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Lessons from Dubai

At the invitation of Cox Richardson Architects and Context Landscape Design, both of Sydney, Simon Leake visited Dubai in September 2008 to advise on the Dubai Waterfront Islands project, which involves the construction of five offshore island groups by dredging.

The Arabian Canal

In recognition that the supply of oil will one day run out, Dubai is creating a business and tourism hub based along the Persian Gulf coast. One current project is the Arabian Canal development, in which a 150-m-wide, 6-m-deep canal is being dug into the desert sands of Dubai, penetrating the interior for many kilometres. The canal will track inland in a U shape from the Dubai waterfront, effectively creating an island. During the construction, over a million cubic metres of sand is being shifted every day.

The Waterfront Islands

Five island groups laid in a crescent offshore and linked to the mainland by causeways are being built. The construction technique relies on a perimeter rock revetment wall infilled by sand.

Completed projects include Palm Jumeirah, a series of islands shaped like a palm tree, and a map of the world (see photo).

The Waterfront Islands will be developed into residential communities, luxury marinas, parks and business centres. As landscaping these islands is a major challenge, Simon was asked to join the team and advise.

Soil resources

Because the dredged marine sand is too coarse to hold any moisture, and the sand excavated from the canal is saline, the normal solution has been to import "Sweet Soil", a fine dune sand, at great cost. Simon found a better solution.

The Sweet Soil is essentially sand and nothing else. Simon’s analyses showed that the canal sand, on the other hand, has an appreciable silt and clay content (around 5%), and can hold more water and nutrients than the Sweet Soil.

However, the canal sand is saline, and so cannot be used in landscaping without desalinating – which needs water. So Simon came up with another low-cost solution.

Water-efficient desalination

Using a technique he used at Bond University in the 1980s, which was in turn based on a technique he remembered from an undergraduate project at university, Simon knew he could desalinate sand in a single irrigation. In the lab he set up an experiment using the canal sand, and found that just one infiltration event was enough to desalinate the profile to a depth of 600 mm, removing most of the salt from the plant root zone. The NaCl content of the sand dropped from about 2000 mg/kg (that’s 0.2%) in untreated sand to about 100 mg/kg in the surface layers. Test plants germinated without problem. All this used just 60 mm of irrigation water.

The net result of using on-site canal sand and water-efficient desalination should translate into considerable cost and greenhouse gas savings for the Waterfront Island landscaping project.

Landscaping the desert

The Dubai coast is a desert environment based on wind-blown sand. It has distinct, highly evolved soils and associated vegetation types that are unique to the desert environment. Even in this dry environment the surface soils are not necessarily saline. Simon identified natural soil landscapes that support nine vegetation types, ranging from beachfront, mangroves and foredunes to hind dunes, shrubland, wadis and oases, and used this analysis as a basis for the landscape design process.

Wadis

One critical feature of the desert environment in Dubai is the wadi. A wadi is a drainage basin that funnels occasional rainwater into natural storage in the sand. The sloping land around a wadi considerably increases the effective rainfall, capturing enough water to support surprisingly abundant natural vegetation, including trees, notably the ghaf (Prosopis cinerea) (photo below) and the date palm (Phoenix dactylifera), with no artificial irrigation. This is remarkable considering that the rainfall is less than 80 mm a year and the potential evaporation is close to 2000 mm! Australia has similar environments.

Artificially constructed wadis can be used to provide water infiltration and storage for landscaped areas. Capping with pebble or cobble mulch allows rapid infiltration without erosion or evaporation, and the fine sand beneath holds the water in plant-available form.

Further reading

http://en.wikipedia.org/wiki/Arabian_Canal

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