Heavy rainfall triggered loess-mudstone landslide and subsequent debris flow in Tianshui, China

Continuous and heavy precipitation triggered a large loess landslide at the southern Dagou Village of Mapaoquan Town, Tianshui City, Gansu Province of China on July 21, 2013. The landslide debris rapidly turned into a debris flow that was deposited in the ravine mouth with the volume of 1.9 × 105 m3. Detailed field mapping, three-dimensional laser scanning, aerial photograph interpretation, and laboratory tests were carried out to study the formation and moving characteristics of the landslide and subsequent debris flow. The results showed that: 1) The peak flow velocity and peak discharge of the debris flow were estimated to be approximately 7.2 m/s and 730 m3/s, respectively. The velocity had a tendency to first increase and then decrease from the head to entrance in the Dagou gully. 2) The analysis of rainfall conditions showed that the effective antecedent rainfall within 7 days and hourly rainfall intensity which triggered the slide-debris flows was 239 mm and 20 mm/h, respectively. Compared with the critical rainfall in this area, the effective antecedent rainfall was found to be more significant in triggering this event. 3) The mean and effective particle sizes were approximately 0.73-1.3 mm and 0.036-0.087 mm, respectively, and had the same distribution along the gully as the velocity, which confirmed the process of the variation in the flow velocity. 4) The scale amplification was very obvious in the formation and moving process of the slide-debris flows.