 So in these set of videos, I'm going to introduce some concepts in hydrogeology. Water movement in soils is really important because it has influences on effective stress and consolidation as well as being widely important for things like water resource management. So the first thing I'm going to talk about is something called Darcy's Low. Now if we have a situation like this where we have two tanks of water and they're connected together via a tube or a pipe filled with with sand, if we left this system long enough, eventually the water in both tanks would reach the same level. What this means is that the water pressure on this side of the system is equal to the water pressure on this side of the system. So it is in effect in equilibrium. So what would happen if we we have the system like this and we then took one reservoir and we load the water level. So we scooped out a bunch of water from that and we had a new water level. So what it does is it lowers the water pressure on this side of the system and we have a relatively higher water pressure on this side. So you can see that water would be driven from this side through the sand and into this side. And eventually if we left it long enough, the water levels will equilibrate again. But what governs the rate of the movement of water through this system? So we're interested in this because well first of all we're interested in water pressures because that has an influence on effective stress. We're also interested in how those water pressures might change over time. And to do that we need to invoke something called Darcy's Law which relates or is an equation for the flow of water through the material. So Darcy's Law says something about flow and that's q and that's flow of water. So meters cubed per second. And it says that this flow is equal to the cross-sectional area of the flow. So in this case if we were looking at the sand in this direction and if it was around a tube we might be looking at a tube that looks something like this. Darcy's Law is interested in the cross-sectional area of the flow path. So it would be this cross-sectional area in this example. And that has units of meters squared. And then we then multiply that by the permeability. So what the permeability says is how susceptible this material is for water flow through it. So and that is given the units of meters per second. So we can see if we multiply these two units together we'll get our units of flow. So when we talk about permeability we often talk about it on a log scale and that's so we can cover a huge range of different permeabilities that that materials have. So something like a clay might have a permeability on the order of 10 to the minus 9 meters per second. Whereas a sand or a gravel might be on the order of 10 to the minus 2 or something meters per second. And you can see that we've got seven orders of magnitude. It's 10 million times different in permeability. So what this equation needs now is some sort of driving force to talk about flow. You can say that we've got a cross-sectional area and a permeability. And those are both specific to the the soil that we're looking at. But we need a driving force. So what drives the flow of water from this side of the the system to this side? And it's the difference between the levels of water. So delta H in this case. And you see that if we had a larger delta H we'd have a more of a driving force pushing water from this side to this side. And what we're also interested in is knowing the distance at which that delta H is operating in. The length of the flow path. And you can see that if we had a much longer tube of sand here, the pressure difference, the difference in water level will be spread over a much larger flow path. So this L is really quite important as well. So what's missing from this is the driving force, which is delta H over the flow path. You can see that actually delta H and L are both in the same units. So ones in meters over meters and they cancel each other out. So the units don't contribute to the units of flow here. So we have the driving force now. And that's called the hydraulic gradient. And it's often written instead of delta H over L, it's written as I, a hydraulic gradient.