Deriving vegetation drag coefficients in combined wave-current flows by calibration and direct measurement methods

Abstract

Coastal vegetation is efficient in damping incident waves even in storm events, thus providing valuable protections to coastal communities. However, large uncertainties lie in determining vegetation drag coefficients (C D), which are directly related to the wave damping capacity of a certain vegetated area. One major uncertainty is related to the different methods used in deriving C D. Currently, two methods are available, i.e. the conventional calibration approach and the new direct measurement approach. Comparative studies of these two methods are lacking to reveal their respective strengths and reduce the uncertainty. Additional uncertainty stems from the dependence of C D on flow conditions (i.e. wave-only or wave-current) and indicative parameters, i.e. Reynolds number (Re) and Keulegan-Carpenter number (KC). Recent studies have obtained C D-Re relations for combined wave-current flows, whereas C D-KC relations in such flow condition remain unexplored. Thus, this study conducts a thorough comparison between two existing methods and explores the C D-KC relations in combined wave-current flows. By a unique revisiting procedure, we show that C D derived by the direct measurement approach have a better overall performance in reproducing both acting force and the resulting wave dissipation. Therefore, a generic C D-KC relation for both wave-only and wave-current flows is proposed using direct measurement approach. Finally, a detailed comparison of these two approaches are given. The comprehensive method comparison and the obtained new C D-KC relation may lead to improved understanding and modelling of wave-vegetation interaction.

Non-hydrostatic modeling of drag, inertia and porous effects in wave propagation over dense vegetation fields

“…Besides drag force, inertia force is the other component in the total force that acts on vegetation (Morison et al, 1950;Chen et al, 2018;Yao et al, 2018). Inertia force is commonly ignored in wave-vegetation modeling because the work done by the theoretical inertia force over one wave cycle is zero if linear wave theory is applied.…”

Analysis of coastal storm damage resistance in successional mangrove species

“…7; Roeber and Bricker 2015;Dasgupta et al 2019). Despite the low flow conditions, the tests are still informative about the drag coefficient, which is known to be reasonably constant when the Reynolds number is above 1000 (2500 in our test; Hu et al 2014;Chen et al 2018). However, higher flow conditions would likely result in strong branch realignment, reducing the frontal surface area and thus drag force on the branch (Vollsinger et al 2005).…”

“…Orbital velocities of 0.25 m s À1 for waves and currents combined with Reynolds number of about 2500). We measured current velocity at half of the water depth, which approximates the depth-averaged velocity (Hu et al 2014;Chen et al 2018). Following basic fluid dynamics (Morison et al 1950), a wave-averaged drag coefficient C D was derived from the peak drag measurements for each scenario:…”

Etudes expérimentales et numériques de la propagation des vagues à travers un réseau de cylindres verticaux en vue de la conception d'un système atténuateur de houle inspiré des racines de mangrove

“…Ce coefficient est dépendant de la nature de l'ouvrage, mais aussi des caractéristiques des vagues. Différentes formulations ont été proposées dans la littérature pour estimer ce CD en fonction de nombres adimensionnels de type Reynolds ou Keulegan-Carpenter (e.g., HU et al, 2014 ;OZEREN et al, 2014 ;LIU et al, 2015 ;LUHAR & NEPF, 2016 ;CHEN et al, 2018). Certaines d'entre elles ont été appliquées dans la présente étude pour évaluer la sensibilité des résultats à l'expression du CD.…”