Measuring the Discharge of River Flood Using Witnesses Movies Found on the Internet
Abstract:The knowledge of the discharge of river during extreme flood is of prime importance for the scientific and the research community. Unfortunately, measuring discharge using conventional methods is impossible because the high velocities and the floating debris endanger the operators and the equipment. The typical time-scale for gauging does not match up to the time scale of the dynamic of extreme flood. Finally, floods are mesoscale events that affect generally several watersheds at the same time, and gauging teams do not have the capabilities for covering the whole region of interest.
Recently, non intrusive method for measuring discharge have been developed and tested in flood conditions. Doppler surface velocity radar and Large Scale Particle Image Velocimetry (LSPIV) showed their efficiency for measuring discharge during extreme events, but those methods need to be deployed by operators and the problems of time-scale and space-scale covering aforementioned are not solved.
In this study, authors present how flood discharge measurement can benefits from the huge development, the last 10 years, of internet and of the on-line sharing of files. Floods are impressive phenomena, and hundreds of witnesses movies can be found on the internet after every important event. The different steps in order to apply LSPIV analysis to witness movie are detailed: (i) selection of the video of interest; (ii) contact with the author of the video; (iii) preparing the video for the LSPIV analysis : stabilization of the images, field campaigns; (iv) LSPIV analysis, providing surface velocity field; and (v)discharge computation.
A case study on the major flash flood of 18 June 2013 of the Gave River at Cauterets, French Pyrennees, is presented. Results show that witnesses movies can bring useful information and allow estimating discharges values. Capabilities and limitations of LSPIV applied to witnesses movies are detailed. Finally, the paper presents an approach conducted within the framework of the FloodScale project in order to involve population in the recording of LSPIV compatible video during flood. A video recording protocol was given to town councils of villages located close to the Ardèche River, France, itemizing different conditions required to get quantitative information from movies.