The Merowe Dam was designed to serve several purposes, namely:  the generation of electricity from its 1250 MW hydropower station; the development of centralized agricultural irrigation schemes (about 300000 ha); and, the protection of the Northern State against devastating high floods of the river Nile.

Signing of the Civil Works Contract: Presidential Palace 2003

Furthermore, the dam will act as a sediment trap, reducing sedimentation at the Aswan High Dam further down- in Egypt.

 Because of the topographical conditions at the dam site, the project features a dam with a total length of about 9.3 km, which consists of the major Dam structures as shown in Table below.

 

The dam structures

Dam  structures

Length (m)

Height (m)

Right  bank dyke

445

15

CFRD - Right  bank

4315

53

Spillway dam

154

67

Non Ėover flow dam

90

67

Power intake dam

300

67

Earth core rock fill Dam  

883

67

CFRD -left  bank

1590

50

left bank dyke

1523

18

The powerhouse, which will accommodate the ten power generating units, was arranged at the downstream toe of the power intake dam. Irrigation outlet structures are provided on the right and left banks to release water for future irrigation developments.

The river diversion scheme:

In view of the very high flood potential of the River Nile during the period from August to October, the diversion of the Nile during the entire construction period of five years represents a very challenging engineering task. A natural two-steam flow regime exists at the project site, namely the main channel (left channel) and the secondary channel (right channel), which were used for the river diversions during construction. The following two river diversion stages were implemented:

Stage 1: Closure of the right river channel and diversion of the Nile through the left river channel over a period of two years (from January 2004 to December 2005).

 Stage 2: Closure of the left river channel by the end of December 2005, and diversion of the Nile through the right river channel until the start of reservoir impounding. The partially completed spillway located in the right river channel was used to discharge the river flows into the tail waters.

On 30 December 2005, one day ahead of the contractual milestone, the 800 m-wide left river channel was successfully closed and the Nile was diverted though the partially completed spillway. During the flood season of 2006, peak flows of close to 11 000 m3/s were recorded, representing a flood with a return period of about 10 years. Upstream of the structures, the water level rose up to el. 259, about 5 m below the crest of the upstream cofferdam and power intake dam. The river diversion scheme has been designed to discharge the 100 year flood safely, featuring peak flow of 13 200 m3/s.

 The spillways:-

As described above, the partially completed spillways were used for river diversion over a period of almost two years. Simultaneously, with the diversion of the Nile through the structure, the individual spillway piers were constructed up to el. 303, and the opening between the piers temporarily closed by steel stop logs to allow for construction of the bottom outlets and the surface spillways. The design, construction and completion of the spillway, as regards the river diversion aspects, may be described as one of the most challenging engineering tasks, as hydrology, hydraulics and construction progress of all structures have to be fine-tuned to minimize the risk of overtopping.
A total of more than 25 major construction stages will be required to close the temporary openings of the spillway finally.

The spillway structure is about 67 m high, and is accommodates 12 bottom outlets and two surface spillways. Each of the bottom outlet openings is 6 m wide and 10 m high, with an invert set at el. 264.0. Two surface spillways are completed, one to the left and one to the right of the bottom outlets, each 15 m wide and with a crest elevation of 280.5. Both the bottom outlets and the surface spillways are equipped with radial gates.
 

The huge construction equipments                      Progress of concrete works

At the maximum reservoir water level of 300, each opening of the surface spillways and the bottom outlets discharge 2271 m3/s and 1292 m31s, respectively, resulting in a combined discharge capacity of 20,046 m3/s, slightly above the required discharge of 19, 900 m3/s. The hydraulic performance of both the surface spillways and bottom outlets was confirmed by hydraulic model tests on a scale of 1 to 40, which were conducted by the University of Innsbruck, Austria. In addition to the discharge capacity of the spillways, a further 1500 m3/s can be discharged by  the six low level sediment sluices which are arranged below the power intake opening.

The spillway structure is protected against scouring by a 13m deep concrete key and a 30 m-long and 2 m-thick steel-reinforced concrete slab. The left and right down stream retaining walls are constructed on top of secant pile walls. This massive protection was chosen mainly in view of the concentrated flow during the stage 2 river diversion.

 The Earth Core Rock fill Dam (ECRD):-

The main dam of the Merowe project is a classic earth core rock fill dam (ECRD) with a central earth core (zone 1), fine and coarse filters (zones 2 and 3) and upstream and downstream rock fill shoulders (zone 4).

For the construction of the cofferdams, random rock fill (zone: 7) was used. The ECRD is founded on alluvial sediment, which are up of 30 m thick. To avoid seepage underneath the layer, a one m-thick plastic concrete cut-off wall is provided, which penetrates about 4 m into the bedrock. On top of the cut-off wall a cushion of highly plastic material is provided, to avoid stress concentrations in the wall and cracking of the core

To achieve the required factors of safety, the upstream and downstream shoulders were sloped at 1V:2H and 1V:1.8H, respectively. With a total length of 883 m- and a height of 64 m, the ECRD has a total volume of 8.25 x 10 m3 . The upstream and down- stream cofferdam were designed to protect the construction pit against the 100 year flood.

Completion of work on the spillway bridge     

  On 12th Dec. 2003, the Riverís course was diverted to the right side, in preparation for construction works at the River bed. Since that day the Nile course has been made to pass on the left side of Merowe Island.

  A temporary sand-made wall was built within 48 hours of continuous work to block the Nile path completely. Then the temporary assisting barrier was built, and the water was pumped out from the area between the two blockages, and the area was dried out to make  possible for the construct work to start.

  During the period from 5-8 August 2004, a delegation from the Hydro-project Institute reviewed the excavations for the Dam's basement. The High Technical Consultancy Committee was also briefed  on the preparations to start the Concrete Part.

  On Tuesday 9th Nov. 2004, the Contractor started to pour the first concrete mix in the Damís Body.

 Installation of turbine housings inside the concrete dam

4.6 The Concrete Faced Rock fill Dams:-           

Concrete faced rock fill dams (CFRDS) were selected on both the left and right banks for economic reasons. Because of the prevailing topographical conditions, the CFRDs extend some 1590 m to the left and 4315 m to the right. The zoning of both CFRDs is conventional, consisting of a transition zone, fine, coarse and, and random rock fill. To facilitate the compaction of the transition zone, and the construction of the concrete face slabs extruded concrete curbs were placed throughout, which proved to he very beneficial. The upstream and downstream slopes are inclined at 1V:1.3H and lV:I.5H, respectively. The total volume of both CFRDs is 5.3 x 10 m3.

The concrete plinth was founded mostly on moderately weathered to sound rock, which required excavations to a depth of between 3 and 5 m. Seepage control was achieved by a single row grout curtain with an average depth of about 20 m. Because of the lengths of the plinths, slip forms were used throughout.

Of particular interest is the interface between the ECRD and the CFRD, which required the design and construction of a 48 m-high concrete interface structure.

Towards the ECRD, this structure is sloped to provide a watertight pressure joint between the core and the concrete wall. Towards the CFRD, the slope of the structure becomes gentler (1V:0.4H) to minimize differential settlements along the parametric joint.

The power intake dam:

The power intake dam is a steel-reinforced concrete structure 300 m long and 67 m high, which accommodates ten power intakes. Each of these intakes is designed for a rated discharge of 300 m3/s, and is equipped with a trash rack, stop logs and a submerged roller gate with a 8.5 m clear width and 10.5m height, Downstream of the roller gate, a transition zone conveys the water from the rectangular gate opening to the circular cross-section of the penstocks. The penstocks have inner diameters of 8.5 m, with a steel thickness of 18 to 32 mm, depending on the head. They are fully embedded in steel reinforced concrete. The so-called GIS Building, which accommodates the 500 kV-GIS switchgear, is placed on the downstream slope of the power intake dam.

Originally, the power intakes were designed for a minimum reservoir water level of el. 290, which was lowered at a later design stage to 285 m, to allow for operation during extremely dry years. This design adjustment called for extensive hydraulic model test at a scale of 1:40, to study the required submergency to avoid vortex formation and air entrainment.

From the structural point of view, the power intake dam could not be dimensioned as a conventional concrete gravity dam. The 11.5 m-wide and 13 m-deep blockouts for the penstocks changed the structural behaviors of the concrete dam significantly, and required the introduction of steel-reinforced shear walls between the penstocks to transmit the loads into the rock foundation safely.