The day Wadi Dayqah roared

26 January 2011

In a country where waterfalls are almost unknown, the discharging spillway of the Wadi Dayqah dams in Oman was a dramatic spectacle. Saleh Hamood Al Harthy, M J Hieatt, J K Hall and M Wheeler describe how the dams have been tailored and tested by two major cyclones

The recently constructed Wadi Dayqah dams in Oman are set in desert terrain against a mountainous backdrop. Despite the arid climate both the start and completion of construction were marked by tropical cyclones bringing torrential rainfall and dramatic floods. The first cyclone, Cyclone Gonu, brought unprecedented rains and flooding to Oman. The second cyclone, Cyclone Phet, completed the filling of the reservoir within hours and resulted in the first discharge from the spillway on a scale that may not be seen again in our lifetime.

The Sultanate of Oman has few permanent surface water resources. Wadi Dayqah is unique in Oman in that there is some flow throughout the year and studies pursued by the Ministry of Regional Municipalities & Water Resources (MRMWR) in the early 1970s identified Wadi Dayqah as a potential water source. Its catchment area is approximately 1700km² and average annual rainfall within the catchment is 148mm. Much of this rain comes from thunderstorms that can give rise to flash floods. The wadi has for centuries been the source of irrigation from a system of aflaj, open irrigation canals fed from a diversion structure across the wadi. This ancient system is now being fed from the new reservoir and assured of a continuing supply.

Providing fresh water has long been a major challenge faced by Omani generations and aflaj are an important heritage illustrating the diligence and determination of the Omani people in building a civilization in a harsh environment. Aflaj rise from within the mountains, flowing down in channels and passing through the hills and to the plains. Some date back more than two thousand years, during which time Omanis developed special tools and means that enabled them to maintain these aflaj and create new ones to meet the growing subsistence demands and the development of agriculture, which has always been an important part of Oman’s economy despite being short on rainfall.

Dams have played an important role in the development of water resources in the Sultanate of Oman. Recharge dams have resulted in an ongoing improvement in the quantity and quality of water in some areas over recent years due to the water impounded in floods and held back to allow recharge of groundwater reservoirs. Surface storage dams in the mountainous areas have provided water to residents in the adjacent areas thus leading to a secured settlement of people in their areas, in addition to the agricultural, livestock and economic development within these communities.

The work on dams in Oman is under the Dams Department of the Ministry of Regional Municipalities and Water Resources. The Ministry has constructed over 30 groundwater recharge dams distributed in the various regions and governorates of the Sultanate with a capacity to capture over 1000Mm3 of flood waters. Further groundwater recharge dams are being constructed. There are more than 60 small surface storage dams in mountain areas with more under construction.

Wadi Dayqah dam is the Sultanate’s tallest dam and one of the region’s major water projects. It will provide water to supply the Governorate of Muscat. Construction of flood protection dams is also in progress: a major protection dam has been completed at Salalah plain in Dhofar Governorate and construction is currently underway on a protection dam in Wadi Addy to guard against the risk of floods in the capital, Muscat. The present construction projects are part of an on-going programme of dam construction studies and implementation in various governorates and regions.

Construction projects

The Wadi Dayqah project comprises an RCC main dam, rockfill saddle dam, a water treatment works, pumping station and water transfer pipelines to supply local towns and to supplement supply to the capital, Muscat. The contract for detailed design, preparation of procurement documentation and construction supervision of the dams and water supply scheme was awarded in early 2005 to a joint venture led by black-veatch International (UK) with NESPAK (Pakistan) and Su Yapi (Turkey).

The hydrological assessment carried out as part of the design derived a PMF of 18,400m³/sec. The spillway on the main dam was designed to contain the 1 in 10,000-yr flood within the width of the ogee crest, whilst the PMF would be accommodated with overspill over the full width of the dam.

The main dam is 75m high and 400m overall crest length, designed as a gravity dam and constructed of some 600,000m³ of roller compacted concrete. The dam is founded within the narrow limestone gorge through which the wadi flows. An ungated Roberts Type aerated spillway with 200m wide ogee crest and splitter teeth [1] extends over the central half of the dam crest. Physical model tests were carried out to confirm the configuration and dimensions of the spillway crest and downstream apron with scaled discharge rates up to 90m³/sec/m and a depth of over 10.5m over the crest. The adjacent saddle dam is nearly 50m high from base of core trench to crest, with a crest length of some 360m. A 2.1 km long grout curtain extends into the abutments and beneath the foundations of the dams. A fuller description of the construction has been published elsewhere [2].

Tropical cyclone

By June 2007 preparation of the rock foundation was under way when Oman was hit by tropical Cyclone Gonu. The cyclone brought unprecedented rainfall, widespread flooding and damage. One rain gauge on the edge of the catchment recorded 935mm in 24 hours. At Wadi Dayqah the cofferdams were washed away and the excavation filled with coarse alluvium. The peak flow at the site during Cyclone Gonu was estimated to be of the order of 10,000m3/sec; some 40% greater than the original 1 in 10,000-year design flood. The flood hydrology was revisited in the light of this event resulting in a revised 1 in 10,000 design flood of 13,500m3/sec, almost twice the original design figure [3]. The PMF was unaltered. The assessment of long return period floods in climates with infrequent but severe events and relatively short data sets cannot be an exact science and the results were cross-checked by comparison with global experience of extreme events using modified Creager and Rodier curves [4 & 5].

The selection of the design flood return period followed general UK practice for dams as this has historically been the norm in Oman, making the dam Category A. The spillway works were therefore designed such that floods up to the 1 in 10,000 year flood would be accommodated by the main dam spillway and floods in excess of the 1 in 10,000 flood and up to the PMF, will also spill over the dam roadway crest. Some damage will be accepted for PMF conditions, provided the works remained stable. Without any design changes the increase in the 1 in 10,000 year flood resulting from the re-assessment of the hydrology would result in spilling over the dam roadway on more frequent occasions (1 in 100 year flood), thus failing to meet the original design criteria.

The model tests on the 196m long central overspill indicated good hydraulic performances up to a unit discharge of 72.5m³/m/sec and with an unstable, but acceptable, performance up to the maximum unit flow tested of 84.2m³/m/sec. These corresponded to discharges over the central spillway section of 14,200 and 16,500m³/sec respectively. The latter case was the 18,400m³/sec PMF event with additional water passing over the dam road crest.

Options were reviewed to identify practical modifications that would accommodate the revised design floods, meet design criteria acceptable to MRMWR and be initiated during construction with least implications on project cost and programme.

The principal means available for increasing spillway capacity were:

• Increasing depth over the spillway by raising the crest roads on either side.

• Increasing depth over the spillway by lowering the spillway crest.

• Lengthening the spillway crest.

• Re-introducing an auxiliary spillway (part of the original tender design).

• Introducing gates, fusegates or fuse plugs to cater for extreme events.

Reducing the crest level of the spillway would reduce the storage capacity of the reservoir which was unacceptable to the ministry.

Whilst these investigations were being carried out construction continued, including casting the spillway apron and the start of placement of the roller compacted concrete for the main dam.

Following an initial review the ministry opted to follow a recommendation from their panel of experts for the main spillway to be modified to accommodate a flood of similar size to that experienced in Cyclone Gonu and for greater floods to overtop the full width of the dam. After viewing the various options put forward, the panel of experts suggested further consideration of the fusegate principal with a sacrificial pre-cast parapet in place of the upstream hand railing on the raised crest of the dam. The panel’s suggestion was adopted and developed into a counterbalanced pre-cast parapet which replaced the handrailing on the upstream side and could be accommodated within the restricted width of the original crest. This solution provided a practical and inexpensive means of containing the 1 in 10,000-year flood within the spillway whilst not overloading the dam in the PMF. It thus met the originally intended safety criteria in line with internationally established dam safety practice.

The final modifications comprised:

• Lengthening the main spillway crest and apron by 6m towards the right abutment.

• Raising the main dam crest roads and intake tower superstructure by 1.65m.

• Providing a pre-cast parapet wall in place of upstream hand railing.

• Adjusting curvature of the spillway crest to accommodate increased maximum expected depth [6].

• Replacing hand railing with solid concrete parapet on top of the left abutment apron wall.

• Raising the crest of the saddle dam by only 0.31m to maintain freeboard in the PMF.

Main dam stability

An increase in water level in the reservoir increases the loading on the dam. Initial analyses using the parameters used at the time of the original design indicated little scope for accommodating any increase in water load. Although the factors of safety were acceptable for normal operating conditions and 1 in 1000-year flood events, for the PMF the factors of safety for overturning stability fell below the acceptable target value of 1.20. For a PMF flood level of 183.41m, retained within the reservoir by raising the roadway of the main dam, the factor of safety against sliding fell below unity.

Assumptions, such as concrete characteristics, made at the time of design are necessarily conservative. With construction underway it was now possible to use the concrete characteristics for the material in place and to recognise the quality of construction that was being achieved. Check stability analyses for the design modifications therefore adopted an increased concrete density of 2.5 tonnes/m3 with an internal friction angle of 45° and cohesion of 600kPa, and a residual effective cohesion for the concrete in sliding of 100kPa. Separate analyses carried out for the crest road cross-section and for the spillway overflow cross section showed that the cross-sections proposed in the design modifications met the original design criteria for factors of safety under the various defined load conditions.

Design and testing

The counter balanced parapet wall was introduced to contain the 1 in 10,000-year flood, but topple over in the event of significantly larger flood.

The wall was designed to tip when the upstream water level was overtopping the wall by 25 to 30cm, although in practice it will be adequate for the wall to tip with the upstream water level between the top of the wall and 1m above.

A 0.5m wide full scale unit was constructed of reinforced concrete and set up in a purpose built flume to test the theory. The prototype tests demonstrated that the wall units will function as required. Bedding the wall on mortar (as intended by the design) had a significant effect on the water level required to tip the unit. Without a mortar bedding (or other such seal) uplift below the wall made the wall tip prematurely.

The proposed modifications were selected so that they could be constructed using the same materials, methods, equipment and tower crane layouts as were being used to construct the original design.

Water in the wadi had been diverted into the diversion culverts in November 2007 allowing construction of the dam to proceed. At the end of June 2008, when the revised design proposals were finalised, the RCC in the main dam had been placed to approximately 20m above the foundations and intake tower to almost full height. The final pour was on hold pending confirmation of the main dam crest level.

At the downstream toe of the main dam the apron was still under construction. The only area of conflict between the design modifications and the works constructed to that date was at the toe of the right abutment of the main dam. This was readily overcome by relatively minor modifications to the 17m high wing walls. Concreting the body of the main dam was completed in May 2009, six weeks ahead of programme.

First major flood

Full impounding began with the closing of the second diversion culvert in September 2009 The wadi bed level is about 110m asl, the bottom outlet level 127.5m and the spillway crest 172.5m. By February 2010 the reservoir level had risen to 136m asl. The first significant rains then fell and the level rose to 154m in early March and 157m by early May.

During the week commencing 31 May there were warnings of potential heavy rainfall associated with Tropical Cyclone Phet, which was tracking up from the Indian Ocean. A decision was taken by the ministry to open the bottom discharge valve in readiness for a large inflow. The first rains of Cyclone Phet fell at the site in the afternoon of 3 June and continued until about mid-night on 4 June. The rain gauge at the dam site recorded a max 24hr hour rainfall of 423mm and total event rainfall of 450mm at the dam site but it must be stressed that these figures come from a single rain gauge which cannot be held to be representative of the whole catchment.

Run-off was very rapid and available records show the reservoir level rising from 158m asl to a peak of 177.75m asl in approximately six hours. The reservoir level reached spillway crest level for the first time at 9.30am on 4 June and continued to rise. By that time power was lost to the site and the electronic level instruments failed.

Members of the MRMWR Wadi Dayqah dam monitoring team were present at the site throughout the event and made valuable observations, notes and photographic and video records.

The first meaningful flood experienced by the Wadi Dayqah dams resulted in a simultaneous first filling and first major discharge over the spillway. One estimate (based on intermittent readings indicating the rate of rise in reservoir level) has put the peak flood flow entering the reservoir as high as 9000m3/sec although it was probably less than this upper bound figure and closer to the peak outflow. By the time the reservoir reached its peak level the discharge over the spillway was 5200m3/sec as assessed from the rating curve; this discharge is equivalent to between a 1 in 25 and 1 in 50 year event under the revised design hydrology established in the wake of Cyclone Gonu.

A newspaper article reported that ‘In what local residents have described as a humbling experience, raging flood waters flowed a staggering six metres above the crest in an unremitting thunderous roar that lasted several hours.’

Rainfall at the site during Cyclone Phet actually exceeded that reported during Cyclone Gonu by 20% and is estimated to have been greater than a 1 in 200 year rainfall event. However by observation, in the catchment as a whole, rainfall during Cyclone Phet was clearly less than in Cyclone Gonu. Three days after the cyclone, the dams were inspected by representatives from the designers, the contractors, the MRMWR and its panel of experts. The structures of the main dam and the saddle dam are considered to have performed satisfactorily under first filling and during the first meaningful flood.

Seepage from the foundation and abutments of the main dam with the reservoir full followed a similar pattern to that recorded in inspections in March 2010 when the reservoir was half full. As would be expected, seepage flows increased under full reservoir head but were still acceptable and less than predicted following the March inspection.

After Cyclone Phet, seepage flows at the main dam have generally decreased as the reservoir level dropped but an area of concentrated seepage at a jointed limestone outcrop on the right abutment of the saddle dam is being investigated with a view to some further grouting.

Even for engineers accustomed to seeing dam spillways in operation, the sight of the spillway at the peak of the flood during Cyclone Phet left a lasting impression. For the population in a country where waterfalls are almost unknown, in the days following Cyclone Phet, the discharging spillway was a dramatic spectacle and a popular attraction.

An Oman newspaper headline declared: ‘When Wadi Dayqah roared’ and reported that ‘the dam not only held up admirably to Cyclone Phet’s fury, but it also transformed into a breathtakingly striking waterfall attraction, the likes of which Oman has never witnessed before.’ [7].

Saleh Hamood Al Harthy, Director of Dams Department Ministry of Regional Municipalities & Water Resources, Oman; M J Hieatt, Black & Veatch International Ltd, Redhill, UK; J K Hall, Black & Veatch Ltd, Redhill, UK; and M Wheeler, Black & Veatch International Ltd, Chester, UK

The authors thank Under Secretary HE Ali Mohammed Al-Abri and his staff at the Ministry of Regional Municipalities & Water Resources of the Sultanate of Oman for their generous and friendly support throughout the design and construction of this project.

The authors wish to acknowledge the technical and managerial efforts of joint venture contractor Vinci Grands Projets (France) and CCC Oman: they completed construction to a high standard throughout in a remote location and under severe climatic conditions.

The authors also acknowledge the support of Black & Veatch and its joint venture partners NESPAK of Pakistan and Su Yapi of Turkey throughout

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