Field observations along West Africa are of extreme importance for understanding rapid beach geomorphological changes. Here, we conducted comprehensive field surveys to understand the geomorphological and hydrodynamic conditions of Ghana's lower Volta Delta. Over five months, in situ data were collected along 90 km of coast, examining temporal bathymetry, beach profiles, beach width variations, waves, and grain sizes. The primary objective was to determine the factors behind the significant coastal erosion and flooding, destabilizing the coastal ecosystems and communities. Results show that grain sizes are in the range of 0.2 < D50 < 0.5 mm. The beach slope, which varies between the flattest 0.4% at Blekusu, Keta Fort, Atiteti and the steepest 21% at Atorkor, is substantial for the returning floods experienced at some sections. Despite the same regional climatic environment, the study reveals contrasted coastal behaviors, with different sections of the Delta exhibiting distinct alterations in beach profiles and widths. Global wave model data (e.g. ERA5), widely used for coastal assessment in terms of magnitude, are significantly inconsistent with in-situ measurements in shallow water as found this study which could affect modeling of wave dynamics in shallow waters. This work serves as a database for current and future projects.
Article
The State of the Lower Volta Delta Beaches in Ghana from Field Observations
https://doi.org/10.21203/rs.3.rs-4727110/v1
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Nearshore bathymetry
Beach morphology
Digital Elevation Model
Volta estuary
Grain size study
wave variations
Coastal erosion in West Africa and particularly Ghana is a significant environmental issue that poses numerous challenges for the country [1, 2]. The lower Volta Delta (hereafter LVD) coast in the Eastern corridor is influenced by various anthropogenic activities and developmental projects like the construction of dams, jetties, and laying of electrical and telecommunication pipelines [3]. The lower Volta Delta is noted as one of the most vulnerable deltas in West Africa [4, 5]. Thus, the execution of any unplanned work in this dynamic environment could lead to shoreline erosion, floods, loss of available land, and several risks. Understanding the dynamic coastal environment requires a comprehensive study of waves, tides, currents, seabed stratification, seafloor classification, and nearshore bathymetry. This is because any design based on insufficient data can cause infrastructure failure which often leads to difficulties during the construction and operational stages of the infrastructure. Therefore, conducting hydrographic surveys or oceanographic investigations will provide a detailed understanding of the environment and its associated sea states.
Typically, groyne structures installed along coastlines are being damaged due to erosion, storm surges, salinization, and siltation of major river channels needing expensive dredging. Understanding the multifaceted coastal processes in a multi-scale coastal evolution, especially in tropical regions, has been limited due to a lack of available observational data. Recent research by Appeaning Addo et al. [6] reveals that beaches in the Volta Delta are still experiencing erosion at an alarming rate of over 2 meters per year while there is an increase in the frequency of coastal flooding [7]. The acceleration in hazards has been primarily attributed to human interventions through coastal constructions and development, as highlighted in earlier studies [e.g., 3; 5; 8;9] and the limited spatio-temporal information [9]. Additionally, the changes in this coastal region are influenced by a high population [10] and mangrove degradation. Field studies are therefore very crucial in capturing the inputs and data required for proper coastal structure design and maintenance.
In order to promote coastal socio-economic development, it is essential to gain insights into the factors that drive coastal evolution over different periods, especially considering the current vulnerability of many coasts due to climate change. This could be achieved by adopting multiple approaches like the use of echosounders for measuring depth, tide recorders, acoustic-based devices for seismic data collection and seabed studies, side scan sonar to capture seafloor images, and current meters to record the time series variations of the tide. An intensive and large-scale but comprehensive approach that precedes with a greater depth understanding of the natural coastal system, and their combined effects is very necessary.
Largely, a notable knowledge gap exists in understanding the exact conditions within the LVD, leading to its characteristic and rapid changes. Large-scale observational data on parameters like tide, shoreline changes, bathymetry, and topographic beach profile changes are limited. To emphasise, the Delta coastline is about 100 km but so far only satellite imagery and model data have provided data to cover this extent. These freely available satellite and model data are well known for their coarse resolution. There have been some high-resolution works within the LVD but most of them were conducted at highly smaller spatial scales [9;11–13]. In addition, satellite and model data are inadequate for a comprehensive assessment of the distribution of grain sizes, the dynamics of beach cusps, and the evolution of the beach profile. Thus, the precise conditions governing these transitions and the underlying mechanisms remain poorly understood, as well as the influence of the pre-existing beach morphology [14]. Field experiments will provide a comprehensive understanding of the study area's characteristics, including beach elevation, slopes, and hydro-morphodynamic changes, crucial for erosion control, safe beach landing, and scientific coastal dynamics research.
The main goal of this study is to assess the morphological current state of the LVD coast and its underlying wave-driven dynamics, utilizing a wide array of field data. This involves an in-depth measurement of the substantial variabilities in topography, sediment characteristics, beach widths, waves, and bathymetry for both river and nearshore areas.
Topographical profile changes
The range of elevation values above mean sea level vary significantly across the 16 transects, reflecting the rapid changes in the coastal geomorphology. Elevations ranged from 1 to 13 m, with the Woe having the lowest elevation of 1.5 m and the Atorkor having the highest elevation of 13 m, depending on the specific geological and geographical characteristics of each transect. Figure 1 reveals that some beaches are gentle and wide while others are steep and narrow. Most of the gentle beaches, like those at Keta Fort, Whuti, Woe and Fuveme, are the widest. Conversely, steep beaches, such as Atorkor and Avume, have narrower widths. The topography of all the beaches along the LVD is diverse. These variations are influenced by factors such as sediment size, wave energy, and coastal geomorphology.
Beach width and slope variation
Beach width measurement is crucial data as it provides insights into active beach zone width, which can vary significantly due to tides, storms, and sediment transport. The beach width was measured by determining the horizontal distance from the lowest land-water interphase to the closest vegetation at the back beach. The monthly variation in beach width was evaluated using RTK-DGPS survey equipment (resolution is 1cm), measuring the horizontal distance for each beach profile at the different 16 sites, as shown in Fig. 2a. The study found a range of beach widths between 20 and 140 m, influenced by erosion, wave energy, sediment availability, and human interventions like coastal structures and beach nourishment projects. This is vital for the effective planning and management of coastal development and natural resources. The beach slope varies between 0.4% and 21% along the 90 km coastline. The beaches of Blekusu, Keta Fort, and Atiteti are the flattest with beach slopes of less than 0.7%. The steepest beach was noted at Atorkor with a slope of 21%. Grain sizes were observed from 0.2 < D50 < 0.5 mm. In terms of how long a beach will exist (hereafter beachlife), Table 1 shows the rates of erosion verses the beach widths. It is emphasised that about 70% of the entire beach is eroding up to a maximum of 4.5 m/year that would lead to disappearance of an entire beach.
| Site | Monthly slope (%) | Average slopes (%) | D50 (mm) | Mean widths (m) | Shoreline changes m/yr | ||||
|---|---|---|---|---|---|---|---|---|---|
| May | Jun | Jul | Aug | Sep | |||||
| Aflao | 4.8 | 4.5 | 6.1 | 2.7 | 4.2 | 4.4 | 0.2 | 56.5 | -0.7 (± 0.4) |
| Denu | 3.5 | 3.2 | 4.4 | 2.2 | 3.7 | 3.4 | 0.4 | 69.0 | -1.2 (± 0.4) |
| Seven Sea | 6.9 | 7.8 | 6.3 | 5.5 | 4.8 | 6.3 | 0.4 | 28.8 | -2.0 (± 0.4) |
| Adina | 4.5 | 6.5 | 6.7 | 10.1 | 7.6 | 7.1 | 0.2 | 53.9 | -1.2 (± 0.6) |
| Blekusu | 0.1 | 0.3 | 0.2 | 1 | 0.6 | 0.4 | 0.3 | 74.9 | -3.6 (± .8) |
| Keta Port | 1.5 | 3.4 | 5.5 | 7.7 | 1.5 | 3.3 | 0.2 | 34.5 | 2.3 (± 0.9) |
| keta Fort | 0.4 | 0.9 | 0.4 | 1.5 | 0.4 | 0.7 | 0.2 | 114.2 | 3.8 (± 2.4) |
| Tegbi | 2.4 | 1.9 | 3.1 | 4.4 | 1.8 | 2.7 | 0.4 | 78.7 | 1.4 (± 0.7) |
| Woe | 1.8 | 1.3 | 1 | 2.4 | 0.2 | 1.4 | 0.5 | 73.3 | 1.6 (± 0.6) |
| Avume | 8.0 | 1.7 | 9.9 | 1.6 | 14.5 | 11.1 | 0.4 | 26.7 | -1.6 (± 1.0) |
| Anloga | 5.0 | 4 | 6 | 6 | 4.3 | 5.1 | 0.2 | 76.7 | 1.9 (± 0.9) |
| Whuti | 0.0 | 0.8 | 0.6 | 6 | 0.6 | 1.6 | 0.5 | 122.1 | -2.6 (± 1.6) |
| Atorkor | 14.8 | 3.4 | 17.4 | 29.3 | 21.5 | 21.3 | 0.4 | 20.2 | -3.3 (± 0.9) |
| Dzita | 3.7 | 3.5 | 4.8 | 37.9 | 4.1 | 10.8 | 0.4 | 62.8 | -4.5 (± 1.1) |
| Atiteti | 1.9 | 1.4 | 1.1 | 1 | 1.6 | 0.6 | 0.3 | 67.3 | -1.7 (± 1.0) |
| Fuveme | NaN | 2.4 | 1.5 | 2.7 | 4.7 | 2.8 | 0.4 | 51.9 | -2.7 (± 1.0) |
Sediment characteristics along the beach
Figure 2b represents the sediment characteristics based on their grain sizes along the LVD coast. In order to adequately understand sediment and grain size distributions, D10, D50 and D90 were estimated. These are fundamental in beach sand characterizations. D10 signifies the diameter beneath which 10% of the particles in a sample are smaller, whereas D50 serves as a representation of the median or typical particle size, denoting the diameter under which 50% of the particles fall. D90 indicates the diameter below which 90% of the sample's particles lie. The most used is the D50. Erodibility is tied to soil properties thus, different soils exhibit different erodibility (sand, clay). Areas like Keta and Anloga with smaller D50 will likely experience significant erosion than the rest of the beaches. These measurements were used to describe the particle size distributions along the beach. Table 2 displays the mean grain sizes at each location. Sediment sizes between 0.20 and 0.29 mm were regarded as fine sand while 0.3–0.4 mm were of medium sand sizes. Most of the beaches are made of medium sand with only about 43% of the beach being fine sand. Only the sand at the beach of the Keta Fort is well sorted (0.001 mm), the rest of the beaches are moderately sorted at about 0.002 mm. Thirteen out of the sixteen beaches are very coarse skewed (higher than 0 mm) while the remaining three are very fine skewed (less than 0 mm). Fuveme and Whuti beaches are extremely platykurtic while the rest are very platykurtic.
| Transect | Mean size (mm) | Sorting (mm) | Skewness (*10− 1 mm) | Kurtosis (10− 1 mm) |
|---|---|---|---|---|
| Aflao | 0.246 | 0.002 | 0.007 | 0.005 |
| Denu | 0.326 | 0.002 | − 0.006 | 0.005 |
| Seven Sea | 0.326 | 0.002 | − 0.006 | 0.005 |
| Adina | 0.243 | 0.002 | 0.007 | 0.005 |
| Blekusu | 0.278 | 0.002 | 0.008 | 0.005 |
| Keta (Port Area) | 0.247 | 0.002 | 0.007 | 0.005 |
| Keta Fort | 0.208 | 0.001 | 0.006 | 0.004 |
| Tegbi | 0.323 | 0.002 | 0.006 | 0.004 |
| Woe | 0.333 | 0.002 | 0.005 | 0.005 |
| Avume | 0.333 | 0.002 | 0.006 | 0.005 |
| Anloga | 0.244 | 0.002 | 0.007 | 0.005 |
| Whuti | 0.373 | 0.002 | -0.022 | 0.020 |
| Atokor | 0.326 | 0.002 | 0.006 | 0.005 |
| Dzita | 0.322 | 0.002 | 0.006 | 0.005 |
| Atiteti | 0.245 | 0.002 | 0.007 | 0.005 |
| Fuveme | 0.393 | 0.002 | 0.006 | 0.014 |
River and nearshore bathymetry patterns
In general, the nearshore ocean bathymetry was measured on 15 km along the beach from the estuary. Two sets of bathymetry data were collected already; bathymetry of the nearshore surf zone between the estuary (Fuveme) and Dzita beaches; and bathymetry of the river from the sea to 15 km in the land (Fig. 3). The ocean depths range from 0.5 to 9 m, with the Dzita side being deeper than the Fuveme section. This was used to discuss the Fuveme coastal area which usually faces floods because there are hardly any floods in the deeper areas of Dzita. The Volta River's bathymetry, on the other hand, showed depths from 0.5 to 17 m.
When we compared measured bathymetry to that of the General Bathymetric Chart of the Oceans (GEBCO) data. The GEBCO data does have relatively poor temporal resolution and as indicated in Fig. 3, at some distance to the beach, about 2 km, there are no GEBCO signals as well. Figure 3 shows that GEBCO data does not extend to all areas which are now covered by water. Figure 3 generally presents the topo-bathymetric continuum; i.e., the elevations from the back beach to the 9 m water depths. With this region, GEBCO is unable to capture data.
Drone flights were performed on the same section where bathymetry was collected to obtain a topo-bathymetric continuum. Digital terrain models (DTMs) and orthophotos were generated from mosaic aerial photos. DTMs provided changes in elevations in three dimensions (3D). The beach area that was captured by the drone includes back beach lagoons (small water bodies isolated from the sea during major sediment supply into the coastal segment) as well as houses and vegetation along the study area. The main geography of the region is depicted in Fig. 3. Figure 3 indicates that the nearshore back beach elevation is up to 30 m above mean sea level. Thus, extending to the bathymetry, the total elevation of the nearshore ranges from − 9 m to 30 m about the MSL. Repeated orthophotos and DEMs showed areas of erosion, loss of sediment in volumes and beach profile changes within this surveyed area.
Biophysical observations of the estuary/Volta River
Temperatures measured on different months (March - July 2022) across the river/estuary were significantly different (post hoc; p < 0.05). Average temperature ranged from ≈ 27 to 30 oC with May 2022 recording the highest and July 2022, the lowest temperature. Salinity ranged from 0.04 to 7 ppt, with the highest in July and the lowest in May. Salinities recorded on different sampling months were significantly different across the length of the estuary. On average, surface conductivity ranged from 0.1 to 12.3 mS/cm. Highest conductivity was recorded in July. The lowest was recorded in May. Amount of dissolved oxygen (DO) in the water ranged from ≈ 2–7 mgl-1, with the highest DO occuring in May 2022 and the lowest in July 2022. The oxygen content of the water varied significantly for the sampling months (≈ 3–4 mgl-1) across the length of the estuary (post hoc test p < 0.05). The range of pH in the estuary was ≈ 7–8; pH was highest in March 2022 (≈ 8 ± 0.1) and lowest in (≈ 7 ± 0.1). The acidity of the water in the estuary changed significantly on the different sampling months.
Wave conditions within the LVD: ERA 5 data vs buoy data
ERA 5 wave data is utilized as the primary data in various works [ 9, 15–16] to evaluate the impact of waves on beach changes and erosion within this coastal area. ERA 5, the fifth generation of the European Centre for Medium-Range Weather Forecasts, is climate reanalysis dataset covering 1979-present. It is generated by the Wamdi Group's WAM model since 1988, providing daily hindcast data for offshore significant wave height (\(\:{H}_{s}\)), direction (Ɵ), and peak period (\(\:{T}_{p}\)). It provides data on many parameters including offshore significant wave height (Hs), direction (Ɵ), and peak period (Tp), with a resolution of 0.5o*0.5o. Even though these researchers used some algorithms [15] to convert offshore conditions to nearshore waves (Fig. 4b) at breaking point, there remains significant discrepancies. A comparison between ERA 5 and the measured data is shown in Fig. 4a as well as Hs at breaking point [15] against ERA 5 (Fig. 4b). As shown in Fig. 4a, only 40% (R ~ 0.63 or r2 = 0.4) of the variability in in situ measurements is consistent with ERA 5. As Hs increases, the two data sets disperse into high values and low values with no correlation between the two data sets.
Hazards within the LVD
The LVD coastline is subjected to natural and human-induced stresses, manifesting as persistent coastal flooding [13], subsidence at a rate of 4 mm/yr [7], coastal erosion at a rate of over 2 m/year on average, and the gradual depletion of its wetlands and water bodies [9]. For the reported subsidence and topographical changes, it remains unclear if it is only due to water abstraction within the Delta for farming purposes. There have been reports of basement faulting and the fact that the Keta Basin is a fault-controlled Mesozoic/Tertiary sedimentary basin [18; 19] within the Delta. Coastal erosion has largely been attributed to a decline in sediment supply from the Volta River after the Akosombo Dam construction, which has heightened the susceptibility of the delta to ocean wave impact [20; 6]. For instance, about 300 inhabitants were displaced in Fuveme between 2013 and 2017, and now the community is wiped out. According to Babanawo et al. [21] other areas such as Agavedizi, Amutinu, Salakope and Blekusu have been affected by serious coastal flooding and erosion issues. Our results, as indicated in Fig. 3, show the beaches are reducing in widths and their slopes are getting steeper.
Along the entire stretch of the LVD coastal segment, there are several coastal protection structures, which include groynes and revetments/seawalls [1, 3]. These coastal structures have influenced the flow of sediments along the LVD coast thereby rendering some areas of the delta coastline deficient in sediment supply and flow. There is also a notable net increase in built-up areas primarily from the conversion of marshland, dense vegetation, and cultivated lands into saltpans. Between 1991 and 2007, cultivated land increased by 125.51 km2 over 15 years [22], while dense vegetation and marshland lost 151.37 km2 and 2.44 km2, respectively, due to the conversion of marshland, dense vegetation, and cultivated lands.
The Keta Lagoon is experiencing a decline in water quality due to increasing agricultural activities. This is particularly in areas near Anloga, Agovi, Avume, Akakagbo, Fiahor, Woe, and Zedevu towns, where extensive farming practices, reliant on manure and inorganic fertilizers on exposed beach sands, are prevalent. This is causing a significant portion of the lagoon to become unsuitable for aquatic life, with nearly 90% experiencing eutrophication. Coastal erosion has caused the loss of sea turtle nesting grounds around Fuveme and the other areas along the Delta. Furthermore, the plankton index of biotic integrity underscores the overall poor environmental conditions, especially in the aforementioned areas, where the diversity of phytoplankton and benthic macroinvertebrate species has significantly diminished [23]. Most recently, the LVD has also been reported to be affected by land subsidence of about 4 mm/yr [7] which can eventually lead to an increase in flooding from relative sea level rise in the next few decades. In all, the LVD is currently impacted by coastal challenges and demands immediate attention.
Consequences of the coastal processes
Comprehensive data gathering is crucial for understanding the present state of the LVD and providing a complete management approach. The present study collected in situ data on five key parameters: nearshore bathymetry, wave parameters, sediment grain sizes, beach profiles and digital elevation of the beach. All these factors collectively contribute to our understanding of the coastal dynamics, safety, and resilience of the delta and are useful in understanding various stressors of the LVD. Thus, these factors will be pivotal in guiding coastal management, engineering efforts and research efforts dedicated to the safeguarding of coastal ecosystems and communities.
Figure 3 displays the cross-section elevation profiles from the backbeach to the shoreline or a bit underwater for the study from May to September 2023. As shown, there is no consistent trend for the delta, as each beach profile behaves differently depending on the beach morphology. For instance, at places like Keta Fort, Fuveme, Woe, and Wuti, the beach profiles (Fig. 3) are longer but changing rapidly through erosion or accretion. According to Gazi et al. [24], wider and gentler beaches are less likely to experience erosion than steeper beaches. In contrast, the beach profiles in rest of the places are narrow and steep (Table 1). On a temporal scale, most of the beaches in the delta (e.g., Fuveme, Keta Fort, Blekusu and Woe) show erosion in proportion to changes in beach width; this pattern is consistent over the months. Coastal erosion is observed over this short period, with Fuveme exhibiting the greatest changes in beach width and erosion.
Sediment variations in grain sizes measured as D10, D50 and D90 provide valuable insights into the sediment characteristics. Grain size observation reveals that all beaches in the delta are mostly made up of medium and fine sands, making them easily susceptible to erosion [25]. The entire studied area is sandy and the degree of sorting in the delta is generally consistent. The moderately sorted nature of the beach sediments in the delta also points to previous reworking and mixing, where sediments have been transported and deposited more than once [26]. However, this requires further validation with multiple grain size data.
The study assesses nearshore bathymetry Fig. 3 within the river and 15 km along the beach area, which plays a pivotal role in comprehending wave patterns, energy dissipation, and coastal currents [27], and aids in the planning and construction of essential structures like breakwaters, jetties, and harbors. The beach area along the estuary is of great importance because it has frequently exposed sand bars and experiences significant erosion over time yet there is no in situ bathymetry data to help understand these frequent changes. Collection of the nearshore bathymetry of the delta environment is crucial, as it provides valuable insights into sediment transport patterns and coastal erosion [28]. Fuveme Beach, located about 1 km from the estuary, is frequently flooded due to its gentle nearshore profile. On the other hand, Dzita, about 15 km from the estuary, does not experience flooding as its depth is deeper and wave overtopping is limited. Bathymetry data reveals these differences in flooding experiences. However, it is worth noting that the General Bathymetric Chart of the Oceans (GEBCO) (Fig. 3), a globally accessible data source, lacks information on the surf zone environment [29], highlighting the need for local observations to support global data like GEBCO.
This paper establishes that wave data within the nearshore shallow environments is significantly different to those obtained from offshore deeper waters, emphasising the importance of depth in wave data. The result indicates that about 60% of the ERA 5 data does not correspond with wave buoy data. This agrees with the previous studies [12;30–31]. It underscores the significance of collecting in situ data within the LVD, as all the parameters measured are important in the design and maintenance of already existing coastal infrastructure and projects that are aimed at protecting beaches in the LVD.
Future of the Volta Delta.
Globally, sea level rise has an effect on all deltas [32]. However, future sea-level rise varies by region, depending on the chosen climate scenario. Within the LVD, the anticipated rise in mean sea level will worsen flood events that occur during extreme coastal events [7]. Unfortunately, understanding the LVD system is challenging due to its high spatial and temporal variability, involving hydrodynamics, morphology, sedimentation, and human activities, and limited data available for these processes. Figure 5 shows the beach retreat relative to the current beach widths at the current rate of increasing temperature and rising sea levels. Shoreline changes were obtained from Digital Earth Africa (https://www.digitalearthafrica.org/platform-resources/services/coastlines). The platform provides coastline monitoring service from satellite data to map specific locations of the African coastline through time (Fig. 5a). The past changes in the coastline and areas of concern for the future are provided. Using interactive map, averaged shoreline changes (Fig. 5b) are computed from satellite imagery between the year 2000 and 2024 at all the 16 sites (Fig. 5b). The potential of beaches disappearing or surviving was computed (Fig. 5c) as the ratio of beach width to erosion rate at each of the 16 local sites. Below zero means the current beach will disappear or be covered by water after a number of years depending on the erosion at each site; and above 0 m means the beach will remain, be stable, or advance seaward. It can be seen that about 50% of the beaches will likely disappear in 50 years with the formation of new beaches or not at all. In 80 years, about 70% current beaches of LVD may be covered by water. The estimated age of these beaches could be shortened with uncontrolled climate change. That notwithstanding, 20% of the beaches (Keta port to Anloga) will remain stable or accreting beyond all these years. The areas that are accreting are on the Cape and also have significant sea defense systems. The longshore movement of littoral sediment remains important, particularly at the middle portions (Keta to Avume) of the study area where the wave directions are most oblique. There is less erosion along this part. The resilience of the middle part of the beach suggests a convergence of sediment both from the east and west, which requires further exploration into details.
To understand the details, numerical models must be developed, calibrated and validated with these widely measured in-situ and local field data [33] for utilisation within the LVD to provide adequate information for adaptation purposes. In addition, climate change should be prioritised in the development of coastal engineering solutions.
Significance of in-situ data collection
Coastal areas play a central role in the lives of many coastal dwellers because they drive the dynamics of large ecosystems. In situ studies are of huge importance for modeling and forecasting extreme events and vulnerability of coastal ecosystems. Thus, it is imperative to monitor coastal areas, and to assess or in some cases predict their impact on our society. There is so much about the world coastal areas that we still don’t know. The reasons behind this are related to the fact that coastal data is very scarce and expensive to collect. Different technologies have been used to collect this data, from remote sensing to in situ sensors. Yet, there are several challenges to consider and solve, before an effective and accessible collection of coastal in situ data is achieved. On one hand, models are very often limited by a lack of data: wave data are often downloaded from global or regional databases that do not adequately reproduce variability at the coast; bathymetry is often not optimal, and there is virtually no validation data for the models.
On the other hand, when vulnerability at the coast is well predicted, it can limit the loss of human life and material at the coast: better knowledge of the environment's dynamics is a guarantee for the implementation of effective warning systems at the coast, and so on. Remote sensing has greatly increased the amount of coastal data available, and recent advances in this area have also resulted in substantial gains in data quality and reliability. However, remote sensing is not enough to collect data on all the relevant coastal variables, since it only covers the surface mostly with poor resolution, many temporal and spatial gaps. For this reason, in situ observing and monitoring systems still remain key to study the coast.
In reality, in situ data is critical for a complete understanding of the coastal area. Our results show that in situ sensors have the ability to gather meaningful data, such as topography, sediment characteristics, bathymetry, waves and seawater temperature, among others, in a much better resolutoin than remotely sensed data. We noticed in Fig. 6 that in situ bathymetry represented the nearshore up to the shoreline while GEBCO data is about 1.5 km far away from the shoreline. For this end, several research activities must be conducted to further develop coastal-observing devices.
In developed countries, there are several initiatives fostering the growth of in situ data availability but this is not same in the global south due to funding issues and technical expertise. This current work therefore is very timely for the LVD within West Africa.
The continual collection of data is important, particularly for developing models for local applications. It is also important to collect in situ data in this observation scarce region to improve our understanding of coastal evolution. This field experiment holds significant scientific value as it marks the first comprehensive data-gathering effort along the Volta Delta within the Bight of Benin, West Africa. Several projects, decision-makers, and scientists will make use of this data to calibrate their models. The study, executed in 2023, provides a comprehensive description of the morphodynamical characteristics of the study area and identifies the factors influencing these changes. Nearshore and riverine bathymetry data collection, topography, sediment grain size distribution, and wave measurements in the study area have been achieved. The research experiment provides a foundation for future extensive and long-term investigations along the LVD and promotes localized investigations. The quantitative and in-depth insights from nearshore topo-bathymetry constitute a pivotal building block for the establishment of enduring and robust observation and analysis for beach sediment and profile dynamics. Our wave and bathymetry data revealed the limitations inherent in global data sources which have in ability to record within the surf zone near the shoreline. These surveys are expected in the coming years to support the broader West African coast as part of the West Africa Coastal Area - mapping Vulnerability, Adaptability and Resilience in the changing Climate (WACA-VAR) project.
Site Description
Over the years, the LVD has been categorized by its distinct geomorphic characteristics. The VD is defined as the area below the 5-meter contour line, located within the Keta Basin [6]. The present study investigated 16 communities spread along a 90 km Ghanaian coastline from Aflao near Togo to Fuveme near the Volta estuary. The specific communities involved are Aflao, Denu, Seven Sea, Adina, Blekusu, Keta Port, Keta Fort, Tegbui, Woe, Avume, Anloga, Whutti, Atorkor, Dzita, Atiteti and Fuveme (Fig. 2b). All these areas are characterized by sandy beaches, and the Keta Lagoon complex, extending throughout the Anyanui area into the Volta Estuary. In recent times, the area has shown rapid changes in the beach morphology changes with the creation of new islands along the Delta due to the intermittent breaching of some beaches by the lagoon. In the last ten years, the LVD has experienced continuous flooding. Some communities such as Fuveme have been completely displaced [6]. Other communities such as Agavedzi, Amutisnu, Blekusu and Salakope also experience frequent flooding [21]. The coastal area is marked by a moderate plateau upper shoreface and steep lower beach face and experiences two primary types of waves: one originating from seas generated locally due to weak monsoons and another from swells generated by storms in the southern Atlantic Ocean. Tides are semidiurnal with a tidal range of around 1 m, resulting in gentle tidal currents [34]. The climate is classified as dry and equatorial, with coastal strands and mangroves being the predominant vegetation. The LVD experiences two major rainy seasons: May to July, and August to early October [6]. Fishing and farming of vegetables and root tubers as well as trading are the main economic drives of the indigenes.
Topography and Beach Profile variation along the LVD coast
Beach profiles, defined as topographic or elevation changes on individual transects between dune vegetation and the lowest water line, are measured for all 16 communities (Fig. 2). To ensure accurate and representative data, the collection period spanned over five months, from May to September 2023. The measurement started in May 2023. This was done using a Real-Time Kinematic Differential Global Positioning System (RTK-DGPS) coordinated to the geoid at mean sea level (MSL), with already established ground control points at each of the beaches. Accurate elevation data is essential for modelling sea-level rise impacts and assessing the vulnerability of coastal areas to inundation.
Aerial imagery of the beach
Aerial photos of the beach topography between Fuveme and Dzita were taken by DJI Phantom 4 Pro drone surveys. The flights were taken at 100 m above the ground level such that it could provide a large cover of the beach. The surveys were conducted during low tides to minimize errors associated with tide oscillations. Blurred and disoriented images were manually filtered to eliminate processing problems. Sixteen Ground Control Points (GCP) were set up throughout the sites (from Fuveme to Dzita) to correct the drone photos. The GCP coordinates were obtained using the differential GPS (D-GPS) method in the Ghana Meter Grid coordinate system. The D-GPS has an accuracy of ± 4 mm. Each Ground Control Point was observed for about 30 min on average to reduce Root Mean Square Error (RMSE). Flight missions were scheduled using drone deployment [35]. Flight paths were set to longitudinal and lateral to allow for the overlapping of images and 3D reconstruction of the terrain using the structure from motion (SfM) approach and Multi-View Stereo algorithms on Agisoft Metashape Software [36]. All of the images were mosaiced into orthophotos and a DEM was produced.
Sediment Grain Size measurement along the LVD
For each of the sixteen (16) transects, sediment samples were collected on the lower water line (LWL), the berm top, and the back beach. As demonstrated in Akwetey [37], 200 grams of air-dried samples were sieved through a 0.05 mm (50 µm) mesh sieve in the laboratory and later soaked with a 10% sodium hydroxide solution to isolate the particles from each other. Particles finer than 50 µm were further removed through the 50 µm separations. The bulk of the samples were transferred into pre-weighed aluminium foil trays and dried in an oven at 105°C until a constant weight was attained. Dried samples were again weighed (the weight loss accounts for the particles less than 50 µm). 100 g of the dried samples were weighed and sieved through a set of sieves of different mesh sizes (1 mm, 0.5 mm, 0.125 mm, 0.63 mm) and a collector (pan; <0.63 mm) at the base. Samples retained in each sieve were collected, weighed and recorded. Particles smaller than all the sieves were retained in the pan. The weight of each sieve fraction was calculated as a percentage of the original soil sample used as indicated in Eq. 1.
Using the method by Yankson [38] and Akwetey ([37]), the Mean Particle Size (MPS) was also calculated as:
Where x = mean size of the sediment samples (mm) and Y = corresponding percentage composition. The MPS obtained was compared to the Wentworth classification of particle grades.
Bathymetry and biophysical conditions
The study used a single beam echosounder (Valeport MIDAS surveyor echo sounder) to measure water dept variations in the Delta's nearshore area on an annual basis. The echo sounder, with a unique 'fuzzy logic' DSP, provides accurate readings, with data logging at ± 0.01 m or ± 0.02% accuracy and a resolution of 1 cm based on single channel high frequency. The bathymetry was measured from the Volta River estuary to Dzita, covering about 15 km. Figure 3 shows the bathymetry of the selected area that was measured, and bathymetry from the global model GEBCO. In addition, biophysical parameters including temperature, salinity, dissolved oxygen, conductivity, chlorophyll-a and pH were measured in the dry and wet seasons from the estuary into the river.
Nearshore wave conditions
In July 2023, nearshore wave data (Fig. 6) was measured directly off the Cape Saint Paul Lighthouse at Woe, one of the communities along the LVD. This wave data was retrieved from a wave rider buoy (Sofar Spotter Buoy) situated 2.9 km offshore at 20.45 m water depth. The buoy, moored at latitude 5.805750 and longitude 0.983133 degrees, was deployed on July 1, 2023, and held by floaters and a catenary loop of sinker weights. Additionally, four markers were installed 50 m away from the main setup (buoy), consisting of floaters and anchors and serving as signs for seafarers like artisanal fishermen and commercial vessels. The minimum, mean, and maximum wave height (Hs) were 0.7,1.07, and 1.73 m, respectively. The wave direction is oblique between 100 and 200º from southwest. The minimum, mean, and maximum wave peak periods (Tp) were 4.66, 9, and 14.62 s, respectively. Wind speeds are moderate O (10 m/s).
Funding
This study was funded by the National Geographic Society’s Harmony Coast and IRD-JEAI’s Coast-Under-Control projects at the Center for Coastal Management, University of Cape Coast.
Author Contribution
1. Conceived and designed the experiments -BDA and EKB;2. Performed the experiments to collect data- EKB, DA, PNJQ, EK, GTO, GAO;3. Analysed and interpreted the data - EAtt., RA, NOE, KAKN;4. Contributed materials, analysis tools or funding data – BDA, EA;5. Wrote the paper – BDA, OAD, RA
Data Availability
The data to this paper can be retrieved with https://doi.org/10.5281/zenodo.10554597
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No competing interests reported.