Introduction to Ocean Sciences

Chapter 13: Coasts

Guide to Reading and Learning

Most of us are very familiar with the beach and with a variety of other types of coastline, but do you know why these variations occur? Even more important, do you know that coastlines are constantly changing, that rising sea level is constantly pushing the shoreline landward, and that many human attempts to protect coasts from erosion are ultimately futile? In this chapter you will learn that coasts change constantly, that waves are important in forming and maintaining beaches, and that rising sea level is likely to be one of the most difficult problems caused by greenhouse effect–induced global climate change.

Unless you are lucky enough to live on the coast, you have probably only visited it periodically and perhaps mostly in summer, when you can enjoy playing and relaxing on the beach and in the water. You probably take for granted the “fact” that the beach you love to visit each spring break or summer will always be there just as it was last year. However, you may not know that the beach may be much narrower and steeper next time you visit, and that perhaps there is almost no beach in the winter months and that it  returns only in the spring.

If you have seen groins, piers, breakwaters, and other shoreline structures you may never have asked why they are there and how they function. You may also be unaware that many are poorly designed and that none of these structures can provide more than temporary protection for beaches and coastal infrastructure and houses they are meant to protect from the sea.

Most of you will have heard about the flooding that inundated the city of New Orleans when the levees designed to keep out the ocean failed after hurricane Katrina. Do you know why these levees where built in the first place? Have parts of New Orleans always been below sea level? Will rebuilding the levees to make them stronger protect New Orleans in future? Why do many scientists say we should abandon New Orleans and move people elsewhere rather than rebuild the levees?

You will learn the answers to these questions and more in this chapter and, will most certainly look at the shore and the beach with a very different perspective the next time you take that summer vacation. Interactions between the oceans and the edge of the land are endlessly fascinating in many ways that you will be able to enjoy after studying this chapter.

Chapter 13 Essential to Know 

Critical Concepts used in this chapter

CC.2, CC.4, CC.8

13.1 Processes That Form and Modify the Coastline

  • The coast is the strip of land between the line where the water meets the land and the inland location where the ocean no longer has any environmental influence.
  • The characteristics of a coastline are related to the balance of various formation and modification processes at each particular location.

Formation of Coasts

  • New coast is formed when the relative levels of the land and ocean change or when landmass is added or removed.

Tectonic Processes

  • New coast can be formed almost instantaneously when an earthquake uplifts or drops a section of coastal land or the seafloor.
  • Volcanic lava flowing into the ocean or building underwater volcanic mountains can create new coast and even new islands.
  • Existing coast can be destroyed when continents collide or by vertical motions of the land edge during earthquakes.

Landslides     

  • Landslides from cliffs can create new coastline if the slide reaches the shoreline.
  • Landslides can simultaneously destroy old coast and create new coast when a section of coastal land falls into the ocean. Huge landslides have occurred when the steep flanks of volcanic hot spot and emergent oceanic ridge volcanic islands have collapsed. Such landslides can cause massive tsunamis and are know to occur on average about every 10,000 years.

Isostatic and Eustatic Sea-Level Changes

  • Sea level has risen about 120 m in the past 18,000 years, creating new coast on what was previously land.
  • Historically sea level has oscillated between about 40 m above and about 130 m below its present level.

Glaciers

  • Glaciers scour steep sided valleys and leave a glacial moraine at their lower end. When the glaciers retreat and sea level rises these valleys are flooded by the sea to form deep fjords with steep coasts and a shallow submerged sill at the ocean end.

River-Borne Sediments

  • When river valleys have become filled with sediment, the suspended sediment load is carried by the river  deposited to form deltas at the river mouth that can extend the land out into the oceans..

Biological Processes

  • Coral reefs can develop into new coastline if uplifted by tectonic or isostatic processes.

Modification of the Coast

  • All coasts are subject to constant modification. The rate of modification depends on the nature of the coastal land, especially its susceptibility to erosion.

Waves

  • Breaking waves are the principal erosional force on coastlines.
  • Wave energy is concentrated on headlands, so wave action tends to erode headlands faster than coastline within bays.
  • Continuous wave erosion tends to remove headlands and straighten the coastline.
  • As waves erode away the base of coastal cliffs sections of the undermined land periodically collapse.
  • Wave erosion of headlands can form seacaves, sea arches, and seastacks where the rock has varying susceptibility to erosion.

Tides

  • Tidal range determines the height range over which wave erosion occurs.
  • Wave erosion is more concentrated where tidal range is small.
  • Tides are particularly important in transporting sediment and shaping estuaries and wetlands.

Winds and Weather

  • Onshore winds can carry sand from beaches to form sand dunes.
  • Offshore winds can erode coastal soils and deposit the particles in the ocean.

Vegetation   

  • The type and extent of vegetation affect the rate at which wind, wave, and water erosion occur.
  • Rooted aquatic plants, including sea grasses and mangroves, help prevent erosion of mudflats.

13.2 Beaches

The Littoral Zone

  • Beaches have many different conformations but each has a foreshore and backshore, and may have one or more berms and scarps that stretch along the beach at different heights above the water level.
  • The foreshore is the area that between the location reached by waves at high tide and the lowest point exposed at low tide.
  • The backshore is the low-slope area of the upper beach that is normally dry even at high tide.
  • Berms are areas on the backshore where the generally flat and gradual downward slope of a beach steepens abruptly. They are created by storm waves at the location to which the storm waves reach and remove sand. Where there is more than one berm on a beach, the highest berm was created first, is the oldest, and is often called the winter berm. Berms are destroyed by any storm whose waves reach higher up the beach than the berm’s location.
  • Scarps are found at the upper end of the foreshore. They are abrupt, almost vertical faces of sand usually only a few centimeters high. Scarps are formed by waves that cut away sand from the beach up to the location that the waves normally reach at high tide.
  • More than one scarp may exist on a beach. If so, the upper one was created first, at a time when the tidal range was higher than when the lower one was cut.

Sources of Beach Materials

  • Most beaches consist of sand from local sources such as rivers and cliff erosion.
  • Most sand consists of weathered solution-resistant silicate minerals.
  • In some areas, especially on volcanic islands, beaches may consist of green, red, or black sand particles that are derived from largely unweathered local rocks.
  • In some tropical areas beaches may consist mostly of the remains of calcareous organism hard parts.
  • On most beaches sand consists of grains that are all within a small range of grain sizes and is said to be well sorted.

Longshore Drift

  • Sand is transported along the coast by longshore drift caused by waves. The waves move up the beach at a slight angle as they break but the water then runs directly perpendicular down the beach causing a small lateral displacement of the suspended sand grains.
  • Because waves are continuous longshore drift can be as fast as 1 km per day and several hundred to several thousand cubic meters of sand can be moved past a given point on a beach each day.
  • Sand is transported along the beach until it reaches a submarine valley where it is transported offshore down the valley.

Wave Sorting of Beach Sands

  • Sand on most beaches is well sorted by wave action.
  • The highest waves are capable of bringing some large particles to the beach from shallow offshore locations or from along the coastline.
  • The slower velocities of the backwash as the broken wave flows back down the beach cannot resuspend the larger particles so they remain on the beach.
  • Smaller particles can be resuspended by the backwash, removed from the beach, and transported offshore.
  • Particles on a beach are restricted to the size range between the largest particles that can be transported to the beach by waves and the largest particles that can remain on the beach and not be resuspended by the backwash.

Seasonal Changes in Beach Profiles

  • Beach shape changes seasonally. Sand is transported offshore and deposited in longshore bars when wave action is strong in winter and the beach becomes narrower and steeper.
  • Longshore bars formed during normal winter storms are shallow enough to cause waves of exceptionally strong storms to break offshore and so protect the coast from erosion.

Beach Slope and Grain Size

  • Beaches with larger grained sand are steeper than beaches with finer grained sand. In large grain beaches, water percolates between the grains into the sand and reduces the backwash flow rate. Thus more sand is transported onshore to make the beach steeper until increased gravity due to the greater slope compensates for water lost in the backwash.

13.3 Barrier Islands and Lagoons

  • Many coastlines are dominated by elongated barrier islands composed primarily of sand. These barrier islands often enclose shallow lagoon where mangroves, sea grasses, and marsh grasses are often abundant. These sheltered lagoons also provide habitat for the juveniles of many marine species.

Formation of Barrier Islands

  • Some barrier islands may be formed from spits that become elongated until they close off a lagoon. Others may be formed originally as a longshore bar built by storm waves when the water depth is increased by a storm surge. However, these formation mechanisms are uncertain.
  • Barrier islands retreat as sea level rises. Storms wash sand off the beach over the island to accumulate on the inshore side. Some barrier islands have retreated a distance equal to their entire width in the past several decades.

Future of Barrier Islands

  • Barrier islands are retreating as sea level rises, and rising sea level appears to be important to their formation and maintenance.
  • If sea level were to remain stable, barrier islands would likely disappear as their lagoons slowly fill with sediments.
  • If sea level falls no new barrier islands may form.
  • If sea level rises faster, barrier islands will retreat faster.

Development on Barrier Islands

  • Barrier islands protect the mainland from erosion.
  • Because barrier islands are constantly retreating, development on these islands is subject to erosion and to increasingly frequent storm wave damage.
  • Seawalls and other attempts to prevent barrier islands from retreating are at best temporary solutions, especially if the rate of sea level rise increases.

13.4 Beaches and Human Structures

  • Many beaches are eroding and being depleted of sand because of rising sea level or reduction of sand inputs from rivers due to upstream dams. The beach can be replenished but will erode again. If the replenished sand is taken from offshore bars the beach will be exposed to further storm wave erosion. If the replenished sand is a different grain size from the original beach it may erode more easily or make the beach steeper and narrower.

Seawalls

  • Seawalls are often built parallel to the shore on the backshore, where they can protect the coast from storm wave erosion. As sea level rises or the barrier island retreats, the beach is eroded away in front of the wall and erosion continues at either end of the wall until the waves can undermine the wall or erode it from the side.

Groins and Jetties

  • Beach erosion is often countered by building a groin out from and perpendicular to the shore through the wave breaking zone. Sand accumulates on the upstream side of the groin but is lost on the downstream side, causing the beach to become narrower. Often a line of groins is built along a long stretch of coastline. Groins cannot restore sand to the beach. They temporarily retain some sand and redistribute it on the upstream side affect the overall rate of loss of sand..
  • Groins and jetties built to protect harbors interfere with the longshore drift. Usually this results in sand buildup inside the harbor and sand loss from the beach on the downstream side of the harbor. Usually the harbor must be periodically or continuously dredged to remove sand, which is usually deposited downstream to replenish the downstream beach.

Harbors

  • Breakwaters are often built parallel to the shore in water deeper than the breaking wave zone. Breakwaters block waves from reaching the shore behind them, interfere with the longshore drift, and cause sand to build up on the beach, reducing the size and depth of the harbor. Again, dredging is usually needed to maintain harbor depth.

13.5 Coral Reefs and Atolls

  • Reef building corals grow only in water temperatures greater that about 18oC. They can only live symbiotically with photosynthetic dinoflagellates called zooxanthellae, so they can only grow in shallow water where light is available.
  • Coral reefs form around islands in tropical and subtropical regions but are sensitive to high turbidity and variable salinity so they do not grow near most river mouths.
  • Fringing reefs grow upward as sea level rises, or the land edge subsides, and become barrier reefs. Around islands that sink isostatically or eustatically below sea level the barrier reef can become an atoll. Because they depend on rising sea level, barrier reefs are especially abundant in today’s oceans but they were not abundant at other times in the geological past.

13.6 Wetlands

  • Tidal wetlands in estuaries and bays are formed where wave action is low and organic-rich sediments can accumulate. Mangroves, sea grasses, or marsh grasses are found in many such wetlands. Wetlands provide food and shelter for many marine species, particularly in their juvenile stages.

13.7 Deltas

  • Deltas form when sediments fill river valleys, so additional suspended sediment is carried through the river and accumulates in fan-shaped wetlands outside the river mouth. Deltas are highly productive farming areas because of the abundant supply of river-supplied detritus in the soil. However, high levees have been built in many deltas to prevent flooding. This practice causes the land to subside and become progressively impoverished because the supply of riverborne organic matter is cut off.
  • As sea level continues to rise and the land protected by levees continues to sink and/or erode, levees will need to be stronger and higher to compensate for the higher pressure they must contain as the height difference between the water level outside and the protected land inside the levee increases.
  • At some point in the future, the cost of engineering levees of adequate strength will become prohibitive and the protected land will have to be abandoned. This land will eventually fill with sediments to become ecologically valuable wetlands again. However, the longer the levees are retained, the longer it will take for this recovery process to occur.

Critical Concept Reminders:

CC.2 Isostasy, Eustasy, and Sea Level (pp. 343, 365)

  • Earth’s crust floats on the plastic asthenosphere. Sections of crust rise and fall isostatically as temperature changes alter their density or as their mass loading changes. This, in turn, causes isostatic changes of sea level. Eustatic changes of sea level occur globally when the volume of water in the oceans changes or when the volume of the ocean basins themselves change. Sea level changes create new coasts. Oceanic crust cools progressively after it is formed and sinks because its density rises. Thus, the hot spot volcanic islands slowly sink after they move away from the hot spot. To read CC2 go to page 5CC.

CC.4 Particle Size, Sinking, Deposition, and Resuspension (pp. 348, 351, 354, 355, 356–57, 366)

  • Suspended particles in the ocean sink at rates primarily determined by particle size: large particles sink faster than small particles. Once deposited, particles can be resuspended if current speeds are high enough. Generally large particles are more difficult to resuspend, although some very fine particles may be cohesive and therefore, also difficult to resuspend. Sinking and resuspension rates are primary factors in determining the grain size characteristics of beach sands and sediments at any given location. To read CC4 go to page 12CC.

CC.8Residence Time (p. 365)

  • The residence time of seawater in a given segment of the oceans is the average length of time the water spends in that segment. In restricted arms of the sea, such as lagoons behind barrier islands and fringing reefs, residence time can be long, in which case the nutrients may be depleted, limiting the growth of corals and other marine species. To read CC8 go to page 19CC.

 

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