Magnitude estimation of a landslide-triggered debris flow in the Serra do Mar Mountain Range, Brazil

Debris flows represent great hazard to communities and infrastructures, since they move quickly and are very destructive. In Brazil, debris flows mainly occur in the Serra do Mar Mountain Range, where thousands of casualties were reported in the last two decades due to these phenomena. This study aims at estimating the magnitude of a debris-flow event that occurred in Serra do Mar on February 2017, at the Pedra Branca watershed in the State of Paraná. Debris-flow magnitude refers to the volume of material discharged during an event and is an important aspect of debris-flow hazard assessment. The Pedra Branca event was initiated by rainfall-triggered shallow landslides, damaging local oil pipelines and farms. The magnitude estimation is based on the combination of empirically based equations and the geomorphic features of the debris flow, acquired from in situ and aerial investigation. 28 cross-sections were made along the river channel, considering post-event channel width, erosion and accumulation depth, as well as depositional features. Sediment sources and accumulation areas were identified and delimitated based on high-resolution (1:500) aerial drone photographs. The results indicate that the landslides that initiated the event released approximately 26,884.5 m³ of sediments (V_i) into the main channel of Pedra Branca and that the volume eroded (V_e) and accumulated (V_d) along the channel are, respectively, 82,439 m³ and 22,012 m³. The estimated total solids volume (V_s) is 87,274 m³, assuming that V_s = V_i + V_e - V_d. Moreover, considering a solids concentration of 57% calculated according to empirically-based equations for Serra do Mar, the debris flow had a total magnitude of 153,113 m³. These estimations suggest that the February 2017 debris flow mobilised great volume of material and that 15% of the total volume accumulated on the channel bed, which can be remobilised by future events. Further research on debris-flow dynamics and recurrence at the Serra do Mar Mountain Range is recommended to mitigate future hazards.