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Cost-Effective Ways to Increase Discharge Capacity at Spillways

By Francois M. Lempérière and Jean-Pierre H. Vigny

Dam engineers can choose from a number of strategies for increasing discharge capacity at existing spillways. Several non-conventional alternatives are available that provide additional protection at relatively low cost.

Many dam owners encounter a need to increase the safety of existing structures against floods that exceed the original design event. There are numerous approaches for achieving this, some of which tend to be used in certain regions of the world more than in others. A dam owner’s choice of a solution is affected by physical and economic conditions, but also by the limitations of local tradition and experience.

As part of its mission to promote the sharing of knowledge and experience concerning dam projects, the non-profit international organization HydroCoop assembled information on the design, suitability, and cost of solutions used worldwide for increasing dam safety against extreme floods. This information is based on laboratory test reports, technical conferences, and investigations by committees of the International Commission on Large Dams (ICOLD). As part of this investigation, HydroCoop evaluated several non-conventional alternatives for adding spillway capacity that provide additional discharge capacity at relatively low cost.

Non-conventional methods foradding discharge capacity

Adding spillway capacity within the constraints of the original design flood typically involves the loss of storage or construction of gates, with a direct or indirect cost in the thousands of U.S. dollars per cubic meter per second (cms). To reduce the cost of adding discharge capacity per cms, a dam owner can increase reservoir levels and accept limited damages in the case of very rare floods.

Ways to increase reservoir levels include:

Any of these solutions may prove to be practical, depending on project configuration, local material and labor costs, and the amount of additional capacity needed. The following sections provide details on each alternative.

Improving the embankment crest

Raising the reservoir level above the original design flood level can be a low-cost way to increase the discharge capacity of an existing spillway. Embankment improvements associated with this approach may include:

The cost to gain one unit of discharge capacity may be readily calculated from the weir flow formula and the geometry of the dam. The capacity of a spillway may be computed by:

The coefficient of 2 is approximate and ranges from 1.7 to 2.2, in metric units.

An approximate formula for the discharge capacity gained by raising the dam crest by 1 meter may be derived from Equation 1. This formula is:

The cost per cubic meter of discharge can then be estimated by:


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