Matrix gives answers on beach project

In a bid to protect Dubai’s coastline from continued erosion, Dubai Municipality is working on a beachfront protection scheme. Zoe Naylor learns how a soft engineering matrix approach is paying off for DM’s Coastal Management Section.

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By  Zoe Naylor Published  March 18, 2006

|~|113proj200.gif|~|Work continues on the Jumeirah Open Beach protection project. The US $27 million job has been engineered using an objective decision-making matrix, taking into account environmental, social and economic factors.|~|In a bid to attract a growing number of tourists and provide for local residents, Dubai is capitalising on its natural assets of sun, sea and sand. But what happens when the sand element of this holidaymakers’ trinity is prone to severe erosion, particularly on Jumeirah Open Beach, one of Dubai’s premier beach destinations? “While the Arabian Gulf around Dubai may seem a fairly benign water body, the winds that run along the length of the Gulf during storm events can result in some fairly significant waves — up to 5m high in extreme cases,” says Dr. Gary Mocke, marine expert, Dubai Municipality Coastal Management Section. Dubai’s 70km-long linear coastline is also subjected to a natural sediment transportation system that moves from Abu Dhabi to Sharjah. This, combined with infrastructure developments related to trade, fishing and tourism have gradually taken their toll on the natural coastal processes of waves, tidal currents and sediment transport. And in many cases — as with the stretch of beach running from Jumeirah fishing harbour to Dubai Marine Beach Resort (DMBR) — it has led to coastal erosion. This particular section of the Jumeirah coastline has gradually been eroded over the years, says Dr. Mocke. “Historically there was a beach here, but because of the sand-trapping effects of the fishing harbours over the years, the sand supply to the beach on the north side has been slowly cut.” In the early 1990s the decision was made to put in structures to hold the beach in place. After these structures were put in, they created embayments. But pinch points also formed in between the embayments i.e. areas where the sand is redistributed to the two sides of the structure. The current phase of the Jumeirah Beach protection project began in 2000, the same time as DM’s Coastal Management Section (CMS) was established. Although DM recognised that a sustainable solution was required for the area, an initiative was launched by the CMS to come up with an interim measure to mitigate the pinch point effect until a longer-term solution could be found. “We revisited what the engineering consultant had previously come up with — but we found that it was high in cost with many negative impacts. We also didn’t want to put in anything permanent because we knew the central area would have to be properly stabilised, so we put in soft rock technology,” explains Dr. Mocke. This consisted of sand-filled geotextile bags with a vandal-resistant outer layer that can be coloured to resemble sand. The sand adheres to it so it blends in with the beach — people can sit and walk on it. “We saw this as an intermediate measure until we came up with the final solution.” Along with engineering consultants Mouchel Parkman, DM carried out studies to gauge the effects of different structures and the beach’s response. Extensive numerical modelling by DM and DHI consultants was complemented by physical modelling carried out in laboratories at HR Wallingford near Oxford in the UK, to study wave penetration and circulation within harbours and marinas. The laboratory models involved building replicas of proposed coastal structures to a scale of 1:35. “For the very complex shoreline response modelling, anthracite (ground coal) was used, which is the closest thing you can use when you scale down to replicate sand.” A vast amount of survey data was gathered over time, including wave, current and sediment transport conditions of the stretch of coastline. Much of this data is available to the general public in real time via the CMS web site www.dubaicoast.org. The modelling study was structured in such a way that the laboratory and computer-aided models worked together to complement one another, and to support the decision-making process. A variety of solutions were proposed, including designing a major breakwater to protect the central point of the beach; increasing the length of an existing breakwater; building a new breakwater and adding revetments; and adding a submerged reef so waves would dissipate over the top of it. “In total, over 40 options were put forward and we narrowed this down to 22 sensible ones. Proposals came from a variety of sources — including the police and rescue services — and we held workshops with the consultants,” explains Dr. Mocke. One of the biggest challenges was to choose not only the most appropriate option, but also the one that would also provide maximum cost-benefits. To achieve this, DM came up with an option evaluation matrix approach, an objective decision-making tool designed to refine and select the best solution. Each of the concepts was fed into the decision matrix and then compared to the baseline i.e. the existing situation. The concepts were then scored against a number of characteristics including sedimentary — does it hold the beach?; structural stability — will it stand up to wave action?; and safety for swimmers — how strong are the currents? Dr. Mocke says that the tool has also helped to cut costs in the crucial design-optimisation stage of the project: “The design they had in the 1990s cost around US $46 million (AED169 million) for marine structure works only, whereas going through the matrix process we were able to reduce the cost significantly to stand at less than $27 million. The extensive testing gave us the confidence to reduce both the length and height of breakwater structures, which offered visual as well as cost benefits.” After lengthy evaluation using the decision matrix, eight of the 22 options proceeded to the physical model testing stage, and two options were eventually taken to final testing. But before a decision could be made on the final option, DM and the project team had to factor in the announcement of Nakheel’s World development. “By the time the World project was announced, our design was already at a relatively advanced stage, so we then had to model the structure from Nakheel’s planning layouts taking into account where it would be and how the waves from it would affect the coast.” While Dubai’s offshore developments have prompted speculation that they are contributing to coastal erosion, Dr. Mocke says this is not necessarily the case. “The natural Shamal wind direction in this area is from the north-west, and over time the beach tries to obtain an equilibrium configuration that is perpendicular to the dominant wave direction. Everywhere along the coast the beaches exposed to the open Gulf try to obtain this orientation, with the rate of coastal erosion being related to the amount of shoreline deviation from the equilibrium shape. But Nakheel’s developments have actually reduced a lot of that movement because they are sheltering the shoreline from direct wave action.” He says there may be some localised effects towards the sides of the wave sheltering zones where the sediment pattern may have been changed, but much of the action has been stabilised because the developments act as massive breakwaters. “Through our monitoring programme and modelling we have a good handle on what’s happening here, allowing us to implement coastal management measures that anticipate the predicted future evolution of the coast,” adds Dr. Mocke. After extensive modelling and number crunching, DM and Mouchel finally decided upon the best solution for the 1.5km stretch of Jumeirah Open Beach. Phase one involves a combination of marine construction, including using the existing breakwater, shortening it and adding horn-shaped structures to it to help hold the beach in place. At the northern end of the Open Beach development, main contractor Overseas AST is extending the existing DMBC breakwater by 200m, which will make it around 450m long in total. Part of the original structure had to be demolished before work could begin. “We’re at the final stages of protecting the head of this breakwater with concrete armour units,” says Matt Thompson, project manager, Overseas AST. Gabbro rock from Fujairah is being used to form the basis of the structure. The next section is the ‘T’ breakwater, which will remain unchanged, but will have some landscaping and rehabilitation work done, including the addition of an upmarket themed restaurant on the water’s edge. “The following stage of marine construction work is the 300m-long breakwater sitting offshore,” says Thompson. “Horn-shaped extensions are being added to this either side which will take it to a total length of 480m. The main section of this is completed and we’re doing the external facing around the corners. But the actual breakwater structure itself is operational,” adds Thompson. The final part of phase one is to add nourishment sand into each of the embayments. This means placing over 1 million m3 of beach sand to create the desired beach widths. This has proved to be a challenge, since there is a massive shortage of beach-quality sand along the coast of Dubai. “It’s very important to get the right texture and colour,” explains Dr. Mocke. “To date we’ve been getting the sand through the non objection certificate (NOC) permitting process for any construction work within one km of the coast.” This means that if contractors remove sand during excavations for tower basements, for example, they cannot simply dispose of it since the bulk of it is good quality beach sand. Previously this sand could be sold, as it fetched a very high premium for finishing and plastering if washed. “But now, through the regulations we have in place, there is a government decree that the sand cannot be disposed of and a contractor has to bring it to where we demand it be put on the coast,” says Dr. Mocke. Much of the sand over the past year has come from the Dewa excavations near Jebel Ali, and has been taken to DM’s Jumeirah site via trucks. To date, around 340,000m3 of sand has been used to replenish the beach, and the marine construction work is around 85% completed. The project is now gearing up towards moving into phase two, which is the landscaping and cosmetic stage. This will include paving for the promenades, food courts, sports facilities, a skate park, changing rooms, a beach market, an amphitheatre for open-air activities and a jogging track. “Phase two will kick in as soon as the marine work is finished,” says Dr. Mocke. “It is in final design stage at the moment and should be out to tender later this month.” As Dubai races ahead with its expansion plans, the Jumeirah frontage in particular is likely to play a key role in attracting tourists to the emirate. Jumeirah Open Beach already attracts 1.2 million visitors a year and is currently the only public beach with facilities that people can visit for free. A rapid increase in tourist numbers — coupled with a projected 300% increase in population over the next 20 years — means the emirate’s coastal resources are likely to face massive pressures. By implementing the matrix approach, DM and the project’s consultants are going a long way towards optimising and protecting one of Dubai’s best natural assets.||**||

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