This paper analyses some of the main processes that occur during a flash-flood with a two-dimensional (2D) model (InfoWorks RS with a 2D module). The data collated in the Valency River after an extreme rainfall event in the Boscastle area, in the north coast of Cornwall (UK) in August 2004, are used to analyse such processes. The paper focuses on blockage of structures, which clearly influences flow distribution and water levels in the floodplain and on changes in flow paths by collapse of some structures, which have a relatively local influence. Changes of channel dimensions due to geomorphological processes as scour and deposition are also studied, although their influence is reduced by the effect of bridge blockage. Understanding the processes that occur in the river during a flash-flood contributes to the development of risk models to reduce vulnerability.