Evaluation Of The Major Dam Types

Essay add: 22-10-2015, 20:34   /   Views: 139

Many parts of the planet suffer from water shortages due to lack of rain. One of the most serious consequences of this effect is drought. The problem of drought combined with the effect of climate change and the increase of the population, who increasingly demand a greater quantity of water, can cause catastrophic consequences in some regions of the planet.

One solution to this problem may be the storage of groundwater. If water from runoff is stored in underground reservoirs, then the serious problem of water scarcity can be reduced. The groundwater storage is one of the best ways to cope with these problems in arid and semi arid regions.Conventional surface storage offer disadvantages in arid countries. In the first place a large amount of water is lost by evaporation. In addition to this, this water can be contaminated.

Groundwater dams offer a viable alternative in these regions of the planet. Different techniques have been used throughout history to store groundwater, but two main types are considered in this work. Groundwater dams offer a solution in these regions where there is a water scarcity problem.BACKGROUNDIn arid and semiarid areas, water systems are characterized by limited and irregular precipitation. The water supply is scarce and a large quantity of water is lost by evaporation. Also, water demand in these areas has increased within the last few years due to an intensive agriculture and tourism industry.

Some of the risks of this effect are the reduction of the water table of the aquifers, seawater intrusion, loss of water quality and soil quality, damage to river ecosystems and wetlands, and even the sustainability of uses of affected soil.The rational exploitation of groundwater is a key element in the economic development of a country, area or region. In regions with arid or semiarid climate, groundwater also has a strategic interest. The intensive and uncontrolled exploitation of groundwater may result in certain cases, in environmental problems that encourage processes of desertification (Salman, S., 1999).

In sandy soils, the lowering water tables can provoke erosion and formation of dunes.In very arid areas land irrigation with waters with high salt content may require the abandonment of land due to salinisation of agricultural soils (Jackson, A. et al., 2000). In this way, the optimal use of the potential for groundwater recharge is an important factor in helping to ensure supply. Therefore, the availability of procedures to estimate the magnitude of the potential recharge and identify the most appropriate sites is very important.The main purpose of the management of aquifer recharge is to achieve the sustainable exploitation of groundwater, to ensure supply and the future of water resources in the area.Different techniques exist for the management of recharging aquifers; one of the most important is the construction of dams in order to modify the water channel.

Within this group of groundwater dams we can consider two main types: subsurface dams and sand storage dams (Kresic, N., 2007). These dams can store from a few hundred up to several million cubic metres, although if the dam is constructed in a narrow valley this capacity will be lower than that (Murty J., 1998).In regions with arid and semiarid climates the construction of small conventional surface dams causes in many occasions large evaporation losses. Subsurface dams are an important alternative provided against evaporation. Subsurface dams are underground walls whose function is to increase the water table of the aquifer upstream the dam blocking the flow of water.Sand storage dams are structures that are built transversely to the direction of flow of the channel, causing sediment accumulation upstream of the dam. These sediments form a highly permeable aquifer, which can be considered as an artificial aquifer that contains the runoff water.

They increase the storage capacity forming an artificial aquifer upstream of the dam. These dams do not retain completely the flow of water, but it holds back part of it and allows some of the flow through them.SUBSURFACE DAMSFigure 1 General principle of subsurface damUnderground dams can generate artificial groundwater reserves. This method is applicable to rivers with a temporary regime in arid regions. Sometimes they capture large amounts of water and can be very attractive in countries with problems with water shortages (Clarke, R.,1991). Figure 1 shows the basic theory in an underground dam.For its construction there must be fairly long alluvial valley excavated in a geological non aquifer formations such as granite (Paepe R. et al, 1990).

The alluvial sediment layer thickness should be between 10 and 20 meters. It is also important that the smaller channel of the river temporarily embedded in the alluvial valley is not too deep.For the construction of underground dams is sufficient to dig a trench across the valley, at right angles to the direction of the underflow, in the alluvial sediments layer until reaching down the bedrock or any other solid, which is impervious (Kresic, N., 2007). Next, a wall is built in the trench which can be a brick wall, dry stone or masonry (Paepe R. et al, 1990), covered with a waterproof front. After that, the trench is refilled by the materials excavated before. The wall cannot be higher than the top of the bedrock as this would provoke a drainage down the valley and the idea is to make the structure as impervious as possible.The water in the underflow dam will be stopped by the wall and the saturation surface will go up to the top of the wall where will emerge (Mati, B., 2006).

In this way, a water reservoir is built due to the porosity of the alluvial sediments nature and the water can be captured by wells if the permeability of the wells is optimal.In arid areas, the underground dams have two major advantages over classical dams. In the first place the wall is built within a trench and is supported by the materials surrounding the wall. This is much simpler than to build a dam on the surface which requires a more complicated engineering work.

And second, the water stored in the aquifer underground is protected from evaporation (Yoo K. et al, 1994). Also, another advantage of subsurface dams are that the alluvial sediment work as a filter and in that way the quality of water is better, free from contamination and parasites.SAND STORAGE DAMSFigure 2 General principle of a sand storage damThe other important type of groundwater dams are the sand storage dams. This type of dam is typical of arid or semiarid regions where there is usually a rainy season and dry season, so the rivers are not permanent (Bonell M. et al, 1993).

The basic concept of sand dams is that in the rainy season runoff water is stored in the basin with the sediment transported. Thus originates a deposit of sand with a capacity to store a large amount of water. In this way the water stored in the aquifer during the rainy season can be used in dry season (Bonell M. et al, 1993). As water is stored underground this prevents that water evaporates or become contaminated.

Water quality is pretty good since being filtered through the sand would improve the quality. Sand dams are a very cost effective (Sen Z., 2008) method of storing water in arid and semi arid areas. They also provide clean water and improve the environment raising the water table.During different episodes of rain the river carries large quantities of sediment. During the dry season large amounts of sediment will be created resulting from erosion. Then, at the beginning of the rainy season these materials are transported and deposited by water reaching the dam as a result of the decrease of speed.

As the river flow rate decreases, larger particles are deposited. Thus, smaller particles remain in suspension and are carried over the dam. Sometimes a layer of silt is deposited immediately adjacent to the dam.

For that reason the height of the dam should not be too high as it could provoke the silt to settle. If this process is repeated over several episodes the body of sand will get closer and closer until it reaches the dam where will fill the entire volume behind the dam. The dam is built in successive stages (Stephenson, D., 2004).Some of the advantages of sand dams are recharge of groundwater over an area where several sand dams are constructed in a channel. Sand dams produce land for harvesting.The big problem in some arid and semi arid areas in the planet is that a high percentage of runoff water is lost.

Normally in these areas rain happen as a short event, and also if the soil is made of silt or clay, this make it very difficult for water to infiltrate.The sand dam is built across the riverbed of a seasonal river, on top of the rock. A trench is excavated and a reinforced concrete wall is built. In the first stages of the construction of the dam it fills up with water, but after a few seasons will fills up with sand. The river carries sediment and the sand is deposited upstream the dam while the clay and silt is taken down the stream.

The sand is like a filter that provides clean water free of contamination. Sand dams can hold up to 30% (Unger, P., 2006) of water behind the dams. So this makes water available when there is a drought.

While water is stored in a sand dam is also recharging the surrounding area improving the environment in the area. Sand dams can solve water scarcity problems in some parts of the planet creating a system of sand dams along a river. Within years, the system can store enough water that can regenerate the whole area, creating a microclimate. In order to make a sand dam efficient some points must be taken. If the dam is not properly constructed there will be leakages.CONCLUSIONSGroundwater dams have proved to be a viable solution in regions with arid or semi arid climate.

The advantages are many and are a very cost effective alternative to conventional dams. Some of the advantages are no loss of water by evaporation, the risk of contamination is reduced and water quality is even improved since the aquifer performs as a filter that cleans the water. One of the critical points of building the dam is that if it is not properly constructed then leakages may occur and then the purpose of the dam will be useless.

Two main types of dams are described, subsurface dams and sand storage dams. Both types work very similar, the idea is to hold back the water upstream the dam in a permeable aquifer that sits on top of the rock which is impervious. In this way water is stored and collected by different ways.

The water is clean and is available for long time, especially in dry seasons.

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