482
Climatic Geomorphology
The inherent risk of flooding is increased by constructing close to rivers and by the
agricultural use of alluvial soils; this increases the population close to rivers and their
floodplains.
One of the most catastrophic flood events of recent history occurred in the spring of
1887 when the Yellow River or Huang Ho (China) broke its banks, killing between 1.5 and
7 million people (Davis, 1992). It affected 2000 towns and villages and material losses
were huge. Similarly, in September of 1911, the Yangtze flooded, affecting four Chinese
provinces, killing over 200,000 people and leaving half a million homeless. On the 17th
September 1954, in Farahzad, Iran, 2000 pilgrims who were camped in a narrow gorge
were killed when a flash flood hit them. In semi-arid Spain, the worst ever flooding
occurred in Valencia in November of 1957 with 86 deaths, and in Tarrasa-Sabadell, in
September 1962, where 267 people lost their lives (Martfnez et al., 1987). L6pezBermfidez et al. (2002) developed an historical analysis of the floods which occurred in the
Spanish Mediterranean zone.
The fight against flooding has gone on throughout history in arid areas. The town of
Calatayud in the Province of Zaragoza, Spain, was founded by the Arabs at the
beginning of the eighth century, and it partially occupied an alluvial fan exposed to
frequent flooding during storms (Guti6rrez, 1998). With the aim of reducing this risk, a
weir and channel were built that deviated the water from the Barranco de la Rfa and the
Rambla de Ribota (Fig. 19.19). In the Barranco de la R6a basin, the runoff over a
surface area of 6.3 klTl 2 (total basin area 8.2 km 2) was diverted by Arab engineers. As
the population grew and settled closer to the River Jal6n, mainly in the 13th and 14th
centuries, the risk of flooding increased. During this time, or perhaps a little later, the
Balsa de Valparaiso reservoir was constructed close to the fan apex to alleviate the
problem. After the 16th century, a tunnel nearly 1 km long was excavated that drained
the water towards the Barranco de Longia, and in so doing some 98 per cent of the
runoff of the Barranco de la Rtaa was diverted from its drainage basin, avoiding the risk
of flooding. The Barranco de las Pozas, with its smaller drainage area, still causes
problems for Calatayud (Guti6rrez, 1998). In the neighbouring town of Daroca in the
Province of Zaragoza, the Mina tunnel, also about 1 km long, was excavated in 1555, to
divert water from the main throughway, which passed through what is now the Calle
Mayor (Main Street) where it caused serious flooding.
To avoid or at least reduce the chance of flood disaster, many and costly projects
are undertaken in the world's arid regions, although recent studies suggest that the
engineering solutions adopted are not always very adequate (Cooke et al., 1982).
Dikes, dams and ditches are all part of these projects. Small dams at the headwaters of
channels reduce erosion and control the runoff over small areas, whereas large dams
downstream allow the attenuation of large rises in water levels (Costa and Baker,
1981). Dams, however, do not provide control over flooding, but rather "partial or
specific protection" (Leopold and Maddock, 1954). It is also very important to control
human use of floodplains with adequate legislation. Each local government should have
detailed information concerning areas at risk of flooding. Flood alert systems have
been improved by the installation of automatic hydrological information systems
(AHIS), the aim being to establish emergency alerts that minimise flood damage
(Ortiz, 1993).
Climatic Geomorphology
The inherent risk of flooding is increased by constructing close to rivers and by the
agricultural use of alluvial soils; this increases the population close to rivers and their
floodplains.
One of the most catastrophic flood events of recent history occurred in the spring of
1887 when the Yellow River or Huang Ho (China) broke its banks, killing between 1.5 and
7 million people (Davis, 1992). It affected 2000 towns and villages and material losses
were huge. Similarly, in September of 1911, the Yangtze flooded, affecting four Chinese
provinces, killing over 200,000 people and leaving half a million homeless. On the 17th
September 1954, in Farahzad, Iran, 2000 pilgrims who were camped in a narrow gorge
were killed when a flash flood hit them. In semi-arid Spain, the worst ever flooding
occurred in Valencia in November of 1957 with 86 deaths, and in Tarrasa-Sabadell, in
September 1962, where 267 people lost their lives (Martfnez et al., 1987). L6pezBermfidez et al. (2002) developed an historical analysis of the floods which occurred in the
Spanish Mediterranean zone.
The fight against flooding has gone on throughout history in arid areas. The town of
Calatayud in the Province of Zaragoza, Spain, was founded by the Arabs at the
beginning of the eighth century, and it partially occupied an alluvial fan exposed to
frequent flooding during storms (Guti6rrez, 1998). With the aim of reducing this risk, a
weir and channel were built that deviated the water from the Barranco de la Rfa and the
Rambla de Ribota (Fig. 19.19). In the Barranco de la R6a basin, the runoff over a
surface area of 6.3 klTl 2 (total basin area 8.2 km 2) was diverted by Arab engineers. As
the population grew and settled closer to the River Jal6n, mainly in the 13th and 14th
centuries, the risk of flooding increased. During this time, or perhaps a little later, the
Balsa de Valparaiso reservoir was constructed close to the fan apex to alleviate the
problem. After the 16th century, a tunnel nearly 1 km long was excavated that drained
the water towards the Barranco de Longia, and in so doing some 98 per cent of the
runoff of the Barranco de la Rtaa was diverted from its drainage basin, avoiding the risk
of flooding. The Barranco de las Pozas, with its smaller drainage area, still causes
problems for Calatayud (Guti6rrez, 1998). In the neighbouring town of Daroca in the
Province of Zaragoza, the Mina tunnel, also about 1 km long, was excavated in 1555, to
divert water from the main throughway, which passed through what is now the Calle
Mayor (Main Street) where it caused serious flooding.
To avoid or at least reduce the chance of flood disaster, many and costly projects
are undertaken in the world's arid regions, although recent studies suggest that the
engineering solutions adopted are not always very adequate (Cooke et al., 1982).
Dikes, dams and ditches are all part of these projects. Small dams at the headwaters of
channels reduce erosion and control the runoff over small areas, whereas large dams
downstream allow the attenuation of large rises in water levels (Costa and Baker,
1981). Dams, however, do not provide control over flooding, but rather "partial or
specific protection" (Leopold and Maddock, 1954). It is also very important to control
human use of floodplains with adequate legislation. Each local government should have
detailed information concerning areas at risk of flooding. Flood alert systems have
been improved by the installation of automatic hydrological information systems
(AHIS), the aim being to establish emergency alerts that minimise flood damage
(Ortiz, 1993).