now we just checked the cans let's look at checking your understanding so let's do a little quick check all right now a wave has a wavelength that's what l stands for of 50 meters and it's traveling in water with a depth of 30 meters so that's not talking about what its wave base is this is talking about the depth of the ocean to the surface of the bottom right 30 meters is this a deep water wave a shallow water wave or you can't determine go ahead and try that out pause it here and try that out okay i hope that you actually did try that out and that you pause the video and if you came up with this being a deep water wave you are correct all right the way you do this is your wavelength is 50 meters okay so that's the distance between consecutive crests you know that your wave base which is something that you need to know in this one to determine is 25 meters how do i know that because your wave base is equal to one half the wave length so one half of 50 is 25 meters now i compare 25 with 30. 25 meters is not as deep as 30 meters meaning my wave base with the wavelength of 50 meters is going down 25 meters and it's not hitting the bottom so it would be like out here right here this is about 25 meters deep here's 30 meters i'm not touching the bottom therefore i'm a deep water wave okay so deep water wave is your answer if this were a 70 meter wavelength then that would be a 35 meter wave base and therefore that would look more like this where oh it's actually hitting the bottom therefore um because the depth of the bottom of the surface of the ocean is actually shower shallower than 35 meters therefore you end up with a shallow water wave so there you have it you have a difference between shallow water waves and deep water waves let's take a look back at that check your understanding so hopefully you understand this question now this is something that you'll definitely see on the final and for the rest of your worksheet we're going to cap it off with a review video from a professor that i actually took my introductory oceanography course from keith meldahl and uh i'll upload that it kind of goes over everything that we just talked about in a short five minute clip so i want you to check that out try and finish off the rest of that worksheet there's still a couple more pages left and we might end up going over the last page after this review so go ahead take a look at this review and then we'll get back together okay let's go now hi oceanography students this short video is going to help you with your ocean waves field trip and help you answer the questions on the pre-lab exercise dolphins figured out that waves are fun long before people ever did dolphins are really the original surfers but people eventually figured it out too figured out that waves are fun and waves can also be dangerous and there are therefore there are a lot of good reasons to study the science behind waves and to understand their behavior and this is what we're going to be doing on our field trip to the pier we're going to be measuring waves that formed far out at sea probably formed by big storms and we're going to understand try to understand how those waves changed as they went from the deep water into the shallow water where we're going to see them and where surfers are riding them now some basic wave measurements you need to understand uh wave height is the vertical distance from the trough of the wave to the crest of the wave wavelength is the horizontal distance from one grave crest to the next wave crest and then wave period is a time measurement it's the time between two successive wave crests measured in seconds and you're going to be making this measurement at the pier when one wave crest comes by you're going to be starting your stop watches and when the next wave crest comes by you're going to be stopping your stopwatches and that will be a measure of the wave period and just just so you know typical wave periods for ocean swells arriving at our beaches in southern california 12 13 14 15 seconds those are the kinds of numbers that you can expect to get now this graph shows us an important relationship between the wave period and the wavelength and the wave speed and basically what the graph tells us is that waves with longer periods are also also have longer wavelengths and also move faster and this turns out to be a useful relationship because the wave period is a fairly simple number to measure now this is the relationship that we find when waves are out in deep water before they slow down near land um you know so for example if you were to measure a wave period of 14 seconds that corresponds to a wave speed of about 22 meters per second or say 70 feet per second if you go over here and a 14 second period also corresponds to a wavelength of a little over 300 meters or say about a thousand feet so the message from the graph is that longer wavelength waves with longer periods also move faster this turns out to be important because most waves are formed by big storms far out at sea and what happens is when you go to a storm it's a very chaotic ocean where many waves are moving out from the storm but the waves aren't all moving at the same speed the faster waves with the longer periods and the longer wavelengths are moving out ahead of the slower waves with the shorter periods and the shorter wavelengths and so if you let those waves separate from one another and this process is called dispersion waves separating according to their speed period and wavelength what happens is if you go away from a storm let those waves travel they'll separate and form very uniform groups of waves that we call ocean swells you can see here the distances between the crests are very very regular and that's because they've all been traveling at the same speed because they all have the same period and wavelength and so when those big swells arrive say at the coast they begin then to change in a in in a particular sorts of ways that we're going to try to understand a very important number that you want to know uh and and understand is called the wave base this is the depth at which the orbital movement of the waves begins to feel friction with the bottom and that forces the waves to begin to slow down the wave base is a number that is a depth of water that is equal to one half the wavelength so for example if you have waves with a wavelength of 500 feet their wave base is going to be half of that or 250 feet that's the depth at which they're going to feel friction with the bottom and begin to slow down and as waves slow down in shallow water and by shallow water i mean water deeper the shallower than the wave base their wave speed decreases and that causes their wavelengths to decrease the waves begin to bunch up as they catch up to each other you can see on this picture how the wave lengths how the waves kind of squeeze together the squeezing together of the wavelengths causes the wave heights to increase which is what causes them to eventually break but an interesting thing is that the wave period actually stays the same so when you measure the wave periods at the pier those waves had the same period when they were far out at sea in deep water and that'll be useful for us to be able to use that number to figure out some things about the behavior of the waves when they were in deep water so i hope you have a good time at the pier and see some beautiful waves and get some good measurements and don't forget your surfboards have fun so don't forget your surfboards is right and uh keith he's the uh a legend in oceanography for me because uh he introduced the topic to me hopefully uh you know um some of you might take this uh oceanography with you and you always think wow i had that teacher snort him right so hopefully it passes along um and now we are back at the worksheet so if i scroll down we already did this we talked about deep water wave and shallow water wave keith just recapped that be able to draw along this okay oh from here to here consecutive wave crests right that's the wavelength okay here's your trough here's your crest wave base is half the wave bait wavelength that would be down here you could draw that in we already talked about this and we did this exercise make sure you can read this figure i'm sure it'll come back up all right um here this is something that i'm hoping you did while you watch keith or me talk about this stuff so for deep water waves specifically this s as the wave period increases right so that's the time between consecutive crests so instead of 10 seconds maybe it's 15 seconds it's asking what happens to the following variables well wavelength your wavelength is going to increase your wave base so how far down it reaches right since your wavelength is increasing your wave base will also increase and meaning it'll reach further down into the depths of the ocean and your wave velocity will also increase so as wave period increases all of these increase now let's take a look at shallow water waves so the difference once again between deep water and shallow water is that your wave base is touching the sea floor and your wave base is equal to one half the wave length so now we're going into shallower and shallower water so what happens to the following variables as waves enter shallower and shallower water like when they approach a beach right that's when you get to see them so in this case your wave velocity since it's starting to interact with the sea floor your wave base your wave velocity will start to decrease okay your wave length will start to decrease as well because since the wave in front is first becoming a shallow water wave it's kind of like a car coming up to a red light right the car in front of you starts breaking first or the wave in front of you starts to slow down first then the wave behind is still moving at the same speed or the car is and it has a little bit of time to react before it starts feeling the bottom or starts pressing the brakes therefore your wavelength decreases because your waves start to bunch together your wave height increases because that energy on the bottom is now pushing your waves up starting to make them taller and taller until maybe they break and that's where i'll be hanging out right maybe you too and then your wave period this is the most interesting factor here is that your wave period stays the same okay your wave period from deep water waves to shallow water waves no matter how deep the water is your period will stay exactly the same so a 14 second period deep deep sea stuff is the same 14 second period in shallow water you might ask well how is that if things are closer together well yeah things get closer together because your wavelength decreases right but your wave velocity also decreases so since they're closer together and moving slower they have the same time between each wave consecutive crest okay now if you have that totally understood that's great and uh if you don't please email me and we'll talk more about it okay now one last thing down here is uh this graph here another figure okay so on the y-axis you have wavelength in meters so zero all the way up to 400 meter wavelength and on the bottom you have wind speeds this is to get you thinking about how fast the wind has to to blow in order to create waves of particular wavelengths right so you can see an example here right see the arrows above waves with 150 meter wavelength can be formed by winds blowing steadily for several days at 42 knots across the open ocean how do we get that well we see a wavelength of 150 meters maybe we measure that right and then if you follow this arrow here so you go straight across from here once again you hit the line on the graph and you read straight down that right there is about 42 and that's where i got that 42 knots across the open ocean so let's do a couple more all right if you got a wavelength of 200 meters what is its wave speed or wind speed right so wind speed in knots if i go up here l is 200 if i run across straight across i hit this graph and boom it looks like it runs right down to 50 knots so my answer here would be 50 knots okay another one if you want if we started with the wind speed we know oh it's blowing 25 knots steadily right now what kind of waves are going to be created from this well if it's blown at 25 knots 25 knots would be right here in the middle we could run straight up from here hit this bar on the graph or this line on the graph and read straight across to the left and that's about 50. so it create that 25 knot win 50 meter wavelengths okay so between this figure and the one above please know them both and let me know if you got questions about that other than that just keep checking out this worksheet and this is the kind of stuff that i'll be asking you on your test okay um and uh have a great day whatever you're doing maybe night or morning and we'll see you next time we'll recap a little bit about what we did but we're also going to go into some more wave features as well as tsunami so we'll see you then