Impact of boat generated waves on near-bank flow in rivers
Abstract
Boat induced waves generate additional stress on banks and should be included in bank erosion predictive models. The objective of this study is to investigate the role of boat generated waves on near bank flow in a fluvial environment. The experiment was carried out in a sand bed river with lateral bars (Batiscan River, Québec, Canada). We used a longitudinal array of four bi-directional electromagnetic current meters (ECMs) perpendicular to the bank to measure mean and turbulent properties of the horizontal and vertical flow velocities. We also used an optical sensor to quantify water level fluctuations and two optical backscattering sensors (OBSs) to measure suspended sediment fluxes generated by the waves. The OBSs were set back to back, one facing the river bank (inner looking) and the other towards the river (outer looking). Measurements of the instantaneous velocity variable allowed us to estimate shear stress induced by wave generated oscillatory and turbulent flow. The waves were generated using a 16 ft long boat equipped with a 40hp motor. The effect of three different distances between the bank and sailing line (5, 10, 15 m) and of three different speeds of the vessel (7, 11 and 15 km/h) was evaluated. Five replications of each combination of distance and speed were realized for a total of 45 passages. We estimated cross-correlations between hydraulic and turbidity variables and performed spectral analyses in order to separate oscillatory and turbulent flow induced by boat generated waves. We also tested different methods to estimate shear stress in order to link flow and turbidity properties. While wave height, oscillatory flow strength and near-bed shear stress values increase with boat speed and decrease with distance of the boat from the shore, the role of turbulence is important with respect to the oscillatory movement. The suspended sediment response to the passage of the waves increased in intensity with turbulent shear stress and it was often unbalanced between the inner and outer looking OBS suggesting that the bank changed its morphology. This study provides quantitative relations between wave dynamics and plumes of suspended sediment that could help to develop mitigation measures in fluvial environments where vessel traffic represents a major issue for bank erosion and retreat.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMEP23B0807P
- Keywords:
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- 1825 HYDROLOGY / Geomorphology: fluvial;
- 1862 HYDROLOGY / Sediment transport