# Question Video: Understanding How Tidal Barrages Work Physics

The four diagrams in the image show a cross section of a tidal barrage. When water from one side of the barrage passes through the turbines to the other side, the turbines generate electricity. In which two of the diagrams shown will water flow from one side of the barrage to the other?

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### Video Transcript

The four diagrams in the image show a cross section of a tidal barrage. When water from one side of the barrage passes through the turbines to the other side, the turbines generate electricity. In which two of the diagrams shown will water flow from one side of the barrage to the other?

Okay, we see these four diagrams labelled (a), (b), (c), and (d). Each diagram shows a cross section of a tidal barrage. On one side, on the left in each case, we have water from the sea. And then, on the right side, we have water from the river estuary behind the barrage. An estuary, by the way, is a transitional zone where water goes from being in a very large body of water, such as a sea or an ocean, into a smaller narrow one, such as a river or a bay. But anyway, based on what we see in these four diagrams, we want to identify which two of them will have water flowing from one side of the barrage to the other. In other words, in two of these four diagrams, water will not tend to flow. And in two of them, it will.

When we think about what makes water flow through a barrage in the first place, we can recall that it has to do with an imbalance of water pressure on either side of the barrage. For example, let’s take a closer look at diagram (b). In this diagram, we see that the height of the water on the sea side is less than the height of the water on the river estuary side. This has an impact on the pressure on water on either side of this tidal barrage. If we think about a bit of water right here on the sea side that has a certain height of water column above it pushing down on it, that height of water directly translates into some amount of water pressure at that point. And the taller the stack of water on top of that point, the greater the pressure will be.

If we consider a point at the same elevation on the river estuary side, we can see that this point has much more water stacked on top of it, so to speak, whereas on the sea side, we have this much water pushing down on that spot. On the river estuary side, we have this much. This means that the water pressure on the river estuary side will be greater than that on the sea side. And if there’s an imbalance of pressure, that means water will tend to move. Since the water pressure on the river estuary side is greater, water will tend to flow from right to left through the turbine like this.

We see then that, at least in the case of diagram (b), water will tend to flow from one side of the barrage to the other. This was because of the imbalance in water pressure on either side of the barrage, which had to do with the uneven water heights on either side. And that’s the key. Uneven water heights on either side of a tidal barrage create uneven pressures. And when the pressure on either side is not the same, water will tend to move through the barrage.

So then, looking at the other diagrams, we want to find a situation where water level on either side of the barrage is again uneven, like it is in diagram (b). In diagram (a), we see these water levels are the same, so the water pressure will be the same and no water will flow. And it’s the same thing in diagram (c), even though, compared to (a), the water level is higher in this case. The important point is that it’s the same on either side of the barrage, so water won’t tend to flow through it. But when we come to diagram (d), we see that, here, the water level on either side is different. That difference, like we’ve seen, will create a pressure imbalance. In this case, the greater pressure will be on the sea side. So the water will tend to flow through the turbine in this direction.

Knowing this, we now have the answer to our question. In diagrams (b) and (d), water will flow from one side of the barrage to the other. And we saw that this is due to a pressure imbalance which is created by uneven water heights on either side of the barrage.