https://orcid.org/0000-0001-5121-4197
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ABSTRACT
The paper reports the results of laboratory experiments to investigate the effect of vegetation patch mosaics on hydraulic resistance. Experiments were run for seven levels of vegetation coverage with square patches of flexible plastic grass in aligned and staggered configurations and a wide range of hydraulic conditions. Hydraulic resistance was substantially higher for staggered than aligned configurations, particularly for intermediate ranges of vegetation coverage. The results indicate that hydraulic resistance differs between regimes of isolated roughness flow, wake interference flow, and skimming flow. Two types of models are proposed to predict hydraulic resistance (i.e. Manning’s coefficient n) for aligned and staggered configurations, one as a function of the nondimensional spatially-averaged hydraulic radius and another as a function of relative submergence and surface area blockage factor. To account for the effects of vegetation patch alignment, an additional factor α is introduced. This work provides comprehensive datasets and models that can be used to improve the prediction of hydraulic resistance in open-channel flows with vegetation patches.
Acknowledgements
The authors gratefully acknowledge the assistance of Roy Gillanders and Benjamin Stratton for technical support during the experiments and two anonymous reviewers for their comments and suggestions. The authors also thank the following master’s students: Valentina Boscolo, Ross Kemlo, and Nico Bettio, for their help during the experiments.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Notation
| A | = | wetted area (m2) |
| BSA | = | surface area blockage factor (–) |
| Bx | = | cross-sectional blockage factor (–) |
| Bx,ave | = | flume-averaged cross-sectional blockage factor (–) |
| bx | = | length of a patch (m) |
| by | = | width of a patch (m) |
| B | = | flume width (m) |
| c1 | = | coefficient in multivariate regression model n = exp(c1BSA – c2) × (H/hv)c3 (–) |
| c2 | = | coefficient in multivariate regression model n = exp(c1BSA – c2) × (H/hv)c3 (–) |
| c3 | = | coefficient in multivariate regression model n = exp(c1BSA – c2) × (H/hv)c3 (–) |
| Fr | = | Froude number (–) |
| g | = | gravitational acceleration (m s−2) |
| k1 | = | exponent in the relationship n ∝ (Bx)k1 (–) |
| k2 | = | exponent in the relationship n ∝ (H/hv)k2 (–) |
| k3 | = | exponent in the relationship n ∝ (〈Rh〉/hv)k3 (–) |
| L | = | flume length (m) |
| hv | = | mean deflected height of a patch (m) |
| H | = | mean water depth (m) |
| m | = | proportion of unvegetated cross-sections along the channel (–) |
| n | = | Manning’s resistance coefficient (s m−1/3) |
| na | = | Manning’s resistance coefficient for aligned configuration (s m−1/3) |
| nb | = | Manning’s resistance coefficient for benchmark configuration (s m−1/3) |
| ns | = | Manning’s resistance coefficient for staggered configuration (s m−1/3) |
| Nt | = | total number of patches in the flume (–) |
| Nx | = | number of patches rows along the flume (–) |
| Ny | = | number of patches across a vegetated cross-section (–) |
| Ny,s | = | number of patch sides across a vegetated cross-section (–) |
| Q | = | mean flow rate (L s−1) |
| Re | = | Reynolds number (–) |
| Rh | = | hydraulic radius of unvegetated cross-section (m) |
| 〈Rh〉 | = | spatially-averaged hydraulic radius (m) |
| Rh-veg | = | hydraulic radius of vegetated cross-section (m) |
| S | = | flume bed slope (–) |
| u* | = | shear velocity (m s−1) |
| U | = | cross-sectional mean velocity (m s−1) |
| α | = | correction factor for Manning’s n (–) |
| Δx | = | patch spacing in the x direction (m) |
| Δy | = | patch spacing in the y direction (m) |
| ν | = | water kinematic viscosity (m2 s−1) |
| φ | = | porosity (–) |