東北地域災害科学研究

第58巻（2022）

111

Assessment of river mouth variability after jetty and groyne construction: Acase study of the Volta River mouth, Ghana
Stephan Korblah Lawson1, Hitoshi Tanaka2, Keiko Udo3, Nguyen Trong Hiep4, and Nguyen Xuan
Tinh5
Abstract
The response of the Volta River mouth to the construction of a jetty and groyne system wasstudied using satellite images from Landsat and Sentinel-2. Prior to the construction of these coastal
structures, the river mouth sandspits exhibited a cyclic morphological pattern which included sandspit
elongation, spit breaching and deposition of the breached updrift spit onto the downdrift beach. However,a new morphological regime has been observed at the river mouth. Results from the image analysis has
shown that, intrusion of the sandspit into the estuary, development of offshore shoals, accretion anderosion of the sandspits due to an alteration of the sediment budget are some morphological changes thathave occurred at the river mouth.
Keywords: Coastal structures, river mouth, sandspit, satellite image analysis, Volta River mouth.
1. INTRODUCTION
In many coastal regions, jetties and groyne systems have been utilized in beach stabilization andflood related projects. These coastal structures also facilitate tidal exchange and channel navigation
especially at river mouths or inlets. In the design phase of these projects, several attempts are made to
predict the interactions of these structures with prevailing hydrodynamic conditions. In most cases,
unforeseen alterations are observed at such locations. One of such cases is the construction of the Pranburi
Jetties in Thailand. A study by Phanomphongphaisarn et al. (2020) showed that the post construction
period disagreed with the simulations in the design phase of the project which resulted in the constructionof three detached breakwaters north of the jetty. This was done through the use of satellite images.
Another study by Tanaka & Lee (2003), at the Shiribetsu River mouth in Japan revealed a decrease inwave set-up after a jetty was constructed.
The Volta River mouth was chosen for the study mainly because of the presence of a jetty on thewestern side of the river mouth. Also present on this side of the river mouth is a groyne system whichcovers a 16 km stretch with each groyne spaced at 800 m. These structures were constructed as part ofthe Ada Sea Defence Project (ASDP) from 2012 – 2016. The project was aimed at controlling the severe
beach erosion, restricting the growth of the updrift sandspit, improve tidal exchange and navigation(Mensah & FitzGibbon, 2013).
2. STUDY AREA AND ENVRIONMENTThe Volta River mouth is situated at Ada-Foah on the eastern coast of Ghana, West Africa (Figure1). The river drainage area is approximately 400,000 km2 which is made up of catchment areas within
Mali, Burkina Faso, Ivory Coast, Ghana, Togo and Benin. Several hydro-electric and irrigation dams
have been constructed on the Volta River network. The Akosombo and Kpong dams are the closest dams
located upstream of the river mouth which have affected the flow rate and fluvial sediment supply to theriver mouth. Prior to dam construction, mean monthly flow rates over 4000 m3/s were recorded and thissubsequently reduced to about 2000 m3/s after dam construction. A study by Amenuvor et al. (2020)
indicates the reduction of the annual fluvial sediment supply from 180×106 to 13×106 m3/year due to damoperations.

The coastal zone around the river mouth is marked by a micro-tidal environment with tides being
semi-diurnal. The average tidal range is 1m with spring tides at 1.28m and neap tides at 0.64m (Allersma
1,2,4,5
Department of Civil and Environmental Engineering, Tohoku University, 6-6-06 Aoba, Sendai
980-8579, Japan.
3International Research Institute of Disaster Science, Tohoku University, 468-1 Aoba, Sendai 980-8572,Japan.

112& Tilmans, 1993). The significant wave heights along the river mouth ranges from 0.5 to 3.30 m basedon wave data retrieved from the WaveWatch III model. The dominant wave direction is southwest fromswells generated in the Atlantic Ocean. In addition, the West African Monsoon affects the wave heights,
with the largest wave heights generated from March-October. The induced longshore currents along thiscoast is unidirectional from the west to east and the average longshore sediment transport at the Volta
River mouth is about 1.25 ×106 m3/year as determined by sandspit and wave analysis (Lawson et al.,2021).

Figure 1. Map of the Volta River mouth at Ada-Foah, Ghana.
3. MATERIALS AND METHODS
3.1 Data Collection and Shoreline Extraction
The primary dataset used in this study were satellite images retrieved from the Google EarthEngine repository. These included images from Landsat 5 (TM), Landsat 7 (EMT+), Landsat 8 (OLI) andSentinel-2 (MSI). The image collection period for the study extended from March, 2002 to July, 2021.
This was chosen due to the significant morphological evolutions observed at the river mouth before andafter the completion of the ASDP.
The shorelines for the study area were extracted using the Coastsat toolkit (Vos et al., 2019). The
toolkit uses a supervised image classification and a sub-pixel resolution border segmentation to extract
shorelines with a horizontal accuracy of 10m (Vos et al., 2019). The image classification stage enables
the definition of the sand, water, white-water and other land features whereas the sub-pixel segmentation
stage uses the Modified Normalized Water Index (MNDWI) to categorize the water and land pixels. The
shoreline is then defined using Otsu’s thresholding and the Marching Squares algorithms.
3.2 River Mouth Variability Analysis
To ascertain the river mouth variability before and after the construction of the jetty and groyne
system, the river mouth sandspits were divided into three sectors. These included Sector 1 on the updrift

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sandspit (red), Sectors 2 (yellow) and 3 (green) on the downdrift sandspit (Figure 2). Furthermore, thesandspit areas for each sector was estimated for the study period to depict accretion and erosion trends at
the river mouth. For this analysis, the areas for Sector 1, 2 and 3 were denoted as A1, A2 and A3,
respectively (Figure 2).

Figure 2. Definition of the three sectors used in the river mouth variability analysis.
4. RESULTS AND DISCUSSIONS
4.1 River Mouth Morphological Change
Within the study period, a number of morphological changes were observed at the Volta Rivermouth from the collected satellite images. The jetty at the river mouth was constructed with the purposeof halting the lateral migration of the updrift sandspit from the west to the east (Figures 3a and 3b). This
resulted in an increase in the width of the river mouth which facilitated an efficient tidal exchange andchannel navigation. In the years following the construction of the jetty, there has been the frequent
formation of offshore shoals which could be attributed to changes in the water depth at the river mouth
and wave breaking. These shoals move landward and attach to the downdrift sandspit which led to a
reduction in the width of the river mouth (Figures 3c to 3e). In addition to this, the landward growth of
the updrift sandspit is observed (Figure 3c to 3e). This phenomenon has manifested in the intrusion ofboth sandspits into the estuary (Figure 4). A more recent development at the river mouth is the multiple
breaching of the sandspits which has led to widening of the river mouth (Figure 3f). These distinct
changes at the river mouth give an indication of the river mouth’s response to alterations in the sedimentbudget at the river mouth by the ASDP.

Figure 3. Satellite images showing the various morphological evolutions at the Volta River mouth(2002 – 2021).

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Figure 4. Shoreline variation of the Volta River mouth (2016 – 2021).
4.2 Variation of Sandspit Area
By plotting the sandspit area for the three defined sectors of the river mouth, the accretion anderosion trends can be assessed. Figure 5 shows the time variation plot of the sandspit area for Sector 1.
Regression analysis for this sector after the jetty and groyne construction shows a sandspit area change
rate of 1.3×104 m2/year from 2016 to 2020. The increment in sandspit area within this period is as a resultof the intrusion of the updrift spit into the estuary induced by sediment bypassing the jetty and the actionof breaking waves.

Figure 5. Updrift sandspit area variation for Sector 1.
With respect to Sector 2 on the downdrift spit, the regression analysis revealed an area change
rate of 4.1×104 m2/year from 2013 to 2019 (Figure 6). This gain in sandspit area was mainly due to theattachment of the offshore shoals to this section of the downdrift sandspit. Sector 3 on the other handshows the exact opposite of both Sectors 1 and 2. The analysis showed a sandspit area change rate of 6.4×104 m2/year from 2013 to 2021 for Sector 3 (Figure 7). This corresponds to the shoreline retreat
observed in Figure 4 for this sector of the river mouth. Table 1 shows a summary of the sandspit areachange rate for the three sectors of the river mouth.

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Figure 6. Downdrift sandspit area variation for Sector 2.

Figure 7. Downdrift sandspit area variation for Sector 3.
Table 1. Summary of sandspit area change rates for the three defined sectors.
SectorSandspit area changerate (m2/year)

Remarks1

1.3×104

Spit intrusion

24Spit intrusion

4Erosion3

4.1×10-6.4×10

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4. CONCLUSION
From this study, the variability of the Volta River mouth has been investigated after theconstruction of a jetty and groyne system. The study employed the use of remotely sensed images from2002 to 2021. The results of the study for three sectors of the river mouth showed that while the sandspit
area for two sectors showed a positive trend (spit intrusion), the third sector was characterized by anegative trend (erosion). Furthermore, the findings from the image analysis is unable to determine if thejetty and the groyne system are the main cause of erosion on the third sector since chronic erosion along
this section of the of the river mouth has been an existing predicament. As such, further analysis would
be required to properly understand the variability of the river mouth through the use of advanced
numerical models. The results from the image analysis in this study could serve as a basis for conducting
such future studies.
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