Irrigation & Hydrology (General Engineering) (SSC JE)

In gravity dam with uniform width of base, whenever the reservoir is full, the resultant tends to fall on the toe side of the center and the maximum compressive stress is at the toe.
• When the reservoir is empty, the resultant is on the heel side of the center point and the compressive stress becomes maximum at the heel. The middle third rule has to be satisfied both for the full dam and empty dam conditions.

Elementary profile of a gravity dam –
• The shape of the elementary profile of the dam is a right angled triangle.
• An elementary profile of a gravity dam is the theoretical shape of its cross-section when it is subjected to only three main force.
• Self-weight, water pressure and uplift pressure. Moreover, the elementary profile has zero top width and no freeboard. The right angle triangle is most suitable section for the theoretical profile.
• The elementary profile is hypothetical because anactual gravity dam has some minimum top width and freeboard and it will also be subjected to forces other
than the three main forces considered in the elementary profile.

River Training Works–The works or measure taken to control flood, erosion and sediments to manage the river to flow smoothly in required direction are called river training work.

The different river control–
1. Direct and guide the river to flow in the desired direction.
2. Control and regulate the river bed and bank.
3. Increase the low water depth.
4. Control the erosion of banks.
5. Control the deposition of silt and sediment on river bed.

Note–The very essence of river control or training work is to stabilize the river along a desired alignment.

Hence, when the resultant force lies in the middle onethird region, there is no tension any where at the base of the elementary profile of a gravity dam.

• Hyetograph is a group of rainfall intensity versus time taken.
• It is the plot of the rainfall intensity drawn on the ordinate axis against time on the abscissa axis.
• The hyetograph is a bar diagram.
Hydrograph–A graph showing variation of discharge with time is known as hydrograph.
• Isohyet–The line joining places of equal rainfall.
• Mass curve–It is plot of cumulative or accumulated rainfall depth versus time.

A mass curve of rainfall is a plot of cumulative depth of rainfall against time. The steepness of the curve indicates the intensity of rainfall. A horizontal portion of the curve indicates that there was no rainfall during that period.
• A mass curve of rainfall is very useful in extracting the information on the duration and magnitude of a storm.

Reasons for groundwater depletion–
• Increased demand
• Limited storage facilities
• Green revolution
Inadequate regulation of groundwater–
• Encourages the exhaustion of groundwater resources without any penalty.

Runoff water–Run-off water refers to the water falls during rainfall (precipitation) and goes back to the source e.g. sea, ocean etc. In this way, a large amount of fresh water gets wasted. So the greater problem of water conservation is to reduce the amount of run-off water.

Rain water harvesting– It is the collection and storage of rain, rather than allowing it to run off. Rain water is collected from a roof-like surface and redirected to a tank, cistern, deep pit (well shaft or borehole), aquifer or reservoir with percolation. So that it seeps downs and restores to the ground water.