東北地域災害科学研究第57巻（2021）

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Long-term morphological evolution of the Volta-Delta River mouth in
Ghana using satellite images
Stephan Lawson1, Nguyen Trong Hiep2, Hitoshi Tanaka3
Abstract
The Volta-Delta River mouth at Ada, Ghana drains one of the largest river basins in West Africa.
The river mouth sandspits have undergone enormous morphological changes due to both natural andhuman activities. Before the construction of a groyne system from 2012-2017 on the western side of theriver mouth, cyclic sandspit elongation and breaching events were observed. To fully understand these
cyclic events, satellite imagery from Landsat, Sentinel-2 and Google Earth were acquired from 19842020 and analyzed to depict the morphological processes occurring at the river mouth. The study
examined the river mouth morphology before and after the construction of the groyne system.
Keywords: Breaching, Elongation, Erosion, Groyne system, Sandspit, Volta River mouth.
1. INTRODUCTION
Globally, deltaic coastlines are subjected to alarming rates of vulnerability with respect to erosion
and shoreline retreat. Human engineering works in the drainage basin of deltas have influenced thedestabilization of deltaic coastlines by impeding sediment supply to these areas (Syvitski et al., 2009).
The presence of well-developed sandspits at the mouth of river deltas provides massive benefits withregards to urbanization, floodplain inundation prevention and biodiversity which requires the study ofchanges ensuing at such locations (Pradhan et al., 2015). Inadequate studies in relation to morphologicalchanges at river mouths exposes them to huge vulnerabilities.
The Volta River mouth is geographically located at Ada in the Greater Accra Region of Ghana
between latitudes 5°46'15.06" to 5°46'28.44" North and longitudes 0°39'46.03" to 0°41'31.46" East. The
river mouth drains the Volta basin with a catchment area of approximately 400,000 km2 and a total river
network of about 1,500 km (Fig. 1(a)). The river mouth is characterized by two well defined sandspits
which are highly morphological in nature (Fig. 1(b)).

Figure 1: Map of study area (a) Volta River basin (b) Volta River mouth, Ada, Ghana.

1MSc. Student, Department of Civil and Environmental Engineering, Tohoku University, 6-6-06 Aoba,
Sendai 980-8579, Japan.
2PhD. Student, Department of Civil and Environmental Engineering, Tohoku University, 6-6-06 Aoba,
Sendai 980-8579, Japan.
3Professor, Department of Civil and Environmental Engineering, Tohoku University, 6-6-06 Aoba,
Sendai 980-8579, Japan.

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Upstream of the river mouth are two hydro-electricity dams which have influenced the reduced
river discharge and sediment load by 23.23% and 92.32% respectively (Amenuvor et. al., 2020). Anotherengineering structure in the vicinity of the river mouth is a groyne system on the western side of the rivermouth which was constructed between 2012-2017. The groyne system protects a total stretch of 16km of
the coast against the intense erosion rates and displacement of coastal communities (Roest, 2018). Before
the two-phased construction of the groyne system, erosion rates were estimated at about 6m/year which
justified the urgent need for protection measures for Ada and its environs (Bollen et al., 2010). The goalof this study is to analyze observed morphological trends at the Volta River mouth using remote sensing
images and highlight the driving forces for these processes.
2. MATERIALS AND METHODS
2.1 Data Collection
In this study, historical satellite imagery was acquired from Landsat (5, 7, & 8) and Sentinel-2
satellite missions from 1984-2020 which served as the main source of images. These images weresupplemented with Google Earth images to fill the no-data gaps from the Landsat and Sentinel-2 sources.
A total of 164 images were acquired for the study period and used to perform the long-term morphologicalanalysis. The Landsat and Sentinel-2 images which originally have a spatial resolution of 30m/pixel and20m/pixel respectively, were down-sampled to 15m/pixel and 10m/pixel respectively using bilinear
interpolation to improve the accuracy of detected shorelines (Vos et. al., 2019). Furthermore, theresolution of supplementary images from Google Earth were 2m/pixel and 20m/pixel. A summary ofimages acquired for the study are presented in Table 1 below.
Table 1: Details on acquired satellite images for the study area.
Image Source

No. of Images

Resolution

Landsat (5,7 & 8)
Sentinel-2
Google Earth5080
3430m/pixel
20m/pixel
2m/pixel & 20m/pixel

Down-sampled
Resolution
15m/pixel
10m/pixel-2.2 Image Processing
Shoreline detection was carried out using two approaches for the main and supplementary sourcesof images. For the Landsat and Sentinel-2 images, the water-land boundary was defined using theModified Normalized Difference Index (MNDWI) developed by Xu (2006). The MNDWI values range
between -1 and 1 with negative values corresponding to water pixels and positive values corresponding
to land pixels. In the case of the Google Earth images, all images were rectified into a single coordinatesystem (World Geodetic System-84) using an affine transformation with the baseline 271 degrees fromthe North. The shorelines were then detected using the difference in colour intensity of the wet and drysand.
2.3 River Mouth Sandspit Analysis
To conduct detailed studies on the morphological evolution of the of the Volta River mouth,
quantitative analysis was conducted using the acquired satellite images. Parameters of the sandspit used
to achieve this include the alongshore coordinate of the updrift (x1) and downdrift (x2) sandspits and theircorresponding y-coordinate values (y1 and y2 respectively). The areas of the updrift (A1) and downdrift(A2) sandspits were also defined. Definition of these parameters are shown in Fig. 2.

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Figure 2: Definition of sandspit parameters used in sandspit analysis.
3. RESULTS AND DISCUSSIONS
3.1 Pre-Groyne Construction Morphological Trends (1984 – 2012)
Visual inspection of remotely acquired images in the pre-groyne construction period gives anindication of the morphological processes at the Volta River mouth (Figs. 3 and 4). As shown in Figs.
3(a) and 3(b), development of the western sandspit (updrift) is sustained by the accumulation of thelongshore sediment transport and elongates in the eastward direction. This confirms the direction oflongshore sediment transport coming from the west to the east and makes the western sandspit thedominant sandspit at the river mouth.
Upon reaching an equilibrium length, the western sandspit undergoes breaching events whichleads to the transfer of sediment from the breached sandspit to the eastern sandspit (downdrift) and other
downdrift areas (Figs. 3(c) to (d)). Once this cycle ends, the natural morphological process is repeated(Figs. 3(e) to (g)). This natural process has been halted through the stabilization of the western sandspit.
This was done by the construction of the groyne system to ensure efficient tidal exchange, facilitate
navigation through the channel and trap the incoming longshore sediment transport to alleviate the intense
erosion experienced at coastal communities around the river mouth.

Figure 3: Pre-groyne construction morphological period (1984 – 2012).

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Figure 4: Volta River mouth shoreline variation (1984 – 2012).
3.2 Post-Groyne Construction Morphological Trends (2013– 2020)
Figures 5 and 6 show a change in morphological trends from the pre-groyne construction periodwith the most influential factor being the presence of the groyne system. Figures 5(a) and 5(c) shows theriver mouth sandspit conditions after the first and second phases of groyne construction, respectively.
Owing to the stabilization of the western sandspit, the formation and landward migration of shoals in theebb delta is observed and its subsequent attachment to the eastern sandspit (Fig. 5(b) to (f)). Thisphenomenon has led to the intrusion of both sandspits into the estuary area of the river mouth.

Figure 5: Post-groyne construction morphological period (2013 – 2020).

Figure 6: Volta River mouth shoreline variation (2013 – 2020).

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3.3 River Mouth Sandspit Features
From the quantitative analysis of sandspit properties, a detailed explanation of the morphologicalprocesses at the river mouth can be performed. In Fig. 7(a), the elongation of the western sandspit isobserved up until a maximum length of x1 = 9672m (1988) where the sandspit was breached (BR-01).
This cyclic process was once again repeated with the sandspit elongating to a length of x1 = 5013m (2008)
where it was breached (BR-03). With respect to the eastern sandspit, it is observed that no appreciablechanges in terms of elongation trends occurred making it the non-dominant sandspit at the river mouth.
However, a breach (BR-02) occurred after reaching a length of x2 = 2810m (2003) (Fig. 7(b)).

Figure 7: Alongshore x-coordinates of updrift and downdrift sandspits.
Analysis of the alongshore sandspit y-coordinates (y1 and y2) revealed sandspit intrusion trends
in the post-groyne construction period (2013-2020). As shown in Figs. 8(a) and 8(b), intrusion of bothsandspits into the upstream section of the river mouth can be observed. This trend may tend to pose
futuristic problems such as narrowing of the river mouth and hence, leading to an inefficient tidal
exchange between the estuary and the ocean.

Figure 8: Alongshore y-coordinates of updrift and downdrift sandspits.
The final quantitative parameter investigated was the area of sandspits (A1 and A2). In Fig. 9(a),the area changes of the western sandspit follows a similar trend as the elongation trends which further
supports the dominance of the western sandspit at the river mouth. Pertaining to the eastern sandspit, its
unstable nature is depicted in Fig. 9(b). This is because of the deposition of sediment from the westernsandspit when breaching events occur. This condition also leads to the formation of a small lagoon on theeastern sandspit.

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Figure 9: Sandspit area changes of updrift and downdrift sandspits.
4. CONCLUSION
From this study, the morphological evolution of the Volta River mouth has been outlined byutilizing freely available remote sensing images from 1984-2020. Analysis of river mouth morphologicalvariations before and after the construction of the groyne system was performed. Results from image andsandspit analysis showed the natural morphological condition of the river mouth from 1984-2012 where
the dominant sandspit was characterized by elongation and breaching events. After groyne construction(2013-2020), a new trend of morphology was observed with both sandspits intruding into the estuary.
The sudden change in river mouth behaviour clearly highlights the response of a river mouth sandspit
when the longshore sediment transport is trapped through engineering activities.
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