A drag reduction method by polymer additives was tested for the first time in a large scale open-channel watercourse. Ten and a half tons of a water soluble polymer were injected during 15 consecutive hours in the upstream section of an irrigation canal in steady state, leading to a 20 ppm concentration of polymer in the water. The evolution of the water depth was measured every 10 min for 18 h along ten sections further downstream, up to a distance of 26.3 km from the injection section. The water depth at all sections remained constant until the arrival of the polymer, at which time it strongly decreased, sometimes with a slight water-depth increase beforehand; the depth then remained constant as long as the polymer injection remained. A maximum water depth reduction of 26 cm (i.e., 17%) was measured at the first cross section (2 km downstream from injection). The water depth reduction decreased to 10% and 3% at 10 and 20 km downstream from the injection, respectively. However, further downstream, at a distance of 26.3 km, the water depth increased by 5%. This paper also discusses the environmental impacts of polymer injection through analysis of samples taken from the water and bed material before and during the experiments.
Urban agglomerations face the risk of overflowing rivers due to intense urbanization in flood-prone areas and the climate change effects. Despite the important protective measures deployed to reduce the fluvial flooding risk, additional efforts are still needed. This work aims to propose a new complementary non-structural protection measure, used to reduce the river flooding risk. The study is part of the NABRAPOL (NEBRASKA POLYMER) project, which aims to improve knowledge of the drag reduction effect by adding polymers in open-channel flows. The addition of polymers, even in limited concentrations, allows high friction to decrease with the typical Manning coefficient reduced up to 45%. An application case on a real watercourse is presented in this article. Two measurement campaigns are carried out on a river along 30 km. Experimental devices are deployed, and non-intrusive hydraulic measuring instruments are installed at the study field. Surface velocities are evaluated by the Large-Scale Particle Image Velocimetry (LSPIV) technique, and water depth is measured using ultrasonic radar sensors over the river. Measurement results show that the addition of 20 ppm of polymers in the flows results in a marked drag reduction by decreasing the water depth to 18% of its initial depth. The drag reduction technique by addition of small concentrations of polymers can be considered as a new and effective method to reinforce the measures already deployed in the flood risk management strategy since it allows the water depth to be decreased thus avoid overflowing rivers in the extreme flooding event.
Résumé :Cet article présente une technique de mesure de la vitesse en surface d'un écoulement fluvial pour en déduire le débit. La Technique LSPIV (Large Scale Particule Image Velocimetry) permet de mesurer les vitesses en surface de l'écoulement par analyse de séquence d'image. Les potentiels de cette technique sont présentés dans un cas d'application en laboratoire : Ecoulement entrant dans un modèle réduit d'une bouche de métro. Les résultats de mesure sont comparés à ceux produits par un calcul numérique effectué par le logiciel Fudaa-Reflux.
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