Development of an Integrated Coastal Vulnerability Index for the Ivorian Coast in West Africa

This study assesses the vulnerable state of the 566-km Ivorian coastal area using the physical (geomorphology, coastal slope, coastal retreat rate, relative sea level rise and wave/Tide energy) and socio-economic (coastal population density, harbor, airport, road, land use and protected area) factors as indicators. This enabled an Integrated Coastal Vulnerability Index to be determined for the Ivorian coastal zone. This Index could be defined as the weighted average of indexes based on physical and socio-economic factors. The study revealed that vulnerability of the western and the eastern coastlines of Cote d’Ivoire are strongly influenced by human activities, while physical forcing affects significantly the vulnerability of the central section. The relative vulnerability of the different sections depends also strongly on the geomorphology, wave energy, coastal population density and land use factors. The west and central sections of the coastline are more resilient than the eastern section when integrating physical and socio-economic factors. The Integrated Coastal Vulnerability Index, based on physical and socio-economic factors, appears to be more appropriate for coastal vulnerability assessment. These results could be useful in the development of adaptation strategies to increase the resilience of this coastal area and then extended for West Africa Coastal Areas Management.


Introduction
The coastal zones correspond to the transition area between land and sea. These coastal areas provide vital ecosystem services and help buffer the coastline against severe extreme events.
In West Africa, the coastal zone concentrates about 31% of the population and the main infrastructures [1]. This coastal area is exposed to a variety of natural and potentially damaging events that increases the vulnerability and risk levels along the entire West African coastline [2]. The low-lying coastal areas of West Africa (as for instance Lagos and Abidjan) are particularly exposed to sea-level rise and the increase in storm intensities and frequencies. In these two cities, more than 3,000,000 of people are exposed to climate change and extreme events impacts [3]. Extreme events have frequently occurred during these last three decades and have often induced a severe coastline retreat and human life and infrastructures losses. Recently, a strong storm eroded more than 12 m of Ivorian coastline during two days in August 2011 [4]. High tides and heavy precipitation also increase vulnerability in the coastal zone through permanent processes (such as erosion) and rapid but temporary phenomena (such as coastal flooding) [5].
Recent study assessed the vulnerability of the Ivorian coastline based on physical parameters including geomorphology, coastal slope, coastal retreat rate, relative sea level rise and wave/Tide energy [6]. This study revealed that the vulnerability of the Ivorian coastline increases eastward and is controlled by geomorphology and frequent extreme events such as energetic swell waves [4]. In addition, socio-economic activities could increase the vulnerability of this coastal area because changes induced by socio-economic activities occur often much more rapidly than those related to physical factors [7].
The factors derived from socio-economic activities considered in West Africa coasts are: coastal population density, harbor, airport, road, land use and protected area [8] [9]. The use of these factors is justified by the important role in generating incomes for national economy or severity of pressure in the case of the Ivorian coastal zone. The West African countries are vulnerable to sea level rise, particularly where growing cities with high population density are situated in the coastal zone. Most of these countries undergoing rapid population growth, urbanization, coastward migration and associated socio-economic growth, countries are experiencing dramatic coastal change (e.g. [10]). Airport, road and harbor are the major infrastructures that seem more exposed to the impact of extreme events and accelerate sea-level rise due to their proximity from the coastline (e.g., the distance between the coastline and the Abidjan airport not exceed 1.5 km [11]). Moreover, the protection infrastructure of the harbors can contribute to coastline retreat rate [12] [13] by modifying sediment supply Eastward (case of the Ivorian coastal zone) and they are exposed of energetic wave impacts [14]. Land use expresses the reduction of the natural resilience of the coast and the exposure of agriculture, industrial and urbanization activities. Finally, pro- The innovation here comes from the integration of a proportional coefficient relative to socio-economic and physical factors in the vulnerability index. This Integrated coastal vulnerability index illustrates the degree of exposure of coastal areas to the impacts of physical and socio-economic factors.
Despite of the impact of these factors on the coastal environment, no serious

Materials
In order to quantify the Ivorian coastal vulnerability index to socio-economic pressure and its combination with physical parameters, relevant data were gathered from various sources. The coastal vulnerability index (CVI) data ( Figure   1) was taken from [6]. The socio-economic datasets were obtained from government agencies such as Centre National de Télédétection et d'Information Géographique (CNTIG) and Institut National de la Statistique (INS) as well as journal articles. The distribution of socio-economic data along the coast is provided by CNTIG and [21]. The spatial distribution of the dataset is presented in Figure 2. The main infrastructures such as airport, harbor, military base and oil and gas activities are located in Section 3 (Abidjan, Cape Three Points). These six parameters were then computed to assess the Ivorian socio-economic coastal vulnerability index (SVI).

Coastal Vulnerability Index
The coastal vulnerability index firstly consists of classifying qualitative (geomorphology, coastal slope) and quantitative factors (shoreline, sea-level rise, mean wave and mean tide height) as new no-dimensional "risk" variables. Each risk variable ranges between 1 and 5 and illustrates its degree of vulnerability.
Then, the CVI was calculated for each coastal zone through the following equation:  N = 6 is the number of physical parameters; x 1 is associated to the geomorphology, x 2 is the coastal slope and x 3 represents the shoreline change rate. The relative sea-level rise, the mean tidal height and the mean significant wave height correspond respectively to x 4 , x 5 , and x 6 .
In this study, the CVI data are only available for the three sections because of the lack of high resolution coastal retreat rate and tidal gauge datasets.

Socio-Economic Vulnerability Index (SVI)
In order to quantify the SVI for each main city of each coastal section, the qualitative (airport, road, harbor, land use and protected area) and quantitative (coastal population density) factors have been classified as new variables ranged between 1 and 5 which could illustrate the degree of vulnerability ( where N = 6 is the number of socio-economic parameters; v 1 , v 2 , v 3 , v 4 , v 5 and v 6 represent population density, airport, road, harbor Land use and protected area respectively. The assessment of coastal vulnerability based on physical and socio-economic factors require also the identification of the different parameters which affect mostly the vulnerability of each coastal section. It represents key information that will help to elaborate the appropriate adaptation measures for the vulnerability reduction risk relative to each coastal section. The mean SVI and CVI were computed for each coastal section defined previously. The ratio (SVI norm /CVI norm ) of the normalized SVI (SVI norm ) and the corresponding normalized CVI (CVI norm ) enabled the identification of the main factor (physical or socio-economic) which affects significantly the vulnerability of the corresponding section. If the ratio is lower than 1, the vulnerability of the associated section depends strongly on the physical factors. If this ratio is equal to 1, then the coastal section is equitably affected by both physical and socio-economic factors. Finally, if the ratio is higher than 1, then, the influence of the socio-economic factors represents the main vulnerability factors.

Integrated Coastal Vulnerability Index (ICVI)
The integrated coastal vulnerability index illustrates the degree of exposure of coastal areas to the impacts of physical and socio-economic factors. The new approach of the coastal vulnerability index called integrated coastal vulnerability index (ICVI) computed in this study can be expressed by the Equation (3) as follows: where CVI ij and SVI ij represent respectively the physical forcing for the section i and the human pressure for the city i includes in section j. ij a is the ratio of normalized SVI and CVI of the section j.
In the previous sub-section, we noted that the ratio of the SVI normalized and CVI normalized enabled the identification of the type of factor that governs the vulnerability of a given coastal section.
Finally, each coastal zone could then be classified as low, moderate, high or very high-risk categories vulnerability based on the quartile ranges of the SVI (resp. ICVI) data [22]. If the SVI (resp. ICVI) is weaker than the 25 th percentile, then the associated area falls into a low vulnerability risk. Moderate and high vulnerability represented the SVI (resp. ICVI) between the 25 th and 50 th percentiles and 50 th and 75 th percentiles respectively. Very high vulnerability is characterized by SVI (resp. ICVI) greater than the 75 th percentile.

Results and Discussion
This section is composed of three parts. The results of the Ivorian Socio-economic

Influence of Socio-Economic Factors on Ivorian Coastline
The impact of human activities on the Ivorian coastal area is quantified by computing the SVI of this area. The SVI illustrates the degree to which coastal areas are threatened by socio-economic factors. This analysis is undertaken by creating a SVI for each main coastal city. As described in the ''Methods'' sub-section, socio-economic factors are transformed into new variables, which are summarized in Table 3. According to this table, the degree of vulnerability associated to land use factor is high for most of the cities except Tabou, Grand-Lahou and Jacqueville. The population density decreases westward. Socio-economic factors are therefore low compared to that observed in the eastern coastal zone. Most of the socio-economic factors have a high degree of vulnerability in Abidjan. As for the Abidjan city for instance which is densely populated, the very high presence of industrial, commercial (port and airport) and urban infrastructures justify its very high degree of vulnerability. The construction of these infrastructures and the uncontrolled urbanization are carried out to the detriment of the factors of resilience of the coast (vegetation cover). The area around the Tabou (western coastal side) city has the weaker degree of vulnerability because of the low population density, lack of modern infrastructures and protected areas. Figure 3 represents the spatial variability of the Socio-economic Vulnerability Index (SVI). This Figure shows that the SVI increases eastward from 0.58 (Tabou) to 22.82 (Abidjan). The 25th (1.54), 50th (4) and 75th (6.17) percentiles of all SVI show that the mean value of the Ivorian coastal zone SVI (5.79) falls within the high-risk category. According to the classification based on percentiles  In the western coast, San-Pedro (6.12) and Sassandra (4.47) cities fall into high risk-category. The high degree of vulnerability observed at Sassandra is controlled by the degradation of land use and its low natural resilience (absence of protected area).
Finally, Dabou (6.32) and Abidjan (22.82) cities located in the eastern coastal section have the highest socio-economic vulnerability indexes due mainly to socio-economic factors: these two cities fall in very high risk category. When considering each zone, the cities which shelter important trade infrastructures (harbor and airport activities) or concentrate high density population like San-Pedro and Abidjan seem to be the most vulnerable (Figure 3).
The port cities of San-Pedro and Abidjan have been classified in high and very high risk categories respectively, which mean higher exposure to inundation, erosion and others coastal hazards. This result is confirmed by [23] and [3] who have noted that Abidjan is one of the most vulnerable African cities in term of population and assets exposures. The very high presence of industrial, commercial (port and airport) and urban infrastructures in Abidjan explains its very high degree of vulnerability. The construction of these infrastructures and the uncontrolled urbanization are carried out to the detriment of the factors of resilience of the coast. The very high vulnerability of Dabou city is due to the high human concentration and the reduction of the forest area in favor of cash crops such as rubber, palm oil as noted by [24] and the absence of resilience factors such as protected areas and forest cover. The destruction of forest, especially mangrove forests, along the east Ivorian coast [25] reduces its ability to protect the coast against energetic swell wave action.

Identification of the Main Factor of Vulnerability Associated to Each Coastal Section
The identification of the main factor (physical or socio-economic) that governs   The high vulnerability observed in the eastern coastal areas is due to the low coastal elevation [6], natural increasing of the coastal population and coastward migration [3]. The Integrated vulnerability indexes for different coastal cities will increase over the years due to rapid population growth and the number of coastal economic activities [24], continuous sea-level rise, increasing of southern wave and extreme coastal events. This increase could be particularly significant in harbor cities such as Abidjan and San-Pedro as in other coastal cities due to natural population growth and coastal migration. The Abidjan city, for instance, appears to be one of the most exposed in terms of population and economic assets in Africa, based on scenarios for physical and socio-economic factors as noted by [23]. The high risk of the eastern section requires urgent measures to protect the coastal environment and its resources. This coastal protection could include relocating near-coastal inhabitants to preserve life and to safeguard coastal infrastructure and ecosystems from the threats of extreme events.

The Ivorian Integrated Coastal Vulnerability Index
The results of the ICVI are in accordance with the literature of sea-level rise such that low-lying areas are more vulnerable to impacts of climate change and associated factors such as sea-level rise [3] [6] [24] and human pressure as is the case in east of Ivorian coastline. This new index could be improved by including high-quality and high resolution coastal retreat rate, tidal gauge, socio-economic, marine and soil pollution and rainfall data or by weighting the input variables.
The ICVI method could be used to evaluate the vulnerability of the entire Gulf of Guinea coastline to climate change and non-climate factors.
This study could particularly contribute significantly to the West Africa Coastal Areas Management Program (WACA) which aims to assist West African countries to sustainably protect their coastal areas and enhance socio-economic resilience to the effects of climate change.

Conclusions
This article evaluated and estimated the vulnerability of the Cote d'Ivoire coastal Journal of Environmental Protection zone due to both physical and socio-economic factors.
The Coastal vulnerability index (CVI) computed for each zone according to the physical factors shows that the relative vulnerability of the different sections depends strongly on the geomorphology and on the wave energy factors. This vulnerability increases spatially westward taking account of their CVI values.
The socio-economic vulnerability index (SVI) carried out for each coastal section increases eastward. This vulnerability is mostly controlled by coastal population density, the irrational use of land, uncontrolled urbanization and the presence of commercial, industrial and urban infrastructures. This study showed that the degradation of sections 1 and 3 is mainly due to human pressure. In contrary, the center section of the coast is more exposed to the impacts of physical factors.
A development of an integra ted coastal vulnerability index (ICVI) based on physical and previous socio-economic factors is also carried out in this study. The ICVI appears to be more appropriate for assessing the degree of exposure of a given coastline to the impacts of these factors due to the combining of the CVI and SVI indexes.
The spatial variability of the Ivorian ICVI increased eastward. The western section of the coast appears more resilient than the eastern side, which appears to be the most exposed to various risks. This eastern section deserves special attention from the Ivorian government and the end users.