The determination of the peak flow and the corresponding hydrograph for predefined 
return periods is of crucial importance in the common practice of public 
administration dealing with natural hazard and land use planning.
The availability of measured runoff data and past evidences are often very poor or 
even nonexistent. Therefore the estimation of design valued in vary small, steep and 
ungauged basins, prone to flash floods events with intensive sediment transport and 
debris flows remains challenging with considerable difficulties and uncertainties.
The existing link between basin morphology and hydrologic response provide since 
years a unique and coherent framework to improve the performance of predictive 
modelling in absence of direct data. The concept of Geomorphic Instantaneous Unit 
Hydrograph (GIUH) and its representation by means of the width function has been 
proposed by many authors and recently introduced as independent module in the JGRASS 
distribution.
An extensive study and a sensitivity analysis of the module have been carried out in 
different small catchments in Southern Switzerland, in which a hydrologic estimation 
of design flood was required.
The results show that the parameterization of the module, based on geomorphic 
assumptions and user skill, is able to produce reasonable results for design and land 
planning issues.
Furthermore, the application of a recently developed debris flow initiation criterion 
in steep torrents allowed extending the hazard analysis to these events and therefore 
establishing a coherent and general framework for further practical analysis and 
request.