Geological – Geomorphological Features of River Catchments in Flood Susceptibility Assessment (on the Example of Middle Struma Valley, Bulgaria)

The geological-geomorphological features of the river catchments control flood propagation by influencing the surface runoff and its redistribution to surface and underground flow. The paper analyses the geological-geomorphological susceptibility to floods in a part of the river Struma valley – Municipalities Kresna and Strumyani (Bulgaria, EU). The following parameters are considered: lithology (presented by water permeability, capacity and type of rock reservoirs), distance from streams/river, slopes and fluvial landforms. Raster layers for each one of these parameters are created in geographic information system (GIS) environment and susceptibility rates are assigned. The flood susceptibility map is elaborated by spatial weighted sum overlay of the raster layers. The results show that the susceptibility is the highest in the low part of the valley, south of Kresna. The susceptibility of flooding is not equal over the whole area of the floodplain but depends on the retention capacity of the alluvial deposits and the hydro-geomorphological state of the floodplain. The river bank height above the water level and the river bed type are also taken into consideration by application of fuzzy logic. Using the fuzzy logic minimizes some imperfections of the initial data and gives good results in data-scarce areas. The fuzzy logic model is designed as a two-level hierarchical system with three inputs and one output. The output gives the complex assessment for geological-geomorphological flood susceptibility of study area regarding the interaction of the three inputs. The results of the current research aims to direct the attention of planning experts and decision-makers to the importance of geological-geomorphological parameters in flood management and can be considered as a first step of flood hazard assessment.