Coastal erosion affects all countries along Africa's Atlantic coast, with flooding being the second most significant threat after erosion. In Cameroon, this phenomenon is particularly alarming, especially along the Kribian coast and in the Wouri estuary. Cape Cameroon in the Wouri estuary is experiencing severe erosion. The question arises whether the current erosion in the Wouri estuary is due to internal factors or part of a broader dynamic affecting all Atlantic African coasts. This inquiry aims to understand the causes of coastal erosion in the Wouri estuary. To address this, we used a large-scale map based on Landsat images (30m spatial resolution) covering the entire Wouri estuary and a smaller-scale map based on Pleiades images (0.5m spatial resolution) focusing on Cape Cameroon. The Digital Shoreline Analysis System (DSAS) in ArcGis 10.7.1 facilitated this analysis. Aerial photographs from 1916 to 2016 helped assess changes in Douala and their impact on mangroves over time. Diachronic land cover analysis employed Landsat images and the maximum likelihood algorithm. The Landsat results indicate that coastal erosion affects over 75% of the Wouri estuary's coastline, with average recession rates ranging from ± 3m to ± 11m between 1975 and 2020. Hotspots of regression include Cape Cameroon, the coastal spit around Manoka Island, and the area near the APD. High-resolution images from Pleiades and Google Earth confirm the severity of coastal erosion. The northern tip of Cape Cameroon is retreating at an estimated rate of + 15m/year, with the most significant recession measured at the tip of Cape Cameroon, estimated at 53 meters. A small accretion of + 5m is visible at Toubé, spanning no more than 100 meters.
Case Report
Hydrogeomorphological dynamics and erosion of the soft coasts in tropical Africa, the case study of the Wouri estuary, Cameroon
https://doi.org/10.21203/rs.3.rs-4907645/v1
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hydrogeomorphological dynamic
Coastal erosion
soft coasts
tropical Africa
Wouri estuary
Cameroon
Gulf of Guinea
Land-sea interface zones with complex ecology are areas that change quickly and are connected to dozens of factors (Daeden, 2015). One thing is that since the seas got bigger 6,000 years ago, the coasts are moving away. According to the International Geographic Union (Fabbri, 2012), 70% of coasts are experiencing coastal erosion, 20% are stable, and only 10% are getting firmer. It's hard to figure out what causes coastal erosion because there are many different environmental problems involved. The coastal retreat is caused by human activities. They cause global warming and rising sea levels (Cazenave et al., 2015). They also keep sediments upstream by building dams, making it harder for sediment to move downstream (Le Lay & Germaine, 2017), and harm natural ecosystems (Fehri, 2011); Andrieu et al., 2019).
Africa is a continent where man has a strong influence on natural processes. For example, mangroves are a good barrier to coastal erosion on the African coast and in another side, (Bocquet & Tech 2018); (Mitra, 2020); Duke et al,. (2007).
Global warming is causing water levels to rise, which could cause flooding (Douglas et al., 2008; Molua (2009); Klutse et al., 2018); Toulmin, (2009); Weber et al., (2018). On the other hand, these coasts are subject to strong anthropogenic pressure and deforestation. Rabehi et al (2023) report a recession of more than 5m/year around the Mediterranean. The situation is similar in the soft coasts of the far north of Morocco (Salmon et al., 2010), in Algeria (Benkadja & Boussag, 2015), and on the French coast (Rey et al., 2019).
Cameroon is no different from other countries. Mfombam Nsangou (2016) says that of the 134 849 m of coastline in southern Cameroon, 118 020 km (87.52%) is eroding, 16 620 m (21.33%) is accreting, and 320 m (0.24%) is rising looked at how the coast of Cameroon has changed. After finding areas that had been eroded, they found that the retreat was more than + 0.54m/year between 1986 and 2015 Ondoa et al., (2015). They found that the Cameroonian coast has a lot of social and economic activities, and there is a lot of water in the Wouri estuary, Kribi, and Campo (Tchindjang, et al., (2019). According to Fossi Fotsi et al., (2019) and Ellison, (2012), the Wouri estuary is one of the hotspots of this coastal erosion. This erosion would hurt the estuary disproportionately.
This estuary, which is the subject of this research, is also a home of one of the biggest mangrove forests in Africa (UNEP, 2007). It is the biggest port in Central African (Ovono, 2017). The mangrove ecosystem is heavily populated by humans (Dzalla Nguangué, 2013; Din et al. Joanna and Zouh (2012) also say that intertidal mangroves have also been affected by the relative rise in sea level.
This study aims to characterize environmental changes in the Wouri estuary between 1975 and 2024: (i) coastal morphology (ii) coastline dynamics to identify/quantify eroding and/or accreting coastal sections, (iii) identifying and analyzing erosion factors in the Wouri estuary. The methodological approach combines remote sensing (using Landsat Pleiade and Google Earth images), GIS and field observations. The DSAS V5 tool associated to ArcMap 10.5® are used to map coastal erosion, with theoretical support from the work of Sparfel, (2011), among others.
This overview enables us to discuss the possible erosion factors in the Wouri estuary, and their specific features compared with other coasts in Atlantic Africa.
II-1 Geographical and administrative situation of the Wouri estuary
The Wouri estuary is largely located in the Wouri department, whose capital is Douala, and the Sanaga Maritime department, whose capital is Edéa. The location is between 9° 28' 00" and 9° 28' 30" N, then 3° 54' 20" and 3° 54' 40" E (Fig. 1). The estuary covers an area of approximately 1200 km² and open to the Atlantic Ocean (Ndongo et al., 2015); he is divided into the communes of Douala 1, 2, 3, 4, 5, 6, Mouanko, Tiko and Dizangué.
The main city of the Wouri estuary is Douala. It occupies the highest elevation on either side of the Wouri. The city is experiencing rapid growth. In 1916, there were approximately 15,255 inhabitants (Mainet, 1983) In 1976, the population of Douala had reached 400,000, making it a true metropolis (Hatcheu, 2003) According to BUCREP, (2010) statistics, the estuary currently has over 5 million inhabitants, a large proportion of whom live in the economic city of Douala. Douala is also home to 60% of Cameroon's industries, as well as the largest port in Central Africa in terms of maritime traffic. Douala is also the home to 60% of Cameroon's industries.
II-2 Land use in the Wouri estuary
The estuary is a coastal area occupied by a large mangrove forest (24% of Cameroon's mangroves), the most important of which have a high conservation value. These include the Douala-Edéa National Park and the Mabé Nature Reserve. The mangrove consists of mangrove trees, mainly Rhizophora spp., but also Avicenna germinans and Nypa fruticans (Din, 2008). An Atlantic forest on the south-eastern shore of the estuary plays an important role in protecting and conserving biodiversity. Little subsistence farming takes place on Manoka Island and in part of the Douala-Edéa reserve (Duka et al, 2007; Bojang & Ndeso-Atanga, 2009). There are visible environmental impacts (Dzalla Ngangué, 2013) in this highly anthropogenic environment.
II-3 Climatic characteristics
The Wouri estuary area is located in the equatorial climatic band, with extremely abundant rainfall (Douala: 4,025 mm/year) and widely distributed throughout the year, creating very characteristic conditions for weathering by leaching (Lafond, 1965). Locally, it lies in the coastal agro-ecological zone with monomodal rainfall, which is characterised by higher rainfall than anywhere else, according to the National Climate Change Adaptation Plan (MINEPDED, 2015).
This coastal equatorial climate has a short dry season (November to March), and a long rainy season (April to October). The region receives 3,500 to 4,500 mm of rainfall per year, with peaks in July and August, with an average temperature of 26.4°C. (Din et al., 2015).
The solar radiation intensity ranged from 3.04 kWh/m2/dr to 5.41 kWh/m2/dr in the city of Douala, with an average value of 4.31 kWh/m²/dr. Minimum and maximum values are recorded in the rainy season (July to September) and maximum values in the dry season (December to February) (Mbiadjeu-Lawou et al., 2017).
II-4 The hydrogeomorphological context of the Wouri estuary.
The information on hydrogeomorphological dynamics used in this section comes mainly from the literature review. In general, four major rivers supply water to the estuary. These are the Sanaga, the Dibamba, the Wouri, and the Moungo, from east to west. According to a report by the Netherland Economic Institute (NEI, 1993), fluvial inputs of sediment into the estuary are estimated at 1 million tonnes per year, 70% of which occurs during the rainy season. This results in moderate sedimentation. Most of the sediment load feeds the marshes of the mangrove estuary.
II-4-1 The Sanaga River
Until 1896, the Sanaga merged with the Wouri estuary (Morin & Kuété, 1988). Now, it only flows into the estuary during exceptional periods of intense flooding, as it now discharges to the south-east of the Wouri estuary. In the Adamaoua region, this river is considered to be Cameroon's water reservoir. A vast sedimentary basin is formed by a succession of plateau, bounded to the west by the Cameroon Volcanic Line (CVL), a succession of volcanic ranges, and to the north by the Adamaoua plateau. The climate here is subtropical, with annual rainfall ranging from 900 to 1,500 mm. The length is 918 km and the average flow rate is 2072 m3/s, six times the rate of the Wouri. The flow was put at 520 m3 /s at low water and 6,000 m3/s at high water by Kouandji Bekoumb (2016) More than a quarter of Cameroon is covered by its catchment area (Dubreuil et al. 1975) The Precambrian basement or "base formation" dominates the geology of the Sanaga basin. Clay and clayey-sandy sediments cover almost the entire surface catchment area. The Sanaga flows through a highly anthropogenic catchment with high rainfall, particularly in its western part. More than 2.5 million cubic meters of silt and clay transported annually by the Sanaga come from the west (Fig. 3). The Bamiléké and Bamoun tributaries (location/catchment area) have a turbidity of over 950 g/m3 (Morin, 1981) Human activities and the changing seasons affect the mobility of these sediments (Ndam Ngoupayou et al., 2016) There is a high sediment load at the Sanaga's mouth, which shows its high sediment load. The Souelaba headland, which can be up to 200 m wide, is reworked by marine currents once at the mouth. The island of Manoka is protected by this low ridge, with a forest covering, that's between 5 and 6 meters high.
II-4-2 The Dibamba River
To the south of the Wouri river lies the Dibamba basin. It rises in the western highlands and drains a series of small hills along the lower Sanaga watershed (maximum altitude 1,053 m, with Mount Moukak). The terrain drained is exclusively weathered crystalline basement, which is weathered. The Dibamba River is 150km long and has a catchment area of 2,400km2. The estimated flood flow at its entrance ranges from 480 m3/s (Olivry, 1986) to 1,000 m3/s (Kouandji Bekoumb, 2016). The flow drops to 70 m3 /s at low water during the dry season. The average annual rainfall in the region amounts to 2,660 mm per year. The average annual run-off is around 1,600 mm/year, giving an average of 125 m3/s (Olivry, 1974). In the section closest to the city of Douala, the river is heavily used for sand extraction. The river joins a tidal arm in the south-eastern part of the estuary and flows into the Wouri estuary, passing through the mangroves to the south of Douala.
II-4-3 The Wouri River
The Wouri is the second-largest river in the region, supplying water to the Wouri estuary. The basin covers 21,600 km2 (Darnault, 1947) and the area surrounding Douala covers 11,700 km2. Two main tributaries feed it. The first is the Nkam, which comes from the mountains in the north-western part of the basin. There are sharp topographical contrasts in the Makombé, which drains a terrain of sharp topographical contrasts. Most of the Wouri basin has a geological cover consisting of basement formations represented mainly by biotite gneiss and secondarily by anatexites and syntectonic granites. It is surrounded to the north and west by volcanic chains formed by a thick cover of Pliocene and Quaternary sediments (Gèze, 1941). The sedimentary load is made up of unconsolidated volcanic material.
The Wouri's average flow at its mouth is 140 m3 /s at low water and 2,000 m3 /s at high water, according to Kouandji Bekoumb (2016). It has an average gradient of 5.7 m at the outlet.
II-4-4 The Moungo River
Mountain massifs around the Monts Rumpi, which reaches 1,768 m, are the source of the Moungo River, which is 150 km long and rises from the slopes of the mountain massifs around the Monts Rumpi (Pollard et al., 2004). The Mungo has a surface area of 4,200 km2. According to Mahé & Olivry (1995), it has an average cumulative flow of 10 m3 /s at low water and 1,300 m3 /s at high water. It flows into the Bay of Mudeka through a delta (Mbesse, 2013).
For around 100 km, the river is navigable in its southern part, then flows through the coastal plain before entering the mangroves, where it divides into numerous small channels that empty into the Wouri estuary. The geography of the catchment is diverse. A portion of Cameroon's volcanic line and Mt Cameroun are drains in the upstream part of the Moungo, which flows over bedrock. Conglomerate, sandstone (Basic Sandstone), dark grey shaly clay, marl rich in organic matter and thin limestone beds (Belmonte, 1996; Ntamak-Nida et al., 2010) mark its catchment (Nguene et al., 1992; ECL, 2002). Continental and fluvio-deltaic deposits dominate at its mouth, with some intercalations of marine facies covered by mangroves.
There are several other major rivers that contribute sediment to the Wouri estuary, in addition to these major rivers. These are the Benoe, Ombe, Tole, and Mokota (Fig. 1), which rise on the slopes of Mount Cameroon and drain a number of mountains, including Bokesi Mount (495 m), Ebongo Mount (588 m), Engel Mount (581 m) Mount Cameroon is the main summit and is a Quaternary volcano made up of basalts (oceanite), trachy-basalts (hawaiite), tranchyte, tephrite and phonate Dedzo et al., 2011. It peaks at over 4040 m. The northern edge of the estuary is marked by it.
II-5 Intrinsic description of the Wouri estuary
There are multiple straight channels separated by sufficiently vegetated banks with large accumulations of sediment at the outlet of the Wouri river, well before the installation of the port. The accumulation of sediment in the form of banks leads to the division of the initial channel, causing the divergence of flows and, consequently, a local reduction in flow velocity (Wiederkehr et al., 2008). The Wouri forms a sufficiently calibrated straight channel, with tidal channels on either side, before entering the ocean to the south of the autonomous port of Douala (APD) (Fig. 3).
The soil in the estuary is ferralitic in the landward parts and hydromorphic sandy loam on the coastal fringes (Awalou et al., 2014). The original forest vegetation has been degraded by human activity and consists mainly of grasses, shrubs, fruit trees, arable species, and mangroves around the Wouri.
II-5-1 Geomorphological configuration around the estuary itself
Mt Cameroun, an active volcano rising to 4095m, dominates the geomorphological context of the estuary. The Wouri estuary is crowned by a series of volcanic mountains, all belonging to the Cameroonian volcanic arc (Fitton, 1987).
The Moungo catchment area contains four mountain ranges: Mount Bahosi (1,487m), Ngongonja (609 m), Mount Mumdeme (461m), and Mount Ngamokoli (322m). There are 12 mountain ranges in the Wouri catchment area, the most important of which are Mount Manengouba (Dongmo et al., 2001) (2,411 m), Mount Lahke (1,407 m), Mount Nlonako (1,653 m), and Mount Moulak. Lastly, the largest number of ranges are located in the Sanaga catchment area (more than 26) Mount Toukwet (1130 m), Mount Lekete (1086 m), Mount Nolanga (813 m), etc. are some of the most important ones (Fig. 4).
The geological context of the Wouri estuary.
There are three prominent geological entities at the level of the Wouri estuary: (1) a crystalline zone, formed of migmatites and granites (Weecksteen 1957), developed towards the NE dating from the Palaeozoic; (2) the Quaternary volcanic zone of Mt Cameroon, corresponding to the highest peaks and essentially basaltic to the NW. The basin is composed of loose, low-lying coasts stretching for at least 220 km from the mouth of the Sanaga River to the Wouri estuary. Mt Cameroun is the most active volcano in the Cameroon volcanic line, an active tectonic-magmatic alignment oriented N30°E, comprising 6 major volcanoes in the oceanic domain and more than 7 in the continental domain (Ngounounoa et al. 2006). The Mt Cameroon morphology of these cinder cones plays a significant role in shaping the shoreline (IUCN/WWF and WCS, 2015).
Sand and clay make up the soil in the Wouri estuary. It's a fragile substrate, but it's also protected and compensated for by nature. There is a natural barrier against coastal erosion provided by a thick layer of mangroves and coastal forest. Four major rivers, the Sanaga, Dibamba, Wouri and Moungo, with significant sediment loads, supply the estuary with sediment (Fig. 5).
The Douala sedimentary basin, composed of low-lying unconsolidated coasts stretching for at least 220 km from the mouth of the Sanaga River to the Wouri estuary, is where the Wouri estuary emerges from.
Inside the estuary, altitudes vary from 0 to 16m (Fig. 1), with relatively flat relief made up mainly of loose rock that is highly vulnerable to erosion.
The Wouri estuary lies at the heart of the Douala sedimentary basin, centred on an old PanAfrican mylonite syncline, along the axis of which flows the Wouri River (Morin & Kuété, 1988). In this environment, differences in altitude are small and are essentially made up of the remains of sandy strips and small islands of Mio-Pliocene sands and gravels, sometimes bearing a ferruginous armourstone, as in the middle of the Docteur (Boye et al. 1975).
Sand and Paleogene-Neogene alluvium cover the eastern shoreline of the Wouri estuary, from the mouth of the River Nyong (Takem, 2010), recent volcanic formations of the Mount Cameroon region are extended to the west by the islands of Bioko and Soto. The sedimentary deposits are coherent, resulting in a degree of heterogeneity in the shape of the coastline. The morphology of these structures plays an important role in the appearance of the coastline (IUCN, WWF, and WCS, 2015).
Two authors have divided the Wouri estuary into three distinct geological entities. A crystalline zone composed of migmatites and granites (Weecksteen, J957) developed towards the northeast; a volcanic zone corresponding to the highest peaks and primarily basaltic (Gèze, 1941&1943).
II-5-2 Marine context in the Wouri estuary
II-5-2-1 The tide in the Wouri estuary
The Wouri estuary is where the strongest tides in Cameroon are found (Ondoa et al. 2018) Tide heights in the estuary vary greatly; they are semi- diurnal and asymmetrical (Onguene et al., 2014) There is an average amplitude of 2.5 meters towards the port However, surges are frequent during windstorms and are highly variable in the estuary (on average 2.8 m towards the Port and 1.7 m towards Cape Cameroon) The tides are amplified in the shallow parts of the estuary at the level of the Wouri and Dibanda rivers (Onguene et al., 2014).
II-5-2-2 Prevailing winds in the Wouri estuary
From 1960 to 2001, we have been able to understand the wind directions and speeds in the Wouri estuary by analyzing the annual and seasonal boreal wind rose for the Cameroon coast.
The seasons affect the winds in the Wouri estuary. In the winter, the prevailing winds are SW, S, and SSW. The same trend will continue in the spring, with the only difference being that W winds (16 km/h) will make their appearance and S winds will decrease in intensity. In the summer, the SW and SSW winds dominate, followed by the SWS and S winds. In autumn, the SW winds take over again, with more than 24 km/h, followed by the WSW and SSW (16 km/h) winds Keugne Signe, 2018). An annual report shows that SW winds dominate (Fig. 6). The predominance of SW winds on the Cameroon coast, coming from the Atlantic Ocean, was supported by Morin and Kuété (1988) and Leduc-Leballeur, (2012).
The overcoast (Ondoa et al., 2018) varies from near the Port Autonome de Douala to Cap Cameroun.
The trade winds (regular intertropical wind between 23°27 north and 23°27 south) are very active in the Gulf of Guinea, blowing regularly from east to west from subtropical high pressure (subtropical ridge) They occur mainly between December and March, during the dry season.
II-5-2-3 Currents at work in the Wouri estuary
In the estuary, the main current is longshore drift. Sediment transport and accumulation in the outer estuary are influenced by it. The waves generated by the local SW wind and long-period swells that affect the entire West African coast make it essentially conditioned. Two types of swell have been identified (Ondoa, 2018) The NNW swell is caused by easterly winds in the region west of Namibia, while the other is caused by westerly winds in the southern Atlantic Ocean around 50oS (Ondoa, 2018) Swell height varies significantly with a maximum occurring in June-July-August and a minimum occurring in December-January-February (Ondoa, 2018).
In the Wouri estuary, a counter-current situation occurs. It brings together river currents related to the Wouri, Moungo, Dibamba, and Sanaga rivers (Fig. 7), as well as ocean currents (mainly longshore drift and large SW swells) The considerable quantities of clay, sand, and silt carried by the latter tend to accumulate in the estuary because of this convergence of several currents, which limits the speed of the coastal currents and influences the fluvial hydro-sedimentary dynamics. Fluvial currents are weak at Douala at less than 0.70 m/s (NEI, 1993).
All things considered, large waves influence this coastline, and the materials brought in by the rivers (sand and kaolin clays, etc.) are absorbed by the south-east longshore drift and a west-east coastal current (Morin and Kueté, 1988) Longshore drift and coastal currents play an important role in the remobilization of sediment, in the form of longitudinal sediment transport (Le Berre, 2017).
II-5-2-4 Variation in bathymetry in the Wouri estuary
The presence of channels causes significant variations in the depth of the Wouri estuary. The analyzed area has a total surface area of around 117,2 ha (source: Navionic data processing) The water levels downstream of the estuary are lower than those at its entrance (Fig. 9) A 200-meter-wide and 650-meter-long channel reaches depths of over 20 m in places in the most downstream part of the estuary, to the southwest of Cape Cameroon. There is a wider and deeper channel in the southwest part of the estuary, which gradually narrows to the level of the autonomous port of Douala (APD). There are two arms to this main channel, one of which borders the island of Manoka to the north, and the other is 25m deep, reaching the city of Douala. The access channel to the port is permanently dredged.
Close to farmland surrounded by mangrove forests, the lowest bathymetric values range from 2 to 8 meters (Fig. 8). The speed of waves and their height at shore are affected by this reduction in depth. Tidal currents have little or no effect on the outermost parts of the estuary, where longshore drift is greatest, as a result. They are only allowed on the barrier beaches.
At Cap Cameroun, coastal erosion was mapped, as well as throughout the Wouri estuary. A spatial resolution of 0.5 m was used in the first case. Landsat images were used in the second case.
III-1 Pleiades and Google Earth images
A series of images from several sources are used. A series of images from several sources are used. Pleiades (2013 & 2018) and Google Earth (2000 and 2016) images with identical resolutions were used to map coastal erosion on the Cape Cameroon coastline (Table II). This is the section of the estuary where erosion is the most intense.
Table I
Characteristics of Pleiades (2013 & 2018) and Google Earth (2000 and 2016) images used in small-scale coastal erosion mapping of Cape Cameroon, Wouri-Cameroon estuary.
| Cape Cameroon | Images Pléiades 1 |
|---|---|
| Date | 16 December 2018 |
| Resolution | 0.5 m |
| Sensor | Pléiade |
| Identifier | DS_PHR1A_201812160944155_FR1_PX_E009N03_0622_03147 |
| Cape Cameroon | Pleiades 2 images |
| Date taken | 19 April 2013 |
| Resolution | 0.5 m |
| Sensor | Pléiade |
| Identifier | DS_PHR1B_201304190944536_FR1_PX_E009N03_0721_01956-1 |
| Cape Cameroon | Google Earth 1&2 images |
| Date taken | 23/11/2000 & 3/05/2016 |
| Resolution | 0.5 m |
III-2 Landsat images process
Landsat images are used on a large scale to analyze coastline dynamics between 1975 and 2024 (49 years) and to classify land use. The 1975 image has been resampled to reduce its spatial resolution to 30 m, which is compatible with the other images. To facilitate analysis, the estuary has been segmented into six zones (Fig. 8).
III-2 Coastal erosion mapping method
It uses the DSAS tool, which is grafted onto ArcGis software. Parameterization at several stages is required for the production of erosion models.
III-2-1 Extraction of coastal lines
The process was automated for images with minimal cloud cover on the Wouri estuary (1975; 2017 and 2018) (Fig. 9). There were no manual corrections made to the sections of the line concerned in the other images with clouds. The classification was done using ArcGIS 10.7.1. The result is then transformed into a vector and a polyline. The undesirable parts are trimmed.
III-2-2 Treatment of different coastal lines
The shoreline is formed by merging the various linear features. Around this merge file, a buffer of 100 meters was created for this project. The terrestrial part of the buffer (Offshore) created is automatically digitized to define the baseline. This is the reference point for the calculations of shoreline dynamics.
III-2-3 Treatment under DSAS
They're 50 meters apart. The transects1 are 500 m long. There's a 50m straight line with a 30m pixel error. The resolution of each image is reflected in these error margins. The orientation of the transects was defined by the 'Onshore' option. The Onshore option means that the transects generated will be oriented towards the ocean, and the End Point Rate (EPR) calculations will be based on the oldest coastline.
III-2-4 Calculation of statistics and quantification of coastline recession (EPR, NSM, LRR)
DSAS performs several calculations on global changes in coastline positions (Thieler et al., 2005). The annual rate of coastline change is calculated for each period. In this research, we used the End Point Rate (EPR) method. This End Point Rate method is the distance on the transect between two coastlines, the most recent and the oldest, divided by the number of years separating these coastlines (Himmelstoss et al 2018) based on the formula:
R is the speed in meters per year (m/year), D is the distance in meters, and Te is the time elapsed between the oldest and the most recent coastline (years).
The uncertainties of the annual rate of change (in meters per year) are defined, together with the 95% confidence intervals LCI95 or WCI95 (According to (Himmelstoss et al 2018), LCI is the Confience Interval of Linear Regression–LCI%, where % is the CI value entered in the Calculate Rates window; WCI Confience Interval of Weighted Linear Regression–WCI%, where % is the CI value entered in the Calculate Rates window). The uncertainty value considers both positional uncertainties associated with natural influences on the position of the coastline (wind, waves and tides) and measurement uncertainties (e.g. digitization or global positioning system errors) (Ruggiero et al., (2013). We based our work on the USGS report of 2006, which expresses the error mages from the equations:
The rate of shoreline changes for a single transect is calculated as the quadrature sum of the uncertainties for each year of shoreline position divided by the number of years between shoreline surveys:
Up1i and Up2 are the uncertainties of the position of the first and second shoreline (year 1), respectively, at transect i, calculated by Eq. 2 or 3. For the linear regression method (long term), the uncertainty of the shoreline change rate of a single transect URi is found here as a percentage at 90% on the linear (Ruggiero, 2013). Table II summarizes the different margins of error calculated.
Table II
Errors linked to the model.
| Date | 1975–2000 | 2000–2016 | 2016–2024 |
|---|---|---|---|
| Pixel error (Ep) | 2 | 5 | / |
| RMS ortho-rectification (ERMs) | 15 | 22 | 12 |
| Digitisation error (Ed) | 7 | 15 | / |
| Planimetric error (PE) | / | 23 | 23 |
| Total error (Et) | 15 | 38 | 10 |
| Measured errors (Em) (m) | 42 | 56 | 30 |
| Annual error (Ea) (m/44 years) | / | 0,65 | |
| Uncertainties in calculations (ECI) (m/year) | 2 | 7,2 | 0,52 |
IV-1 Mapping coastal erosion in the Wouri estuary using Landsat images
Large-scale cartographic processing of coastal erosion was based on Landsat images from Landsat. For an expressive representation, we opted for a division into four periods: 1975–1986, 1986–2000, 2000–2016, and 2016–2024. The maps are characterized by erosion rates expressed in m/year (or the final recession rate EPR). The distance of shoreline movement is divided by the time interval between the two shorelines to arrive at these rates. It's measured in meters per year. Positive values > 10 indicate accretion while negative values 10 indicate erosion. Positive values > 10 indicate accretion while negative values 10 indicate erosion. There are retreats and advances between − 10 and 10 between the two values.
IV-1-1 Large-scale coastline dynamics between 1975–1986 in EPR
The Wouri estuary began to show signs of eroding between 1975 and 1986, but they remained uneven. Over a distance of around 1.5 km, zone 1 is the most striking point. Then there's zone 3 between Pointe Olga and Malanda Massadi (Fig. 10) and the Dongo zone at Pointe Malimba. The area around Manoka shows erosion, though it's scattered. On the west coast of the island, it can be seen near the Number Two Creek area, and in the south-western part, near the Epaka I area. Zone 6 (the coastal spit around Manoka Island) is experiencing erosion between Souélaba Point and the village of Youmé.
During these 11 years, statistical values for coastline dynamics are relatively low in some areas of the estuary. The average End Point Rate (EPR) reveals a retreat of -13m in Zone 1, while Zone 2 remains completely stable. Zone 3 exhibits a significant retreat of -40m, and Zone 4 shows a minimal retreat of -2m. Zone 5 experiences a retreat of -25m and an average annual erosion rate of -11m/year. In contrast, accretion is observed in several zones: Zone 1 (+ 21m), Zone 2 (+ 17m/year), Zone 3 (+ 42m), Zone 4 (+ 3m), Zone 5 (+ 2m), and Zone 6 (+ 2m). Notably, Zone 3, corresponding to the area on either side of the Douala Autonomous Port, is the most eroded zone during this period. This timeframe coincides with major works to widen the banks of the Wouri, aimed at increasing the storage capacity of the Douala Autonomous Port.
IV-1-2 Coastline dynamics between 1986–2000 (14 years)
Between 1986 and 2000, widespread erosion occurred throughout the Wouri estuary, with varying degrees of severity (Fig. 11). A maximum erosion zone is observed between Cape Cameroon and Kangué in Zone 1. In contrast, the western part (Mboko) exhibits minimal accretion. Notably, the eastern portion of Zone 2, previously experiencing accretion, is now undergoing significant erosion. The Kombo Moukoko region is shifting from stability to increased instability. In Zone 3, substantial accretion is observed, particularly around the APD channel. The tidal arm southwest of Douala shows minimal erosion. However, erosion has intensified south of Olga Point, especially north of Malanda Massadi. Zone 4 is also experiencing erosion, while the town of Epassi is undergoing accretion. Zone 5, encompassing Manoka Island, exhibits significant variability, with the sector towards Epaka remaining eroded. The northern part of the island, towards Dahomey, shows signs of erosion. At the base of the spit, Zone 6 is expanding southward but eroding towards Souélaba.
Over the 14-year period, statistical analysis of the figure above reveals the following average values for coastline dynamics: Zone 1 exhibits a recession of -25m and accretion of + 8m. Zone 2 shows a recession of -11m and accretion of + 21m. In Zone 3, the coastline retreated by an estimated − 13m and accreted by + 35m. Zone 5 recorded a recession of -16m, compared to 15m of accretion. Finally, Zone 6 shows a recession of -21m and accretion of -23m. Notably, Zone 1, home to Cape Cameroon, the estuary's main economic hub with a population of over 6,000, is the most eroded zone during this period. This area, also encompassing Toubé and Kangué, is densely populated and has a relatively high bathymetric value (4m), making it vulnerable to strong marine currents and severe erosion. Furthermore, the area is primarily occupied by a cosmopolitan fishing community, which exerts significant pressure on the mangrove ecosystem for housing, fish smoking, and timber (Mbevo, 2016).
IV-1-3 Coastline dynamics between 2000–2016 (16 years)
During this 16-year period, the dynamics in the estuary changed again. In zone 1, erosion appears to have been reduced only in the localities of Cape Cameroon and Petit Toubé (Fig. 12) The people of this locality testified that erosion intensified in this area from the year 2000 onwards, destroying houses and forcing people to move inland. Tataw et al (2021) estimate that around 123 houses were destroyed between 2000 and 2017 in Cap Cameroun. Zone 2 shows no signs of erosion anymore; stability and accretion dominate. The same thing happens in zone 3, where the coastline appears to be stable and growing. To the south of the airport, towards Youpwé and to the south of Malanda Massina, the only traces of erosion can be seen. There's more accretion in Zone 4. The island of Manoka follows the same trend. Number Two Creek is the only area that has been eroded. Zone 6 is where the dynamics change completely. Overall, the zone is gaining and stable.
Over this 16-year period, zone 1 remains the most eroded, with an average retreat of -17 m, compared with an average accretion of + 21 m. Zone 2 has once again remained stable. In fact, this zone is sufficiently protected to the north by the Moukalan Tanda point and to the south by the Cap Cameroun point; in addition, the bathymetry is sufficiently low (2 m), which seriously slows the speed of the marine currents that reach it. Zone 3 shows an average retreat of -14 m compared with an accretion of + 28 m. Zone 4 has remained stable. This zone has almost the same characteristics as zone 2. Zone 5 shows an average recession of -11 m and an accretion of + 10 m. Zone 6 does not show any recession, but is rather accreting (+ 32 m).
IV-1-4 Coastline dynamics between 2016–2024 (08 years)
Over the past four (04) years, there has been an increase in the number of eroding sectors in the Wouri estuary (Fig. 13) zone 1 was eroded overall. Along with Cape Cameroon and Petit Toubé, Kangué and Mboko are also deteriorating. The decline in Zone 2 contrasts with the previous period. The same holds true for zone 3, which has undergone remarkable transformations, with the appearance of numerous zones of extreme erosion. There is little accretion at its tip towards the ocean and on its northern side, even though Zone 4 is eroding at its right and left ends. The same thing happens to the island of Manoka (zone 5) which experiences significant erosion, albeit with a few pockets of moderate accretion and stability. On the other hand, Zone 6 is more likely to become fatter, especially at its base, facing the ocean, and at its tip, near the locality of Souélaba.
In this recent 08-year period, zone 1 shows an average retreat of -16 m, compared with an accretion of + 11 m. Zone 2 is experiencing erosion, with an average retreat of -19 m and an average accretion of + 10 m. This zone, whose main locality is Kombo Moukoko, has seen recent demographic growth. The population has risen from around 1,210 in 2010 to over 3,000 in 2024. Most of the population are fishermen. Zone 3 still holds the record for retreats, with − 39 m compared with + 14 m of accretion. This period coincides with the major works to widen the site of the autonomous port of Douala. Zone 4 shows little erosion, with a rate of -8 m and + 9 m of accretion. Zone 5 shows − 21 m of recession compared with + 22 m of accretion. Finally, zone 6 shows an average recession of -9 m and accretion of more than 40 m.
This analysis, based on low spatial resolution Landsat images, provided a general trend in coastal erosion in the Wouri estuary. The Cap Cameroun area, which is currently a hot-spot for coastal erosion in the estuary, needs to be analysed using high spatial resolution images. The following session will therefore zoom in on this area using Pleiades and Google Earth images.
IV-2 Mapping coastal erosion at Cape Cameroon using Pleiades and Google Earth images
The shoreline in the Wouri estuary recedes the most in Cape Cameroon. Mapping large areas has shown this so well. This approach, based on very high-resolution Pleiades images and Google Earth aerial photos, aims to produce more refined statistics, in order to quantify the retreat of the coastline with as little error as possible.
IV-2-1 Evolution of the coastline between 2000 and 2016 (16 years), using Pleiades images from 2013 and Google Earth aerial photographs from 2000 & 2016.
The tip of Cape Cameroon has suffered severe erosion over the last 16 years. As far south as the town of Toubé, it has spread. Cape Cameroon remains the center of erosion. The coastline became appreciably firmer after the town of Toubé (Fig. 14).
Quantitatively, the magnitude of the recession is -5m to the north of Cape Cameroon and − 32m to the south of the said locale. The recession at Petit Toubé was − 8 m. The linear length to the north of Cape Cameroon shows a slight increase in accretion. An elevation of + 2 m was observed only at the southern end (Fig. 15).
IV-2-3 From 2016 to 2018 (2 years)
The coastal strip remains eroded in this period. The northern end of Cape Cameroon has a recession estimated at + 15 m. There were two accretions to the north of Cape Cameroon. The most significant recession is measured at Cape Cameroon and is estimated to be 53 meters. At Toubé, a small accretion of + 5 is visible over a distance of no more than 100 meters. The recession of + 40 m was reached to the south of Toubé, where erosion increased over a distance of more than 2 km. Figures 16 and 17 show the behavior of the coastline during this analysis period.
If we look at the results obtained with Landsat and those obtained with Pleiades, we can see that there is consistency between the two. In the period 2000–2016, both image sources showed erosion at Cape Cameroon. The tip of Cape Cameroun and the town of Toubé were the main points of control. As a result, Cape Cameroon continues to exhibit the highest rate of shoreline retreat within the Wouri estuary. The remains of houses destroyed by coastal erosion in the intertidal zone were seen on an on-site visit in March 2024 (Fig. 18).
IV-3 Comparison of mapping results with field observations in the Wouri estuary
The previous analyses have been complemented and refined by field observations, testimonies from long-time residents of the intertidal zone, and Google Earth aerial photos from 2000 and 2016. Field observations along the Cameroonian coastline have enabled us to identify remnants (such as house foundations, tree trunks, etc.) that attest to the existence of coastal erosion at Cap Cameroun. A total of five field visits were conducted: two in 2014, one in 2015, one in July-August 2018, and one in May 2019. The most striking spatial evidence in the Wouri estuary is the presence of the FM radio antenna at Cap Cameroun, built in 1992 in the middle of the territory, now located over 300m offshore. Similarly, colonial remnants, such as the prison where Cameroonian resistance fighter Douala Manga Bell was held, provide tangible evidence. Built by the Germans around 1910, this reinforced concrete structure still stands on the eastern shores of Manoka Island, bordering the coastal spit (Souélaba Point). This prison, a veritable indestructible fortress, is now situated almost 40m into the ocean (Fig. 18).
Given the extent of erosion at Cape Cameroon and other locations, the situation in the Wouri estuary is even more concerning. A correlation seems to be established when comparing this erosion phenomenon with the explanatory factors discussed above. Existing literature supports this idea.
V-1 Coastal dynamic in the Wouri estuary:
Long before this work, several authors had already mentioned coastal erosion in the Wouri estuary. Tataw et al (2021) used Landsat images from 1984 and 2017, Google Earth aerial photographs from 2000 and 2016, and GPS surveys to analyse the dynamics of the coastline between Cap Cameroun and Toubé in the Wouri estuary. Between 2016 and 2017, Cap Cameroun showed an average retreat of 1.4m. The study also revealed an average rate of change in the coastline of Cap Cameroun of 452.4 m, between 1984 and 2017, with an annual average of 13.70 m. This shows that Cap Cameroun has experienced a higher rate of shoreline retreat than any other in the coastal region. The annual recession for the zone as a whole would be 9.56/year, while that of Cap is 13.70 m, with a difference of 10%, i.e. a difference of 4.14 m. At Toubé, between 1984 and 2017, changes in the coastline were measured at different rates, with an average recession rate of 213.75 m. The main factors identified were the strong presence of fishermen, who weaken the environment by cutting mangroves, and the action of longshore drift (Salem, 2014).
The work by Fossi Fosti et al (2019) in the Wouri estuary covers the period 1948–2012. They used topographic surveys from the archives of the French Naval Hydrographic and Oceanographic Service (SHOM) on the Wouri estuary from 1948 to the present day to analyze the retreat of the coastline in the Wouri estuary as a whole. In zone 1, which corresponds to Cap Cameroun, an average recession of 3.2m/year was estimated between 1948 and 1996, i.e. 48 years. For the same zone, we obtained an average recession of 3.10 m/year between 1975 and 1986 (11 years). On either side of the PAD port facilities, these authors mention a recession of -0.6 m/year on the east bank of the port and − 5.8 m around the port site. For zone 2 (taken as a whole), we found an average recession of 3.69m/year. For zone 5 around Manoka Island, the same authors conclude that erosion is low, with an annual recession of -0.4/year. On the coastal spit (zone 6), towards Souélaba point, they obtain a recession of 1.4 m/year. In contrast, we calculated a higher average recession of 3.3 m/year around Manoka Island and 2.28 m/year on the spit. Between 1996 and 2012, according to these authors, the Wouri estuary was fairly stable, except for the areas around Cap Cameroun and a thin section of Pointe Olga. Erosion is said to have increased at Cap Cameroun, with an average retreat of 5.8 m/year, 3.2 m/year around the PAD, 2 m/year on Manoka Island and 2 m/year on the coastal spit. Between 2000 and 2016 (the period closest to theirs), we quantified a recession of 4 m/year at Cap Cameroun, 4.53 m/year around the PAD, 1.74 m/year around Manoka Island, and 1.8 m/year on the spit. These results are similar, although we did not use the same sources of input data. Similarly, the boundaries of the zones are not the same, as we extended the analyses to the tidal inlets and the entire coastal strip around the Mabé mangrove reserve. In any case, the Wouri estuary is undergoing permanent and disproportionate erosion. The main factors cited by the authors are natural processes such as tidal action, climate change, and human activity.
Ondoa et al (2018) also assessed changes in the coastline along the entire Cameroonian coastline. After identifying eroded areas, they quantified an average retreat of more than 0.54m/year over the entire coastline between 1986 and 2015. The main factors they identified were the predominance of socio-economic activities on the Cameroonian coast, as well as tidal action, which often reaches more than 2 m in the Wouri estuary. In the Douala area, which is made up of sedimentary rocks, Domguia (2016) quantified a recession of around 4.78 m, or a linear recession rate of 31.86 cm/year, based on Landsat images from 1978, 2000 and 2015.
Ellison and Zouh, (2012) worked on the Manoka Island area, which corresponds to zone 5 delimited as part of this work. As base data, they used Landsat images from 1975, 1986, 2000, 2007 and 2010. These dates are similar to those used in this study, with the exception of 2007 and 2010. The results show that the coastline in this zone has retreated by an estimated +-30m over the last 35 years. Occasionally, average retreats of -0.79m/year to -3.4m/year were recorded. This regression of the linear area rhymes with the destruction of the mangrove, which is estimated to have shrunk by 11% compared with the area it covered in 1975. Climate change is also cited as one of the main causes.
Over Cameroon, Brière (2005) and Di Paola et al., (2014) cite a similar connection between the hydrodynamics of the adjacent beaches in Anglet and Southern Italy. The predominant action of the tides on sediment dynamics is highlighted. Tomety, (2013) examined the coastlines of Benin, Côte d'Ivoire, and Ghana to discover a strong correlation between tides and ocean currents in the destruction of shorelines. During high tide, the most destruction is observed. Cape Cameroon is currently experiencing the same situation that this is the same situation that Cape Cameroon is in. The temporal variations over several years of cumulative erosion are not explained by the fact that erosion over a year corresponds to the occurrence of high tides. Over time, the extent of eroding areas varies greatly both on the Atlantic coast, as at Cape Cameroon, and in the interior of the estuary, even though the tides have not changed. For example, in the Wouri estuary, the extension of perod.
V-2 Mains factors of coastal erosion
V-2-1 Development, urbanization, dredging and coastal erosion on Cameroon's coastline
Cameroon's coastline is well-developed when it comes to coastal development. The construction of the APD in early 1881 marked the starting point in the Wouri estuary (Fossi Fotsi et al., 2019). The width of the Wouri channel was narrowed by the port extension out to sea. This will lead to erosion on the other side of the channel.
The expansion and extension of the city of Douala over the existing mangrove zones will reduce the area occupied by the tide in this part of the estuary (Fig. 19). The urban fabric is estimated to have grown from 2,478 hectares in 1975 to 18,614 hectares in 2018 (Mbaha and Tchounga, 2020) Significant sediment removal and the resuspension of previously deposited sediments are caused by regular dredging operations in the APD boat channel ship channel (Fossi Fotsi et al., 2023). The void created by this removal increases the depth of the channel, which could lead to a narrowing of the unquantified tidal wave in the Wouri estuary (Augris, 2001). The speed of tidal propagation is dependent on the configuration of the seabed. The thicker the sand on the bottom, the slower the waves and tides get.
Wouldn't it be a good idea to deposit dredged sediments in areas with a sediment deficit, such as Cape Cameroon, instead of selling them or dumping them out to sea? These sediments could be reclaimed and then pumped back to eroded sites (Ozer et al., 2017).
V-2 Sinking of the coastal spit at Souélaba headland.
The strong erosion at Cape Cameroon is explained in this paragraph. The results of the coastline dynamics study and observations made in situ indicate that the deepening of the spit around Manoka Island could partly explain the coastal erosion at Cape Cameroon.
The estuary closes off when this spit of coastline deepens, encouraging erosion towards Cape Cameroon on the opposite side of the estuary. The dynamic nature of this spit, whose width ranges from 60 to 80 meters, has already been highlighted by Morin & Kueté, (1988) The drift currents at work in this estuary are diverted towards Cape Cameroon by this narrowing. Figures 9, 11 & 15 shows that the maximum erosion at Cape Cameroon occurred between 2000 and 2017, corresponding to the most spectacular escalation of the spit at Souélaba headland.
The strong erosion at Cape Cameroon could also be favored by the deep bathymetry in this area. The Navionic bathymetric map shows a depth of -4 m around Cape Cameroon, and a channel with a depth of more than 20 m is located just offshore. In this zone, the waves will be less attenuated than in others. Actually, the Kombo Mokoko point (to the north of Cape Cameroon), which is = 2m deep, has a more stable line. The amplitude of the waves is lower in the interior of the estuary because this point is set back. This explains why the coastline is stable at this point.
V-3 Marine dynamics and its influence on the morpho-dynamics of the coastline
Most of the other estuaries worldwide see their balance disturbed by human activities, such as dredging and enlarging the channel leading to the port of Douala, and the extraction of sands, which alters the morphology of the estuary and consequently its hydrodynamics. The action of ocean and river currents that meet in the estuary is one of the natural factors that make up the whole picture. The hydrodynamics of the mouths of Cameroon are affected by these factors, but the most likely consequences are an amplification of the tides and an increase in sediment transport along the channel (Temmerman et al., 2013). Rising sea levels will further accentuate this trend (Nichols, 2018). The quantity of sediments transported to the mouths of Cameroon is affected by the changes in land use in the various BVs and the construction of a dam on the Sanaga (Cox et al. 2021b, 2022). The development of agro-industrial activities will increase the sediment flow towards the estuary, while the dams retaining sediments upstream will decrease it (Raburu & Okeyo-Owuor, 2006; Ndam Ngoupayou et al., 2007); Akoachere et al., 2019). No quantitative assessment of the processes involved is prevented by the lack of regular measurements of the sediment load of rivers.
Changes in turbidity, nutrients, and their dispersion can be caused by all of these processes (Ndam Ngoupayou et al., 2016) As a result, the marine resources of the estuary are impacted, as evidenced by the interviews conducted with this community by Semboung Lang (2017) The reduction in biodiversity and the decrease in fish populations are caused by the reduction in biodiversity. Thus, the evolution of fishing in the Wouri estuary is marked by significant environmental difficulties, necessitating responsible handling of this vital resource for local populations (Din et al, 2017; Fusi et al., 2016). Thus, the evolution of fishing in the Wouri estuary is marked by major environmental challenges, which require sustainable management of this essential resource for local communities.
V-4 Degradation of plant cover/and increased coastal erosion in the Wouri estuary
Stabilizing coastal areas is dependent on mangroves. It favors accretion through its root system, which fixes the soil; it slows down the tide and ipso facto constitutes a natural barrier against coastal erosion; it intercepts water from rainfall and considerably reduces its flow (Ellison and Zouh, 2012). The degradation or deforestation of it appears to be a factor in the vulnerability and amplification of coastal erosion (MINEPDED, 2018).
The mangroves of the Wouri estuary are in danger (Dzalla Nguangué, 2013). The agro-industrial activity on its edges and the growth of fishing populations have made this anthropized area fragile. In Cape Cameroon, Manoka, in short, the estuary, the destruction of the mangrove is a reality (Ellison et al., 2012). There is a connection between coastal erosion and destruction (Fossi Fotsi et al., 2019). The 2018 land use map shows that areas of stagnant water outside of the usual channel are where the mangrove has disappeared. In 1985, such a correlation had already been established by Syaka Sadio, (1985), who raised the issue of man's responsibility for increasing erosion in Senegal through land clearance for energy needs, land cultivation, etc. Ackermann et al. (2006) emphasize the crucial role of vegetation cover in stabilizing the Sine-Saloum delta in the same country.
The FAO has raised the alarm about the degradation/degradation of the mangrove in Cameroon, estimating the annual loss of over 300 ha (FAO, 2006) The Mabé Mangrove Reserve, where Cape Cameroon is located, has lost an area of more than 1,162.25 ha over the past 30 years, representing a loss rate of 13.68% of its total area (Mbevo, 2019) As shown in Fig. 20, these mangroves are used for smoking fish, building housing and many other purposes.
V-5 Population growth and coastal erosion in the Wouri estuary
The steady population growth in the Wouri estuary is having a significant impact on the natural environment. This demographic dynamic appears to be driven by Douala, the main economic city. The population is growing, needs are increasing, and pressure on natural resources is increasing. This is what's happening in Cape Cameroon, as the number of fishermen grows. Cape Cameroon accounts for more than half of the fish supplies for the city of Douala. In situ fish smoking techniques are environmentally destructive. The mangrove wood is used abusively. Dzalla Nguangué (2013) has already highlighted the high level of human activity in the Wouri estuary and the resulting environmental impacts. Marine intrusion, flooding soil pollution, and coastal erosion are some of the impacts. Population growth and coastal erosion are not unique to the Cameroonian coast, however. It's a global one. Looking at the islands of the Indian Ocean.
Cazes-Duvat (2005) notes that coastal erosion is worsening as a result of population growth, and that population growth is aggravating coastal erosion and accelerating the degradation of coral reefs. There are coral reefs.
The authors of Côte d'Ivoire, Touré et al (2012) highlight the impact of human activities on the evolution of coastal areas in Port-Bout Bay. Natural hazards, including coastal erosion, are thought to be caused by demographic growth, which is the main factor in coastal dynamics.
V-6 Dynamics related to the Sanaga and its repercussions on morpho-dynamics in the Wouri estuary
The Sanaga River is the largest in Cameroon, covering over 140,000 km2 with a length of 918 km. Until the 18th century, it flowed directly into the Wouri estuary, but has since shifted 11 km south, only feeding the estuary during exceptional floods. This impacts the dynamics of the Wouri, as the sediment supply from the Sanaga during these overflow events greatly influences the estuary (Morin & Kuété, 1988; Ondoa et al., 2018).
The Sanaga's hydro-sedimentary kinetics have also been significantly altered by the construction of several dams, including the Mbakaou reservoir dam (6°18'16" N, 12°48'30" E), the Bamendjing reservoir dam (5°41'55" N, 10°30'03" E), and the Songloulou hydroelectric dam (4°04'41" N, 10°27'54" E) (Ndam Ngoupayou et al., 2016). These dams have modified the river's flow and sediment transport.
The complex interplay between the Sanaga River and the Wouri estuary is not yet fully understood. The Sanaga plays a dual role, both supplying sediment during floods and being impacted by human developments along its length. Further research is needed to elucidate these dynamics and their implications for the broader regional ecosystem. Understanding the Sanaga's evolving relationship with the Wouri is crucial for effective management and conservation of this vital waterway.
The Sanaga River is the largest in Cameroon, covering over 140,000 km2 with a length of 918 km. Until the 18th century, it flowed directly into the Wouri estuary, but has since shifted 11 km south, only feeding the estuary during exceptional floods. This impacts the dynamics of the Wouri, as the sediment supply from the Sanaga during these overflow events greatly influences the estuary (Morin & Kuété, 1988; Ondoa et al., 2018).
The Sanaga's hydro-sedimentary kinetics have also been significantly altered by the construction of several dams, including the Mbakaou reservoir dam (6°18'16" N, 12°48'30" E), the Bamendjing reservoir dam (5°41'55" N, 10°30'03" E), and the Songloulou hydroelectric dam (4°04'41" N, 10°27'54" E) (Ndam Ngoupayou et al., 2016). These dams have modified the river's flow and sediment transport.
The Mapé reservoir dam (6°01'56" N, 11°18'18" E), built in 1988, is an impoundment dam located in the Bankim district, 11 km north-east of Magba. It is located on the Mapé River. The purpose of this regulation is to regulate the level of the Sanaga River in order to increase hydroelectricity production from downstream dams.
The construction of the Lom Pangar dam took place between 2012 and 2016 (5°22'17" N, 13°30'45" E). This is an embankment dam with a section containing a gravity dam on the Lom, located about 88 kilometres north of Bertoua in the East Region of Cameroon. A major tributary of the middle course of the Sanaga is located on the Nachtigal dam, which is currently under construction. In September 2020, construction began.
The most downstream part also features the Édéa dam. It was constructed in 1950 and completed in 1954. Between 1966 and 1976, the power station was extended and rehabilitated in 2008. The capacity of the plant was increased from 4 to 265 MW. The sediment that no longer reaches the shoreline and feeds the spit is trapped by these dams. The development of this zone and the erosion observed between 1975 and 1986 may be due to this factor.
When it comes to dams on the African continent, we're thinking about the Akossombo dam, constructed in 1963 on the Volta in Ghana, which is said to capture more than 90% of the sediment on its way to the Togolese coast. Major erosion and scouring of the piers of the Accra-Lomé international road bridge were caused by it (Rossi, 1996).
The dams on the Sanaga aim to regulate the flow and therefore flooding by regulating the flow. If there is a significant reduction in flooding, there should be less overflow from the Sanaga into the Wouri Estuary, which would encourage coastal erosion. The study by Dzana et al. (2011) shows, however, that the development of reservoirs and their operation up to 2004 did not significantly affect downstream peak flows. The construction of new dams may have changed this.
The reduction in sediment input from dams on the Sanaga could be offset by increased sediment input from deforestation, slash-and-burn farming and the expansion of agroindustries in the Sanaga watershed. The Sanaga watershed has been heavily deforested for over 30 years. This is likely to increase the load of suspended and bottom sediments. This has been indicated by several authors (Boeglin et al., 2000; Sigha-Nkamdjou et al, 2005; NDam Ngoupayou et al, 2007; Dzana et al., 2011).
Finally, there is another factor affecting the sediment load transported by the Sanaga: climate change. Climatic variability strongly affects the suspended load, even in humid tropical regions (Liénou et al., 2008). The most important factor here is the amount of rainfall during the dry season. The decrease in rainfall leads to a significant water deficit in the soils and a drop in river flow (Dzana et al., 2011). In the early 1970s, there was a major and widespread water deficit in the Sahel. This deficit had a major impact on hydrology and solid transport in Cameroon (Sigha-Nkamdjou et al, 2005; Sighomnou et al, 2007; Ndam Ngoupayou et al, 2007). Climate change is affecting not only the Sanaga River, but all other West African rivers (Mahé et al., 2005).
V.7 Dynamics of other rivers: Mungo, Wouri, Dibamba, and others
These three rivers have catchment areas of an order of magnitude smaller than the Sanaga, and therefore their sediment inputs are smaller. However, there are no dams located along these rivers. There are varying geographical influences on accretion in the Wouri estuary. Zone 1 is directly influenced by the rivers that drain Mount Cameroon. These young terrains are much less altered and consolidated than the rest of the African craton, and they incise a formidable relief. Even if it hasn't been quantified, the associated sediment movement must be significant. The Moungo is influencing Zone 2 and the western part of Zone 3, draining the young volcanic terrain. This is the only river in the estuary for which there is an estimate of suspended solids transported: 46.5 mg/l (Ndam Ngoupayou et al., 2007). An erosion rate of 17t/km2/year is calculated by these authors. There is a catchment that is also occupied by a preserved forest, but whose land is much older than the Ntem further south. The central part of zone 3 is influenced by the Wouri, while the western part is influenced by the Dibamba. Finally, zones 4, 5, and 6 are affected by the Sanaga.
At the end, climate change and land use are some of the factors that affect the sedimentary flows of these rivers. These rivers were affected by the drought of the 1970s, which affected the Sanaga. This sediment supply could be linked to the reduction in coastal erosion documented between 1975 and 1986. Changes in land use have had a major impact on the Dibamba with the development of the city of Douala and agro-industries. The foot of Mount Cameroon is where these changes in land use are significant.
V-8 Dynamics linked to bathymetry and its impact on morpho-dynamics
The Wouri estuary has different bathymetric levels. They diminish from the open sea to the banks. The areas where the depths are greatest experience major tidal movements that attack the land and subject it to coastal erosion. There is ample evidence of this provided by Navionics bathymetric maps. There is a bathymetric value of + 4m in the Cape Cameroon region, where the estuary experiences the greatest erosion (Komba et al., 2019). The area of deposition of cream of mud along the channel migrates seasonally between periods of high and low water, which shows that bathymetric levels vary with the seasons. The estuary's immediate environment is fed by three main rivers, which provide an average annual inflow of 28.109 m3 /year. We get a lot of sediment in the estuary during the rainy season, which affects the bathymetry (Komba et al., 2019).
Finally, this work on coastal erosion in the Wouri estuary has led us to analyse the phenomenon and quantify the dynamics and morpho-dynamics of the coastline between 1975 and 2024. The factors amplifying these dynamics are also addressed and discussed in the light of field knowledge and existing literature. It appears that the hot-spot of coastal erosion in the estuary is the Cape Cameroon locality, with retreats of more than 15 m/year in places. The main factors are demographic growth in the estuary, destruction of the mangrove swamps, the action of the tide and marine currents, and human development in the estuary, reflected in the spatial expansion of the city of Douala and port development. Far from being a case typical of the Cameroonian coastline, the issues addressed in this study are of universal concern to all the countries that share the ocean coastline. From the Senegalese coast to the Nigerian coast, coastal erosion is present everywhere. It is even more active on the coasts of West Africa, particularly in Benin and Togo. This is why the World Bank has financed a vast project (the WACA project) to curb erosion in this area. The need to coordinate solution initiatives is proving decisive, in order to avoid spreading the phenomenon to other areas.
Author Contribution
Philippes Mbevo Fendoung contributed to the conceptualization, methodology, mapping, analysis, and writing of the article following his fieldwork. Aurélia Hubert-Ferrari's role consisted of constructing the methodology, revision, editing, and analysis. All the authors read and approved the version. Aurélia Hubert-Ferrari’s role involved constructing the methodology, review, editing, and analysis. All authors have read and agreed to the published version of the manuscript.
Data availability:
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.
Funding
This research did not receive any external funding.
Conflicts of interest:
The authors declare no conflicts of interest.
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- Transects are orthogonal lines generated automatically in DSAS, with a specified spacing (50 m if necessary) along the baseline. They are equidistant and perpendicular to the baseline.
No competing interests reported.