||A friction model is used for predicting the risk of human penetration of fragile vegetation and bird breeding sites in a nature reserve in central Belgium. The basic components of the study are the terrain classification and the determination of friction values. Different sets of friction values are proposed: based on expert estimation, on walking speed, on energy consumption estimation and on willingness to trespass. The results are compared with spatial data derived from visitor’s observations and interviews. The model is to be improved in a later stage by incorporating the effects of visitor's goals, and of attractors and detractors such as vistas, free roaming cattle or physical challenges. The outcome of this study will be used as basis for the evaluation, and eventually of the redesign, of the current management decisions provided in and around the reserve. Also it should help in following-up the effects of the rapidly changing vegetation and terrain conditions on the behaviour of visitors.