Recurrence of damaging floods and role of population number in a karstic area (Bari, southern Italy)

In karstic areas cases floods are rare, short, and sudden but the effects are often very dramatic. Flash floods (local floods of great volume and short duration, defined by the USA National Weather Service as floods caused by heavy rainfall in a short period of time, generally less than 6 hours) are extremely dangerous. Recent years have seen several flood-related casualties in Italy, in 2003, 2005 (when the last catastrophic flood happened in the karstic area of Bari), and 2007. Flash flood warnings are intrinsically difficult due to the short time available to warn people living next to the river. Our goal in this study was to contribute to the understanding of the relationship between flood events in karstic areas, in terms of recurrence, exceptionality and/or intensity, anthropogenic effects, mainly in terms of population number variations, and damages. Some authors have shown that urbanisation, land use changes, and hydraulics works must be taken into account to really understand the effects of past flood events. A comparative analysis of these data can thus allow us to understand if the trend of flood damage is steady or increasing and to determine whether it is affected by the growing density of population and facilities in flood-prone areas.The approach was based on three types of analyses: the historical analysis, based on historical data, the hydrological and the geomorphological analysis, both based on recent monitored or measured data. The historical analysis was focused on identifying and comparing two kinds of data: the series of past flood events over a period sufficiently wide to define the flood recurrence period and the main steps of anthropogenic development, called the anthropogenic role. This role was expressed using the population number and needs and considering the facilities in flood-prone areas. The hydro-geomorphological analyses were based on two types of data. Long time series of the annual maximum rainfalls of short duration were used to assess the return period of high intensity rainfall that caused studied flood event(s) for which rainfall measurements were available. The geomorphological analysis considered rainfall, hydrogeological characteristics of outcropping rocks and soils, and geomorphological data on the drainage basin to calculate net rainfall and peak flow characteristics. The historical analysis of floods and damages highlighted four flood periods, distinguished on the basis of the recurrence period and the level of damages. The hydro-geomorphological analyses showed the peculiarities of the 2005 flood event. The maximum rainfall return period for the 2005 event was due to exceptional three-hour cumulative rainfall. The study case was a sub-catchment where very high rainfall intensities were observed. The rainfall return period was found to be roughly in the range of 50-100 years, considering a cumulative rainfall duration (3-hour) about equal to the concentration time. This study shows that main modifications of flood recurrence and level of damages can be justified considering the variation of population number.