Dredging and causeway construction

During 1983–85, a navigation channel was dredged across the shoal to facilitate navigational access. The main navigation channel extends from north to south, branching out with several extensions in the north, totalling 31 km in length. The channel is 125 m wide and 6–7 m deep (i.e. some 3 m deeper than adjacent seabed), and its excavation resulted in a total volume of approximately 12 Mm³ of carbonate sediment, consisting mostly of sand (70%), with 22% gravel and 8% fines (Aces, 2016).

Approximately 10 Mm³ of this dredged material was used for the construction of a 17-kilometre-long causeway that protects several oil pipelines and power cables, and serves as a road connection between the oil production areas and the main island. The causeway is 50–150 m wide and has a ‘height’ of 5.1 m above the adjacent seabed or approximately 3.3 m above mean sea level (MSL), assuming average water depth near the causeway to be in the order of 2.8 m, and MSL being 1.2 m above Chart Datum. The remaining ~2 Mm³ of dredged material was used for land reclamations at the production wells and along the island.

There are no historical records of environmental impacts of the construction of the causeway (early 1980s), but more recent environmental assessments describe the marine ecosystems at Mubarraz as being in good health (Deltares, 2006; DOME, 2007). Remote sensing analysis suggests that the seagrass meadows, which cover approximately 4,000 hectares of the shallow waters of Mubarraz shoal, recovered well in the years following the completion of dredging and causeway construction, having fully recovered by 1990 (DAMCO Consulting, 2018). The seagrass beds at Mubarraz support significant populations of green turtles (Chelonia mydas), dugongs (Dugong dugon) and humpback dolphins (Sousa plumbea). Coral reefs around Mubarraz shoal suffered multiple bleaching events in 1996, 1998, 2002 and 2017 due to thermal stress related to climate change and are now in a poor condition similar to most reefs in the southern Gulf (Burt et al., 2019). Two culvert gaps were constructed in 2007 to reduce perceived barrier effects of the causeway and enhance connectivity, providing opportunity for the exchange of water, organisms and larvae.

Mangrove planting

The first mangrove planting campaigns at Mubarraz were carried out during the startup phase of causeway construction and oil operations during 1983–87. These plantings were conducted with seedlings of the mangrove species Avicennia marina obtained from a mangrove tree nursery in mainland Abu Dhabi, along with some propagules of Rhizophora stylosa (from Pakistan), Rhizophora mucronata (from Malaysia) and Ceriops tagal (from Thailand). The primary focus of these early planting efforts was environmental enhancement (‘greening’), implemented at numerous locations, being only successful with Avicennia marina in sheltered corners of the causeway and island, showing very low survival with the other species and at other locations.

Following increasing shoreline erosion along the western side of the causeway in the mid-2000s, a new ‘Working with Nature’ approach was adopted for the mangrove planting works. A tree nursery was established under the shade of trees at the south-western edge of the main island with a capacity of 80,000 seedlings. Nursery practices followed general guidelines from international best practice manuals, further optimised over time according to what worked best under local conditions. Propagules to seed the pots in the nursery are collected annually over a two-week period in mid-September from mature mangrove stands on Mubarraz, obtaining best results with three propagules planted per pot in a soil mixture of sand, peat moss and plant potting mix. Seedlings are watered and nurtured in the nursery for approximately six months until they are large and strong enough to be planted in the field. A site survey in March 2005, assisted by satellite imagery analysis, was conducted to guide site selection for new mangrove planting efforts.

At the start of the program in the mid-1980s, most planting was done in a random fashion along the outer shorelines of the causeway and island without any specific site preparation. Although this yielded reasonable success at some sheltered locations in the north-west corner of the causeway (at West Mubarraz), most initial planting efforts along the exposed stretches of the causeway failed completely.

Therefore, an alternative approach had to be developed here to provide the plantings with initial protection from erosion. Artificial (tidal) channels 6–9 m wide were excavated with a backhoe along the length of the causeway, leaving a protective berm or bund of dredged material between the planting site and the open sea. The channels had passages to allow for the entry and drainage of seawater with the tide and varied in length from <100 m to several kilometres. Channels were dug to a depth considered suitable for mangrove growth, generally between mean high water (MHW) and mean high water spring (MHWS) (Lewis 2005; Van Loon et al., 2016). Because these channels did not directly face the seashore, except for their intakes, the plantings were relatively protected from wave action and seagrass wrack accumulation. Compacted soil in the channels was loosened with rakes or spades to facilitate plant root penetration and soil aeration. Soil quality was further improved by the addition of peat moss prior to planting. Since 2006, the channels have been planted annually by staff from the oil company. Tens of thousands of mangrove seedlings have been planted at 1 m spacing, covering different areas each year. By 2019, a total of ~500,000 seedlings had been planted. Plans for the coming years are to continue planting more mangroves, wherever possible, until most of the shoreline of the causeway is fringed on both sides by a protective belt of mature mangrove vegetation.

A recent assessment of the overall success of the mangrove planting program at Mubarraz revealed survival rates of planted mangroves (from seedling to mature trees) to be in the order of 26% (averaged over all years), with well-established mangrove vegetation (totalling some 140,000 trees and saplings) now lining approximately 7 km of shoreline, covering 16.5 hectares (Erftemeijer et al., 2020). Successfully planted stands of mangroves were characterised by high tree density (mean: 8838 trees ha-¹), stunted tree height (mean: 3 m), narrow stem diameter (mean: 5 cm), moderate basal area (5–37 m² ha-¹), mean pneumatophore density of 589 m-², and healthy recruitment (>14,000 natural seedlings ha-¹) (Erftemeijer et al., 2020). The overall survival of 26% (or 32% if recently planted seedlings are included) of all planted seedlings at Mubarraz is comparable to natural survival rates for mangrove seedlings in the field, as reported in the general literature (see Erftemeijer et al., 2020).

Colonisation by fauna

Established mangrove stands have been successfully colonised by benthic invertebrate fauna, as indicated by high densities of crab holes (up to 638 m-²) and gastropod snails (up to 429 m-²). Mubarraz currently supports a significant diversity of birds with 50 species recorded to date. This can be attributed almost entirely to the creation of new land, by the reuse of dredged material, and successful establishment of mangrove vegetation, previously absent from Mubarraz. At least six of these bird species have been confirmed to breed on the island, owing to the presence of the mangrove vegetation, while several others (including 12 shorebird species) visit the island during the migratory season. The oil company also facilitated the construction of 12 artificial nesting facilities for Ospreys that are being used for breeding every year by multiple pairs of this impressive fish-eating bird of prey (with maximum daily counts of 21 birds). These observations suggest a significant level of faunal colonisation of the artificially created mangrove stands on an island that did not have any vegetation before, underscoring the importance of the planted mangroves for biodiversity conservation.

Hydrological investigations and channel modification

New plantings along a further 9 km of the causeway’s shoreline are currently underway. The bed levels in some tidal channels had to be adjusted in order to provide optimal tidal inundation conditions for these new plantings, based on detailed hydrological investigations in 2019 (DAMCO Consulting, 2020) following recommendations described in Van Loon et al. (2016). Using water level loggers (complemented with auto-levelling measurements), the daily duration of tidal inundation at successful and unsuccessful planting sites were measured to establish the relationship between planting success and hydrological conditions (tidal inundation as function of bed level relative to MSL). The results of these hydrological investigations indicated that mangrove planting efforts at sites that experience tidal inundation between 180–450 minutes per day, which at Mubarraz corresponds to channel bed levels of 27–42 cm above MSL, have been generally successful. Sites inundated for more than 500 minutes per day were generally unsuccessful, apparently exceeding the mangroves’ tolerance limit for waterlogging, leaving them insufficient time at low tide to bring enough oxygen back into the soil through their breathing roots for survival. These site-specific findings are now being used to guide the design of new channels and modify the depth of unsuccessful (existing) channels to further optimise replanting success at Mubarraz.

Shoreline protection

The protective berms of dredged material along the mangrove channels occasionally required maintenance to repair sections affected by storm damage. Following planting, the young mangrove vegetation will not offer much shoreline protection initially, but as the mangrove stands develop and expand over the years, they can substantially reduce wave height and energy, and contribute to protection from erosion, depending on tree density, stem diameter and the density of their pneumatophores. It is expected that over the coming decade, these mangroves will have grown and established themselves sufficiently enough to withstand significant wave energy and protect these shorelines from erosion after the protective berms of dredged material separating the channels from the sea have gradually eroded away. Meanwhile, a few critical stretches of the causeway’s shoreline have been protected with sheet piling. The costs of such hard engineering solutions, which require maintenance and have a limited lifespan, were six times more expensive than the costs of the mangrove planting approach. This clearly underscores the potential costeffectiveness of nature-based solutions such as the use of mangrove vegetation over conventional engineering approaches for shoreline protection. Similar findings were reported by Winterwerp et al. (2016) who successfully restored mangroves as a cost-effective sustainable climate-adaptive solution to reverse severe coastal erosion along the north coast of Java, Indonesia, with associated socio-economic benefits for the local community.

Beneficial reuse of dredged material for mangrove habitat

There are other examples similar to the work showcased here for Mubarraz Island. The last few decades have seen a rise in such beneficial use application of dredged material for the rehabilitation and creation of coastal wetlands worldwide (Turner and Streever, 2002). Particularly in the USA (largely through the work of the US Army Corps of Engineers) there is a considerable body of well-documented experience with the establishment of ‘dredged material wetlands’, i.e. the intentional restoration or creation of wetlands using dredged material, both in freshwater/estuarine and coastal/ marine environments (Bridges et al., 2018). It is important to note that few dredged material wetlands would be built if there was no need to dispose of material resulting from navigationrelated dredging operations.

In Florida, more than 250 spoil islands (1–2 hectares each) from historic dredging operations (1930–1970) at the Indian River, Yankee Town, Pitchlacha-scotee Rover, Tampa and Caloosahatchee River, were naturally colonised by mangroves, saltmarsh and terrestrial vegetation, following processes of natural succession and are often used by waterbirds for breeding (Lewis and Lewis, 1978). Two larger dredged spoil islands south of Alafia River mouth (Florida), which initially functioned as dredged spoil disposal facilities, were later planted with marsh grass (Spartina) and colonised by mangroves and are now managed as a bird sanctuary, hosting more than 12,000 breeding pairs of 20 waterbird species (Robin Lewis III, pers. comm., 2017). In Singapore, two new mangrove areas encompassing a total area of 13 hectares were created on dredged material to compensate for the loss of habitat due to the construction of an offshore landfill site at Pulau Semakau (Chuan, 2009). The site was protected by a rock perimeter bund, underlain with geofabrics, and engineered to correct tidal positioning with an excavated system of artificial tidal creeks. Initial mangrove test plantings all died as the fresh mud was too acidic, but after a delay of four months to let the seawater neutralise the soil pH, the two areas were successfully planted with 400,000 mangrove seedlings (Chuan, 2009). There are similar sites at Lake Maracaibo in Venezuela (Pannier and De Pannier, 2008) and at Navachista Bay and Chiapas lagoon in Mexico (Chargoy and Hernández, 2002), all involving dredged material deposits that were either actively planted with mangroves to help stabilise the deposits or colonised naturally over time.

More recently, there is also an increasing attention to explore nature-based approaches to coastal protection from climate-change related threats worldwide (Mink and Sansoglou, 2020), including the Gulf region (Burt and Bartholomew, 2019). These form part of a wider paradigm shift in the waterborne infrastructure development industry that are inspired by larger initiatives such as ‘Working with Nature’ by PIANC (www.pianc.org/working-with-nature), ‘Building with Nature’ in the Netherlands (www.ecoshape.org; Van Eekelen and Bouw, 2020) and ‘Engineering with Nature’ in the USA (https://ewn.el.erdc.dren.mil; Bridges et al., 2018). A variety of innovative projects implemented under these programmes also involved the beneficial reuse of dredged material from capital or maintenance dredging operations.

Conclusions

This paper describes a unique case study of beneficial reuse of dredged material from the excavation of a navigation channel in the Arabian Gulf for pipeline protection, causeway construction and creation of mangrove habitat for environmental enhancement and shoreline protection. Over a 30-year period, Abu Dhabi Oil Co. (Japan) Ltd employees planted half a million nursery-raised mangrove seedlings along the shores of the island and causeway. Initial mangrove planting success was typically highest at sheltered locations along the causeway, with little or no seedling survival at wave-exposed sites at risk of erosion. The somewhat ironical paradigm emerging here is that mangroves require sheltered conditions for their initial establishment, but once well established provide effective protection from shoreline erosion. The innovative approach adopted by the more recent mangrove planting campaigns at Mubarraz (i.e. excavating parallel tidal channels along the length of the causeway, leaving a protective berm of dredged material between the mangrove-planting site and the open sea) seems to be successful in overcoming this establishment bottleneck.

Owing to the innovative approach and continual planting at Mubarraz over many years, mangrove vegetation has been successfully established along nearly 7 km (~20%) of the causeway, with new plantings along a further 9 km underway. The resulting belts of mangroves along the shores of the causeway, once fully matured, are likely to contribute substantially to its stability and offer protection against the erosive forces of waves and storms. Well-established stands of mangroves have attracted a significant biodiversity (including benthic invertebrates and 50 bird species) previously absent from Mubarraz.

Key factors that drove the success of this program included:

• mass production of seedlings in a tree nursery;
• soil treatment – raking of hard consolidated soil to facilitate plant root penetration and addition of peat moss;
• persistent planting efforts (annually) over many years with strong commitment and support from company and staff;
• the unique design of parallel tidal channels protected by a berm of dredged material; and
• ensuring the right depth of the channels to provide appropriate tidal inundation for the mangroves.

Less successful aspects of the work included fencing to prevent ingress of plastic and debris into the mangrove plantations, planting of different mangrove species, and remote monitoring of the plantations through in situ (interval) camera systems, all of which were discontinued.

This case study demonstrates that planting of mangroves on dredged material is feasible, even under the extreme climatic conditions in the Arabian Gulf and offers a cost-effective nature-based solution for shoreline protection, being six times cheaper than sheet-piling or other hard engineering solutions, with added benefits for biodiversity conservation.