So the boundary layers at the base of flow strongly influence sediment transport. So if we redraw our water flow here, we have our dead surface and we can make a plot of our flow speed. And we have L here, 0 here, this is Z, which is distance above the bottom of the flow. We have our average flow speed coming down here and we have our boundary layer in the zone where the friction with the base of the bed starts influencing the flow speed. So that is our graph from before. So this is if you have a flat bed. So let's put a fairly large grain on our bed here. So this is a sand grain. And one of the things that if you notice, so this is the diameter of our grain up here, I'm going to draw that. The flow speed that the bottom of the grain experiences is 0. And the flow speed that the top of the grain experiences is significantly higher. So the force on the grain varies with height on the grain. And so here you have a force that's pushing downstream from the flow. And in contrast, at the bottom of the grain, you have a friction force pushing up this way. So it's pretty easy to imagine that if this was a flat bed, if the flow pushes hard enough, that grain can end up rolling. Yeah. So that's one of the way sediment can be transported that's influenced by the flow speed. And also if you think about it, this difference in force at the top represents a shear force. And so the concept, one of the key concepts related to this boundary layer at the bottom is that it creates a shear force that we call often. So you end up with forces are higher at the tops of the grains and lower lower in the grains. And then at some point there's friction pushing back. So what we do is it's related to the beds. We call this a bed shear stress. That strongly influenced by that variation in velocity and forces on the grain that are related to the flow. So the bed shear stress is related to more than just the boundary layer because you have to look at all the forces on the grain. So you can look at the bed shear stress more broadly. So I'm going to draw a slipping surface this time. And we can look at all of the forces on a grain. So here's my grain here. All right. There's a force of gravity. And then there's the differential flow on the top. And there's friction on the bottom. So the steeper the surface is, the more the gravity adds a component of pushing the grain down slope. The more friction there is, the more resistance there is to that grain actually moving. So the bed shear stress depends on a number of things. So the amount is influenced by the slope of the surface. And then it's influenced by the factors that affect the flow and the friction. And one of those is actually the bed roughness. And it's influenced by the flow speed. And it's also influenced by how much turbulence you have and what the Reynolds number is. So the Reynolds number also depends on flow speed. So these two are closely related to each other. But in general, as an overarching principle, we can do a higher bed shear stress leads to more sediment transport. And the bed shear stress increases with slope. So increasing slope produces higher bed shear stress, increasing bed roughness increases the turbulence. So it also causes an increase in the bed shear stress, same with the flow speed. And the same with the Reynolds number. So it's the bed shear stress is a useful concept for understanding how the fluid and surface characteristics influence sediment transport. Thanks for watching.