Dams & Reservoirs
Only 0.1% of earth's water supply that includes both surface water and groundwater is reasonably available for our use. And conservation of this water is vital for mainly two reasons 1st: its naturally purified water from evaporation from oceans. 2nd: it can flow under the force of gravity. So that means any drop of that water if you won’t take care of you will lose it. So what you may expect from this video is what engineers do so that we don’t lose that water. Groundwater is competitively much safer to drink as surface water. Why can't we just continue using groundwater? Why do we need to collect surface water? From the dawn of human civilization, we have been using groundwater for most of our purposes. Let’s understand a little bit about in order to answer that question. Now where this groundwater gets collected at what we call them aquifers. So What are these aquifers? They are porous stratum of soil or rock in which groundwater collects. Now the water from these aquifers can be obtained in two ways. First is to dig a well down up to the aquifers. Allow that water to get collected well from down. And then take the water out and use it. Well, the second is to find a spring. It’s a place where an aquifer intersects the surface resulting in the ground flowing back on the surface. Collecting water from these groundwater sources was okay. But in today’s modern world our water consumption is very high and ground sources can't cope with such high demand. Another reason is at some places groundwater aquifers are at greater depth to reach them.
So we left with one choice: to collect surface water and then treat it then it would be as safe as groundwater to drink or any other use. And with modern treatment technology, it has become economical as well. Some cities fulfilled water demand by drawing water directly from lakes or by a river flowing across. This water flow rate changes with the season so in order to stabilize water demand throughout the whole year we make dams as a source of a reservoir. Which receives a variable inflow of water but holds enough water to provide steady outflow.
No other civil engineering structure exemplifies the grandeur of a dam. So let’s talk about dams. A dam is an engineered structure designed to impound water by blocking the flow of the watercourse. Dams are used for water supply, irrigation, flood control and of course electricity generation. Apart from all this dam also creates recreation areas. The type of dam depends on 1) it’s principal construction material like concrete, masonry, soil and rocks 2) its structural configuration like an arch dam or a gravity dam. We will start with the arch dams. When we see an arch in a building or abridge the load which is applied to it, it's vertical. And an arch carries a load primarily in compression. That’s why it has to be made up of materials like concrete or stones which indeed have a very high compressive strength. And whenever we make an arch structure it becomes very important to give it some lateral support. What do I mean? Let’s understand with an illustration see in 1st figure when the applied force on the arch the outward force develops in their ends as a result arch collapses. So in order to stop the arch from collapsing, we have to provide some lateral support so that it won’t fail. Which you can in figure no 2
Arch Dams:
Now in arch dams, the same concept is taken but the alignment of the arch is sideways. As you can see in the image. Here everything works the same as any other arch structure difference is the load which is acting on this arch is hydrostatic pressure which is acting laterally in direction. Because we know from the knowledge of fluid mechanics the pressure on a submerged body acts perpendicular to the surface of the submerged body. Which in this case dam acts like a submerged body. here dam is paced vertical so the pressure will act horizontally. As you may have noticed, the lateral support to this arch dam is provided with the help of hard rock of the river valley. And this is why can only make this dam where solid rock is available for lateral support. By the way, this picture is of The Idukki Dam, located in Kerala, India.
Gravity dams:
Gravity dams as the name suggests get its structural stability primarily from its weight. Primarily there are two types of gravity dams. Based on construction material concrete, masonry, soil or rock fill on the other hand. Sardar Sarovar dam in Gujarat is the fine example of a concrete gravity dam. So how do these dams carry the load? Of course, it’s not curved. So it's not the arch action which is giving its stability. Let’s use a diagram to answer this question. Because the dam held vertical. The hydrostatic pressure acting on it is horizontal. Now if the concrete is sufficient to carry such a large load it would fail either by sliding or by turning. Now the base of this dam is fixed on hard bedrock. So there are no chances of sliding. Now the tendency of a force to cause rotation is called the moment. Now the hydrostatic force will try to turn this dam. Weight of this dam will create a counter moment to stabilize the dam. We can write it in mathematical form as well. The moment is Mass x Distance (perpendicular distance). Now the moment because of pressure force is at the bottom tip of the dam. To overcome this moment the dam should have adequate weight as well as enough distance from the bottom tip of the dam. This is the reason why concrete gravity dams are massive as well as wide. If we change the construction material of this gravity dam to some natural materials for example rock, soil, sand we call it earthen dam. But It works on the same principle of concrete gravity dams.
Let’s understand earthen dams with a diagram at the centre of this dam. We use clay for its extremely fine-grained soil and because it’s fine-grained it shows cohesive nature means it is capable of holding its shape without crumbling or breaking. And also its highly impervious means it doesn’t allow water to pass through it. But this dam cant be made entirely from clay because its material is easily erodible. It doesn’t have the mass to resist hydrostatic pressure. Thus we have to fill on both sides of the core with the compacted fill of rocks or soil if the fill is soil then its called earth fill earthen dam and if the fill is of rocks then its called the rock-filled earthen dam. On the upstream side of earth-filled earthen dam its provided layer of stones. And on the other side vegetation cover is provided to stop the erosion of soil. And you can see it in this picture. You can see there is also a concrete structure at the side of his dam. This is called a spillway. A spillway is a structure used to provide the controlled release of flows from a dam or levee into a downstream area. Now when the excess amount of water comes during heavy rain the excess amount of water released from the spillway. So that it won’t damage the dam.
Now you must be wondering which type of dam should be used when. In general concrete, dams are more preferable when stored water is deep. Also depends on the topography of the dam. Either your dam is just going to be used for hydroelectricity, or just for storage of water. When the deep valley is there and rocks around are hard the arch dam is ideal. Somewhat wider valley with high walls and concrete gravity dams is preferable. A very broad valley with a lower valley is generally called for earth dams. Thank you. Hope you understood everything we explained.