Shoreline Changes and Erosion Protection Effects in Cotonou of Benin in the Gulf of Guinea

Abstract

Coastal erosion has been a threat to coastal communities and emerged as an urgent problem. Among the coastal communities that are under perceived threat, Cotonou located in Benin, West Africa, is considered as one of the most dangerous area due to its high vulnerability. To address this problem, in 2013, the Benin authorities established seven groynes at east of Cotonou port, and two additional intermediate groynes have recently been integrated in April 2018. However, there is no quantitative analysis of groynes so far, so it is hard to know how effective they have been. To analyze effectiveness, we used optical satellite images from different time periods, especially 2004 and 2020, and then compared changes in length, width and area of shoreline in Cotonou. The study area is divided into two sectors based on the location of Cotonou port. The difference of two areas is that Sector 2 has groynes installed while Sector 1 hasn't. As result of this study, shoreline in Sector 1 showed accretion by recovering 1.20 km² of area. In contrast, 3.67 km² of Sector 2 disappeared due to coastal erosion, although it has groynes. This may imply that groynes helped to lessen the rate of average erosion, however, still could not perfectly stop the coastal erosion in the area. Therefore, for the next step, we assume it is recommended to study how to maximize effectiveness of groynes.

1. Introduction

Dolan et al. (1991) defined that shoreline coincides with the physical interface of land and water. In the future, coastal erosion will very likely to be intensified because of global warming. In fact, the average global temperature has increased by approximately little more than 1°C (NASA, 2019), and the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) published that global sea level will rise by up to 60 cm by 2100 in response to ocean warming and glaciers melting. Fig. 1 shows the sea level difference from 1993 to 2020. It can be seen that the sea level change is not constant. In some Regions (Western Pacific and Western Atlantic), sea level has risen up to three times faster than the global mean. Fig. 1 also presents that the Gulf of Guinea also rises by 4 mm from 1993 to 2020. The sea level rise can accelerate coastal erosion, as shown in Fig. 2. Dune beach erosion volume and/or shoreline retreat increases significantly in the 2070s under 100-year return periods storm and a sea level change at Son Bou of Spain. In particular, according to the IPCC (2007) report, Africa is one of the most vulnerable continents to climate variability.

Fig. 1. Sea level difference from 1993 to 2020. Sea levels in the Gulf of Guinea, West Africa, rose by 4 mm per year from the UHSLC Fast-Delivery database (https://uhslc.soest.hawaii.edu/). Each circular symbol represents the amount of sea level increase.

Fig. 2. Dune-beach erosion volume of Son Bou beach, Western Mediterranean, taking into account the sea level rise and the 10-year return periods (RP) of wave (dark blue line, left axis) and 100-year RP (orange line, right axis) under RCP8.5 Scenario (Enríquez et al., 2019). Significant wave height (Hs), period (Tp) and duration (h) for 10 and 100- year are 2.9 m, 8.26 s and 91 h, and 3.96 m, 9.5 s and 120.5 h, respectively.

Human influence in the coastal area has been generally identified as a major cause of shoreline morphological change, which ultimately drives coastal erosion. Boer et al. (2019) found that 44 km² of the African coast changed from 1984 to 2018 and 23 km² and 21 km² was deposited and eroded, respectively. The largest rate of change was 1.99m per year and the coastal land area change was 4.3 km² with a rate of change of 0.11 km² per year. According to Touré et al. (2012), the West African coasts are undergoing a significant erosive process, which can be impressive in some places with average retreats exceeding 10 m per year.

Optical remote sensing has been applied to beach monitoring. The shoreline is in an intersection of coastal land and water surface, depending on sea level, wave, swell, tides and near-shore currents. Therefore, there are limitations in an application of satellite data for automated shoreline extraction mainly using wetness, because radiometric accuracy and spatial resolution from satellites are not enough to discriminate shoreline indicators in small spatial scales (centimeters to meters) of beach face (Boak and Turner, 2005; Lipakis et al., 2008; Toure et al., 2018). Other survey methods of shoreline changes are in-situ measurements such as landmarks, Global Positioning System (GPS) and terrestrial Light Detection And Ranging (LiDAR). An ideal system for beach monitoring is to measure a field environment consisting of sediment transportation and oceanic and atmospheric forces. But it is effective to a local area and are difficult to achieve. In general, remote sensing is recommended for a general understanding of beach balance in a wider area, although it does not provide a detailed mechanism of shoreline changes.

For the research area, manual shoreline extraction is better to use a shoreline indicator of wet/dry line with lower values of RMSE (Root Mean Square Error) (Lipakis et al., 2008; Minghelli et al., 2020). The wet/dry lines were obtained and compared using high-resolution images in Google Earth. Thus, in this paper, we are going to investigate the effectiveness of groynes by exploring changes in length, width and area of shoreline in Cotonou, Benin.

2. Data

1) Research area

The coastline of Benin, 125 km long, is located between the longitudes 1.5° E and 2.5° E and the latitudes 5° N and 6° N and facing the South Atlantic Ocean as in Fig. 3. The beach dynamics are dominated by the influence of waves of moderate energy (mean wave height = 1.36 m, mean peak period = 9.4 s) coming from mid-latitude with an S-SW incidence (incidence on the coast between 4 and 9°), (Almar et al., 2015; Laibi, 2011; Laibi et al., 2014). As a result, sediment transport is primarily driven by waves which have the direction of South/Southwest and South/Southeast. Tides are semi-diurnal with a micro to meso-tidal ranging from 0.3 m to 1.8 m for neap and spring tides (Abessolo Ondoa et al., 2016).

Fig. 3. (a) Benin in the Gulf of Guinea. (b) Cotonou of Benin in the Gulf of Guinea. Cotonou Port is in between Sectors 1 and 2, a set-up for a detailed investigation.

Cotonou is the economic capital of Benin and almost half of Benin population lives in this city described in Fig. 3. But Dodman et al. (2012) explained the shoreline of Benin has a fragile ecosystem exposed to the negative effects. Boer et al. (2019) compared a map of Cotonou between 1963 and 1987. In the east of the Cotonou port, the coastline retreated by 400 m between 1963 and 1997 according to Codjia (1997). Based on a detailed analysis of remote sensing data and verified ground truth, Kaki et al. (2011) observed a coastal erosion of nearly 500 m between 1963 and 2005 in the same area. The shoreline has eroded by 400 m at a maximum speed of 16 m per year, with a total loss of around 1.12 km² of land. Even after 28 years Sector 2 as in Fig. 3 has been eroded by nearly 485 m.

Fig. 4. Procedure of a shoreline extraction and analysis.

The research area is divided into Sector 1 and Sector 2 by Cotonou port (Fig. 3). Sector 2 has eleven groynes installed from 2013 to 2018 while Sector 1 has only a breakwater that acts like a groyne. Sector 2 has been significantly eroded since the early 1960s. So Benin authority has been finding proper measure. That is the groyne installation. It is shore protection structures which used for the purpose of trapping littoral drift or retarding erosion of the shoreline. Benin authorities built seven groynes at Sector 2 as in Fig. 3. In 2013, seven groynes were installed one at every kilometer from the Groyne 1 as in Fig. 10, and two additional groynes (the first one between Groynes 1 and 2 and the second one between Groynes 6 and 7) have been integrated at April, 2018. On the right side of the Sector 1 shoreline, an extension of the breakwater was installed in 2010 as a coastal erosion prevention policy. However, this did not prevent fundamental erosion. Because this have an effect on local sediment distribution (Longueville et al., 2020). As result, residents in the risk area could experience mitigation of coastal erosion. The poorer the people, the more difficult it was to move away from the erosion zone (Longueville, et al., 2020). Also the erosion area was not only a residential area for residents, but also an area of economic activity, making it more difficult to move.

2) Satellite data & method

Cloud-free high-resolution optical images, obtained from Google Earth, were used to extract shorelines and compare their changes in different time periods (Mar. 25, 2004, Nov. 8, 2011, Mar. 21, 2012, Jul. 6, 2015, Oct. 12, 2020). The RGB images with a spatial resolution of 1.1 m are corresponding to the zoom level 18 of Google Earth and are prepared in Geo-Tiff format according to a sector of the research area. Next, image to image registration was performed using the ENVI program, and the accuracy of the image was improved by setting the RMS (Root Mean Square) value to 0.05 m or less. Using the images, the shoreline indicator expressed as the wet/dry line was extracted manually between the wet pixels and land pixels, distinguished from the color, the shape and the texture of the RGB shore images using the QGIS program. A wet/dry line on the beach is called the wave run-up maxima where are no longer under the influence of the sea.

The Cotonou shoreface is divided into a steep part and a gradual part and has a tidal range between 0.3 m and 1.8 m. As illustrated in Fig. 5, the Cotonou coast is composed of both a gradual shoreface (α) and a steep shoreface (β). As an indirect method, photographs were used to obtain α-0.45 and β20-25. The shoreface affected by the tidal change is a slope area of β, and is in the range of 0.83 to 3.91 m in cross-shore direction. Therefore, it was assumed that shorelines extracted from satellite data are not affected by the tide for this study.

Fig. 5. Cotonou shoreline position in a vertical 2D case. Straight line represents a shape of cross-shore plane and the dashed line is virtual horizons to compute αand β.

3. Shoreline change of Sector 1

1) Width and area of shoreline change between 2004 and 2020

As shown in Fig. 6, there is a clear difference between the 2004 (blue line) shoreline and the 2020 (yellow line) shoreline. From this Fig. 6, it was observed that the shoreline stretch to the sea. This zone has been significantly eroded since 2004. In 2010, an extension of the breakwater was installed on the right side of the coastline as a policy to prevent coastal erosion. They interrupt the force of the ocean waves and prevent the sand from being washed away. As a result, the coastline before 2004 was restored. Also the Fig. 6 shows that the shoreline changes greatly from east to west. This means that erosion progressed more actively as it moved to the right.

Fig. 6. Shoreline comparison for Sector 1 between the years 2004 and 2020. Blue line and yellow line represent the date of image March 25, 2004 and October 12, 2020, respectively.

Fig. 7 shows the width extension in Sector 1 in 2004 and 2020. The width extension difference from the west to the east of the shoreline, and the largest difference is the western 1/3 of the entire research area. The minimum difference in width is 46.87 m, the maximum difference is 164.96 m, and the average is 108.9 m. The increase appears to be the installation of breakwater extensions in 2017.

Fig. 7. Width distribution between shorelines of 2004 and 2020 in Sector 1. The minimum and maximum distance are 46.87 m on the left side and 164.96 m near the breakwater, constructed in 2010. Width variations increases one out of third from the shoreline. From that point, a rate of shoreline variation is constantly maintained. The width average is 108.9 m. The coast of Benin showing the breakwater extension.

Fig. 8. Area (green patch) increased by sediment deposition after extension of the breakwater. Increased area from 2004 to 2020 is about 1.2 km².

In 2004 and 2020, the difference between the coastal areas of the Sector 1 was 1.20 km², and the coastal area was larger in 2020 than in 2004. Figs. 7 and 8 show that the width and area of Sector 1 increased after the extension of the breakwater was installed between 2013 and 2017. This was based on the Trujillo, A. P (2005). The breakwater prevented erosion and increased the coastal area by 1.2015 km².

2) Shoreline comparison of Sector 1 from 2004 to 2020

Fig. 9 shows the shoreline extraction plots for 2004, 2011, 2012, 2015, and 2020 in Sector 1, and through this, we can observe the shoreline change. A comparison of extracted shorelines by inserting them into the 2020 satellite image shows that the 2004 shoreline is located inland and gradually erosions toward the seaside. The changes to the shoreline from 2004 to 2011 and from 2015 to 2020 were significant, but the changes to the shoreline from 2011 to 2015 were minimal. It can be estimated that the effect of the breakwater extended from 2013 to 2017 is more effective than the breakwater installed in 2004. Breakwaters prevent erosion by blocking the flow of sediment along the coast Trujillo (2005). As erosion takes place, the transmitted sediment begins to deposit again from the breakwater, acting like a groyne. As a result, the part where the breakwater is installed is deposited, and on the other side, the coastal erosion is rapidly progressing due to the obstruction of the current flow due to the breakwater.

Fig. 9. 2004, 2011, 2012, 2015, and 2020 shoreline extraction in Sector 1. The largest changes are between 2004 and 2011 and between 2015 and 2020.

4. Shoreline change of Sector 2

1) Width and area of shoreline between 2004 and 2020

Fig. 10 presents the 2004 and 2020 shoreline changes in Sector 2, the area where the groyne was installed. It can be seen that erosion was observed in 2020 compared to 2004. However erosion appeared where there was no-breakwater impact. This seems to be the same as in Sector 1, which, according to the Trujillo (2005), appears to have caused the sediment flow and wave direction change. The place where the most erosion occurred is on the right side of groyne No. 11. Although it is difficult to solve the fundamental erosion problem by installing the groyne, it can be seen that it is effective in reducing the speed.

Fig. 10. (a) Shorelines for 2004 (blue line) and 2020 (pink link) and distance between shorelines in Sector 2. (b) There are 11 groynes. Among the groynes from No. 3 to No. 11, except No. 4 and No. 10, they were installed in 2013, and No. 4 and No. 10 were additionally installed in 2018.On the right side of the 11th groyne there is a large change in width.

In order to more effectively compare the uneven Sector 2 shoreline change, it was divided into three Regions as in Fig. 11. From west to east, Region 1, Region 2, and Region 3. Region 1 has 2 groynes, Region 2 has 9, and Region 3 has none. Erosion is shown in Sector 2 as a whole, but the rate of change is the largest in Region 3, especially where there are no groynes. Comparing the average width difference, Region 2 and Region 3 increased 6 times and 18 times in Region 1, respectively. In Fig. 10, the closer the groyne from No. 3 to No. 11, the less erosion is caused by the groyne effect, and the more erosion occurs on the east side of the 11th groyne.

Fig. 11. Distance between 2004 and 2020 shorelines for Regions 1, 2, and 3 for further clarification. Sector 2 divided according to the width variation. From Region 1 to Region 3, the minimum, maximum, and average values increase. Region 2 (78.84 m) has an average value of 6 times greater than Region 1 (12.91 m) and Region 3 (222.69 m) has an average value of 18 times greater than Region 1.

2) Shoreline comparison of Sector 2 from 2004 to 2020

Sector 2 brought about changes to the coastline by installing 7 groynes in 2010 and 2 additional in April 2018 (as illustrated in Fig. 10). Sector 1 has been steadily increasing since 2004 (as in Fig. 8), but Sector 2 is repeating erosion and recovery. This can be confirmed in Fig. 13, which shows changes in Sector 2 in 2004, 2011, 2012, 2015, and 2020, and records 2004-2011 erosion, 2011-2012 recovery, 2012-2015 recovery, and 2015-2020 erosion. As a result, the coastline in 2004, when no groynes were installed, was the widest. However, it is difficult to confirm that the coastline has been eroded by the installation of groyne alone, since factors of sea level increase due to global warming can also affect erosion.

Fig. 12. Area changes in Sector 2 between 2004 and 2020. Coastal area changed in Sector 2 2004 and 2020. Coastal erosion increased with groyne installation. In particular, the effect is evident in Region 3 (Fig. 12).

Fig. 13. Changes in the shoreline from 2004 to 2020. Changes to the shoreline from 2004 to 2020 were noticeable, but not significant from 2011 to 2020. In Region 3 (Fig. 12), change in shoreline are distinguished. In Area A (Fig. 11), shorelines are distinctly distinguished. This groyne is a significantly change in the shoreline due to the effects of breakwater.

3) Shoreline change in Sector 2

Fig. 14 compares the impact of groyne in Region 1 of Sector 2. The area of Area A in 2020 is 4.68 km², which is the result of 16 years of deposition from 2004. Although the land coverage has increased in the Fig. 14, it can be seen that the settlement has moved away from the coast. This can be presumed to be due to the fact that after the breakwater acting as a groyne was installed in Area B, the change of wind driven current occurred and the residents felt a threat to the habitation on the coastline, where deposition and erosion were repeated, and the residents moved more toward the land as in illustrated Fig. 14.

Fig. 14. Shoreline of Area A (Sector 2 in Fig. 8) difference between 2004 and 2020. In 2020, coastal areas were deposited. Residential area was disappeared in 2020. Deposition area is 4.68 km².

Deposition occurred in Area B as well as in Area A, where the deposition area increased (Fig. 15). However erosion occurred at the back (in the west) of the breakwater while deposition occurred at the front. This can be inferred that the shoreline located behind the breakwater is not protected because a diffraction phenomenon occurs in which part of the wave energy passing through the side of the breakwater is transmitted to the rear (Lee et al., 2005).

Fig. 15. Shoreline of Area B (Sector 2 in Fig. 8) difference between 2004 and 2020. The breakwater has been transformed. Coastal area has increased due to sediment deposition. The deposition area is 1.00 km².

5. Concluding Remarks

As the intensity and speed of climate change increases, the frequency of storms and rainfall is also increasing, and the number of countries concerned about coastal erosion is increasing. Especially in West Africa, where poor countries are concentrated, the concern is even greater. To overcome the problem, the governments of these countries are working hard to come up with appropriate countermeasures through cooperation with international organizations. In this study, one of the countermeasures, the effectiveness of using groynes was examined with shoreline changes.

Sector 1 of Cotonou showed a shoreline advance of 108.9 m on average, showing the area increase of 1.20 km². It seems that the breakwater extension played an important role to prevent coastal erosion and trap sediments transported from the western part.

Sector 2 is a general state of shoreline retreat and 12 groynes were installed to move sandy beaches seaward. But the groyne is locally effective and on its right side erosion is getting worse. The average erosion rate of Region 3 is 222.69 m, while Region 1 and Region 2 where the groynes are installed are 12.91 m and 78.84 m, respectively.

This study wanted to explain the effect of groyne related to shoreline changes, but the results could not fully explain the effect due to difficulties in field data collection. Therefore, in the future, wave diffraction near the breakwater using SAR images will be continuously performed with a support of more high resolution satellite images.