Flooding is the most universal of natural hazards. It occurs on every continent and is a potential threat wherever there is a rainfall or coastal hazards. With the exeption where rainfall is never more then very light, every watershed is a potential site for flooding. The most noted floods are associated with the world’s great rivers. However, the lesser floods in smaller rivers or upstream tributaries may cause cumulatively more damage, even though receiving less public attention.
The most common flood type occuring in Greece is the flash flood type. Flash floods are local floods of great volume and short duration. A flash flood generally results from a torrential rain or “cloudburst” on relatively small and widely dispersed streams. Runoff from the intense rainfall results in high flood waves. Discharges quiqly reach, a maximum and diminish almost as rapidly. Flood flows frequently contein large concentrations of sediment and debris. Flash floods are paricularly common in mountainous areas and desert regions but are a potential threat in any area where the terrain is steep, surface runoff rates are high, streams flow in narrow canyons and severe thunderstorms prevail.
Rainfall – runoff modelling in warm, low-latitude, arid region catchments is rendered challenging because of enviromental heterogenity in space and time, witch results in the production of irregular and extreme flood events. Arid region rainfall may occur in one of two seasons, one during winter and the other during summer. Winter rainfall events often cover large areas and have a long duration per storm, while summer rainfall events are more localised and are o shorter duration and higher intensity. These localised, high intensity stormas produce flash floods in arid areas, where interception capacity is almost negligible.
Arid region floods are characterized by steep rising limbs, which indicate a short time to peak often int the form of “bores”advancing on a dry bed.
This creates another difficulty for flood prediction since most of the available physically based numerical techniques cannot handle sudden changes in flood depth, which cause kinematic shocks owing to numerical instability.