Increasing Strickler coefficient by the drag reduction technique in open-channel flows

The drag reduction method by polymer additives is generally used to reduce friction losses in pipes over long-distances. The interactions between the polymer and the turbulent flow structures tend to change the velocity profile close to the walls by adding an elastic sub-layer between the viscous layer and the logarithmic zone of the boundary layer. This new sub-layer prevents the destabilization of the boundary layer. The gain in linear head losses can reach up to 80%, depending on the roughness of the walls and the concentration of the polymers. The application of this technique to sewer networks and the considerable effect on the reduction of water depth led us to study experimentally the drag reduction in an open- channel flows. Two measurement campaigns are performed on a laboratory flume for different discharges and several polymer concentrations. The surface flow velocities are measured by the LSPIV (Large Scale Particle Image Velocimetry) technique before and after the polymer injection. Backwater curves will be represented for smooth and rough flume walls. The addition of polymers even at low concentrations causes a sharp reduction in friction with a Strickler coefficient increased to 45% and 37% respectively for smooth and rough wall configurations and a decrease in water depth up to 17%. The drag reduction method by adding polymers seems to be a promising technique for limiting freshet levels and controlling river floods.