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The Mediterranean region under climate change

 | 
Jean-Paul Moatti
, 
Stéphane Thiébault

Part 3. Adaptation, resilience, conservation of resources and prevention of risk

Sub-chapter 3.4.1. Urbanization and land use as a driver of flood risk

Freddy Vinet, Mohamed El Mehdi Saidi, Johnny Douvinet, Noômène Fehri, Wafa Nasrallah, Wahiba Menad et Samira Mellas

Texte intégral

Introduction

1Obviously, floods have natural causes – heavy rainfall in the case of flash floods – nevertheless, their consequences depend strongly on urbanization and land use. On both sides of the Mediterranean, regions have been subjected to fast changes which have led to a concentration of assets in valleys and coastal areas (Plan Bleu, 2008). Changes in land use play a predominant part in the “risk production process” i.e. in the increasing exposure of human activities to flood risk.

2This chapter gives an overview of these land changes and their impact on flood risk, based on documented cases in the Mediterranean.

Recent catastrophic floods: land use in question

3Among the growing damage linked to floods, urbanization and more generally un-adapted land use is often called into question. Three examples taken from three Mediterranean countries (Tunisia, Morocco, Algeria) illustrate the impact of land use with its considerable human and material toll.

The evolution of the global disasters toll

4Such changes can be seen in the evolution and variability of natural disaster annual assessments (fig. 1). Since 1970, the cost of natural disasters has quadrupled, essentially due to an increase in the number and value of exposed assets. Since the mid-1990s, costs have been subjected to high inter-annual variability that does not only concern floods. The years with a heavy toll are those where highly urbanized areas were hit (e.g. Katrina in 2005). The high variability of damage is therefore more indicative of the spatial concentration of activities and population than of increasing natural variability.

Figure 1
Evolution of insured and non-insured losses for “natural” disasters.
Source: MunichRe.

Recent floods in the Mediterranean showing the impact of urbanization

5Tunisia regularly suffers from torrential floods with considerable human losses (fig. 2). On 13 October 2007, heavy rainfall hit Greater Tunis (70 to 180 mm in 24 hours). 16 people died (Fehri et al. 2009), mostly on the GP8 road to the north of Tunis. Through careless construction, increasing numbers of obstacles (buildings, roads) had been built on riverbeds. Hydraulic works were often under-sized and badly maintained.

Figure 2
Human toll of main flood events in Tunisia (1962-2011).

6Like Tunisia, Morocco has suffered from numerous lethal floods since the mid-20th century (fig. 3). The one that hit the Ourika valley (503 km², High Atlas of Marrakech) was the deadliest in the country’s modern history.

Figure 3
Main deadly floods in Morocco since 1990.

7In the afternoon of 17 August 1995, intense rainfall hit the upstream basin of Ourika to the south of Marrakesh. The peak discharge reached around 1,000 m3/s in the gorges. This was where the damage was the most severe. The estimated human toll ranged from 289 to 730 people (according to sources) among the 35,000 holidaymakers in the valley that day (Secrétariat d’Etat auprès du Ministère de l’Energie, des Mines, de l’Eau et de l’Environnement, 2008). This toll can be explained by the exposure of both the population and holidaymakers on the river bed. Restaurant terraces were set into the wadi’s minor bed. Furthermore, the overcrowding of the valley late in the summer afternoon, combined with the narrowness of the only road, blocked several cars with their occupants. Trapped in their vehicles, the latter did not survive.

8The most emblematic case of torrential floods in an urbanized zone remains that of Bab-el-Oued. The huge rainfall of 9-10 November 2001 (263 mm in two days, and above all 70mm in two sequences of 30 min.) triggered both human (781 deaths and 115 missing) and material (3,721 destroyed or damaged houses) losses which had never been observed here before. The most important damage occurred at Bab-el-Oued, situated in the western part of greater Algiers. In two decades (1985-2005) the basin of Wadi Koriche experienced considerable urbanization-up to 78% of its surface was built upon. The basin presents a high predisposition to runoffs in case of intense rain. The inner-basin networks (25 km of roads, 7.8 km of pipe works) served as collectors and enabled the propagation of flows towards the Bab-el-Oued area. The main river bed had been piped and the Chevalet-Triolet highway was built on top of it. Setting up a road network instead of a hydrographic one removed it from the landscape. Gradually, people forgot about it and tended to use the wadi’s bed. Most of the victims were car-drivers trapped by the flood on the highway, and people at the Bab-el-Oued market.

9These emblematic events show the part played by urbanization in the increased cost of flash floods. The catastrophic tolls invariably result from inconsiderate occupation of riverbeds by economic activities, housing or roads.

Land use transformations, a key factor in flood related damage

10Since the early 20th century, world population growth has been intense. In 100 years (1915-2015), the world’s population rose from 1.9 to 7.4 billion people. It doubled between 1970 (3.7 billion) and 2015 (7.4). It ranged from an “empty” Earth to a “fully populated world” – “full” in the sense that we have reached the limits of the Earth’s system capacities. This increase in the population was accompanied by deep land transformations which determine the extent of flood-related damage.

Demographic growth and urbanization

11Countries on the north and south sides of the Mediterranean basin have experienced high demographic growth (fig. 4). For example, Morocco’s population increased from around 26 million inhabitants in 1994 to nearly 33.8 million in 2014, i.e. over 7 million more people within 20 years. This demographic growth came with spatial redistribution. The population is concentrated in the cities of coastal areas and valleys. The urbanization rate went from 48.6% of the population in 1990 to 60.3% in 2014 (Haut-Commissariat au Plan, 2015).

Figure 4
Urban population in Mediterranean countries. Source: Plan Bleu, 2008.

12Most major cities in North Africa experienced demographic growth boosted by rural-urban migration during the first decades of independence. The Greater Tunis population rose from 194,000 inhabitants in 1921 to 561,000 in 1956 and 2.4 million currently. Simultaneously, urbanized areas expanded from 3,387 in 1957 to 28,000 ha in 2012 (source: l’Agence d’Urbanisme du Grand Tunis (AUGT).

13In the south of France, the population increased in the plains and regressed considerably on higher lands (Cevennes, Pyrenees).

14At the end of the 19th century, the population of the Languedoc (fig. 5a) was balanced between populated countryside and a network of midsize cities. At the end of the 20th century (fig. 5b), the population was concentrated on the coast, big cities and communication axes.

15The attractiveness of coastal areas is naturally explained by touristic and industrial activities. In Morocco, 70% of the country’s tourism is concentrated on the Atlantic and Mediterranean coasts. The coast is also home to most industrial activities. This trend should continue, particularly with the development of the “Tanger Med” harbor (the biggest in the Mediterranean basin) and the extension of the “Nador West Med” port. However, the concentration in low areas cannot be narrowed down to the appeal of maritime activities. Indeed, many so called “coastal” cities have no major activity linked to the sea (e.g. Montpellier in southern France). The attractiveness of low lands is mainly explained by the proximity to roads.

Figure 5
Spatial distribution of population in Languedoc (southern France) in 1881 (fig. 5a) and 2005 (fig. 5b).

Population growth and flood risk stakes

16Demographic growth and urbanization brought about land and society transformations that induced changes in the assessment of flood risk. As the exposed assets increase and move, social vulnerability and resilience evolve.

17The first consequence of these land transformations is the increase in assets in flood-prone areas. As in North Africa, the French population has increased in the last 50 years. It rose from 46 to 66 million inhabitants between 1962 and 2016. As a consequence, real estate increased by 8 million dwellings. Housing demand was met according to urban planning regulations where environmental risk questions remained marginal until the 1980s. Urbanization decisions were based on the available land, in relation to transport networks and activities, without really taking flood risk into account, except some local cases where risk awareness was high.

18The assessments of numbers of housing or people living in flood-prone areas must be taken with caution as they vary considerably according to the chosen context. In France the number of people living in flood-prone zones comprises between 6 million and 17 million according to the Evaluation Préliminaire du Risque Inondation body (EPRI) following the setup of the 2007 European directive. French Mediterranean departments are among the most exposed (see ONRN website www.onrn.fr). The Alpes-Maritimes (+9,200 dwellings in flood-prone zones in between 1999 and 2008), the Var (+8,300 dwellings) and the Vaucluse (+5,800 dwellings) are among the six departments most affected by real estate demand in France.

19In Morocco, although the population is concentrated on the coast, the inner valleys have experienced important growth of urban perimeters as in the Souss valley, where newly built zones continue to encroach on the wadi bed, and as in the Jorf and Tarrast neighborhoods in the south of Agadir (Rehaimi, 2013; Saidi et al, 2013). Such encroachments expose numerous populations who are sometimes already vulnerable due to their socioeconomic precariousness.

Changes in runoff conditions

20Besides the increase in exposed assets in risk-prone areas, urbanization results in changes in local runoff conditions. Such modifications are now well known (e.g. Zevenbergen et al. 2010). Apart from the increase in impervious areas, basin urbanization is characterized by changes in runoff conditions by longitudinal and transversal obstacles. In addition, the disappearance of rivers in the urban landscape contributes to forgetting the risk and encourages the occupation of minor and major river beds. The watershed of the El-Ghrich El-Greb wadis north of Tunis is a good example of the impact of expansion and densification of urban areas on runoff coefficients (Fehri and Zahar, 2016). From the 1950s onwards, this watershed experienced rapid urban expansion. So much so that buildings and roads now take up over 70% of its total surface. The exposure of the road network is also a growing danger factor (fig. 6).

Figure 6
Crosscheck between watercourses and roads in the southern suburbs of Tunis.

How to control flood risk?

21Faced with the increase in assets in flood-prone areas, the sometimes relative, yet real failure of land use management must be noted. Land demand has been too high and changes in land use too sizeable and rapid to be controlled in risk-prone areas. Land use control tools have been set up, particularly in France but their efficiency has been limited. Moreover, they do not efficiently address the question of existing buildings in flood-prone zones.

22Debates tend towards integrated flood risk management, from prevention to post-crisis, via crisis management preparedness and warning devices. However, structural measures (i.e. aiming at controlling flood hazard: dams, dikes, river bed recalibration) remain often the first considered type of response to risk whereas non-structural measures struggle to become widespread, except for warning and crisis management which are increasingly efficient.

Structural measures are still dominant

23The set of flood prevention tools has broadened considerably (Andjelkovitch, 2001), but despite international requirements for comprehensive risk management plans (2007 European Directive; UNISDR, 2005, UNISDR, 2015), the temptation to use structural measures (such as dikes) as the solution to all flood risk problems is still high, whether in Morocco, Algeria, France or Tunisia.

24After the Bab-el-Oued floods, the newspaper Le Quotidien claimed: “40 million dinars for a protection plan. Bab-el-Oued will never experience another flood”. The newspaper article presented projects commissioned or considered by the State to protect Algerian cities from floods. At the same time, new buildings were built (after the disaster of 10 November 2001) such as a primary school in the minor bed of Wadi Koriche in Bab-el-Oued.

25For several decades Morocco has relied on a dam building policy (150 great dams storing 18 billion m3 of water), initially designed to fight drought. In these dams, important parts were reserved for flood water reduction.

26These works are expensive and do not guarantee 100% protection. They may even be an additional risk factor during exceptional events (dam or dike break). The only 100% efficient prevention method is to refrain from locating vulnerable activities and buildings in flood-prone areas. This may seem obvious but it does not appear to have been incorporated at every level of land use planning.

The difficult control of land use

27Land use control is a fundamental point of risk reduction policies and practices (Pottier et al. 2005). Existing regulations in France (R 111-3, Submersible Surface Plan (PSS)) were rarely applied until the setup of the Exposure to Predictable Natural Hazard Plans (PER in French) in 1984 and the Risk Prevention Plan (PPR in French) in 1995. When these plans are approved, they are generally respected but local authorities sometimes use stratagems to delay their application. The Ministry of Ecology (CGDD, 2009) showed that in 424 French cities, nearly 100,000 dwellings were built in flood-prone areas between 1999 and 2006. Demand is also high from local authorities to urbanize areas “protected” by dikes.

Living with water?

28For the millions of people living in flood-prone areas, living with water is an undeniable fact. In this case, they must adapt existing buildings in order to limit damage costs and ensure the safety of people (Kelman, 2007). In urban renovation areas, there have been attempts to take the risk into account within the redefinition of urban space. Activities are spread out according to their vulnerability, in decreasing order, as the flood hazard increases. Thought has been given to this question (November et al. 2011) and the French Ministry of Ecology has initiated reflections on how to build safely in prone areas (Bonnet & Morel, 2016).

Conclusion

29The information provided by retrospective studies on past evolutions of frequency and intensity of heavy rainfall in the Mediterranean and models developed to anticipate these evolutions in the future is uncertain (Soubeyroux et al. 2015; Tramblay et al. 2012). However, the socioeconomic and demographic human evolutions that the Mediterranean basin has experienced over the last 50 years have proved far more decisive. The land use transformations have constituted a real “risk production process” through the increase and the spatial concentration of exposed assets and the evolution of vulnerabilities.

30These land use changes are linked to broader changes in our societies. They are disconnected from the risk issue but they have repercussions for the population’s exposure. They are not only quantitative (rise of assets at risk) but also qualitative. They affect the degree and nature of social and territorial vulnerabilities facing floods. Our societies are not vulnerable in the same way as they were 50 years ago, nor in the same places. The vulnerability of traditional housing (adobe bricks, cob) facing floods is thus reduced by the use of concrete but reappears in other forms (precarious housing) and other places. Conversely, the vulnerability linked to migration increases with the often anarchic densification of networks.

31Demographic pressure is not expected to decrease before 2050. It has been forecasted that at this date, there will be near 170 million inhabitants on the European side (versus 140 in 2005) and over 300 million on the east and south coasts of the Mediterranean basin (151 in 2005). Near 2030, around 42 million extra dwellings will be necessary, mainly in cities (source: http://planbleu.org). This will also be followed by the aging of the population including in the South (Coudert, 2002). Research shows that the elderly are more vulnerable in the face of flood risk (Jonkman & Kelman, 2005; Vinet et al. 2012).

32Prevention disconnected from the land use control issue is doomed to fail. Above all, the question of flood risk management and prevention is a territorial issue with a heavy social and political resonance. The choices made in this field may determine the risk of future and forthcoming disasters.

33While the potential impacts of climate change are certain for some and more hypothetical for others, social and land use evolutions are decisive in the process of risk production. Urbanization, population growth, social transformations, and housing evolution are key parameters in flood risk evolution. Beyond any consideration about climate change, it is urgent to take disaster risk reduction measures – and preferably non-structural measures that are more resilient in the face of climate change.

Bibliographie

References

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Bonnet F., Morel J.F. (DIR.), 2016
Atout risques. Des territoires exposés se réinventent. Marseille, Parenthèses, coll. territoires en projets, 176 p.

CGDD, 2009
Croissance du nombre de logements en zones inondables. Le point sur. Commissariat Général du Développement Durable, 6:1-4.

Coudert E., 2002
Une approche régionale de la population et de l’urbanisation en Méditerranée, rétrospective et projections à 2025. In Carrière J.P. (ed.): Villes et projets urbains en Méditerranée, PUFR, 135 p.

Fehri N., Samaali S., Abaza K., 2009
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Fehri N., 2014
L’aggravation du risque d’inondation en Tunisie: éléments de réflexion. Physio-Geo, 8: 149-175.

Fehri N., Zahar Y., 2016
Etude de l’impact de l’extension et de la densification du tissu urbain sur les coefficients de ruissellement dans le bassin versant des oueds El-Ghrich et El-Greb (Tunis) par l’application de la méthode SCS aux évènements de septembre 2003. Physio-geo, 10: 61-79.

Haut-commissariat au Plan, 2015
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Jonkman S. N., Kelman I., 2005
An analysis of the causes and circumstances of flood disaster deaths. Disasters, 29 (1): 75-97.

Kelman I., 2007.
Decision-making for flood-threatened properties. In Begum S et al. (eds.). Flood risk management in Europe, Springer, Dordrecht, 534 p.

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Les crues de l’oued Issil en amont de Marrakech (Maroc), un risque naturel récurrent. European scientific journal, 9 (23): 189-208.

Menad W., 2012
Risques de crue et de ruissellement superficiel en métropole méditerranéenne: cas de la partie ouest du grand Alger. Thèse de doctorat en géographie. Université Paris Diderot, 330 p.

November V., Penelas M., Viot P. (dir.), 2011
Habiter les territoires à risque. Lausanne: presses polytechniques et universitaires romandes, collection «espaces et sociétés», 252 p.

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Les perspectives du Plan Bleu sur le développement durable en Méditerranée. PNUE/PAM, Sophia-Antipolis, 27 p.

Rehaimi H., 2013
le système de transport et la ségrégation sociale de l’espace urbain: cas du grand Agadir. Thèse de l’université Ibnouzohr, Agadir, 354 p.

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Sendai framework for disaster risk reduction 2015-2030.35 p.

Vinet F., Lumbroso D., Defossez S., Boissier L., 2012
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Urban flood management. CRC Press Routledge, 340 p.

Table des illustrations

Légende Figure 1Evolution of insured and non-insured losses for “natural” disasters.Source: MunichRe.
URL http://books.openedition.org/irdeditions/docannexe/image/23910/img-1.jpg
Fichier image/jpeg, 181k
Légende Figure 2Human toll of main flood events in Tunisia (1962-2011).
URL http://books.openedition.org/irdeditions/docannexe/image/23910/img-2.jpg
Fichier image/jpeg, 273k
Légende Figure 3Main deadly floods in Morocco since 1990.
URL http://books.openedition.org/irdeditions/docannexe/image/23910/img-3.jpg
Fichier image/jpeg, 239k
Légende Figure 4Urban population in Mediterranean countries. Source: Plan Bleu, 2008.
URL http://books.openedition.org/irdeditions/docannexe/image/23910/img-4.jpg
Fichier image/jpeg, 176k
Légende Figure 5Spatial distribution of population in Languedoc (southern France) in 1881 (fig. 5a) and 2005 (fig. 5b).
URL http://books.openedition.org/irdeditions/docannexe/image/23910/img-5.jpg
Fichier image/jpeg, 430k
Légende Figure 6Crosscheck between watercourses and roads in the southern suburbs of Tunis.
URL http://books.openedition.org/irdeditions/docannexe/image/23910/img-6.jpg
Fichier image/jpeg, 180k

Auteurs

Cadi Ayyad University, Morocco
Geographer, Cadi Ayyad University, Morocco.
m.saidi@uca.ma

Avignon University, France
Geographer, University of Avignon, France.
johnny.douvinet@univ-avignon.fr

University of Manouba, Tunisie
Geographer, Manouba University, Tunisia
fehri_n@yahoo.fr

University of Manouba, Tunisie
Géographe, UR Biogéographie, Climatologie Appliquée et Dynamique Érosive, Faculté des Lettres, des Arts et des Humanités, Université de la Manouba, Manouba, Tunisie

Hassiba Benbouali University, Algérie
Hassiba Benbouali University, Algeria
menadwahiba@yahoo.fr

Montpellier-3 University-IRD, France
Geographer, University of Montpellier, France.
mellas.samira@gmail.com

© IRD Éditions, 2016

Conditions d’utilisation : http://www.openedition.org/6540