6 th European Conference on Severe Storms (ECSS 2011) , 3 - 7 October 2011, Palma de Mallorca, Balearic Islands, Spain MODELLING FLASH FLOODS IN SMALL FORESTED MOUNTAIN WATERSHEDS USING GEOGRAPHICALLY REFERENCED DATA Mihai Daniel NiŃă 1 , Ioan Clinciu 1 1 Faculty of Silviculture and Forest Engineering – Transilvania University of Brasov, Sirul Beethoven No. 1 Brasov, Romania, nita_mihai_daniel@yahoo.com 1 Faculty of Silviculture and Forest Engineering – Transilvania University of Brasov, Sirul Beethoven No. 1 Brasov, Romania, ioan_clinciu@yahoo.com I. INTRODUCTION Severe storms which appear in mountainous areas in Romania produce highly unpredictable flash floods. Many mathematical models, rather expeditious than analytic, are used in predicting this kind of phenomena (Drobot, 2007). II. RESEARCH LOCATION AND METHODOLOGY This study used a highly analytic method for modelling flash floods produced in Valea Băii experimental watershed (fig, 1). FIG. 1: Valea Băii experimental watershed location For calculating the flood hydrograph we choose for the use of sensors-logger, which record the pressure of the water column above it (Clinciu, 2001). The sensor was placed in the lake formed upstream a hybrid crested spillway placed on the main course of the river (fig. 2). FIG. 2: View from the front, back and top of the hybrid spillway installed on the main course of Valea Băii watershed (Photo: NiŃă, 2009) For modelling the flood hydrograph the method is based on mathematical quantification of soil potential accumulation of water and uses geographically referenced data on: soil texture, soil porosity, soil humidity, antecedent rains, soil cover and watershed morphometry. Many modelling methods, despite of soil importance in producing the surface and subsurface flow, do not use too many data for characterizing this influence. Therefore the prognosis quality is enhanced in the presented study with the geospatial data on soil. The method itself (called MPA – Potential Accumulation Method) predicts the flash floods phenomena by creating a synthetic hydrograph (Gaspar, 1997). The flood wave simulation is based on following cell raster characteristics: slope, soil cover, rainfall intensity, overland flow distance, and the discharge is calculated based on rainfall intensity and derived runoff coefficients. The input data for the application of MPA in isochronous mode are: - Raster: digital elevation model, satellite images, orthophotoplan; - Vector: watershed, the river network, forest compartments (or categories of land in the watershed). The calculation involved the following steps: - Determination of the four factors which quantify the land use effect, average slope effect, the effect of rainfall intensity and the effect of traffic in the watershed; - Determination of effective porosity using data on the soil; - Determination of calculation soil depth; - Determination of maximum retention capacity and effective retention; - Derivation of potential accumulation of water in the soil; - Estimate the initial retention and infiltration losses; - Estimate the actual drained layer; - Creating flood hydrograph and calculating peak discharge. MPA implementation problem in reconstructing the flood hydrograph is to establish a relationship for determining the flow depending on the drainage time, on watershed characteristics and on the characteristics of rainfall data. Therefore we used flow coefficient determined by the MPA in a rational equation formula type to convert the value of drained layer in the discharge water value. t i t S K Q ⋅ ⋅ ⋅ = 6 1 m3/s where: K is the average drainage surface factor contributing to calculation section and is based on a correction coefficient