Map of the Konkouré River watershed in Lower Guinea 

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Context 1

... the water related disasters (floods, droughts, tropical storms) are already a major challenge for sustainable human development (Ajibade et al., 2013; Okada et al., 2014; Tingsanchali, 2012). Facing with a deficient flood management, climate changes, deforestation, large zones of Africa continent are exposed to water related disasters (Descroix et al., 2012; Padi et al., 2011; Runge and Nguimalet, 2005). Located south-west of West Africa, Guinea covers an area of 245,857 km2. It is a coastal country with 350 km of Atlantic coast, halfway between the Equator and the Tropic of Cancer, between 7°05' and 12°51' north latitude and 7°30' and 15°10' west longitudes. Many major rivers in West Africa have their source in Guinea: Niger, Senegal, Koliba (Corrubal), Kolenté (Great Scacie), Cavaly, Diani, Gambia, Makona etc. Two natural regions are particularly affected by flooding due to their geomorphology, hydrological functioning of their rivers and rainfall intensity. These are Upper Guinea and Lower Guinea (Maritime Guinea), the last one being affected both by seawater inundation and flooding by coastal rivers (Capo et al., 2009). Coastal regions of Guinea and neighboring countries, Guinea Bissau and Sierra Leone, have a large area of mangrove swampy plains. The loss of such surfaces due to flooding and erosion relate with the rise in sea level and global climate change threatens to derail the economy of the coastal countries of West Africa (Capo, 2006). The morphology of lower coast favorizes its exposure to marine and environmental problems, the greatest concern is coastal erosion and flooding resulting from the combined action of several factors, including heavy rains that increase flood coastal rivers, stagnant waters of the city due to rain at the inner gentle slopes, the rise of the river water by sea water during high tides. The main objective of this study is the delineation of flooding zones, combining GIS technology and the measurement of height of water in Guinea, particularly in Lower Guinea (Maritime Guinea). These areas must be generally localized; the existence of a database from geographic information systems (GIS) on the water level and the flow of the water is very important and necessary for floods modeling (Anoh et al., 2012; Furdu et al., 2013; Liu et al., 2013). This mapping using GIS is very useful to support the politics and urban management confronted with an urban explosion, which is presently taking place. Opening out of the Atlantic Ocean by a front of around 300 km, the administrative area of Lower Guinea (Guinea Maritime) occupies a strip having about 150 km average width, between the foothills of the Fouta Djallon plateau in the east and the ocean to the west (Fig. 1). With a surface of 43,730 km2, it includes almost 36% of the country's population thanks to the presence of the capital Conakry. It has eight prefectures in addition to the capital Conakry, the prefectures of Boffa, Boke Coyah, Dubreka Forécariah Fria and Kindia Télimélé. Lower Guinea is characterized by abundant rainfall, generally higher than 2,000 mm/year, with a pronounced dry season of at least five months, but also by a poorly differentiated river system, widely open to the ocean through many channels and estuaries. This drainage network is affected, sometimes over distances of up to one hundred km, by high amplitude tides exceeding 5m in estuaries of the North, the phenomenon being favored by very low slopes, through widely developed mudflats (Rossi et al., 2001). For the purpose of this study, the spatial data analysis was made through ArcGIS 10.0. Other maps were used from the results of the "Mangrove Observation Project", a joint project between the Guinea Ministry of Agriculture and Livestock Breeding, Ministry of Fishing and Aquaculture, the University of Bordeaux III, and other research institutions (Rossi et al., 2001). The data for the water level were obtained by old measurements presented by Capo (2006). The starting layer containing the region to be used as the basis for analysis in ArcGIS was made using the SHP files (Shape files) of Guinea on Mapmakerdata (- website-eu-west- 1.amazonaws.com/library/stacks/Africa/Guinea/inde x.htm), Divas-gis.org (Diva-GIS, 2014) and data on ponds and rivers of Africa from the database of the FAO/Aquastat (Aquastat, 2014). The data presented in this study are based on the work done by Sylvain Capo on the estuary of the River Konkouré Guinea, presented in his PhD Thesis (Capo, 2006). For his work, five tidal platforms have been installed in the estuary of Konkouré between December 2000 and July 2001 by a team of U.S. DIVHA IRD. These devices were designed to control simultaneously and continuously the water levels at stations K12, K14, K15, and K18 (Fig. 2). Each tidal platform was composed of: a base of 9m 2 with one stake 1 to 2 m long at each corner; three vertical elements 1m 2 surface, the upper and lower elements having a height of 3m, the intermediate element a height of 2m; a measurement well; a protection cubicle for the recorder. Each station was equipped with a level recorder type OTT Thalimedes (float-operated shaft encoder water level sensor) configured with a scan time of 1 minute and storing the values of water height every 10 minutes (Capo, 2006). Water levels were measured from downstream to upstream (respectively K15, K14, K18 and K12) in the arms of Konkoure and Sankine Rivers. The tidal wave in K15 has an almost sinusoidal shape with a slight asymmetry due to the relatively low depth on the Guinean coast. A systematic analysis of water levels between stations K15, K14, K18 and K12 was performed. In spring tide, the amplification of tide amplitude from downstream to upstream of about 35% was observed. The average spring tidal range in K15 is 2.67 m; in K14, further upstream on the arm of Konkouré, the average amplitude is 2.96 m; in K12, the amplitude is 3.67 m. The same phenomenon operates in the arm of a Sankine, however, with less deformation than for Konkoure. The average neap tidal range in K15 is 1.85 m; the tidal range goes to 2.16 m in K14; the amplitude reaches 2.91 m upstream (K12). The tide is amplified from downstream to upstream of 57%. In Fig. 3 it is represented the water level during a sequence of three consecutive spring tides, for a period of high flow for Konkouré River. It stresses the deformation suffered by the tidal wave when it enters the estuary during early flood. The influence of Konkouré flow on water levels in the estuary occurs from the upstream of the estuary to the median estuary during floods, as shown in Fig. 4. Water levels observed in K15 and K18 during floods are not changed, while the low-water in the estuary upstream (K12) is raised about 2 meters. The presence of a high river flow swells the water level at low tide. To K12 station, the river effect becomes predominant on water levels in flood. The results of the mapping analysis and simulation from ArcGIS are presented as maps to observe and identify the limits of risk areas during the rising waters or in case of floods. The data from the measurements of water level were analyzed and compared with data from four main maps: the map of hydrographic network, the hypsometric map, the precipitation map and the land use map, in order to identify the flood risk areas. The used maps and the results of the analysis are presented in the following sections. Using the existing maps and other information from the literature, a map of the regional hydrographic network was realized, identifying the watershed of the Konkoure River, as the main river of the region which can be correlated with the tide level measurements. The map, presented in Fig. 4 is extended outside the Low Guinea region in order to show the entire watershed. The hypsometric map of Guinea, presented in Fig. 5, was collected from the Wikipedia web page (Wikipedia). The altitudes in the region of study range from 0 to 1200 m, the highest peaks are encountered especially in the East and South-East that the Lower Guinea. These are low-lying areas that facilitate early flooding due to rains but also by the rise of water in the continent. The zone climate is tropical with two seasons. Rainfall is relatively high, with annual values exceeding 4000 mm/year in the area of Conakry. This is one of the wettest regions in West Africa with a large inter-annual variability (Rossi et al., ...

Context 2

... the water related disasters (floods, droughts, tropical storms) are already a major challenge for sustainable human development (Ajibade et al., 2013; Okada et al., 2014; Tingsanchali, 2012). Facing with a deficient flood management, climate changes, deforestation, large zones of Africa continent are exposed to water related disasters (Descroix et al., 2012; Padi et al., 2011; Runge and Nguimalet, 2005). Located south-west of West Africa, Guinea covers an area of 245,857 km2. It is a coastal country with 350 km of Atlantic coast, halfway between the Equator and the Tropic of Cancer, between 7°05' and 12°51' north latitude and 7°30' and 15°10' west longitudes. Many major rivers in West Africa have their source in Guinea: Niger, Senegal, Koliba (Corrubal), Kolenté (Great Scacie), Cavaly, Diani, Gambia, Makona etc. Two natural regions are particularly affected by flooding due to their geomorphology, hydrological functioning of their rivers and rainfall intensity. These are Upper Guinea and Lower Guinea (Maritime Guinea), the last one being affected both by seawater inundation and flooding by coastal rivers (Capo et al., 2009). Coastal regions of Guinea and neighboring countries, Guinea Bissau and Sierra Leone, have a large area of mangrove swampy plains. The loss of such surfaces due to flooding and erosion relate with the rise in sea level and global climate change threatens to derail the economy of the coastal countries of West Africa (Capo, 2006). The morphology of lower coast favorizes its exposure to marine and environmental problems, the greatest concern is coastal erosion and flooding resulting from the combined action of several factors, including heavy rains that increase flood coastal rivers, stagnant waters of the city due to rain at the inner gentle slopes, the rise of the river water by sea water during high tides. The main objective of this study is the delineation of flooding zones, combining GIS technology and the measurement of height of water in Guinea, particularly in Lower Guinea (Maritime Guinea). These areas must be generally localized; the existence of a database from geographic information systems (GIS) on the water level and the flow of the water is very important and necessary for floods modeling (Anoh et al., 2012; Furdu et al., 2013; Liu et al., 2013). This mapping using GIS is very useful to support the politics and urban management confronted with an urban explosion, which is presently taking place. Opening out of the Atlantic Ocean by a front of around 300 km, the administrative area of Lower Guinea (Guinea Maritime) occupies a strip having about 150 km average width, between the foothills of the Fouta Djallon plateau in the east and the ocean to the west (Fig. 1). With a surface of 43,730 km2, it includes almost 36% of the country's population thanks to the presence of the capital Conakry. It has eight prefectures in addition to the capital Conakry, the prefectures of Boffa, Boke Coyah, Dubreka Forécariah Fria and Kindia Télimélé. Lower Guinea is characterized by abundant rainfall, generally higher than 2,000 mm/year, with a pronounced dry season of at least five months, but also by a poorly differentiated river system, widely open to the ocean through many channels and estuaries. This drainage network is affected, sometimes over distances of up to one hundred km, by high amplitude tides exceeding 5m in estuaries of the North, the phenomenon being favored by very low slopes, through widely developed mudflats (Rossi et al., 2001). For the purpose of this study, the spatial data analysis was made through ArcGIS 10.0. Other maps were used from the results of the "Mangrove Observation Project", a joint project between the Guinea Ministry of Agriculture and Livestock Breeding, Ministry of Fishing and Aquaculture, the University of Bordeaux III, and other research institutions (Rossi et al., 2001). The data for the water level were obtained by old measurements presented by Capo (2006). The starting layer containing the region to be used as the basis for analysis in ArcGIS was made using the SHP files (Shape files) of Guinea on Mapmakerdata (- website-eu-west- 1.amazonaws.com/library/stacks/Africa/Guinea/inde x.htm), Divas-gis.org (Diva-GIS, 2014) and data on ponds and rivers of Africa from the database of the FAO/Aquastat (Aquastat, 2014). The data presented in this study are based on the work done by Sylvain Capo on the estuary of the River Konkouré Guinea, presented in his PhD Thesis (Capo, 2006). For his work, five tidal platforms have been installed in the estuary of Konkouré between December 2000 and July 2001 by a team of U.S. DIVHA IRD. These devices were designed to control simultaneously and continuously the water levels at stations K12, K14, K15, and K18 (Fig. 2). Each tidal platform was composed of: a base of 9m 2 with one stake 1 to 2 m long at each corner; three vertical elements 1m 2 surface, the upper and lower elements having a height of 3m, the intermediate element a height of 2m; a measurement well; a protection cubicle for the recorder. Each station was equipped with a level recorder type OTT Thalimedes (float-operated shaft encoder water level sensor) configured with a scan time of 1 minute and storing the values of water height every 10 minutes (Capo, 2006). Water levels were measured from downstream to upstream (respectively K15, K14, K18 and K12) in the arms of Konkoure and Sankine Rivers. The tidal wave in K15 has an almost sinusoidal shape with a slight asymmetry due to the relatively low depth on the Guinean coast. A systematic analysis of water levels between stations K15, K14, K18 and K12 was performed. In spring tide, the amplification of tide amplitude from downstream to upstream of about 35% was observed. The average spring tidal range in K15 is 2.67 m; in K14, further upstream on the arm of Konkouré, the average amplitude is 2.96 m; in K12, the amplitude is 3.67 m. The same phenomenon operates in the arm of a Sankine, however, with less deformation than for Konkoure. The average neap tidal range in K15 is 1.85 m; the tidal range goes to 2.16 m in K14; the amplitude reaches 2.91 m upstream (K12). The tide is amplified from downstream to upstream of 57%. In Fig. 3 it is represented the water level during a sequence of three consecutive spring tides, for a period of high flow for Konkouré River. It stresses the deformation suffered by the tidal wave when it enters the estuary during early flood. The influence of Konkouré flow on water levels in the estuary occurs from the upstream of the estuary to the median estuary during floods, as shown in Fig. 4. Water levels observed in K15 and K18 during floods are not changed, while the low-water in the estuary upstream (K12) is raised about 2 meters. The presence of a high river flow swells the water level at low tide. To K12 station, the river effect becomes predominant on water levels in flood. The results of the mapping analysis and simulation from ArcGIS are presented as maps to observe and identify the limits of risk areas during the rising waters or in case of floods. The data from the measurements of water level were analyzed and compared with data from four main maps: the map of hydrographic network, the hypsometric map, the precipitation map and the land use map, in order to identify the flood risk areas. The used maps and the results of the analysis are presented in the following sections. Using the existing maps and other information from the literature, a map of the regional hydrographic network was realized, identifying the watershed of the Konkoure River, as the main river of the region which can be correlated with the tide level measurements. The map, presented in Fig. 4 is extended outside the Low Guinea region in order to show the entire watershed. The hypsometric map of Guinea, presented in Fig. 5, was collected from the Wikipedia web page (Wikipedia). The altitudes in the region of study range from 0 to 1200 m, the highest peaks are encountered especially in the East and South-East that the Lower Guinea. These are low-lying areas that facilitate early flooding due to rains but also by the rise of water in the continent. The zone climate is tropical with two seasons. Rainfall is relatively high, with annual values exceeding 4000 mm/year in the area of Conakry. This is one of the wettest regions in West Africa with a large inter-annual variability (Rossi et al., ...