Hydrometeorological and Hydraulic Factors and Problems Related to Floods in Arid Regions of Spain

U.S.- Italy Research Workshop on the Hydrometeorology, Impacts, and Management of Extreme Floods Perugia (Italy), November 1995 HYDROMETEOROLOGICAL AND HYDRAULIC FACTORS AND PROBLEMS RELATED TO FLOODS IN ARID REGIONS OF SPAIN Juan B. Marco Universidad Politecnica de Valencia Departamento de Ingenieria Hidraulica y Medio Ambiente 46071 Valencia SPAIN Abstract. Meteorological and hydrological aspects and hydraulic problems related to extreme floods, which occur in arid regions of Spain, are reviewed. Small coastal basins with steep slopes and little vegetation cover are characteristics of these regions in which "flash floods" develop. They are produced by large convective rainfall, resulting in very intense soil erosion and deposition. The flow in alluvial cones is basically two-dimensional, unsteady, and often supercritical. Using mathematical models that incorporate these features are typically prohibited because of the cost associated with topographical precision requirements. Since a large number of dams has been constructed and are operational in Spain, management of reservoirs considering the potential of flood events has become an important problem. 1. GEOGRAPHIC INTRODUCTION Spain is a large (505,000 km2) and high country with an average altitude of 540 m.a.s.l., crossed by important orographic systems running East to West. The Iberian system and some of the eastern mountains complete the division of the Peninsula into two unequal portions. Basically, the Iberian Peninsula can be thought of as a large plateau, encircled by mountains on three sides and gently tilting toward Portugal. Due to this orography, most large Iberian watersheds (the Rivers Duero, Tagus, Guadiana and Guadalquivir) flow to the Atlantic Ocean, while the River Ebro (86,000 km2) forms the only large Mediterranean basin. Along the eastern and southern coasts of Spain, a series of small basins are located between the mountains and the Mediterranean Sea. These are steep, fast-response watersheds. Among them, the Jucar (18,757 km2) and the Segura (14,432 km2) are important, not only for their medium size but mostly for their intensive water resources exploitation. Possessing the mildest climate, southeastern Spain has almost completely exhausted its water resources as a result of highly intensive irrigated agriculture, in addition to population and domestic consumption. Located between the Atlantic Ocean and the Mediterranean Sea, the climate of Spain is controlled by Atlantic frontal rains, crossing over Europe from South-West to North-East. Only one out of three fronts, however, affects the Spanish plateau. Only the North-West and the North Coast (the so-called "wet" Spain) have a mean rainfall close to or over the European average (1,473 mm/yr, on 54,000 km2). Most of the national territory belongs to the so-called "dry" Spain with rainfall ranging from 400 to 600 mm/yr; that is, a semi-arid climate. The Mediterranean Coast leeward from the mountains, however, is very rarely affected by frontal rains, to the point that southeastern Spain receives less than 350 mm/yr average precipitation. Almeria (229 mm/yr) is the most arid, heavily desertized region of Europe. However, as in most arid countries, convective storms are extremely severe in summer all over dry Spain and heavy, convective mega-scale phenomena are present all along the Mediterranean coast from Gibraltar to Catalonia, with huge recorded precipitations (greater than 600 mm/day). Due to being land-locked between the Sahara and Europe, the Mediterranean Sea is overheated by the end of the summer. The energy and moisture supply is guaranteed to fuel this type of large-scale convective phenomenon. The Spanish population (39 million), which is sparse by European standards, is heavily concentrated along the coast. On the other hand, the four regions occupying the plateau and central Spain comprise 70% of the nation's extension but they are occupied by only 10% of the total population. This process has occurred during this century due to the concentration of industry, cash-crop agriculture and tourism (9.6% of the Spanish GDP) along the coast. The narrow coastal plains, especially in the Mediterranean area, are overcrowded by the development of linear urban settlements and transportation corridors along the coast. Needless to say, after this introduction, flash floods along the Mediterranean coast are the main flood problems. Flood problems in the plateau are definitely secondary, since towns have neither developed nor encroached on the flood plains. The major rivers cut deep canyons before entering Portugal. Flood problems in these rivers are all related to management of the large hydropower developments located on the border with Portugal. On the other hand, along the Mediterranean coast, the following three factors converge: •. high intensity convective rainfall; •. small, steep basins which are heavily differentiated and have very short response times; and •. heavy population and economy encroachment on the coastal alluvial plains. 2. FLOODS IN SPAIN 1955-1995 Between 1957 and 1972, Spain had 15 years of accelerated economic growth, largely fuelled by the tourist boom. Large population migrations took place from rural central regions to coastal towns and to Madrid. Urban growth was fast, uncontrolled, and, largely, unplanned, without any prior development of the necessary infrastructure. On occasions, it even meant the disappearance of the local drainage network, which consisted of a system of ephemeral streams. The flooding of Valencia in October 1957 anticipated the problems that later became generalised. A 2,500 m3/s flow of the River Turia (6,395 km2) flooded the entire downtown area of Valencia, Spain's third largest city in terms of population, and resulted in the deaths of 86 people. The new urban developments with the worst urban conditions were the most affected. However, the flooding of the Vallés area in 1962, the industrial suburban belt of Barcelona, was symbolic of the situation created by the rural exodus that was taking place. The Riera de Rubí dry bed on the outskirts of Barcelona, with a drainage basin of only 400 km2, carried/transported 1,200 m3/s and killed 973 people. Most of them were living in the dry bed of the Riera in pitiful buildings. All the 2 encroaching factories were washed away. Without reaching such large disaster proportions, the Catalonian rieras have produced flash floods from the Ter (1970) to the Francolí (1994) dry beds. The 1963 flooding in southeastern Spain, without involving such a large economic and death toll highlighted a different aspect of the Mediterranean floods. The Rambla de Nogalte carried as much as 28% of sediment by volume. The River Almanzora and other ephemeral streams carried similar mudflows in this heavily deserted region. For almost 15 years, however, flooding spared the large population centers. It can be said that in 1982 the flooding problem had been neglected and virtually forgotten, totally unlike other aspects of Spanish economy and politics where frantic changes had been taking place. After October 1982, nothing was ever the same. On October 20, two large mega-scale convective storms hit the Jucar basin and the southeastern watersheds, including the River Segura. Rainfall on the middle Jucar basin was extensive, involving an area of around 5,000 km2 that received over 300 mm precipitation in a single day. The only recording gauge located at the Cofuentes Nuclear Power Plant registered 600 mm, most of which fell in just six hours. On the mountains, unofficial records of around 1,000 mm were estimated. As a result, a flood with an estimated peak of 9,800 m3/s was produced. Most of the rainfall event took place during the night. By dawn, the flood wave had already reached the Tous Dam, which was still under construction but already in service. The Tous Dam was a 60 m high rockfill dam, completed in its first phase to hold 50 Hm3 to feed the intake of a canal. When the flood occurred it was full. The dam had no provisions for any type of night vigilance. When the engineers arrived, there was no electricity to open the gates. The emergency generators were stacked by the roadside and the flood started to spill over the closed gates and the embankments, cutting an access to the spillway and, hence, preventing the gates from being blown open by explosives. At 19.12 hours, a breach opened up and the top part of the dam collapsed. A flood peak of 15,200 m3/s was produced. The valley downstream was already flooded to a depth of 2 m on the extensive flood plain, due to high tributary flows and water overflowing the dam. By nighttime, the water had reached a depth of 4.5 m on the plain and was entering the second story of buildings. The 8-km wide flood plain was fully inundated. It should be noted that due to its heavy, solid load, the main channel of the River Jucar, capable of holding only 200 m3/s, stands above the plain. Downstream from Alzira, where the valley narrows again to 1 km, a system of levees protected the lowest alluvial plain, mostly an area of rice paddies and lagoons. The flood broke the levees and two diverging streams branched until the flood reached the sea over a coastal stretch more than 40 km wide. The population affected by the flood was about 300,000. Thirty-eight people were killed, nine of them in the flood plain and the rest on roads mostly in the upper basin. Economic damage was evaluated at 800 million U.S. dollars. It must be noted that the flooding that occurred at Alzira, where almost 5 m of water covered the entire city, lasted less than 48 hours. At the same time, a more heterogeneous mega-scale rainfall produced a flood in the Segura basin and adjoining small basins.

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