2.1. Understanding Blue-Green Infrastructure (BGI) and Current Trends
BGI, albeit used less frequently, is an umbrella term used in planning (often in landscape planning) and is closely related to the concept of “green infrastructure” (GI) [31
]. It combines the concept of green (including blue) networks [32
] and ecological networks [33
]. Scholars mostly draw on a standard definition given by the European Commission –
“a strategically planned network of natural and semi-natural areas with other environmental features designed and managed to deliver a wide range of ecosystem services. It incorporates green spaces (or blue if aquatic ecosystems are concerned) and other physical features in terrestrial (including coastal) and marine areas. On land, green infrastructure is present in rural and urban settings.”
Compared to its earlier predecessors, more notably GI, BGI is relatively a new term which was first used for describing the planning efforts in Sao Paolo, Brazil to create a network of “green and blue” infrastructures in response to flood risks [35
]. A systematic review of literature on blue, natural, ecological, and green infrastructure from 1989 to 2015 [36
] suggests that there is a shift of focus observed in the transitioning from green, or natural, infrastructure to BGI. The later seeks an integrated approach, utilizing different types of eco-systems and associated eco-system services. It is a shift away from a comparatively simple “land use view” [37
], towards recognizing more flexible eco-system service-based solutions [38
] that do not only include green and blue elements and processes but also take into account man-made interventions, such as permeable pavements, bioswales, retention basins, and constructed wetlands as an integrated whole. The present paper aligns with the later body of works and the definition provided by Ghofrani that BGI is
“an interconnected network of natural and designed landscape components, including water bodies and green and open spaces, which provide multiple functions such as: (i) flood control, (ii) water storage for irrigation and industry use, (iii) wetland areas for wildlife habitat or water purification, among many others.”
This later approach to BGI moves away from earlier engineering discourses that proliferated through the conceptualization of green infrastructure mostly for the management of water in urban areas. A range of nomenclature and acronyms were used to define different, or sometimes very much the same, elements, designs, and purposes [41
]. Studies have used different nomenclature and acronyms for water management in literature since 1980s [14
]. The approach to BGI in the later period, more closely evolves from the concept of water sensitive urban design (WSUD) [13
] which shows a clear emphasis on the ‘blue’ elements (e.g., rainfall and flood) to describe the infrastructure [41
]. Liao et al. [42
] further clarifies that BGI is a particular type of green infrastructure involving a network of landscape systems, which often combine both natural and artificial materials, and is purposefully designed and managed to provide (storm) water-related ecosystem services (see also Fletcher et al. [14
], for a review of the concept).
By adopting the term BGI, the present paper shifts away from an earlier fragmented and to some extent elective approach to blue elements as indicated by European Commission’s definition (“or blue if aquatic ecosystems are concerned” [34
]). Through the use of the term BGI, this paper attempts to recognize the active agencies of and adequate sensitivity to the water elements in planning urban infrastructure. In this regard, the paper also acknowledges the notion of “intentional landscapes” [43
] (p. 133) in encapsulating BGI, not just comprising of natural landscapes but also made out of man-made elements. This approach to BGI helps a greater recognition of an array of ecosystem services (e.g., water purification, heat retention, as well as cultural and economic benefits) with a higher sensitivity towards human interventions, i.e., planned or designed urban spaces [44
]. BGI, in this paper, is approached as purposeful and intentional, not just the remnant or leftover landscapes but designed and deployed primarily for social, economic, and environmental benefits [45
], yet without compromising but sustaining natural processes [47
Many studies have focused on BGI, but most of them seek to highlight the benefits of BGI [48
]. In countries such as the Netherlands and Sweden, BGI is well accepted; however, the theoretical considerations of how the concept of BGI can inform the practice of planning remain undeveloped. BGI represents a conceptual shift from conventional approaches to water management, emphasizing the natural landscape [49
] to provide “resilient and adaptive measure to deal with flooding by mimicking pre-development hydrology” [50
] (for example, detention and retention techniques). This approach reduces stress on ‘grey’ infrastructure in urban areas [51
]. Thus, planning for BGI is significantly different from conventional planning that historically relied too much on ‘hard’ built infrastructures, such as streets, sewage and drainage systems, and utility lines [31
]. From the experiences of the Malmo City Council in Sweden [52
], it is recommended that a careful balance is needed between hard and soft elements for a successful BGI through the incorporation of blue and green infrastructure among existing land uses where sealed or paved surfaces can be effectively minimized. Research also reinforces that planning for BGI should aim for a “symbiotic” relationship between the city and its region [53
]; the emphasis should be on enticing more polycentric development and multiple densities across the entire region. Overall, there is a call for a more integrated mix of blue, green, and grey elements in which “the boundary between the natural and the technical networks is blurred” [53
] (p. 9).
In the rest of the paper, a range of literature informs the investigation, analysis, and the overall approach of designing the BGI networks in the context of Dhaka, Bangladesh. Aligning with Ahern’s [54
] recommendation, the paper adopts a ‘multi-scalar’ planning approach to implement BGI integrating both micro (i.e., land parcel and neighborhood) and macro levels (i.e., urban and peri-urban region). Scholars have suggested the ‘scale’ in terms of regional/urban and private /public component in a given urban area [55
]. Some key studies [56
] propose for three spatial relationships that need to be taken into account when implementing BGI: site-specific elements, linkages, networks and connectivity [58
], and other broader (regional) scale landscape elements. This paper also follows Hansen et al. [59
], who recommend promoting ‘multifunctional’ BGI for high-density urban areas. BGI is well suited for dense urban areas as it reduces the need to upgrade or expand conventional stormwater/drainage system, both spatially and financially [42
]. Following Hansen et al. [59
], the first step of designing a BGI network involves a systematic “spatial assessment” of the urban morphology to identify all blue and green spaces at the ‘site-level’ as well as ‘city-wide’ that could be strategically organized in the future to meet multiple functions (p. 99). The second step focuses on the ‘connectivity’ of blue and green corridors as a critical component. The third step combines the two to envisage a multi-scalar solution that incorporates Dhaka’s urban morphology and the dynamics of water.
2.2. Context: Water and Dhaka’s Urbanization
Dhaka—one of the world’s most densely populated megacities with 17 million people—is geographically situated in the deltaic plain of three major rivers: Padma, Brahmaputra, and Meghna, and surrounded by tributaries of these major rivers (see Figure 1
). To describe Dhaka’s historical context, in this section, we mostly rely on Rahman’s edited collection [60
] that collate facets of Dhaka’s urban social and spatial transformations. Dhaka traces back its origin in the 17th century as a trading hub of the Mughals. As water played a principal role in transportation at that time, the city was not only structured by the bodies of water that flowed through it, but also culturally constructed as ‘the river city’. At this time, the city’s boundaries were clearly defined by the tributaries: Buriganga to the south, Turag to the north and west, and Balu to the east, as well as the swampy lands all around.
The story of Dhaka is similar to many highly urbanized cities of contemporary time such as Delhi and Shanghai that neglected their water systems to find room for growth, if seen from a morphological point of view. The city underwent different phases of development throughout history. In the 16th century Mughal rule, the development followed the river Buriganga in the east-west axis. During the British rule of the 17th century, the city started to grow toward the north because of the relocation of the industrial zones and the connection with railway lines. The 18th century was all about the coexistence of the city and the river where the city development respected the river course and used it as the network of the main transportation system. After the independence of Bangladesh in 1971, the city expanded in all directions, maintaining the tradition of a centralized capital city with a concentration of all governmental and private developments. The historical flooding event of 1988 put 75% of the city under water (see Figure 2
). As a result, the western riverbank was protected by dam construction, which in the later phases caused dense urbanization process along the dam, keeping minimum room for the river. With the development of other grey infrastructures, waterways eventually became a forgotten backyard of the city.
The phenomenon of rural-urban migration has worsened the situation as the city doubled in size from 1990 to 2005 [61
]. The U.N. predicts, by 2025, the population of Dhaka will cross 20 million making the city larger than Jakarta, Mexico City, or Shanghai today [62
]. New residents move to Dhaka every day, seeking residence in the low-lying lands—by filling up water retention basins, river beds, or even portions of a river itself [63
The process of neglecting water in urban planning and development brought about both physical and metaphorical consequences. People started perceiving water as an element to neglect, polluting and encroaching its banks indiscriminately without any concern of environmental impacts on the overall city landscape. The waterways that used to clearly define the limit of the urban–rural condition of the city are blurred today. The city continues growing in all directions, keeping bare minimum porosity in the urban tissue. The city’s green landscape is also a distant memory with more than 1000 ponds, canals, and parks replaced by houses, workplaces, and markets; now, only 5 percent in old parts and 12.5 percent in new parts of the city is composed of green areas [64
]. The most common trend in Dhaka is to build multi-storied buildings on smaller parcels with minimal open space preserved. Private developers are filling up low-lying areas, including wetlands and marshlands, specifically on the eastern part of the city (see Figure 2
). Such development patterns are resulting in severe scarcity of pervious land for rainwater to permeate and recharge the ground. Since the surface runoff cannot accumulate on the eastern lower side of the city, it causes high amounts of water clogging all around the city during major rain events. The inner part of the city that used to have water channels connecting the rivers on both sides has been occupied over time. Subsequently, there is not sufficient surface water corridors to channel the water from the inner part of the city to the downstream areas. High rainfall coinciding with a high-water level in the river quickly floods the city because stormwater cannot be naturally drained through the remaining water system that is in part due to the insufficient drainage system. As a result, frequent flooding events have increased in the last decade. The city now struggles relentlessly for the survival of its few remaining water courses and very often fails to withstand the pressure of the urbanization process that either completely ignores or mostly replaces the natural system with asphalt surfaces.
This paper addresses the opportunity to rethink how to accommodate future inhabitants of the city without necessarily compromising the blue (and green) infrastructural elements (e.g., water and green/open spaces) while possibly reviving those that have been lost. The recognition of the importance of blue and green natural elements as a structural tool is evident in the recent Draft Dhaka Structure Plan 2016–2035 [65
], where significant spatial strategies have been premeditated to re-organize the future growth development pattern. The structural plan proposes to identify special zones for protecting and preserving natural areas for both flood management and recreational purposes. Notably, the plan identifies strategies for reclaiming the illegally occupied flood-flow zones in the city to be implemented in the coming years. In the context of the future urban development strategy, the following sections explore the potential of BGI networks—both at macro and micro scales.