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

About Dams

Back to safetySafety - Floods

A dam is designed to hold back water that flows into the valley behind it, and to control the amount of water that passes through it. Hydrologists estimate the quantity of water that will flow into a reservoir. Civil engineers use this information to choose the location and height of the dam. The dam has to be high enough to ensure that it will not be overtopped by excess flood water, unless it is designed to do so.

Floodwater passing over the spillway of Treig dam near Fort William in ScotlandFloodwater passing over the spillway of Treig dam

Accidental overtopping could damage property downstream. Worse, it could wash away part of the downstream side of an embankment dam, affecting its stability, and causing it to collapse. Then, a catastrophic flood could occur as the water is released from the reservoir, causing death and destruction where the people live downstream.

Model of the spillway for Trieg Dam

A scale model of the spillway for Trieg Dam

Floodwater is usually passed through a reservoir using a spillway. Where people and property are at risk, it is important that spillways can safely pass the largest flood imaginable. Hydrologists estimate the size of this flood by studying past rainfall records for the area and considering recent climate changes. They use the information to design new spillways and to check existing ones. Often, small-scale models are used to check the design.