Physical Geography - Version 1 Unit 16 Shaped by Coastal Processes

Figure Coral Beach in . Image by Jeremy is used under a license . UNIT 16 SHAPED BY COASTAL PROCESSES Goals Objectives of this unit Describe the origins of longshore currents and longshore drift Explain why some coasts are more affected by erosion than others and describe the formation of coastal erosional features , including stacks , arches , cliffs , and platforms Summarize the origins of beaches , spits , bars , and barrier islands Explain the various mechanisms of change and the implications for coastal processes Compare the positive and negative implications of human interference with coastal processes GEOGRAPHY

SHORELINES Most people love shorelines . They love panoramic ocean views , they love sandy beaches on lakes , they love to swim and surf and go out in boats , and they love watching the giant waves crash onto rocky shores . While an understanding of coastal processes necessary for our enjoyment of coastal regions , it can make our time there much more interesting . But an understanding of coastal processes is critical to people who live near a coast , or those who like to spend a lot of time there , because , to be safe and avoid damage to infrastructure , we need to know how coastal processes work . We also need to understand the processes in order to avoid some of the possible consequences of changes that we might like to make in coastal areas . Waves Waves form on the ocean and lakes because energy from the wind is transferred to the water . The stronger the wind , the longer it blows , and the larger the area of water over which it blows ( the fetch ) the larger the waves are likely to be . The important parameters of a wave are its wavelength ( the horizontal distance between two crests or two troughs ) its amplitude ( the vertical distance between a trough and a crest ) and its velocity ( the speed at which wave crests move across the water ) Wavelength Wave velocity Trough Amplitude Figure The Parameters of Water Waves . Image by Steven , BY . The typical sizes and speeds of waves in situations where they have had long enough to develop fully are summarized in the table below . In a situation where the fetch is short ( say 19 on a lake ) and the wind is only moderate ( 19 ) the waves will develop fully within hours , but they will remain quite small ( average amplitude about 27 , wavelength ) On a large body of water ( the ocean or a very large lake ) with a fetch of 139 and winds of 37 , the waves will develop fully in 10 hours the average amplitude will be around and the average wavelength around 34 In the open ocean , with strong winds ( 92 ) that blow for at least 69 hours , the waves will average nearly 15 high and their wavelengths will be over 200 Small waves ( under a meter ) tend to have relatively shallow slopes ( amplitude is to of wavelength ) while larger waves ( over 10 ) have much GEOGRAPHY

steeper slopes ( amplitude is to of wavelength ) In other words , not only are large waves bigger than small ones they are also generally more than twice as steep , and therefore many times more impressive . It is important to recognize , however , that decrease with distance from the area where the waves were generated . Table The Duration times listed are the Minimum Required for Waves to Develop Fully Wind Fetch Duration Amplitude Wavelength Wave Wave Speed period Velocity 19 19 37 139 10 56 518 23 74 42 136 92 69 212 Relatively small waves move at up to about 10 and arrive on a shore about once every seconds . Very large waves move about five times faster ( over 50 ) but because their wavelengths are so much longer , they arrive less frequently about once every 14 seconds . As a wave moves across the surface ofthe water , the water itself moves up and down and only moves a small amount in the direction of wave motion . As this happens , a point on the water surface describes a circle with a diameter that is equal to the wave amplitude . This motion is also transmitted to the water underneath , and the water is disturbed by a wave to a depth of approximately of the wavelength . Wavelength Amplitude the wavelength Figure The Orbital Motion of a Parcel of Water ( Black Dot ) as a Wave Moves across the Surface ( image by Steven , BY ) The wavelength depth of disturbance of the water beneath a wave is known as the wave base . Since ocean waves rarely have wavelengths greater than 200 , and the open GEOGRAPHY

ocean is several thousand meters deep , the wave base does not normally interact with the bottom of the ocean . However , as waves approach the much shallower water near the shore , they start to feel the bottom , and they are affected by that interaction . The wave orbits are both flattened and slowed by dragging , and the implications are that the wave amplitude ( height ) increases and , the wavelength decreases ( the waves become much steeper ) The ultimate result ofthis is that the waves lean forward , and eventually break . Amplitude increases and wavelength decreases wave feels Surf zone the bottom , Figure The Effect of Waves Approaching a Sandy Shore . Image by Steven , Waves normally approach the shore at an angle , and this means that one part of the wave feels the bottom sooner than the rest of it , so the part that feels the bottom first slows down first . The waves ( with crests shown as white lines in the diagram below ) were approaching at an angle of about to the beach . The waves first reached shore at the southern end ( right side of the image ) As they moved into shallow water , they were slowed more at the southern end , and thus gradually became more parallel to the beach . In open water , these waves had wavelengths close to 100 In the shallow water closer to shore , the wavelengths decreased to around 50 , and in some cases , even less . Figure As Waves Approach the Shore , They are refracted to Become More Parallel to the Beach . Image by Steven , GEOGRAPHY

Even though they bend and become nearly parallel to the shore , most waves still reach the shore at a small angle , and as each one arrives , it pushes water along the shore , creating what is known as a longshore current within the surf zone , or the areas where waves are breaking . Figure The Generation ofa Longshore Current by Waves approaching the Share at an Angle image by Steven , BY . Another important effect of waves reaching the shore at an angle is that when they wash up onto the beach , they do so at an angle , but when that same wave water washes back down the beach , it moves straight down the slope ofthe beach . The water , known as the swash , pushes sediment particles along the beach , while the water , the backwash , brings them straight back . With every wave that washes up and then down the beach , particles of sediment are moved along the beach in a zigzag pattern . The combined effects of sediment transport within the surf zone by the longshore current and sediment movement along the beach by swash and backwash is known as longshore drift . Longshore drift moves a tremendous amount of sediment along coasts ( both oceans and large lakes ) around the world . Path of a sand grain on the beach I water onto the beach Figure The Movement of Particles on a Beach as a Result and Backwash . Image by Steven , BY . GEOGRAPHY

A rip current is another type of current that develops in the area and has the effect of returning water that has been pushed up to the shore by incoming waves . As shown in Figure , rip currents flow straight out from the shore and are fed by the currents . They die out outside the surf zone but can be dangerous to swimmers who get caught in them . If part of a beach does not have a strong unidirectional current , the rip currents may be fed by currents going in both directions . Figure Rip Currents on Beach in Central Chile . Photo by is in the public domain . Tides are related to very but waves on the ocean surface ( and to a much lesser extent on very large lakes ) that are caused by variations in the gravitational effects of the Sun and Moon . Tide in shoreline areas vary quite dramatically from place to place . On the west coast of Canada , the tidal range is relatively high , in some areas as much as , while on most ofthe east coast the range is lower , typically around A major exception is the Bay of between Nova Scotia and New Brunswick , where the daily range can be as great as 16 Anomalous tides like that are related to the shape and size of bays and inlets , which can significantly enhance the amplitude of the tidal surge . The Bay of has a natural oscillation cycle of hours , and that matches the frequency of the rise and fall of the tides in the adjacent Atlantic Ocean . Bay , on north coast , has a similarly high tidal range . As the tides rise and fall , they push and pull a large volume of water in and out of bays and inlets and around islands . They do not have as significant an impact on coastal erosion and GEOGRAPHY

deposition as wind waves do , but they have an important influence on the formation of features within the intertidal zone . OF COASTAL EROSION DEPOSITION Large waves crashing onto a shore bring a tremendous amount of energy that has a significant eroding depositional effect , and several unique erosion depositional features form on rocky sandy shores . Erosional When waves approach an irregular shore , they are slowed down to varying degrees , depending on differences in the water depth , and as they slow , they are bent or refracted . That energy is evenly spaced out in the deep water , but because of refraction , the energy of the waves which moves perpendicular to the wave crests is being focused on the . On irregular coasts , the receive much more wave energy than the intervening bays , and thus they are more strongly eroded . The result of this is coastal straightening . Arches and sea caves are related to stacks because they all form as a result of the erosion of relatively rock . An arch in the River area of western is shown below . This feature started as a sea cave , and then , after being eroded from both sides , became an arch . During the winter of , the arch collapsed , leaving a small stack at the end ofthe point . If you look carefully at the upper photograph you can see that the hole that makes the arch developed within a layer of relatively soft and weak rock . GEOGRAPHY

Figure An Arch in Tilted Sedimentary Rock at the Mouth of the River , July 2012 . Bottom The Same Location in June 2013 , Showing that the Arch Has Collapsed . Photo by David Murphy in Physical Geology , Depositional Some coastal areas are dominated by erosion , an example being the Pacific coast of Canada and the United States , while others are dominated by deposition , examples being the Atlantic and Caribbean coasts of the United States . But on almost all coasts , both deposition and erosion are happening to varying degrees , most of the time , although in different places . This is evident in the Malibu area where erosion is the predominant process on the sedimentary , while depositional processes predominate within the bays . On coasts , the coastal sediments are still being eroded from some areas and deposited in others . The main factor in determining if a coast is dominated by erosion or deposition is its history of tectonic activity . The coasts of the United States along the Atlantic and the Gulf of Mexico have not seen significant tectonic activity in a few hundred million years , and except in the northeast , have not experienced uplift . These areas have relatively little topographic relief , and there is now minimal erosion of coastal bedrock . On coasts that are dominated by depositional processes , most ofthe sediment being deposited typically comes from large rivers . An obvious example is where the Mississippi River flows into the Gulf of Mexico at New Orleans another being the Los Angeles River flowing out to the Pacific . GEOGRAPHY

On a sandy marine beach , the beach face is the area between the low and high tide levels . A berm is a flatter region beyond the reach of high tides this area stays dry except during large storms . Foreshore ) Breaker zone Swash zone High tide level Beach face Law tide I Figure The Components ofa Sandy Marine Beach . Image by Steven , Most beaches go through a seasonal cycle because conditions change from summer to winter . In summer , sea conditions are relatively calm with , waves generated by distant winds . Winter conditions are rougher , with , amplitude waves caused by strong local winds . The heavy seas of winter gradually erode sand from beaches , moving it to an underwater sandbar offshore from the beach . The gentler waves of summer gradually push this sand back toward the shore , creating a wider and flatter beach . Winter ( rough weather ) beach Sand stored offshore Sand stored on the beach Summer ( calm weather ) beach Figure The Differences Between Summer Winter on Beaches in Areas Where the Winter Conditions are Rougher Waves Have a Shorter Wavelength but Higher Energy . In winter , Sand from the Beach is Stored in Sand Bars . Image by Steven , GEOGRAPHY

A spit , for example , is an elongated sandy deposit that extends out into open water in the direction of a longshore current . A spit that extends across a bay to the extent of closing , or almost closing it off , is known as a bar . Most bays have streams flowing into them , and since this water has to get out , rarely a bar will completely close the entrance to a bay . In areas where there is sufficient sediment being transported , and there are islands , a tombolo may form . Tombolo Prevailing wind bar Figure Depiction Bar a Tombolo . Image by Steven , In areas where coastal sediments are abundant and coastal relief is low ( because there has been little or no recent coastal uplift ) it is common for barrier islands to form . Barrier islands are elongated islands composed of sand that form a few kilometers away from the mainland . They are common along the Gulf Coast from Texas to Florida , and along the Atlantic Coast from Florida to Massachusetts . North of Boston , the coast becomes rocky , partly because that area has been affected by a crustal rebound . Some coasts in tropical regions ( between and ) are characterized by carbonate reefs . Reefs form in relatively shallow marine water within a few hundred to a few thousand meters of shore in areas where there is little or no input of clastic sediments from streams , and marine organisms such as corals , algae , and shelled organisms can thrive . The associated biological processes are enhanced where upwelling currents bring chemical nutrients from deeper water ( but not so deep that the water is cooler than about ) Sediments that form in the back reef ( shore side ) and fore reef ( ocean side ) are typically dominated by carbonate fragments eroded from the reef and from organisms that thrive in the area that is protected from wave energy by the reef . GEOGRAPHY

Back reef Reef Fore reef Figure Through a Typical Barrier or Fringing Reef . Image by Steven , CHANGE change has been a feature on Earth for billions of years , and it has important implications for coastal processes and both erosional and depositional features . There are three main mechanisms of change , as described below . Eustatic changes are global changes related either to changes in the volume of glacial ice on land or to changes in the shape of the sea floor caused by plate tectonic processes . For example , changes in the rate of spreading will change the shape of the seafloor near the ridges , and this affects . Over the past years , there have been approximately 125 of eustatic rise due to glacial melting . Most ofthat took place between and years ago during the major melting phase of the North American and Eurasian Ice Sheets . At around years ago , the rate of glacial melting and rise decreased dramatically , and since that time , the average rate has been in the order of . Anthropogenic climate change led to an accelerating rise starting around 1870 . Since that time , the average rate has been , but it has been gradually increasing . Since 1992 , the average rate has been . GEOGRAPHY

Sea Level Rise A Santa , Rio de . as i Meltwater Pulse i 60 upper bound , Australia A , Jamaica Last Glacial Tahiti Peninsula . 120 ' lower bound Thousands of Years Ago Figure Eustatic the Past 24 ka . rise is Global due to Melting Glaciers . from Foundation , Change changes are local changes caused by subsidence or uplift of the crust related either to changes in the amount of ice on the land or to growth or erosion of mountains . Almost all of Canada and parts of the northern United States were covered in thick ice sheets at the peak of the last glaciation . Following the melting of this ice , there has been an isostatic rebound of continental crust in many areas . This ranges from several hundred meters of a rebound in the central part of the Ice Sheet ( around Hudson Bay ) to 100 to 200 in the peripheral parts of the and Cordilleran Ice Sheets . In other words , although global sea level was about 130 lower during the last glaciation , the regions were depressed at least that much in most places , and more than that in places where the ice was thickest . Tectonic Change Tectonic changes are local changes caused by tectonic processes . The subduction of the Juan de Plate beneath British Columbia is creating tectonic uplift ( about ) along the western edge of Vancouver Island , although much of this uplift is likely to be reversed when the next large earthquake strikes . Coastlines in areas where there has been a net rise in the geologically recent past are commonly characterized by estuaries and . A glacially eroded or modified valley GEOGRAPHY

that extends below sea level and connects to the ocean . Filled with seawater , depths may reach more than feet below sea level . The largest Alaskan are more than 100 miles long and more than miles wide . Also spelled Fiord . The rocky , yet steep coast of Fjords National Park lies along the southeastern side of the Peninsula in Alaska . This region of the Peninsula is part of the terrane that collided with Alaska about 65 million years ago the bedrock is primarily composed of partially metamorphosed muddy and shales from deposit with intrusions of granite and granodiorite . The Harding sits astride the Mountains and supplies ice to outlet glaciers that carve deep valleys into the soft bedrock . Figure Aerial Photo of Puget Sound a Fjord . Image by , ESA , HUMAN INTERFERENCE WITH SHORELINES There are various modifications that we make in an attempt to influence beach processes for our purposes . Sometimes these changes are effective and may appear to be beneficial , although in most cases there are unintended negative consequences that we do recognize until much later . An example is at the beach near Malibu , which has been armored with and concrete blocks in an attempt to limit the natural erosion that is threatening the properties at the top of the cliff . As already noted , the unintended effect of this installation will be to starve the beach of sediment . As long as the armor remains in place , which might be several decades , there is a GEOGRAPHY

risk that the spit will start to erode , which will affect many of the organisms that use that area as their habitat and many of the people who go there for recreation . Figure The Broad Beach Riprap is Designed to Retain Sand . Image by Ralph Daily BY . As shown below , a series of ( structures parallel to the shore ) were built in the and sand has accumulated behind them to form the bulge on the beach . The would have acted as islands and the sand has been deposited in the water behind them had the breakwater not been worn down . Figure Santa Monica Harbor Pier , 1936 . Image in Introduction to Oceanography by Paul Webb , BY . GEOGRAPHY

Groins have an effect that is similar to that of , although groins are constructed perpendicular to the beach , and they trap sediment by slowing the longshore current . Figure Fields Interrupt the Flow , Worsening Erosion Down the Beach . Image by is in the public domain . Most of the sediment that forms beaches along our coasts comes from rivers , so if we want to take care of the beaches , we have to take care of rivers . When a river is dammed , its sediment load is deposited in the resulting reservoir , and for the century or two , while the reservoir is filling up , that sediment can not get to the sea . During that time , beaches ( including spits , bars , and ) within tens of kilometers of the river mouth ( or more in some cases ) are at risk of erosion . GEOGRAPHY

UNIT 16 SUMMARY Waves form when the wind blows over water . The size of the waves depends on the wind speed , the area over which it is blowing , and time . The important parameters of a wave are its amplitude , wavelength , and speed . The water beneath a wave is disturbed to a depth of halfthe wavelength , and a wave is slowed when it approaches shallow water . A longshore current develops where waves approach the shore at an angle , and swash and backwash on a beach move sediment along the shore . The combined effect of these two processes is sediment transport by longshore drift . Coasts that have experienced uplift within the past several million years tend to have irregular shapes and are dominated by erosional processes . Wave paths are bent where the coast is irregular and wave energy is focused on . Rocky are eroded into sea caves , arches , stacks , and sea cliffs , and the areas around these features are eroded into platforms . Over the long term ( millions of years ) irregular coasts are straightened . Coasts that have not been uplifted for tens of millions of years tend to be relatively straight , and are dominated by depositional features , although deposition is also important on irregular coasts . Waves and longshore drift are important in controlling the formation of beaches , as well as spits , bars , and barrier islands . Beaches can be divided into zones , such as foreshore and , and beach shapes typically change from season to season . Carbonate reefs and carbonate sediments form in tropical regions where there is little input of clastic sediments . The relative levels of the land and sea have significant implications for coastal processes and , and they have been constantly changing over geological time . Eustatic changes are global in effect and are typically related to glacial ice formation or melting . isostatic changes are local effects caused by uplift or subsidence of continental crust , typically because of the gain or loss of glacial ice . Tectonic changes are related to plate interactions . Net rise leads to the development of estuaries and , while net level drop creates uplifted marine terraces and beaches . Humans have a strong urge to alter coasts for their convenience by building , and other barriers . Although these types of features may have economic and other benefits , they can have both geological and ecological implications that must be considered . GEOGRAPHY