Floods are one of the most common natural disasters causing many casualties and significant economic damage around the world. Hurricane Florence and Typhoon Mangkhut in 2018 were only the last reminder of the disruption caused by the catastrophic floods. Despite significant safeguards to reduce fluvial flooding risk, further efforts are still needed. This study is part of the NABRAPOL (NEBARSKA POLYMER) project, which aims to improve the knowledge of the drag reduction effect on free surface flows, a new protection technique used to reduce the rivers flooding risk. This article presents a state of the art in terms of fluvial flow modeling in the presence of polymers to strengthen flood risk management. A modeling approach for flows mixed with polymers is described. A case study on the watercourse modeling containing a flow without and with polymers is presented. The modelling results show that the addition of polymers to the flows watercourse results in a marked drag reduction by decreasing the water depth up to 18% of its initial depth. This technique can be considered as an efficient new method for strengthening non-structural protection measures against river floods risk.

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.