Flood hazard mapping can be performed using different methods that, generally speaking, can be grouped into four main categories: historical and paleohydrological methods, hydrogeomorphological methods, hydrological-hydraulic methods, and the recently developed dendrogeomorphological methods.
These groups of methods are not mutually exclusive and, in fact, they should be used complementarily. Unfortunately, in the case of ramblas, this combination is really difficult. On one hand, paleohydrological methods are not suitable for so small and torrential catchments as ramblas are and, on the other hand, hydrological-hydraulic methods demand a large amount of hydrologic data, non available in most of these systems. Indeed, many authors assert the inadequacy of these methods to predict extreme floods in Mediterranean small catchments.
Anyway, hydrogeomorphological method is nowadays achieving more relevance. It is based on the location and typology of landforms and sediments generated during floods in order to delineate flooding areas and identify processes. It is a qualitative approach that gives a realistic image of the processes and it is enough to make decisions, with a minimum effort, in the 80% of the instances. Studies developed in the Mediterranean region of southern France and north-western Spain have demonstrated the effectiveness of this method in ephemeral streams, where channel and floodplain morphology are highly variable and changeable over time and, in addition, hydrological information is scarce or nearly inexistent.
This work presents a method for mapping flood hazard in two Mediterranean small catchments -Barranc de Carraixet and Rambla de Poyo-, based on hydrogeomorphological interpretation. A synthetic hydrogeomorphological cartography was obtained supported by previous studies (carried out by Camarasa, Carmona and Ruiz) and taking into account the forms and processes developed during the great flood event of October 2000. Thirteen different landforms related to flooding processes where identified and valuated in terms of hazard, ranking from levels 1 to 8, in which level 1 represents the highest hazard (streams and critical points) and level 8 the safety areas (mountains and longshore bars).
Hydrogeomorphological method has proved to be highly effective for mapping flood hazard in this kind of torrential ephemeral streams, where standard hydrological and hydraulic methods do not work properly. This method allows working with scarcity of hydrological data and, what is more, is quite adaptable to any change in flood area morphology (either natural or man made)
