Wave transmission and drag coefficients through dense cylinder arrays: Implications for designing structures for mangrove restoration

A Gijón Mancheño,W Jansen,W.S.J Uijttewaal,A.J.H.M Reniers,A.A Van Rooijen,T Suzuki,V Etminan,J.C Winterwerp

doi:10.1016/j.ecoleng.2021.106231

A Gijón Mancheño, W Jansen + Show 6 more

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https://doi.org/10.1016/j.ecoleng.2021.106231

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Abstract

Mangrove vegetation constitutes a natural coastal defence against waves and erosion. Despite their protective role, mangrove ecosystems have experienced continuous degradation over the last decades due to human causes. At retreating mangrove coastlines, bamboo structures are built to create new habitat for mangrove colonization. Existing structures have experienced mixed rates of success due to the lack of a scientific basis in their design. Optimizing future structure designs requires investigating the effect of the bamboo poles on waves. We consequently conducted laboratory experiments to measure wave transformation, hydrodynamic forces, and flow velocities inside cylinder arrays, mimicking bamboo poles, with varying cylinder configurations and orientations. The experiments provided relationships for wave transmission, wave reflection, and the drag coefficients for configurations with volumetric porosities between n = 0.64 − 0.9. Configurations with a small lateral spacing (causing higher blockage) and a relatively longer streamwise spacing (causing less sheltering) exhibit larger forces and dissipation per element. Such arrangements enable optimizing wave dissipation at locations where the wave direction has low variability over the year. Placing the poles horizontally instead of vertically increases the forces and wave dissipation per element in relatively deeper water. Based on the experiments, we developed a conceptual analytical model that predicts wave reflection and dissipation through cylinder arrays, including blockage and sheltering. The model can reproduce the influence of cylinder arrangement on wave transformation, and it suggests that accurate predictions of sheltering and wave reflection are important to find optimal designs. Overall, these results provide useful insights on how to model and optimize the design of structures for mangrove restoration.

Highlights

Mangrove ecosystems have gained considerable interest as a coastal protection tool during the last decades
The best agreement between predictions and measurements is obtained when using the empirical characteristic drag velocity to the power of three (Fig. 10, d). These results suggest that the bulk drag coefficients derived from wave transmission measurements and those derived from force measurements are related, but they are not directly exchangeable
The present study investigates wave transmission and hydrodynamic forces acting on groups of cylinders with volumetric porosities between n = 0.64 − 0.9

Summary

Introduction

Mangrove ecosystems have gained considerable interest as a coastal protection tool during the last decades. Their protective value results from their ability to reduce waves (Quartel et al, 2007; Bao, 2011) and to promote sediment accumulation (Van Santen et al, 2007). They can constitute an efficient coastal defence under climate change sce narios (Temmerman et al, 2013; Cuc et al, 2015). When sedi ment is transported landwards by the filling tide, the lower wave stirring behind the structures enhances sediment settlement, favoring coastline expansion and mangrove colonization

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