Plant stiffness and biomass as drivers for drag forces under extreme wave loading: A flume study on mimics

Abstract

Moving water exerts drag forces on vegetation. The susceptibility of vegetation to bending and breakage determines its flow resistance, and chances of survival, under hydrodynamic loading. To evaluate the role of individual vegetation parameters in this water-vegetation interaction, we conducted drag force measurements under a wide range of wave loadings in a large wave flume. Artificial vegetation elements were used to manipulate stiffness, frontal area in still water and material volume as a proxy for biomass. The aim was to compare: (i) identical volume but different still frontal 2 area, (ii) identical stiffness but different still frontal area, and (iii) identical still frontal area but different volume. Comparison of mimic arrangements showed that stiffness and the dynamic frontal area (i.e., frontal area resulting from bending which depends on stiffness and hydrodynamic forcing) determines drag forces. Only at low orbital-flow velocities did the still frontal area dominate the force-velocity relationship and it is hypothesised that no mimic bending took place under these conditions. Mimic arrangements with identical stiffness but different overall material volume and still frontal area showed that forces do not increase linearly with increasing material volume and it is proposed that short distances between mimics cause their interaction and result in additional drag forces. A model, based on effective leaf length and characteristic plant width developed for unidirectional flow, performed well for the force time series under both regular and irregular waves. However, its uncertainty increased with increasing interaction of neighbouring mimics.

Livestock grazing alters multiple ecosystem properties and services in salt marshes: a meta‐analysis

“…; Paul et al . ). Unsurprisingly, AGB, canopy height and cover were reduced in the presence of livestock, with a general trend of stronger effects at higher stocking density or duration of grazing (Figs b and ) and within forb‐dominated plots (Fig.…”

“…Vegetated coastal regions reduce wave energy more effectively than bare mudflats (M€ oller et al 1999;Shepard, Crain & Beck 2011), with taller, denser vegetation being most effective (M€ oller et al 2014;Paul et al 2016). Unsurprisingly, AGB, canopy height and cover were reduced in the presence of livestock, with a general trend of stronger effects at higher stocking density or duration of grazing (Figs 3b and S3) and within forb-dominated plots (Fig.…”

Wave Attenuation by Spartina Saltmarshes in the Chesapeake Bay Under Storm Surge Conditions

“…More flexible stems might be bent by high orbital velocities, extending them in the flow direction, and therefore reducing the actual length that is affecting the flow (Losada et al, ). On the other hand, the S. alterniflora stiffness is much higher in comparison to the stiffness of those more flexible species, and it might result in neglecting the bending angle under these hydrodynamic conditions (Paul et al, ). As stated by Maza et al (), the stems of this cordgrass act like a cantilever, and therefore, they do not align with the flow and have larger impact on the waves.…”

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

“…Wave energy dissipation by vegetation (hereafter referred as WDV ) is affected by incident wave height ( H , Méndez and Losada, 2004;Bradley and Houser, 2009 ), wave period ( T , Augustin et al, 2009;Suzuki et al, 2012 ), the ratio of water depth to vegetation height in the water ( h/h v , Ysebaert et al, 2011;Yang et al, 2012 ), drag coefficient ( C D , Henry et al, 2015;Losada et al, 2016a,b ), stiffness ( Bouma et al, 2005;Luhar et al, 2017;Paul et al, 2016 ) and stem frontal area of plants per unit height (i.e. N * b v , N is the number of stems per unit area and b v is the stem diameter, Augustin et al, 2009;Fonseca and Cahalan, 1992;Nepf, 2012Nepf, , 1999Ozeren et al, 2014 ).…”