I. Coastal Processes & Landforms

A. Introduction

• coastal areas are attractive locations for populations

• commerce, tourism, fishing, access to sea routes

• 2/3 of the Earth's population lives near/in coastal areas

B. Overview of Exogenic Forces & Actions Affecting Coastlines

• coastlines are in a state of dynamic equilibrium

• land, ocean, atmosphere, Sun, & Moon, act together to generate:

• tides, currents, waves, erosional features, & depositional features

C. Coastal Environment

• "littoral zone" - the specific coastal environment - it spans the region between the high water line during a storm, to the ocean depth where storm waves are no longer able to move sediments along the ocean floor (~200 ft.)

• "shoreline" - the contact point between the sea & the land

• "coast" - land adjacent to the shoreline

• "mean sea level" - a calculated value, based on the average of tidal levels recorded hourly at a given site over a long period of time (at least 19 yrs.)

• mean sea level varies because of ocean currents, waves, air temperature, air pressure, & ocean temperaure

D. Coastal Actions

(1) Tides

• defn - patterns of daily oscillations in sea level - produced by the astronomical relationships among the Sun, the Moon, & the earth

Process:

• gravitational force - force of attraction between the mass of the Earth & the mass of the Moon; & the mass of Earth & that of the Sun

• recall: gravitational force is inversely proportional to the distance between two bodies ( & directly proportional to their sizes)

• (Sun's influence is 46% (~ 1/2) that of the moon)

• inertia - tendency of a body at rest to stay at rest; & for a body in motion to stay in motion & move along a straight line

• the force of gravity is greater on the side of the Earth facing the Moon & Sun (near-side) - (gravity overcomes inertia)

• the force of gravity is weaker on the opposite side of the Earth - (inertial forces are greater)

• the ocean ( & atmos.) are pulled outward on the near-side of the Earth (high tide)

• on the far-side, the ocean remains in position (is left behind) - an opposing tidal buldge exists (high tide)

• the Earth's surface rotates into and out of these tidal bulges

• every 24 hours & 50 minutes, any given point on the Earth's surface rotates through two high tides ("flood tides") & two low tides ("ebb tides")

• "tidal range" - the difference between consecutive high & low tides

• "spring tides" - results in greatest tidal range - occur when the moon & sun are in conjunction (line up) with the Earth

• "neap tides" - a lesser tidal range - Moon & Sun are neither in conjunction nor in opposition with the Earth - gravitational influences are offset & counteract each other

• "tidal bore" - an incoming tidal current that is constricted as it enters a river channel forming a measurable wave - can cause considerable turbulence

(2) Waves

defn - undulations of ocean water produced by the conversion of solar energy to wave energy;

• wind friction across the surface of the ocean produces undulations of the ocean water called waves

• these waves travel in groups ("wave trains")

• waves are produced in a stormy area of the sea

• waves radiate outward in all directions from these storm generating regions

• the ocean is crisscrossed with intricate patterns of waves travelling in all directions

• waves seen at the coastline may have been produced by a storm in one of the generating regions

• "swells" - regular patterns of smooth, rounded waves in open water (ranging from ripples to very large waves

• deep water waves tend to have long wavelengths

• water molecules within these deep water waves are not moving forward (migrating) very much

• instead, move along a circular path; & transfer energy from molecule to molecule - "waves of transition"

• the diameter of the circular path decreases with increasing depth

• as the wave approaches the shoreline, the wavelength shortens due to frictional drag - (the speed of the circular motion is slowed & becomes more elliptical)

• a waves reaches a point where its height exceeds its vertical stability & then breaks as it approaches the shore "breaker" ("waves of translation")

• "wave refraction" - wave bending along coastlines

• wave refraction along coastlines tends to straighten coastlines:

• wave energy is concentrated on headlands (protruding, more resistant rocks)

• wave energy is diffused as it diverges in coves & bays

• thus, different sections of the coastline are subjected to variations in erosion potential

• "longshore currents" (littoral current) - a zigzagging current that forms parallel to the beach as waves arrive at an angle to the shore - is also a function of wind direction

• is generated in the surf zone, transports sand, gravel, & sediment along the shore

• "beach drift" - sand, gravel, & shells, that are moved by the longshore current in the general direction of the longshore current

• "tsunamis" (seismic sea wave) - travel at high speeds across the ocean (375-500 mph)

• have long wavlengths (~ 60 miles)

• but have small heights (~ 1 m)

• formed by sudden & sharp motions in the sea floor (earthquakes, submarine landslides, eruptions of undersea volcanoes)

• their wavelength shortens as they approach the coast; & the wave height increases (50 ft)

• can have devastating affects on coastlines

E. Sea Level Changes

• changes in sea level expose coastlines to tidal and wave processes

• sea level changes result from either tectonic processes or glacio-eustatic forces

• it is not always easy to determine which of these two processes is responsible for the resultant changes in sea level

• as the global climate warms up or cools down the quantity of water stored as ice in Antarctica, Greenland, & mountain glaciers fluctuates

• if all the ice sheets were completely melted, sea level would rise 215 ft.

• sea level has been rising as the climate has warmed over the last 10,000 years (Holocene Epoch) - sea level has risen 430 ft. since the peak of the Pleistocene Ice Age (18,000 yrs. ago)

• just 100 years ago sea level was 15 inches lower along the coast of S. Florida

• with global warming (due to enhanced G.H. effect) sea level could rise 20 feet during the course of the next century

• most scientists believe, however, that the rise will be less than 6.5 feet - yet devastating consequences!!!!!

• a rise in sea level of just 1 foot will move shorelines inland an average of 100 feet

• Intergovernmental Panel on Climate Change (IPCC) - sea level changes of 1-4 feet

Erosional Coastal Processes & Landforms

• erosional coastlines are those coastlines where erosional processes dominate

• erosional coastlines tend to be rugged, have high relief, and be tectonically active

(e.g., the Pacific coasts of the North and South American Continents)

• erosional landforms: sea cliffs (from undercutting action of the water), sea caves, sea arches, and terraces

Depositional Coastal Processes & Landforms

• depositional coastlines are those coastlines where deposition of material dominates

• are located near onshore plains of gentle relief - sediments from a variety of sources are readily available - are not generally associated with tectonically active coastal margins

• erosional processes do occur, particularly in association with storms

• a common feature of a depositional coastline is a beach

• a beach is defined as the portion of a coastline where an accumulation of sediment is in motion

• "beach drift" - material, sand, gravel, and shells that are moved by the longshore current in the effective direction of the waves

• human intervention