Hydraulic Structures

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Reservoir site with the presence of permeable rocks reduces the water tightness of the reservoir. Reservoir It should also have minimum percolation losses. The catchment area should avoid or exclude water from tributaries carrying high percentage of silt in water. Catchment Area There is no special site requirement for the spillway if it is to be built inside the dam. Spillway Therefore construction materials should easily be available either locally or near vicinity of the site so as to reduce the transportation cost.

Selection of Site for Dam The choice and selection of dam at a particular place in the river may depend on the following factors. Hence such foundations are suitable only for earth dam or low concrete gravity dam. They are not suitable for rock fill dam. Geology and Foundation Conditions The local availability of sand, gravel, crushed stone suggests concrete gravity dam. However, if coarse and fine grained soils are available locally, an earth dam may be suitable. When separate site of spillway is available earthen dam may be preferred.

In case of large capacity spillway, an overflow concrete gravity dam having overflow section in the middle will be the best choice. Spillways are invariably provided in all the dams and often called safety valve for the dam. It is necessary to provide a spillway of sufficient capacity so as to avoid water from overtopping the dam.

Overtopping of dam may lead to failure of dam resulting in serious damage to the property. It is a major component of a spillway. It regulates and controls the surplus water from the reservoir. It does not allow the discharge of water below from reservoir level and allows the discharge of water below from reservoir level and allows only when water surface in the reservoir rises above that level. The discharge channel may be the downstream face of spillway itself, or open channel excavated along the ground surface or a closed conduit placed through or under a dam.

Components Parts of a Spillway High Voltage water coming through Spillway may cause serious damage to the toe of dam and to the adjacent structures.

This high energy of flow must be dissipated before it flows back to river. For this, energy dissipaters are provided However entrance channels are provided to draw water from reservoir and convey it to control structure. Similarly Outlet channels are provided to carry the spillway flow in river channel below the dam. Similarly, outlet channels are provided to carry the spillway flow to river channels below the dam.

The spillways may be classified in the following types. This type of Spillway is suitable for low earth dam, low concrete masonry gravity dam or low thin arch dam. It is not suitable for high Dam. Free Over-fall The overflow water is guided smoothly over the crest and profile of spillway. This type of spillway is more preferable on valleys where width of river is more to provide sufficient crest length and river bed can be protected from scouring at reasonable cost.


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Ogee or Overflow Spillway It discharges the surplus flood through a steep sloped open channel. The crest of spillway is kept normal to its centre line. It consists of a discharge channel to the river. Trough Spillway It is best suited for non rigid dam like earthen dam. Side Channel Spillway Types of Spillways Siphon Spillway: When available space is limited and surplus discharge is not large siphon spillway is often preferred. It is based on siphon action in the shape of an inverted pipe. Usually siphon spillway is provided in concrete gravity dam through its body.

Siphon Spillway The arrangements provided to dissipate this energy are known as energy dissipaters. Roller Basin B. Deflector Bucket C. Flip Bucket D. Non-radial spillway and sluice buckets E. Schoklitsch Dissipater An additional storage can be obtained by providing gates on the spillway crest. However, during floods, these gates are lifted to make full use of spillway capacity. Great care is needed to be taken while operating crest gates of earth dams so as to avoid the overtopping of earth dam. They consists of wooden panels usually 1. They are hinge at bottom and supported by struts to resist water Pressure.

These wooden panels can be raised or lowered from an overhead cableway or a bridge. Its shape is just as sector of a circle properly braced and hinged at the pivot. The load of the gate, water etc. An operating plateform is provided to lift the the gate by means of ropes and chain Radial Gates or Tainter Gates The drum gate consists of circular sector in cross section formed by skin plates attached to internal bearing.

The entire section may be raised or lowered such that upper surface coincides with the crest of spillway. The buoyant force due to head water pressure underneath the drum assists in its lifting. The drum Gate needs large recess and hence is not suitable for smaller spillway. Drum Gates The sediment particles try to settle down to the river bottom due to the gravitational force, but may be kept in suspension due to the upward current in the turbulent flow which may overcome the gravity force.

The bigger suspended particles and most of the bed load, therefore, gets deposited in the head reaches of the reservoir. The disintegrated rocks and loose soil form the sediment which is carried by the river and gets deposited on the reservoir bed near the base of the dam.

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The sediment mainly consists of sand and silt. The process of deposition of sand and silt in the reservoir is designated as reservoir sedimentation. Causes of Sedimentation in Reservoir The following are the causes of sedimentation Characteristics of soil in Catchment area: If the catchment area is composed of loose soil, then it may get easily eroded and get carried away by the river. On the other hand, if the soil of the catchment area is hard and rocky, the river would not be able to carry sediment.

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Topography of the catchment Area: If the catchment area consists of steep slope, then it will develop high velocity of flow which will cause more erosion of the surface soil thereby making the river carry a lot of sediment. Intensity of Rainfall in Catchment Area: If the intensity of rainfall in catchment area is high, then it will increase the rate of run-off and the river will carry more sediment. The following are the causes of Sedimentation Cultivation in Catchment Area: The intensive cultivation in the catchment area will make the soil loose and rain water will carry a lot of sediment to the river.

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Vegetation cover in Catchment Area: If the catchment area is covered with vegetation cover like grass, plants, forest area, etc. If there is no such vegetal apron or cover, the soil may be easily eroded and the river will carry much sediment. This zone is termed as delta. The fine sediments remain in suspension for a considerable time and are carried towards the dam.

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These sediments are deposited at the foot of the dam. The floating debris is collected above delta. The clear water circulates the middle portion of the reservoir. Thus, the life of reservoir depends on the trend of the sedimentation. Effect of Sedimentation If it is found that a tributary carries heavy sediment, the reservoir site should be shifted towards the upstream to avoid the tributary.

Initially a reservoir of less capacity is constructed. When the reservoir gets silted, gradually the capacity should be increased stage by stage, by increasing the height of the dam. Selection of a Reservoir Site Removal of Sediments So the cultivation should be done in a planned manner and necessary measures should be taken to avoid the loose soil getting carried away by the rain. Here, the check dams serve the purpose of detention reservoir where the heavy sediments get settled down.

Construction of Check Dams It is generally composed of loose soil or permeable rock formation.

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These slips should be stabilized by suitable methods so that the debris may not be carried by flowing water. Construction of Contour Bunds Density Currents A lot of sediment load can, thus be passed out of the reservoir, if it is possible to locate the dam outlets and sluiceways in such a fashion, as to vent out the density currents. Depending upon which, the following losses may occur from such a reservoir.

Reservoir Losses The evaporation loss from the reservoir under the given atmospheric condition can easily estimated by measuring the standard pan evaporation and multiplying the same by the pan coefficient.

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  4. These losses in fact vary from place to place and from season to season, and hence monthly values of these losses are usually determined. All these methods are based upon the efforts made to reduce the evaporation rate from the surface of water bodies by physical or chemical means, since the basic meteorological factors affecting evaporation cannot be controlled under normal conditions.

    The following methods are generally used for evaporation control. They certainly depend upon the type of soil forming the reservoir. But in certain particular cases, when the walls of the reservoir are made of badly fractured rocks or having continuous seams of porous strata, serious leakage may occur. Sometimes, pressure grouting may have to be used to seal the fractured rocks.

    The cost of grouting has to be accounted in the economic studies of the project, if the leakage is large. It is equivalent to the elevation of the spillway crest or the top of the spillway gates for most of the cases. This level may be fixed by the elevation of the lowest outlet in the dam or may be guided by the minimum head required for efficient functioning of turbines. Storage Zones of a Reservoir Water stored in the reservoir below the minimum pool level is known as the Dead Storage, and if it is not of much use in the operation of the reservoirs.

    The useful storage may be subdivided into conservation storage and flood mitigation storage in a multipurpose reservoir. The maximum level to which the water rises during the worst design flood is known as the maximum pool level.

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    Artelia has a wealth of experience in the technical aspects of such schemes free surface flows, unsteady flow conditions, water hammer, regulation, etc. The Group also enables developers to benefit from its experience in major hydraulic structures such as pumping stations, lift stations, water intakes and outlets, tunnels, vortex shafts, surge shafts, reservoirs, etc. In addition, Artelia has well-tested know-how in the field of protecting linear infrastructure against floods, especially cases such as a road or rail bridge crossing a river.

    With a view to improving drinking water supply in the wilaya of Algiers, the SEAAL has launched a project to rehabilitate seven drinking water pumping stations that play a major role in the supply of drinking water to the wilaya. The first objective for VNF is to develop a tool adapted to the sustainable management of its network: this tool must provide sufficient data on the state of its structures and thereby enable it to plan and optimise its maintenance policy by ranking the measures to be taken by order of priority.

    The current water resources tube wells are insufficient and it has proved essential to look for a resource located further away from the site. Share :.

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