Behind the scenes of the Kashimbila Multipurpose Dam

Behind the scenes of the Kashimbila Multipurpose Dam

The construction of the Kashimbila Multipurpose Dam has had a significant positive impact for Nigeria and is one of its key showcase presidential projects. The benefits to the local community have been immense and range from job creation to potable water supply, electrification, and an improved transmission network.

Kashimbila Multipurpose Dam at full flood Kashimbila Multipurpose Dam during construction

The preliminary design of the Kashimbila Multipurpose Dam on the Katsina-Ala River in the Taraba state in Nigeria was initially focused solely on the dam’s functioning as a buffer in case the natural embankment of Lake Nyos in Cameroon failed.

Lake Nyos is a volcanic crater lake around which the ash embankment has been moving and settling. Should the embankment fail, the extreme flooding that would follow would endanger the lives of a significant portion of the people in south-eastern Nigeria. As the design process progressed through a more holistic and multipurpose approach, the capacity of the dam was increased to provide irrigation and potable water to the surrounding towns and villages, as well as for the generation of hydropower.

The Nigerian government appointed SCC (Nigeria) Ltd to design and construct the flood protection dam, which was subsequently changed to provide a multipurpose use. SCC appointed Zutari to review the concept design of the spillway proposed by the Federal Ministry of Water Resources before later appointing them to conduct the detail design of the dam.

Following discussions with the various stakeholders, Zutari proposed several optimisations to the project, which resulted in a safer, more affordable, and better optimised solution for all parties. These included an increase in the capacity of the proposed 6 MW hydropower generation to 40 MW, as well as optimised routing of the transmission line to improve the electricity network and distribution in eastern Nigeria. The full project included: • a 36 m high composite dam (clay core rockfill and concrete gravity spillway) • a 40 MW hydropower station and associated substation capable of releasing 260 m3/s (four units, each with a rated flow of 65 m3/second) • the Kashimbila substation equipped with two 30 MVA triple winding transformers to connect the four generators as well as two 15 MVA for the local rural electrification supply • a 132 m long bridge to connect the two riverbanks • outlet works for the irrigation supply, potable water supply, and river releases • 210 km long double-circuit 132 kV transmission lines with OPGW with lattice towers • 45 km long double-circuit 33 kV transmission lines with OPGW with lattice towers • 45 km of 33 kV reticulation network supplying

multiple small towns and villages in the surrounding areas with electrical supply • two new 60 MVA 132/33 kV substations • two new 15 MVA 33/11 kV substations • rehabilitation and upgrade of the 132/33 kV

Yandev substation where the new Kashimbila line interconnected to the national transmission network • development of a 2 740 ha commercial irrigation scheme comprising centre pivots and flood irrigation.

PROJECT TEAM

Ultimate client: Nigerian Federal Ministry of Water Resources and Federal Ministry of Power Zutari’s client: SCC (Nigeria) Professional team: Zutari Main contractor: SCC (Nigeria)

Optimising the project

Zutari’s innovative, creative and out-of-the-box thinking led to several changes in the initial concept of the project. These changes resulted in a multipurpose project that better serves the people of Nigeria and the communities in the area of the project.

From the inception of the detail design in 2012, Zutari used 3D design to improve the quality of design and presentation to the construction team and, more importantly, to integrate the various elements of the project, as well as to ensure a zero-clash approach, which facilitated the construction and minimised disruption.

Using 3D models, finite element models for analysis were also developed for various components of the project for optimisation purposes. This resulted in significant cost savings to the project, not only on the dam and hydropower structure itself but also on the transmission towers.

Notable design features

The initial spillway design comprised a conventional spillway and spillway return channel in the far-left bank, which resulted in an excavation volume of more than twice the volume of material needed to construct the embankment. At the time, the designers had also not fully developed the river diversion strategy.

Understanding the hydrology was key to optimising the project. With a river flow varying from ≈25 m3/s in the dry season and peaking at ≈1 500 m3/s every year during the wet season, the construction of the embankment in the riverbed presented significant challenges.

Zutari’s approach was to eliminate the conventional spillway in the left bank and instead construct a composite dam with a concrete gravity spillway on the right bank, where the bedrock was shallow, and where it would also be possible to divert the river through 10 culverts left open in the mass gravity section, each 5.5 m wide by 7.5 m high. These culverts would then be closed during the dry season when the flow is lowest to commence impoundment.

In order to reduce the volume of concrete required for the 18 m high right bank training walls, which were also retaining walls, Zutari used tie-back slabs to help support the wall, together with passive anchoring in both the excavated face and vertically to connect the slab and the rock foundation. Thereafter, the backfilling could commence to reach the final platform level.

The design and construction of the foundations of the transmission towers also followed an unconventional approach, as the contractor was remunerated a lump sum for the tower foundations. Finite element analysis was used to optimise the volume of concrete, backfill and loads, mainly at the strained towers, in order to assist SCC, Zutari’s client, by optimising the concrete foundations. This required a sloping top for the tower foundation, which reduced the total volume of concrete on the foundations alone by more than 1 000 m3 .

The type of turbines installed also played a key role in the project's success. Taking overall due consideration of the project, Zutari selected vertical compact axial turbines (CAT) units. This arrangement, offered by only a few suppliers worldwide, was based on the ability to pre-assemble much of the turbines in the factory, ensuring easy assembly and embedding on-site. The vertical CAT also provided easy and direct access to the runner and turbine guide vanes for inspection and maintenance purposes, whereas such access is impossible with conventional Kaplan turbines.

Kashimbila substation

A project for the people

Several challenges were experienced throughout the design and construction period but were all overcome using a teamwork approach, which resulted in the first filling of the dam in 2017, followed by the successful commissioning of the power station, transmission line and substations in February 2020.

“The solution was one that provided an inspirational story to many people of Nigeria and resulted in a project designed and constructed in accordance with international best practice, yet adapted to the local conditions,” says Bertrand Rochecouste Collet, project director, Zutari.

Nigerians can now enjoy additional power on their very strained grid, potable water and power supply to the surrounding community and can look forward to the construction of the irrigation schemes, which is soon scheduled to commence.

Power station construction viewed from upstream