Coastal Systems Wave Physics Longshore transport, wave refraction. Coastal Features Coastal Erosion and Mitigation
Waves form as wind blows over the ocean surface. Waves increase in size with the following factors:
- greater wind velocity
- greater amounts of time that the wind blows (wind duration)
greater fetch : the distance of water over which the wind blows
Why is the Pacific (CA, HI, Australia) better for surfing than the Atlantic?
In waves far out at sea, in deep water, the motion of water in the waves is circular. As waves move by, the water moves in a circle. You can observe this by watching the way an object (like the seagull in this figure) moves as the wave passes. Such waves are called waves of oscillation . The wave form itself moves, but the water does not have net forward motion -- instead the water travels in circles.
Orbitals: Water doesn’t move (is not displaced) but the energy travels THROUGH the water. Water molecules circulate in orbitals as the wave passes. Note, wave length, height (H) and wave base (L/2). At wave base water is not effected by wave energy.
We measure the sizes of waves by:
wave height -- the vertical distance from low point (trough) to high point (crest)
wavelength -- the horizontal distance from one crest to the next crest
Note in this figure the circular motion of the water, and notice that the motion becomes less with depth. At a water depth equal to half the wavelength , you can see that there is no more water motion. This is called WAVE BASE—movement of water particles is not felt below wave base.
Wherever they form out at sea, waves eventually will come into shore. When this occurs some important changes take place. At a water depth equal to half the wave length, the waves begin to slow down (or feel bottom ) due to friction with the sea floor. (Recall from the previous slide that water motion in a wave occurs down to a depth equal to half the wavelength. Therefore waves feel friction with the bottom and slow down when the depth becomes half the wavelength.)
The shallower the water, the slower the waves move. This causes the waves to bunch up (wavelengths decrease) and grow taller (wave heights increase). Eventually the waves move slower than the water orbiting within them, causing the waves to tumble toward the shore, or break . Breaking waves are called waves of translation because the water in them moves landward (rather than in a circle), hitting the shore with great force.
The goal of waves is to fill in depressions and erode back headlands extending out to sea. A well worked coastline is flat (I.e. atlantic). As waves move onshore, they slow down in areas around the headlands (shallow water). By slowing down, the wave bends (refraction) and most of the wave energy is exerted on the headland.
Refraction erodes headlands. The rest of the wave that is still in ‘deep’ water gently rolls up the beaches (depositing sand and filling them in).
… wave refraction….
Wherever waves approach the shore at an angle , they bend in toward the shore somewhat, because the part of the wave closer to shore, in shallower water, moves slower than the part farther from shore, in deeper water. This bending of waves is called refraction .
Longshore Drift When waves strike the shore at an angle, it causes a current of water to flow along the shore in the direction of the waves . This is called a longshore current . The longshore current, along with the waves breaking on the beach at an angle, cause littoral drift: the movement of large amounts of sand along the shore in the direction of the waves .
This photograph shows the waves hitting the beach at an angle. Therefore the direction of the longshore current/drift (the movement of water) and the littoral drift (the movement of sand) is along the beach toward the viewer. We measure littoral drift by the volume of sand moved in cubic meters per year or cubic yards per year.
Depositional Beach Environments Depositional coasts include the eastern seaboard of the U.S. Here, the coasts form gentle slopes with the continental shelves. Continental shelves are wide in these areas (coincidental that they are passive continental margins?).
Barrier Island Cross Section
Barrier island are places of change and motion. Islands grow and shrink depending on the amount of sediment available and global conditions like change in sea level. An island that is migrating toward the mainland is termed retrograding. In most cases an island that is retrograding undergoes a process known as "island rollover". This process commonly occurs on islands that are experiencing erosion due to a stifled sediment supply. Two major types of evidence that show an island is rolling over are washovers and exposed marsh mud on the beach.
Barrier Island systems are not STABLE!!! But in fact are dynamic and motile. During storms, overwash deposits form. Waves essentially push beach material behind the barrier island system moving it TOWARDS the coast. Problems arise when beach front properties try to stabilize the barrier island.
This figure shows how Hog Island, Virginia, has changed over the past 150 years, as a result of hurricanes and wave erosion. The dashed lines show the former shorelines of the island. Entire towns, such as the once-thriving community of Broadwater, have disappeared into the ocean as the barrier has shifted toward the mainland.
Barrier Island Systems run parallel to the mainland. Sediments that supply the barrier island system arise from deposits from streams that discharge into the ocean. Outer Banks, NC (left), Sea Bright, NJ (right). In many cases, barrier islands are thin (~3 miles wide, if that). Standing on a sea wall in seabright, you can see the bay water…. Its about 4 city blocks wide!!!
Long Beach Island, NJ riddled with beautiful beach homes. Note that from bay (left) to ocean (right) is just a few city blocks. The only way to get onto LBI is via only a few bridges. Evacuations can be hectic.
Littoral drift builds up very large features over time. In this photograph of San Diego, you can see the large sand spit (about 10 miles long) called Coronado Island, that encloses San Diego Bay. The sand spit formed by northward (to the left in the picture) littoral drift of sand from the Tijuana River in Mexico to the south (out of view to the right). San Diego Bay forms an excellent natural harbor as a result, and is the main reason why this area is a major U.S. Navy base. SAND SPIT (Coronado Island)
Oftentimes, because of a long shore transport current, barrier islands may have hook like extensions. These ‘spits’ are deposits of sand. Sandy Hook is connected to the mainland by a thin strip of land at its southern end. You can see where the older ‘tips’ of sandy hook were once.
Human structures may interfere with the movement sand by littoral drift . In this figure, the rock jetties built to create an open inlet block the movement of sand moving by littoral drift. The sand builds up on one side of the jetties, and gets eroded on the other side because no sand comes along to replace the sand that moves away. The result: severe shore erosion on the “downstream” side of the jetties.
The effect of jetties on littoral drift is dramatically illustrated in this figure. In the 1930s jetties were built at Ocean City, Maryland, to maintain a channel connection to the ocean. The direction of littoral drift here is from north to south (top to bottom in the figure). By stopping the sand from coming south, the jetties deprived Assateague Island of the sand it would normally have received. As a result, the island has eroded and shifted landward by more than half a kilometer in places. (The red line shows the former location of the island.)
Groins are built specifically to trap sand being carried by longshore drift and build up the beach updrift of the structure. Unfortunately these structures starve downdrift beaches of sand, and erosion on those beaches is accelerated. The only way to save downdrift beaches is to construct more groins, and therefore groins tend to multiply into groin fields.
This photograph shows how littoral drift was interrupted by the rock groins that stick out into the water. Sand accumulates on one side and erodes on the other. What is the direction of littoral drift here? Shore erosion in this area of coastal New Jersey has caused damage to roads and private property.
Erosional Coastal Features Common to areas that are tectonically active (I.e. pacific coast), erosional coasts do not have wide continental shelves offshore. Tectonic uplift and downdrop of coastal areas can result in stranded marine terrace structures or drowned stream valley coastlines (such as San Francisco). Erosional features are evident in these types of coasts.
Some shorelines, particularly on the western coast of the U.S., are dominated by sea cliffs rather than beaches. The tops of these cliffs form prime real estate. But sea cliffs are subject to erosion by both wave undercutting and mass wasting . How long is that nice ocean view shown here going to last ?!
Cliffs usually meet the sea in these areas and as such, are subject to wave erosion. Recall the goal of waves is to erode back headlands and fill in depressions. Tectonic movements of coastal areas however do not allow for wide sandy beaches to develop.
Erosion of headland areas form sea arches as seen below. The high energy waves that strike headlands erode a tunnel. Over time, the tunnel ceiling will become weak enough and fall.
Once the ceiling falls, you are left with a projection of land off shore (originally part of the sea arch). These sea stacks are bombarded with waves and eventually will erode to sea level.
Tombolos are extensions of sand, deposited from long shore transport, between a sea stack and the mainland.
Protecting the Coasts: the beaches are moving!
Mitigation of Shore Erosion
Human activities contribute to shore erosion in the following ways.
1. River dams trap sand that would otherwise be carried down rivers to the shoreline. This cuts off one of the main supplies of new sand to beaches.
2. Jetties and groins stick out from the shoreline and block sand moving by littoral drift. The result is shore erosion on the down-drift side of the structures.
3. Seawalls or revetments cause breaking waves to bounce hard back toward the sea, carrying away sand and eroding the beach.
Mitigation of shore erosion may be accomplished by:
- beach replenishment : adding sand to eroded beaches; generally an expensive and short-term solution, since the sand often erodes away within a few years
- setting structures (houses, roads) far back from the edges of eroding sea cliffs
designing groins and jetties so that they allow sand carried by littoral drift to pass through
Installation of sand dunes (with vegetation), breakwaters.
Sand dunes are a natural mechanism built up by winds, that buffers the impact of storm waves. During storms, dunes may be eroded away for the most part, but the back dune areas remain protected. Typically grasses grow on dunes and the root systems help to stabilize the sediments preventing wind erosion. This is why you are supposed to KEEP OFF THE DUNES!
To minimize the movement of sediments due to long shore transport, groins (a bunch of rock material piles) are extended out perpendicular from the coastline. The idea is that they will trap sediments from long shore transport, build up over time and sediments will pour over to the adjacent beach upstream of the current, widening the beaches. This has yet to happen. In fact, LST continues whether groins are placed or not, so usually sediments build up on one side and erosion continues on the other. Sediment supply is essentially cut off to updrift sides of the groin (here erosion predominates). The right image shows a groin field; once one is constructed, then others must be to avoid sediment loss.
Jetties function in a similar way as groins, however they usually extend further out to sea and are constructed along inlets. LST (littoral drift) can fill in inlets, preventing ships to travel.
Another way to prevent sediment erosion is to place RIP RAP along the beach. Large objects, remember, require more energy to transport. It makes for a lovely scenery no?
Typically a last resort is to erect a seawall (rocks and cement) or bulkhead (metal, see next slide). Basically, seawalls increase erosion rates because waves crash on them with high energy and retreat with high energy, taking sediments (whatever little beach is present) with it. They are erected ONLY to protect the homes behind them. Beach loss is inevitable.
Or millions of taxpayer dollars can be used to pump sediment from offshore, onshore and pushed around with a bulldozer to widen the beach (REPLENISHMENT).
Critical Zone on Sandy Hook filled by beach nourishment conducted by the US Army Corps of Engineers.
Beach Replenishment, Ocean City, MD.
Before (top), during (left) and after replenishment (right). If the sand was lost once, it will be lost again.
Long Branch, NJ before and after replenishment. It’s a temporary quick fix to a problem that can only be solved by moving out.
Sea Bright, NJ before and after
After all is tried, the beaches will move. These houses were bought out by the government. We can not make the MOST DYNAMIC NATURAL ENVIRONMENT ON EARTH, stable (although we try). Did I mention beaches generate the most tourism revenue than ANY OTHER natural environment? Lots of money and lots of politics!