Hi! We are back again!
This time we are here to share with you about dams!!

In short:

By the way, before we start, let us set you thinking!
Do you know what dams are?
What are dams for?

Well, dams, they are, structural barriers built to obstruct or control the flow of water in rivers and streams. They are designed to serve two broad functions. The first is the storage of water to compensate for fluctuations in river discharge (flow) or in demand for water and energy. The second is the increase of hydraulic head, or the difference in height between water levels in the lake created upstream of the dam and the downstream river.
By creating additional storage and head, dams can serve one or more purposes:
- Generating electricity
- Supplying water for agricultural, industrial, and household needs;
- Controlling the impact of floodwaters; and
- Enhancing river navigation.
They can be operated in a manner that simultaneously augments downstream water quality, enhances fish and wildlife habitat, and provides for a variety of recreational activities, such as fishing, boating, and swimming.

In detail:

How dam works?

A typical dam is a wall of solid material built across a river to block the flow of the river thus storing water in the lake that will form upstream of the dam as water continues to flow from the river upstream of the dam.
The main purpose of most dams is to create a permanent reservoir of water for use at a later time. The dam must be watertight so that water does not leak out of the dam and escape downstream. An essential part of a dam is therefore the "impermeable membrane", ie the watertight part of the dam that prevents water leaking out. As we shall see later, it is not necessary that the entire dam wall be watertight. The natural earth or rock on which the dam is built (ie the dam foundation) must also be watertight, as must the river valley in which the storage reservoir forms. If these natural areas (dam foundation and storage area) are not watertight then water could leak out of the reservoir even if the dam itself is watertight.
As well as being watertight a dam must also be stable ie the dam wall must have sufficient strength to firstly, stand permanently under its own weight especially when at least part of the dam wall is saturated with water and secondly, resist the water pressure in the lake upstream of the dam. This water pressure exerts a force on the dam wall tending to push it downstream. The higher the dam, the greater the depth of water stored behind the dam and the greater the water pressure on the dam wall. The dam must also have sufficient strength to resist other forces to which it may be subjected from time to time eg shaking from earthquakes. The threat that earthquakes pose to dams varies widely depending on the region of the world in which the dam is located.
A dam must have some way of releasing water in controlled amounts as it is needed ie an outlet valve of some type. Depending on the purpose of the dam the water may be released into a pipeline to supply a city with water, or into a hydro-electric power station to generate electricity or the water may simply be released into the river bed downstream of the dam and allowed to flow naturally down the river, eventually to be pumped out and used for irrigation of crops further downstream. The outlet valve must be connected via a pipe or tunnel to some type of intake structure where the water is actually drawn from the storage reservoir.
When the river on which the dam has been built floods a very large volume of flood water will flow into the storage reservoir. Usually this is very, very much more water than can be released through the outlet valve. A dam must have some means whereby these large volumes of flood water can flow around the dam without causing damage to the dam itself; ie a spillway which, in most cases, is an open cut channel large enough to carry the flood water around the dam. If the dam is built of concrete the spillway may form part of the dam wall itself. However, if the dam is built of earth and/or rock fill (ie soil and broken rock) the spillway must be a separate structure because flood waters cannot be allowed to flow over the top of a fill (or embankment) dam which would be quickly washed away by the flood water if this was to happen.

A large dam project may involve many types of construction apart from building the dam wall itself eg tunnelling for diversion or outlet works; road building to replace roads flooded by the reservoir; quarrying to obtain rock fill and other construction materials; excavation of open cuts for the spillway, access roads and road deviations

There are mainly 4 major classes of dams are based on the type of construction and materials used: embankment, gravity, arch, and buttress.

1. Embankment.
Embankment dams typically are constructed of compacted earth, rock, or both, making them less expensive than others that are constructed of concrete. Consequently, more than 80 percent of all large dams are of this type. Embankment dams have a triangular-shaped profile and typically are used to retain water across broad rivers.
2. Gravity.
Gravity dams consist of thick, vertical walls of concrete built across relatively narrow river valleys with firm bedrock. Their weight alone is great enough to resist overturning or sliding tendencies due to horizontal loads imposed by the upstream water.
3. Arch.
Arch dams, also constructed of concrete, are designed to transfer these loads to adjacent rock formations. As a result, arch dams are limited to narrow canyons with strong rock walls that can resist the arch thrust at the foundation and sides of the dam.
4. Buttress.
Buttress dams are essentially hollow gravity dams constructed of steel-reinforced concrete or timber.

Do you know how dams are constructed?

Careful planning throughout the siting, design, and construction of dams is necessary for optimal utilization of rivers and for preventing catastrophic dam failure. These planning phases require input from engineers, geologists, hydrologists, ecologists, financiers, and a number of other professionals.
Designers must first evaluate alternative solutions and designs for meeting the same desired objective, whether the goal is to allocate water supply, improve flood control, or generate electricity. Each alternative requires a comprehensive cost-benefit analysis and feasibility study for evaluating its physical, economic, ecological, and social impact.
Once an alternative has been selected, a number of important considerations enter into the design and construction of the dam. These include:
- Hydrological evaluation of climate and streamflows;
- Geologic investigation for the foundation design;
- Assessment of the area to be inundated by the upstream lake (also called a reservoir) and its associated environmental and ecological impacts;
- Selection of materials and construction techniques;
- Designation of methods for diverting stream flow during construction of the dam;
- Evaluation of the potential for sediments to accumulate on the reservoir bottom and subsequently reduce storage capacity; and
- Analysis of dam safety and failure concerns.

When planning is all done, here comes the construction!

Construction work on a dam project is carried out at the dam site itself and also at other locations such as spillway site, rock fill quarry, clay and gravel borrow areas and road works which may be near the dam site or, in other cases, may be considerable distances from the dam site.From a construction point of view the dam site consists of the river bed area which is normally submerged by the river and the abutments or valley sides above the river bed. No work can be carried out in the river bed area until the river has been diverted, usually through a tunnel or channel. After diversion the river bed area is dry and construction of the dam wall can commence.
Before the dam wall can be built two important preliminary steps have to be completed: the dam foundation area must be stripped of overburden (ie material, usually soil, which is unsuitable for dam construction must be removed and disposed of), and the foundation for the dam must be prepared. Foundation preparation usually consists of two quite different activities: removal of pockets and seams of weak material and replacement of this material with "dental" concrete, so called because the work resembles filling cavities in teeth; the second aspect of foundation preparation is grouting. Grouting involves drilling holes (often to quite great depths, equal to the height of the dam in some cases) and then pumping these holes full of cement grout (a mixture of cement and water). The purpose of grouting is to fill up open cracks in the rock foundation on which the dam is to be built so that water will not leak out of the reservoir after the dam is finished.Once the foundation preparation is complete the construction of the dam wall itself can commence by hauling, dumping and compacting construction materials such as clay and rock fill. Compaction is carried out to ensure that the fill, as placed, will have the maximum possible density. Properly compacted fill will have the desirable engineering properties of high strength and low settlement over time, and, in the case of clay fill for a dam core, low permeability, which is important to reduce the likelihood of failure by piping or internal erosion.

A typical construction sequence for a fill dam is:

Stage 1: Excavate diversion tunnel and build coffer dams. The milestone of diverting the river through the diversion tunnel marks the end of this stage.

Stage 2: Strip dam foundation of overburden. Carry out foundation treatment and grouting. Excavate and haul fill construction materials from their sources and place and compact in the dam embankment. The end of this stage is marked by the milestone of closure of the diversion tunnel to start the storage of water in the dam reservoir. Excavation of the spillway will also be under way during this stage.

Stage 3: Complete outlet works, spillway and all other parts of dam project.
Management of floods during construction is one aspect always given considerable attention by the engineers designing the dam. Before the design of the dam can be completed engineering geologists have to find sources of suitable clay, gravel and rock which can be used as construction materials with which the dam wall can be built.

In fact, a dam project passes through four phases during its life;
- Investigation phase
- Design phase
- Construction phase
- Operation and Maintenance phase

When a dam is put into operation, or commissioned, water is released from the upstream reservoir over a spillway or through gates in a manner to satisfy intended objectives. Operating rules for maximizing power generation, for example, include maintaining hydraulic head. In contrast, water levels in flood control reservoirs must be periodically reduced to allow for new storage during anticipated periods of flood hazard. Operating issues, however, can easily become complex and highly politicized and may be difficult to resolve. This is particularly true for river systems containing several reservoirs, for dams serving multiple purposes, and in cases where adverse social, ecological, and environmental impacts are significant.

Do you know what dams are for?

Dams are usually built for one or more of the following reasons:

1) To provide a supply of water for towns, cities and mining sites; eg Warragamba Dam, Australia is the main water supply dam for the city of Sydney.

2) To contain and store waste (tailings) from mines; eg Omai Tailings Dam, Guyana, South America which stored waste from a gold mining operation.

3) To provide a supply of water for the irrigation of crops; eg Burrinjuck Dam, Australia which was built as the main head water storage for the Murrumbidgee Irrigation Area in New South Wales.

4) To generate electricity in hydro-electric power stations; eg Itaipu Dam, Brazil is the largest hydro-electric power station in the world.

5) To help control or mitigate floods; eg the Tennessee Valley Authority dams in the U.S.A. which help control floods on the Tennessee, the lower Ohio, and the lower Mississippi Rivers.

Many dams are multipurpose and most dams have at least some flood mitigation effect in addition to their primary purpose. Dams built specifically for flood control may have some of their storage capacity kept empty during normal river flow conditions so that space is available to store excess water inflow under flood conditions. The flood mitigation effect of a dam is such that the downstream river height at the peak of the flood is reduced but, after the peak has passed, the river levels usually remain high for a longer period than would have been the case if the dam had not been built. This is because excess flood water is only stored behind the dam temporarily and is slowly released from the dam in the days and weeks after the flood peak has passed.

Citations:

1) Linsley, Ray K. et al. Water Resources Engineering, 4th ed. New York: McGraw-Hill, 1992.
2) Mays, Larry W. Water Resources Engineering. New York: John Wiley & Sons, 2001.
3) Morris, Gregory L., and Jiahua Fan. Reservoir Sedimentation Handbook. New York:McGraw-Hill, 1998.

Hope that you have a clearer idea what dams are and what they are for!
So see you again soon!
Signing off!

Hydroelectricians:D