Reinforcement measures are applied by water boards during critical conditions in an ultimate attempt to prevent flooding. However, the effect of these measures on the reduction in flood risk or failure probability of the flood defence is often unknown. This research focusses on the effectiveness of grass revetment reinforcement measures applied during critical conditions at the outer slope of river dikes. Reinforcement measures applied during critical conditions are characterised by time constraints and human errors. The effect of the reinforcement measure on the reliability of the flood defence is studied. A probabilistic model is developed to assess the failure probability of the reinforcement measure and the time needed to install the measure. A model named OPSCHEP-model is used to quantify the failure probability due to human action. The effect of the measures on the failure probability of the flood defence is calculated by a crude Monte Carlo simulation. It is found that the reinforcement measures are most effective for known weak spots in the flood defence. Weak spots that are to be found during emergency dike inspection result in a much lower reduction in the failure probability of the flood defence. The level of preparedness of the water board organization largely determines how effective the measures are in reducing the failure probability.