To ensure Sur in the sultanate of Oman can rely on better flood protection in the future, over the past few months one of the largest valley dams in the country – the Fulaij Dam – has been constructed just outside the city.
From the beginning of 2014 STRABAG International GmbH implemented this immense project on the Arabian Peninsula. The company brought in another renowned Austrian business, Braun Maschinenfabrik, as experts in the provision of hydrotechnical steel structures. Braun supplied the components for the dam required to control the run-off of stored water after flooding. The Fulaij Dam went into operation officially mid-April this year.
Floods and inundation are probably not the first thing people associate with the sultanate of Oman on the eastern side of the Arabian Peninsula. The country is actually located in a semi-arid region, which means protracted stretches of dry weather are periodically punctuated by phases in which more rain falls than evaporates. With this in mind, it is understandable that water is a precious commodity in a country like Oman. Hence, over the past decades the government has invested heavily in the construction of reservoirs and dams to allow rainwater to be directed to – and stored in – dry valley ‘wadis’. In parallel, valley-mouth dams have been built to protect the cities from repeated flooding. The main causes of such inundation are offshoot storms of monsoon rain that cross the Indian Ocean and meet the coastal mountains in Oman, where they cause voluminous precipitation and, consequently and not uncommonly, extreme flooding. In 2007 in particular, Oman was hit by devastating flooding caused by the arrival of cyclone ‘Gonu’ on land. Meteorologists ranked it as a once-every-500-years flood. Not only did it cause millions of dollars of damage, it also claimed several lives. This natural disaster, and cyclone ‘Phet’, which hit Oman in 2010, convinced the officials to invest even more mindpower, manpower, material and money in the realisation of flood protection projects.
Complex protection system
The 70,000-inhabitant city of Sur, not far from the easternmost point of the sultanate, was severely affected by ‘Gonu’ in 2007. Back then, large areas of the city were completely inundated. Based on a viability study conducted in 2009 and 2010, the plan to build the new Fulaij dam in the location of a former ground water supplementation dam was developed to avoid suffering devastation on a similar scale in the future. According to details provided by Dr. Reinhard Schmid (Ing.) of STRABAG International, the concept foresaw a system of four (and later just three) upstream dams and the widening and strengthening of the riverbed in the city. This should lead to the volume of water flowing through the city being reduced from 1800 m³/s to around 800-900 m³/s – that is, by roughly a half. The Fulaij Dam is to be the largest and most powerful defence for the city of Sur, serving a catchment area of 684 km². Before the remaining dams are completed it will be able to protect the city from a once-in-200-years flood. Subsequently, the system should be able to deal with the scale of flooding that occurs once in 500 years. The solution was a 1.2 km long earth-fill dam and a 230 m concrete structure for controlled water release. The mammoth project was awarded to STRABAG International GmbH and was implemented by their subsidiary in Oman.
Concrete structure with steel gates
According to Dr. Reinhard Schmid (Ing.) the main structure consists of a 23 m high earth-fill dam sealed with an inner tarmac core, a milled concrete diaphragm wall and water-side embankment reinforcements made of hydrotechnical armour stones. The earth-fill dam can be flooded via the concrete crown and the linked-up underwater gabion steps along around 700 m. It is estimated this infrastructure could even withstand the kind of flooding that occurs once every 10,000 years without being damaged.
The concrete structure is 230 m wide and was built on the left-hand side of the valley, where all the necessary plant control components can be found. The main part of the system is considered to be the stepped concrete floodwater discharge solution with a run-off section 140m across and designed to deal with a maximum flow volume of 3900 m³/s. The release gate structure has a key function. It is fitted with 4 hydraulically driven, wheel-mounted steel doors that are 4.6 m wide and 3.6 m high, as well as a dam base water discharge system with a further two smaller slide-in protection units serving as shut-off valves. These components were manufactured in the Austria town of Vöcklabruck at the Braun Maschinenfabrik. They produced the entire hydrotechnical steelwork needed for the Fulaij Dam, the steel shut-off gates, above-the-surface and underwater protective plating, and all the hydraulics and controls. This was an exciting and challenging project for the Upper Austrian experts in steel building for dams, whose solid gates and tried and trusted steel hydrotechnical structures are in demand far beyond the Austrian borders. These Austrian steel builders have developed their reputation on their spirit of innovation and the provision of unsurpassed quality.
Components from Upper Austria
Before components were installed on site they were sent to Artelia Consultants’ hydraulic test lab in Grenoble for physical testing to guarantee their functionality required for the solution planned. STRABAG’s plan proved to be both functional and efficient. Overall technical supervision of the project on site was the responsibility of Artelia Consultants. “Artelia technicians also visited our factory and took the opportunity to see the manufacturing process at close quarters. Braun Maschinenfabrik is now recognised by Artelia as a certified manufacturer”, Braun’s Head of Sales Thomas Oberanzmair said. As an experienced industry insider he is completely familiar with every challenge the contract poses, having been involved in the project from the very first discussions in 2013 to the moment the system commenced operation. Initially he made two advance trips to Oman to clarify technical and financial details. At the end of 2015, STRABAG Oman L.L.C. awarded the contract for the provision of dam steelwork to the Braun Maschinenfabrik. “The components were built in our works in the spring and summer of last year, then transported to Oman on a freight ship. Our assembly team began work on the protective plating and reinforcements last autumn. Subsequently, the dam gates were delivered this February and installed over the period up to the end of March. The plant went online on the 11th April this year”, stated Thomas Oberanzmair. The assembly team showed its vast experience as installation work ran smoothly and was completed on schedule, despite the extremely hot and humid conditions.
Challenge to be 100% ready-for-action
Guaranteeing 100% technical and operational availability for the hydro-plant steel structures made the task even more challenging – mostly due to the extreme weather conditions. Temperatures around the city of Sur can vary between -10 and +60 degrees Celsius. Sandstorms are a common hazard and the air is very salty, since Sur is located at the edge of the Gulf of Oman. Thomas Oberanzmair stated: “It’s important to realise that sometimes it doesn’t rain for three years, but when the rain does come the entire set-up must be in complete working order. Alternatively, it may be that the water gates need to be operated in short intervals.” For these reasons the components had to be specially designed and equipped for the task. The wheel-mounted gates were fitted with a special irrigation system to ensure the sealings were dampened before being put into operation. The high temperatures outdoors necessitated the implementation of a special hydraulic oil cooling unit. The head of sales at Braun Maschinenfabrik also explained: “Ease of maintenance was another key goal for those responsible for the project, so a special maintenance room was integrated into the infrastructure.”
A milestone for Sur
STRABAG Oman L.L.C. worked on the implementation of this mammoth project from the beginning of 2014. According to Dr. Reinhard Schmid. (Ing.), the project required excellent communication and a large degree of flexibility in terms of construction work, particularly with a view to the risk of flooding during the building phase, but also due to the proximity of the building operation to existing buildings, machinery and local public infrastructure. The entire volume of excavation for the 1.2 km valley dam encompassed around 1.2 million m³ of loose earth and 300,000 m³ of rock. In total the project used up 180,000 m³ of concrete and 7,000 metric tons of steel. In order to broaden the riverbed in was necessary to excavate around 3 million m³ of loose earth and install gabion mattresses to secure the channel banks along 2.3 km through the heart of the city. The new Fulaij Dam and, most importantly, the 6 km of riverbed expansion through the city, required precisely detailed planning, immense flexibility and sophisticated, well-coordinated publicity.
The concrete required for the construction of the dam was produced in a concrete mixing plant situated just 300m from the site. “The coarse-grain concrete produced by the plant wasn’t easy to pump to the site, so it was mostly lifted to the sections in which concrete was needed using mobile cranes and large-scale concrete vessels. Because of the high daytime temperatures, it was necessary to pour the concrete into moulding structures at night”, explained Dr. Reinhard Schmid.
When the Fulaij Dam went online this April it was a moment of great significance for the city of Sur, which is now far better protected from the consequences of violent floods. The city’s inhabitants no longer need to be fearful of what could happen when torrential rain comes. From now on, cyclones of the category of Gonu will pose a far lesser threat than was previously the case.