The Effect of Biofilms on Turbulent Flow Over Permeable Beds

Despite an increasingly large body of work advancing our understanding of flow interactions occurring at the interface of a turbulent flow overlying a permeable bed, little is known concerning how such flow may be affected by the presence of biofilms, which exist in nearly all aquatic environments. This study quantifies the effects on flow exerted by biofilms grown over experimental laboratory permeable beds until biofilm detachment, and then compares this to the residual effects after its detachment. The investigation is conducted in a flow channel by immersing two‐dimensional permeable beds with idealized geometry and different porosities in order to explore different bed permeabilities. Sequences of increasingly higher flow velocity conditions, followed by lower flow, were considered to explore the effect of detachment. Measurements were performed using particle image velocimetry. The total wall shear stress and friction velocity were found to increase in the presence of pregrown biofilm, and decrease after biofilm detachment, when compared at the same pump frequency. The dimensionless Reynolds stresses, at constant pump frequency, collapsed for different bed configurations in the outer layer, while for the inner layer, the presence of biofilm led to a decrease in dimensionless Reynolds stress. Quadrant analysis shows that this decrease was primarily due to a reduction in strong Q2 contributions. These results suggest that models for flow and transport over permeable media in aquatic environments cannot neglect the role of biofilms in modifying turbulence. Key Points The effect of biofilm presence on turbulence in the free flow is due to a combination of reduced bed porosity and change in geometry and roughness Presence of biofilm increases Reynolds stresses in the outer layer, scaling with wall shear stress Presence of biofilm dampens dimensionless Reynolds stresses in the vicinity of the permeable bed