Debris flows are extremely rapid gravity-driven mass movements of saturated sediment in concentrations between 60% to 80% by volume that move along steep channels, eroding and entraining material, typically terminating in a fan-shaped deposit. Debris flows are responsible for causing innumerable deaths and extensive damage across the world. The mobilisation of rainfall-triggered landslides is the primary cause of such flows. Estimating the debris flow travel distance or runout is essential for managing this hazard.

The conventional approach to debris flow runout analysis idealises the triggering of landslides across the source area into an instantaneous event. When this idealisation is done, a key characteristic of debris flows, their tendency to propagate in surges or waves, is overlooked. This study analyses the effect of accounting for surging on runout estimation by spatially and time-resolving debris flow events. A prime candidate for such a study is the Mocoa Debris Flow of 2017, a tragedy that involved 273 shallow landslides mobilising into a debris flow, resulting in the death of more than 300 people and the devastation of local infrastructure, Sarmiento et al. (2019). First, the landslide inventory was analysed to assess the scale of the disaster. Then, a novel method to spatially resolve events based on stream orders is implemented, after which a runout analysis is performed for different spatially resolved scenarios using a depth-averaged numerical model. Next, the timing and distribution of landslides are assessed based on a four-day storm period using a process-based landslide susceptibility model. This assessment determines the relative volume of each debris flow surge. The surges are then incorporated into a time-resolved runout analysis. The results of both the spatially and time-resolved runout analyses are compared. We find a marginal difference in the estimate of runout based on critical performance criteria, such as area coverage ratio, in favour of the spatially resolved analysis.

This study concludes that incorporating the phenomenon of surging caused by non-simultaneous landslide events does not improve the forensic analysis of the Mocoa Debris Flow runout. One of the main limitations of this study is the absence of data measured during the event to confirm the extent of surging due to non-simultaneous landslides. A possible avenue for future research would be varying the period of rainfall that is considered by reevaluating initial groundwater conditions.