**1. Introduction**

Floods have become more recurrent and usual events in several countries [1]. Compared to the figures of the 1990s, the number of floods has almost doubled in the world since the 2000s [2]. They represent a threat with a major impact in terms of victims. In 2018, floods accounted for 50% of people affected by natural hazards [3]. Floods also have severe consequences in terms of economic loss and material damage [4].

The upsurge in floods can be explained by various factors, including climate change [5], which generates changes in precipitation regimes and intensity [6], and often manifests in torrential rains. Intense precipitation can cause flooding in small river basins and in rivers [4]. Extreme events in Africa [7], Europe [8], and Asia [9] are examples of the significance of climate change in increasing flooding. However, floods are not exclusively linked to climate change, but also to urbanization dynamics [10]. Jha, Bloch, and Lamond argue that regardless of climate change, urbanization can increase the risk of flooding [11]. With an emphasis on exposure and vulnerability, we would like to highlight this aspect in this paper.

**Citation:** Ramiaramanana, F.N.; Teller, J. Urbanization and Floods in Sub-Saharan Africa: Spatiotemporal Study and Analysis of Vulnerability Factors—Case of Antananarivo Agglomeration (Madagascar). *Water* **2021**, *13*, 149. https://doi.org/ 10.3390/w13020149

Received: 29 October 2020 Accepted: 7 January 2021 Published: 10 January 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Urbanization generally leads to an increase in impervious surfaces, which limits the possibility of water infiltration in the soil and increases the volume of water runoff on the surface [12]. Additionally, urbanization is often accompanied by an artificialization of urban rivers, which further increases the risk of water overflows [13,14]. This modifies existing land use not only inside cities but also in the outskirts [15].

In 1900, 15% of the world's population lived in urban areas [16]. Currently the proportion is more than 50% [17]. The numbers are increasing by 200,000 people a day, or 70 million people a year, and the proportion is estimated to reach 70% in 2050 [17]. This urban growth increases the demand for housing and land to build [16]. Given the competition for urban land, some people are tempted to build on areas exposed to risks [18,19].

Controlling exposure to floods implies a combination of urban planning and management of drainage systems. It requires follow-up of spatial planning policies [11], because risks are partly related to governance [1]. A lack of planning or poor planning can lead to an increase of informal installations and constructions, often exposing vulnerable residents to risks [10,20]. Lower-income residents usually do not have access to services and infrastructures that could mitigate the problems [21]. On the other hand, extending the drainage system should go hand-in-hand with any increase in built spaces, and it should be resilient [22] by having the capacity to evacuate water in the face of flooding. Without an adequate, sufficient, and well-maintained drainage system [23], urbanization cannot be sustainable.

Africa is one of the two continents in the world most affected by floods [24]. Floods are the most frequent disaster and remain a threat, especially in Sub-Saharan Africa (SSA)'s cities [25,26]. At the same time, the continent contains a population that is growing twice as fast other regions in the world [27]. Beyond this high growth, management and planning remains a problem throughout the continent and particularly in the region south of the Sahara [23,28]. The absence or ineffectiveness of disaster management plans and the inadequacy of basic systems, infrastructures, and services contribute to increasing vulnerability of urban areas [11]. The inability to accommodate a fast-growing population in decent conditions explains why constructions are located on unsuitable and dangerous sites, exposing cities to natural disasters [18], including floods. The deficiency of the drainage systems means a part of the population is affected by floods [14].

In this study, we show that urbanization leads to increased exposure of populations and constructions to floods and tends to add to their vulnerability. In order to reduce exposure and vulnerability to floods, it is important to recognize all aspects related to flooding, including socioeconomic factors that explain why flood-prone zones keep attracting a part of the population. We thus adopt a co-evolutionary perspective in order to better understand the long-term bi-directional relations between flood exposure, urban expansion, and vulnerability [29,30].

This paper is centered on the agglomeration of Antananarivo, the capital of Madagascar. Apart from the extreme climatic hazards the country is exposed to every year [31], it has most of the characteristics of SSA's cities mentioned above, in particular growing urbanization. According to the Institut National de la Statistique de Madagascar (INSTAT), nearly 5 million Malagasy people lived in urban areas in 2018 [32]. Rapid urban expansion is a problem due to the lack of planning [33]. It is associated with drainage problems plaguing the country. Insufficient capacity and poor functioning of the drainage network due to clogging with solid waste and deterioration are among the causes of floods [34]. The growing urban population is settling more and more in flood-prone areas [35], with the majority in informal spaces with limited services [36]. All of these factors contribute to the vulnerability of low-income groups.

The study starts from the birth of the agglomeration and proceeds with an analysis of its demographic growth, and then the evolution of the built-up areas. Two case studies on a finer scale are presented in order to better understand the socioeconomic conditions of urban areas located in lower areas of the city. These sites were selected based on the identification of sensitive areas affected by flooding during the 2018 rainy season by Service Autonome de Maintenance de la Ville d'Antananarivo (SAMVA). They are among the black spots of the city, since they are flooded every year. They have a similar urban dynamic and

socioeconomic situation but differ in terms of the motivation of the people living there. This makes the comparison relevant. The case study analysis is followed by a discussion of results, conclusions, and limitations of the research.

#### **2. Study Area: Agglomeration of Antananarivo**

The agglomeration of Antananarivo, also called Greater Antananarivo, is located on the central Malagasy highlands (Figure 1). It covers an area of 76,800 ha and in 2018 had about 2.9 million inhabitants according to INSTAT. On the administrative and institutional level, it brings together the Urban Community of Antananarivo (CUA) composed of six boroughs forming the city of Antananarivo and 37 peripheral municipalities. The whole is located in the Regions of Analamanga and Itasy and is subdivided into 571 neighborhoods called Fokontany.

**Figure 1.** Location of agglomeration of Antananarivo.

Located at an altitude between 1200 and 1500 m above sea level, Antananarivo is characterized by a wide variety of landforms. It is made up of a set of elevated areas with steep slopes to the south, lower areas to the east and center, and a vast alluvial plain in the north and west (Figures 2 and 3).

The plain is drained by the Ikopa and its tributaries (Figures 2 and 3), flowing mainly from the south, southeast, and east to the northwest [37]. Upstream, the flow is more fluid, because the rivers face areas with steep slopes. The river slows down and generates water retention in the lower parts upon its arrival in the plain. This is due to the slight slope of about 0.25% [38] as well as the confluence of the rivers. The topography of the site and its hydrographic network make it very vulnerable to flooding. Almost a third of the urban area is occupied by flood-prone areas (Figure 3).

These flood-prone zones (Figure 3) were produced from a combination of the topographic wetness index (TWI), a soil moisture index, and the stream power index (SPI), an index characterizing the intensity of surface runoff. They are extracted from calculations carried out based on a digital Shuttle Radar Topography Mission (SRTM) model with geographic information system (GIS) software. These two indices are important parameters in flood sensitivity analysis [39].

After the last confluence in the northwest, the Ikopa flows to a single point [34], characterized by a succession of rock outcrops that reduces the water evacuation capacity and generates the formation of alluvial deposits at the level of the plain [40]. In the CUA, the plain forms a polder surrounded by dikes that protect it from overflowing rivers. However, as the river levels are often higher than its level during the rainy season, it is very sensitive to flooding [41].

**Figure 2.** Relief of study area.

**Figure 3.** Flood-prone areas.

The first hydraulic infrastructures date from the royal period in the 17th and 18th centuries, consisting of river embankments and canals constructed to protect the plain and ensure the evacuation of irrigation water [42]. Currently, the drainage system is denser and more complex. It is structured along three main channels with multiple functions: drainage of rainwater, wastewater, and sewage, as well as irrigation of the agricultural plain. These are the Andriantany, C3, and GR (Génie Rural) channels (Figures 4 and 5). Primary canals, open drains, and buried pipes of various dimensions are connected to these three main channels. Pumping stations and retention basins have been added to this to ensure operation.

At the CUA level, the Andriantany and C3 channels are the main drainage channels. The upstream Andriantany runs through the western part of the city to the pumping station to the northwest and collects water from the hills and eastern plain. It collects rainwater, but also wastewater, from some Fokontany [37]. Downstream, it takes up water from the pumping station and drains to its point of confluence with the Ikopa [43]. As for channel C3, it collects water from the southern plain and agricultural drainage flows as well as excess flows from Andriantany [41]. The GR channel irrigates the plain [44] and acts as a drain during the rainy season [45].

Despite the existence of this drainage system, water tends to accumulate in the plain. At the pumping station, the flow remains paltry compared to the total flow to be drained [46]. Moreover, the increased intensity and volume of runoff due to soil sealing accelerates the degradation of the drainage system [47]. These problems cause flooding during the rainy season.

The peripheral municipalities are not connected to the main drainage network. In these municipalities, water is channeled through sanitation devices along the road network that evacuate rainwater, concrete or earth canals, and drainage ditches leading into the

natural environment. In some communes, a collective sanitation system does not even exist [34].

**Figure 4.** Agglomeration's main drainage system.

**Figure 5.** Urban Community of Antananarivo (CUA) watershed.

#### **3. Materials and Methods**

#### *3.1. Collected Data*

The demographic data used for this work were collected from a general population census based on administrative divisions provided by INSTAT conducted in 1993. For 2018, the figures are based on estimates, since the current general census is not yet official.

Built spaces correspond to structures that host housing, service, industrial, and economic activities. Those represented in this study were digitized based on historical maps and aerial photos. The first base map are maps of Antananarivo from 1953 and 1975. They were provided by Foiben-Taosarintanin'i Madagasikara (FTM), a public establishment in charge of cartography and geographic information in Madagascar. It covers the Greater Antananarivo area except for a few communes to the north and west of the agglomeration. The map representing the areas built in 2006 and 2017 was developed through vectorization of aerial images provided by Google Earth.

The flood-prone zones used in this study are those described in Figure 3 and explained in Section 2. These data were provided by UN-Habitat Madagascar.

Data related to the drainage system came from two organizations that specialize in sanitation and flooding in Madagascar, Autorité pour la Protection contre les Inondations de la Plaine d'Antananarivo (APIPA) and SAMVA.

Table 1 shows the details of these data.

**Table 1.** Data used in the analysis. INSTAT, Institut National de la Statistique de Madagascar; FTM, Foiben-Taosarintanin'i Madagasikara; APIPA, Autorité pour la Protection contre les Inondations de la Plaine d'Antananarivo; SAMVA, Service Autonome de Maintenance de la Ville d'Antananarivo.


In order to cross-check all of the data, cartographic work was carried out. Analysis, modeling, and display of results was done using QGIS geographic information system software.
