Featured Article Summary #266

A dam is a structure that holds back water or slows its flow. There are four main types: gravity dams use their heavy weight, embankment dams are made from earth and rock, buttress dams have a concrete face supported by triangular supports, and arch dams use curved walls that push pressure onto the valley sides. Dams are built for many uses, including irrigation, hydropower, drinking and industrial water, flood control, recreation, navigation, and even fish farming. Some of the earliest dams were built in the Middle East and Turkey thousands of years ago, and engineering methods became more scientific in the late 1700s, helped by better building materials and machinery in the 1900s. Many dams also include features like locks for ships, fish ladders, spillways to prevent overflow, and outlets to release water for draining or maintenance. Dams can be removed for safety or environmental reasons, but they can also fail, and these failures have helped improve the rules for designing safer dams. “Dam” generally means any structure that holds back water or stops flooding, but in civil engineering it usually refers to structures that block a river or raise the water level. This article uses the stricter meaning, so it excludes dikes, levees, and diversion dams that redirect water without increasing levels. The word “dam” comes from Middle English and traces to Germanic languages, with roots meaning “to stop up” and “to set or put in place.” Dams can be classified by what they are made of and how they are built. Human-made dams include embankment, gravity, buttress, arch, and composite types, while natural dams can be formed by rockslides, volcanic activity, or beavers. The most common dams are embankment dams, which are built by compacting natural materials like rock, clay, sand, gravel, and soil into a wide, mound-shaped barrier. They are usually cheaper than concrete dams and can be built on softer ground because their broad base spreads the weight, with major examples including Tarbela, Nurek, and Chicoasén dams. However, they can leak because they are permeable, so seepage must be controlled with drainage, grout, and an impervious layer to prevent “piping,” where small channels grow and can eventually cause failure; modern designs also use layered materials, and the 1970s CFRD type adds concrete slabs sealed on the upstream face. Gravity dams hold back water mainly using their own weight, historically built with masonry but now usually made of concrete, which makes them more expensive than earthfill embankment dams. Their shape is typically roughly triangular, and designers follow rules like the “middle-third” to ensure the forces keep the dam’s pressure within the safer middle of the base, with carefully chosen thickness and slopes. They must be built on strong bedrock and protected against leaks or failure from uplift pressure caused by seepage, using measures such as grout curtains and drainage systems.