(a) Observed incision point of the top arc surface of the failure surface where debris flow occurs. It is the critical slip point at which ω reaches the FL as the soil absorbs water. The failure arc expands because the soil flow occurs slowly as ω reaches the FL. The sliding angle is the critical angle θ_FL corresponding to FL. (b) Side-slope failure due to rainfall at an open excavation site in Kampala, Uganda, in 2008. It is a site with horizontal slats plus a slope cut. Six workers were killed, and five workers were injured in the accident. At least ten people were buried (excerpt from news by Elle Decor Italia (Marelli, 2018))