One of the consequences of rainfall and hydrological variability in semi-arid regions is, without doubt, the rapid saturation of the water courses and dry riverbeds, etc., their overflowing and the consequent flooding. A large number of such phenomena are due to the flooding of rivers whose sources are in mountain ranges lying close to the sea and parallel to the coast. The devastating effects are caused, mainly, by three factors:
The speed at which saturation takes place, causing the flash flood phenomenon,
The high velocity of the flood waters as a result of the steep slopes of the riverbeds, and
The large quantities of solid discharge.
The effects of flooding get worse because of the extremely flat morphology and the absence of adequate drainage systems on some floodplains, deltas and marshland, serve to make the effects of flooding even worse. Difficulty in removing some water, means that the land is submerged for longer periods making crop-loss more likely and more serious.
In these regions, flooding constitutes a natural disaster with far-reaching effects, both material and human. Therefore, such zones are obliged to have flood prevention schemes which use the most effective measures for each specific case, so that the effects of floods can be reduced. Such measures can be either structural (reservoirs, river channelisation, dikes, floodways for water courses, etc.), or non-structural (flood warning and prevention systems, management of flood areas, insurance, etc.). It must also be taken into consideration that some of these measures produce negative impacts on the river; for instance, river channelisation can disrupt the existing physical equilibrium of watercourses (alteration of hydraulic variables and parameters, modification of riparian vegetation etc.).
The existing major seasonal unevenness of rainfall distribution in Spain produces very serious flooding when rivers overflow their banks, flooding the surrounding areas as well. This amounts to a major natural disaster (Berga, 1995) both in terms of average damage per year, whether this be material (70,000*106 Pts/year) or human (50 victims/year), and this has made it necessary to draw up flood plans which use the most effective measures in each specific case. If flooding cannot be prevented, at least the adverse effects can be reduced to a minimum, and such measures can either be structural (reservoirs, dikes, diverted water courses, etc.) or non-structural, the latter are also called management measures.
The document entitled "Las Inundaciones en España. Informe General (MOPU, 1983)" (Flooding in Spain)" contains studies carried out on a national level, for the 10 river basins on the mainland, which deal with aspects concerning the following: a) identification and analysis of past floods and b) definitions and classifications of zones where flooding is a potential risk.
The document refers to 2,438 floods which have taken place in the last five centuries, and these statistics reveal that an average of approximately five floods have occurred per year throughout the Spanish mainland. The Guadalquivir and Ebro Basins prove to be those which are most susceptible to this phenomenon, suffering the consequences of about one flood per year, and these are followed by the Júcar, Segura, Sur and Douro. Using the information contained in this document, it was possible to pinpoint a total of 1,036 zones that are at risk from flood damage. Furthermore, a classification criterion was established on the basis of the degree of risk involved, and maps were drawn up to a scale of 1:200,000, in which the boundaries of risk areas were plotted, and the priorities established in each zone were appropriately included in each map. There are nearly seventy zones where a serious risk of flooding exists, which gives a good idea of the scale of the problem.
Among the non-structural methods taken in Spain to prevent floods or alleviate their effects are systems based on prevention and warning; some of those worth mentioning are the SAIH (Sistema Automático de Información Hidrológica) [Automatic Hydrological Information System] Program, implemented by the Dirección General de Obras Hidráulicas (D.G.O.H.) [Hydraulic Works Administration]. This service is operational in approximately 50% of Spanish territory. The SAIH is a means for informing the River Authorities, in real time, of the hydrological and hydro-meteorological situation in their basins. This knowledge is obtained by recording, transmitting, elaborating and presenting the hydro-meteorological and hydrological variables for strategic points in the basin. Although the SAIH was initially designed to be concerned only with flood prevention and warning, the system was soon realised to be able to be applied to an automatic and permanent hydro-meteorological service for the River Authorities for only a slight cost increase. Therefore the system would be able to: a) predict and monitor floods, b) optimise and manage water resources, c) improve dam safety and d) improve the hydro-meteorological and hydrological databases (Pedrero, 1995).
A preliminary report concerned with the occurrence of river floods and the description of flood-prone areas in Portugal was made by LNEC (1992). The report presented at first a brief discussion of causes and effects of floods, then some management measures and, a preliminary map of flood risk areas at a scale of 1:50,000.
According to the report, main risk areas in Portugal have been identified as those produced either by a discharge with a return period of 100 years or by the biggest known flood. The influence of a possible dam-break has also been taken into account. The identification of three big flood plains in Portugal, Vouga, Mondego and Tejo catchments, torrential regimes in Algarve which cause flooding, and urban flood problems taking place near the largest cities have been the main outcomes of that work.
The known floods represented in the risk map are those which took place in the last 30 years in the lower reach of river Mondego, the area of Lisbon, the mouth of river Tejo and in the Algarve area.
In Sardinia, rainfall is also extremely irregular and occasional storm events with high runoff cause flooding with damage to crops, roads and buildings.