Insights on the growth and mobility of debris flows from repeat high-resolution lidar

How debris flows erode and deposit material along their paths is difficult to determine in natural settings due to the lack of warning and the rapid pace at which they occur. Post-event field measurements or controlled flume experiments are commonly used to evaluate debris flows between the head and the distalmost deposit. Increasingly available high-resolution lidar data provide another means to evaluate the entrainment and deposition of material during a debris flow. This study utilizes submeter lidar before and after a debris flow event in Polk County, North Carolina, to evaluate for volumetric growth and decay of 54 rainfall-triggered debris flows that occurred during a convective storm on May 18, 2018. Debris flow evolution can be characterized by three discrete phases when viewed according to a volume-distance plot: (1) the initiating debris slide, (2) the entrainment phase, and (3) the depositional phase. The rate of debris flow growth is highest during the first phase, nearly linear during the second, and negative during the third phase. When normalized by distance along the flow, the growth rate decays according to a power law E=aX(b) (r(2)>= 0.97). This new power law relationship may indicate differences in initiating landslide process and should be leveraged in future runout modeling studies.