30 of 50 states abut a major body of water. and those 30 states contain 85% of the nation's population.

active margin: a coast line at which there is an active plate boundary. characterized by cliffs above the waterline and a narrow continental shelf. and a relatively steep drop to oceanic depths off shore.

passive margin: A coast line that is far removed from active plate boundaries. characterized by a broad continental shelf and development of broad beaches and sandy offshore islands.

waves and currents are the principal forces behind natural shoreline modification. wave are induced by the flow of wind across the water which sets up small undulations of the surface. wave are actually the rising and falling of water molecules in circular orbits that grow smaller with depth.

wave base: the depth at which water motion due to waves is negligible.

the most effective erosion along the shore is through wave action.

milling: (abrasion) type of wave erosion caused when sand and other sediments carried by waves erode the land

tides: the periodic regional rise and fall of water levels as a consequence of the effects of the gravitational pull of the sun and moon on the earth.

the earth's rotation tends to cause a bulge in this water mass, greatest near the equator where the velocity of the surface is most rapid and decreasing near the poles.

the combined effects of gravity and rotation cause two bulges in the water envelope: one facing the moon where the moon's gravitational pull is greatest and one on the opposite side of the earth where the gravitational pull is weakest and the earth's rotational forces dominate

spring tide: tidal extremes are greatest when the sun moon and earth are all aligned, and the sun and moon are pulling together. the tides are called spring tides, they have nothing to do with season.

neap tides: when the moon and sun are at right angles to each other the difference in high and low tides is minimized. these are called neap tides

the lithosphere is so rigid and the drag of the viscous mantle so great that tens of thousands of years after the ice age (when the weight or glaciers forced the continents down into the athenosphere) the lithosphere is still springing back. up to 2 cm/ year in Scandinavia

in basins being filled by sediment, like the gulf of Mexico, the ground is sinking under the weight.

reasons why the water level is rising:

ice caps are still melting

global warming will cause existing water to expand as it warms up

locally pumping large volumes of water from the ground can cause sink age

wave-cut platforms: a distinctive coastal feature that develops where land is rising and/or the water level is falling is a set of wave cut platforms. the eventual product is a series of terraces. these develop most visibly on rocky coasts rather than those consisting of soft material

drowned valley: caused when the sea level rises and begins to fill the flood plain of a river or valley that was previously above land

the shifting of sand or its removal from a low-lying beach is often caused by longshore currents. when wave approach a beach at an angle they do not move strait up the beach, rather they move sideways to dome degree and carry sediment with them, moving the beach over time.

the net result is littoral drift, sand movement along the beach in the same general direction as the motion of the longshore current.

one commonly used method of preventing beach erosion is the construction of one or more groins or jetties-- long narrow obstacles set more or less perpendicular to the shoreline. currents slowed by such barriers tend to drop their sediments and pick up more on the other side. giving the beach a scalloped appearance

one consequence of damming large rivers is the trapping of sediment behind the dam, causing the rapid erosion of beaches down stream on the coast where fewer sediments are received.

replacing sand on beaches can be hazardous the wildlife if the wrong type of sand is used. ex. smaller grained sand can cause over clouding of the water and kill plants

jutting points of land, or head lands are more actively eroded than recessed bays because of wave refraction deflection of the waves around irregularities in the coastline.

wave refraction occurs because waves touch bottom first as they approach the projecting points, and faster waves are focused inward onto the irregularity. the long term tendency is toward a rounding out of angular coastline features.

sea wall: a solid wall of concrete or other material or a pile of large boulders (riprap) or other blocky debris. generally placed parallel to the coast line

barrier islands: are long, low, narrow islands paralleling a coastline somewhat offshore from it. it is not known how or why they form. they provide important protection for the water and coastline near them. because of this they are venerable them selves. subject to high energy waves on their seaward side, they tend to shift 2 m per year towards the coast.

an estuary is a body of water along the coastline, open to the sea, in which the tide rises and falls and in which fresh and salt water mix to create brackish water.

water circulation in estuaries is often limited, this makes them especially venerable to pollution.

demand for fresh water by humans is diminishing the flow into estuaries and causing the salinity to increase markedly.

the problem with estuaries is not that natural changes affect us, but that we are affecting changes in the estuaries.

one of the most ambitious estuary filling projects is occurring in the Netherlands it is called: Zuilder Zee

Rise of Sea Level:

much of the coastal erosion plaguing the united states is the result of gradual, but sustained sea-level rise, probably from the melting of the polar ice.

the rise is currently estimated at 1/3 m or 1 ft. per year

this small rise is particular threatening for 2 reasons

1 many coastal areas slope very gradually or are nearly flat. so that a small rise in sea level results in a relatively large retreat of the shore line

2 the documented rise of atmospheric CO2 levels suggests that increasing greenhouse effects will cause the ice caps to melt more rapidly. some estimates put the estimated accelerated sea level rise at 1m per year.

consistently rising sea levels are a major reason why shoreline-stabalization efforts repeatedly fail.

storms and coastal erosion

the low air pressure associated with a storm causes a bulge in the water surface. this, coupled with strong onshore winds, can result unusually high tides during the storm, a storm surge. when the exceptional force of waves during a storm is coupled with this rise in water level, the coast can be severely eroded. the bern the area directly above the shore line (sand dunes), may even be over washed.