Finisterra - Revista Portuguesa de Geografia (Dec 2001)
As cheias no sul de Portugal em diferentes tipos de bacias hidrográficas
Abstract
THE FLOODS IN THE SOUTH OF PORTUGAL IN DIFFERENT KINDS OF DRAINAGEBASINS – The regime of the Portuguese rivers depends on the space and time variation of rainfall. Portugal has clear regional contrasts in the geographical distribution of rainfall. The NW and the Central Mountain Range (Cordilheira Central) are the regions with more rainfall. The NE and the south are the driest regions. Therainfall regime is very irregular. The monthly rainfall regime is clearly Mediterranean with autumn-winter rains (November-March) and an extremely dry summer. The river flows are also very irregular, with severe droughts and surprisingly high flood discharges. These characteristics tend to worsen from NW to SE. The southern rivers have specific discharges 6 to 7 times inferior to the ones of theNW, greater irregularity (the flow in years with more rainfall may surpass 100 to 240 times the flow in driest years), a more severe drought (6 months), almost all are temporary, and flood peaks (200-300 times the average flow) can reach extremely high values.In the twentieth century, floods were responsible for the highest rate of casualties in natural disasters in Portugal, followed by earthquakes: one death for every seven were due to floods. The type of floods known as «progressive floods» mainly affects the big hydrographic basins, such as the River Tagus basin, due to the large flooded area. This kind of flood is caused by heavy rainfall periods connected to the western zonal circulation, which usually lasts several weeks. The dams’ basin system reduces flood frequency, especially in autumn when reservoirs still manage to absorb the highflows after the summer dry period, but cannot «tame» the river. It has even contributed to an increase of the peak flow, as in the 1979 flood. Flashfloods are another kind of floods that occur in Portugal and, unlike the former, are dangerous and deadly, such as those in 1967, 1983 and 1997. They affect the small drainage basins and are caused by heavy and concentrated rainfalls, due to convective depressions (cold pools especially active or depressions caused by the interaction between polar and tropical circulations), namely in the south of the country (Lisbon region, Alentejo and Algarve). In the small drainage basins with a natural regime (uninfluenced by a dam), it is interesting to verify the existence of a trend in these extreme phenomena over the last decades. There has been a clear intensification of flood importance during autumn months, in contrast with an accentuated diminishing in winter and spring months.This trend concerns us mainly for two reasons. Firstly, the rainfall concentration in fewer months lessens its availability in the other months and requires a greater storage capacity. Secondly, this concentration means a bigger rainfall intensity in autumn, with the worsening of the number and intensity of floods and a greater soil loss. Deforestation, soil impermeability, chaotic urbanisation, building on floodplains, the blocking up of small creeks, or their canalisation, the building of walls and transverse embankments along the small creeks courses that work as dikes, contribute to the aggravation of this kind of floods. The floods in rivers of southern Portugal are here analysed, and range from the big drainage basins (River Tagus, 80 100km2), to the smaller ones (Cobres stream, 700 km2; Garganta stream, 1 km2). Also discussed are the human causes that have contributed to increasing the consequences of the floods in small catchments.
