TOPIC 5b - COASTAL ENVIRONMENTS AND BEACH PROCESSES

 

I. Physical Characteristics of Coastal Margins - Coastal margins are strongly influenced by plate tectonics, sea level variations, surface ocean and atmospheric circulation, and local climate.

A. Passive plate margins - sometimes called trailing edge coasts.

1. broad continental shelves.

2. low lying continental margins (no active tectonism or volcanism). Weathering and erosion have had time to reduce any high topographic relief.

B. Active plate margins - sometimes referred to as collision coasts.

1. narrow (to nonexistent) continental shelves. Slope leads quickly into a trench.

2. Active volcanism and/or mountain building creating coastal margin with high relief.

C. Influence of sea level variations on coastal margins.

1. On a scale of thousands of years (and longer), sea level has shifted up and down by more than 100 m due to changing climates and plate tectonics. The position of sea level over time will alter the shape of a coastline.

2. If sea level rises, flooding an eroded irregular surface, the resulting shoreline will be irregular or submergent.

3. If sea level falls, the smooth, wave-planed shelf is exposed and the shoreline will be relatively straight and emergent.

4. In some areas, deltas and coral reefs will make the effect of changing sea level hard to discern because they will dynamically change and respond to the sea level variations.

 

II. Coastal Process Model - an attempt to relate coastal form and its variation to basic Earth processes.

A. Factors that contribute to the appearance of a coastline on a short time scale. (Plate tectonics/sea level act at long time scales.)

1. Wave energy

2. Sediment supply

3. Tides

B. From these factors, we can develop a simple diagram that relates coastal forms to two major factors : wave energy and sediment supply. We can neglect tides on most open coastal margins.

C. This model can be tested using actual U.S. Corps of Engineers data. They record annual wave data and sediment transport at many coastal localities. With this information and notes on coastal form, we can construct a more sophisticated diagram:

D. It is evident that when wave energy is in excess of what is required to move the available sediment, that a coastline will erode, retreat, and form sea cliffs. When sediment supply roughly balances the wave energy, beaches are typical. When sediment supply dominates, deltas form and extend seaward.

 

III. Coastal Environments - Beaches and Barrier Beaches

A. Beach systems basically extend from back-beach dunes and sea-cliffs offshore to water depths where sand is regularly moved by waves. This is typically 10-15 m off southern California.

B. Beach Zones.

1. The Offshore Area where typical wind waves will interact with the ocean bottom and form sand ripples.

2. The Surf Zone occurs where waves form surf and water turbulence. This is a zone of sand bars and scour troughs.

3. The Transition Zone contains reformed breakers; coarse particles accumulate; near the low tide line.

4. The Swash/Backwash Zone is the beach face with a smooth rising slope up which the uprush of water from breakers moves (swash), and then runs back down to the ocean (backwash).

5. The Berm is the terrace built up by the wave swash. Its top surface may slope shoreward.

6. Backbeach Dunes occur when wind off the beach carries sand shoreward.

7. Sea Cliffs that are inactive may lie inboard of the backbeach or dunes. With a change in climate or sea level, parts of the beach may be eroded away and the sea cliff will then become active.

C. Energy Sources in the Beach Environment

1. Surface waves are the primary source of energy in the beach environment. They typically refract as they enter the Surf Zone and entrain sediment in the breakers as they move shoreward. Longshore currents and rip currents are formed as a consequence of the onshore wave activity.

2. Tides also influence the position of the Surf-Berm beach zones creating a broader zone in which waves are active along the beach margin. Changes in tides cause alternating intervals of deposition and erosion on the shore face and variations in the local beach water table.

3. Seasonality of wave energy. During the southern California winter, strong waves move sand offshore forming bars and leaving narrow berms. In the summer, gentle waves move sand back toward shore, forming wide berms and leaving smaller bars.

D. Sediment Supply

1. Beaches are normally composed of sand which comes from weathered rock via rivers. The sand is deposited by rivers when they reach the ocean, so that beaches are wide near river mouths. The littoral current transports sand away from the river mouths and along the shore. In some places, there is no sand available to build beaches. In those cases, the beaches may be composed of gravel or cobbles, volcanic ash or pumice, or even sea shells.

2. Seasonal variations can also occur in sediment supply. Sediment which supplies the beach environment arrives in part due to longshore currents and littoral drift. The sediment may come from erosion of the beach upcurrent, but much of the sediment comes from river sediment discharge on the coastal margin. This sediment may have a strong seasonality depending on local climate.

3. There can also be longer term variations in sediment flux and river discharge due to shifting climate patterns or changes in river channel.

4. Because of variations in sediment supply and incoming wave energy, the process of beach erosion and deposition is continuous. The longterm net change in the beach will depend on which aspect of the process is dominant.

E. Sea Cliffs

1. Ocean cliffs are classified as either active or inactive. An active cliff undergoes erosion as the wave energy is expended at its base. An inactive cliff has a beach at its base that lessens the erosive activity of the waves.

2. Wave erosion on active cliffs carve sea caves and arches in the cliff rock. When enough erosion takes place so that the arch can no longer support its weight, the bridge collapses, leaving a column of rock, called a sea stack, standing in the water.

F. There are four basic types of beaches around the world.

1. Coastal Plain Beach (most common) - Coastal plain beaches are the most common beach type and indicative of a passive margin. There tends not to be resistant bedrock near the coastline and no well-established offshore barrier islands or bars.

2. Barrier Beach - Barrier beaches are notable for the presence of large permanent barrier sand bars offshore. These bars are evidence of episodic extremely strong wave action that carries sand from the beach environment offshore to the barrier bars. This heavy wave action is usually associated with Hurricanes!

3. Pocket Beach - Pocket beaches are common in active margin settings where hard bedrock is subject to wave action. Rock that is especially hard and resistant to erosion forms headlands and the intervening pockets become beaches. These are very common in California.

4. Spit - A spit develops where longshore drift has to turn a corner, whether man-made or natural. The corner creates a still-water "shadow" from the wave energy. With a lowering of wave energy, the longshore current cannot carry as much sediment and it dumps some. When enough sand accumulates so that it is visible above water, it is called a spit.

G. Human Impact of the Beach Environment

1. Artificial Beach Structures: Beach shape is often altered by building structures that interfere with wave action and the littoral current. A structure perpendicular to the beach is called a jetty or a groin; a structure parallel to the beach is called a breakwater.

2. A groin is a rock barrier built out into the water from the beach. Sand collects and builds up on the "updrift" side of a groin, whereas on the "downdrift" side, sand erodes, narrowing the beach. A jetty is a special type of groin that keeps sediment out of a shipping channel.

3. If a breakwater is built parallel to the mainland beach, longshore drift may pile up behind the breakwater, forming a sand bridge to the island. Such a sand bridge is called a tombolo.

4. Sea walls and revetments are constructed parallel to sea cliffs to prevent wave erosion. A sea wall is a concrete barrier; a revetment (or riprap) is a barrier of rocks and boulders wired together. Both types of barriers absorb the wave energy that would otherwise be expended on the cliff.

 

IV. Coastal Environments - Deltas

A. Deltas form where sediment supply predominates over wave energy.

1. Deltas are typical on coastlines with large rivers. In fact, all major rivers will typically build deltas at their mouths. The most notable exception is the Columbia River in Washington/Oregon which enters a high energy coastal environment.

2. The vast majority of deltas are found on passive margin (trailing edge) coastlines. There is a typical and clear association between passive margins, wide coastal plains, and large drainage basins. This produces large rivers and yields large sediment supply.

3. Examples of important deltas around the world.

a. Nile Delta (N. Africa)- This delta is the classic example of a delta which was scientifically studied by the ancient Greeks.

b. Other examples: Niger Delta (W. Africa), Amazon Delta (E. South America), Mississippi Delta (S. North America), Indus and Ganges Deltas (S. Asia), Yellow and Yangtse Deltas (E. Asia).

4. Deltas are natural sites of agricultural development. Floods deposit new sediment each year. Delta is near base level and has shallow water table.

5. Deltas represent the surface of thick sediment bodies which have been built up over millions of years. As new sediment is added, the entire sediment pile subsides to compensate.

B. Processes of Delta Formation

1. As a river enters an ocean or lake, the jet of sediment-laden water diffuses and slows.

2. Sediment deposits at the border of the diffusing jet forming bars and levee (lateral bars flanking the channel).

3. As channels grow, they extend their natural levees as well. Levees are built up by successive floods which overtop the channel banks and deposit more sediment on the levees and in the low flood plain areas between channels. Wetlands form in these broad areas.

4. Marshes form on mud plains that build up between delta channels. Salt water plants first colonize the mud flats. As sediment and plant debris (peat) collects, it builds up the marsh surface and new plants that are less tolerant to salt water invade the flats near high tide level. The marsh surfaces continue to build outward and upward. When the surface is above tide levels, grasslands develop which make excellent farm land (beware of big floods).

5. As channels build outward, they may also bifurcate (divide) forming a network of distributaries.

6. As channels build out, the gradient of the channels must be maintained by sediment deposition and so the channel-levee system must build up as well as seaward.

7. When a channel has built above the surrounding delta plain, the next major flood can overtop and cut through the levees to open a new channel which may build a new system. The old channel is eventually abandoned and filled with bog and meadow deposits. This process creates fan-like deltas as channels shift, divide, and build outward over thousands of years.

8. A delta's structure is like a layer cake of shelf marine sands at the base, then overlying sandy channels with muddy levees and adjacent mud and peat marshes, and a top layer of fresh water meadow soils.

 

V. Coastal Environments - Estuaries and Lagoons

A. Estuary Environments

1. Estuaries are semi-enclosed embayments open to the sea at one end. The drowned mouths of rivers formed by rising sea level and Scandinavian fjords formed by ancient glaciers extending out to sea at times of lower sea level are classic examples.

2. Estuaries are places where fresh water meets and mixes with ocean salt water.

3. Estuaries are characteristic of low-gradient, broad coastal plains and broad shelves on a passive margin.

4. Estuaries are strongly affected by tides and river discharge.

5. Estuaries are short-lived geologically and soon fill with sediment.

B. Estuarine Circulation

1. Fresh water from rivers moves outward (within the estuary) and over denser salt water.

2. Mixing occurs at the base of the fresh water layer as it moves seaward over salt water. The upper layer thus becomes saltier as it moves seaward.

3. Salt water continues to flow inward under the fresh water to replace what has been lost through mixing with the surface layer.

4. Thus there is normally a net inflow at the bottom of an estuary and net outflow at the surface. The water column in the estuary is stratified chemically except where strong tides may cause intermittent complete mixing.

5. Sediment particles in river flow are flocculated (aggregated) into particle clusters when they meet salt water (ionic effects of dissolved solids). They then settle into the lower layer and are transported back into the estuary. The finest particles become trapped at the boundary between fresh and slat water forming a 'turbid' layer.

6. Tides move in and out of the estuary and can temporarily move the mixed and turbid layers either inshore or out to sea.

7. The combined effects of tides, mixing, and intermittent strong river discharge can be to trap pollutants in the estuary.

B. Lagoons - often associated with estuaries and always with barrier islands.

1. Lagoons have to river input. They are formed from sea water flooding behind a barrier or by a barrier building out and cutting off a coastal sea area.

2. Located on broad low gradient coastal plains and broad shelves of passive margins.

3. Tides dominate lagoonal flow. Restricted mouths of lagoons can cause strong tidal forces. Tide water must enter and leave through narrow inlets or breaks in the barrier.

4. Surface water evaporates in lagoons creating denser saltier water which sinks to the lagoon bottom. Surface salt water flows in to replace the denser water and the denser bottom water flows out of the lagoon at the bottom. This is the reverse of estuarine circulation. Larger restricted coastal oceans and seas behave the same way. The best examples are the Mediterranean Sea and Red Sea.

5. Both estuarine and anti-estuarine circulation operate as pumps moving nutrient-rich water into lagoons or estuaries to supply rich biological communities.

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