ARATTANO, M., GRATTONI, P.,, 2000, Using a fixed video camera to measure debris-flow surface velocity, Second International Conference on Debris-flow Hazard Mitigation_ Mechanics, Prediction, and Assessment, pp. 273–281, Taipei, Taiwan, 16-18 Agosto, 2000,
URL: http://www.cnr.it/prodotto/i/249666

In hydraulics several methods have been developed and used to measure flow velocity and discharge in natural channels. Velocity measurements are made through the use of current meters, through methods that appeal to the principle of critical flow (weirs, Venturi flumes, Parshall flumes etc.), that make use of chemical tracers, that resort to floating objects thrown in the channel or that are based on the surveying of the tracks left in the channel after the occurrence of a flood event (slope area method, super elevation formula). Debris flows cannot be easily measured through all these classic techniques. Debris flow fronts contain large boulders and their tails, which are certainly more fluid, remain charged with pebble-sized fragments. This impedes the use of current meters, weirs and flumes that would be damaged or destroyed. Moreover the sudden, often unpredictable occurrence of debris flows makes it very difficult for operators to be present at the moment of their occurrence. This impedes also the use of chemical tracers and other techniques that require an operator on the scene of the event when it occurs, such as throwing floating objects in the channel and measure their velocity. The difficulties in performing good rheological measurements of these flows makes also uncertain the use of methods based on the surveying of the tracks left after the occurrence of the events. Only remote sensors are therefore suitable for measurements of debris flows velocity (Itakura and Suwa, 1989). A pair of ultrasonic sensors placed at a known distance along the torrent have been used as a method to obtain mean front velocity of debris flows. Spatial filter velocimetry and electromagnetic Doppler speedometers have been used for continuous measurements of surface velocity. Since the installation of fixed video cameras has become more and more common throughout the world to obtain visual information on debris flows, the development of image processing techniques may provide another, cheaper solution to the problem of measuring superficial velocities of the flowing mass of a debris flow. Videos of debris flows, shot automatically through these fixed video cameras, are in fact relatively easy to obtain and not too expensive. In 1995 a debris flow prone torrent on the Eastern Italian Alps has been instrumented with a fixed video camera shooting the channel slantwise. This video-camera is triggered by an ultrasonic sensor installed 150 m upstream. During the 1996 summer two debris flows occurred in this torrent that were both shot by this video camera. Topographical surveys were subsequently carried out to provide the data for the calibration of the camera and of the scene, to be used in the processing of the recorded images. A simple method, based on the direct computation of the mapping between 2D image points and points in the 3D space, has been developed to perform measurements of surface velocity. This method, that was preferred to a more complex geometric calibration, has provided results accurate enough, as it has been possible to show comparing measurements of characteristic features of the debris flows with measurements of the same features performed through the use of a pair of ultrasonic sensors installed along the monitored reach. The description of the image processing techniques employed will be presented in this paper together with the results of the measurements performed.

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