During the night of the 21 August 2015, a debris flow took place in one of the presently most active ravines of the Pyrenees, the Portainé torrent (Eastern Pyrenees, Spain), and caused considerable damage to the road. Detailed information was gathered from the debris flow monitoring system recently installed in the torrent and field surveys. The monitoring system measures ground vibration at geophones and flow height at an ultrasonic sensor. Meteorological data showed that the debris flow was triggered by a convective rainstorm with a total rainfall amount of 29 mm. All these data provide information on the debris flow occurrence and dynamics. The analysis of the data showed that the debris flow had three different surges and mobilised a total volume of about 2130 m3. The front velocity in the monitoring reach was somewhat small (about 2 m/s) and the peak flow discharge comparatively low (about 13 m3/s). In addition, the debris flow was back-analysed by a numerical model calibrated by the observed event. The results of the simulation showed that a Voellmy fluid rheological model can represent considerably well the recorded and observed measurements, and the best-fit values were µ = 0.28 and C = 8 m0.5/s.