Turbulence and Bedload Transport in Submerged Vegetation Canopies

Abstract

Using a constant channel velocity, $U$, flume experiments investigated how canopy density ($ah$, with canopy frontal area per unit volume $a$, and canopy height $h$) and submergence ratio ($H/h$, with $H$ the flow depth) impacted near-bed velocity, turbulence, and bedload transport within a submerged canopy of rigid model vegetation. For $H/h$ < 2, the near-bed turbulent kinetic energy (TKE) was predominantly stem-generated. As $ah$ increased, both the near-bed TKE and bedload transport rate ($q_{\mathrm{s}}$) increased. For $H/h$ > 2, the near-bed TKE was insensitive to $ah$ and $H/h$, because of a trade-off between decreasing stem-generated turbulence and increasing canopy-shear-generated turbulence, as $ah$ and $H/h$ increased. However, the near-bed velocity declined with increasing $ah$ and $H/h$, such that, even with a constant TKE, $q_{\mathrm{s}}$ also declined. These trends highlight that both TKE and velocity were important in controlling bedload transport. Models to predict velocity, TKE, and bedload transport were developed and validated with measurements. The models were then used to explore conditions more relevant to the field, specifically with constant energy slope ($S$) and flexible vegetation. For a constant energy slope, $U$ increased as $ah$ decreased and as $H/h$ increased, which in turn influenced the in-canopy velocity and TKE. The highest $q_{\mathrm{s}}$ occurred with the greatest $H/h$ and smallest $ah$, corresponding to the highest $U$ and greatest contribution of canopy-shear-generated turbulence, reflecting the importance of canopy-shear-generated turbulence in submerged canopies. The lowest $q_{\mathrm{s}}$ occurred with smallest $H/h$ and highest $ah$, corresponding to the smallest $U$ and least contribution of canopy-shear-generated turbulence.