New rulers for estimating the magnitude of catastrophic debris flows

On July 25, 2020, at 05:25, a catastrophic debris flow occurred in the Keke gully, southwest China, causing a loss of ~ $5,000,000. To provide a basis for research and check-dam construction, accurately estimate debris-flow magnitude is essential. Field investigation revealed that the debris flow left massive boulders and continuous mud traces in the narrow gully. However,  efficiently extracting data from these boulders and mud traces and translating it into the magnitude of debris flows remains challenging. Here, we present an executable framework called the "ruler" to estimate debris-flow magnitude, utilizing DJ Magic4, AgiSoft, Rhino, ArcGIS, and Pycharm. Extracting 408 boulders and 3582 sections from a digital orthophoto map and digital elevation model of a 7-km gully. Establishing a boulder-based velocity and cross-section-based discharge ruler based on the modified manning's formula and empirical hydrodynamic formula. This ruler was cross-referenced and validated against traditional field investigation methods. The estimation results show that the Keke gully debris flow was a significantly large and dilute debris-flow event; the peak discharge was ~ 842 m³/s, the total volume was ~ 3,440,000 m³, and the total volume of the deposit was ~ 127,000 m³. Our rulers addressed the lack of debris-flow magnitude data records, particularly in areas where accumulation fans are not obvious or observations are limited. The intelligent historical debris-flow magnitude estimation proposed in this study allows for more efficient debris-flow prevention and control, thereby reducing the damage caused by such events.