The British Dam Society
Caring for dams, people and the environment

About Dams

Back to safetySafety - Monitor behaviour

Although dams look solid, they do move under the forces of nature, and water seeps through tiny pores (or spaces) in their structure. The weight of an embankment dam actually causes its own height to reduce over a number years. This is called settlement. A reservoir pushes a dam over a little and tries to make it slide on its foundation. Dams contract and expand as their temperature changes throughout the year. Civil engineers ensure the stability of dams by designing them to minimise water seepage and the effects of natural forces.

Survey

Changes in the behaviour of a dam occur slowly, so dams are monitored regularly using scientific instruments that can measure very small changes. This way any undesirable behaviour can be detected well before it causes a major problem. Some instruments are permanently installed in dams, while others are taken to the site to make measurements.

These are some the things that are monitored.

Seepage and leakage

Leakage or increased seepage can indicate that there is a crack in a dam. In embankment dams, it could indicate that internal erosion of the fill material is taking place.

Settlement

Levels on the crest of an embankment dam are measured, using optical instruments, to check whether there has been a reduction in the dam's height. Settlement of the crest can be a warning of problem occurring within the body of the dam. The crest level is designed to be a certain height above the normal full reservoir level. This ensures that the dam is not overtopped by excess floodwater or by waves on windy days.

Tilting

Pendulums are installed in vertical shafts in concrete dams to measure how much the dam is tilting over.

Horizontal Movements

A dam can move horizontally, in either the downstream or upstream direction. This movement can be measured on the crest using optical or computer-based equipment. Excessive movements can indicate that the dam is sliding and becoming unstable.