 Okay, so the Bernoulli effect can lift grains up into the flow. And two different things can happen to those grains once they're in the flow. One is they can fall back to the bottom of the bed, or they can remain suspended in the flow. So, in this video I'm going to talk about bed load and suspended load transport of grains. And the bed load are the ones that do return back to the bed or roll along the bed, or the suspended ones are the ones that stay in the water. So if we look at our grain on the surface here, we have our variation in flow speed above the height of the bed. So this is our average flow speed in the bed and our flow depth L. We have the Bernoulli effect, which overcomes the force due to gravity, and it creates a lift fluid force, right? So the grain gets lifted up into the flow if that fluid force is large enough. So if I draw it a little bit smaller, actually we can do it up here, so once the grain is lifted up into the flow, you end up with a difference in the pressure distribution. So for the Bernoulli effect we have a low pressure at the top and a medium pressure at the bottom, and then you have high pressure pushing the grain downstream. And that's because of this variation in the flow speed. Once the grain is up in the flow, the flow speed is the same at the top and the bottom. So you end up with a low pressure, both the top and the bottom of the grain, with pressure high on the upstream side, while it's going this way, and a low pressure on the downstream side. So in general, we have our fluid forces going off this way, and we still have gravity pulling down on the grain. So if you add these two forces together, the trajectory of that grain is downward. So I'm going to erase our old grain here, the force on our old grain here, so I have more space. So if you add these two forces together, the trajectory of the grain is actually back down towards the bed. So we can look at the trajectory of one of these grains without all the forces. Here, so we have our surface, we have our grain, and the grain gets lifted into the flow from the Bernoulli effect. And then once it gets above the boundary layer, once it gets above the boundary layer, the forces on the top and the bottom of the grain are the same, and it falls back down to the bed here. So here the forces are different, which is lifting the grain up here, they're the same, the top and the bottom, in terms of the fluid forces, but you still have gravity pulling it down, and you have the fluid force pushing it down straight. So grains that behave like this are called saltating grains, and the process is saltation, and that actually has the same root as the word salt, which means something like to dance, I think it's in Latin. Now that's because if you dump salt out of a salt shaker, the little grains bounce and pop around, and in this case you have the grains, the saltating grains sort of bouncing and leaping around, but moving down straight. So we've also talked about this where you have a grain, and maybe the Bernoulli effect is not large enough to pick it off the bed, but you have the force at the top of the grain is large enough to cause the grain to roll. So these two methods, so the blue is, we call it traction, it's basically moving like maybe a tractor track or roller, it's just rolling along the bottom. And so these grains, these two types of grains both stay very close to the bed, and then we call it, so we call them bed load. And that's an engineering term that's related, the load is sort of a measure of how much sediment gets transported that way. So this is one of our key methods for the way sediment is transported, and it has these two parts, saltation and traction. We have another option where if we have a grain that gets lifted up by the Bernoulli effect again, and if it's a small enough grain and there's enough turbulence up here, the force of the grain here will have gravity pulling it down, but you can have upward bursts of the fluid and the turbulence that counteracts that gravity. And so the grain can actually get buffeted by turbulence, so in this particular case, turbulence in the flow keeps the grain from settling down. And so in this particular case, the grain is in essence suspended in the fluid, and we call this suspension transport. So the process is suspension, and it would be suspended load in engineering terms here. So as you can imagine, or as you know, smaller grains with smaller mass have a lower, smaller force pushing them downwards, so it's much easier for smaller grains to be incorporated as suspension sediment transport in the flow because gravity is lower, and they're more susceptible to that turbulent flow. So these two types of transport have pretty different implications for sediment. The grains that are in bed load can get caught in irregularities in the bottom of the flow, whereas the suspension load can get transported further out. As the flow speed slows down, we know from the metals number, the turbulence goes down, and so as the turbulence goes down, the effect of gravity on the suspended particles increases and they do settle out. So there's a really nice relationship between the size and mass of grains and how they are transported within the fluids. Thanks for watching.