WP4: Dimensions for Artificial Seagrass

1. Starting point

Observation of the environmental conditions on WP3 provides a starting point towards the development of the artificial seagrass (ASG). Existing seagrass meadows in the area of interest are important sources of information regarding the environmental conditions needed for the seagrasses to grow and survive. Additionally, the conditions on selected sites for restoration are going to be observed and recorded. Simultaneously, the ASG needs to be produced and tested; it must be able to deliver the sheltering and filtering capacities while also being biodegradable so that the restoration process can be completed without the need of structure extraction. Prototype manufacturing is the focus of WP5, on which material scientists take the lead.

2. Assignment

In order to provide the shelter and filtering capacity needed for the actual seagrass to grow, the ASG needs to be able to withstand hydrodynamic forces induced by waves and currents by itself. This means, a proper dimensioning must be developed to achieve enough energy dissipation (Figure 1).

Figure 1. Picture: Leibniz Universität Hannover

Furthermore, the ASG should be bundled with an appropriate base layer able to hold the artificial meadow, while at the same time providing a suitable habitat for the actual eelgrass to grow. An anchoring system strong enough to cope with hydrodynamic loadings will also be designed to keep the base layer in place. All of these points constitute the focus of WP4, whose aim is to use the information collected on WP3 to dimension the ASG, and subsequently work with WP5 to merge the ASG, the base layer and the anchoring system into a viable, ecologic and sustainable solution. Figure 2 shows all the aforementioned components together.

Figure 2: Base layer, anchoring system and artificial seagrass. Picture: Leibniz Universität Hannover

3. Accomplishment

To do so, experiments both on a wave flume channel as well as a flow circuit will be carried out to test the hydrodynamic bearing capacities of the ASG and base layer prototypes against waves and currents, respectively. Figures 3 and 4 show the facilities to be used during the execution of the experiments; figure 5 and 6 shows the experimental setup planned for these experiments. Seagrass meadows have an effect on both waves and currents.

Figures 3-6. Pictures: Leibniz Universität Hannover

Many studies have tried to determine this numerically and experimentally; however, the fluid-structure and structure-fluid interaction is still not perfectly understood. The experiments will thus assess both interactions to determine:

The energy dissipation capacities of ASG meadows while seeking affinity with Zostera marina. This will provide an idea of the ecosystem service provided by the seagrass with respect to coastal protection and mitigation.
The effect of waves and currents on the growth capacity of the plant regarding the correct settlement of seeds and the capacity to stay in place during growth. The ASG should ensure the survival of the seeds and the good growth of the plant.

Additionally, the base layer will be tested against the drag forces coming from both the flow and the drag from leaves. The anchoring systems proposed range from simple gravity systems consisting on heavy dead weights, to actual frictional anchors buried in the soil, as shown in Figure 1.

The success of the dimensioning process will require a close cooperation between the coastal engineers and the material scientists. Following the framework of this project, a 2-way information-sharing path – based on constant feedback between partners – is the methodology chosen to obtain the best possible results: defined dimensions and density for an ASG meadow with a proper base layer and anchoring system able to not only resist the hydrodynamic forces, but also facilitate the growth of actual seagrass, and thus achieve the objective of restoration.