Investigation of the bulk drag coefficient of nature-based breakwater by flow experiments

Abstract

Nature-based solutions for nurturing the coast, particularly mud and mangrove coasts, have gained significant attention and are emerging as a new trend in the context of coastal engineering. In this study, the bulk drag coefficients of a permeable structure, a brushwood fence, are investigated and assessed by Darcy-Forchheimer flow experiments. Two samples, assembled from a number of bamboo cylinders with two different densities (607 and 1209 cylinders/m²) and an average diameter of 0.02 m, are installed inside a square tube (26 cm × 26 cm). A steady flow discharge is forced through the samples, which naturally creates a hydraulic loss (a difference in water levels) between the upstream and downstream sides. The forces created by hydraulic gradient, a ratio of hydraulic loss to sample widths, are then investigated as the drag forces, in which the bulk drag coefficients (C¯d) plays a vital role in this study. The findings from this investigation conclude that the bulk drag coefficients depend on the flow stages (represented by Reynolds number, R_e) and reach significantly high values at low R_e but become stable at high R_e. Our findings of C¯d indicate to have a similar trend to previous studies, that is, 2.0 for R_e > 2000 (turbulent flow) for cases with a lower density of 607 cylinders/m², while C¯d is greater than 2.0 for R_e < 2000 (laminar flow).