Elements of passive geomorphic flow control in rivers*

Geomorphodynamically active rivers evolve in space and time due to continuous erosion and deposition processes that mark the interactions among the turbulent flow, the migrating bedforms and the wall shear stress transporting the sediments. In uniform conditions, various bedforms result from instabilities amplifying small disturbances in the bathymetry depending on the grain size and hydraulic characteristics. Forced response however can be obtained imposing spatial discontinuities or asymmetries in the flow. We explored experimentally how asymmetric submerged porous plates as well as hydrokinetic turbine arrays, or yawed turbines lead to a local time averaged morphodynamic effect triggering a forced response far from the disturbance, and thus non-local in nature. Observations in different scaled flumes suggest that i) the spatial distribution of scours and deposits may be imposed in the average bathymetry; ii) the interaction between energy harvesting devices and river morphodynamics can lead to a positive impact, facilitating stream naturalization by modulating the shear stress distribution in the channel; iii) the design and siting of energy harvesting devices can be envisioned to target power efficiency and bank protection.