CHAPTERS 13 - SHORELINES

WAVES AND WAVE ENERGY

Wave energy is directly related the "size" of a wave. These factors include:

wave height - the "amplitude" increases with increasing "wave energy"
wave length - also increases with increasing "wave energy"
wave period - time interval between the passage of crests, also increases with increasing "wave energy"

Wave energy is dependent on:

wind speed - faster wind results in greater surface drag, which translates into more energy into sea's surface.
wind duration - wind energy is "cumulative" - the longer the wind blows, the larger the waves.
fetch - the longer the distance that wind blows on the sea's surface, the greater the wave energy.

Where on earth are these conditions the strongest?

Most ocean waves evolve in South Atlantic Ocean (speed, duration, fetch)
Storms (tropical storms and mid-latitude winter storms produce great waves)

CONCEPTS ABOUT WAVES

WAVE BASE - the depth to which wave energy penetrates below the surface. Wave base is related to the size of waves; "wave base equals about one half of the wave length." Wave energy is dissapated by grinding action against the sea bottom. The interaction of wave against the sea bottom results in THREE KINDS OF WAVES depending on water depth...

Wave of Occilation - deep ocean waves; nothing stops them from travelling great distances. Wave energy moves in "circles"
Breakers - The wave length of a wave shortens as it approaches the shore due to "drag on the sea bottom." Shorter wavelength translates energy into greater amplitude - at @20 degrees waves become too steep and they "break."
Wave of Translation - shallow water waves that dissapate energy on the sea bottom as it travels onward to shore after it breaks.

SWASH - spash up the beach

BACKSWASH - return runnoff from swash.

WAVE EROSION

Wave energy translates into erosion and the break down of rock materials. Abrasion by Wave energy results in the development of shore features:

sea cliffs on the headlands
beach deposits in the lower energy bays

WAVE REFRACTION - wavelength shortening causes wave diffraction (the "bending" of waves as they move toward shore). The result is that a LONGSHORE CURRENT develops along shore flowing in the same direction as the incoming waves. This results in:

LONGSHORE TRANSPORT: longshore current carries sediment along the beach, contantly moving sand on the beach.

BEACH DRIFT - the path of sand particles up and down the beach - the net movement of sand grains are down current.

DIFFERENT KINDS OF COASTS

GENTLE SLOPE SHORELINES: such as the Atlantic Coast, have gentle, sandy beaches;
MOUNTAINOUS COASTLINES: such as thePacific Coast - mountainous coastlines display features such as sea cliffs, sea arches, sea stacks, and wave-cut platforms.

ESTUARY - a "drowned" river valley caused by changes in sea level. Examples include the Hudson River, the Chesapeake, and the lower Delaware River and Bay.

BARRIER ISLANDS - sand-built islands that parallel a gentle slope shoreline.

Features associated with inlets and river mouths:

baymouth bar - buildup from tidal inflow and outflow
SPIT - accumulation from "longshore drift"; examples include Sandy Hook, NJ and Rockaway Beach, NY.

Beach Profile

offshore bars
beach - sand (and gravel - "flotsom") - is subdivided into:
- lower shoreface - always submerged in swash
- upper shoreface - affected only during high tides and storms
- shore dunes - wind blown sand; natural defense against storms
- lagoon (or estuary) - slackwater behind barrier; bounded by tidal flats

Shoreline Erosion Problems

Sea level has been rising since the end of the Pleistocene Epoch as the great continental glaciers have been melting.

Hurricanes and winter storms (with great windspeed, fetch, and duration) cause sand erosion.

STORM SURGE: the "piling" of water in advance of a storm.

The removal of dunes removes sand reserves and the "natural shore defenses" against storm wave and current erosion.

Man made structures to prevent beach erosion

Manmade structures built to protect the shore often do more harm than good to existing structures and the environment if improperly constructed. Groins, jetties, breakwaters, and seawalls are built to minimize the impact of wave energy, but, in doing so, disrupt the long shore transport of sediment.

GROINS - walls built perpendicular to the beach designed to "stabilize the beach"; they trap sand on the up-current side, but shut off the sand supply to the down-current side (resulting in erosion, i.e. the destruction at Westhampton Beach, NY after winter storms.)

JETTY - very large"groins" built on either side of an inlet to prevent sand from filling an inlet.

BREAKWATERS - large walls built parallel to shore several hundred yards offshore. These structures make calm waters for boat harbors, but calm waters result in siltation in harbors.

SEAWALLS - Large rock or concrete walls built along the back beach to protect property. Seawalls "increase" wave energy during storms and cause beach sand loss, resulting sometimes in the complete loss of the beach (and sometimes the seawall!). E.G. Seabright, NJ.

Economics of beach nourishment

Beach nourishment is the process of restoring sand to the beach by dredging or trucking in sand.

"sand tax" - a small tax added to housing and motels along developed beaches used to "replenish" sand to beaches prone to erosion; it is cheaper than rebuilding after storms, keeps insurance costs down.

TIDES

Tides are "bulges" caused by gravitational attraction of the moon and sun on water as earth rotates. (see Figure 13.15 in text)

SPRING TIDES - the highest tides occur when the moon and sun "pull together" during new and full moon cycle.

NEAP TIDES - moon and sun at right angles

Tidal Currents - the bulge of the tide flows as a current landward and seaward as sea level rises and falls.

FLOOD TIDE - tide comes in
EBB TIDE - tide comes out

TIDAL DELTAS - tidal waters moving in and out of an inlet on a barrier creates a tidal delta.

REEFS

Reefs form in warm water shallow water settings. A "reef" is any obstruction to shipping. A biogenic "reef" is a buildup of "skeletal remains" of calcareous coral, shells, etc. along coasts were sediments from rivers are not significant. Biogenic reefs grow "rapidly" (measured in inches to feet per century). The majority of sediment produced by a "reef" is "excrement" - algae, shell, and coral consumed by fish and invertebrates.

TSUNAMIS

Tsunamis are giant waves caused by earthquakes at sea or along sea coasts; typically only a "Pacific Ocean" problem, but could occur anywhere. The size of a tsunami depends on earthquake intensity and location; earthquakes and volcanic eruptions primary cause.

The islands of Hawaii are prone to tsunamis (partly because they are in the middle of the Pacific Ocean. The shores facing the Alaska side of the islands are particularly prone to giant tsunamis (because Alaska frequently has great quakes). The islands of Hawaii also are prone to massive underwater landslides that trigger tsunamis. One such landslide on the side of the "Big Island" (Hawaii) may be responsible for a deposit of coral 2000 feet up on the side of Lanai (another Hawaiian Island). Thie implications of such deposits is quite foreboding for the future of modern island resorts!

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