Dear Editor,

The article on sedimentation entitled ‘Overloaded’ contained a lot of useful information from respected experts in the field. However, I think that one of the most significant recent, conceptual advances in the philosophy of the design of dams and reservoir sedimentation management has not been dealt with.

The Research on an Economic and Engineering Model on Reservoir Sedimentation Management programme (RESCON) was developed by the World Bank for use in reservoir sedimentation management and is based on the life cycle management approach, which is a completely new concept in dam design, construction and management.

Dams and their reservoirs are generally very large facilities that are difficult to decommission at the end of their lives. It is therefore not advisable to follow the same design approach for designing dams as for designing other infrastructure. The conventional approach when designing infrastructure is to adopt a ‘design life’, which is usually the number of years during which the capital investment is paid and during which period the benefits are required to exceed cost. This approach works for conventional infrastructure, such as roads, because it is relatively easy to refurbish such facilities. When a road reaches the end of its design life it can be resurfaced, and used as if it was virtually new.

However, this is not the case with dams. Should a dam be designed for a design life of, say, 50 years and it essentially loses its storage capacity during this period, it is almost impossible to refurbish the facility to store water again. If the dam has not been designed to accommodate sediment management, it is often abandoned at the end of its design life. A fatal flaw in following the design life approach when planning and designing dams, which was historically the case, is that the question as to what to do with the dam and reservoir at the end of its ‘design life’ has been ignored.

It is therefore necessary to develop a new design philosophy when designing future dams by adopting a life cycle management approach instead of the conventional design life approach. The objective with the life cycle management approach is that the sustainable use of infrastructure, ie the dam, its reservoir and ancillary facilities, in addition to the sustainable use of resources, is emphasised. When following the life cycle management approach, the design objective is to design infrastructure that can be used in perpetuity.

The difference between a life cycle management approach and a design life approach is illustrated in the figure on the right. The linear progression of the conventional design life approach commences with planning, and is followed by design, construction and maintenance. When following this approach the designer normally ignores the need for perpetual use of the facility. Current pressure by environmental demands often requires such facilities to be decommissioned. Once decommissioned, the facility needs to be replaced with another, often in less desirable circumstances.

The life cycle management approach, in contrast, aims at providing a facility that can be used in perpetuity. Such an approach also commences with planning, and is followed by design and construction. However, when designing a dam by following this approach, it is designed in a manner that will allow operation and maintenance procedures that facilitates perpetual use of the dam, reservoir and ancillary facilities. This can be done by for example providing low level gates and designing the dam and reservoir to allow flushing on a regular basis.

The World Bank funded development of the RESCON model which can be used to identify the technically feasible and economically optimal sediment management techniques that can be used for implementing the life cycle management approach when designing new dams, or when retrofitting existing dams to allow for sustainable use of the infrastructure. My personal opinion is that it should be a critical element in future design approaches to dams.

Yours sincerely

George Annandale

President, Engineering and Hydrosystems,

Denver, Colorado, US

Editor’s note: The RESCON model was developed under the leadership of Alessandro Palmieri from the World Bank, with George Annandale, providing guidance on assessment of the technical feasibility of potential sediment management techniques. Professor Farhed Shah from the University of Connecticut provided guidance on the economic optimisation used in the computer program. For more information contact Alessandro Palmieri on u-apalmieri-worldbank-org-u, or George Annandale on u-gwannandale-worldnet-att-net-u