Conservative estimates of forces from debris flows on barriers are computable using the 'free field approach', assuming that flows are channelised until impact. However, this approach neglects the reduced flow velocity and depth due to lateral spreading in basins, which depends on flow type. In this study, physical dam-break tests using (i) water and (ii) dry sand running out onto an unconfined planar surface are performed, and then used to evaluate rheological models coded into a depth-averaged SPH program. A numerical parametric study on material type, material volume, channel length (both affecting the Froude number Fr) and lateral confinement is then performed. For water, the spreading angle correlates with Fr and gives a conservative solution for impact force and spreading. Debris spreading depends on material volume and is thus scale-dependent. The internal strength of arrested debris enables obstacle formation, inhibiting downstream motion, and forcing higher volumes of oncoming material to flow laterally. Moreover, impact force requirements for terminal barriers could be reduced by >50 % when compared to 2D free-field flows, provided the barrier is placed sufficiently far downstream (>5 m). Three-dimensional analyses of flows entering basins should complement simplified two-dimensional analyses commonly adopted by engineers, potentially reducing overly-conservative designs.