Hydrodynamics over Permeable Beds

Free-surface water flows are ubiquitous in nature. As greater river flood risk is expected because of climate change, widespread river monitoring becomes necessary. This could be achieved via remote sensing techniques based on the physical linkage between flow and free surface. Empirical relationships currently describe this linkage, limiting their applicability as changing the scales is not possible.

An experimental campaign aimed at deepening the understanding of the surface-flow linkage was conducted using simulated permeable armoured river deposits. The turbulent flow field was quantified using a 3D Particle Tracking Velocimetry system, whilst the dynamic free surface was studied using a 2D Digital Image Correlation system.
The free surface was found to be dominated by non-dispersive turbulence-induced features and gravity-capillary waves when the free-surface velocity is below the minimum phase velocity of gravity-capillary waves in still water. Conditions with surface velocities above this threshold see the water surface dominated by gravity-capillary waves having the same wavenumber of stationary waves and gravity capillary-waves with different wavenumbers.
In the body of the flow only advective turbulent features were observed below this limit. Above this threshold, most turbulent features advected with the surface velocity, however, features with frequency and scale of the observed gravity-capillary waves were also discovered close to the water surface.

Results for both the free surface and the flow field clearly indicate the existence of two flow regimes where the free-surface velocity represents the discriminating factor: when the free-surface velocity is below the minimum phase velocity of gravity-capillary flow and free surface are dominated by effects of turbulence; when the free-surface velocity is above the minimum phase velocity of gravity-capillary the free surface is governed by gravity-capillary waves and standing waves, while resonance phenomena and turbulence effects occur in the flow field.