A practical method for forecasting rockfall ditch effectiveness deterioration

Rockfall ditch effectiveness is a common component in risk calculations for cut slopes. Therefore, understanding ditch effectiveness deterioration is vital for forecasting changes in slope risk. However, studies analyzing changes in ditch behavior over time are limited. In this study, we propose a practical method for forecasting ditch effectiveness deterioration based on a conceptual model. The model assumes that ditch effectiveness is zero once a talus pile of rockfall debris has formed to its angle of repose. Using initial ditch effectiveness estimates, rockfall frequency estimates, and assuming linear deterioration of ditch effectiveness as a function of volume in the talus pile, ditch effectiveness can be forecasted. To evaluate the conceptual model, rockfall trajectory numerical models were developed for different cut slope and ditch geometries. Ditch-filling was simulated by approximating talus pile geometry as a triangle. The talus pile angle was increased incrementally until the angle of repose was reached, and ditch effectiveness was recorded as proportion of rockfall retained. Numerical modeling results can be approximated with our conceptual model for steeper cut slopes (∼4V:1H), while the conceptual model is conservative relative to modeling results for shallower cut slopes; we attribute this to the transition from bouncing- to rolling-dominated rockfall trajectories.