Use-mix intensity and open-space ratio for sustainable urban form: the case of Dire Dawa, Ethiopia

Numerous academics in the field of urban morphology have their theoretical roots in the Conzonian school, which separated urban form into four components: building structure, street pattern, land-use pattern and plot pattern (Conzen, 1960; Larkham and Conzen, 2014; Oliveira et al., 2018; Van Nes et al., 2012; Ye, 2015). These Conzen's morphological elements help to assess urban form for a better understanding of the built environment by way of the historical method of constructing cities (Ahmadi et al., 2009). Similarly, Madanipour (1996) defined urban morphology as a systematic study of urban form, which includes the shape, plan, structure and functions of the built fabric of towns and cities. Based on the concept of the Conzonian school, which is a qualitative one, recent urban morphology studies sought to exclusively or in combination, measure these components depending on their purpose. The objective of this study is to examine the sustainability of the land-use transformation trend based on one of the components of Conzenian urban form elements through the morphogenesis of the case city, Dire Dawa, Ethiopia. In addition, it attempts to examine results from the perspective of the planning interventions (formal and organic) and the location of the case area with respect to the whole city. To fulfil this objective, the mixed-land use and open-space ratio (OSR) are used to investigate the city's move towards sustainable land-use development by comparing the intensity of mixed-land use and the OSR of the parcels under investigation over time.

Many recent studies have emphasised on sustainable urban forms such as: significance of compact urban forms (Rice, 2010); linking nature with urban forms (Alizadeh et al., 2019), development of sustainable smart cities (Sameer et al., 2022) and sustainable urbanism as an evolving trend (Balsas, 2022). This study, however, attempts to look at sustainable urban form through the transformation of mixed-land-use intensity and OSR within the morphogenesis eras of Dire Dawa, a second-tier city in sub-Saharan Africa.

Many cities in Africa, especially in sub-Saharan Africa, were not planned and purposefully established to have uniformity; rather, they developed in a piecemeal manner, therefore the cities represent both organically developed and formal planned structures (Musakwa and Van Niekerk, 2013). The case study on Dire Dawa, which has been established by Ras Mekonen, the father of the late Emperor Haile Silasie I, in 1902 on the Eastern part of Ethiopia, has exhibited similar characteristics (Gebreyesus, 2019). Dire Dawa city was initially established to function as an intermediate maintenance and inspection station for the Ethio-Djibouti railway (Wolde-Michael, 1973). It is the second largest city in Ethiopia (UN-Habitat, 2008; Worldometer, 2022), and the only chartered after Addis Ababa (Łykowska, 2011). It hosts the largest industrial park of the country (Gebreyesus, 2019). In addition, the city serves as an import and export gateway through Port Djibouti due to the newly built standard gauge train station, dry port and a highway. This significantly improved the social and economic importance of the city not only to the country but also to the Eastern African region as well.

Urban land-use studies in Africa mostly investigate horizontal spatial dimensions, leaving out the vertical land-use perspectives. Nevertheless, the planning of the city appears to be nothing more than a standard zoning rule with various colours reflecting different land-use categories, both during situational analysis and stages of proposal development. However, the vertical approach to land-use in addition to a horizontal one, adds a new dimension to land-use studies.

Apart from the contextual peculiarity, this research study is novel because to the best of the authors’ knowledge, most previous studies and approaches focused on street density, linkage, junction and land-use intensity and disregarded OSR. In addition, they address this issue separately from the perspective of density. Thus, this study aims to address the sustainability of land-use development in Dire Dawa by considering representative cases from different development eras since its establishment.

Mixing complementing land uses has been an increasingly significant objective in urban development interventions in recent decades due to its sustainability benefits. This mixed-land-use density has been proven to affect travel behaviour, health outcomes and neighbourhood-level quality of life, which in general influences the sustainability of development of an area (Manaugh and Kreider, 2013). Musakwa and Van Niekerk (2013) have also illustrated the significant positive correlation between the intensity of mixed-land use and sustainable urbanism. Besides, Chiaradia et al. (2013) argued that walking is most strongly related to metrics of mixed-land-use intensity among others, and the number of destinations within walking distance, which in turn has implication on sustainability. Although there is no study conducted on the optimum level of mixed-land-use intensity that suffices for sustainable urbanism, this study considers that an area with better mixed-land-use intensity is liable to achieve sustainable urban form. Apart from this, existence of open spaces in one's area is also one of the indicators of sustainability as it allows better circulation, gives more space for recreation and breathing (Manaugh and Kreider, 2013).

Although most studies had based their assessment on neighbourhood level, this study explains the perspective of plot and neighbourhood level. In addition, an assessment of change in OSR during the different spatial transformation eras of the city is made to comprehend the trend of open-space provision and ultimately to assess the sustainability of the development trend. Besides, the comparison considers the formally planned areas and the organically developed parts of the case city within a morphological era were appropriate. As a result, this study will have a multi-faceted contribution to both the academic world and for practical urban planning and design interventions.

The method employed in this paper is a case study. A case study is an empirical approach that explores a current occurrence – the ‘case’ – in-depth and within its real-world environment, particularly when the boundaries between phenomenon and context are unclear (Yin, 2018). It requires a thorough investigation, by combining various observations (Gerring and Cojocaru, 2016). Similarly, Gillham has defined a case study as one that analyses a study in order to answer particular research questions and seeks various pieces of evidence (Gillham, 2000), which has made this study method unique. In addition, Scheer (2017) further emphasised that case studies are commonly used in urban morphology research to compare information about individual areas and formulate theories based on observed trends.

The function of the built fabric, which is land/building use is mostly considered as a determining element of urban form (Conzen, 1960; Conzen et al., 2012; Larkham and Conzen, 2014; Oliveira et al., 2018).Van den Hoek (2008) studied mixed-use index (MXI), which emphasises on the built fabric used by different researchers in recent years (Carpio-Pinedo et al., 2021; Manaugh and Kreider, 2013; Van Den Hoek, 2009; Van Nes et al., 2012; Ye and Van Nes, 2014a, 2014b; Zhuo et al., 2019). Some studies have modified or combined it with other methods depending on the specific need of individual studies. Some have also linked it with sustainable development denoting the mixed-land use as one aspect of sustainable urban form (Dawodu et al., 2022; Ferrari et al., 2022; Yigitcanlar et al., 2015). This study used MXI, which is a quantitative-based analysis it is a tool or instrument that helps to assess the concept of ‘mix’, or the functional composition and mix of applications categorising the sets of different functions under three categories as living, working and amenity (Van den Hoek, 2008). Besides, Spacematrix, a spatial analysis method developed by Berghauser Pont and Haupt (2010), is employed to detect the OSR of the parcels in the case areas throughout the various morphological eras of the city.

The case city, Dire Dawa is the second largest city in Ethiopia and is located on the eastern side of the country, which is also a part of the horn of Africa (see Figure 1). First, methods to urban growth management depend on understanding the causes, timing, impact of the urbanisation process, and its driving factors (Longley and Mesev, 2000). This study thus attempts first to assess the density of mixed-land use in the selected sites of this case city in both horizontal and vertical manner employing MXI. Second, an examination of the sustainability of the urban form is made through land-use development trend analysis of the city during its different morphological eras. Finally, an assessment of the OSR provision of the different period developments is made to complement the assessment of the sustainable urbanism development trend. This is made through a comparison among the selected case areas.

Sustainable urban forms can be compared over time within the same city or across cities at the same time (Pili et al., 2017). However, this research considered the case study of a single city across time. A city-wide level study by Bekkering et al. (2019) would be possible in cases where there are available data that describe development in each period. However, in this study the required data are detailed pertaining different periods, so it is difficult to address this at a city-wide level. To serve this purpose, the researchers sought to focus on specific case areas and classify the morphogenesis period of the city into several morphological eras. This enabled one to comprehend the morphological features of each of the city's development phases and to determine the city's current morphological position. Then, for each of the morphological eras, the land-use patterns were identified. Additionally, this strategy assisted in identifying the plot patterns and building structure of each case area, allowing for reconstruction of each case site in conjunction with historical maps, pictures, and Google timeline images.

The trend in the plot and neighbourhood level of the different land uses was evaluated, which finally divided the morphology of the city into three different morphological eras. This was done based on six representative samples.

The governance eras of the case city are inextricably linked with those of the country, and thus preferred to be used to classify the periods of morphogenesis of the city. In this respect, the first one is the early morphogenesis era including both the early establishment period of the city (from 1902 to 1935) and Italian occupation (1936–1941). The second one is the intermediate morphogenesis era encompassing the Imperial era following the Italian occupation (1942–1974) and the Dergue period (1975–1991). The last one is the recent morphogenesis era which encompasses the period since 1992. The selected sites seem to be concentrated on the eastern part of the city although some developments are taking place on the western part of the city (see Figure 2).

Identifying land uses of each of the parcels in the case areas was difficult, especially during the first two morphological transformation periods of the city. The archival resources did not provide the uses of parcels during the early periods. The problem aggravated further due to less resilient nature of the land and building uses in comparison with other urban form elements. To tackle this problem, researchers have considered master plans developed for the area during the early establishment period by French architects and later during the Italian occupation by the Italians assuming that the plans have been implemented with few alterations. Desk review and in-depth interviews were conducted to supplement this and to minimise errors. The assumption was made based on the assessment conducted in the Kezira area (one of the case sites) which has witnessed the implementation of the first plan despite some alterations which are being made in recent periods. Due to the similarities in the interventions made, early evolutionary period and the Italian occupation era are accounted as one morphological era.

For the periods after the withdrawal of Italians, surveys of the earlier and the current uses of the premises were conducted by the householders and rectified by the cadastral data of the city. Shares of the different uses in mixed-use plots were identified based on building floor areas.

After having identified the land use, the building footprints and the heights of the buildings within each plot in the case study sites, it was possible to identify the floor space index (FSI) and the ground space index (GSI). These helped to identify OSR based on the identified figures for each FSI and GSI. According to Pont and Haupt (2010) OSR is sometimes known as spaciousness, a figure that quantifies the amount of unbuilt space at ground level per square metre of gross floor area. This figure indicates the amount of non-built space that is under pressure. OSR is measured in m²/m².

Therefore, a comparative analysis of the case study areas is made so that it could be possible to examine the trend of development of the city from a sustainability perspective. The level of correlation of the land-use intensity and the OSR in the different morphological epochs of the case city helps to identify the cross-case scenario and establish a theory based on the different periods. In doing so, the formal planning context of the sites and the location implication of the case areas are addressed. These have significant influence on the land-use intensity of the city among other factors. The influence of these factors is also identified in the process. The result is thus displayed in a map and graph which signifies the level of correlation identified from the study. These are also described in narrations supported by theoretical and empirical evidence from studies conducted so far.

The data are arranged chronologically in accordance with the morphological eras investigated in this study. The goal of this research study is to examine the sustainability of the morphological transformations seen throughout the city's morphogenesis depending on land-use intensity at both the plot and neighbourhood level and the trend in the provision of OSR at each plot. It attempted to evaluate the relationships between the functional intensity of plots and neighbourhoods along with their respective OSR and of course make a comparative analysis across the chronologically ordered case areas.

The early morphogenesis era encompasses the period from the early establishment of the city up to the withdrawal of Italians. The selected case areas representing this morphogenesis era are Megalla and Kezira where the former one is an organically developed part while the latter is formally planned part of the city. Both are located in the historic core of the city separated by the Dechatu dry riverbed. As shown in Figure 3, the early stages of Dire Dawa's morphogenesis from the standpoint of land-use intensity acknowledged the importance of mixed-land use in both the formally planned and spontaneously evolved parts of the city. Megala, the organically developed section, appears to have placed a greater emphasis on higher land-use intensity during the city's early establishment period. Conversely, Kezira, which was fundamentally established based on the city's train station, has some level of land-use intensity than its organically developed counterpart. Megala's organic development has enabled it to accommodate several functions, establishing working, living and amenity at a plot level. There are still a significant number of dual-use plots in addition to the monofunctional ones located on the periphery, which are mostly used for residential purposes. Conversely, in the planned component of the city, Kezira, there are a few dual-functioning plots among the bulk of single-use plots.

The next morphological era considered in this study includes the period running from last Imperial administration of the country which starts from the withdrawal of Italians up to the Durge period. This morphological era is represented by two distinct cases, Legehar and Sabiyan. Both of these cases are formally planned areas. Due to the strict control of the government it was difficult to find a full-fledged organically developed settlement in this period. In terms of location, the case area representing the final Imperial era, Legehar, is situated in the eastern segment of the city's current geographical configuration, but the governance period representing the Dergue administration, Sabiyan, is placed close to the city's current geographic centre. In both cases, as indicated in Figure 4, dual functioning mixed-land uses are located on distinct plots inside the case areas, while it is uncommon to find a multi-functional plot that combines three of the purposes, dwelling, amenity and working. In the Legehar neighbourhood, few traces of a tri-functional plot living, working and amenity have been discovered. However, the majority of plots are monofunctional, with the living function taking precedence. In the case of Sabiyan, this is an area where it aims to compete with the city's historic core, Kezira, and Megala, for the position of central function. Almost all of the dual mixed purposes are a combination of amenity and living, followed by working and living; it is uncommon to find a plot that functions as spaces of both working and amenity.

The other morphological era considered in this study is the city's recent transformation period since the end of the Dergu socialist administration. This is an era of growth when the city experienced many changes resulting in a massive expansion that allowed the city to more than double its former size. Due to the size of the geographical sphere of the city's growth, two case areas were chosen, Mariam Sefer and Gende Gerada. To ensure that the samples represent the morphological era's development scenario, the former one is taken from the formally planned development while the latter one represents the organically developed part.

The first site, as indicated in Figure 5, Mariam Sefer, hosts a mix of real estate, government-managed condominium housing, and privately developed commercial and service premises. This case area developed in a formally planned manner and is located in between the city's historic core (Megala and Kezira) and the city's geographic centre (Sabiyan area). While it is uncommon to find a plot that serves all three functions, amenity, working and living, simultaneously in this case areas, dual functional plots are prevalent. As previously indicated, the majority of dual-functional uses involve a combination of living and amenity, however, others represent a combination of working and living. Nonetheless, it is uncommon to find a combination of working and amenity at the same time. There are also plots dedicated to a specific purpose, such as housing, amenity or working.

On the contrary, the Gende Gerada case area is situated on the city's northeastern outskirt, where the town expanded haphazardly without a formal planning intervention, though some regularisation measures have been made. This region is dominantly residential; however, some plots have a mix of various purposes, indicating a greater land-use intensity than the Mariam Sefer in terms of entertaining the three functions in the same plot. The plots designated for amenity purposes take up the majority of the plot size, followed by working areas. Plots with a dual use have a bigger area than those exclusively allocated for dwelling purposes. In relative terms, multi-functional plots occupy a bigger area than dual-purpose plots.

Apart from the plot level evaluation, a neighbourhood assessment is conducted to further understand the mixed-land-use intensity on a larger geographical scale and to get a new perspective on the issue. Similarly, OSR is also discussed, which takes both the GSI and the FSI into consideration. The mean of the OSRs of each plot is used to calculate the neighbourhood-level OSR.

At a neighbourhood-level assessment, during the early morphological period, it is determined that the Kezira district, which is the historic core and the formally designed portion of the city, has the highest degree of mixed-land use. While Megala, the city's organic historic centre, advanced to the next rung on the ladder of use mix. The neighbourhood-level intensity of mixed-land use in the intermediate period (the last Imperial and the Socialist periods), on the contrary, has recorded a lower level of intensity than the early morphogenesis period with minor distinctions between the two observed cases (see Figure 6). The Sabiyan case area seems to have a higher level of mixed-land use, while Legehar appears to have a lower level. Similarly, the recent transformation period has indicated a lower rate of mixed-land-use intensity where there are some distinctions in between the considered two cases of these morphogenesis periods.

The OSR as shown in the city's various morphological phases in Table 1 indicates a general decline in the proportion of open space provided from the early to the present morphological periods. There is an exception in the instance of the Gende Gerada region on the periphery, which has evolved spontaneously with no formal planning intervention, allowing for a high OSR. Otherwise, there is a general downward tendency in the existence of OSR from the distant past to the present, particularly in formally designed neighbourhoods.

The purpose of this research study is to determine the sustainability of the land-use transformation trend observed throughout the morphogenesis of the case study. Additionally, it examines the effects from the standpoint of the planning intervention (formal and organic) and the case area's position with respect to the inner part of the city. To achieve the research objective, mixed-land-use intensity and OSRs of the case areas are compared over time to determine the city's move towards sustainable urban form in the study area. The study is made based on two spatial spheres, one at plot level and the other at neighbourhood scale so as to visualise the scenario from different perspectives. The discussion is presented in sequence beginning with the plot level examination and progressing to the neighbourhood level.

During the first stage of development (1902–1941), Kezira and Megala are the two case sites that have been the city's significant development centres and continue to be the historic urban cores. Both of these case areas, Kezira and Megala, are located in the historic centre of the city and have demonstrated a distinct historic progress in their growth. The former is the result of a formal plan related to the construction of a train station in this area, while the latter is an organic equivalent that had less official planning intervention. Formal planning practice of that timewas more inclined towards land-use segregation and implementation of stringent zoning laws. However, mixed-land-use approaches are preferred planning principles in recent years (Dempsey et al., 2010; Grant, 2002; Van Den Hoek, 2009). The results of these case areas indicate that the planning intervention made during this period have influenced the level of mixed-land-use intensity of the formally planned area while the organic one follows the traditional development trend where urban settlements are developed in a mixed manner responding to demands.

Van Nes et al. (2012) has described that the historic core of cities tends to accommodate all the three functions, working, amenity and living in comparison to the other parts of a city. However, as has been noted from the plot level examination of mixed-land use, it seems here that formal planning act and the period of development got stronger influence on the urban form of these case areas than the centrality issue.

The other cases representing the intermediate morphogenesis period, the latter Imperial governance after the evacuation of the Italians (1941–1973) and the Dergue period, a socialist Ethiopia (1974–1991), are Legehar and Sabiyan areas of the city. The case areas do have all the three functions, on individual plots. Furthermore, there is more land-use mixes, particularly of residential and amenity, in both case areas, indicating fewer, if any, instances of a blend of the three uses in one plot. Because of possible negative externalities, planners have historically sought to segregate land uses (Dempsey et al., 2010; Van Den Hoek, 2009). Both of these places were developed in a planned way, with regular plot divisions based on the plans produced for each development era. This has resulted in less mixing of all three uses at the same time.

The fundamental difference between these two cases is the proportion of the use mix, with the former reflecting a bigger percentage of living than amenity and the latter reflecting a more equal distribution of spatial use of mix of living and amenity. According to Ibrahim and Omer (2014), locational importance plays a pivotal role in defining the shapes of urban areas, with one having a stronger significance over the other. Similarly, the disparity in the share of the mix of uses in the cases of Legehar and Sabiyan results from the two cases’ different locations: the former is located near the city's outskirt to the east, while the latter is located at the geographic centre of the current expanse of the city. Sabiyan case area is thus fighting with the historic core to establish itself as one of the city's subcentres, but Legehar is unable to attract developers due to its location, which does not allow for future growth in this direction. In both situations, formal planning practice has hampered the occurrence of a multiple use mix at the plot level. However, locational variances have contributed to variations in the intensity of mixed-land use, implying that location may also have an impact on the assignment of sustainable land uses across a city region.

The last two cases that illustrate the recent morphological era since 1992 are two due to the reason that this morphological era has witnessed extensive spatial development coverage. The case areas to represent this morphological era are St. Mariam Sefer and Gende Gerada localities. St Mariam Sefer is located at a good proximity to the centre in between Sabiyan, the geographic centre of the city; and Megala and Kezira, at the historic core of the city. This case area developed following a strict and formal plan intervention. Gende Gerada is located on the northeastern outskirt of the city which developed organically with no formal planning intervention. The former does not show evidence of a blend of the three functions, but the latter has seen this on multiple occasions. This morphological era indicated a clear division made by formal planning act in the mixed-land-use distribution. As seen earlier, studies have emphasised the significance of location and its influence on land-use mix (Dong et al., 2019; Van Nes et al., 2012). Similarly, formal planning interventions do have an influence on the forms of settlements (DeLisle and Grissom, 2013; Van Den Hoek, 2009). The intensity of mixed-land use exhibited in this morphological era could be attributed to those two factors: the first is connected to the official planning process in the two locations, and the second is related to their location.

In the second sphere of research, the study examines the trend in the level of land-use mix and the trend in the OSR in relation to neighbourhood-level sustainable development. Choguill (2008) describes neighbourhoods as the fundamental building blocks of cities, with the total sustainability of a city dependent on the neighbourhood's sustainability. Thus Choguill promotes sustainability at neighbourhood level. Rice (2010) also added that walkable mixed-use neighbourhoods would be more suited to sustaining a sustainable urbanism. Although plot-level land-use mix has its own significance in accessing living, amenity and working at once, neighbourhood-level land-use mix also contribute to accessing these different functions in a walking distance. Therefore, apart from the plot-level examination, neighbourhood level assessment of the scenario would help to give a a high-level insight of the issue.

The neighbourhood-level assessment, during the early morphological period, indicated that the Kezira district has the highest degree of mixed-land use; while Megala, the city's organic historic centre, advanced to the next tyre. This is mainly because the city had only these two areas to accommodate all its activities during the first morphological era, and that is why the level of use mix in the area at neighbourhood level is high.

Mixed-land use at plot level is rare in the planned historic core and higher at neighbourhood level. On the contrary, in the organic historic core of the city the use mix both at plot and neighbourhood levels is very high. Van Nes et al. (2012) suggest that the historic core of cities tend to accommodate all the three functions, working, amenity and living in comparison to the other parts of a city and this is reflected at neighbourhood levels, though was not prominently wittnessed at plot levels.

On the contrary, the neighbourhood-level intensity of mixed-land use during the intermediate era (the last Imperial and the Socialist periods) was lower than the one exhibited during the early morphogenesis period, with small differences between the two observed case scenarios. Sabiyan seems to have a larger degree of use diversity, while Legehar appears to have a lesser degree of use diversity dominated by living function. This period's development was mainly guided by a formal plan. The main reason for this would basically be the location of the two sites where the former is located in the expansion area while the latter is located in a confined area (bounded by a large compound of a textile factory). In this morphological era, thus, the observed scenarios at plot level and neighbourhood level do exhibit a similar situation.

Similarly, the recent transformation era, where St. Mariam Sefer and Gende Gerada are the case areas, has shown a decreasing trend of mix intensity, with some differences between the two examples analysed during this morphogenesis period. The data indicated that the former case area, which is developed in a formally planned manner and located at close proximity to the inner city, has a better use mix than the latter one which is developed in an organic manner and located at the periphery of the city's existing expanse. The neighbourhood-level examination of this period seems basically influenced by both location and formal planning practice.

Apart from the land-use mix perspective, the other dimension of analysis is the level of OSR found in each of the case study neighbourhoods. In this regard, the study clearly indicated that there is a general decline in the share of the OSR across periods of morphological eras. An increasing trend in the level of built-up can be taken as the major cause for this scenario.

In general, examination of the mean density of mixed-land use at the neighbourhood level indicates that there is some pattern of growth in the intensity of use mix throughout the course of the city's existence. Kezira and Megala, which form the city's ancient centre, have the largest density of uses, followed by Sabiyan and St. Mariam Sefer, and finally Legehar and Gende Gerada. According to the neighbourhood-level analysis, moving out from the historic centre of the city to the periphery results in a decline in the intensity of mixed-land use regardless of the planning interventions made and the morphological periods.

Many scholars suggested mixed-land uses and spatially concentrated functions as one aspect of sustainable urban form (Dawodu et al., 2022; Ferrari et al., 2022; Westerink et al., 2013). Madanipour (2011), on the other hand, mentioned various elements in his research on sustainable development and urban form in Tehran, such as the existence of diversified pattern of land use, which contributes to the sustainable development of urban areas, among others. Furthermore, Talen (2011) also mentioned that a mix of land uses is one of the pillars for guaranteeing a sustainable urban form. As this sustainable land-use form increases, transit and pedestrian use also increase (Frank and Pivo, 1994). In addition, empirical evidence from Japan, the United Kingdom, and the United States demonstrates that it is possible to obtain the benefits of land-use mix by offering a diversity of uses within walking distance at the neighbourhood level, hence promoting sustainability (Sharifi and Murayama, 2014).

Sustainable cities are priorities for many governments’, but are especially critical in developing countries facing dangerously high rates of urbanisation (UN-Habitat, 2009). Considering this, most emerging cities and municipalities strive for sustainable development, though little is being done to operationalise the notion. Clearly, new concepts and strategies are required to assist in the management of sustainable land use in those fast-growing cities (Musakwa and Van Niekerk, 2013).

The basic concept of sustainability dates back to 1970s following the publication of the Brundtland report (Komeily and Srinivasan, 2015).

Furthermore, sustainable neighbourhood planning gained momentum since the turn of the century (Sharifi and Murayama, 2014). In this regard, Madanipour (2011) has signified the important conjugation of urban form and sustainability basically from the perspective of land use. Besides, Madanipour (1996) also described the significance of addressing urban form studies on the basis of the origin and evolution of their urban fabric across time. Similarly, this study attempted to examine land-use intensity based on the MXI through different time periods of morphological eras. It also assesses the sustainability of the land-use transformation of the cases that shape sustainability both at the neighbourhood level and the plot by integrating it with OSR.

At neighbourhood-level proximity to multiple functions at a walking distance is considered as a good indicator of better level of use mix (Ferrari et al., 2022; Sharifi and Murayama, 2014), which thus could be accounted as an indicator of sustainable urban form. Mixed-land use is also regarded as one of the cornerstones for urban sustainability (Carpio-Pinedo et al., 2021). With this notion, the assessment of the land-use development trend of the city indicates that there is a general decrease in the trend of intensity of mixed-land use which reflects a similar decrease in the attainment of sustainable urban form. This tendency is also observed in the trend of OSR provision which thus makes the situation worse.

Over-strict zoning of land uses has resulted in a variety of unforeseen consequences (Song et al., 2013). Studying the sustainability of land-use transformation across different time periods has great significance. These can be observed from the effects of mixed-land use on the three different pillars of sustainability: economic, social and environmental. From an economic point of view, a diversified land-use pattern tends to reduce travel times, improve accessibility, reduce travel costs and promote a more diverse work mix. Additionally, it promotes spatial and community integration, as well as physical activity, from social dimension. It also promotes environmental sustainability by encouraging non-motorised mobility, increasing transit use, and reducing car miles driven, resulting in reduced emissions and energy consumption, as well as land consumption (Musakwa and Van Niekerk, 2013). Thus, the presence of a trend of varied range of land uses promotes sustainable urban growth.

Although Ethiopia and the rest of sub-Saharan Africa did not witness the industrial revolution in its entirety, the era's planning system had a profound effect on them as a result of their adoption of European and North American planning theories. As a result, their urban planning and design practices are more inclined to adopt mixed-land use as a critical component. However, it is important to mention here that organic developments in these countries advocate mixed-land use in general, which is important to improve sustainability.

However, compatibility for the mix of different uses must be closely watched in order to further streamline the degree of sustainability for the mix of uses. Additionally, there should be an optimum level of usage mix that would be deemed appropriate for a certain location. These two areas need further studies in order to bolster the development of sustainable land use in sub-Saharan Africa's ever-changing urbanisation scenario.

Finally, it is recommended that formal planning practices need to consider balancing plot-level and neighbourhood-level mixed-land-use intensity to achieve sustainability in urban areas. Furthermore, maintaining a healthy level of OSR at the neighbourhood level, learning from empirical experiences of organic development and integrating it into formal planning exercise help to achieve urban forms in developing countries.