Howdy everyone and welcome to module 11 where we'll talk about more inclement weather with the topic of hurricanes. But before we can get into hurricanes, we must spend some time talking about the coasts and other coastal hazards that are seen here. If we want to truly understand the the full impact that we see with storms, tides, sea level rise, and hurricanes as a whole. So, you probably noticed that I always like to start with discussing why we should study certain things. So, let's start with why we should study coastal processes and hazards. In the United States alone, most of the largest cities are on coasts and that includes the coast of the Great Lakes as well. Surprisingly, 75% of our population lives in coastal areas. Now there are some terms that will come up during our discussion. So let's start with some basic definitions. The coast refers to the zone from water to a major change in landforms. And the coastal zone is constantly modified by waves and human activity. The shore is the zone from high to low water. And the shore line is the line that marks the contact between land and sea. The shore is affected by a complex interaction between sea level and local geology. The beach refers to the material that is in transit along the shore. That's important to remember that this material is indeed in transit. Long shore currents are the currents parallel to the coast and longshore currents transport sediment all along the shore. Some coastal processes that commonly occur are storms, which we have a whole separate video on with hurricanes, wind, waves, tides, and long shore currents, sea level changes, and human land use changes. These processes shape coastal morphology, which is constantly shifting and is a constant battle between erosion and deposition. Coastal features affected include beaches, bluffs, underwater bars, sea cliffs, coastal dunes, and barrier islands, just to name a few. Normal ocean waves travel along the ocean atmosphere interface and are caused by the frictional stress of wind on the water surface. Waves are the visible evidence of energy that is passing through a medium which is water in this case. These wind generated waves provide most of the energy that shapes and modifies shorelines. So it's important that we understand them. The size of a wave depends on the velocity of the wind, the duration of the wind, and the fetch, which is the distance the wind blows across open water surfaces. Some components of simple non-breaking waves are shown here in this figure. The crest is the top of the wave and the trough is the bottom of a wave. Halfway between the crest and the troughs is the still water level which is shown here with this dotted line. Wave height is the vertical distance between a trough and a crest. And the amplitude is the vertical distance between a trough and the still water level. Or we could do the crest and the still water level as well. This means if the wave height is 15 m, so from the trough to the crest 15 m that the wave really only extends 7 and 12 m below the water surface. Wavelength is the horizontal distance between two successive crests or you could do two successive troughs as well. The wave period is the time it takes for one full wavelength to pass. And we can actually calculate a wave's velocity by dividing the wavelength by the wave period. Most waves derive their energy and motion from the wind. When wave height increases by a factor of two, the wave energy increases by a factor of four. The height, length, and period of a wave depends on wind speed, length of time the wind has blown, and the fetch, which again is the distance that the wind has traveled across open water. The waves we see in the ocean are the result of shearing of wind energy transferred to the surface water. Wave action only affects surface waters. The wave energy is what moves forward, not the actual water itself. As a wave moves through the water, the water passes the energy along by moving in a circle. This is called circular orbital motion. This motion decreases downward to a depth equal to about half the wavelength. And we call this the wave base. Beneath this wave base, water molecules are not affected by the wave at all. Here's a figure that very nicely shows the passage of a wave. Again, it is the energy that is moving forward, not the actual water. The water molecules experience orbital motion. So a toy boat would essentially bob up and down as the wave moves forward. Same is true for ships out in the deep ocean. The deeper the wave base, the more volume of water is involved in the wave. Remember this is why tsunamis are so powerful. The wavelength of a tsunami is much much longer than that of a regular wave. So it carries a much much larger volume of water. So wind generated waves increase in size with an increase in wind speed. Wind speed and fetch determine the frictional force and ultimately the wave height. Large waves are the result of high velocity, steady winds blowing across a wide area with no obstructions. Now, as a wave approaches the shore in shallower water, it begins to slow by friction because the wave base begins interacting with the sea floor. This causes the wavelength to decrease and the wave becomes taller and steeper. The wave eventually collapses due to over steepening forming a breaker as shown here. As a wave approaches shore, it will experience wave crowding, which is when the wave starts to feel the bottom. As the water gets shallower and waves slow down and bunch together. As the bottom progressively gets shallower and shallower, the base of the wave is slowed even more. And the base is slowed more than the crest. And that begins leaning that begins causing the wave to lean forward as its height is increasing and the wave becomes steeper. Eventually, the wave crest becomes unstable and spills over into the trough to form a breaker as it enters the surf zone. This forward movement of the wave pushes water toward the shore into the swash zone. Breaking waves rush up onto the beach and swash before retreating back into the sea as backwash. So to recap, deep water waves are unaffected by wa water depth. When waves approach the shallower water at the shore, friction will start to influence their behavior because the base of the wave interacts with the sea floor. As the waves feel the bottom, the waves will begin to slow down. Faster deep water waves can catch up and the wave starts to grow higher. As the wave gets higher, it becomes too steep and eventually collapses or breaks, creating surf, which is the turbulent water created by breaking waves. Waves cause both erosion and deposition. Irregularly, irregularities in the shoreline or changes in the seafloor can change the shape and direction of the waves. This can cause the bending of waves towards the shore or refraction. Here is a beautiful example of wave refraction. Wave refraction occurs where coastlines are not straight. A beach is an accumulation of sediment found along the landward margin of a water body. That can be along oceans or lakes and seas. Wave erosion occurs because the breaking waves exert a great force. Atlantic waves in the winter average about 10,000 kilograms per square meter. This force becomes even greater during storms. The erosion is caused by the wave impact and pressure, but also abrasion, which is the grinding action of water with rock fragments. This abrasion can be very can be very intense in the SER zone. Have you ever noticed all those tiny broken shell fragments in the surf zone? This is a consequence of abrasion. Remember, wave refraction is the bending of a wave that causes waves to arrive nearly parallel to the shore. Wave refraction has some consequences for the movement of sand on a beach. The wave energy is concentrated against the sides and ends of headlands or land that sticks out. And the wave energy is weakened within bays. You probably have noticed this if you've ever vaced on the beach. The bays have a gentler surf zone, which makes swimming ideal here, where on the sides of land that sticks out, you have more violent breaking waves. Over time, wave refraction can straighten irregular shorelines. As the waves approach nearly straight on, refraction causes the wave energy to be concentrated at the headlands which results in erosion and the energy is then dispersed in bays resulting in deposition. This is why you have beach deposits along a bay and usually rocky cliffs at the headlands. Now on to long shore currents. Long shore currents in shallow water move parallel to the coast. This combined with the approaching waves creates a zigzag path of water particles and fine suspended sediment parallel to the shore and transports sediment along the beach face in a zigzag pattern as well by swash and backwash. Waves seldom approach the shore straight on, but rather at an angle, causing the sediment to be transported along the beach face in a zigzag pattern called beach drift. Long shore currents easily move fine suspended sand along the coast. Both rivers and coastal zones move water and sediment from upstream to downstream. And beaches are often characterized as rivers of sand. Remember, the beach is the material that is in transport. Beaches are fragile and constantly moving. So, a river of sand is a perfect way to describe them. The two components of the transport system are beach drift and long shore currents which are created by breaking waves that approach the beach at an angle. These processes transport large quantities of material along the beach and in the surf zone. Rip currents flow in the opposite direction of breaking waves. Most backwash from waves move back to the open ocean as sheet flow along the ocean bottom. Rip currents are concentrated movements of this backwash on the ocean surface. Let me show you what I'm talking about. Here's a figure that shows rip currents very nicely. Rip currents are the narrow currents of water flowing through gaps and sand bars lying just offshore where the back wash is concentrated. Rip parents are really important to understand because they cause about a 100 deaths in the US each year. Now, how to identify them is really important so you can be sure to avoid being caught up in one. They can usually be recognized by the way they interfere with incoming waves or by the sediment that is often suspended within them like shown here. But if you do get caught in one, let it sweep you out past the feature that is causing it. Once past it, swim parallel to the beach and then diagonally back towards shore. Here's another rip current that is extending outwards from the shore, and you can recognize it by the interference with incoming waves. This is what you should be looking for. There are a lot of features of a shoreline. Some coastlines can be rocky like Big S on the left where resistant rock meets the shore or they can be sandy like the Bahamas on the right. Beaches can be wide or narrow and they can form all kinds of pattern, shapes, and features. Shoreline features depend on several factors. These factors include their proximity to sediment laden rivers, the degree of tectonic activity, the topography and composition of the land, the prevailing winds and weather patterns, and the configuration of the coastline. Waves cause erosion, transport, and deposition, which we already briefly mentioned. They cause erosion by wearing away headlands and filling in bays to straighten out coastlines. They transport the material along the coast by long shore currents and out to sea. They also deposit sand and other materials. All along coastlines, erosion and deposition is in a continuous battle. Winds, waves, tides, currents, and storms are constantly modifying the coastline. People on the other hand tend to want a coastline or beach to stay the same though. Here you can see a rocky coastline in Oregon where a more resistant rock forms the headland while a less resistant rock forms the bay and sandy beach in the foreground. This is how geology can dictate the shape and features of the shoreline. Some erosional features we see include wave cuts, wave cut platforms, and marine terraces. Wave cut cliffs, or I know them better as just bluffs, originate by the cutting action of the surface against the base of the coast. Wave cut platforms are flat bench-like surfaces that are left behind by a receding cliff. And a tectonically uplifted wave cut platform is known as a marine terrace. Here's an example of a wave cut platform below and a marine terrace above in New Zealand. The wave cut platform is exposed at low tide. The marine terrace used to be a wave cut platform, but some kind of tectonic event caused it to be uplifted. Some more erosional features that we see and are super cool to look at are sea arches and sea stacks. These occur when the headlands are the focus of wave erosion due to wave refraction. The rocks in the headlands do not erode at the same rate, similar to the differential weathering we talked about a few modules ago. Soft and fractured rock erode faster than hard rocks forming sea caves. A sea arch forms when two sea caves meet. And a sea stack forms when the arch of a sea arch falls. Here's an example of sea arches and a sea stack along the coast of Portugal that resulted from vigorous wave attack on a headland. Shorelines will grow in width if more sediment is deposited than eroded. Currents carry sediment onto and off the beach. And sand bars can form offshore during storms. Longshore currents, remember, are what is depositing or transporting sediment parallel to the beach in the surf zone. Here are some transport and depositional features we see. This is Tamales Bay, north of Point Reyes in California. On the left is an image prior to storms. On the right is that same location, but after some heavy storms. These storms transported and deposited huge amounts of sand along the shoreline, which widened the beach drastically. Okay. So, just like rivers where we have features created by erosion and by deposition, so do coasts. Here are some examples of depositional features we see along coast such as spits, bars, and tumbolos. A spit is an elongated ridge of sand extending from the land into the mouth of an adjacent bay. Spit means spine in this case, which kind of makes sense when you look at it. A baymouth bar is a spit that extends across a bay to seal it off from the ocean. A tombolo is a ridge of sand that connects an island to the mainland or another island. And I have never actually heard of this word until I read your textbook. So, this is a new term even for me. Holo means mound. So, it's literally a mound of sand connecting features. Here's a high altitude image of a well-developed spit in Bymouth Barmouth Bar along the coast of Martha's Vineyard in Massachusetts. You can see the spit is that elongated ridge of sand that juts out from the land into the mouth of a bay. And it's photographed a little closer there in B. Baymouth bar is this very long sand bar that completely crosses the bay, sealing it off from the open ocean. These usually form across bays where currents are weak. For example, we have a delta here that is depositing river sediment. And because the currents and waves are weak, it allows this bar to form here. Here are some examples of tmbolos. Now that I know what these features are called, they are pretty neat. These form in the same way as a spit. Barrier islands are features we see of low ridges of sand that are parallel to the coast, usually about 3 to 30 km offshore. They are found mainly along the Atlantic and Gulf Coast plane. Most of them are only 1 to 5 km wide and 15 to 30 km long. Their formation is still not quite understood, but they probably form in several ways. Some originate as spits. Some originate from sand piled up offshore. And some are flooded sand dunes from the last glacial period. Here's an example of a barrier island in North Carolina. There are nearly 300 barrier islands up and down the Atlantic and Gulf coasts. North Carolina has many barrier islands and are great excellent examples, but the more famous one here in Texas is Galveastston. I haven't been here yet, but I'm hoping to do some exploring there in a few weeks. I know a lot about its history, which I will talk about in one of the later later videos in this module. Galveastston is believed to have started forming about 5,000 years ago as wave action, especially those generated by storms like hurricanes that pushed back the shoreline, destroyed and reworked ancient barrier islands, and piled up sand deposits along the coastline and across from the mouths of bays. The newly developed sand piles gradually built up seawward and began to protrude out of the water. Over time, the island gradually grew as waves and long shore currents deposited more and more sand. Pretty cool. But these features are super fragile and are by no means permanent. As the sand is constantly being transported and reworked, the structures built here are at the mercy of mother nature and her everchanging mind. So we have something called the sediment budget along coasts. Sediment is brought into the system by rivers, by the erosion of cliffs and marines, and by wind. Sediment moves along the coast as a result of beach drift. And sediment leaves the system by being blown off the beach, by sinking into deeper water, or by being carried out by the long shore current. Shorelines are constantly evolving and continually undergo modification. If the shoreline remains tectonically stable, like on passive margins, for example, the east coast of the United States, the shoreline erosion will eventually produce straighter coasts. Now there are two types of coast and we call them emergent coasts and submergent coasts. Emergent coasts develop because of uplift of an area which is common along active margins like the west coast of the United States or a drop in sea level. Some examples of emergent coasts include almost the entire California coast and the Hudson Bay. The features commonly found here include wave cut cliffs or bluffs, wave cut platforms, and marine terraces. A submergent coast is caused by subsidance of land adjacent to the sea or a rise in sea level which is common all along the Atlantic coast. The features commonly found here are estuaries which are drowned river mouths and highly irregular shorelines. Here's a perfect example of an estuary. The lower portions of many river valleys were flooded by the rise in sea level that followed the end of the ice age. This created large estuaries like we see in the Chesapeake and the Delaware Bay bays. So let's compare America's coasts. The Atlantic and Gulf Coast are considered passive margins because there are no active tectonic processes occurring here. No plate boundaries close by, no continental collision, no subduction, no major transforms. These coasts develop barrier islands which help absorb the full force of major coastal storms. Now, the Pacific coast is considered an active margin because there are active tectonic processes occurring here with the subduction of the Wandafuka plate offshore Washington, Oregon, and Northern California. The biggest problems we see on the Pacific coasts are actually shrinking beaches. Dams on rivers here prevent sediment from reaching the coast, and the thinner beaches are unable to protect the cliffs, which we have so intelligently built houses on. For some reason, erosion along the Pacific coast is sporadic because coastal storms here are also sporadic. And we will actually talk about this coastal erosion in our next video. I'll see you there.