River ice jams[edit]
An ice blockage on a river is more often called an ice jam but sometimes an ice dam. An ice jam is a dam on a river formed by blocks of ice.[4] Defined by the International Association of Hydraulic Research (IAHR) Working Group on River Ice Hydraulics an ice jam is a "...a stationary accumulation of fragmented ice orfrazil that restricts flow." on a river or stream. This definition includes what some scientists call an ice dam as a "...bottom accumulation of anchor ice...".[5]
Ice jam floods are less predictable and potentially more destructive than open-water flooding and can produce much deeper and faster flooding. Ice jam floods also may occur during freezing weather, and may leave large pieces of ice behind, but they are much more localized than open-water floods. Ice jams also damage an economy by causing river-side industrial facilities such as hydro-electric generating stations to shut down and to interfere with ship transport. The United States averages 125 million dollars in losses to ice jams per year.[6]
Ice jams on rivers usually occur in the springtime as the river ice begins to break up, but may also occur in early winter during freeze-up. The break-up process is described in three phases: pre-break-up, break-up and final drive.[7] Pre-break-up usually begins with increased springtime river flow, water level, and temperatures fracturing the river ice and separating it from the shore. Changes in river height from dam releases may also affect the pre-break-up. During the break-up, the ice in areas of rapids is carried downstream as an ice floe and may jam on still frozen sections of ice on calm water or against structures in the river such as theHoneymoon Bridge, destroyed in 1938 by an ice jam. Smaller jams may dislodge, flow downstream and form a larger jam. During the final drive, a large jam will dislodge and take out the remaining jams, clearing the river of ice in a matter of hours. Ice jams usually occur in spring, but they can happen as winter sets in when the downstream part becomes frozen first. Freeze-up jams may be larger because the ice is stronger and temperatures are continuing to cool unlike a spring break-up when the environment is warming, but are less likely to suddenly release water.[8]
Three types of natural ice jams can occur:
- a surface jam, a single layer of ice in a floe on calm water;
- a narrow-channel or wide-channel jam; and
- a hanging jam, the accumulation of river ice at slow current areas which only occur during freeze-up.[9] Ice jams also occur at sharp bends in the river, at man-made objects such as bridge piers, and at confluences.[8]
Northerly flowing rivers tend to have more ice jams because the upper, more southerly reaches thaw first and the ice gets carried downstream into the still-frozen northerly part. Three physical hazards of ice jams are 1.) The ice floe can form an ice dam and flood the areas upstream of the jam. This occurred during the 2009 Red River Flood and the 2009 Alaska floods. 2.) After the ice dam breaks apart, the sudden surge of water that breaks through the dam can then flood areas downstream of the jam. 3.) The ice buildup and final drive may damage structures in or near the river[1] and boats in the river.
Ice jams may scour the river bed, causing damage or benefit to wildlife habitats and possibly damage to structures in the river.[6]
Early warnings of an ice jam include using trained observers to monitor break-up conditions and ice motion detectors.[8]
The prevention of ice jams may be accomplished by
- weakening the ice before the break-up by cutting or drilling holes in the ice;
- weakening the ice by dusting it with a dark colored sand; or
- controlling the timing of the break-up using ice breakers, towboats, hovercraft, or amphibious excavators. However, the movement of migratory fish is known to be related to freeze-up and break-up, so affecting ice break-up may affect fish migration.
Where floods threaten human habitation, the blockage may be artificially cleared. Ice blasting using dynamite may be used, except in urban areas, as well as other mechanical means[10] such as excavation equipment, or permanent measures such as ice control structures and flood control.
Ice jams on a lake or ocean occur during the spring break-up if wind-driven ice piles up along a shorelinePower generation plant[edit]
Main article: Hydroelectricity
As of 2005, hydroelectric power, mostly from dams, supplies some 19% of the world's electricity, and over 63% of renewable energy.[55] Much of this is generated by large dams, although China uses small-scale hydro generation on a wide scale and is responsible for about 50% of world use of this type of power.[55]
Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator; to boost the power generation capabilities of a dam, the water may be run through a large pipe called a penstock before the turbine. A variant on this simple model uses pumped-storage hydroelectricity to produce electricity to match periods of high and low demand, by moving water between reservoirs at different elevations. At times of low electrical demand, excess generation capacity is used to pump water into the higher reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine. (For example, see Dinorwig Power Station.)
Spillways[edit]
Main article: Spillway
A spillway is a section of a dam designed to pass water from the upstream side of a dam to the downstream side. Many spillways have floodgates designed to control the flow through the spillway. There are several types of spillway. A service spillway or primary spillway passes normal flow. An auxiliary spillway releases flow in excess of the capacity of the service spillway. An emergency spillway is designed for extreme conditions, such as a serious malfunction of the service spillway. A fuse plug spillway is a low embankment designed to be overtopped and washed away in the event of a large flood. The elements of a fuse plug are independent free-standing blocks, set side by side which work without any remote control. They allow increasing the normal pool of the dam without compromising the security of the dam because they are designed to be gradually evacuated for exceptional events. They work as fixed weirs at times by allowing over-flow for common floods.
The spillway can be gradually eroded by water flow, including cavitation or turbulence of the water flowing over the spillway, leading to its failure. It was the inadequate design of the spillway which led to the 1889 over-topping of the South Fork Dam in Johnstown, Pennsylvania, resulting in the infamous Johnstown Flood (the "great flood of 1889").[citation needed]
Erosion rates are often monitored, and the risk is ordinarily minimized, by shaping the downstream face of the spillway into a curve that minimizes turbulent flow, such as an ogee curve.
Dam creation[edit]
Common purposes[edit]
| Function | Example |
|---|---|
| Power generation | Hydroelectric power is a major source of electricity in the world. Many countries have rivers with adequate water flow, that can be dammed for power generation purposes. For example, the Itaipu Dam on the Paraná River in South America generates 14 GW and supplied 93% of the energy consumed by Paraguay and 20% of that consumed by Brazil as of 2005. |
| Water supply | Many urban areas of the world are supplied with water abstracted from rivers pent up behind low dams or weirs. Examples include London, with water from the River Thames, andChester, with water taken from the River Dee. Other major sources include deep upland reservoirs contained by high dams across deep valleys, such as the Claerwen series of dams and reservoirs. |
| Stabilize water flow / irrigation | Dams are often used to control and stabilize water flow, often for agricultural purposes and irrigation.[56] Others such as the Berg Strait dam can help to stabilize or restore the water levels of inland lakes and seas, in this case the Aral Sea.[57] |
| Flood prevention | The Keenleyside Dam on the Columbia River, Canada can store 8.76 km3 (2.10 cu mi) of floodwaters, and the huge Delta Works protects the Netherlands from coastal flooding.[58] |
| Land reclamation | Dams (often called dykes or levees in this context) are used to prevent ingress of water to an area that would otherwise be submerged, allowing its reclamation for human use. |
| Water diversion | A typically small dam used to divert water for irrigation, power generation, or other uses, with usually no other function. Occasionally, they are used to divert water to another drainage or reservoir to increase flow there and improve water use in that particular area. See: diversion dam. |
| Navigation | Dams create deep reservoirs and can also vary the flow of water downstream. This can in return affect upstream and downstream navigation by altering the river's depth. Deeper water increases or creates freedom of movement for water vessels. Large dams can serve this purpose, but most often weirs and locks are used. |
Some of these purposes are conflicting, and the dam operator needs to make dynamic tradeoffs. For example, power generation and water supply would keep the reservoir high, whereas flood prevention would keep it low. Many dams in areas where precipitation fluctuates in an annual cycle will also see the reservoir fluctuate annually in an attempt to balance these difference purposes. Dam management becomes a complex exercise amongst competing stakeholders.[59]
Location[edit]
One of the best places for building a dam is a narrow part of a deep river valley; the valley sides then can act as natural walls. The primary function of the dam's structure is to fill the gap in the natural reservoir line left by the stream channel. The sites are usually those where the gap becomes a minimum for the required storage capacity. The most economical arrangement is often a composite structure such as a masonry dam flanked by earth embankments. The current use of the land to be flooded should be dispensable.
Significant other engineering and engineering geology considerations when building a dam include:
- Permeability of the surrounding rock or soil
- Earthquake faults
- Landslides and slope stability
- Water table
- Peak flood flows
- Reservoir silting
- Environmental impacts on river fisheries, forests and wildlife (see also fish ladder)
- Impacts on human habitations
- Compensation for land being flooded as well as population resettlement
- Removal of toxic materials and buildings from the proposed reservoir area
Impact assessment[edit]
Impact is assessed in several ways: the benefits to human society arising from the dam (agriculture, water, damage prevention and power), harm or benefit to nature and wildlife, impact on the geology of an area (whether the change to water flow and levels will increase or decrease stability), and the disruption to human lives (relocation, loss of archeological or cultural matters underwater).
Environmental impact[edit]
Main article: Environmental impacts of reservoirs
Reservoirs held behind dams affect many ecological aspects of a river. Rivers topography and dynamics depend on a wide range of flows, whilst rivers below dams often experience long periods of very stable flow conditions or sawtooth flow patterns caused by releases followed by no releases. Water releases from a reservoir including that exiting a turbine usually contain very little suspended sediment, and this in turn can lead to scouring of river beds and loss of riverbanks; for example, the daily cyclic flow variation caused by the Glen Canyon Dam was a contributor to sand bar erosion.
Older dams often lack a fish ladder, which keeps many fish from moving upstream to their natural breeding grounds, causing failure of breeding cycles or blocking of migration paths. Even the presence of a fish ladder does not always prevent a reduction in fish reaching the spawning grounds upstream. In some areas, young fish ("smolt") are transported downstream by barge during parts of the year. Turbine and power-plant designs that have a lower impact upon aquatic life are an active area of research.
A large dam can cause the loss of entire ecospheres, including endangered and undiscovered species in the area, and the replacement of the original environment by a new inland lake.
Large reservoirs formed behind dams have been indicated in the contribution of seismic activity, due to changes in water load and/or the height of the water table.
Dams are also found to have a role in the increase/decrease of global warming.[60] The changing water levels in reservoirs are a source for greenhouse gases like methane.[61] While dams and the water behind them cover only a small portion of earth's surface, they harbour biological activity that can produce large amounts of greenhouse gases.[62]
Human social impact[edit]
The impact on human society is also significant. Nick Cullather argues in Hungry World: America's Cold War Battle Against Poverty in Asia that dam construction requires the state to displace individual people in the name of the common good, and that it often leads to abuses of the masses by planners. He cites Morarji Desai, Interior Minister of India, in 1960 speaking to villagers upset about the Pong Dam, who threatened to "release the waters" and drown the villagers if they did not cooperate.[63]
For example, the Three Gorges Dam on the Yangtze River in China is more than five times the size of the Hoover Dam (U.S.), and creates a reservoir 600 km (370 mi) long to be used for flood control and hydro-power generation. Its construction required the loss of over a million people's homes and their mass relocation, the loss of many valuable archaeological and cultural sites, as well as significant ecological change.[64] During the 2010 China floods, the dam held back a what would have been a disastrous flood and the huge reservoir rose by 4 m (13 ft) overnight.[65]
It is estimated that to date, 40–80 million people worldwide have been physically displaced from their homes as a result of dam construction.[66]
Economics[edit]
Construction of a hydroelectric plant requires a long lead time for site studies, hydrological studies, and environmental impact assessments, and are large-scale projects by comparison to traditional power generation based upon fossil fuels. The number of sites that can be economically developed for hydroelectric production is limited; new sites tend to be far from population centers and usually require extensive power transmission lines. Hydroelectric generation can be vulnerable to major changes in the climate, including variations in rainfall, ground and surface water levels, and glacial melt, causing additional expenditure for the extra capacity to ensure sufficient power is available in low-water years.
Once completed, if it is well designed and maintained, a hydroelectric power source is usually comparatively cheap and reliable. It has no fuel and low escape risk, and as an alternative energy source it is cheaper than both nuclear and wind power.[67] It is more easily regulated to store water as needed and generate high power levels on demand compared to wind power.
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