Morphological Patterns and Drivers of Urban Growth on Africa’s Wetland Landscapes: Insights from the Densu Delta Ramsar Site, Ghana

Abstract

The morphological aspects of urban growth on wetlands in Africa are under-researched. Using the Densu Delta Ramsar site in Accra, Ghana, as a case study, this paper analyses the morphological patterns and drivers of urban growth and its impact on wetlands. Data were obtained through remote-sensing, ground truthing, and limited key informant interviews. The analysis combined land use/land cover, building coverage and spatial autoregressive and ordinary least square regression techniques with the aid of ArcGIS version 10.8.2, QGIS version 3.34 and STATA version 17 software. The findings reveal that urban growth at the Ramsar site follows discernible spatial patterns consistent with the spreading pancake, village magnet, and ribbon development models. However, the primary force behind these patterns is growing demand for land to meet housing needs, aided by the failure of state institutions to perform their land use control and wetland protection functions. To achieve sustainable urban development, there is an urgent need to ensure effective wetland management by enforcing existing land use, development control, and wetland protection measures. This calls for the strengthening, resourcing, and closer collaboration of the relationships between the various state agencies responsible for urban planning and wetland management. There is also the need to engage and sensitise political leaders to increase their commitment to implementing wetland protection and pro-environmental policies.

1. Introduction

Over the last few decades, urbanisation has become a major developmental issue of global significance due to its magnitude, speed, and impact. For instance, between 1950 and 2018, the share of the world’s population living in urban areas grew from 30% to 55% [1]. Although sub-Saharan Africa is still the least urbanised region in the world, it is urbanising faster than any other region, with its urban population growing at about 3.6 million per annum [2]. As of 2014, 8 out of the 10 most rapidly-urbanizing countries in the world were in sub-Saharan Africa, namely Rwanda, Niger, Mali, Burundi, Burkina Faso, Uganda, Eritrea, and Tanzania [3].

Physical expansion of cities and towns is required to accommodate the growing urban population, as well as release the potentials of urban centres as engines of economic growth. As has been noted “the physical growth of cities is not an anomaly, but a process that has characterized cities throughout history” [4] (p. 46). Indeed, urban growth has long been associated with industrialisation and economic development [5]. However, it can also lead to several environmental and socio-economic consequences that are inimical to sustainable urban development if it is not effectively managed. In sub-Saharan Africa, much of urban growth has been unplanned and organic [6]. Such growth negatively impacts the green periphery—protected ecologically significant areas within the peri-urban zone that provide vital ecosystem services to urban dwellers [7]. The green periphery, particularly wetlands, often becomes encroached upon by the ever-growing built-up areas of poorly planned cities [7,8,9].

Wetlands are transitional ecosystems that bear the characteristics of both aquatic and terrestrial environments, where the land is covered by shallow water or the water table is near the land surface [10]. They include “areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres” [11]. Wetlands help in providing water and livelihood to humans, providing recreation, sustaining plant and animal lives, controlling erosion and floods, facilitating purification and the storage of surface water, recharging aquifers [12,13,14], and slowing down global warming through carbon sequestration [10,12,15]. Notwithstanding their critical importance, wetlands are increasingly being lost to rapid urbanisation. For instance, about 64% of the world’s wetlands have been lost since 1900 [16].

The urbanisation process can be in harmony with the environment, including wetlands and other ecosystems, but only when urban growth is consciously managed in a sustainable manner. This requires a good understanding of the nature and patterns of urban growth, the underlying forces, and the structure and form they create [17]. Elsewhere, scholars have rigorously undertaken quantitative research on the morphological patterns and drivers of urban growth on wetlands and other ecologically important landscapes [18,19]. However, little work in this area has been performed on urban wetland landscapes in Ghana and, for that matter, sub-Saharan Africa. This has partly contributed to the environmentally unsustainable way the region’s growth has proceeded, sometimes manifesting in the fast depletion of its urban wetlands. To contribute to the filling of the knowledge gap, this paper combines geospatial and statistical techniques to quantitatively analyse the morphological patterns and drivers of urban growth on the Densu Delta Ramsar site in Ghana.

Like the rest of the developing world, Ghana has been experiencing rapid urbanisation for some time. As Figure 1 shows, the country’s urban population increased from just 179,119 (representing 7.8% of the total population) to 17,472,530 (representing 56.7% of the total population) between 1921 and 2021. At an annual growth rate of 2.6%, it is projected that the country’s urban population will reach 37,518,000 by 2050, representing 73.2% of the national population [20].

The high tempo of urbanisation is fuelled by both natural increases and rural-to-urban migration [4,26], resulting in fast growth in the physical expanse of existing cities and towns. This poses a major threat to ecologically-sensitive areas in the country’s urban areas, including the Densu Delta Ramsar site. The site, located in the Greater Accra Metropolitan Area (GAMA), exemplifies African wetlands that have been adversely affected by rapid urbanisation. The present study analyses the magnitude, speed, and morphological patterns of urban growth at the site and the extent to which this growth has been driven by proximal factors such as the presence of the city of Accra, road network, and nearby village magnets. Findings and lessons from the study will enrich scholarly discourse on urban growth on wetlands and inform sustainable urban planning efforts. Methodologically, the study adds to existing knowledge by combining Aggregate Building Coverage Ratios (ABCRs) with Land Use/Land Cover (LULC) analysis in quantifying and assessing the patterns of urban growth. While LULC is commonly used in similar studies (e.g., [26]), the use of ABCRs to quantitatively model the morphological patterns of urban growth is a novelty. Although this study shares a similarity with the one conducted by [27] regarding the use of geospatial and regression techniques to analyse the influence of spatial drivers on urban growth, the reliance on ABCRs in the current study marks a point of departure from their methodology.

The study concludes that urban growth patterns at the Ramsar site are consistent with the spreading pancake, village magnet and ribbon development models, although the primary force behind the growth is increasing demand for land for residential development. This is exacerbated by the failure of state institutions to perform their land use control and wetland protection functions.

2. Conceptualising Urban Growth from a Morphological Perspective

Urban growth refers to an increase in an urban area’s population and physical size. Urban population growth and growth in the volume of socio-economic activities lead to the physical expansion of the urbanised area [4,26]. Physical expansion involves changes to the size, shape, and composition of the urban form through infill development, vertical layering, and horizontal spread [28]. Infill development is the inward expansion of a city through the development of patches of undeveloped spaces within the existing built-up area. This, if well planned, can be a land-use-optimisation strategy to deal with the leapfrog-development form of urban sprawl [17]. Vertical layering refers to upward expansion through the construction of high-rise buildings, especially in and around the city centre, while leaving enough green open spaces among the buildings [28]. Because of its capital-intensiveness, pyramidal expansion is more common in high-income cities than low-income cities. Horizontal spread refers to the lateral expansion of the built-up area, which is associated with low-income cities where land prices are relatively low and buildings are generally short [28].

Through hypothesising the drivers of urban growth at the Densu Delta Ramsar site, this study adapts three of the morphological growth models used by [27] to analyse peri-urban growth patterns in Accra: spreading pancake, ribbon development, and village magnets.

2.1. Spreading Pancake

Proximity to the city centre influences urban growth and form. Accessibility to the centre (in terms of travel time) is crucial for residents because this part of the metropolitan area has high concentrations of urban services and jobs (livelihoods), which they need to access. Therefore, locations near the centre experience more intense growth than the edges [17,29]. However, over time, as a population, economic activities and demand for space grow, space at and around the centre becomes exhausted, and growth spreads horizontally to the urban fringe in a manner akin to a spreading pancake [27] (see Figure 2a). Reminiscent of von Thünen’s 1826 agricultural land use model and the monocentric bid-rent model popularised by William Alonso, Richard Muth, and Edwin Mills in the 1960s and 1970s, the spreading pancake model assumes the existence of a dominant central city that drives outward urban growth in concentric waves across a metropolitan area [17,30]. Thus, based on the spreading pancake hypothesis, locations at the Densu Delta Ramsar site that are closer to Accra, the central city, are expected to experience more significant amounts of physical growth than locations that are farther from the city.

2.2. Ribbon Development

Ribbon development occurs when the horizontal spread of the contiguously built-up area of a metropolitan area happens axially along arterial roads, leaving swathes of undeveloped or rural landscape between them [17]. On one hand, urban residents seek to have unimpeded access to the city centre, where jobs and socio-economic amenities exist in high concentrations; on the other hand, they seek to enjoy cheap spacious living environments available at the urban peripheries [17,27,31]. These two conflicting demands can only be met through the mediating effect of a reliable transportation system that allows residents to have their dwellings in suburbia and commute to the city centre or elsewhere within the metropolitan area at affordable costs in terms of travel time and money spent (see Figure 2b). Thus, transportation technology and infrastructure improvements and growing automobile ownership have contributed significantly to the horizontal expansion of cities and metropolitan areas [28,31,32]. For instance, the expansion of Shenzhen, China, which is influenced by the development of transport corridors, follows this pattern of urban growth [32]. In low-income metropolitan areas, most commuter trips are made via a few paved arterial roads connecting the central city to other parts of a country or region in radial patterns. While these radially-oriented major arterials are discretely spaced, engineered circumferential arterials, minor arterials, and connectors are either non-existent or unmotorable, especially in the peri-urban parts of metropolitan areas [17]. Under such conditions, it is only rational for developers to site residential, industrial, commercial, and other structures along or close to major arterial roads to enhance accessibility and the value of their properties. Therefore, in this study, we expect locations near arterial roads at the Densu Delta Ramsar site to experience higher concentrations of urban growth than locations farther away.

2.3. Village Magnets

Village magnets are pre-existing peri-urban communities that attract rapid growth in the population and physical development as part of the spillover effect of the growth of an urban area. In Africa, residential development in the peri-urban zone often precedes the provision of basic infrastructure such as access roads, piped water, electricity, and educational facilities due to a lack of planning and a lack of local governmental financial and managerial capacity to keep pace with rapid urbanisation concerning infrastructure development [33]. As a result, peri-urban communities, with some of these critical facilities and services, tend to attract urban residents seeking cheap land for housing construction. Thus, over time, these pre-existing settlements “become the nuclei of fast-growing densely populated pockets surrounded by slow-growing sparsely populated areas” [27] and eventually merge with the contiguously built-up area of the city (see Figure 2c). In this study, the immediate environs of peri-urban settlements located at the Densu Delta Ramsar site are expected to attract faster urban growth than other locations.

3. Materials and Methods

3.1. The Study Setting

The 5893-hectare Densu Delta wetlands were designated at a Ramsar site in 1992 [34]. Located between latitudes 5°29′45″ N and 5°34′20″ N and longitudes 0°15′33″ W and 0°23′16″ W, the site straddles the Ga South and Weija-Gbawe Municipalities in the Greater Accra Region of Ghana (see Figure 3). The Gulf of Guinea also bounds it in the south and the Weija Lake (an artificial lake created by constructing the Weija Dam on the Densu River) in the northwest. The eastern half of the protected site, where the core wetlands are located, is generally flat and low-lying, with an elevation of less than 5 m above sea level. However, the western half is primarily hilly, rising to 100 m above sea level.

The wetland ecosystem within the protected area includes a delta formed by the Densu River, lagoons, salt pans, freshwater marshes and scrubs, and streams [35]. During the dry season, water flow from the Densu River to the wetlands is interrupted by the Weija Dam, located approximately 11 km upstream [36]. However, during the rainy season, excess water from the dam is discharged downstream and this, together with an increased volume of water from surrounding streams and surface runoff, causes the delta and the lagoons to overflow their banks, leading to flooding in low-lying communities such as Gbegbeyise and Glefe. The seasonal flooding also brings detritus, nutrients, and pollution [37]. The vegetation at the site comprises about 136 plant species made up of 50 flowering plant families, including mangroves, reeds, and sedges [35]. The site also serves as a habitat for about 35,000 birds belonging to 57 species; reptiles such as snakes, lizards, and turtles; mammals; marine and freshwater fish and shellfish species; and other invertebrates [35,38].

Notable communities within the protected area are Bortianor, Kokrobite, Oshieyie, Aplaku, Tetegu, Gbegbeyise, and Glefe, with all existing before the Ramsar site designation. Spillovers from the growth of Accra have resulted in rapid population growth in these and other peri-urban communities. For instance, available census data show that the combined population of the above communities was only 3669 in 1970 but increased dramatically to 28,216 in 2000 and 87,852 in 2010. Similarly, the combined population of four communities close to the protected area—Malam, McCarthy Hill, Mpoase, and Oblogo—increased from 3954 to 50,530 between 1970 and 2010, respectively.

The main economic activities at the protected area before its Ramsar site designation included fishing, crop farming, and salt winning [35]. However, crop farming is currently limited, while industrial and commercial activities have become predominant. Predominant examples among them include Panbros Salt Industries Ltd., the oldest and largest salt production company in the country, several hotels, beach resorts, and many retail and service operators. The rapid growth in population and non-farm economic activities has accelerated the construction of residential and other buildings, leading to significant loss and degradation of the wetlands.

3.2. Study Methodology

The aim of the study, which commenced in 2023, is to analyse the magnitude, speed, and morphological patterns of urban growth at the Densu Delta Ramsar site. It also analyses the extent to which this growth has been driven by proximal factors such as the presence of the city of Accra, road network, and nearby village magnets. To achieve this, satellite images were acquired and analysed using various geospatial and statistical techniques, which were supplemented by ground truthing and limited interviews by the researcher during visits to the study area.

3.2.1. Land Use/Land Cover Analysis

The loss of the Ramsar site to urban growth was preliminarily assessed though spatio-temporal analysis of land use/land cover (LULC) data. The LULC analysis spanned a 20-year period, from 2003 to 2023. The choice of this period was informed by the fact that most of the urban growth at the site occurred during this period.

Data were obtained from the United States Geological Survey’s (USGS’s) Landsat images [39]. Data for 2003, 2008, and 2013 were obtained from Landsat 7 images while data for 2018 and 2023 were from Landsat 8 and Landsat 9, respectively (

Table 1. Description of Landsat data used for LULC Analysis.

Year	Landsat Mission	Sensor	Spatial Resolution	Path/Row	Date of Acquisition
2003	7	ETM	30 m	193/056	12 February 2003
2008	7	ETM	30 m	193/056	10 February 2008
2013	7	ETM	30 m	193/056	6 January 2013
2018	8	OLI_TIRS	30 m	193/056	12 January 2018
2023	9	OLI_TIRS	30 m	193/056	20 December 2023

Source: [39].

). With the aid of version 3.34 of the QGIS software, the images were processed and classified using the supervised classification method. The classes were vegetation (including reeds, shrubs, grass, and croplands), water (including rivers, streams, lagoons, ponds, and salt pans), bare land (scraped surfaces, sandy surfaces, etc.), and built-up areas (including buildings, roads, and other artificially constructed physical structures).

The accuracy of the classified images was assessed by comparing them to reference vector layers obtained from samples of ‘ground truth’ sites in the study area based on high-resolution GoogleEarth images for the various years. The method involved gauging the level of agreement between the classified images and the reference images based on the following metrics: Producer’s Accuracy, User’s Accuracy, Overall Accuracy, and Kappa Coefficient.

3.2.2. Using Geospatial Technology to Compute ABCRs

The study used ABCRs (aggregate building coverage ratios) to measure and analyse the magnitude, speed, and pattern of physical urban growth at the study site between 2013 and 2023. ABCR is a modified version of the building coverage ratio (BCR). BCR is a zoning concept used to regulate the intensity of urban land use. It is defined as the ratio of the coverage area of the building(s) or other physical structures erected on a single plot to the total plot size when viewed vertically from the sky [40,41]. Similarly, the term ABCR is used in this study to refer to the ratio of the aggregated coverage area of all building(s) erected in a sector to the total sector area. A sector comprises multiple plots.

The generation of data for the computation of ABCRs was conducted using the ArcGIS (version 10.8.2) software to digitise the coverages of all existing buildings at the study site based on high-resolution satellite images extracted from the Google Earth application for the years 2013, 2018, and 2023. The same software and images were used to digitally divide the site into ‘sectors’ using roads, streams, and other physical barriers as boundaries. This is illustrated in Figure S1.

Based on the digitised data (in the form of shapefiles), sector and building coverage areas were electronically computed in ArcGIS 10.8.2, which in turn allowed the ABCR of each sector to be computed as

$$ABCR=\left({\displaystyle \frac{\sum _{i=1}^n{A}_i}{B}}\right)\times 100$$

(1)

where

Ai_i = Coverage area of building i (in m²);

B = Sector area (in m²);

0 ≤ ABCR ≤ 100.

3.2.3. Variable Description

The main variables used for data generation and analysis included ABCRs, changes in ABCRs, and proximity to the three urban growth drivers—Accra, arterial roads, and village magnets. The major arterial, a segment of the N1 highway connecting Accra to the southwestern part of the country and Abidjan (Cote d’Ivoire), forms the northern boundary of the Ramsar site. In contrast, the minor arterial is a 15.8-km paved secondary road connecting communities at the site to the N1 highway. The pre-existing peri-urban localities serving as village magnets include Kokrobite, Oshiyie, Bortianor, Aplaku, Glefe, and Gbegbeyise (see Figure S1).

In addition to the growth drivers, the possible effects of natural features on urban growth were controlled for. These include steep terrain and nearness to marshy areas, both of which were assumed to inhibit growth (see

Table 2. Description of the variables.

Variable Name	Description/Measurement	Variable Type
Outcome variables:
ABCR13	ABCR in 2013	Continuous
ABCR18	ABCR in 2018	Continuous
ABCR23	ABCR in 2023	Continuous
CHNGE1	Absolute change in ABCR, 2013–2018	Continuous
CHNGE2	Absolute change in ABCR, 2018–2023	Continuous
CHNGE3	Absolute change in ABCR, 2013–2023	Continuous
Predictors:
ACC	Distance from Accra (km) squared	Continuous
VILL	Located within 1 km of a village magnet	Dichotomous (Yes = 1, No = 0)
ROAD1	Located within 1 km of a major arterial road	Dichotomous (Yes = 1, No = 0)
ROAD2	Located within 1 km of a minor arterial road	Dichotomous (Yes = 1, No = 0)
WET	Located within 1 km of the wetland core	Dichotomous (Yes = 1, No = 0)
ELEV	Elevation exceeding 50 m above sea level	Dichotomous (Yes = 1, No = 0)

Source: Author’s construct.

).

3.2.4. Statistical Analytic Techniques

With the sectors serving as units of analysis, the study employed tables, graphs, maps, and other descriptive statistical tools to analyse the magnitude, patterns, and trend/rate of physical growth based on the outcome variables listed in Table 2. To test the spreading pancake, ribbon development, and village magnet hypotheses, multivariate regression techniques were employed to establish the statistical significance of the growth drivers on the outcome variables. This began with fitting an ordinary least square (OLS) multiple linear regression model, which is of the form

$$Y=\alpha +{\beta }_1{X}_1+{\beta }_2{X}_2+{\beta }_3{X}_3+\dots +{\beta }_k{X}_k+ϵ$$

(2)

where

Y = outcome variable; $X_i=$ predictors; $\alpha$ = intercept; ${\beta }_i=$ coefficient of $X_i$; $ϵ$ = error term

Various diagnostic procedures were conducted to check for five multiple linear regression assumptions: normality, linearity, non-multicollinearity, multivariate normality, homoscedasticity, and absence of autocorrelation. A Jarque–Bera normality test showed that the predictor ACC violated the normality assumption and this was resolved by squaring the variable (renamed as ACC2). There were no serious violations of the linearity assumption.

Multicollinearity tests using variance inflation factors (VIFs) showed that the variable WET was considerably correlated with the other predictors. It was therefore dropped from the model.

Although Jarque–Bera tests for normality showed some violation of the multivariate normality assumption, visual examinations of kernel density histograms showed the extent of violation was minimal. Moreover, non-normality of residuals does not seriously affect the F-statistic regression coefficients and other related hypothesis-testing procedures in terms of significance and power [42].

The visual inspection of scatterplots of residuals against fitted values of the outcome variables and formal Breusch–Pagan/Cook–Weisberg tests showed no severe violation of the homoscedasticity assumption. However, as expected, a visual inspection of choropleth maps of ABCRs and ABCR changes revealed some clustering in the distribution of these variables, implying a possible spatial autocorrelation. A formal diagnosis was conducted using Moan’s tests. Moran’ I, which is based on spatial weighting, is defined as

$${\displaystyle \frac{n}{W}}\ast {\displaystyle \frac{{\sum }_i{\sum }_j{w}_{ij(x_i-\overline{x})(x_j-\overline{x})}}{{\sum }_i{(x_i-\overline{x})}^2}}$$

(3)

where

n is the number of spatial units indexed by i and j;

x is the variable of interest with $\overline{x}$ being the arithmetic mean of x;

w is a matrix of spatial weights;

W is the sum of all w_ij.

A joint Moran test based on contiguity and inverse distance weighting matrices showed no statistically significant spatial dependence in three outcome variables: CHANGE1, CHANGE2, and CHANGE3 (see Table S1). Therefore, the OLS linear regression method was maintained for these variables, with the model being specified as

$$\mathrm{C}\mathrm{H}\mathrm{A}\mathrm{N}\mathrm{G}\mathrm{E}1={\alpha }_1+{\beta }_{11}\mathrm{A}\mathrm{C}\mathrm{C}2+{\beta }_{12}\mathrm{R}\mathrm{O}\mathrm{A}\mathrm{D}1+{\beta }_{13}\mathrm{R}\mathrm{O}\mathrm{A}\mathrm{D}2+{\beta }_{14}\mathrm{V}\mathrm{I}\mathrm{L}\mathrm{L}+{\beta }_{15}\mathrm{E}\mathrm{L}\mathrm{E}\mathrm{V}+{ϵ}_1$$

(4a)

$$\mathrm{C}\mathrm{H}\mathrm{A}\mathrm{N}\mathrm{G}\mathrm{E}2={\alpha }_2+{\beta }_{21}\mathrm{A}\mathrm{C}\mathrm{C}2+{\beta }_{22}\mathrm{R}\mathrm{O}\mathrm{A}\mathrm{D}1+{\beta }_{23}\mathrm{R}\mathrm{O}\mathrm{A}\mathrm{D}2+{\beta }_2\mathrm{V}\mathrm{I}\mathrm{L}\mathrm{L}+{\beta }_{25}\mathrm{E}\mathrm{L}\mathrm{E}\mathrm{V}+{ϵ}_2$$

(4b)

$$\mathrm{C}\mathrm{H}\mathrm{A}\mathrm{N}\mathrm{G}\mathrm{E}3={\alpha }_3+{\beta }_{31}\mathrm{A}\mathrm{C}\mathrm{C}2+{\beta }_{32}\mathrm{R}\mathrm{A}\mathrm{D}1+{\beta }_{33}\mathrm{R}\mathrm{O}\mathrm{A}\mathrm{D}2+{\beta }_{34}\mathrm{V}\mathrm{L}\mathrm{L}+{\beta }_{35}\mathrm{E}\mathrm{L}\mathrm{E}\mathrm{V}+{ϵ}_3$$

(4c)

However, for the outcome variables ABCR12, ABCR17, and ABCR22, serious spatial autocorrelation problems were found (see Table S1). This was resolved by using spatial autoregressive (SAR) models in place of OLS regression models.

The SAR model is of the form

$$Y=\alpha +{\sum }_{i=1}^k{\beta }_i{X}_i+\lambda Wy+{(I-\rho W)}^{-1}ϵ$$

(5)

where

Y is the outcome variable; $\alpha$ is the intercept; $X_i$ are predictors; ${\beta }_i$ are coefficients $X_i$; $Wy$ is a spatial lag of Y based on weighting matrix $w$ with $\lambda$ as its coefficient; and ${(I-\rho W)}^{-1}ϵ$ represents a spatial lag of the autoregressive error term $ϵ$ with $\rho$ rho being the autocorrelation parameter and I being an identity matrix.

Thus, based on the variables under consideration, the SAR model was specified for each of the three outcome variables (ABCR12, ABCR17, and ABCR22) as

$$\mathrm{A}\mathrm{B}\mathrm{C}\mathrm{R}12={\alpha }_1+{\beta }_{11}\mathrm{A}\mathrm{C}\mathrm{C}2+{\beta }_{12}\mathrm{R}\mathrm{O}\mathrm{A}\mathrm{D}1+{\beta }_{13}\mathrm{R}\mathrm{O}\mathrm{A}\mathrm{D}2+{\beta }_{14}\mathrm{V}\mathrm{I}\mathrm{L}\mathrm{L}+{\beta }_{15}\mathrm{E}\mathrm{L}\mathrm{E}\mathrm{V}+{\lambda }_1{W}_{ABCR12}+{(I-{\rho }_{1}W)}^{-1}{ϵ}_1$$

(6a)

$$\mathrm{A}\mathrm{B}\mathrm{C}\mathrm{R}17={\alpha }_2+{\beta }_{21}\mathrm{A}\mathrm{C}\mathrm{C}2+{\beta }_{22}\mathrm{R}\mathrm{O}\mathrm{A}\mathrm{D}1+{\beta }_{23}\mathrm{R}\mathrm{O}\mathrm{A}\mathrm{D}2+{\beta }_{24}\mathrm{V}\mathrm{I}\mathrm{L}\mathrm{L}+{\beta }_{25}\mathrm{E}\mathrm{L}\mathrm{E}\mathrm{V}+{\lambda }_2{W}_{ABCR17}+{(I-{\rho }_{2}W)}^{-1}{ϵ}_2$$

(6b)

$$\mathrm{A}\mathrm{B}\mathrm{C}\mathrm{R}22={\alpha }_3+{\beta }_{31}\mathrm{A}\mathrm{C}\mathrm{C}2+{\beta }_{32}\mathrm{R}\mathrm{O}\mathrm{A}\mathrm{D}1+{\beta }_{33}\mathrm{R}\mathrm{O}\mathrm{A}\mathrm{D}2+{\beta }_{34}\mathrm{V}\mathrm{I}\mathrm{L}\mathrm{L}+{\beta }_{35}\mathrm{E}\mathrm{L}\mathrm{E}\mathrm{V}+{\lambda }_3{W}_{ABCR22}+{(I-{\rho }_{3}W)}^{-1}{ϵ}_3$$

(6c)

The generalised spatial two-stage least squares (GS2SLS) estimator, robust to violations of the assumption of normally distributed residuals [43], was used to fit the SAR model.

3.2.5. Ground Truthing and Interviews

In addition to generating geospatial data from satellite images, the study involved ground truthing and key informant interviews. The ground truthing exercise involved the research team paying visits to the study site to relate the remotely sensed data with features on the ground. The visits, which were necessary to aid data interpretation, offered the team the opportunity to directly observe the state of the wetlands, the nature of the buildings constructed, and other human activities taking place at the site, as well as to identify the use of the structures constructed at the site. In all, the uses of 163 randomly selected completed buildings were identified.

The key informant interviews were conducted with purposely selected officials of four key state agencies responsible for protecting wetlands against uncontrolled urban growth: the Environmental Protection Agency, the Wildlife Division of the Forestry Commission, the Water Resource Commission, and the Ga South Municipal Assembly. The interviews focused on reasons for physical development at the study site despite its official status as a protected Ramsar site.

4. Results

4.1. Land Cover Depletion at the Ramsar Site

The LULC analysis shows a high level of agreement between the image classifications and the reference (ground truth) maps. The Overall Accuracy was 85.4% in 2003, 85.7% in 2008, 90.9% in 2013, 90.8% in 2018, and 92.4% in 2023 while the Kappa Coefficient was 0.80, 0.79, 0.84, 0.84, and 0.86 for the respective years. These accuracy results are reflected in the Producer’s and User’s Accuracy statistics for the individual LULC classes. The image classifications show a dramatic transformation of the Densu Delta Ramsar site over the last 20 years. As Figure 4 and Figure 5 show, all the four landcover classes have undergone either an increase or a reduction in size, with vegetation and built-up areas showing the most remarkable changes. For instance, while the built-up area more than tripled between 2003 and 2023, vegetation declined by 77% during the same period. There were also considerable declines in bare land and water (see Figure 5).

Change detection analysis reveals that, between 2003 and 2023, all the four land cover classes lost and gained land from each other (see Figure S3). As expected, the largest change, amounting to 1912 hectares, was from vegetation to the built-up area, followed by bare land to built-up area (789 hectares). Overall, the built-up area gained 2907.4 hectares and lost only 28.24 hectares while vegetation gained only 277.3 hectares and lost 2214.2 hectares. Also, water lost 493.7 hectares and gained only 81.5 hectares.

4.2. Magnitude and Trend of Building Coverage

The data show considerable growth in the number of buildings and building coverage between 2013 and 2023. In 2013, the site had 43,243 buildings, which increased to 45,659 in 2018 and 46,850 in 2023, representing an overall increase of 8.3% between 2013 and 2023. As Figure 6 shows, this translated into growth in the total building coverage (land take) from 626.8 hectares in 2013 to 789.6 hectares and 855.3 hectares in 2018 and 2023, respectively. It also means that the building coverage growth rate rose from 32.6 hectares per annum between 2013 and 2018 to 13.15 hectares per annum between 2018 and 2023.

The field data show that the buildings constructed at the site were predominantly residential structures. Out of the 163 randomly selected buildings directly observed on the field as part of the ground-truthing, as many as 144 (or 88%) were residential structures. In comparison, the remaining 12% included churches and mosques, school buildings, and commercial buildings. Also, building construction occurred on both dry ground and at locations where the landscape is low-lying, waterlogged, and highly flood-prone.

4.3. Spatial Pattern of Building Coverage

Between 2013 and 2023, building construction was more concentrated at specific locations than others. Locations with exceptionally high concentrations included the eastern edge of the study site, which happens to be closest to Accra. High concentrations were also recorded around Glefe, Gbegbeyise, and the other village magnets and along the arterial roads (see Figure S4). These spatial patterns were confirmed by the ABCRs. From 2013 to 2023, ABCRs were persistently high in sectors within the corridors of the major and minor arterial roads, except those located along the Kokrobite–Tuba Junction stretch of the minor arterial, where there was little building construction (see Figure 7). In all, sectors along the eastern boundary of the study site recorded a mean ABCR of 30.5 in 2023, compared to the overall mean of 21.5.

4.4. Growth in Building Coverage

Between 2013 and 2023, about 92.2% of the 102 sectors recorded positive growth in ABCR, while the remaining 7.8% recorded either zero or negative growth. However, most of the growth occurred between 2013 and 2018, during which period, the average sector recorded an ABCR change of 27.9%, compared to the 2018–2023 period, when the corresponding change was only 8.1%. As Figure 8 shows, the absolute increase was more significant in sectors located along the segment of the major arterial east of Old Barrier and the Old Barrier–Kokrobite stretch of the minor arterial, as well as those located around village magnets or near Accra. The fact that sectors with the highest ABCRs in 2013 recorded the most significant increase in ABCR means that, between 2013 and 2023, already built-up sectors experienced considerable consolidation in building construction.

While the spatial distribution of absolute ABCR change showed clear patterns, relative ABCR change was substantially uniform, with 67 out of the 102 sectors recording a 25–50% increase. As Figure 9 shows, there was a slight spatial variation in percentage growth in ABCR between 2013 and 2023, except in the western end of the study site and areas along the Old Barrier–Tuba Junction stretch of the major arterial where nine sectors recorded between −35.3% and 0.8% change. The relatively slow pace of ABCR growth in the western end could partly be attributed to the hilly terrain. This part of the study area also included a 40-hectare protected farmland (near the village of Tuba), forming part of the Tuba Irrigation Scheme. It is also significant to note that Glefe, Gbegbeyise, and Tetegu communities, together with their immediate environs, where the landscape is low-lying, waterlogged, and highly flood-prone, experienced almost the same amount of relative ABCR growth as other locations. This indicates significant encroachments on the core wetland zone.

4.5. Statistical Significance of the Effects of Growth Drivers

Data presented under the preceding sub-sections suggest, rather descriptively, that the hypothesised growth drivers (i.e., proximity to Accra, arterial roads, and village magnets) influenced the spatial pattern and growth of building coverage at the study site. This sub-section presents the results of OLS regression and SAR models to ascertain the statistical significance of these revelations. As indicated earlier, the SAR technique was used to assess the effects of the growth drivers on the spatial distribution of ABCRs, while OLS regression was used to assess their effects on ABCR changes.

4.5.1. Effects on the Spatial Distribution of ABCRs (SAR Models)

The results in

Table 3. Spatial autoregressive (SAR) models of ABCR.

Predictors	Model 1 (y = ABCR12)		Model 2 (y = ABCR17)		Model 3 (y = ABCR22)
	Coefficient	Std. err.	Coefficient	Std. err.	Coefficient	Std. err.
$Alpha\left({\alpha }_i\right)$	13.086 ***	2.297	18.448 ***	2.772	20.662 ***	3.008
ACC2	−0.020 ***	0.008	−0.032 ***	0.009	−0.037 ***	0.010
VILL	3.886 *	2.084	5.343 **	2.420	5.835 **	2.599
ROAD1	4.539 ***	1.732	4.681 **	1.976	4.864 **	2.115
ROAD2	−1.305	1.777	−1.707	2.102	−1.617	2.275
ELEV	−0.965	1.560	−2.057	1.780	−2.399	1.905
$Lambda{(\lambda }_i)$	0.283 **	0.139	0.270 **	0.136	0.257 *	0.137
$Rho{(\rho }_{i)}$	0.444 *	0.253	0.428	0.275	0.429	0.279
Wald test of spatial terms [chi2(2)]	17.1 ***	--	13.5 ***	--	12.4 ***	--
Pseudo R2	0.289	--	0.379	--	0.406	--
Wald chi2(6)	37.650 ***	--	55.020 ***	--	60.210 ***	--
Number of observations	102		102		102

* p ≤ 0.10; ** p ≤ 0.05; *** p ≤ 0.01. Source: Authors’ construct.

show that ACC2, VILL, and ROAD1 significantly influenced the spatial patterns of ABCRs throughout the period 2013–2023. For instance, ABCRs were higher in sectors close to Accra (ACC2) than those located farther away from the city. In other words, as suggested by the spreading pancake hypothesis, the SAR models show that distance from Accra negatively impacted ABCR values, after accounting for spatial autocorrelation and the effects of other growth drivers. Similarly, sectors located within 1 km of a village magnet (VILL) or the N1 highway (ROAD1) recorded higher ABCRs than those located elsewhere. That is, in consonance with the village magnet and ribbon development hypotheses, proximity to the five village magnets and the N1 highway positively impacted ABCRs, even after accounting for spatial autocorrelation and proximity to Accra. However, the effects of ROAD2 and ELEV were found to be statistically insignificant as their p-values exceeded the 5% alpha level.

4.5.2. Effects on ABCR Change (OLS Regression)

Table 4. Ordinary least square (OLS) regression models of ABCR change.

Predictors	Model 1 (y = CHANGE1)		Model 2 (y = CHANGE2)		Model 3 (y = CHANGE3)
	Coefficient	Std. err.	Coefficient	Std. err.	Coefficient	Std. err.
${\alpha }_i$	4.865 ***	0.910	1.058 ***	0.251	6.518 ***	1.139
ACC2	−0.011 ***	0.003	−0.003 ***	0.001	−0.016 ***	0.003
VILL	0.693	0.742	0.028	0.190	0.862	0.929
ROAD1	−0.873	0.612	−0.321 **	0.153	−1.137	0.766
ROAD2	0.179	0.589	0.280 *	0.150	0.453	0.737
ELEV	−1.255 ***	0.493	−0.329 ***	0.126	−1.656 ***	0.617
ABCR13	0.101 ***	0.035	--	--	0.156 ***	0.044
ABCR18	--	--	0.067 ***	0.008	--	--
R2	0.4892	--	0.7434	--	0.5482	--
Adjusted R2	0.4570	--	0.7272	--	0.5196	--
F-test (6, 95)	15.17	--	45.87	--	19.21	--
p-value	0.000	--	0.000	--	0.000	--
No. of observations	102	--	102	--	102	--

* p ≤ 0.10; ** p ≤ 0.05; *** p ≤ 0.01. Source: Authors’ construct.

shows the influence of ACC2, VILL, ROAD1, and ROAD2 on changes in ABCR between 2013 and 2018 (Model 1), 2018 and 2023 (Model 2), and 2013 and 2023 (Model 3) after controlling for terrain and base-year ABCRs. It shows a significant negative relationship between ABCR change and distance from Accra (ACC2) in all three periods. That is, in line with the spreading pancake hypothesis, sectors closer to Accra registered higher amounts of absolute growth in ABCR than those located farther away. During the period 2018–2023, sectors located within 1 km of the N1 highway (ROAD1) registered lower amounts of absolute growth in ABCR than those located more than 1km away. This was probably because sectors within the immediate vicinity of the highway were nearing the point of saturation, so building construction had slowed down there. The data also show that sectors with elevations (ELEV) exceeding 50m above sea level registered lower absolute growth in ABCR than those on lower grounds. However, proximity to the village magnets (VILL) and the minor arterial (ROAD2) had no statistically significant influence on ABCR change.

4.6. Institutional Response to Urban Growth at the Ramsar Site

Ghana has established a policy framework and institutional arrangements for protecting wetlands in line with its signing up to the Ramsar Convention on Wetlands. However, the fact that physical development continues at the study site suggests that the government’s wetland protection measures are either not enforced or ineffective. Key informant interviews revealed several bottlenecks that constrain the capacity of the responsible institutions to play their wetland protection roles. First, the interviews revealed inadequate coordination and collaboration among the key state institutions (such as the Municipal Assembly, the Wildlife Division, and the Environmental Protection Agency) responsible for wetland protection.

Secondly, the interview revealed a lack of cooperation from political leaders, chiefs, and other customary landowners, which discourages technocrats from enforcing land use control and wetland protection measures. For instance, respondents complained that political leaders lack the will to enforce land use control regulations and that some of them protect developers who encroach on wetlands by interfering with public officials who attempt to enforce the law. For example, in the Ga South Municipal Assembly, one official stated that “sometimes when a developer decides to put up a building on a reserved land and public officials attempt to demolish it, they are prevented by politicians from doing their work” (Public Official 1, 2023). Also, customary landowners such as chiefs and family heads were accused of disregarding wetland protection policies and zoning regulations by demarcating and allocating building plots in wetlands to developers.

Thirdly, the respondents claimed that the wetland protection institutions lacked adequate human resource, transport, and logistical capacity. For instance, one respondent mentioned that conservationists, planners, and development control officers were handicapped by a lack of vehicles and staff to undertake field monitoring activities at the protected site. Consequently, these institutions appear overwhelmed by the enormity of spontaneous physical development at the site.

5. Discussion

5.1. Loss of Natural Landcover to Uncontrolled Urban Growth

The results presented above show clearly that urban growth at the Densu Delta Ramsar site is environmentally unsustainable. This manifests in the massive loss of vegetative cover to the built-up area, which has come as a result of uncontrolled construction of buildings and infrastructure to meet the needs of Accra’s rapidly-growing population. For instance, building count and coverage grew steadily between 2013 and 2023 and this occurred both on dry ground and at locations where the landscape is low-lying, waterlogged, and highly flood-prone. The findings in the current study are consistent with earlier ones by [36] who reported on widespread illegal building encroachments, wetland drainage, deforestation, ecosystem degradation, pollution from indiscriminate waste disposal, and biodiversity loss at the site, all due to uncontrolled urban growth. In Kumasi, Ghana’s second largest city, wetland loss and degradation are commonplace due to encroachments by residential developers [9] and indiscriminate waste disposal and other activities of informal economic operators such as artisans and traders [8]. Elsewhere around the world, similar findings have been made by various researchers. For instance, in the Ndop Central Subdivision of the Ngoketunjia Division in the North West Region of Cameroon, Kometa et al. estimate that the built-up area increased from 3.9 km² in 1999 to 11.7 km² in 2017 while wetlands declined from 11.5 km² to 7.5 km² during the same period [44]. In Fuzhou, China, Cai et al. reported on the dramatic conversion of wetlands to industrial developments [19].

The loss of natural landcover at the study site and other locations to unrestrained urban growth have negatively impacted natural wetland ecosystems. Other studies confirm that, in addition to being illegally reclaimed for building construction, parts of the core wetland landscape of the Densu Delta Ramsar site, which inhabit vital wildlife and vegetation, have been degraded through the indiscriminate disposal of solid and liquid waste and other environmentally hazardous human activities [38,45]. For example, some residents of the Glefe community had reclaimed land from the Glefe Lagoon for residential development by filling the lagoon with solid waste and laterite [45]. These destructive activities have resulted in the decline and disappearance of “nursing and breeding places for fish and other species especially in the mangrove forested areas” [38] (p. 47). At the Sakumo Ramsar site, similar encroachments have significantly reduced the flood control capacity of the potential of the wetland and led to the loss of livelihood for fishing and farming households [46].

At the current growth rate, building coverage at the Densu Delta site is expected to increase by 57% in the next decade. Most of this growth will occur as in-fill development within the existing built-up area but will also involve further encroachments on the remnants of natural landcover. This will lead to further deterioration and loss of the wetlands and heightened exposure of residents to the risk of environmental hazards such as flooding. Also, the fact that almost 9 in 10 buildings are residential structures suggests that demand for dwelling space for the spillover population from Accra is a major socio-economic force driving urban growth at the study site. Therefore, the achievement of SDG 11 (sustainable cities and communities) will be undermined if drastic measures are not taken to check urban growth at the site.

5.2. Effects of Growth Drivers

The study has also shown that urban growth at the study site follows some discernible patterns consistent with the spreading pancake, village magnet, and ribbon development models [27]. In line with the spreading pancake model, sectors located near Accra had significantly higher concentrations of building counts and building coverage than those located farther away. These findings can be attributed to the fact that physical development at the study site results from the unchecked horizontal spread of the contiguously built-up area of Accra, a common phenomenon in low-income metropolitan areas [28]. In addition, the data show that sectors close to the city registered greater amounts of growth in building coverage between 2013 and 2023 than those located farther away. In other words, there was a negative relationship between the magnitude of change in building coverage (ABCR) and distance from Accra. This is because horizontal spread involves leapfrog development at the initial stages but, over time, densification is achieved through infill development. However, this kind of infill development is more of an organic process than a consciously planned land-use-optimisation strategy [17]. As [17] observed in Kumasi, this pattern of growth results in serious encroachments on wetlands and other landscapes that are not suitable for physical urban developments.

The data also reveal that the general distance–decay feature of the spreading-pancake pattern of urban growth was punctuated by pockets of village magnets where physical development was relatively dense. As has been pointed out, peri-urban villages close to large cities tend to attract urban residents seeking cheap land for housing construction, leading to pockets of relatively fast and dense growth surrounded by slow and sparse growth [27]. The presence of this phenomenon at the study site was evidenced by the fact that sectors located near Bortianor, Kokrobite, Oshieyie, Aplaku, Gbegbeyise, and Glefe recorded significantly higher concentrations of both building counts and building coverage than those located elsewhere. However, the data show that the effect of the village magnets on the spatial distribution of physical development disappeared over time as development became ubiquitous. This was demonstrated by the fact that, although proximity to the six village magnets influenced the spatial concentration of building coverage, it had no significant effect on the change in building coverage during the period under consideration (2013–2023).

The data also show clearly that the effect of transportation infrastructure on urban growth, as observed by several authors [28,31,47], played out at the study site. Sectors located near the N1 highway registered higher concentrations of building counts and coverage than those located farther away. This is consistent with the ribbon development model, which suggests that the horizontal spread of the built-up area of a metropolitan area occurs axially along transportation corridors [17,27,31,47]. For instance, George Owusu observed that upgrading the N1 and other highways connecting Accra to the rest of the country to multi-lane dual-carriage roads had contributed to sprawl development along these roads’ corridors, including the study site [47]. Indeed, in both GAMA and the Greater Kumasi Metropolitan Area (GKMA), about 90% of residents lived within four kilometres of a highway [17,27]. The authors attributed this to the inadequacy, poor connectivity, and deplorable nature of secondary and access roads, which make the few paved highways the only reliable routes for most residents to commute to the city centre and other parts of the metropolitan area [17]. In Aydin, Turkey, ribbon development along the Denizli–Izmir highway was fuelled by attraction of residential and economic establishments to the road, due partly to relatively low costs [48]. In the United States, it has long been established that highway development and a shift from public transport to private automobiles by commuters drove urban sprawl along highways within suburban areas [49]. Similarly, road and rail transport, together with proximity to urban cores, have influenced urban growth in the Randstad metropolitan region of the Netherlands [50] and the Jeddah city in Saudi Arabia [51]. The implication of the empirical verification of the ribbon development model is that wetlands that are located close to highways will always remain threatened unless effective measures are taken to protect them from urban encroachments.

It is worth noting that, while confirming these earlier findings in part, the current study also shows that the relatively high concentration of physical development within road corridors diminishes over time. According to our data, there was a negative correlation between proximity to the N1 highway and the change in building coverage between 2013 and 2023. That is, although sectors located within 1 km of the highway initially registered relatively high concentrations of buildings, these sectors registered lower amounts of new buildings constructed during the period than elsewhere. This was because development spread outward from the highway as the road corridor became saturated.

In summary, the magnitude, speed, and pattern of urban growth are driven by factors such as proximity to a central city, highways, and pre-existing peri-urban villages. These factors are so influential that their presence is enough to attract physical development to ecologically-sensitive areas such as wetlands, even when such sites pose hazards like flooding. Therefore, the presence of these growth drivers cannot be ignored when devising policies and strategies to protect wetlands and other ecological assets.

5.3. Institutional Failure and Implications for Environmental Sustainability

To a large extent, the potency of proximity to Accra, village magnets, and the N1 highway in driving urban growth played out at the study site, which was almost unhindered due to the failure of state institutions to control and regulate physical development. This failure has been attributed to inadequate institutional coordination and collaboration and a lack of political will to support and resource these institutions to perform their land use control and wetland protection functions. This confirms other studies that found poor governance and weak institutions to be responsible for the failure to effectively manage Africa’s urban environments [52,53]. For instance, in Kumasi, responsible state agencies have failed to effectively enforce existing regulations and laws designed to protect wetlands, which has been attributed to a lack of political commitment, low capacity, and poor institutional coordination [52]. In Tanzania, Materu et al. reported on threats to the country’s wetlands due to uncoordinated policies and the lack of a comprehensive legal framework for wetland protection [54]. The destruction of wetlands and other protected ecosystems due to ineffective land use control in human settlements has also been reported in New Mexico, Mexico [7].

Continual urban growth at the study site amid institutional failure to control physical development has dire consequences for environmental sustainability. In particular, the reclamation of land from the core wetlands (including lagoons and marshes) for building construction and the generally widespread physical development in low-lying areas will not only lead to the loss of ecosystem benefits of wetlands but also to the increased frequency and scale of flooding and other environmental hazards. Therefore, there is an urgent need to tackle the problem of wetland loss resulting from uncontrolled urban growth from a governance and institutional perspective. In other words, effective governance should be seen as the bedrock upon which environmental sustainability and other pillars of sustainable urban development stand.

6. Conclusions

This study sought to add to existing knowledge on how uncontrolled physical development contributes to the loss of wetlands by analysing the morphological patterns and drivers of urban growth at the Densu Delta Ramsar site. The findings show that urban growth at the site follows discernible morphological patterns that are consistent with the spreading pancake, village magnet, and ribbon development models. These patterns are created by the growing demand for land for construction to meet the housing and infrastructural needs of Accra’s growing population, considering proximity of the site to Accra as well as the presence of the NI highway and pre-existing peri-urban villages. The presence of these factors, amid the failure of responsible state institutions to perform their land use control and wetland protection functions, has led to massive encroachments on the Ramsar site. Unless drastic measures are taken to curb further physical development, the loss of ecosystem benefits of wetlands, increased frequency and scale of flooding, and other dire environmental consequences are bound to become worse.

In addition, the study has demonstrated the utility of aggregate building coverage ratios based on high-resolution satellite images in combination with the often-used land use/land cover method in the spatiotemporal analysis of urban growth. The main strength of the aggregate building coverage ratio method is that it considers the intensity of physical development and also allows for formal empirical testing of urban growth models such as the spreading pancake, village magnet, and ribbon development models.

To achieve sustainable urban development and thus contribute to the achievement of SDG 11, there is an urgent need to devise and effectively implement land use, development control, and wetland protection measures that minimise the environmental threats posed by the urban growth drivers to urban wetlands. These measures should include the relevant local governments collaborating with other wetland protection agencies and stakeholders to formulate and enforce special zoning ordinances for all Ramsar sites and other significant wetlands. Such zoning ordinances should consolidate, build upon, and adapt to local conditions existing policies and regulations on wetland protection. It is also suggested that the central government should support local governments to acquire such wetlands from customary landowners and pay fair compensations to them. In addition, there is the need for the strengthening, resourcing, and closer collaboration of the relationships between the various state agencies responsible for wetland management. There is also the need to engage and sensitise political leaders to increase their commitment to implementing wetland protection and pro-environmental policies.

The main limitation of the study is that it did not assess the impacts of urban growth on the bio-chemical dynamics or the biodiversity composition or health of the wetland ecosystem, neither did it go into detail in analysing the complexities associated with the governance aspects of wetland protection and management. Its main strength lies in contributing to the understanding of environmental scientists, urban planners, and policy makers on how the morphological drivers of urban growth can threaten urban wetlands.