Assessment of different worst-case flood scenarios for Planinsko Polje (Slovenia)

Planinsko Polje is a typical example of structural polje located in South Slovenia that encounters regular flooding. The floods can occur several time per year and create a more than 10 km² large temporary lake. Even if the polje surroundings are densely populated, local people have known since centuries the average level reached by the majority of the floods and avoided constructing below it. However, the occurrence of severe events with maximum water levels far above common values is still possible. While such floods are unusual, they can last for months and drastically affect the life in the polje vicinity. As recent climatic projections expect an increase of extreme meteorological events in a near future, a resulting increase of severe floods in Planinsko Polje might be foreseen. This work aims to investigate the circumstances under which extreme flooding is occurring in Planinsko Polje. The method combines an analysis of historical flood records with a detailed description of five particular high-water events that happened during the last ten years. Then, different worst-case flood scenarios are proposed and tested with a discretized numerical model. The significance of maximum water levels and flood duration arising from modelling results is discussed in light with historical records of extreme floods. Results show that very-high flooding in Planinsko Polje is the consequence of a simultaneous reduction of the ponor drainage capacity with a combination of extreme meteorological events occurring within the polje recharge areas, while the aquifer downstream is already saturated with water. The method has practical applications for people living near Planinsko Polje and can be generalized to other poljes around the world.