Journal Article

Turbulence and Bedload Transport in Submerged Vegetation Canopies

Zhao, Tian, Nepf, Heidi

2024

Overview

Using a constant channel velocity, U $U$, flume experiments investigated how canopy density (ah $ah$, with canopy frontal area per unit volume a $a$, and canopy height h $h$) and submergence ratio (H/h $H/h$, with H $H$ the flow depth) impacted near‐bed velocity, turbulence, and bedload transport within a submerged canopy of rigid model vegetation. For H/h $H/h$ < 2, the near‐bed turbulent kinetic energy (TKE) was predominantly stem‐generated. As ah $ah$ increased, both the near‐bed TKE and bedload transport rate (qs ${q}_{\\mathrm{s}}$) increased. For H/h $H/h$ > 2, the near‐bed TKE was insensitive to ah $ah$ and H/h $H/h$, because of a trade‐off between decreasing stem‐generated turbulence and increasing canopy‐shear‐generated turbulence, as ah $ah$ and H/h $H/h$ increased. However, the near‐bed velocity declined with increasing ah $ah$ and H/h $H/h$, such that, even with a constant TKE, qs ${q}_{\\mathrm{s}}$ 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 $S$) and flexible vegetation. For a constant energy slope, U $U$ increased as ah $ah$ decreased and as H/h $H/h$ increased, which in turn influenced the in‐canopy velocity and TKE. The highest qs ${q}_{\\mathrm{s}}$ occurred with the greatest H/h $H/h$ and smallest ah $ah$, corresponding to the highest U $U$ and greatest contribution of canopy‐shear‐generated turbulence, reflecting the importance of canopy‐shear‐generated turbulence in submerged canopies. The lowest qs ${q}_{\\mathrm{s}}$ occurred with smallest H/h $H/h$ and highest ah $ah$, corresponding to the smallest U $U$ and least contribution of canopy‐shear‐generated turbulence. Plain Language Summary By reducing current, aquatic plants provide many ecosystem services, including nutrient and carbon retention, mitigation of beach and riverbank erosion, and creation of habitat for aquatic organisms. In this study, measurements and modeling were used to define the range of conditions for which submerged vegetation can reduce sediment erosion, relative to unvegetated beds, and specifically reduce the rate at which sediment is carried along the bed (known as bedload transport). In the lab, the channel velocity was held constant, and two regimes were identified. When vegetation extended through more than half of the water depth (water depth/canopy height <2), bedload transport was enhanced compared to unvegetated conditions with the same depth and channel velocity. In contrast, when vegetation extended through less than half of the water depth (water depth/canopy height >2), bedload transport was reduced compared to unvegetated conditions. The lab experiments were used to develop a model to predict bedload transport under field conditions and flexible plants. The model demonstrated that submerged vegetation can diminish erosion, offering a useful guide for river and coastal restoration. Key Points For constant channel velocity, submerged canopies can enhance or reduce bedload transport, depending on their degree of submergence With increasing submergence, the source of near‐bed turbulence shifts from stem wake to the canopy shear layer at the canopy top Bedload transport was best described by a hybrid combination of mean and turbulent velocities

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Aquatic ecosystems / Aquatic habitats / Aquatic organisms / Aquatic plants / aquatic vegetation / Bank erosion / Beach erosion / Bed load / bedload / Canopies / Canopy / canopy height / canopy turbulence / Ecosystem services / Flumes / hydraulic flumes / Kinetic energy / Nutrient retention / Plant cover / River banks / River erosion / Riverbanks / Sediment / Sediment transport / Shear / Stems / submerged aquatic plants / Submerged plants / Submergence / Transport rate / Turbulence / Turbulent flow / Turbulent kinetic energy / Vegetation / Velocity / water / Water depth