 Okay, so now on this video we're going to connect the boundary layer and the flow conditions at the boundary layer with sediment transport. And one of the key things is the Bernoulli effect, which is how the flow overgrains creates a low pressure on the upper and downstream sides of the grain, and that low pressure helps lift the grains up into the flow. Okay, so we're going to analyze the forces on a grain bit by bit. So here we have our surface, and we have a grain on that surface here. So we will use purple for our forces. So the first thing we have is that gravity is holding the grain downward. So we have our force from gravity. And we also have various amounts of pressure. So I'm going to use P for pressure, and pressure comes from molecules colliding against each other and against objects. And so, for example, if you have a grain sitting in a water, those water molecules are jiggling even when the flow speed is zero, and they're jiggling and they're bumping up against the grain. And because our flow speed down near the bottom here is zero, the molecules are just jiggling in place, and you end up with a pressure that's pushing in on the grain. Okay? So you have, we'll just say it's a medium pressure, even though the flow speed is very low here. So we'll say that our grain reaches somewhere up into the boundary layer, and the flow speed will be faster at the top of the grain than at the bottom of the grain. So here I just showed the molecules jiggling. Here they're jiggling, but they're going downstream, and they're colliding with the grain. And that creates some extra pressure, and the higher up on the grain, the stronger the downward direction of the flow is, and the less the molecules sort of jiggle above the grain where they're not actually impinging on the grain, they're going downstream relatively fast compared to the other grains. Okay? So on the upstream side of the grain here, and our flow speed left to right, what we end up with is a high pressure on the upstream side of the grain. Now, as that flow impinges on the grain, it can't actually flow through it. It goes around it, and it's still being pulled downstream. So we end up with a low pressure on the downstream side, because any water molecules coming from around the grain are still moving downstream. And then we have this really interesting area right on top here, and this is where the Bernoulli effect comes in. When particles are moving really fast past an object, they exert very little force on that object, and you end up with a low pressure active on the grain top itself. If you put something across the flow like this, you'd feel a very strong pressure on it with all the molecules colliding right into your hand, but if you put your hand sideways, the very few molecules collide to that surface, and it creates a low pressure. And that's actually how airplanes fly. And so you have the airplane wing, and it's shaped so that the flow goes over the top of the wing, and almost none of the air molecules collide with the top of the wing, and it creates this lift, this low pressure, this suction that pulls the airplane up into the air. And so if you look at an airplane wing, they're shaped like this, so the air in this case would be going this way, this distance versus this distance, this distance is longer, and that makes the air flow faster over the top of the wing, and so there's a lower pressure at the top of the wing than the bottom, and that pressure difference is so big that it overcomes gravity, so 747s, heavily loaded, can still fly using this pressure difference effect. It's also possible to feel it if you're just driving in a car with a window open, things can get sort of sucked out the side of the car because the side window is the air is moving very fast past them, and it creates this low pressure that can pull things, pull things out of the window. So we have a medium pressure relative to a low pressure in this zone, and a high pressure here, and we end up with a net force from the fluid, which goes up and downstream. So this is the combination of the fluid forces, the downstream force coming from the high flow in this area here, and the lift coming from this Bernoulli effect on the top of the grate.