Flood Hazard in the Classical Karst: The Case of Mucille Polje (NE Italy)

In the north-western area of the Classical Karst (NE Italy), the Mucille depression, after abundant precipitations, is subject to frequent floodings, which become problematic since 2001 as they more frequently affect housing and recreational areas, leading the population to believe that the swallow holes draining the area stopped functioning. The climate changes as well as the increased frequency of intense rainfall events led to evaluate the draining capacity of the swallow holes in order to provide fundamentals for the mitigation measures. The depression is fed by a spring area and drained by two swallow holes one of which is permanently active, while the other functions only during floods. About 24 h after the onset of heavy rains, the whole depressed area is flooded. About 8 days later, the water level begins to decrease, coming back to its initial height in about fifteen days. During floods, while springs and swallow holes discharges measurements are impossible, the extension of the flooded areas has been mapped. The obtained flooded surface together with high resolution DTM coverage allows to calculate the volume of surface water. Consequently, the hydrologic balance can be estimated during the whole event. This study provides meaningful evidences for the design of measures to mitigate the risk. It estimates the discharge of the swallow holes, confirming their efficiency. Nonetheless, it also emphasizes the need to improve their draining capacity, especially considering the unsuspected high outflow of the springs at the onset of the flood.