Resistance and reconfiguration of natural flexible submerged vegetation in hydrodynamic river modelling

Abstract

In-stream submerged macrophytes have a complex morphology and several species are not rigid, but are flexible and reconfigure along with the major flow direction to avoid potential damage at high stream velocities. However, in numerical hydrodynamic models, they are often simplified to rigid sticks. In this study hydraulic resistance of vegetation is represented by an adapted bottom friction coefficient and is calculated using an existing two layer formulation for which the input parameters were adjusted to account for (i) the temporary reconfiguration based on an empirical relationship between deflected vegetation height and upstream depth-averaged velocity, and (ii) the complex morphology of natural, flexible, submerged macrophytes. The main advantage of this approach is that it removes the need for calibration of the vegetation resistance coefficient. The calculated hydraulic roughness is an input of the hydrodynamic model Telemac 2D, this model simulates depth-averaged stream velocities in and around individual vegetation patches. Firstly, the model was successfully validated against observed data of a laboratory flume experiment with three macrophyte species at three discharges. Secondly, the effect of reconfiguration was tested by modelling an in situ field flume experiment with, and without, the inclusion of macrophyte reconfiguration. The inclusion of reconfiguration decreased the calculated hydraulic roughness which resulted in smaller spatial variations of simulated stream velocities, as compared to the model scenario without macrophyte reconfiguration. We discuss that including macrophyte reconfiguration in numerical models input, can have significant and extensive effects on the model results of hydrodynamic variables and associated ecological and geomorphological parameters.& Veerle Verschoren

Simulation study of anisotropic flow resistance of farmland vegetation

“…FathiMoghadam et al (2011) estimated the submerged vegetation roughness of flood plains with physical and numerical models. Verschoren et al (2016) developed a practical approach to quantify the resistance of submerged vegetation. It can be applied in 2D depth-averaged hydrodynamic models typically used in research.…”

Landscapes of facilitation: how self‐organized patchiness of aquatic macrophytes promotes diversity in streams

“…The effects of bed and vegetative roughness on flow velocity are represented by determining hydrodynamic roughness characteristics for each cover type separately using the Chézy coefficient, following the approach of Straatsma and Baptist () and Verschoren et al. ().…”

“…() to account for reconfiguration of flexible submerged macrophytes, to express vegetation resistance as:where C b is the Chézy roughness of the bed, g is acceleration due to gravity (9.81 m/s 2 ), D c is a species‐dependent drag coefficient, A w is the specific plant surface area (total wetted vertical surface area of the vegetation per unit horizontal surface area of the river (Sand‐Jensen , Verschoren et al. )), directly related to plant biomass P i , k v is the Von Kármàn constant (0.41), and H v is the deflected vegetation height (m). Deflected vegetation height varies as a function of incoming flow velocity, due to the high flexibility of submerged aquatic vegetation and reconfiguration at higher stream velocities (Sand‐Jensen , Schoelynck et al.…”

“…Following the approach of Verschoren et al. (), H v is calculated within each vegetated grid cell as the product of shoot length L (m) and the sine of the bending angle α (degrees), using an empirical relationship between bending angle and incoming current velocity based on flume experiments performed on single shoots of flexible aquatic macrophytes (Bal et al. ).…”

Model-Based Analysis of Macrophytes Role in the Flow Distribution in the Anastomosing River System

“…Riparian and aquatic vegetation, depending on their structure, density, coverage, etc. can affect river morphology in different ways, by altering the flow and subsequently the sediment balance through hydraulic resistance [4][5][6]. Numerous studies have described the empirical relationships between biological and physical characteristics in aquatic ecosystems [7,8]; nonetheless, more quantitative understanding, linking current knowledge with predictive models is required [9].…”