Experimental Assessment of the Flow Resistance of Coastal Wooden Fences

Hoang Dao,Tri Mai,Bas Hofland,Marcel Stive

doi:10.3390/w12071910

Abstract

Wooden fences are applied as a nature-based solution to support mangrove restoration along mangrove coasts in general and the Mekong Delta coast in particular. The simple structure uses vertical bamboo poles as a frame to store horizontal bamboo and tree branches (brushwood). Fence resistance is quantitatively determined by the drag coefficient exerted by the fence material on the flow; however, the behaviour of drag is predictable only when the arrangement of the cylinders is homogeneous. Therefore, for more arbitrary arrangements, the Darcy–Forchheimer equations need to be considered. In this study, the law of fluid flow was applied by forcing a constant flow of water through the fence material and measuring the loss of hydraulic pressure over a fence thickness. Fences, mainly using bamboo sticks, were installed with model-scale and full-scale diameters applying two main arrangements, inhomogeneous and staggered. Our empirical findings led to several conclusions. The bulk drag coefficient ( C D ¯ ) is influenced by the flow regime represented by Reynolds number. The drag coefficient decreases with the increase of the porosity, which strongly depends on fence arrangements. Finally, the Forchheimer coefficients can be linked to the drag coefficient through a related porosity parameter at high turbulent conditions. The staggered arrangement is well-predicted by the Ergun-relations for the Darcy–Forchheimer coefficients when an inhomogeneous arrangement with equal porosity and diameter leads to a large drag and flow resistance.

Highlights

Brushwood fences have been applied as an alternative porous supportive structure for mangrove restoration along the Mekong Delta coast
The hydraulic gradient of the incompressible fluid over a fence thickness [13] is applied to obtain the drag force: 1 δp gρ δx where I is the pressure gradient generated from pressure difference, δp, over the fence thickness, δx (m)
The bamboo fence was installed inside a square tube with a cross-sectional area A (26 cm × 26 cm) that was h placed i inside the outer chamber (Figure 2)

Summary

Introduction

Brushwood fences have been applied as an alternative porous supportive structure for mangrove restoration along the Mekong Delta coast. By using mainly natural materials, i.e., bamboo and tree branches, fences are considered as a nature-based solution for the protection of shorelines and mangrove forests. This low-cost structure has become more convenient for application on the extremely gentle coast of the Mekong Delta, whereas solid structures were intensively expensive and technically challenging. Installed in front of the mangrove belt to dissipate wave-current energy, the wooden fences in Figure 1 are assembled with a frame and an inner part [1,2,3]. Even though previous studies observed significant wave energy reduction through field measurements [1,2,5] and simulation studies [6], none of the existing studies concludes that either the frame or the inner part played a

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Results

Discussion

Conclusion