Vegetation, erosion and dams

From http://www.dcr.virginia.gov/sw/dsveget.htm

Introduction

With proper care, earthfill embankment dams have proven to be very effective for many years. Most embankment dams are composed of non-organic material and do not deteriorate appreciably with time. An embankment may even continue to undergo additional consolidation and strengthen with age, once the critical points of initial settlement and initial reservoir fill have passed. Nevertheless, the continuing safety of any embankment dam depends on the integrity of its earthen fill to withstand pressure from the volume of water in the reservoir.

The biggest enemy of any earthfill dam is erosion, either external (water overflow creating ruts or rills on the surface of the fill) or internal (sometimes called "piping"). External, or surface, erosion is generally obvious if one takes the time and effort to look over the dam carefully. Internal erosion is not readily visible and may not be detected until it is too late for corrective action. This can result in an emergency situation and even lead to a catastrophic failure.

Need to control vegetation

A dense cover of low-growing grassy vegetation is recommended because it will provide protection from surface erosion, but its root structure does not penetrate the embankment so deeply as to create a potential path for internal erosion.

The type of grass and its fertilization should be appropriate for local conditions. The proper vegetation should be established and maintained over the entire embankment, outlet, plunge pool and spillway area. Coverage should extend at least 25 feet beyond the abutment contacts and toe of the fill. Regular mowing throughout the year is essential so that the surface can be readily traversed by foot. Potential dam safety problems, such as misalignment, cracks, animal burrows, surface erosion, seepage, sloughing, etc. can be spotted early enough to take corrective action. Therefore, mowing should precede each dam inspection.

The trouble with trees

though woody vegetation such as trees and brush may protect against surface erosion, such growth can cause other, serious problems. These problems develop over years and may go undetected until it's too late. In addition, trees or brush can hide an embankment surface, making inspection difficult.

When trees die, it causes the roots to decay, leaving a cavity within the dam. Water leaking through such a cavity can produce a piping failure. In addition, a tree can be blown over during a severe storm, leaving a large hole in the dam in place of the uprooted root ball.

Because tree root problems occur over a long time, even decades, they often go undetected. Many times, dam failure is described as a sudden event when in fact conditions leading to the failure went undetected or ignored for years.

Control of trees on dams

Trees should never be allowed to grow on the embankment fill, at the outlet, plunge pool or spillway area, or within 25 feet beyond the abutment contacts and toe of fill. Any tree on a dam should be removed, its roots grubbed out and dense grass cover established in its place. Deviation from this standard must be based on a critical assessment by a professional engineer who specializes in dams.

Follow the general guidance below:

Condition	Action needed
Existing dam with trees	Cut and remove all trees. Grub out all roots larger than 1 inch in diameter. Grade to adjacent contour and establish cover as described below.
New dam or existing dam without trees	Establish and maintain dense cover of grass. Maintain a height of 4 to 8 inches with regular mowing to discourage growth of woody vegetation and to facilitate visual inspection for seepage, sloughs or other signs of stress. New dams should have no trees on them.

Removal of trees from existing embankments

A question dam owners often ask is, "Why can't I just cut down the trees at the surface, and then keep the vegetation properly controlled in the future?" This may be acceptable for Class III and Class IV dams if and only if accompanied by a commitment to very carefully monitor the dam and to be prepared for immediate emergency action. Eventually, however, it will be necessary to deal with the decaying roots. In other words, a decision to just "cut and watch" simply postpones the dealing with the underlying problem and may result in an emergency situation or even failure of the dam.

Removal of trees and roots on hazard-potential Class I and Class II dams must be done under the direction of a professional engineer. After cutting and removing all trees and brush, all roots should be grubbed out to assure that no roots larger than 1 inch in diameter remain. Generally, the reservoir needs to be lowered prior to grubbing the roots. The rate of decrease in the reservoir level should not exceed 6 inches per day unless otherwise directed by a professional engineer. Holes resulting from the grubbing operation should be backfilled with well-compacted soil. Upstream slopes should be backfilled with impervious soil, while more pervious soil may be used on the downstream slope. The backfill should then be graded to blend with the surrounding contour, and appropriate grasses should be established on all disturbed areas.

Trees on dams are a serious safety hazard. There is no single "cookbook" solution on the proper way to remove them . . . each case is unique. The advice of professional engineer needs to be sought and followed.

Contact DCR's Dam Safety Program staff at (804) 371-6095
or email dam@dcr.virginia.gov.

Damage Caused to Earth Embankment Dams by Earthquakes in the United States

Scot H. Dahms
4/23/04
ES 767, Global Tectonics, Emporia State University, Emporia, Kansas

The theory of plate tectonics explains that the Earth's crust is broken into many plates. The plates are in continuous contact with each other and move as a whole. They move slowly over the Earth's surface and are propelled by heat generated inside of the Earth. Plates meet along fault zones. Earthquakes are caused by the sudden release of energy from the movement of the Earth's crust mostly along fault zones. Areas along the fault zones may become stuck or locked in place storing a tremendous amount of energy. When the area moves, the energy is released in the form of seismic vibrations causing ground shaking. The sudden release of energy can cause enormous amounts of damage to structures including earth embankment dams. Damage to a dam can cause the dam to fail releasing large amounts of water into a river valley. The result can be catastrophic including loss of life and property.

Most of the early dams were built with no consideration of earthquakes. For instance, dams built by the U.S. Army Corps of Engineers in the Louisville District before 1950 were not designed for earthquake forces because designers did not consider earthquakes probable threats (USACE 1990). As more information of earthquakes was collected, the need to built dams that could withstand earthquakes was recognized. Earth embankment dams may be damaged by earthquakes in several ways including dam movement, liquefaction of fill in a dam, water waves caused by an earthquake over topping a dam, and direct damage caused by a dam being located on a fault.

The dam or parts of the dam may move vertically, horizontally or a combination of both resulting in vertical cracks or settlement of the dam. The West Yellowstone Earthquake of 1959 damaged the Hegben Dam in Montana. The Hegben Dam is an earth embankment dam with a concrete core and the earthquake had a magnitude of 7.5. The earth embankment settled more then a meter and the concrete core cracked in four places. The reservoir floor was tilted but the dam embankment did not fail (Bonilla 1991).

In 2001, the Howard Hansen Dam located northwest of Enumclaw was damaged by the Washington Earthquake. The earthquake had a magnitude of 6.8. The damage consisted of cracks in the embankment and movement of instrument housing buildings on the dam (USGS 2001).
Seismic studies of the areas considered for dam construction increase the knowledge of potential problems that may be caused by earthquakes. For dams that are in place or planned to be built, modifications can be made to the structure to increase the structural stability including crack stopping zones, grouting, adding berms, and flattening of slopes (Babbitt and Verigin 1996).



Liquefaction

Liquefaction of fill in the dam may occur. Liquefaction is the large drop in stiffness and strength of soil due to seismic movements (Byrne and Seid-Karbasi 2003).


As a result, part of a dam may slump and slide off the structure. During the Santa Barbara Earthquake of 1925, the Sheffield Dam in California failed due to liquefaction apparently in the area just below the dam embankment containing silty sand. Little damage and no loss of life resulted. The earthquake had a magnitude of 6.3 and the dam was constructed in 1917 (Bonilla 1991).

Above image is a cross section of the Lower San Fernando Dam before and after the earthquake. Image taken from USGS (1995) with written permission for educational purposes.



The Lower San Fernando Dam located northwest of Los Angeles, California was damaged by the San Fernando Earthquake of 1971. The earthquake had a magnitude of 6.7. Liquefaction in the upstream side led to a major slide involving the core, the crest, and the upstream slope along nearly half the length of the dam.


The dam did not fail because a cracked rim on the downstream side of the dam was higher then the water level in the lake. No loss of life resulted but 80,000 people were evacuated downstream of the dam. The dam was constructed between 1912 and 1915 with hydraulic fill methods and was enlarged in 1930. The older part of the dam included a clay core with outer zones of silty sand (Bonilla 1991). Hydraulic fill methods involve the mixing of fill soil with water, transporting the fill to the dam site by pipelines, depositing the fill and water on the embankment, and allowing the water to drain away. The fill is loose and is susceptible to liquefaction (VTU 2000). Because of the damage to the Lower San Fernando Dam, the California Department of Water Resources, Division of Safety of Dams analyzed over 100 dams. Of these, 60 dams have been physically modified, 19 have permanent storage restrictions, 36 have preliminary restrictions pending mitigation of deficiencies, and 4 have been removed (Babbitt and Verigin 1996).

Photograph of the Los Angeles Dam after the Northridge Earthquake of 1994. Notice cracks in the surface. Photograph used with written permission from USGS (1995) for educational purposes.



Los Angeles Dam in California was built 3,000 feet upstream of the Lower San Fernando Dam in 1976. The information from the damage to the Lower San Fernando Dam was used to design the Los Angeles Dam. In 1994, the Los Angeles Dam was damaged by the Northridge Earthquake. It had a magnitude of 6.6 that is comparable to the San Fernando Earthquake of 1971. The Los Angeles Dam only suffered minor deformation and superficial cracking (USGS 1995).


The recognization of materials susceptible to liquefaction and slumping would allow their removal before the construction of a dam would occur. During the design analysis of Patoka Dam in Indiana, loose fine sand in the foundation was found to be susceptible to liquefaction during shaking from a moderate earthquake. The revised design included removal of the valley fill and placement of the entire embankment on bedrock. The dam was built between 1972 and 1978 (USACE 1990).

Over Topping of Dam by Water

Water waves caused by an earthquake may over top a dam. Water going over the top of a dam would result in erosion to the surface and could lead to failure of a dam. Water waves over topped the Hegben Dam in 1959 (Bonilla 1991).

All dams should be constructed with ample freeboard to withstand waves caused by earthquakes (USACE 1990). For dams that are already constructed, freeboard could be increased by adding to the embankment or by lowering the spillway (Babbitt and Verigin 1996).

Fault Locations

The dam may be located on a fault and have direct damage caused by the movement of the plates. The Great San Francisco Earthquake of 1906 caused damage to the San Andreas Dam. The dam only incurred minor damage but the tunnel spillway that ran through the San Andreas Fault was ruined. The fault offset approximately 2 m in the locality of the dam during the earthquake. The dam was built from 1868 to 1870, before the fault was recognized (Bonilla 1991).

Increased knowledge of where faults are located now affect where and how dams are built. Building of dams across faults should be avoided. If the construction across a fault cannot be avoided, special measures need to be taken to ensure the structural stability of the dam. The Coyote Dam southeast of San Francisco was constructed across the Calaveras Fault in 1936. The dam was designed to accommodate 6.1 m of horizontal and 1 m of vertical fault displacement. The Palmdale Dam was originally built across the San Andreas Fault in 1891. The dam was completely reconstructed in 1969 because of the possibility of faulting. The new dam was designed to accommodate 6.1 m of horizontal and 1 m of vertical fault displacement. The Cedar Springs Dam in California was built over active faults. Because of the faults, the original design of the dam was modified including a reduction in height, shifting of the axis, and changes in construction material zonation (Bonilla 1991).

As more is learned about earthquakes and earth embankment dams over time, damage to earth embankment structures is being reduced. Older dams constructed on poor foundations, with poor fill, or with poor construction techniques are more likely to be damaged or fail. Dams constructed on firm foundations, with appropriate fill, and using current construction techniques are less likely to be damaged or fail. Knowledge from past failures, increased technology, increased knowledge of earthquakes, and improved construction techniques are the primary reasons for better structural stability in earth embankment dams.

References.
Babbitt, D. H. and Verigin, S. W., 1996. General Approach to Seismic Stability Analysis of Earth Embankment Dams. California Department of Water Resources, Division of Safety of Dams.

Bonilla, M. G., 1991. Faulting and seismic activity, in Kiersch, G. A., ed., The heritage of engineering geology; The first hundred years. Boulder, Colorado, Geological Society of America, Centennial Special Volume 3.

Byrne, M. P. and Seid-Karbasi, M., 2003. Seismic stability of impoundments. 17th Annual Symposium, Vancouver Geotechnical Society.

U.S. Geological Survey, National Strong-Motion Program, 2001. Howard Hansen Dam. World Wide Web homepage URL: http://nsmp.wr.usgs.gov/data_sets/20010228_1/20010228_hhd_pics.html.

U.S. Geological Survey, 1995. Fact Sheet-096-95, The Los Angeles Dam Story. World Wide Web homepage URL: http://quake.wr.usgs.gov/prepare/factsheets/LADamStory/

U.S. Army Corps of Engineers, Louisville District, Dam Safety Section. 1990. Earthquake Fact Sheet. Louisville, Kentucky.
Virginia Tech University, 2000. Geotechnical Engineering Photo Album. World Wide Web homepage URL:
http://cgpr.ce.vt.edu/photo_album_for_geotech/.

Typical Failure Modes of Embankment Dams

http://www.des.state.nh.us/factsheets/dam/db-4.htm

Failures of earthen embankment dams or dikes can generally be grouped into three classifications: hydraulic, seepage, and structural.

Hydraulic Failures

Hydraulic failures from the uncontrolled flow of water over and adjacent to the embankment are due to the erosive action of water on the embankment slopes. Earth embankments or dikes are not normally designed to be overtopped and therefore are particularly susceptible to erosion. A well vegetated earth embankment or dike may withstand limited overtopping if its top is level and water flows over the top and down the face in an evenly distributed sheet without becoming concentrated in any one area.

Hydraulic failures may be related directly or indirectly to the following:

• Overtopping - Discussed above
• Wave Erosion - Notching of upstream face by wave action reduces the embankment cross section thickness and weakens embankment material.
• Top Erosion - Erosion of downstream toe of the earth slope caused by misdirected spillway outlet discharge.
• Gullying - Rainfall erosion of embankment slopes. Also caused by traffic from people and vehicles.

Seepage Failures

Most embankments exhibit some seepage. However, this seepage must be controlled in velocity and quantity. Seepage occurs through the earthen embankment or dike and/or through its foundation. Seepage, if uncontrolled, can erode fine soil material from the downstream slope or foundation and continue moving towards the upstream slope to form a pipe or cavity to the pond or lake often leading to a complete failure of the embankment. This action is known as �piping.� Seepage failures account for approximately 40 percent of all embankments or dike failures.

Seepage can also cause slope failures by saturating the slope material, thereby weakening the adhesive properties of the soil and its stability. Burrows or holes created by animals such as the groundhog, woodchuck, or muskrat create voids in the embankment or dike, which weaken the structure and may serve as a pathway for seepage.

Tree roots can provide a smooth surface for seepage to travel along. When trees die, their decaying roots may leave passageways for seepage to concentrate in. Pipes through the embankment may also provide smooth surfaces for seepage to concentrate along as well.

Structural Failures

Structural failures involve the separation (rupture) of the embankment material and/or its foundation. This type of failure is more prominent in large embankment dams. However, it is not exclusive to large dams and similar occurrences may be seen on earthen embankments or dikes in New Hampshire. Structural failure of an earthen embankment may take on the form of a slide or displacement of material in either the downstream or upstream face. Sloughs, bulges, cracks or other irregularities in the embankment or dike generally are signs of serious instability and may indicate structural failure.

Other

Tree growth on an earthen embankment or dike can be a contributing factor in the failure of an earthen structure and part of any one of the three previously described type of failures. Tree growth directly on the crest or top of the structure could lead to a hydraulic failure should the tree be blown over. This may displace embankment material within the root ball creating a low area susceptible to flows from the impoundment. Tree root systems may also create seepage paths through an earthen embankment or dike and structural failure of an upstream or downstream slope could occur with the displacement of a large tree implanted within the earth slope.

For more information relative to the design, construction, maintenance and operation of dams, please contact the DES Water Division Dam Bureau at (603) 271-3406 or email damsafety@des.state.nh.us. General information is available at www.des.nh.gov/Dam/. You may also visit our office at 29 Hazen Drive in Concord, New Hampshire.

Embankment Dam -5-: Building

http://www.britishdams.org/about_dams/buildembankment.htm

Embankment dams are constructed upwards in a series of thin layers. For each layer, the fill material is transported to the dam by trucks and dumped there. Then bulldozers are used to spread the material in a thin layer. The thickness of the layer depends on the material being used. Earthfill layers are about 300 mm thick and rockfill layers

When the dam has a core, that is also constructed in layers to keep it at the same height as the rest of the dam.

The reinforced concrete layer being placed on the upstream face of the Messochora rockfill dam in Greece.

Once the dam has been raised to its full height, a protective layer is formed on its upstream face. This layer protects the dam against damage by waves, and sometimes provides waterproofing too.

Embankment Dam -4-: Forces

Gambar di bawah saya dapatkan di website dari sebuah saluran tivi yang ditujukan untuk anak-anak. Sungguh menarik melihat penjelasan dan gambar ini.


http://www.pbs.org/wgbh/buildingbig/dam/emb_forces.html

Water pushes against the embankment dam, but the heavy weight of the dam pushes down into the ground and prevents the structure from falling over.

Embankment Dams -3-: The History

http://simscience.org/cracks/intermediate/ebnk_hist1.html

Ancient Times

The Sadd-el-Kafara Dam
photo from Schnitter
courtesy of A.A. Balkema (#)

Sadd-el-Kafara
The ruins of the Sadd-el-Kafara embankment dam were discovered over 100 years ago in the Garawi ravine in Egypt. The dam was built around 2600 BC and was 14 m high and 113 m along the crest. It is the oldest dam of such size known in the world. The of the dam was to retain the water from rare, but violent, floods.

The grossly overdesigned cross section of the dam was due to inexperience. Unfortunately, there was no channel or tunnel to divert the river around the dam site while it was being built. As a result, the dam was destroyed while still under construction during one of the rare floods it was intended to control. The consequences of the dam's failure must have been so grave that the Egyptian engineers refrained from further dam construction for about eight centuries.

The dams was only able to handle relatively moderate and frequent floods. As a result, the dam was breached quite often by extreme floods which occurred about once every 50 years. After 1300 years of service, the dam failed once again. This time it was not fixed and most of the 50,000 people whose livelihood depended on it had to move.

The Postmedieval Europe

The St. Ferreol Dam
sketch from Schnitter
courtesy of A.A. Balkema (#)

Saint Ferreol
The proposal of a large water supply reservoir turned out to be the key to build the Languedoc canal in France, which connected the Mediterranian Sea with the Atlantic Ocean. The reservoir would be used to conpensate the losses of water in the canal during the dry season. After the approval by King Louis XIV, the designs for the canal and dam were drawn up in 1662. The construction of the Saint Ferreol dam began in 1666 and was completed in 1675.

The dam consisted of a water retaining wall supported by a downstream embankment. In case of a too rapid depletion of the reservoir, the water retaining wall was stabilized by a lower embankment built against the upstream face. The result may be regarded as an earthen dam with a masonry core. With its height of 36 m, the Saint Ferreol dam remained the heightest embankment in the world for 165 years!

Evolution of Modern Embankment Dams

Embankment dams in the U.S. prior to 1930 had a poor track record. Of those over 490 ft high, almost 10% failed, usually due to overtopping in a flood. Overtopping is when the water level in the reservoir reaches maximum height and begins to flow over the top of the dam.

The South Fork dam in Johnstown,PA was one of the first to use rockfills, or loose rocks, on the downstream face. This dam failed after being overtopped in 1889, kill over 2000 people.

The introduction of the use of hydraulic fill came sometime around 1869. Earth and rocks would be transported by pipe to the site and filled into the core of the dam. There were numerous accidents when cores burst during construction. Vibrating rollers are now used to compact soil and gravel for embankment dams.

Embankment Dams -2-

http://simscience.org/cracks/intermediate/ebnk_char1.html


Wolf Creek Dam
photo courtesy of the Nashville District,
US Army Corps of Engineers

Embankment dams are massive dams made of earth or rock. They rely on their weight to resist the flow of water. Explore the links below to learn more about embankment dams.

Embankment dams usually have some sort of water proof insides (called the core), which is covered with earth or rock fill. Grass may even be grown on the earth fill. Water will seep in through the earth or rock fill, but should not seep into the core.

The main force on an embankment dam is the force of the water. The weight of the dam is also a force, but each material has a different weight, so it is not shown here as one force the way it is on the concrete dams. The uplift force is also acting on the embankment dam, but some of the water seeps into the dam so the force is not the same as on a concrete dam. Roll over the water force to see it on the dam.

The embankment dam is the only dam type we are introducing that is not made of concrete. Embankment dams may be made of earth or rock, both of which are pervious to water -- that is, water can get into it. You see above that the water will seep into the core material and should stop at the seepage line. The core material is usually more watertight that the rock or earth that is on the outside of the dam, but the core material is still not totally impervious to water. Concrete is not truly impervious either, but it does not allow as much seepage as these materials do.

The diagram shown above is only one configuration of what an embankment dam may look like. It could be any combination of earth, rock, and core material in any number of arrangements.

Embankment Dams -1-

http://www.britishdams.org/about_dams/embankment.htm

Embankment dams are made mainly from natural materials. The two main types are earthfill dams and rockfill dams. Earthfill dams are made up mostly from compacted earth, while rockfill dams are made up mainly from dumped and compacted rockfill. The materials are usually excavated or quarried from nearby sites, preferably within the reservoir basin. A cross-section (or slice) through an embankment dam shows that it is shaped like a bank, or hill. Most embankment dams have a central section, called the core, made from an impermeable material to stop water passing through the dam. Clayey soils, concrete or asphaltic concrete can be used for the core. Rockfill dams are permeable. They can have a core or an impermeable cover on the upstream face. Materials used for the cover include reinforced concrete and asphaltic concrete. Embankment dams are usually chosen for sites with wide valleys. They can be built on hard rock or softer soils, as they do not exert too much pressure on their foundations. The main type of dam in Britain is the embankment dam, and there are over 3000 of them. Some of them date back to the 9th Century, including Fishers Pond Dam, near Winchester, which was built by monks to provide fish. The average age of our embankment dams is over 100 years old.

Types of Dams

http://en.wikipedia.org/wiki/Dam

The Hoover Dam, a concrete gravity-arch dam in the Black Canyon of the Colorado River

Dams can be formed by human agency, natural causes, or by the intervention of wildlife such as beavers. Man-made dams are typically classified according to their structure, intended purpose or height.

Based on structure and material used, dams are classified as timber dams, embankment dams or masonry dams, with several subtypes.

Intended purposes include providing water for irrigation or town or city water supply, improving navigation, creating a reservoir of water to supply industrial uses, generating hydroelectric power, creating recreation areas or habitat for fish and wildlife, flood control and containing effluent from industrial sites such as mines or factories. Few dams serve all of these purposes but some multi-purpose dams serve more than one.

According to height, a large dam is higher than 15 metres and a major dam is over 150 metres in height. Alternatively, a low dam is less than 30 m high; a medium-height dam is between 30 and 100 m high, and a high dam is over 100 m high.

A saddle dam is an auxiliary dam constructed to confine the reservoir created by a primary dam either to permit a higher water elevation and storage or to limit the extent of a reservoir for increased efficiency. An auxiliary dam is constructed in a low spot or saddle through which the reservoir would otherwise escape. On occasion, a reservoir is contained by a similar structure called a dike to prevent inundation of nearby land. Dikes are commonly used for reclamation of arable land from a shallow lake. This is similar to a levee, which is a wall or embankment built along a river or stream to protect adjacent land from flooding.

An overflow dam is designed to be overtopped. A weir is a type of small overflow dam that can be used for flow measurement.

A check dam is a small dam designed to reduce flow velocity and control soil erosion. Conversely, a wing dam is a structure that only partly restricts a waterway, creating a faster channel that resists the accumulation of sediment.

A dry dam is a dam designed to control flooding. It normally holds back no water and allows the channel to flow freely, except during periods of intense flow that would otherwise cause flooding downstream.

• Diversionary dams

• Timber dams

• Embankment dams

• Masonry dams

• Gravity dams

• Arch dams

• Steel dams

• Cofferdams

Timber Dams

The timber dam is rarely used by humans because of its short lifespan and the limitation in height to which it can be built. The locations where timber dams are most economical to build are those where timber is plentiful, cement is costly and difficult to transport, and only a submerged diversion dam is required. Timber is the basic material used by beavers, often with the addition of mud or stones.


http://ms.wikipedia.org/wiki/Empangan

Timber dams were widely used in the early part of the industrial revolution and in frontier areas due to ease and speed of construction. Rarely built in modern times by humans due to relatively short lifespan and limited height to which they can be built, timber dams must be kept constantly wet in order to maintain their water retention properties and limit deterioration by rot, similar to a barrel. The locations where timber dams are most economical to build are those where timber is plentiful, cement is costly or difficult to transport, and either a low head diversion dam is required or longevity is not an issue. Timber dams were once numerous, especially in the North American west, but most have failed, been hidden under earth embankments or been replaced with entirely new structures. Two common variations of timber dams were the crib and the plank.

Timber crib dams were erected of heavy timbers or dressed logs in the manner of a log house and the interior filled with earth or rubble. The heavy crib structure supported the dam's face and the weight of the water.

Timber plank dams were more elegant structures that employed a variety of construction methods utilizing heavy timbers to support a water retaining arrangement of planks.

History

A dam is a barrier across flowing water that obstructs, directs or retards the flow, often creating a reservoir, lake or impoundment. In Australian and South African English, the word "dam" can also refer to the reservoir rather than the structure. Most dams have a section called a spillway or weir over which or through which it is intended that water will flow either intermittently or continuously.

Some of the first dams were built in Mesopotamia up to 7,000 years ago. These were used to control the water level, for Mesopotamia's weather effected the Tigris and Euphrates rivers and could be quite unpredictable. The earliest recorded dam is believed to have been on the Nile river at Kosheish, where a 15m high masonry structure was built about 2900 B.C. to supply water to capital of Memphis.


http://www.zzw-niedzica.com.pl/historiaa.htm

The history dates back to the beginning of 20th century, and in particular the year 1905, when the first documents were generated concerning the planned construction of Czorsztyn reservoir.

Picture taken in 1928, showing the Dunajec river below the castles in Czorsztyn and Niedzica

More practical implementation of mentioned general concepts consisted in the construction plans of four reservoirs at four most dangerous mountain tributaries of upper Vistula - in Rożnów and in Czorsztyn, on the Dunajec river, in Porąbka on the Soła river, and in Mucharz on the Skawa. It is little known that the initial plans were submitted for evaluation in 1919. The evaluation was to be made by an eminent specialist and constructor of many similar projects in Switzerland, the former Minister for Public Projects and, later, President of Poland - Gabriel Narutowicz.

In effect of the disastrous flood in 1934, the government decided to build rapidly the Rożnów reservoir as the first hydro-engineering project on the Dunajec. The next planned reservoir was to be built in Niedzica. Appropriate documentation was developed in 1938-1939. In the post-war history, the opinions about the project evolved in parallel to the overall problems of water management and environment protection issues, which were crucial in the area of high natural and cultural value.

Dam

From http://www.m-w.com/cgi-bin/dictionary:
Dam is a barrier preventing the flow of water or of loose solid materials (as soil or snow); especially : a barrier built across a watercourse for impounding water.

http://www.answers.com/dam

Dam

A barrier or structure across a stream, river, or waterway for the purpose of confining and controlling the flow of water. Dams vary in size from small earth embankments for farm use to high, massive concrete structures for water supply, hydropower, irrigation, navigation, recreation, sedimentation control, and flood control. As such, dams are cornerstones in the water resources development of river basins. Dams are now built to serve several purposes and are therefore known as multipurpose. The construction of a large dam requires the relocation of existing highways, railroads, and utilities from the river valley to elevations above the reservoir. The two principal types of dams are embankment and concrete. Appurtenant structures of dams include spillways, outlet works, and control facilities; they may also include structures related to hydropower and other project purposes. See also Electric power generation; Irrigation (agriculture); Water supply engineering.

Dams are built for specific purposes. In ancient times, they were built only for water supply or irrigation. Early in the development of the United States, rivers were a primary means of transportation, and therefore navigation dams with locks were constructed on the major rivers. Dams have become more complex to meet large power demands and other needs of modern countries.

In addition to the standard impounded reservoir and the appurtenant structures of a dam (spillway, outlet works, and control facility), a dam with hydropower requires a powerhouse, penstocks, generators, and switchyard. The inflow of water into the reservoir must be monitored continuously, and the outflow must be controlled to obtain maximum benefits. Under normal operating conditions, the reservoir is controlled by the outlet works, consisting of a large tunnel or conduit at stream level with control gates. Under flood conditions, the reservoir is maintained by both the spillway and outlet works. See also Reservoir.

All the features of a dam are monitored and operated from a control room. The room contains the necessary monitors, controls, computers, emergency equipment, and communications systems to allow project personnel to operate the dam safely under all conditions. Standby generators and backup communications equipment are necessary to operate the gates and other reservoir controls in case of power failure. Weather conditions, inflow, reservoir level, discharge, and downstream river levels are also monitored. In addition, the control room monitors instrumentation located in the dam and appurtenant features that measures their structural behavior and physical condition.

All dams are designed and constructed to meet specific requirements. First, a dam should be built from locally available materials when possible. Second, the dam must remain stable under all conditions, during construction, and ultimately in operation, both at the normal reservoir operating level and under all flood and drought conditions. Third, the dam and foundation must be sufficiently watertight to control seepage and maintain the desired reservoir level. Finally, it must have sufficient spillway and outlet works capacity as well as freeboard to prevent floodwater from overtopping it.

Dams are classified by the type of material from which they are constructed. In early times, the materials were earth, large stones, and timber, but as technology developed, other materials and construction procedures were used. Most modern dams fall into two categories: embankment and concrete. Embankment dams are earth or rock-fill; other gravity dams and arch and buttress dams are concrete. See also Arch; Concrete.

The type of dam for a particular site is selected on the basis of technical and economic data and environmental considerations. In the early stages of design, several sites and types are considered. Drill holes and test pits at each site provide soil and rock samples for testing physical properties. In some cases, field pumping tests are performed to evaluate seepage potential. Preliminary designs and cost estimates are prepared and reviewed by hydrologic, hydraulic, geotechnical, and structural engineers, as well as geologists. Environmental quality of the water, ecological systems, and cultural data are also considered in the site-selection process.
Factors that affect the type are topography, geology, foundation conditions, hydrology, earthquakes, and availability of construction materials. The foundation of the dam should be as sound and free of faults as possible. Narrow valleys with shallow sound rock favor concrete dams. Wide valleys with varying rock depths and conditions favor embankment dams. Earth dams are the most common type. See also Engineering geology; Fault and fault structures.
The designers of a dam must consider the stream flow around or through the damsite during construction. Stream flow records provide the information for use in determining the largest flood to divert during the selected construction period. One common practice for diversion involves constructing the permanent outlet works, which may be a conduit or a tunnel in the abutment, along with portions of the dam adjacent to the abutments, in the first construction period. The stream is diverted into the outlet works by a cofferdam high enough to prevent overtopping during construction. A downstream cofferdam is also required to keep the damsite dry. See also Cofferdam.

Personnel responsible for operation and maintenance of the dam are familiar with the operating instructions and maintenance schedule. A schedule is established for collection and reporting of data for climatic conditions, rainfall, snow cover, stream flows, and water quality of the reservoir, as well as the downstream reaches. All these data are evaluated for use in reservoir regulation. Another schedule is established for the collection of instrumentation data used to determine the structural behavior and physical condition of the dam. These data are evaluated frequently. Routine maintenance and inspection of the dam and appurtenant structures are ongoing processes. The scheduled maintenance is important to preserve the integrity of the mechanical equipment.

The frequency with which instrumentation data are obtained is an extremely important issue and depends on operating conditions. Timely collection and evaluation of data are critical for periods when the loading changes, such as during floods and after earthquakes. Advances in applications of remote sensing to instrumentation have made real-time data collection possible. This is a significant improvement for making dam safety evaluations.
Throughout history there have been instances of dam failure and discharge of stored water, sometimes causing considerable loss of life and great damage to property. Failures have generally involved dams that were designed and constructed to engineering standards acceptable at the time. Most failures have occurred with new dams, within the first five years of operation.