Dams

• Dam is a solid barrier constructed at a suitable location across a river valley to store flowing water.
• A dam is a hydraulic structure of fairly impervious material built across a river to create a reservoir on its upstream side for impounding water for various purposes.
• Dams are generally constructed in the mountainous reach of the river where the valley is narrow and the foundation is good. Dams are probably the most important hydraulic structure built on the rivers. These are very huge structure and require huge money, manpower and time to construct.

Storage of water is utilized for following objectives:

•          Hydropower
•          Irrigation
•          Water for domestic consumption
•          Drought and flood control
•          For navigational facilities
•          Other additional utilization is to develop fisheries

Structure of Dam:

•  Heel: contact with the ground on the upstream side
• Toe: contact on the downstream side
• Abutment: Sides of the valley on which the structure of the dam rest
• Galleries: small rooms like structure left within the dam for checking operations.
• Diversion tunnel: Tunnels are constructed for diverting water before the construction of dam. This helps in keeping the river bed dry.
• Spillways: It is the arrangement near the top to release the excess water of the reservoir to downstream side
• Sluice way: An opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side.

Gravity Dam:

• These dams are heavy and massive wall-like structures of concrete in which the whole weight acts vertically downwards.
• As the entire load is transmitted on the small area of foundation, such dams are constructed where rocks are competent and stable.

Buttress Dam:

• Buttress Dam – Is a gravity dam reinforced by structural supports.
• Buttress - a support that transmits a force from a roof or wall to another supporting structure.
• This type of structure can be considered even if the foundation rocks are little weaker.

Arch Dams:

• These type of dams are concrete or masonry dams which are curved or convex upstream in plan.
• This shape helps to transmit the major part of the water load to the abutments.
• Arch dams are built across narrow, deep river gorges, but now in recent years they have been considered even for little wider valleys.

Earth Dams:

• They are trapezoidal in shape.
• Earth dams are constructed where the foundation or the underlying material or rocks are weak to support the masonry dam or where the suitable competent rocks are at greater depth.
• Earthen dams are relatively smaller in height and broad at the base.
• They are mainly built with clay, sand and gravel, hence they are also known as Earth fill dam or Rock fill dam.

Functioning Principle of Fusegate

The Fusegate System is based on the following concept:

o   Fusegates are free-standing units installed side by side on a spillway sill to form a watertight barrier.

o   They bear against small abutment blocks set in the sill to prevent them from sliding before they are required to rotate (under extreme flood conditions).

There is a chamber in the base of each Fusegate, with drain holes to discharge incidental inflow (due to leaking seals for example).

Figure 1: C/S through a fusegate with moderate overspill.

Figure 2: C/S through a fusegate with inlet well being fed.

Figure 3: Uplift pressure cause the fusegate to overturn

An inlet well on the upstream side of the Fusegate crest discharges water into the chamber when the headwaters reaches a predetermined level. (Well lips on individual Fusegates are actually set at different levels).

During very large floods, water entering the chamber over the inlet well causes an uplift pressure to develop in the chamber.

The uplift pressure, combined with the hydro-static pressure (acting from left to right on the adjacent diagram) is sufficient to overcome the restraining forces and the imbalance causes rotation of the unit off the spillway. The Fusegate is then washed away clear of the spillway by the flood.

If the water level continues to rise after the first breach more Fusegates can rotate, all according to pre-determined upstream water levels until eventually there are no more units remaining and the spillway is free to pass the original maximum design flood. Until rotation of the first Fusegate, (for floods of extremely low risk of occurrence), the user has the benefit of the additional storage.

Each Fusegate has a different overturning level, precisely determined by the height of the water inlet and its own unique stability.

The Hydrological Cycle

Introduction:

Earth's water is always in movement, and the natural water cycle, also known as the hydrological cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Water is always changing states between liquid, vapor, and ice, with these processes happening in the blink of an eye and over millions of years.

This Information page provides an understanding of the hydrological cycle.  It describes the principal stages of the cycle, with a brief description of each stage.

Where is all the Earth’s water?

Water is the most widespread substance to be found in the natural environment and it is the source of all life on earth. Water covers 70% of the earth’s surface but it is difficult to comprehend the total amount of water when we only see a small portion of it. The distribution of water throughout the earth is not uniform. Some places have far more rainfall than others.

Notice how of the world's total water supply of about 332.5 million cubic miles of water, over 96 percent is saline. And, of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground. Fresh surface-water sources, such as rivers and lakes, only constitute about 22,300 cubic miles (93,100 cubic kilometers), which is about 1/150th of one percent of total water. Yet, rivers and lakes are the sources of most of the water people use everyday.

The stages of the cycle are:

• Evaporation
• Transpiration
• Condensation
• Precipitation
• Groundwater
• Runoff

Evaporation:

Water is transferred from the surface to the atmosphere through evaporation, the process by which water changes from a liquid to a gas. The sun’s heat provides energy to evaporate water from the earth’s surface. Land, lakes, rivers and oceans send up a steady stream of water vapour and plants also lose water to the air (transpiration).

Approximately 80% of all evaporation is from the oceans, with the remaining 20% coming from inland water and vegetation.

Transpiration:

The release of water vapor from plants and soil into the air. Water vapor is a gas that cannot be seen.

Condensation:

The transported water vapour eventually condenses, forming tiny droplets in clouds.

Precipitation:

The primary mechanism for transporting water from the atmosphere to the surface of the earth is precipitation.

When the clouds meet cool air over land, precipitation, in the form of rain, sleet or snow, is triggered and water returns to the land (or sea). A proportion of atmospheric precipitation evaporates.

Groundwater:

Some of the precipitation soaks into the ground and this is the main source of the formation of the waters found on land - rivers, lakes, groundwater and glaciers.

Some of the underground water is trapped between rock or clay layers - this is called groundwater. Water that infiltrates the soil flows downward until it encounters impermeable rock and then travels laterally. The locations where water moves laterally are called ‘aquifers’. Groundwater returns to the surface through these aquifers, which empty into lakes, rivers and the oceans.

Under special circumstances, groundwater can even flow upward in artesian wells. The flow of groundwater is much slower than run-off with speeds usually measured in centimeters per day, meters per year or even centimeters per year.

Runoff:

Most of the water which returns to land flows downhill as run-off. Some of it penetrates and charges groundwater while the rest, as river flow, returns to the oceans where it evaporates. As the amount of groundwater increases or decreases, the water table rises or falls accordingly. When the entire area below the ground is saturated, flooding occurs because all subsequent precipitation is forced to remain on the surface.

Different surfaces hold different amounts of water and absorb water at different rates. As a surface becomes less permeable, an increasing amount of water remains on the surface, creating a greater potential for flooding. Flooding is very common during winter and early spring because frozen ground has no permeability, causing most rainwater and melt-water to become run-off.

References:

• http://www.euwfd.com/html/hydrological_cycle.html
• http://en.wikipedia.org/wiki/Water_cycle
• http://ga.water.usgs.gov/edu/watercycle.html
• http://www3.geosc.psu.edu/~dmb53/DaveSTELLA/Water/global%20water/global_water.htm

Fusegates System

Fusegates are an innovative and reliable spillway system patented by Hydroplus. Fusegate system consists of a set of independent units placed on weir sill to form a watertight barrier.  They represent genuine alternative to conventional system, such as gates, flaps or inflatable tubes. Fusegates have successfully been implemented on 40 dams throughout the world, providing solid evidence of their value and effectiveness.

Fusegates are modular freestanding units that can be installed side by side on a spillway’s modified sill (the sill must be levelled to provide the necessary flat structure to accommodate the Fusegate modules). The modules consist of a vertical facing connected to a horizontal slab atop the sill. Watertight seals around the upstream perimeter of each Fusegate make it complete waterproof. Each unit rests on abutment blocks on top of the sill and is weighted with ballast to resist hydrostatic pressure (see figure).

Benefits of this system:

• Greater flexibility of operation
• Low maintenance of Fusegate system
• No human involvement in operation
• Lowest risk of malfunctioning
• Enhance dam safety
• Maximum overflow depth is 140% of Fusegate height
• Fusegate system is a non-mechanical
• Minimize the potential for mechanical gate failure
• To recover up to 2/3^rd of the distance between the ungated sill and crest of the dam without raising M.W.L.

Normal operation:

Installed a top the spillway, Fusegates form a watertight screen and raise the dam’s crest. For most floods, water is discharged over the Fusegates, which act as a free-sill spillway.

Major flooding:

A Fusegate’s pressure chamber starts to fill when the water level exceeds the crest of the inlet well. As soon as drain holes reach their saturation point, pressure under the Fusegate lifts the unit and tips it in the downstream direction. As water rises, Fusegates overturn in sequence. Their tipping points are carefully set to produce this controlled, sequential action.

For More details visit: www.hydroplus.com