The Hoover Dam in Nevada is what most people think about when they hear the word “dam”. With the Lake Mead reservoir behind it, the towering smooth surface of the Hoover Dam is an imposing feature of the southern Nevada landscape.
However, there are thousands of dams around the world serving different functions than the Hoover, and a variety of dam designs that are implemented depending on the use and the landscape. Dam structures are selected based on the individual requirements of each particular site. Some designs have evolved over time and some designs are no longer used. However, all dam designs serve the same basic purpose: to meter the water flow from a river.
Masonry Dams
The majority of modern dams are constructed from masonry materials, particularly concrete. Historically, masonry dams were built using dressed stone and mortar, but innovations in concrete construction have allowed for extremely large dams to be built. There are two general categories of masonry dams:
- Arch Dams - An arch dam has a curved structure that is compressed by the pressure of the water behind the dam. This design is very similar to an arched bridge that is compressed by the weight of the vehicles that travel over it. Arch dams are best used in narrow gorges and can be built to large heights. The Roosevelt Dam on the Salt River is an example of an arch dam.
- Gravity Dams - Gravity dams form a straight barrier across the river, and are designed with a cross section that resists the overturning moment force applied by the water pressure behind the dam. The Grand Coolee Dam on the Columbia river is an example of a gravity dam.
The two design concepts can also be combined into an arch-gravity dam design. The Hoover Dam on the Colorado river is an example of an arch-gravity dam.
Embankment Dams
The earliest known dams are embankment dams: dams created from stone and earth because the materials were plentiful and inexpensive. There are three general type of embankment dams:
- Rock-Filled Dams - Rock-filled dams are constructed from compacted rock and gravel with a water-proof layer constructed on the upstream face of the dam, such as concrete, asphalt, or other materials.
- Earth-Filled Dams - Earth-filled dams at their simplest are just compacted dirt and rock. They can have some design elements such as drain layers depending on the needs of the site. Spillways have to be carefully designed and lined to prevent erosion of the dam material.
- Asphalt-Concrete Dams - Asphalt-concrete dams are a more modern version of the embankment dam that provides more overall stability and resistance to erosion. Asphalt provides plasticity to the dam structure, allowing it to flex under pressure without failing.
Embankment dams are generally employed in locations where the reservoir depth is low and the costs of a masonry dam are prohibitive.
Cofferdams
Cofferdams are temporary structures used to hold back water from a construction site adjacent to or within a water way. Cofferdams generally consist of interlocking steel plates topped with a wale to keep the plates connected and interior support beams to resist the exterior water pressure.
Timber Dams
Historically, timber dams were often built in locations where masonry was unavailable. Timber dams could be quickly constructed using readily available dressed logs. However, timber dams have a short overall lifespan due to decomposition of the wood, and most of the existing timber dams in the US have been replaced over the years.
Steel Dams
Steel dams consist of metal plates that are angled with the flow of water and supported on the downstream side of the dam by struts and support beams with concrete footings. Steel dams are generally no longer in use due to their numerous disadvantages, including corrosion of the steel plates, settling and shifting in the concrete foundations, and fatigue crack growth due to concentrated stresses.
Sources
Structurae - Dams and Retaining Structures
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