Tackling Tarbela's problems4 July 2002
UK-BASED HR WALLINGFORD HAS extensive experience of advising on sediment related river problems.Staff at HR Wallingford have developed a computational model - RESSASS - that simulates and predicts reservoir sedimentation. The model solves the equations describing fluid flow and sediment movement so that predictions can be made for any time in the future.
Based on these calculations, the volume and composition of deposited sediment can be calculated at specified cross-sections through the reservoir. Up to ten different sizes of sediment can be input, and the model is able to predict the impact of sediment flushing.
RESSASS helps engineers to decide whether flushing is feasible and what the detailed impact of this will be on maintaining useful storage. Its development has been part-funded by the UK government and the model has been applied on studies in Hong Kong, India, Vietnam, Zimbabwe and at Tarbela Reservoir in Pakistan.
It is currently being used to investigate sedimentation at Koka dam in Ethiopia.
• Case study - reservoir sediment management at Tarbela dam. Tarbela dam is situated on the river Indus and was built between 1970-75. The dam is vital to the economy of Pakistan and provides 40% of the country's water storage (crucial for irrigation during the dry season) and 35% of the country's energy requirements, via hydroelectric generation.
However, storage capacity in the reservoir has declined by approximately 25% due to sedimentation. The front of the advancing sediment deposit has now reached the dam and there is a risk that, in the near future, intakes may be inundated with sediment.
In 1997, American consulting engineers TAMS (in association with HR Wallingford) were commissioned to examine potential solutions to the problem. The client for the study was the World Bank, on behalf of the Water and Power Development Authority (WAPDA) Pakistan.
RESSASS was used to model the reservoir. Using the results, TAMS and HR Wallingford developed an action plan which involved combining a number of options:
• An operational policy to ensure that risks to the intakes are kept acceptably low.
• Construction of a 100m high underwater dyke to protect the intakes from sediment.
• Building a low level flushing system with a capacity of 7000m3/sec. This would maintain a reservoir capacity of approximately 55% of the original value.
The estimated cost for the constructing the flushing system was (1998) US$700M. This compared with a cost of over US$4000M for a replacement dam.