Rapid assessment of urban mega-gully and landslide events with Structure-from-Motion techniques validates link to water resources infrastructure failures

Abstract. Mega-gullies and landslides pose significant hazards to urban development on steep terrain. Water resources infrastructure failures (WRIFs), such as leaks and breaks in water supply pipes, have been postulated as a trigger of mass movement events but data for validation has been challenging to acquire since earthwork proceeds quickly after events to repair roads and other infrastructure. Urban development in Tijuana, Mexico was monitored for a five-year period to document the occurrence of mega-gullies and landslides, including sediment volumes. A rapid assessment approach was developed based on photogrammetric observations from an unmanned aerial vehicle (UAV) and Structure from Motion (SfM) digital processing. Three hazardous mass-movement events were observed including two mega-gullies and one landslide. Furthermore, all three events were linked to WRIFs. Frequency analysis points to the annual probability of a WRIF-based erosion event in the range of 40–60 %, which is far higher than design levels typically used for urban stormwater infrastructure (5–10 %). Additionally, sediment modelling points to WRIF-based erosion as a non-negligible contributor to sediment generation. These results suggest that WRIFs are a significant contributor to erosion hazards facing urban development on steep terrain, and call for expanded monitoring to characterize the occurrence and modes of WRIF-based erosion events.