Debris flows are rapid, unsteady gravity-driven flows wich can cause vast amounts of damage to infrastructures and endanger populations. It is therefore crucial to be able to properly model the behaviour of these flows, to better predict their impact. The aim of this study is to simulate these flows, determine how the presence of an obstacle in the path of the flow influences the propagation of the fluid, and to measure the pressures exerted by the flow on the obstacle, for various flow regimes. In this paper, we present a 2-D Smooth Particle Hydrodynamics (SPH) model capable of simulating a viscoplastic fluid. The Herschel-Bulkley model, which can accurately describe the behaviour of a muddy debris flow, is introduced in the SPH equations of Ferrari et al. The resulting code is validated by simulating a steady flow. It is then used to simulate a Herschel-Bulkley fluid propagating from a tank onto an incline and past a rectangular obstacle. The simulations confirm the existence of two regimes observed experimentally: a dead-zone regime for small values of the Froude number and a jet regime for large values of the Froude number. Results show that the pressure exerted on the obstacle can be described by a combination of a gravitational component and a kinetic component, the latter taking over as the flow transitions from the dead-zone regime to the jet regime.