Dams

Dams may be classified according to the general purpose for which they are designed. These include storage, diversion, and detention.

Storage dams are built to provide a reliable source of water for short or long periods of time. Small dams, for example, are often built to capture spring runoff for use by livestock in the dry summer months. Storage dams can be further classified by the specific purpose for which the water is being stored, such as municipal water supply, recreation, hydroelectric power generation, or irrigation.

Diversion dams are typically designed to raise the elevation of a water body to allow that water to be conveyed to another location for use. The most common applications of diversion dams are supplying irrigation canals and transferring water to a storage reservoir for municipal or industrial use.

Detention dams are constructed to minimize the impact of flooding and to restrict the flow rate of a particular channel. In some cases, the water trapped by a detention dam is held in place to recharge the subsurface groundwater system. Other detention dams, called debris dams, are designed to trap sediment.

Large dams frequently serve more than one of these purposes and many combine aspects of each of the three main categories. Operation of such multipurpose dams is complicated by sometimes opposing needs. In order to be most effective, storage behind a flood control dam should be maintained at the lowest level possible. After a detention event occurs, water should be released as quickly as possible, within the capacity of the downstream channel. Conversely, the efficient and economic operation of storage and diversion dams requires that water levels be maintained at the highest possible levels. Releases from these reservoirs should be limited to the intended user only, such as the power generating turbines or the municipal water user. Operators of multipurpose dams must balance the conflicting needs of the various purposes to maintain the reliability, safety, and economic integrity of the dam. Operators must use a variety of information to predict the needs of the users, the expected supply, and the likelihood of any abnormal conditions that might impact the users or the dam itself. Failure to do so can threaten even the largest of dams.

During the El Niño of 1983, climate and hydrologic forecasts failed to predict abnormally heavy spring runoff in the Rocky Mountains. Dam operators along the Colorado River maintained high water storage levels, failing to prepare for the potential of the flooding. By the time operators began to react, water was bypassing the dams via their spillways and wreaking havoc throughout the system. Ultimately, the Glen Canyon dam in Arizona was heavily impacted with flood flows eroding large volumes of rock from within the canyon walls that support the dam. Fortunately, the flooding peaked and control was regained before the dam was breached.