Rationale for flood prediction in karst endorheic areas

A large karst area of South-Eastern Italy (Puglia) is characterized by endorheic basins, whose runoff does not discharge into the sea but converges toward internal lowlands and infiltrates or flows into underground cave systems through swallow holes. In such environment whenever intense rainfall events cover large areas and rainfall intensity exceeds the discharge capacity of sinks and swallow holes, significant volumes of runoff are produced and stored on surface causing floods and risks for people and goods. Most of these sinks are often at the end of small independent basins delimited by weak divides and, whenever water storage exceeds the overflow threshold, runoff contributes to downstream areas and, in cascade, large areas may contribute to deepest lowlands. The observation of historical flood events suggests that in such areas traditional methods for the individuation of the design flood event, and in particular of critical rainfall duration, lack of applicability and the worst rainfall condition, for a fixed return time, should be searched accounting for soil hydraulic behaviour and groundwater dynamics. In this paper a rationale for the evaluation of the critical rainfall event and of the flood-prone area for given return period is proposed. A case study is presented showing that for high return period events a "multiple-reservoirs" mechanism is activated that affects the critical rainfall condition as well as the flood extent in the urban areas