Sec 2 IDS, 2010 Coastal Management

Coastal processes Erosion, transportation and deposition These processes are influenced by waves and currents.

Coastal processes Waves are the primary agent of erosion. Erodes the coast by hydraulic action, corrosion, attrition and solution. High energy waves will cause more erosion. Q: What factors determine the energy of waves?

Coastal processes Wave energy is determined by -wind speed -fetch -natural events (such as tsunamis) -human events (such as sea traffic)

Waves form as wind blows over the ocean surface. Waves increase in size with the following factors: - greater wind speed – greater wind energy - greater amounts of time that the wind blows - greater fetch: the distance of water over which the wind blows WAVES as agents of erosion

How large waves cause coastal erosion? Swash is the movement of waves up the shore towards the land. As the waves rush up the shore, they transport materials up the shore. As the swash moves up the coast, it loses energy due to gravity and pulled back into the sea. This backward movement is known as backwash. They transport materials down to the sea. WAVES as agents of erosion

If backwash is stronger than the swash – wave energy is high – destructive waves – removes materials from the beach rather than deposit them Low energy or constructive waves lead to deposition of materials. This occurs when there is a strong swash and a weaker backwash. Destructive waves – plunging breakers Constructive waves – spilling breakers WAVES as agents of erosion

Long-term effect of wave translation: Coastal erosion and beach modification WAVES as agents of erosion

Identify the crest, swash and backwash of the wave. Swash & Backwash

CURRENTS as agents of erosion Tides influence currents. Tides are daily changes in the sea level due to the movement of the Moon and the Sun. Therefore the coast experiences a high/low tide every 12 hours Currents are flows of water that move either horizontally or vertically in a certain direction Changes in tides can create currents capable of removing materials from the coast Materials can be removed when water retreats during low tides

CURRENTS as agents of erosion

Coastal processes - erosion Corrasion – when material carried by the waves are thrown against the coast and breaking the rocks on the coasts Attrition – when materials carried by the waves are thrown against one another, breaking down into smaller, smoother and rounded pieces Solution – soluble minerals in coastal rocks are dissolved and removed by seawater Hydraulic Action – water surges into cracks, joints and compress the air inside which exerts pressure on the lines of weakness. When water retreats, there is a release of pressure as trapped air expands. This repeated action weakens and breaks down rocks

Transportation: Longshore currents Longshore currents: currents flowing parallel to the coast Horizontal movement of a large volume of seawater  Due to oblique waves (waves approaching the coast at an angle)  Due to differences in water temperature  Due to differences in water salinity  Global currents or more local currents

Transportation: Longshore Drifts Swash pushing sediment onshore in an oblique angle Backwash: Back flow of water and sediments perpendicular to shore by gravity Net effect: Zigzag longshore beach drift Zigzag movement is one way in which sediments are moved along the coast The movement of sediments is sometimes helped by longshore currents which results in longshore drift (movement of sediments parallel to the coast) Animation:

The waves are hitting the beach at an angle. Therefore the direction of the longshore current (the movement of water) and the littoral drift (the movement of sand) are along the beach toward the viewer. Littoral drift is measured by the volume of sand moved per year (cubic meters). Transportation: Longshore Drifts

Ethan left his soccer ball along the beach. Where will it likely end up at the beach? Why? Wave crest Transportation: Longshore Drifts

Coastal Erosion ~25% of homes within 500 ft of the world’s coastlines (including shores of the Great Lakes) will fall victim to erosion effects within the next 60 years Shore in 30 years Shore in 60 years Shore Today

Factors Influencing Coastal Erosion Erosion factors  Tropical cyclones  Tsunamis  Tidal actions  Long-term rise of sea level  Human activities

Storms and Coastal Dynamics Storms are caused by low pressure systems –Low pressure cause a bulge of water to form –Onshore winds, associated with storms, approach the shore with unusually high tides called a surge –The exceptional force of the wind driven waves and surge combine to erode beach faces, dunes above the beach, and sometimes cliffs behind the beach –Hurricanes are power examples of these storms

Coastal Erosion: Before Hurricane

Coastal Erosion: After Hurricane U.S. Geological Survey

Cliff Waves and currents act to erode beaches and eventually cliffs Cliffs are undercut –Cliff face then slumps or slides off into the sea –Cliff faces show landward retreat of the shoreline Wave refraction works to erode points of land jutting out into the sea –Wave base interaction with the ocean bottom slow the progress of the wave –Wave base encounters the bottom near a jutting point before the coastline

Cliffs and shore platforms

Cliffs and shore platforms

Cliff erosion: June 2002

Cliff erosion: October 2002

Stacks and Stumps

Headlands and Bays