THERE was a time when it was believed that floods were certain whenever two full moons occurred in the same month. Two full moons occurred in July this year, and perhaps this ought to have given pause for thought to Spanish meteorologists in the weeks that followed.
On the other hand, we know now that the moon, except insofar as it affects the tides, has no relationship with floods; they occur whenever more water accumulates than can comfortably drain away through normal channels.
Hurricanes, earthquakes and other major catastrophic happenings aside, there are three main reasons for a flood. One is melting snow: if the temperature rises suddenly in spring in mountainous regions, it may cause snow which has accumulated during the winter months to melt with great rapidity, and the resulting flow of water is often more than the river network can accommodate.
Secondly there is the kind of flooding with which we in Ireland are familiar. It occurs when heavy and continuous rain over an extended period drowns the landscape over a wide area, and water steadily accumulates to a volume well beyond the carrying capacity of local rivers.
Flash floods, however, are a much more treacherous phenomenon. They occur in regions that enjoy long periods of dry weather, but that are prone also to occasional very heavy thunderstorms. Unlike catchments that are rich in growth, sparsely planted areas, or landscapes where vegetation is reduced at the end of a dry hot summer, respond rapidly to an excess of water. The dry river beds and surrounding valleys are quickly choked by a torrent of water which sweeps along the surface for miles on end, advancing at great speed into neighbouring areas that may not have experienced any rain at all.
Moreover, it is not the water alone which provides the great destructive power of a flash flood. As the water courses down the valleys and gullies, it accumulates soil, sand and mud until the mixture forms a thick porridge which surges along with irresistible force. It scours the valleys of virtually everything, until eventually it slows down and spreads across the landscape like a great sheet of wet concrete. The requirement for extreme dryness in the period before the inundation means that to us in Ireland flash floods are largely a foreign phenomenon.
Flash floods are particularly common in parts of the US, where they claim an average of 160 victims every year. But they are also common in a band along southern Europe that extends from northern Italy, through southern France to Spain. As it happens the Spanish foothills of the Pyrenees are particularly prone. The average August rainfall in the region is perhaps 45 mm for the entire month, but massive thunderstorms triggered by the nearby mountains occasionally provide a very localised downpour of 80 mm to 100 mm in a few hours, and the dry surrounding countryside finds it impossible to cope with the billions of tons of waters thus unleashed.
Although large areas may be potentially vulnerable to flash floods, events at individual spots on the scale of last Wednesday's tragedy are mercifully rare, occurring perhaps once in a generation. Regardless of terrain, it requires a large thunderstorm in the right place at the wrong time to have the undesired effect. But this makes the phenomenon all the more difficult for meteorologists to predict with confidence: using the conventional weather chart combined with satellite images and related tools, it is possible to identify times and places where thunderstorms are likely, but it is almost impossible to be locally specific.
A major help in recent years, however, has been the development of sophisticated weather radar, which is ideally suited to continuously tracking the development and movement of large thunderclouds in real time. Conscious of their country's vulnerability in this regard, the Spanish meteorological authorities have been to the forefront in Europe in the installation of equipment of this kind, and have at their disposal what is perhaps the most extensive weather radar network in Europe.
Radar allows individual storms to be tracked, estimates made of likely rainfall, and warnings issued some hours in advance for locations likely to experience a deluge. On this occasion, however, any such advance warnings were obviously not sufficient to prevent tragedy. Even when accurate predictions of the expected volume of water are available, there remains the complication that each landscape reacts in its own unique way to such extreme events.
As with all unusual weather events nowadays, there have been suggestions that Wednesday's flood may be yet another symptom of the dreaded global warming. We cannot, however, draw this conclusion.