Floods in southern Portugal: their physical and human causes,impacts and human response

Floods have been the most deadlynatural disasters in Portugal during the lastcentury, followed by earthquakes. The typeof flood known as a `progressive flood'mainly affects the larger basins, such asthat of the Tagus River, and results in alarge inundated area. These floods arecaused by heavy rains associated with awesterly zonal circulation that may persistfor weeks. The system of dams within thebasin reduces the frequency of flooding,but cannot `tame' the river. The dam systemhas even contributed to an increase in thepeak flow, as in the 1979 flood.Nevertheless, these floods are not a dangerfor the human population. In contrast, flashfloods are more dangerous and deadlier thanprogressive floods, as demonstrated in 1967and 1997. They affect the small drainagebasins and are caused by heavy andconcentrated rainfall, created byconvective depressions (active cold poolsor depressions caused by the interactionbetween polar and tropical air masses),active in the south of the country, in theLisbon region, Alentejo and the Algarve.Deforestation, soil impermeability, chaoticurbanization, building on floodplains, theblockage of small creeks or theircanalisation, and the building of walls andtransverse embankments along the smallcreeks all contribute to the aggravation ofthis kind of flood.