Comparison of blockage factors in modelling the resistance of channels containing submerged macrophytes

Abstract

The proportion of a river channel containing vegetation (termed the blockage factor) has historically been determined in three ways: cross-sectional, surface area and volumetric. The first two versions are two-dimensional measures. Meanwhile, the threedimensional volumetric version is biased towards deeper sections of a reach. A fourth version of the blockage factor is proposed that does not have such limitations: the multi-cross-sectional blockage factor. Between five and nine cross-sections were sampled to determine the four blockage-factor versions for 35 river sites containing the clumped submergent macrophyte, Ranunculus subgenus Batrachium (water-crowfoot). The ability of these four measures to act as predictors of vegetative channel resistance was then assessed.The vegetated proportion of individual cross-sections was poorly related to the vegetation resistance of a channel reach, primarily due to the high spatial variability of patch-forming macrophytes. The weighted median of all cross-sectional blockage factors measured at each site produced the strongest relationship with vegetation resistance, though this was not significantly better than the volumetric or surface area versions. The resistance model using the surface-area blockage factor gave a very high residual between predicted and calculated resistance for a mat-forming macrophyte species, and this model is unlikely to hold for conditions other than baseflow. Likewise, the volumetric version is not expected to hold for sites that have more depth variability than those measured in this study.

Does the canopy mixing layer model apply to highly flexible aquatic vegetation? Insights from numerical modelling

“…It acts as an additional source of channel resistance and has been shown to alter bulk flow velocities and conveyance [2][3][4], generate turbulence through coherent flow structures [5][6][7][8], modify sediment transport processes [9][10][11] and increase habitat diversity [12,13]. Therefore, a good process understanding of boundary layer flow through and around vegetation is central in predicting the functioning of the fluvial system.…”

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

“…Vegetation resistance can be quantified via empirical relationships or with Environ Fluid Mech hydrodynamic models. In the empirical approach, the overall resistance in a vegetated reach is determined via the relationship between biomass and resistance, [17,18] or between blockage factors and resistance [19]. However, it has been demonstrated that hydraulic resistance is also influenced by discharge; and several authors have investigated n-UR relationships [20][21][22][23].…”

Determining Characteristic Vegetation Areas by Terrestrial Laser Scanning for Floodplain Flow Modeling

“…Furthermore, the characteristic plant area over height needs to be correctly estimated to account for the impact of the vertical vegetation structure on the flow resistance for different stages of submersion [10]. To diminish calibration, recent studies on modeling vegetated flows have focused on physically-based parameterization of woody [7,9,11,12] and aquatic [13][14][15][16] vegetation. The area occupied by vegetation plays a key role in the estimates, and various area-based parameterizations are presented for different types of vegetation: the leaf area index (LAI) [9,17,18], the total area of the trees (Atot) [11], the cross-sectional blockage factor [13,15], and the frontal area per unit volume (AC/V) [19].…”