Development of Calibrated Two-Dimensional Runout Maps at a Columnar Jointed Basalt Rockfall-Prone Site

ABSTRACT: Whitney Terrace, located in southeastern Boise, ID, has experienced rockfall from a columnar jointed basalt cliff. This study presents the procedure used to produce two-dimensional calibrated runout maps. Models were calibrated using the runout distances of 75 existing rockfall boulders. Lumped Mass calibrations yielded simulation results that matched the mean runout distances observed at the site, but did not provide the proper distribution. Rigid Body calibration also matched mean runout distances, but there was no differential in runout between boulder size groups. Rigid Body calibration with forest damping zones produced the most realistic calibrations. The models were considered calibrated when the location of the calibrated boulder was within one-half the standard deviation of the mean runout distance of the furthest group of simulated rocks. Depending on the slope profile, final normal and tangential restitution coefficients varied between 0.4 and 0.55, and 0.72 and 0.98, respectively. The runout maps showed that 50- and 100-kg rocks experienced less runout because of vegetation. It is theorized that steeper slopes and block movement may limit the beneficial effects of vegetation in stopping falling rocks. Larger rocks, 800 and 1500 kg, could be a hazard to the existing subdivision on Whitney Terrace.