Coastal Processes and Landforms Erosional and depositional landforms of coastal areas are the result of the action of ocean waves. Erosional Landforms Sea Cliffs Wave-cut Notches Caves Sea stacks Sea arches Depositional landforms Beaches Barrier Spit Baymouth Bar Lagoon Tombolo
Wave Properties The energy source for both coastal erosion and sediment transport are the ocean waves generated by the frictional effect of the winds incident on the ocean surface (1) Kinetic Energy: The motion of the water within the wave. (2) Potential Energy: Due to the position of water above the wave trough. Wave energy increases with wind speed and fetch
Wavelength Distance from one wave crest to the next Wave height The distance between trough and crest Wave period The time taken for two crests to pass a given point (remains almost constant) λ = V * P The wavelength, λ, is the product of its velocity and period.
Wave motion (a)ocean depth > ½ the wavelength - waves not affected by ocean floor (b) Ocean depth < ½ the wavelength - wave height increases and wavelength decreases The wave becomes more peaked Breakers form Breaking of waves: conversion of potential to kinetic energy Work done on the shoreline
Wave Refraction Straight shoreline - drag exerted by the ocean floor causes waves to break parallel with the shoreline. The direction of travel of a wave varies as it approaches an indented coast. Crests approaching the headlands experience the drag of the ocean floor first, which causes: 1. Increase in wave height 2. Decrease in wavelength 3. Decrease in velocity
The crest of the wave approaching the bay continues without change until it gets closer to shore, bending the wave somewhat toward the overall shape of the shoreline *** Headlands are sites of intense erosion *** Embayments are often sites of deposition (protected from currents, gradual transfer of energy) Mechanical and chemical processes affect rocky shorelines leading to a number of erosional and depositional landforms. Such landforms are affected by: Geology, nature of wave action, tidal ranges, and sea level change
Transport of Sediments by Wave Action Rock particles are eroded from one area and deposited elsewhere. Wave refraction affects this process. Beach Drift: Swash and backwash rarely occur in exactly opposite directions Upward movement occurs at some oblique angle Backward movement occurs at right angles to the beach. This creates lateral movement of particles (beach drift)
Rip Currents: Rip currents form when waves are pushed over sandbars. The weight of excess water near the shore can rip an opening in the sandbar, causing water to rush seaward. Source: NOAA web site
Longshore Currents: Backwash and rip currents often cannot remove enough water to compensate for the buildup of water in the shore zone. A "longshore current" results, which parallels the shore in the overall direction of the breaking waves. The longshore current moves sediment-laden water parallel to the shoreline (longshore drift - together with beach drift, we have littoral drift).
Tides and Tidal Currents: Tides are produced by the gravitational pull of both the Moon and the Sun on the Earth, which stretches solid and liquid surfaces (as well as the atmosphere) Tides are strongest when the sun and moon are wellaligned (spring tides) The Earth s surface rotates into and out of the moon s tidal bulges twice daily (Moon s effect is twice that of the Sun) Incoming and outgoing tides produce currents in opposite directions, but the current in one direction is sometimes stronger than in the other, resulting in sediment transport
Spring Tide Tides enhanced during full Moon and new Moon Sun-Moon-Earth closely aligned Neap Tide Tidal effects of Moon and Sun not additive
Influence of Perigee, Apogee, Perihelion and Aphelion on the Earth s Tides Stronger for perigee and perihelion
Erosional Coastal Landforms Along rugged, high-relief, tectonically-active coastlines Sea cliffs A tall, steep rock face, formed by the undercutting action of the sea Wave-cut notches A rock recess at the foot of a sea cliff where the energy of waves is concentrated Sea Caves Caves form in more erosive sediment when the rock does not fully collapse in a deeply-notched environment
Sea Arches and Sea Stacks Sea Arches form when waves erode from two sides through a thin headland, creating an arch-shaped hole beneath part of the headland Sea stacks, steep pillars of rock not far offshore, occur when sea arches eventually collapse http://www.geog.ouc.bc.ca/physgeog/contents/11m.html
Wave-cut platform Horizontal benches in the tidal zone extending from the sea cliff out into the sea If the sea level relative to the land changes over time (becoming lower with respect to the land), multiple wave cut platforms (terraces) result 1 3 2 4 5 6 http://www.rgs.edu.sg/events/geotrip/cliff.html
Depositional Coastal Landforms Located along land of gentle relief Beaches Beach Formation Process 1. Erosion of bottom of steep slope by waves causes instability 2. Upper portion of cliff collapses 3. Waves crash against reworked material. Deposition of finer material leads to offshore terrace while waves continue to erode and steepen cliff. 4. Offshore terrace gets progressively shallower due to deposition 5. Gentle slope favours constructive waves, transporting material from terrace toward land 6. Deposition at base of cliff forms a beach
Barrier Spit Material transported by littoral drift deposited along ridge, extending outward from a coast in an area with weak offshore currents If the spit grows to completely block an embayment, it is called a bay barrier or baymouth bar A lagoon is a body of water behind the barrier
Bay Barrier
A tombolo occurs when sediment deposits connect the shoreline with an offshore sea stack or island http://www.geog.ouc.bc.ca/physgeog/contents/11m.html