldest part of the harbour becoming unstable.
Also the stability of the guard lock construction after removing the sills had to be checked. This unfortunately was not executed due to the lack of technical data and drawings of the reinforcement. Finally an estimation of 300.000 EUR was made to realise this variant.
The idea for the second variant is to leave the harbour behind the guard lock in the state it is currently in and to construct a new harbour in front of the guard lock. In this way smaller ships can still use the old harbour whereas the ships that cannot enter the harbour anymore can moor in the new harbour as well as even larger ships. In this new harbour then there would also be a place to moor the fishing boats as well as a river cruise ship. Because of strict time scheduling it was decided to only design one of the important structures of the harbour, namely the harbour mole. For this design there were 2 variants to take into account. In the first variant the total mole construction (breakwater + the pier) was made of wood, whereas in the second variant only part of the breakwater was made of wood. The pier, however, was made of concrete. Finally it was estimated that the construction of the new harbour would cost 7 million EUR. Which is a big difference compared to the price estimation of the demolition of the sills in the guard lock. Both variants have their pros and cons. By demolishing the sills and dredging the harbour to a lower level the problem of the harbour is resolved while a smaller/ more optimised version of the other variant could enable more future prospects to be worked out for the harbour by increasing the capacity and attracting new functions to the harbour. This could of course increase the harbour profits.