Waves and Coasts
There are many types of coasts. North of Cape Cod at Acadia National Park in Maine, strong igneous and metamorphic rocks make sea cliffs. To the south, Virgin Islands National Park is famous for coral reefs, although they are now endangered by human-caused climate change and other impacts; the reefs are composed of the skeletons of trillions of tiny animals that have built upward from the sea bottom in shallow, clear, sunlit, oxygenated waters far from sediment that would bury and choke them. In Louisiana, we visited the Delta National Wildlife Refuge with its waterfowl-filled wetlands developed on mud delivered by the Mississippi River. At Cape Cod, we find sandy beaches.
The type of coast depends on many things: the amount and type of sediment the coast receives, how energetic the waves are that hit it, how much the tide goes up and down, the type of rocks, how warm or cold the climate is, and many other factors. We discussed some issues with the muddy delta of the Mississippi River earlier. The biological challenge of saving coral reefs from overheating, pollution and other damage is large and important, but a little beyond the scope of this class. Here, we will concentrate on sandy beaches such as those at Cape Cod, to learn about them and to gain some insights to other types of beaches.
If you watch the waves on Cape Cod beaches or any other sandy beach, you will see that those waves move a lot of water, and a lot of sand. Dig a hole just above the water level during a rising tide, and within a few minutes the hole will be mostly or completely filled with wave-carried sand. Go to the beach during a big storm and you will see immense amounts of sand moved. Hundred-foot-high bluffs may be eroded back several feet during a single storm, or layers of sand many feet thick may be added to the beach or eroded from it in hours. A movie long shown at the Salt Pond Visitors Center includes a series of photos taken of one section of beach before and after a string of storms one winter. The summer beach is an unbroken expanse of sand, but boulders many feet across (more than a meter) are buried in it, and were completely uncovered and then buried again several times during that one winter as the sand was moved off the beach into slightly deeper water and then carried back onto the beach as shown in the short video clip below.
Video: Beach Changes (0:45 seconds)
The energy for moving all of this sand is mostly supplied by the wind, which drives the waves, and to a lesser extent by the tides, which are bulges of water raised by the gravity of the moon and sun and following them in their orbits around the Earth. (We saw earlier that earthquakes and other phenomena can cause tsunamis, which also can move a lot of sand, but significant tsunamis are very rare compared to wind-driven waves, and not a big issue for Cape Cod almost all the time.) Most of the sand transported by waves is simply moved onshore—from the ocean toward the beach—and offshore—from the beach toward the ocean—as each wave comes in and goes out. Most transport is into and out from the beach, rather than along the beach, because most waves turn so that their crests are almost parallel to the beach, and their water motion is almost directly towards and away from the beach. The turning happens because waves go slower in shallower water. If a wave approaches a beach at an angle, the first part to get close to the beach will slow down, allowing the rest of the wave still in deep water to nearly catch up, as shown in the diagram below.
Video: Waves turning toward the beach (1:53 minutes)
Every wave moves sand up and down the beach. On even rather quiet days, if you sit down on a Cape Cod beach in shallow water, you soon will find that sand is piling up in places around you, and being eroded in other places, and that you have sand in your swimsuit, and possibly even in your hair. This in-and-out movement of the sand with every wave dominates the sand transport, and allows for very efficient sorting of the sand by size, taking away pieces smaller than sand pieces, leaving pieces larger than sand in other places, and eventually giving almost all sand on the beach (although sometimes with buried boulders in the sand, or some fist-sized rocks just offshore where you might step on them if you wade into the water). This repeated movement of pieces in waves also knocks the sharp edges off sand grains, sea shells, old bottles, and other material on the beach, making the rocks and sand and shells rounded, and giving pretty “sea glass”.
As described in the video below, if you look out to sea during a winter storm, you’ll see high-energy breakers coming at you. The white caps of the waves rise high, curl over, and crash down, so that some of the water arrives on the beach after coming in through the air rather than washing along the sand. But the water then rushes back toward the ocean along the sand, carrying some sand seaward. Storms, which frequently occur during winter, often move some sand from the beach into slightly deeper water. This transport may remove enough sand to lower the height of the beach surface by many feet or tens of feet during the winter, exposing buried boulders as described earlier. The waves of summer are on average lower in energy, and don’t break and travel through the air as much (occasional hurricanes change this story, but most of the time the story is fairly accurate). The surge of summer waves up the beach is slightly faster than the return flow down the beach, and may carry a tiny bit more sand up than back; the net effect is to bring sand from just offshore back to the beach, burying any beach boulders that were exposed during the winter.
Video: Eroding Winter Beach (0:54 seconds)
Recall from earlier, though, that the waves and the sand come almost but not quite straight in and straight out—there is still some angle. If you are playing in the waves at the beach, and ride a wave in, swim out, ride in, swim out, ride in... after a while you may find you are drifting down the beach away from where you left your towel—even though you were mostly going toward the beach and back out to sea, the waves also were pushing you sideways. In such a situation, we say that you are experiencing longshore drift—you, and the water, and sand, are moving along the shore. Eventually, when the water and sand (but we hope not you!) reach the end of the Cape (at the Provincelands to the north, or Monomoy to the south), some of the sand carried by the longshore drift builds a spit or extension of land, but some of the sand is dumped off into deeper water beyond the reach of waves. This sand is then lost from the above-water part of the Cape, and the Cape has gotten a little smaller. Most references say that the great beach facing the Atlantic is retreating at about 3 feet (1 m) per year, although it may have been a little faster recently, and the panicked rescues of light-houses before they fell into the sea were needed because retreat for a few years was much faster. We’ll look at these issues, and what might be done, after visiting Acadia.
As mentioned above, waves move immense amounts of sand, primarily up and down the beach, but also with a little motion along the beach and eventually off into deep water. In this vintage video, Dr. Alley gets cold feet on Coast Guard Beach, Cape Cod National Seashore, to show you moving sand.
Vintage Video: The Feet (1:17 minutes)
Coasting Down the Coast Slideshow
Come take a trip with us to see a bit on sea-level change, some disasters, and some coastal processes, in some beautiful places. We will discuss these more when we visit Acadia, next.