Protecting Paramaribo against sea-level rise

By Delta engineering

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Fifty percent of the people worldwide live in a city. From all the cities, two third of the cities are vulnerable for sea-level rise (Aerts, 2009). There are a lot of cities at the coast, by an estuary or beneath sea-level. These cities are protected against high water by dikes and dams. Due to the global warming the world has to deal with sea-level rise. One of these cities is Paramaribo, the capital of Surinam. The Surinam River through Paramaribo makes Paramaribo one of the delta cities in the world.

To protect Paramaribo against sea-level rise a solution is designed by Delta Engineering. This solution consists of three sub solutions. The first solution is creating a dike around Paramaribo to make sure the people and the city is protected against the water. But with only this solution the transport by ship is going to decrease until it is not possible at all. So the second sub solution is building a dike between Paramaribo and the other side of the river. There will be sluice placed for the transport by ship. And the third sub solution is creating a dam at the coast of Paramaribo. These three sub solutions together make the solution to protect Paramaribo against sea-level rise. These solution however did raise some questions on how to carry them out. In this article the first sub solution of the dam with a sluice is described and thought out. To come up with a working solutions a few questions have been made up to answer, these can be found below. The main research question related to this solution is:

‘’How can the dam with sluice being realized to Protect Paramaribo against sea-level rise?’’

The sub questions are the following:

1. What soil has Paramaribo in the Suriname river?

2. How can a dike be built?

3. How can the sluice be built?

4. What are the advantages and disadvantages of this construction ?

5. What are the recommendations for further investigations?

Research has been done into the soil structure in the Suriname River. This research has shown that 96% of the soil consists of clay and other fine-mesh sediment. Due to the condition of the ground, a traditional concrete dam is not possible. That is the reason why Geotubes are chosen. These Geotubes are perfectly capable of adapting to the settlements of the soil.

A total length of 2 kilometres of dam will be realized. Therefore, there must be a good focus on the execution technique in order for this project to run as smoothly as possible. As indicated earlier, the soil properties of the available sediment in Paramaribo and surroundings are not conducive to large water-retaining structures. In order to prevent expensive applications such as sediment supply from other countries, it was decided to make use of the available sediment in Suriname in an innovative way. To be precise, Geotubes will be used in this project, in this way the available sediment will be dredged up from the mouth of the "Suriname River" and then pumped into the geotextile bags. As a result, the sediment will be retained in place and is resistant to the many different occurring water forces. This process can be compared to the civil engineering phenomenon; "reinforcing of soil", so the available weak sediment that is provided by the river is reinforced in this way using Geotubes. 

 

The installation of these Geotubes takes place in steps and must be carried out carefully to achieve a well-functioning reversing water construction. First of all, a project area will be plotted by using steel poles, these will be pressed into the ground for temporary use. These steel poles will act as coordination points and anchor points for installing the Geotubes. After that, a filter layer of scour protection will be applied to the sea bottom with a stronger geotextile to protect the Geotubes from erosion on the bottom. When this is installed, an empty Geotube will be rolled out from a pontoon and positioned between the steel poles, when it is in the right place on the water surface it will be anchored with ropes. When the empty Geotube is completely in place, the dredging of sediment from the river will be started, which is mixed with water to pump it into the Geotube. This can take some time before the desired amount of sediment is present in the Geotube and most of the excess water is squeezed out. After the Geotube will be submerged to its desired location, this process will then be carried out several times along the length of the dam to arrive at the correct dimensions of the dam. The Geotubes will be built in pyramid shape to achieve the best strength. Finally, the dam of Geotubes will be covered with another type of geotextile and will then be deposited with landfill stones which will acts as a scour protection.  

 

A calculation model has been drawn up to calculate these Settlements. This allows the settlement

of the soil to be determined by means of probing data. Important input data are the desired height of the dam and the desired material from which the dam is made. This results in iterating to determine the height of the dam and the settlements. This model can be found at our website under downloads.

 

Our wish was to give the dam a height of 10 meters above the bottom. This results in a minimum construction height of 15.40 metres. The dam will sink a total of 5.40 meters in a period of 100 years. By constructing the dam in two phases, the dam will gradually sink and it is practical to compensate for any larger settlements in the second phase by adding extra height.

 

In addition to the dam, the lock can also be calculated using this calculation model. The most important data here are: the dimensions of the lock head, the dimensions of the chamber, the desired navigation depth and the desired safety factor. 

This calculation shows how much ballast water the lock needs and what draft is created. In case a safety factor is applied, the settlement of the lock will also be calculated. 

In addition to the settlement and the calculation for the ballast water, it will also be indicated which horizontal forces occur in the construction. These horizontal forces can be absorbed in the ground by means of folding anchors. The dimensioning of these anchors will have to be carried out by a specialized company. In the first place, adjustments to the calculation methods are not possible. Only advanced users who understand the subject matter are authorized to open the input sheets. The underlying calculation sheets can only be viewed and edited by admin users. If this is desired for you, you can request these rights from the service department of Delta Engineering. 

 

There are a few advantages and disadvantages, as every project has. The advantage of this solution is to build a dam which protects Paramaribo even though the soil does not have a high carrying capacity. The second advantage is that the dam can be built in pieces which ensures that the dam can be raised any time it is necessary.  On the other hand, this method has not been used very often, so there is not a lot of experience with this building method to construct dams. Besides the settlement of this soil is very high. So the dam has to be build higher than needed, what has been said earlier.

To make sure the harbour of Paramaribo can still be reached after building the dam, a lock will be needed. At the moment ships with a length of 210 meters, a width of 35 meters and a draft of around 6 meters are entering the harbour (Gratis AIS-volgsysteem van marineverkeer - VesselFinder, 2022). In the locks design these measurements were taken into account. The lock will be 220 meters long and 40 meters wide. The lock will be 10 meters deep so bigger ships can enter the harbour in the future. Because of the weak soil in the Surinam river, it will be challenging to make sure the lock will not sink into the soil. As a solution to this, the lock will be made like a caisson. This means it will float when no ballast is added. This method will make it possible to ballast the lock in such a way it will touch the soil but not sink into it. Despite the lock will not sink into the soil, it will start to move because of the horizontal force of the water and waves. To fix this problem a few things need to be done. 

To start, folding anchors will be placed at both sides of the lock in opposite direction of the forces. When horizontal forces are pushing against the lock these anchors will get tension in them and transfer the forces into the soil. Not only the anchors will be used to hold the lock in place. At each lock head a concrete wall will be build deeper in the soil. This wall will have two purposes. The first one is holding the lock into place by transferring the horizontal forces into the ground and the second one is to make sure the soil underneath the lock will not get washed away by the seepage. To determine which forces are needed in the anchors and what the draft of the lock will be, calculation models have been made. These are usable for calculating every size of lock by filling in the data in the sheet. The models can be found at the website. Both Lock heads will have 2 doors. These doors will form a point in the direction of the high water. This will cause the water pressure to press the doors shut. To prevent the silt of the river to clog up the lock, a bubble curtain will be made in front of the lock this curtain will make direct the silt towards the shore by Paramaribo. Here the silt can get scooped out using an excavator or a silt pump to transfer it to a location where it can be used for other purposes.  

Geotextile will damage by exposure to sunlight, in combination with high water and wind, the geotextile will rip and float into the ocean. To protect the geotextile from sunlight, the Geotubes will be covered by a layer of rocks. To prevent damage on the Geotubes when applying the scour protection. The dump height will not be higher than 1 meter.  

Because the construction of the dam is a very expensive project, the dam will be built in steps. This is also because the sea-level will not rise to 3 meters in at one time. It will take approximately 130 years to reach this sea-level. It is also not certain that the sea-level will rise up to three meters. If the measures will reduce the global warming there is a chance that the sea-level rise will be lower than three meters. To prevent unnecessary expanses on the dam, the project will be divided in steps. In each step the dam will be made higher.  

To estimate the planning of the construction a table has been made. In this table the sea-level rise is predicted with steps of 10 years. The start of the project will be in 2040, the height of the dam will then be made up to 1 meter. In the year 2110 the second layer of the dam will be built if this is still necessary. The dam will then be built up to 300 centimetres above the normal current water level.  

 

 

Figure  SEQ Figure \* ARABIC 1; Expected sea-level ris

Suriname knows a tropical climate with a lot of rain. The geotextile of the Geotubes will protect the sediment from erosion. Critical places like the connexion of the dam with the shore can be vulnerable for this rain and need extra protection by a geotextile.  
Because of the amount and frequently appearance of the showers the dam needs to be monitored regularly. A hole in the geotextile in combination with the rain can cause big leak in the dam and with that, weak spots.  
Surinam also knows hurricanes (klimaatinfo.nl, 2022). The hurricanes mostly occur in the so-called hurricane season. The hurricane season is from June until November. In these months it is a big risk to do maintenance or construction on the dam. Necessary repairs can be made in these months.  

Flow and waves against the dam can cause erosion of the bed at the toe of the dam. This can cause instability of the dam. The holes can be filled with Geobags. These Geobags are small bags filled with sediment. (TenCate, 2022)

 

Delta Engineering looked at the global approach for setting up a dam against the rise in sea-level in Paramaribo. However, the dam is placed in a river which has to drain water. This water cannot be drained through the lock because this will create a flow hole with a lot of turbulence where ships can no longer pass. The follow-up research will therefore focus on draining water from one side of the dam to the other. This can be done through culverts and pump stations. 

From Delta Engineering, we hereby advise you to install divers equipped with a generator. This will enable energy to be generated in the first years to provide Paramaribo with green electricity. When the sea-level rises and the dam actually turns the water, these generators can be converted into pumps. this allows the river water to be pumped to the other side of the dam. This is about 240m³/s. 

in addition to a follow-up study for the dam, the coastline of Paramaribo can also be examined. Here the coast is currently being reinforced with Mangrove forests and research can be done whether this is sufficient to defend the coast against 3 meters of sea-level rise. 

There is also the possibility to research the connection between the dam and the coastline, also the connection between the dam and the lock will have to be researched. These connections must be strong enough to turn the water from the sea. In addition, it is important that the connection is watertight, if this is not the case, a current hole will appear on the connection, which will result in washout. This investigation should be carried out by a specialized company. One option is to provide the connection with a geo-cloth to prevent swamping.

 

Before these studies are carried out, we recommend that you first of all invest in a good research centre into soil data that is publicly accessible. This will boost voluntary surveys in the future to keep Paramaribo safe from sea-level rise.