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If you’ve ever visited the beautiful river Kennet you might have noticed a strange looking zig-zag structure. Did you know that if this structure were to disappear one fine morning, that the result would be a disaster? Welcome to the interesting engineering behind weirs.
Suppose you have an open channel flow like this. You introduce a simple structure called a weir into the flow. What do you think will happen? Is this your answer? Unfortunately, your answer is incorrect.
This is an open channel flow and it has a water level height of almost 1.4 cm. Now let me introduce this weir and see what happens to this water level height. After the introduction of the weir, at the upstream side water level height is almost 2 cm, it has increased. At the downstream side it is just 4 mm, it has decreased. Why is it so?
Are you wondering why this sudden increase and decrease in water level height happens? We will explore the fascinating physics behind this towards the end of this video.
Anyway, this is an interesting discovery. Can you solve this civil engineering design problem? This landscape is blessed with a beautiful river. However, during the rainy season the river becomes violent and destroys everything. To control this flood where will you introduce a weir? Obviously in the forest area. Introduction of the weir is going to increase water height at that upstream side. This means that after the introduction of the weir flooding will increase in the forest area but at the downstream side - where people live, the water level height will reduce. This means the introduction of a simple weir will reduce the chance of flooding at the downstream side. However, the issue is not yet solved. Let’s go back to the experiment and see what it is.
This a minimum water flow rate through this open channel and you can see that, the water level height is above the notch level is just 8 mm. Now let’s increase the water flow rate. With the increased water flow rate the height above the notch level is around 1.5 cm. Now let’s go for maximum water flow rate, now the water is flowing around 2 cm above the notch level. Inshort when you increase the water flow rate, the water level surface height increases. Is this a good thing for the weir ?
If the water level height increases drastically with flow rate, such weirs can easily cause a flood. This is why an ideal weir should be able to limit the water height increase with the flow rate. What about this Zig-zig weir? Let’s try this in the open channel flow.
This is the zig-zag weir we 3D printed. The water level above the weir level in the case of a zig-zag weir is just 1.3 cm at the maximum flow rate. Remember, the water level was at 2 cm height in the case of normal rectangular weir.
Suppose you have an open channel flow like this. You introduce a simple structure called a weir into the flow. What do you think will happen? Is this your answer? Unfortunately, your answer is incorrect.
This is an open channel flow and it has a water level height of almost 1.4 cm. Now let me introduce this weir and see what happens to this water level height. After the introduction of the weir, at the upstream side water level height is almost 2 cm, it has increased. At the downstream side it is just 4 mm, it has decreased. Why is it so?
Are you wondering why this sudden increase and decrease in water level height happens? We will explore the fascinating physics behind this towards the end of this video.
Anyway, this is an interesting discovery. Can you solve this civil engineering design problem? This landscape is blessed with a beautiful river. However, during the rainy season the river becomes violent and destroys everything. To control this flood where will you introduce a weir? Obviously in the forest area. Introduction of the weir is going to increase water height at that upstream side. This means that after the introduction of the weir flooding will increase in the forest area but at the downstream side - where people live, the water level height will reduce. This means the introduction of a simple weir will reduce the chance of flooding at the downstream side. However, the issue is not yet solved. Let’s go back to the experiment and see what it is.
This a minimum water flow rate through this open channel and you can see that, the water level height is above the notch level is just 8 mm. Now let’s increase the water flow rate. With the increased water flow rate the height above the notch level is around 1.5 cm. Now let’s go for maximum water flow rate, now the water is flowing around 2 cm above the notch level. Inshort when you increase the water flow rate, the water level surface height increases. Is this a good thing for the weir ?
If the water level height increases drastically with flow rate, such weirs can easily cause a flood. This is why an ideal weir should be able to limit the water height increase with the flow rate. What about this Zig-zig weir? Let’s try this in the open channel flow.
This is the zig-zag weir we 3D printed. The water level above the weir level in the case of a zig-zag weir is just 1.3 cm at the maximum flow rate. Remember, the water level was at 2 cm height in the case of normal rectangular weir.
This shows us that it is better to use a zig zag weir in our landscape. The animals in the forest will also have a more peaceful life!
The most important application of the weir is irrigation and recreational activities. For example, take a look at Kennet river. People are enjoying a boat ride. You can even see irrigation channels they have constructed. However, before installing the weir, this was the condition of the Kennet river. As we saw in the experiment, when we install a weir, the water height at the upstream side increases. At the downstream side, the flow remains of low height. Anyway, the introduction of the weir made the upstream side of the river useful to humans.
Now let’s get to the topic you’ve all been waiting for: How is an introduction of a weir able to alter water level heights both downstream and upstream? The answer lies in the conservation of energy. Simply implementing a weir won’t change the energy of the fluid. The energy of the fluid will be constant at these two locations. In fact, the fluid energy remains the same before and after the weir is in place. When you introduce a weir, on the upstream side, the potential energy of the fluid stream increases. This means that to keep the total energy constant on the upstream side, the fluid must reduce its velocity. Since the flow rate is the same, the velocity reduction will result in a higher flow area or higher height of flow. This is why, on the upstream side, the height increase is greater than the original water height.
On the downstream side, all this potential energy gets converted into kinetic energy of the stream. This means that the water speed must increase dramatically after the weir. However, here again, there is no change in the water flow rate. In order to keep the flow rate constant, the flow area or the flow height must be reduced.
These physics also mean that on the upstream side the water flows at a low speed whereas on the downstream side the flow is extremely fast. This seems wild, even though we’ve proven it logically. Let’s test it practically so maybe it will be more believable. This floating object is moving slowly before the weir and speeds up greatly after the weir.
Weirs are designed to control soil erosion, which is one of the main applications of a weir. By regulating the water flow, they can prevent excessive water runoff. Slowing down the flow of water reduces its erosive force, allowing sediment to settle rather than being carried away. The sediments are deposited behind the weir. Ultimately, by reducing the speed and force of water, weirs also assist in conserving soil.
We hope you enjoyed how the introduction of simple looking geometries in the open channel flow helps in modifying the water flow to human’s advantage. Thank you!
The most important application of the weir is irrigation and recreational activities. For example, take a look at Kennet river. People are enjoying a boat ride. You can even see irrigation channels they have constructed. However, before installing the weir, this was the condition of the Kennet river. As we saw in the experiment, when we install a weir, the water height at the upstream side increases. At the downstream side, the flow remains of low height. Anyway, the introduction of the weir made the upstream side of the river useful to humans.
Now let’s get to the topic you’ve all been waiting for: How is an introduction of a weir able to alter water level heights both downstream and upstream? The answer lies in the conservation of energy. Simply implementing a weir won’t change the energy of the fluid. The energy of the fluid will be constant at these two locations. In fact, the fluid energy remains the same before and after the weir is in place. When you introduce a weir, on the upstream side, the potential energy of the fluid stream increases. This means that to keep the total energy constant on the upstream side, the fluid must reduce its velocity. Since the flow rate is the same, the velocity reduction will result in a higher flow area or higher height of flow. This is why, on the upstream side, the height increase is greater than the original water height.
On the downstream side, all this potential energy gets converted into kinetic energy of the stream. This means that the water speed must increase dramatically after the weir. However, here again, there is no change in the water flow rate. In order to keep the flow rate constant, the flow area or the flow height must be reduced.
These physics also mean that on the upstream side the water flows at a low speed whereas on the downstream side the flow is extremely fast. This seems wild, even though we’ve proven it logically. Let’s test it practically so maybe it will be more believable. This floating object is moving slowly before the weir and speeds up greatly after the weir.
Weirs are designed to control soil erosion, which is one of the main applications of a weir. By regulating the water flow, they can prevent excessive water runoff. Slowing down the flow of water reduces its erosive force, allowing sediment to settle rather than being carried away. The sediments are deposited behind the weir. Ultimately, by reducing the speed and force of water, weirs also assist in conserving soil.
We hope you enjoyed how the introduction of simple looking geometries in the open channel flow helps in modifying the water flow to human’s advantage. Thank you!