Planning Strategies and Design Concepts
Waterfront Developments and Public Space
Abu Dhabi’s Eastern Mangroves as a Case Study
2024 Volume 12 Issue 1 Pages 44-58
Details
2024 Volume 12 Issue 1 Pages 44-58
Abu Dhabi’s recent coastal development in the Eastern Mangroves area involves the construction of a new bridge that will connect the main archipelago to newly urbanized islands. This bridge crosses over a major mangrove plantation resulting in a change in its geometry, surrounding water canals, and the marine environment. Over-the-sea bridges require construction and dredging activities that change existing environmental habitats and hydraulic flow. This article examines the changes in the waterfront and canal routes in the study area through satellite images and land use maps. Water flow was modeled by a two-dimensional (2D) hydrodynamic model, based on the Finite-Element Surface Water Modeling System (FESWMS), which simulated the changes in water circulation patterns after construction of the bridge. Our two-dimensional, hydrodynamic model indicated that the bridge’s construction would create stagnant conditions in the water downstream the bridge, potentially resulting in sediment aggradation, which would require continuous dredging of the canal in the future.
Abu Dhabi’s coastline is currently being modified to allow for increased connectivity to its new urban islands. These modifications include bridges and piers that, in the long term, will affect Abu Dhabi’s tidal lagoon geo-environment. This “ocean-sprawl” in the Gulf region attempts to revitalize living city building stock by providing an appealing water and green housing environment to a growing population.
However, heavy engineering structures at coastal locations intercept the material transported from land and can result in sand depletion at the beachfront. Structures set by the coast accumulate sand at their upstream side not allowing beach replenishment, and the action of the tide may result in loss of sand to deeper water. Coastal structures that include groin fields, seawalls, and ports and harbors interfere with the dynamic material balance of accretion and erosion between land and sea, and modify the diffraction of waves around port slits and other man-made structures emplaced in the sea as well as the refraction of waves in nearshore shallow waters (Paleologos, Welling et al., 2019). Dredging, to construct desired channel geometries and over-the-sea bridges, results in modification of flow patterns, sediment transport, and beach accretion or erosion on downstream locations. Finally, the concentration and embedment of human activities within fragile ecosystems creates a huge risk to the well-being of mangroves and marine ecosystems (Cavalcante, Kjerfve et al., 2011; FAO, 2007; Mohamed and Paleologos, 2017).
Little has been written about the challenges that face mangrove plantations and habitats in modern Arab cities, especially in light of the rapid urbanization of these cities. In order to maintain healthy mangroves, it is essential to allow for natural tidal fluctuations. Coastal construction threats include the interruption of tidal and circulation patterns, which in turn affects ecosystems (Shahbudin, Zuhairi et al., 2009; Szatten, Habel et al., 2019). The Eastern Mangroves promenade (Corniche) is one of the most important waterfront parks in Abu Dhabi. A representative of biodiversity and natural heritage, it is a centre of inherent ecologic, recreational, and educational importance, fostering non-motorized boating /kayaking. Similar to other mangrove habitats in Abu Dhabi, these saline water forests are important nursing grounds for fish and animal species. The Eastern Mangroves area combines a hotel and residential units complex, cafes, restaurants, water sports centers, and family outdoor public spaces along an open promenade that allows residents to combine outdoor city leisure activities with natural expeditions. The Eastern Mangroves coastline development replaced an old (1968-70s) military camp in Abu Dhabi and was part of Sheikh Zayed’s vision to create a modern city that takes the environment into consideration (Hashim, 2018). The area has a low-rise urban fabric, predominantly villas, along a waterfront that stretches for 3.5 kilometres and contains walking, jogging, and cycling tracks. The promenade offers trails within natural reserves, which have been enriched by the plantation of mangrove trees in the marshlands and lagoons found between the islands. The part of the channel that is by the new bridge was engineered with large gravel close to the resort, and concrete walls along the pedestrian walkways.
Over the last two years, a bridge was constructed in the Eastern Mangroves area, which crosses over the mangrove forest with the purpose of improving connectivity between the main island of Abu Dhabi and the smaller islands comprising the coastal lagoon environment. Figure 1’s 1 top and middle photos depict the area prior to and after bridge construction. The bridge’s mainland entry point is located at the Anantara Eastern Mangroves area (shown in the lower part of the circled area in the bottom picture in Figure 1), which is at the eastern edge of the Mangrove National Park. The bridge passes over the mangrove area to land on the Umm Yifenah Island (top right of the circled area in the bottom picture in Figure 1), and is further connected to other, recently urbanized islands.
Abu Dhabi’s expansion seaward manifests an understanding that waterfronts present a more appealing environment for diversified economic opportunities through “urban renewal” projects (De Jong, Hoppe et al., 2019; Huang, Pai et al., 2016) than the inland desert environment of the United Arab Emirates (UAE). The connectivity of smaller islands to the mainland is served by seven bridges, including two that run over water for 3.8km. The new bridge at the Eastern Mangrove area extends for 3km over water, and is part of a 300 million USD project that consists of five bridges to connect the newly urbanized smaller islands to the main island of Abu Dhabi. The new bridge is built on piers, replacing an early design proposal of a hanging bridge on large intersecting arches (Construction Week, 2019). The bridge’s six-lane highway crosses over mangrove landscape habitats, providing a juxtaposition between the open sea and the built city space. The more general interest from our current study lies in that it exposes the challenge that exists in several modern coastal cities to balance the conservation of natural resources and a city’s expansion and economic development, which requires infrastructure projects and real estate at the outskirts of or within ecosystems. These issues are investigated in this paper with the focus of analysis on the changes in the channel’s hydraulic flow following bridge construction. The paper provides a modeling prediction of the altered water flow due to the bridge’s construction in a tidal lagoon environment. Finally, we touch upon the impact of the new bridge on the forthcoming mangrove developments.
Mangroves have been recognized as a distinct characteristic of the city of Abu Dhabi, a source of oxygen and aesthetic relaxation in a harsh, desert environment, a buffer from tidal effects and natural protection from beach erosion, and a place where land and marine ecosystems develop and interact (El Amrousi, Elhakeem et al., 2019a; El Amrousi, ElHakeem et al., 2019b. Mangrove cultivation areas are a priority for Abu Dhabi planners as these represent an alternative form of greening the desert city. Mangroves have continuously been nurtured in Abu Dhabi in the last decades, with recent commitments from the Abu Dhabi National Oil Company alone exceeding 10 million seedlings by 2022 (ADNOC, 2022).
The sensitive environment of marshlands and mangrove forests challenges architects to develop solutions that are resilient, sustainable, and more inclusive of and responsive to nature (Lőrincz, Kruppa-Jakab et al., 2021). Providing healthy public spaces and walkability within a metropolitan city involves connectivity, land-use patterns, safety, and quality of the path itself. Clearly, the continuous development of smaller islands around the city of Abu Dhabi and the construction of bridges to connect them to the mainland puts pressure on the mangrove areas and requires exceptions to the designated land use zones. In addition, the lagoon environment will be affected by massive construction activities during bridge and residential buildings developments; the traffic that will be generated by the people using the bridge to travel to the connected islands of Al-Reem and Al-Maryah, and eventually to the opposite side at Jubail Island; and the impact of tens of thousands of residents and visitors. For example, Jubail Mangrove Park was created to provide public access to sensitive environments with minimal disturbance to the natural surroundings. A series of pedestrian boardwalks of 2.6 km (Figure 2, bottom) navigates visitors through a variety of ecosystems that includes the mangrove habitat, intertidal mudflats, salt marshes, and lagoon channels, and exposes them to a natural environment that is very different from the surrounding desert (GHD, 2020). Mangrove plantations can offer a solution for “greening” the rapidly urbanizing city without consuming fresh water, which is a scarce commodity in the UAE’s desert environment Khan and Kumar, 2009). Cultivating mangroves parks and integrating them with housing developments in Abu Dhabi’s newly developed islands reflects the importance of eco-engineering, especially when integrated within the surrounding nature (Banerjee, 2001).
Coastal developments have accelerated over the last decade as a means to relieve population pressure, especially in the Arabian Gulf and Asia where 67% of the most populated coastal cities are found (Chee, Othman et al., 2017). The effects on marine and coastal ecosystems from the artificial structures of this “ocean sprawl” include “placement loss, habitat degradation, modification of sound and light conditions, hydrodynamic changes, organic enrichment and material fluxes, contamination, and altered biotic interactions” (Heery, Bishop et al., 2017). Architecture has a strong potential to affect the environment in a negative or positive way through design and site planning, and modern movements are away from massive, engineered coastal structures and towards “eco-engineering”, which integrates engineering and ecological criteria. Modern urban planning strategies foster more sustainable, eco-engineered coastal structures that enhance urban waterfronts’ ability to provide a habitat suitable for a large diversity of aquatic organisms. Seawall stairs and viewing platforms (Figure 2) are designed to increase nearshore habitat areas and add both horizontal surfaces and microhabitats to the urban waterfront. This is achieved by incorporating exposed aggregate and depressions that may provide natural habitation and create microhabitats, diverse surface orientations, and additional shallow water habitat areas (Dyson and Yocom, 2015). Other measures include the use of ecologically friendly material (Perkol-Finkel, Hadary et al., 2018) and the incorporation of design elements, such as textured seawalls, vegetation and habitat baskets, vertical marine gardens, and seawall stairs, among others, all of which have been shown to promote biodiversity by providing habitats for fish and by reintroducing shoreline vegetation (Dyson and Yocom, 2015; Firth, Knights et al., 2016).
To address the continuous coastline modifications in the lagoon environment of Abu Dhabi (Figure 3), Abu Dhabi’s Urban Planning Council has advocated several sustainability regulations in the construction industry. Since 2010 it has been a requirement that construction activities in the city follow local sustainability regulations, modelled after the U.S. Green Building Council’s LEED (Leadership in Energy and Environmental Design) system. One important aspect of the regulations concerns the protection and enhancement of the native biosphere, including the mangrove plantations that exist along the coastline known as the Eastern Mangroves. In terms of land use regarding the Eastern Mangroves area, Abu Dhabi’s Urban Planning Council “Interim Coastal Development Guidelines” the area of our study was characterized as a protected or proposed protected area and national park intended only for recreation, education, and research activities. The new bridge may have been a later intervention for purposes of expanding new real-estate developments.
In order to evaluate the modifications in the shoreline of the Eastern Mangroves and the hydrodynamic performance of the watercourses one can use either physical or numerical models of site-specific conditions (Blazejewski, Pilarczyk et al., 1995; El Amrousi, Elhakeem et al., 2019a; Elhakeem, Papanicolaou et al., 2017). Numerical models are more adaptable to different environmental and geomorphologic domains than physical models (Crowder and Diplas, 2000; Spasojevic and Holly, 1990). In addition, numerical models are not subject to distortion effects like physical models, where complete geometric, kinetic, and hydrodynamic similarity must be achieved when trying to capture the same flow conditions as those present in the field (Elhakeem and Papanicolaou, 2008). Nonetheless, numerical models must be calibrated and verified against laboratory or field data (Papanicolaou, Elhakeem et al., 2011). Numerous commercially available hydrodynamic models (either 1D, 2D (two-dimensional) or 3D) exist for simulating stream flow around hydraulic structures (Papanicolaou, Elhakeem et al., 2011). Channels of complex geometry cannot be simulated using 1D-hydrodynamic models, which are more appropriate for simulating flow conditions in straight stream reaches. 2D-hydrodynamic models have been deemed appropriate for simulating flow conditions in meandering and fork shallow stream reaches that contain pier structures because they can resolve the large-scale turbulent flow regime and provide spatially varied information regarding the flow around piers (Melville and Coleman, 2000; Papanicolaou, Elhakeem et al., 2011; Wu, Shields Jr et al., 2005). In addition, 2D models require less data for model calibration and verification than 3D models, which are appropriate when depth considerations are important (Spasojevic and Holly, 1990). 2D-hydrodynamic models solve the depth-averaged continuity and Navier–Stokes equations using finite-difference, finite-element, or finite-volume schemes (Spasojevic and Holly, 1990; Wu, Shields Jr et al., 2005).
The 2D Finite Element Surface Water Modeling System (FESWMS) was used for this study, which is part of the commercially available Surface Water Modeling System (SMS) software package (version 12.1). FESWMS was developed by the Federal Highway Administration (Froelich, 2002), and has been used successfully in many riverine applications. The software has been used to simulate flow in river bends where secondary currents were prevalent, as well as to study bank erosion in constructed straight water streams. Based on the FESWMS, we developed a 2D digital model and analyzed its results in order to evaluate the effects of the newly constructed bridge on the seawater circulation in the Eastern Mangroves area in Abu Dhabi.
Detailed sensitivity analyses and error propagation studies have shown that FESWMS provides, in general, satisfactory results for streams with complex geometry and bed-roughness when the model is calibrated against field measurements (Elhakeem, Papanicolaou et al., 2017; Pasternack, Gilbert et al., 2006). In order to provide accurate input information to our model, Google Earth satellite images from 2004-2021 were used to detect changes in the mangrove coastline of the Abu Dhabi Eastern Mangroves area and the Jubail Mangrove Park. Since the mangroves grow along the coastline, the coastline boundary was used to mask the land areas from the satellite images. In addition, the properties of the material used (gravel, concrete, etc.) for the engineered banks of the development near the entry point of the bridge, as well as the extent and type of material of the natural embankments (Figure 2, top), were utilized to input friction parameters to our model.
FESWMS solves the differential forms of the continuity and the momentum equations in the streamwise and transverse directions using the Galerkin method of weighted residuals, providing water depth and depth-averaged velocities in the x- and y-directions at each node in the grid (Froelich, 2002). The governing equations are written in the conservative form, hence, momentum is conserved along the streamline and the model is capable of capturing shock effects. The conservative form was chosen because of its robustness in solving critical and transcritical flow regimes under both low- and high-flow conditions by allowing dry-elements to exist within the computational mesh.
FESWMS solves the following equations simultaneously:
Here, equation (1) is the continuity and equations (2) and (3) are the momentum equations in x- and y- directions, respectively. In these equations, t is time (s); d is water depth (m); ρ is water density (kg/m3); g is the acceleration due to gravity (m/s2); n is Manning’s coefficient of roughness; zw and zb are the water surface elevation and bed elevation above a certain datum (m); q1 and q2 are the unit discharge fluxes (m2/s) defined as
FESWMS requires inputs for Manning’s coefficient of roughness n, and for eddy viscosity ν. Model inputs must be assigned correctly to represent the physical processes occurring in the modeled reach and to produce accurate model predictions. Manning’s n is an empirical coefficient that accounts for the total flow resistance caused by water flow’s interaction with a solid boundary (Elhakeem, Papanicolaou et al., 2017). FESWMS utilizes Manning’s n to account for momentum loss due to bed-shear, which may vary significantly in a stream reach in accordance to bed-bathymetry and roughness. The second input variable used by FESWMS is the eddy viscosity ν. Eddy viscosity accounts for flow resistance due to the internal shear stresses, or the Reynolds stresses of the fluid incorporating the added energy dissipation due to turbulence in the flow. Therefore, eddy viscosity is not a physical property of the fluid, but rather a turbulent characteristic of the flow.
Bridges are known to affect the hydrodynamic conditions at the location where they are created, affecting water velocities both in the vicinity of the structure and downstream, with subsequent impacts on the sediment transported by the water; and the erosion of beaches, which may extend for kilometers downstream a bridge. Lagasse, Thompson et al. (1995) surveyed over-the-water bridges and found that 17–39% of these were “scour susceptible,” and over 6,500 were “scour critical.” The scour and erosion reported in the literature has the potential to destabilize the foundations of bridges, causing bridge collapse and casualties. Furthermore, increased erosion, turbidity, and sediment deposition and accumulation negatively affect aquatic life around and downstream of the project site (Seiyaboh, Inyang et al., 2013). Bishop, Mayer-Pinto et al. (2017) have indicated that ocean sprawl and over-the-water bridges may alter the ecological connectivity by: “(1) creating barriers to the movement of some organisms and resources - by adding physical barriers or by modifying and fragmenting habitats; (2) introducing new structural material that acts as a conduit for the movement of other organisms or resources across the landscape; and (3) altering trophic connectivity.”
With regards to our study, the parameters inserted for the simulations were: average water depth in the channels was set at 8.0 m, average value of the Manning’s coefficient was 0.025, and eddy viscosity was 1.0 m2/s. Concrete forms are characterized by a Manning’s coefficient that is about half of our selection, whereas gravel corresponds exactly to the value used. The top picture of Figure 1 showing sandy soil and the mangroves corresponds to the middle left side of our modeled area shown in Figure 4. This is a tidal lagoon with the mangrove area inundated at high tide. Flood plains and natural channels have higher Manning’s coefficient than the value we selected, which represents an average value of the different channel sides’ materials.
Figure 4 shows the predicted flow velocities from the FESWMS simulations, which indicate a range of 0.5 m/s to 1.6 m/s. Due to the natural slope of the simulated coastal area, higher flow velocities were predicted in the channels close to the shoreline, while lower velocities were predicted in the lagoon near the bridge.
Our results in Figure 4 from the FESWMS simulations indicate extremely low flow velocities in the segment downstream the bridge. Thus, at the lower bank boundary, where the engineered bank exists, velocities along the concrete pathways were calculated to be in the range of 0.4 m/sec to 0.7 m/sec, dropping almost to zero after the bridge along the graveled banks of the resort area. There is a clear stagnation area of almost zero velocity along the piers at the bottom half of the bridge, and the whole lagoon area downstream the bridge appears to have stagnant water.
The US Department of Agriculture (USDA, 2007) “Part 654 Stream Restoration Design National Engineering Handbook” classifies channels based on the interaction of flow and sediment taking place in order to be used as a guide to the design approach. Thus, threshold channels are defined as those where channel boundary material exhibit no significant transport during the design flow. In order to avoid aggradation of sediment, such as of clay, silt, and fine sand, a threshold channel must be designed so that a minimum velocity is maintained, capable of transporting the sediment load through the project reach; and a maximum velocity is not exceeded, to avoid scouring. Alluvial channels are those where there is a significant exchange of material between the inflowing sediment load and the channel’s bed and banks during water flow. The channel examined in our study likely falls under the classification of a transition channel behaving as alluvial at low tide, when there is an adequate sediment supply, and as a threshold channel at high tide. In general, if a channel is to remain non-erodible, a mean velocity of not less than 2.5 fps (0.76 m/s) is permissible to prevent sedimentation and avoid vegetation growth that will decrease flow circulation (Te-Chow, 1958). For a grassed channel the permissible velocity to prevent severe erosion, depending on the type of grass, ranges between 2.5 to 6 fps (0.76 to 1.83 m/s) for easily eroded soils (Te-Chow, 1958). Clearly, the velocities predicted by our model either marginally meet the criteria in some channel sections, or even fall below the recommended guidelines in other sections. Given the complexity of the tidal lagoons that make up Abu Dhabi’s water environment, the construction of structures poses difficulties. The restricted area for water to pass through the structures will initially increase the velocity under them, presenting the danger of scouring, and the sediment aggradation in a low-depth, low-velocity downstream environment may require continuous engineering measures to sustain a hydrodynamically functional situation. Furthermore, concentration of intense human activities at coastal locations, heavy traffic, and housing developments have been known to result in water pollution, debris deposition, and microplastic invasion in marine ecosystems (Castillo, Al-Maslamani et al., 2016). Organic pollutants and metals, as well as microplastic particles, can reach the seawater from inland urban areas via various pathways (Mohamed, Paleologos et al., 2020). These include road traffic that may depose them through air dispersion, particle accumulation from tire and brake wear on road surfaces, and street sweeping (Järlskog, Strömvall et al., 2021). Pollution of the marine environment because of the embedding of communities within a previously natural landscape can arise from the construction of storm water and wastewater systems, which even for the relatively new systems in Abu Dhabi have at times developed significant leakages as a result of the high gypsum and other salt content of the local geology. Pollution can also result from the increase of human activities, such as fishing and the use of shipping and pleasure boats in an enclosed, slow-moving seawater environment. Marine debris, either from transportation networks or generated by communities located adjacent to mangrove forests, can suppress mangrove ecosystems, inhibit tidal flushing, and increase salinity levels inside the tidal channels, eventually reducing species populations and diversity (NOAA, 2016). Finally, other effects on marine and coastal environments from the concentration of anthropogenic activities at coastal locations include changes in sound, light, and seawater temperature; contamination of water and sediment; habitat degradation; and altered biotic interactions and patterns (Mohamed and Paleologos, 2017).
In Abu Dhabi, mangroves as alternative greenery offers the city a new form of urban expansion with more connectivity to waterfronts and raised awareness of the importance of nature. The race for “socioeconomic development” among the Gulf states, that has focused on vast coastal infrastructure projects, has recently been placing an emphasis on the mangrove areas. For Abu Dhabi, this is in part because the highly-salt tolerant Avicennia marina or grey mangrove covers about 155 km2 of the shoreline (Paleologos, Welling et al., 2019). Forthcoming urban developments, moving away from the mainland to surrounding islands, offer a promising venue for low-density housing amidst mangrove landscapes and a nature-based, healthier lifestyle. Such developments contribute to sustainable urbanism and support city branding in the wake of global city competition to attract investors and establish a position as a major international center. The development of mangrove parks offers the city of Abu Dhabi new and more sustainable spaces for communities to live in (Feyisa, Dons et al., 2014; GHD, 2020; USEPA, 2008).
Public spaces created by waterfronts and mangrove habitations encourage the belief that a city has the ability to revitalize its urban fabric and to focus more on sustainable urbanism. Especially in the Gulf States with a harsh hot climate, air-conditioned indoor spaces have become the primary public place for people to congregate, interact, and socialize, increasing the risk of exposure during epidemics and pandemics (Oh and Gim, 2021). Outdoor spaces have been seen to stimulate local economies, contributing significantly to urban and economic revitalization of aged cities (Fekete, Hodor et al., 2021). In addition, such spaces offer an alternative paradigm of public space to the air-conditioned, indoor spaces, where people used to congregate and socialize in the Gulf States, which showed their limitations during the COVID-19 pandemic as spaces that would increase the risk of viral exposure to the community.
Despite all the above benefits that coastal development brings, a lagoon system is a very delicate and dynamic environment, where there is continuous and complex interplay between land and sea, tidal effects, and ecosystems; and which can easily be threatened by minor perturbations to a site’s natural conditions. Especially, mangrove forests survive under very specific conditions where, among other particulars, root exposure needs to alternate between air and water periodically, a tight range of salinity must exist, and nutrients are transported by land and the sea. Construction of coastal roads and of other engineering infrastructure and buildings interrupts a location’s land-sea material and nutrient exchange. Structures within the sea, such as bridges, alter not only hydrodynamic conditions but also the physicochemical characteristics of a site, thus endangering the existence and health of delicate marine ecosystems.
The current study investigated the impact of a major bridge of 3km length emplaced at a mangrove site in Abu Dhabi, at which location the city’s coastal guidelines had previously recommended that the natural habitat be preserved and no engineering projects be implemented. Our numerical study indicated that extremely low flow velocities would be developed downstream the bridge. At the existing bank boundary, specifically the section that had been engineered in the past to create concrete pathways, velocities were calculated in the range of 0.4 m/sec to 0.7 m/sec, and at the graveled banks, where a resort exists, velocities dropped almost to zero. The whole lagoon area downstream the bridge appears to have stagnant water. The predicted velocities do not conform with the US Department of Agriculture’s (USDA, 2007) guidelines for the minimum velocities to be retained to avoid sediment aggradation in an engineered channel. These results come as no surprise as the average depth of this part of the lagoon is very shallow and, together with its meandering nature, made for a natural slow water circulation. Hence, it appears that the engineering intervention at this site will deteriorate the marginal, natural flow situation and continuous engineering measures will be required to maintain the function of the channel in the future.
Finally, and beyond the hydrodynamic situation analyzed here, there exist a large number of factors that are expected to deteriorate the quality of water and ecosystems at this site. The sheer magnitude of human activities planned at this location, including initial construction and later the residence of thousands of people; the heavy traffic on the bridge, which is a connecting artery to several islands; and the solid and liquid pollution that will result from the embedment of large human communities within a lagoon environment are all conditions that may prove to be detrimental for the existence of the mangroves and ecosystems at this location.
Conceptualization: El Amrousi, M., Paleologos, E.K., Elhakeem, M.
Field investigation, Data Collection & Analysis: El Amrousi, M., Paleologos, E.K., Elhakeem, M. Writing (Original Draft Preparation): El Amrousi, M., Paleologos, E.K., Elhakeem, M. Review & Editing: El Amrousi, M., Paleologos, E.K. All authors have read and agreed to the published version of the manuscript.
The authors declare that they have no conflicts of interest regarding the publication of the paper.
