 Beaches are structured by a combination of the waves and the sediment that's available for the beach. So waves redistribute sediment along a shoreline to produce beaches, and the dynamics of their interaction with the sea floor near the shoreline and coming up onto the shoreline shapes the sediment transport of beaches themselves. So waves move in the ocean and when they start interacting with the sediment surface there's a lot of friction and that friction is what causes the waves to break. The friction is slowing down the water flow speed at the bottom of the wave but the top of the wave keeps moving quickly and it over tops the bottom of the wave and that's how you get the turbulence. So one of the things that surfers know very well is that the topography offshore of a beach strongly influences the behavior of how and when those waves break. So when the waves break there's a lot of turbulence and so this zone in here is one where there's often erosion and there's a lot of force of the wave and the water flow moving on shore so a lot of the sediment that gets eroded during the breaker gets washed up the shoreline and the wave as it comes up is called the swash from the break. So the swash zone gets sediment transported onto it when the waves come up and then the sediment gets eroded back or transported back offshore as those waves retreat. The details of how much erosion and onshore transport you have versus offshore transport determines whether the beach is growing through time, it's stable through time or it's eroding away. And so that process of the waves breaking creates layers of sand on the beach that are about parallel to the topography of the beach. So as the size of the waves changes through time as the tides go up and down and storms come and go beaches end up changing their shape but in general the swash zone has approximately parallel layers they dip towards the ocean. As the waves change and erodes the beaches deposits on them but this is these parallel layers that dip towards the ocean are really common for beaches and a lot of times because the grain size varies a lot. There are often specific layers that are well sorted but between layers it can be different. So for example if you have very high waves you might end up with a granule beach whereas at a time during low waves you might end up depositing much finer grained maybe fine sand layers. Now there are times when you have particularly large waves associated with storms and tides and they create, they build up a berm. So the berm is usually created at the maximum height that the swash zone comes up. So this is the max, approximately the max swash reaches this area. So if you have a big wave that comes all the way up here it's likely to transport a lot of sand and then it will stop right here at this berm. And so the layers can extend all the way up into this area but only when you have the very strong waves. Occasionally the waves will actually top over the berm and you can accumulate sediment on the backside here as well. So the waves will be carrying sediment and they can erode as they start coming over the crest the water speed will increase again and they can transport sediment into the back berm area. And so while most of the beach deposits are in layers that are dipping towards the ocean there is this transition to a back berm deposit and if the beach migrates away it will leave this topographic high that is eventually called a beach ridge. So over time you have this deposition on shore but also some of this eroded sediment ends up going offshore and once it goes offshore the energy, the flow speeds are much lower and it becomes more and more difficult for that sand to get transported back on shore. Occasionally with large storm waves some of it might be but on average if you don't, if there's not a process that keeps supplying sand to the beaches they will erode away because there's the sink of sand offshore where the flow speeds are very low. Most areas with beaches the sand comes from rivers flowing in to the coast and then that sand is transported via long shore drift away from the river mouths and so it can be in one direction on the shoreline or it can be both but the beaches are intrinsically an environment that only holds sand temporarily. When humans do things like build dams that reduce the flow of sand into shorelines we're cutting off the supply of sand to maintain healthy beaches and without beaches having replenishment of sand whether it's natural or from humans they are not stable, they will erode through time with the transport of that sand into an offshore environment. Thanks for watching.