published on 2 October 2011 in
The hydrogeological risk
On the sides of watercourses are plains known as flood plains, that get flooded only occasionally after large amounts of rainfall or the sudden melting of consistent amounts of snow masses, which provoke river floods or spates. In these cases the water spills out of the banks of the watercourse occupying the adjacent plains, and subsequently depositing the sediments it was transporting.
So inundations may be caused by a rapid or slow increase in the river flow, in some areas they may be influenced by the phenomenon of high tide, and in the coastal areas they may also be caused by hurricanes, tidal waves or storms at sea. Even characteristic large urban centres situated near big lakes may be involved; these may get flooded following strong precipitations, as for example the towns on the banks of Lago Maggiore, Lake Como and Lake Garda. Other causes of destructive floods may be the collapse of a dam or landslides in lake basins as was the case of the disaster in the Vajont valley near Belluno.
Floods may also occur without the presence of a river when violent precipitations fall on a plain, and the intensity is such that the water cannot be absorbed by the ground or it does not flow away at the same speed as it falls to the ground.
Settlements near the rivers
Man has always built settlements or used the areas near rivers or in the plains that were at risk of flooding. In fact the areas near rivers like the Nile, the Tigris, the Euphrates, the Indus, the Ganges or the Yangtze rivers are fertile areas because of the alluvium deposits. Furthermore, the watercourses also have characteristic economic activities tied to them as for example fishing, transportation of lumber, transport of goods and the production of electricity.
Floods: magnitude and frequency
The variations in the flow of the watercourses are pointed out by means of hydrographs which are graphs that show the relation of the river-flow during the course of time, depending on the rainfall.
For example, following a heavy precipitation, the volume of water present in the river increases till it reaches a maximum limit that is indicated as the peak, and as the hours pass in the case of the smaller basins, and as the days pass in the case of the larger basins, the volume of water that flows in the river decreases till it returns to normal conditions. This trend is represented by a curve that is characterized by a part that rises, a maximum limit (the peak) and a part that descends, indicating a gradual return to the situation of regular flow. In order to defend oneself from inundations, it is necessary to know the flow rate at the peak time during extreme events, so as to then design adequate structures. With the available data and information regarding the floods that took place in the past, probability calculations are made which must also bear in mind the precipitation, the characteristics of the landscape, of the altitude, of the geology, of the pedology and vegetation that are characteristic of the area.
A defence against floods
Embankments – The construction of embankments in the ground is a widespread and not very expensive defence against floods. In some cases the embankments are water-proofed with particular plastic or rubber coatings, with concrete or bituminous material.
Canalization – the riverbeds are widened with the aim of increasing their capacity, or the banks are protected by constructing veritable walls. The consequences of canalization are not always positive, because if on one hand they favour a more rapid flow of the waters out of the area to be protected, they aggravate the situation downstream of the canalized area, where floods may occur which would not have occurred otherwise. Furthermore their environmental impact on the flora and fauna is often negative.
Filling channels – when the flow rate of a river reaches a determined level the filling channels remove water that is returned to the main watercourse further downstream of the area to be protected. Usually the waters are made to flow out into a body of water, as for example another river, a lake or directly into the sea.
In case of floods even barrages, like dams, can be used as tanks to collect the water which is then made to flow out slowly at a later time. Also reforestation helps increase the time gap between the rains and the floods.
Forecast and alarm systems
Forecasting systems and alarms are useful in large basins, but are not very effective in the case of torrents that can swell in a very short amount of time, and in fact the time between the alarm and the inundation would be too short. An alarm can be given after having monitored the propagation of the flood wave at various observation stations from upstream, moving towards the valley, or it can be based on the intensity of the precipitations in a particular area.
An example we would like to illustrate are the emergency actions that were carried out during the flood of 1953 in an area in England, after a storm in the North Sea:
- approximately 12 hours before the event the possibility of the flood was indicated and the monitoring services were intensified;
- the probability of a hazard was notified and the municipalities involved were informed;
- the emergency services were warned, and boats and staff were kept alert to verify any damages in the communication channels, the population living in isolated areas were warned and the evacuation measures were ready;
- the alarm was given, rescue activities were started and all the emergency services were used.
The risk in Italy
Numerous geological formations that are present in the Italian territory are characterized by impermeable rocks such as marl and clays. Their impermeability to water causes a poor infiltration into the ground and an increase in the speed and quantity of water converging into streams. These conditions favour the onset of floods. Furthermore the torrential regimen is characterized by long periods of minimum flow of water and by short and intense peaks at the time of the more intense rainfall. In case of strong precipitations concentrated over a short period of time, surface waters can remarkably increase the flow of mountain torrents and the rivers at the bottom of the valley, causing water to overflow the riverbanks.
These phenomena have also increased due to the abandoning of the hill sides, because the agricultural system of terracing prevented the direct flow of water towards the valley facilitating stagnation and infiltration in the horizontal parts of the land. Furthermore the trend to extend agricultural areas has led to the construction of artificial embankments in order to exploit new land near the banks of watercourses, while it would be useful, in areas at risk, to create vast areas adjacent to the rivers at the bottom of the valley that can fill with flood waters during peak flow, without causing damage. Frequently, in these flat areas, houses and factories are built, which will be surely damaged in case of floods.
After the World War, in Italy, floods were frequent and provoked damages for many thousands of billions of Italian Liras, in fact a good organization of the territorial control bodies, both on a national and on a regional scale, is necessary. The main intervention is to carry out prevention activities, by studying the territory, the dynamics of the basins and of the hydrographical network.
Written by Elisabetta Monistier