Dam Construction

masterwriter(58)

Published in

#stemng

Words

1300

Reading

6 min

Listen

Play

images (5).jpeg
Upper Mississippi River Pools, Locks, and Dams.
Source: wikimedia commons

Dams are structures built to divert or store water for irrigation, water supply, flood control, electric power generation and/or navigational purposes. Dams are classified on the basis of the types and materials of construction. The major types are gravity, arch, butress and earth dam. The first three types are usually constructed of concrete. A gravity dam depends on its own weight for stability and is usually straight on plan even though it may sometimes be slightly curved.
Arch dams transmit most of the horizontal thrust of the water behind to the abutments by arch action and have thinner cross-section when compared with gravity dam. Arch dams are preferably used in narrow canyons where the walls are capable of withstanding the thrust produced by the arch action. The simplest type of the buttress dams in the slab type which consists sloping flat slabs, supported at intervals. Earth dams are embankments of rock or earth with provision for controlling seepage by means of an impermeable core or upstream blanket. Other types of dams include steel, timber, movable and roller dams.

Erection of a Dam

^{^{Stages of construction of the dam of the Grande-Dixence dam, source: wikimedia commons}}

Erection of a dam will usually require the consideration of certain factors since its usefulness when appropriately designed and sited have enormous benefit. On the other hand, it's failure can result in considerable socioeconomic and environmental losses. These factors are geographical because of the resulting modification of the surrounding landscape, they are geological in respect to the appropriateness of the bedrock strata which will reduce seepage losses, they are economical in respect to alternative water resources development options, they are social, such as problems of health and resettlement and finally environmental implications.

Forces on Dams

^{^{forces on gravity dam, source: Wikimedia commons}}

A dam must be relatively impervious to water and capable of resisting gravity (it's own weight), hydrostatic pressure (sediment deposits and flowing water), uplift, earthquake forces, and so on. The forces are transmitted to the foundation and abutments of the dam which react against the dam with equal and opposite force.

Earth Dams

^{^{Benmore (Earth) Dam, Spillway Gates - panoramio, source : wikimedia commons}}

Classification of earth dams is based on the material the bulk body is made of. There are the plain earth dams the bulk of which are made of small clayey, sandy or sandy gravelly soils. The rock and earth fill dams are constructed with gravelly pebble while the rock fill earth are constructed with large size material. An earth dam has usually a trapezoidal cross section with straight or broken sideslopes. Horizontal portions of slope surfaces are called berms.

Earth dams are further classified as homogeneous and heterogeneous depending on whether they are made of one or more kinds of soil materials; as low (>30m), medium (30≤ H ≤ 75m), high (75 < H ≤ 125m) and very high ( ≥ 125m); as filled, hydraulically-filled, rip-rap and blast filled depending on the technique of the construction.

Structural safety and economy of earth dams are ensured by careful analysis of the nature, magnitude and effects of seepage forces. Maximum seepage forces occur with the headwater at the full reservoir level (FRL) and tailwater at the maximum water level corresponding to the passage of design flood. It is also necessary to consider other operating conditions such as the case when headwater is at the surcharge level -line and then draw appropriate conclusions on the design parameters. Drainage devices are provided for collection and removal of seepage waters. The upstream slope of the seepage is riveted for protection against wave action while the downstream slope is riveted for protection against rainfall.

Selection of Dam Type

Selection of dam type at a given site depends on many physical factors such as topography, groovy and foundation conditions, available materials and suitable site for spillway. Selection of dam type is preceeded by consideration of the characteristics of each type of dam with respect to its physical features, the purposes the dam is supposed to serve as well as the economy, safety and other pertinent limitations. Any dam can be constructed on a foundation with sound rock which has no fault or fissures. Availability of suitable materials on site is another important consideration which helps to reduce the construction cost.

Freeboard

This is the vertical distance between the top of the dam and the full supply level in the reservoir. It comprises of the rise in reservoir due to flooding, periodic undulations in the reservoir, the wind effect and wave action. For medium sized reservoirs. Free board is expressed as

Where H is the wave height.

H = 0.32√UF + 0.076-0.24^4√F

F and U are the wind fetch (km) and the wind speed(km h^-1) respectively.

General Approach for Design of Gravity Dams

^{^{Norris-dam-design-tva1, source : wikimedia commons}}

For small dams and preliminary design of large dams, it is adequate to use the simplified approach based on the elastic behavior of concrete. Final design of large dams may require the use of sophisticated methods involving finite element procedures like the assumption of the dimension of the dam and to check the statics and dynamic stability.

To avoid tensile stresses, keep the resultant force, R within the middle third of the base. In the simplified approach, the dam is divided into slices and each slice is assumed to act independent of adjoining slices.

A well designed dam must have acceptable margin of safety with respect to rotation and overturning, translation and sliding, both of which control structural stability and lastly over stress and material failure. This must be satisfied for the dam concrete and the rock formation.

Construction of Gravity Dams

^{^{Shasta gravity dam construction, source wikipedia}}

Before the construction of a dam is started, flow is diverted either by damming the river at the upstream and downstream of the construction zone and providing a diversion channel or by excluding only portion of the construction zone in stages.

The temporary structure used to exclude water from an area to permit construction operation is called a cofferdam. In two dates construction, the flow is diverted to one side of the channel while work proceeds on the other side. After work on the lower portion of one side of the dam is completed, flow is diverted through outlets in this portion. A tunnel or diversion channel may be used where the geology and the topography of the area permits. It is advantageous to schedule construction during low flow periods to reduce diversion problems.

Spillways

^{^{Bonneville Dam spillway cross-section. Source :wikimedia commons}}

A spillway is an overflow device which serves to dispose off excess water behind a dam. This is necessary in order to prevent the dam from being damaged while maintaining the level of water below a desired limit. The spillway can form an integral part of the dam or be built separately. If built as part of the dam, the spillway could be of the overflow, submerged or bottom type. In these types the headwater enters the spillway through a weir, draw of orifice and bottom orifice respectively, gates on the spillway crest, together with sluiceways, permit the operator to control the release of water downstream.

Spillways are named depending on the type of structure used and the manner excess water is conveyed downstream. If the water passes over the crest, it is called an overflow spillway. In a chute spillway the water passes through a chute while in a shaft spillway it passes through a vertical shaft. If the water flows through a channel parallel to the crest after passing over the crest, the spillway is called a side channel spillway. While the overflow spillway is usually used in gravity, arch and buttress dams, the chute spillway is applied in both small and large rockfall or earth dams.

The steps for the design of free-flowing spillway (without gate) are the estimation of the maximum outflow from the reservoir for a given inflow hydrograph, choosing the spillway size and type and routing the inflow with the chosen spillway with the maximum permissible water level (which is fixed by reservoir use and dam height). Knowledge of reservoir routing is very important for spillway design.

The energy of water falling over a spill can cause significant scouring downstream. This is mitigaed by toughening the spillway with concrete blocks, bars or any other energy dissipating device. At the bottom of the dam, the spillway is often built with an outward curvature to direct the flow downstream.

Conclusion

In conclusion, a dam is simply a structure that is made of earth, rock, masonry or concrete which is built across a stream or river and stands as a barrier to obstruct the flow of water.
Dams unquestionably have an essential role to play within the world’s social, political, and economic framework.
Dams are multipurpose structures that can be used for irrigation, drinking water, making electricity, and for recreation and flood control.
We may have drank clean water from a dam's reservoir. Of course we have eating food grown on a farm that was irrigated by a dam's reservoir water. What about the electricity we use? Amazingly, electric power can be generated from a dam which is called hydroelectricity , it is generated from a hydroelectric plant using water stored behind a dam.
Dams also provide enhanced environmental protection, such as the retention of hazardous materials and detrimental sedimentation, dams and locks also provide a stable system of inland river transportation.