Velocity and Drag Evolution From the Leading Edge of a Model Mangrove Forest

Abstract

An experimental study of unidirectional flow through a model mangrove forest measured both velocity and forces on individual trees. The individual trees were 1/12th scale models of mature Rhizophora, including 24 prop roots distributed in a three‐dimensional layout. Thirty‐two model trees were distributed in a staggered array producing a 2.5 m long forest. The velocity evolved from a boundary layer profile at the forest leading edge to a vertical profile determined by the vertical distribution of frontal area, with significantly higher velocity above the prop roots. Fully developed conditions were reached at the fifth tree row from the leading edge. Within the root zone the velocity was reduced by up to 50% and the TKE was increased by as much as fivefold, relative to the upstream conditions. TKE in the root zone was mainly produced by root and trunk wakes, and it agreed in magnitude with the estimation obtained using the Tanino and Nepf (2008) formulation. Maximum TKE occurred at the top of the roots, where a strong shear region was associated with the change in frontal area. The drag measured on individual trees decreased from the leading edge and reached a constant value at the fifth row and beyond, i.e., in the fully developed region. The drag exhibited a quadratic dependence on velocity, which justified the definition of a quadratic drag coefficient. Once the correct drag length‐scale was defined, the measured drag coefficients collapsed to a single function of Reynolds number.

Turbulence Within Natural Mangrove Pneumatophore Canopies

“…In general, velocity profiles, and hence turbulence, are strongly dependent both on the submergence depth and the nonuniformity of the canopy (Horstman et al, ; Liu et al, ). Recently, two laboratory studies investigated turbulence within modeled mangrove forests (Maza et al, ; X. Zhang et al, ). Both studies focused on Rhizophora mangroves with dense prop root structures, whose geometry differs substantially from the pneumatophores of the predominant Sonneratia mangroves considered in this paper.…”

“…Both studies focused on Rhizophora mangroves with dense prop root structures, whose geometry differs substantially from the pneumatophores of the predominant Sonneratia mangroves considered in this paper. Maza et al () and X. Zhang et al () observed two distinct zones of enhanced TKE: one near‐bed zone generated by stem and trunk wakes and another zone formed by velocity shear that was generated by the sharp decrease in vegetation density at the top of the prop roots.…”

“…In general, velocity profiles, and hence turbulence, are strongly dependent both on the submergence depth and the nonuniformity of the canopy (Horstman et al, 2018;Liu et al, 2010). Recently, two laboratory studies investigated turbulence within modeled mangrove forests (Maza et al, 2017;X. Zhang et al, 2015).…”

Applying a New Force–Velocity Synchronizing Algorithm to Derive Drag Coefficients of Rigid Vegetation in Oscillatory Flows

“…This technical note provides a practical set-up to derive both time-varying and period-averaged vegetation drag coefficients (C D ) following the direct measuring method [36][37][38]. Different from previous studies, standard force sensors are applied to compose four synchronized force-velocity measuring systems in the current experiment.…”

“…It derives C D by calibrating its values to obtain the best fit between modelled and measured wave height evolution over vegetation fields. The direct measurement method is a new method, which has been developed since the 2010s [36][37][38]. This method directly applies the Morrison equation and measured in-phase force and velocity data to determine C D .…”