Geol 33 Environmental Geomorphology

J Bret Bennington

Coastal Landforms

Beaches

A beach is an area of sediment accumulation (usually sand) exposed to wave action along the coast. Beaches extend from the low tide level inland to dunes, cliffs, or forests. Generally, beaches for in relatively low energy areas such as bays. High wave energy tends to erode sand and transport it to lower energy areas. Beaches have a characteristic morphology that changes from season to season.
 
 


 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Ridge and Runnel repair

During storms, sand is also carried offshore by waves. During times of normal wave activity, this sand slowly migrates back toward the beach. Waves drag on the seafloor and pull the sand shoreward into an underwater ridge. This ridge advances toward the beach and is eventually joined to it. Between the ridge and the beach is a depression called a runnel. During low tide at Jones Beach the ridge and runnel can often be seen.

In the winter, when storms and waves tend to be larger and more frequent, beaches erode faster than they can self-repair, so they tend to become narrow. In the summer, when storms and waves are smaller and less frequent, the beach is able to build back up and widen.
 
 

Waves

Waves begin as tiny ripples generated as wind blows across the surface of the ocean. These ripples combine and are further pushed by the wind to build in size. Strong winds blowing across large areas of water for long periods of time can create very large waves.

Wave motion is the transmission of energy through water, without any net movement of the water itself. In this way, waves are different from currents.

Wave generation:

Most waves are generated by wind. The distance of open water over which wind blows to generate a wave is called the fetch. The relationship between wind speed, fetch, and wave height is complicated, but empirical descriptions relating the three variables have been formulated.

When wind has been blowing for many hours over a large expanse of water the maximum height of the waves will be equal to:

H = .0023 V2 (meters and kilometers / hr)

Wave height is also dependent on fetch:

H = .34 F0.5 (Stevenson formula, meters and kilometers)

or

H = .0067 VF0.5 (Bretschneider equation for short fetch and high wind velocity)



Wave celerity:

Waves have two important parameters: wave height (amplitude) and wavelength (L). Wave height is the vertical distance from the crest to the trough of the wave. Wavelength is the horizontal distance from one wave crest to the next.

Celerity (velocity) is defined as the time requried for two successive wave crests to pass a point.

C = L / T

The relationship between wavelength, wave period, and depth is given by the Airy Equation:

In extremes of shallow and deep water, this equation can be approximated by simpler solutions to show the following relationships:

In shallow water, wave celerity is governed by the depth of the water. For shallow water conditions (D / L < .04) this relationship is:

For deep water conditions (D / L > .5) celerity is related to wavelength:

Thus, in deep water, waves with long periods will separate out from waves with short periods. This process is called wave dispersion and results in a regularly spaced precession of swells called a wave train. Short waves tend to loose energy rapidly and fade with distance, whereas long waves can travel for thousands of miles with relatively little loss of energy. The origin of the giant surf that is sometimes seen in Hawaii and Southern California in July and August is often the winter storms that occur in the Antarctic Ocean.

Two wave trains with slightly different periods (out of phase) will sometimes approach a coast. The wave trains will interact both constructively (addition of two waves produces a larger wave) and destructively (addition of a wave crest and a wave trough produces a smaller wave) to create a wave train with a distict surf beat of alternating higher and lower waves.
 
 

Why waves break:

As a wave passes through the water, the water molecules travel in a circular orbit that returns them to their original positions. These orbits decrease in diameter with depth. At a depth equal to 1/2 the wavelength the orbits become so small as to be negligible. This depth is called wave base.

As the wave enters water less than 1/2 wavelength deep, the lower orbits begin to interfere with the bottom. This causes the wave to slow down as the lower orbits are drag at the bottom and are flattened. As C decreases, L must also decrease if the period of the wave is to remain the same (which it does). Thus, as the wave slows, its wavelength shortens and the wave height increases due to the conservation of the waveās momentum.

When the wave becomes so steep that it cannot support itself the front of the wave falls forward and breaks. When a wave breaks most of its energy is converted into tubulence called surf.

Breaking waves throw water up onto the shoreline where it then runs back toward the ocean as swash.
 
 

Wave Refraction

Waves almost never approach the coast perpendicular to the shoreline. Generally, the waves that are crashing onshore break at a shallow angle to the beach, even if they approach the shore at a steep angle. This is caused by the phenomenon of refraction.

Waves travelling in shallow water are slowed. When a wave arrives at the coast at a steep angle to the shoreline, the part of the wave the enters shallow water slows, causing the rest of the wave to bend or refract toward the slower part.

Refraction also explains why headlands are eroded faster than bays. The shallower water associated with the headlands causes waves to bend towards the headlands and away from the interveaning bays, causing their erosive energy to be focused accordingly.
 
 

Longshore Drift

As waves break on a beach, they throw water up onto the beach in an area called the swash zone. Because the waves approach at an angle, the water is thrown up at an angle.

However, the water runs back down the beach under the influence of gravity perpendicular to the shore. Thus, the crashing of waves causes water to move along the beach in a step-like fashion in the direction of wave movement. This creates a longshore current.

Sand is transported along the beach in like fashion in a process called longshore drift. Longshore drift erodes and deposits sand continuously along the beach. The sand that is removed from one point along the beach is replaced by sand eroded from upcurrent.
 
 

Tidal Currents

Water level in the ocean rises and falls twice a day in response to the moonās gravitational attraction to produce the daily high and low tides. During high tide water is piled up against the shoreline and flows inland into bays and coastal estuaries, producing a flood tidal flow. During the change to low tide this water flows back out to sea, producing an ebb flow.
 
 

Barrier Islands

The east coast of the United States from Cape Cod to Florida is noted for its system of narrow offshore sand ridges that form barrier islands protecting the coast from waves. Barrier Islands form where this is abundant sand accompanied by a shallow offshore gradient. Between the islands and the mainland are shallow lagoons, bays, or marshes.

Shoreline features related to barrier islands

A variety of barrier-related features are seen along the shoreline of the Atlantic coast. Long Island, because of its abundant supply of sediment, has an extensive system of barrier islands and beaches, exhibiting many of these features.

Bay barriers: continuous barrier beaches that close off the entrance to a bay. In the upper reaches of a bay the bayhead barrier protects lagoon or marshland. Barriers that connect headlands together along the outer reaches of an embayment are called baymouth barriers. The eastern reaches of Long Island contain examples of baymouth barriers where the steeper shelf gradient and more vigourous wave activity have pushed the barrier islands against the indented mainland coast.
 
 


 
 
 
 
 
 
 
 
 
 
 
 
 
 

Barrier spits: beaches that are attached at one end to their source of sediment. Simple spits consist of narrow finger of sand with a single dune ridge that elongates in the downdrift direction. Double spits can form if drift transports sand in two directions across and inlet, or if a baymouth barrier is cut by a tidal channel. Wave refraction at the end of a spit will transport sand to form a recurved spit. Complex spits form when a plentiful supply of sediment is transported by both ocean and bay currents. Multiple lines of dunes can be formed by wind transport of sand across the spit.
 
 


 
 
 
 
 
 
 
 
 
 
 
 
 
 

Tombolos: Islands refract incoming waves causing them to bend inward around the island and converge along the mainland in opposite directions. This produces a net transport of sand in two directions toward the island, resulting in a spit of sand that grows outward from the shore, eventually connecting the island to the mainland by a long, narrow strip of sand called a tombolo. Examples of tombolos on Long Island include Ashroken beach connecting Eatons Neck to Northport and the connection between the northern and southern reaches of Lloyds Neck.
 
 


 
 
 
 
 
 
 
 
 
 

Capes: are barrier islands that project into the open sea to form a right angle shoreline. These are generally large features that are exposed to wave attack on each side, but one side is accreting while the other is eroding. This produces a distinctive series of truncated dune ridges. Examples include Cape Fear and Cape Hatteras in North Carolina and Cape Canaveral in Florida.

Sea islands: these are islands created by the flooding of the mainland by sealevel rise. Many of the necks and most of the islands around Long Island are all sea islands.
 
 


 
 
 
 
 
 
 
 
 
 
 
 
 
 

Beach ridge islands and cheniers: These structures form along low-energy coasts where wind piles sediment into dunes with very little wave erosion. Where sand is abundant beach ridges can develop, forming an island consisting of a distinctive pattern of ridges with intervening wetlands. Cheniers are found along the Gulf coast where sand and silt are blown over salt marshes, forming ridges.

Barrier Island Rollover

Sealevel is slowly rising (as it has been for the past 10,000 years). Currently it is rising at an average rate of 1 foot per century. However, global warming could increase this rate to as much as 3 to 6.5 feet per century.

The most notable current effect of sea level rise is the rollover of barrier islands. As sea level rises, the barrier islands slowly migrate landward (as do the lagoons, bays, and marshes). This landward migration is accomplished during storms that erode the beach and wash the sand over the duneline, depositing it on the middle and backside of the island as washovers. This sand builds up the back of the island and creates a reservoir of sand for rebuilding the wind-blown dune deposits landward of their former position.

Where barrier islands are narrow they can be breached by waves during large storms, creating an inlet. Inlets subsequently are flushed by tides, resulting in the creation of ebb and flood tidal deltas. Usually, inlets are eventually repaired as the barrier beach migrates downdrift to form a spit closing off the breach. The ebb tidal delta then gets reworked to add to the supply of sand along the barrier front and the flood tidal delta becomes a bay bar or marshland area.

Many of our current problems with stabilizing barrier island beaches can be understood in light of rollover. For example, during the recent noreasters that have hit Long Islandās south shore barrier islands, tremendous quantities of sand have been washed over the front of the islands and through breaches in the islands, forming sand deposits offshore in the Great South Bay. What is happening here is that the storms of eroding the front of the islands and building their backsides. In effect, storms are moving the islands landward. As they move in this way they become raised above sealevel and more resistant to storms.

Human response to rollover

After storms, homeowners usually attempt to reclaim their property by bulldozing the sand from the back of the island to the front whence it came. This, in effect, undoes what the natural process was trying to do. Unfortunately, it is inherently a short term solution because the next storm will once again wash some of the sand over the island and some out to sea. Often, structures are built to try to prevent erosion. However, as long as sea level continues to rise, storm erosion will only become worse and worse at a particular shoreline location. If the barrier island is prevented from rebuilding itself on its back barrier side, then it will simply get narrower through time and the dune line will get lower and weaker as more and more sand is swept out to sea. Ironically, the only way to preserve a barrier island is to let it rollover and migrate over time.
 
 

Sea Cliffs, Rocky Shorelines and Reefs

Sea cliffs are created by mass wasting as waves undercut a steep shoreline slope. Along a submerging, steep, rocky coastline irregularities in the shoreline will cause waves to refract and focus at shoreline promontories. These are cut back and along the sides by erosion. Eventually such promontories thin and leave erosional remnants called sea stacks as small islands. Undercutting of shoreline promontories by waves can create arches. Because waves are refracted toward promontories the bays become sites of low energy sand accumulation, forming bayhead beaches.
 
 


 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Bays

Bays are indentations in the shoreline that form pockets of sheltered sea. Again, many bays are shoreline valleys and depressions that were eroded during the last ice age and flooded by rising sea level (for example, the bays of Long Island's north shore and Chesapeake Bay). Bays are often partially closed off to the sea by spits or tombolos (a spit connecting the mainland to an offshore island (an excellent local example is Northport Bay - sheltered on the Sound side by Eaton's Neck and the Ashroken tombolo).

Tombolos form because waves refracted around an island impinge on the shoreline at opposite angles, causing sand to converge directly in the lee of the island. Eventually, this sand builds out as a spit from the mainland toward the island.
 
 

Organic reefs

A reef by mariners ' tradition is any submerged obstruction to navigation. Geomorphologists restrict the use of the term reef to organic buildups of carbonate material found fringing parts of the shoreline in the tropics. The most important Cenozoic reef builders are Scleractinian corals with symbiotic algae (called hermatypic corals) and corraline algae. These organisms can build massive mounds of carbonate and have growth rates capable of keeping pace with most rates of sealevel rise.


 
 
 
 
 
 
 
 

In general, reefs form where the following set of conditions are met:

Coral reefs will not grow in the vicinity of river mouths and they tend to be better developed on the windward side of islands. Most textbooks attribute this to nutrients and oxygen supplied by waves coming off of the ocean. However, studies have shown that water coming in from the open ocean is nutrient and oxygen poor compared to reef water. Thus, it appears that most nutrient cycling and oxygenation is occuring within the reef. Why then the dependence on waves?

Hurricanes erode material off of the front of the reef and redeposit it in the back reef and lagoon. Sediment and talus blocks are also flushed from the reef crest into deeper water by the ebbing storm surge. This may be an important mechanism for ridding the reef of excess sediment and maintaining its vitality.

Coral reefs that are cut off from breaking waves become choked in their own mineral detritus and die. Coralline algae may overgrow a dead reef and cement the reef skeleton in a massive encrustation of carbonate, forming a flat, table-like surface. Bioeroders such as sea urchins (echinoids) carve pockmarks and cavities into the carbonate surface, creating a highly porous texture.

Near the high tide line carbonate sediments react with fresh water outflow from the water table and recrystallize into a solid cementstone called reef rock. The presence of artifacts such as bottles and cans embeded in reef rock attests to its recent origin.

Charles Darwin and the formation of reefs

Darwin observed a variety of reef types in his famous voyage around the world on the HMS Beagle. In 1837 he published a paper titled: "On certain areas of elevation and subsidence in the Pacific and Indian Oceans, as deduced from the study of coral formations". In this paper he explained the three main types of island coral reefs as observed by mariners (fringing reefs, barrier reefs, and atolls) as a genetic sequence caused by the subsidence of volcanic islands. Modern drilling has shown that deep ocean atolls (ring-shaped islands formed of coral reefs) consist of up to more than1000 meters of carbonate overlying volcanic rock - verifying Darwin's theory. For example, Eniwetok atoll was bored in 1951. At 1.25 km depth basalt was encountered. The limestone overlying the basalt was found to be Eocene in age. Thus the atoll is a 1.25 km pillar of carbonate built up on a 3.2 km high deeply drowned volcanic island.

Great Barrier Reef

Not all reefs can be fitted neatly into Darwin's scheme. For example, the Great Barrier Reef off eastern Australia is a linear feature that began with coral growth on the upraised edge of a continental margin parallel fault block.
 
 

Estuaries

An estuary is a coastal wetland where freshwater from runoff and saltwater from tides mix. Most large rivers do not empty abruptly into the sea. Instead, they merge with the sea in a transitional area near their mouths called an estuary. Estuaries have water that is a mix of fresh and salty, called brackish.

Most estuaries are formed in valleys that were carved out when sea level was low during the last ice age and then flooded as sea level rose due to melting glacial ice. Most estuaries are bordered by saltwater wetlands called marshes. Marshes are flushed each day by tidal water flowing in and out from the sea. Tidal water is distributed throughout the marsh by a series of branching channels that form a dendritic system that empties into a main tidal channel open to the sea.

Because saltwater is denser than freshwater the saltwater coming in on the rising tide travels up the estuary as a distinct density layer beneath the overriding freshwater layer.

The lower Hudson River is an excellent example of an estuary.

Deltas

Lobate bodies of sediment deposited by rivers as they empty offshore into the sea or a lake.

Deltas build outward from the shoreline at river mouths - usually prograding the shoreline.

They tend to form in areas of subsidence (rivers flow into depressions), also, weight of sediment depresses crust, causes subsidence and creates accomodation space.

Different types of deltas

Gilbert Deltas - these are coarse-sediment-dominated deltas that commonly form in lakes from the deposition of bed-load sediments. Often seen in glacially deposited strata - can be observed in cliffs on the North Shore of Long Island.

Wave-dominated Deltas - these deltas form along coasts where wave activity is strong and sediment supply is moderate. Waves winnow and redistribute sediment - delta tends to be sandy and arc-shaped.

Tide-dominated Deltas - these deltas are shaped by strong tidal flows that tend to create long, straight distributaries and shore-perpendicular tidal current sand ridges.

River-dominated Deltas - these deltas form where rivers carry large amounts of suspended sediment to shorelines with moderate wave and tide activity. Ex. Mississippi delta.

Also called "bird-foot" deltas due to elongated distributary channels that build out onto the shelf.