Assessment of the application of permeable pile groins as coastal protection

Assessment of the application of permeable pile groins as coastal protection

Briele, A.C.

Stive, M.J.F. (mentor)
Uijttewaal, W.S.J. (mentor)
Zijlema, M. (mentor)
Dusseljee, D. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2014-12-17

Permeable wooden pile groins have a long tradition as being part of the coastal protection in the Netherlands, as well as in Germany and the United Kingdom. They are built to protect beaches from eroding by reducing the current velocity along the beach and thus its ability to transport beach material. This type of groin is most popular among the population; in Zeeland (NL), they are incorporated into the national heritage. In scientific research however, they lack attention. More focus is set on beach processes themselves and on the (supposedly) more stable impermeable rubble mound groins. As several researches stress their potential as soft methods and their advantages over impermeable groins, this thesis intends to put their usage more into focus. By means of generic numerical research and a comparison to available laboratory data and analytical approaches, the understanding of this type of groins is enhanced. This thesis explores the usage of permeable wooden pile groins in coastal waters under the influence of current and wave forcing. Both impermeable and permeable groins are able to reduce the sediment transport capacity and yield sediment accumulation. Impermeable groin are researched on reasonable well, as they are supposed to be more stable and their effects easier predictable. Important disadvantages are also originating from their impermeable structure: they deflect the approaching current causing strong offshore directed flows and large scale eddies between neighbouring groins. The paths of the sediment transportation will lead to a zig-zag profile of the shoreline, eventually causing erosion landward of the original shoreline and outflanking of the groin. Permeable groins reduce the original current velocity originating from the tide or breaking waves while not changing the general flow field. As a consequence, the sediment transport capacity reduces and dangerous rip-currents are prevented to a great extent. The thesis provides insight into the theoretical behaviour of the flow field around groins based on available laboratory experiments and theoretical considerations. This is enriched by a numerical research. For the forcing by a current, an analytical model is developed which provides the global change in the water level upstream and downstream of a stream. Further, the change in the velocity behind the groin and next to the groin can be obtained. In numerical models available for hydraulic engineering and research, the direct implementation of permeable pile groins is not explicitly provided. For most large scale hydrodynamic and morphology models the flow around a groin is on a too small spatial scale to be of interest compared to the whole flow domain. However, there are tools available to approximate the effects of permeable groins. The effect of the groins on the hydrodynamics is evaluated by means of the numerical model SWASH (Simulating WAves til SHore). This relatively new model is developed with the intention to reproduce both small and large scale flows in shallow water. In SWASH, two possible tools are available of which one on basis of a Morison equation proved to be suitable for the representation of groins on a laboratory scale. The numerical research evaluates the effect of permeable pile groins on the flow field when subject to a forcing by either a constant current originating from the tide, or the like, or from breaking waves. It is evaluated that many effects caused by the presence of groins on the flow field can be reproduced on a flat and an inclined (beach) bottom, both under a current or a wave forcing. For the wave forcing at a beach profile, a more detailed analysis is done on the effect on the flow field and bottom shear stresses under variations in the physical parameters. This study gives the range of boundaries under which permeable groins are applicable and their consequent effect on the flow field.

groin
groyne
permeable

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Student theses

master thesis

(c) 2014 Briele, A.C.