Experimental evaluation of flow-structure interaction with artificial seagrass: recommendations for restoration and field applications

Abstract

Coastal ecosystems are of extreme importance for the environment and human livelihoods based on their ecosystem services: they provide habitat for diverse fauna, supplying food, protection, and nursing grounds for a myriad of species; they provide a self-sustaining source of nourishment and income for the growing coastal communities; they sequester carbon more efficiently and in greater quantities than any land ecosystem; they trap sediments and reduce energy from waves, currents, and tides, thus protecting the coast and the inherent communities and habitats. Despite these services, coastal ecosystems have seen rapid decline over the past century, mostly due to the rapid human population expansion and the ensuing destructive infrastructure. One of the most affected ecosystems are seagrasses, which unfortunately do not receive the attention of more conspicuous ones such as coral reefs and mangrove forests. Nonetheless, recent research has shown that, although they only cover 0.2% of the ocean floor, seagrasses can sequester 10% of the ocean carbon (a.k.a. blue carbon), exceeding the rate of any land cover. Moreover, according to the United Nations (UN), half of the human population will be living near coastal areas by 2030, while already more than half of the population lives in urbanized areas, meaning more infrastructure, and hence more ecosystem destruction. This deadly trend has led to the disappearance of more than a third of the seagrass cover since the late 19th century.

To counter this, several restoration efforts have been initiated. Within this thesis, the emulation of seagrass ecosystem services is proposed and investigated as a restoration solution. Seagrasses facilitate their own growth by adapting to their surroundings, affecting the ambient hydrodynamics to promote their own survival. Reproducing this behavior through artificial structures in either virgin or former seagrass-covered habitats should then promote seagrass establishment and proliferation. In turn, the restored seagrass should provide these services itself, achieving a self sustaining habitat. Following this concept, a feasible solution was conceived: artificial seagrass (ASG) patches, comprising flexible shoots fixed to a likewise flexible natural geotextile. The ASG should be biodegradable so no harmful substances are introduced into the environment. Successful field deployment then requires lucid understanding of: a) the ecological provisions of seagrass occurrence and survival; b) the physical processes involved in flow-structure interaction; and c) the properties of biodegradable materials. While research on a suitable biodegradable yet flexible material for marine deployment ran parallel to this work, an interim solution was employed by testing plastics of different mechanical properties. These should serve as basis for the development of the final biodegradable product. ASG patches of different geometries were then tested in state-of-the-art, large-scale hydraulic facilities. The focus lied on the interaction between incident hydrodynamics and the submerged ASG,with the ecological provisions used as boundary and target conditions.

The experimental research presented here was subdivided into unidirectional and oscillatory flow experiments, analyzing the mechanical response of ASG and the consequent effect on incident hydrodynamics. The results showed that flow needs to be developed within a meadow before maximum flow attenuation is reached, whereby a minimum length of one meter proved to be enough to reach full development. Flow attenuation then caters for shelter for growing seagrass. Attenuation was shown to be higher under unidirectional flow, with more than 50% flow reduction reached, but it also depends largely on incident flow velocity. Wave heights and in-canopy flow were also readily dampened by ASG, while allowing for enough circulation for nutrient transport, which in turn can foster growth. Furthermore, a discretely anchored base layer was shown to be susceptible to loads depending on the wave propagation direction and number of anchors. The flexibility of the ASG and underlying mat makes generalization of these results through computer models rather complicated. Nonetheless, current accepted models can be used to gain insight into the response to similar flexible mats. For now, safety factors could be employed to design prototypes to be tested in the field.

Alongside the results presented here, plenty of research has shown how geometric properties of submerged elements affect flow, with rigidity likewise playing an important role. Flexibility has continued to be the underbelly, making generalization of the processes all the more complicated. Further, the lack of field experiments and prototypes means that there is no real paradigm on field deployment of submerged flexible structures, and no appropriate approach to model submerged flexible vegetation is available. This ultimately means that no comprehensive guidelines for restoration exist, as only a few approaches are somewhat developed. Hence, the results presented here are intended to: i) expand the current knowledge regarding flow-structure interaction with fully flexible submerged elements; ii) provide a prototype which can be deployed in future field projects; iii) build a bridge between science, ecology and engineering to provide a sound, interdisciplinary solution contributing to climate change mitigation; and iv) provide an overview of the state-of-the-art in green engineering for practitioners and stakeholders to form the basis for restoration guidelines towards the well-being of coastal communities around the world.