|Creating scale models for testing Ecosystem-based coast protection designs|
Schot, D. (2019). Creating scale models for testing Ecosystem-based coast protection designs. Thesis. NIOZ Royal Netherlands Institute for Sea Research: Yerseke. 68 pp.
The combination of land subsidence and sea-level rise has led to the fact that about 60% of the Netherlands is at risk of flooding. Some 3700 kilometres of flood defences have been made to prevent that from happening. The currently used techniques lead to land subsidence and reduced water quality. In addition, raising the dykes to cope with sea-level rise is an inefficient enterprise. Therefore, it is time to look at alternative ways of protecting the coast.Those alternatives might be found in ecosystem-based coastal protection. There are many different forms of ecosystem-based coastal protection possible, depending on the specific circumstances. One of the more common ways is the creation of wetlands.It is known that wetlands can mitigate waves and reduce storm surges. However, little is known about the role of these ecosystems on a dyke breach. The goal of this project was to get new insights into this matter. This could have been done using computer models. However, computer models are inefficient and not very convincing to the general public. Therefore, the different techniques will be tested using physical models in a flume setup. The creation of this flume was the sub-goal of this project. The focus was put on breach dynamics. The setup consists of a water tower, a test tank, and a series of overflow tanks. Because the sea is massively big in reality, the water level will hardly drop when there is a dyke breach. To simulate this the water will flow from the water tower to the test tank through floater switches. Once the dyke is breached the water will flow into the series of overflow tanks, placed below the test tank. The speed at which these tanks fill is used to determine the flow of the water. This is done by pressure sensors. The experiments will be filmed for analysis and communication purpose. To get to the ideal top view a metal and wooden framework was made. A colourant was added to the water to make the depth of the water clear.The height scale of the dykes and ecosystem was 1:100, the width scale was 1:200. Both the seafloor as the polder was assumed to be at a height of -2m. The first test was one with just the dyke. This is the baseline and was used to finetune the system. The next experiments were done with wetlands in front of it. Tests were done for both different heights as lengths of the wetland. This was done by using trespa plates and was done both with and without vegetation. If vegetation is not included there does not seem to be a significant difference between marshes of different lengths. There is a major difference when it comes down to the age (height) of the marsh. When vegetation is included there is a clear difference between the short and long marsh in the discharge. When looking at the velocity of the water through the breach one can only conclude that more research is required.The key conclusion is that wetlands have a positive impact on a dyke breach. Both the height as the length of the marsh seems to be relevant. In this setup, a high, long and vegetated marsh lead to a reduction of discharge of 30% compared to no marsh. In reality, this could save lives in the case of a dyke breachAn important observation was the hydraulic jump. Its location might say something about where most of the erosion would take place in real life. More research is required on this topic.The scaling on the forelands was given only limited attention during this research. It is recommended that for further research the scaling is taken into account very seriously. Most practical issues were solved along the way. Some of them are however still unresolved. It is recommended that a close look at these issues is taken before doing more experiments.