Modeling Overland Sediment Movement from an Extreme Hazardous Rainfall Event

An extreme rainfall event (the largest on record) in Columbia, SC occurred in a 24-hour period on October 4, 2015. A total of 21.49" of rainfall occurred in the Gill's Creek watershed during a 3-day period between October 3rd and 4th. This rainfall event resulted in numerous dam failures, riverine flooding, and large erosion events. The McCrady National Guard Training Base, located near Columbia, SC experienced substantial damages to the sand-clay roads throughout. Heavy rains, overland flow and stream flow caused substantial movements (sheet flows over 1-m in depth) of alluvium. To aid in base management for assessing road stability and accumulation areas airborne LiDAR data was acquired after the event and analyzed in a change detection study with LiDAR data prior to the event. The contributions of this research are in 1) the development of a change detection modeling approach for LiDAR data that incorporates the spatial variation in change/no-change thresholds and 2) application of the LiDAR-based approach to the sediment movement in an extreme rainfall event. The spatial variable threshold model was based on vegetative cover classes, flightline bias, LiDAR-point density, LiDAR scan angle, and topographic slope. Field-based data (301 observations) for calibrating and validating the model was in the form of high precision/accuracy GNSS RTK data, total station surveyed data, manual boreholes (16) of sediment depth, and field reconnaissance. The overall accuracy of the pre- and post-event LIDAR data were 18-cm (Accuracyz).