INTRODUCTORY
OCEANOGRAPHYINTRODUCTORY
OCEANOGRAPHY
|
|
|
|
In the second part of this lesson, we will explain the equilibrium of a beach, the various changes in sealevel and the effects of artificial barriers on the movement of sand.
As shown in Table 12.1, there is a direct relationship between grain size and the slope of the beach. For instance, when a beach is composed of granules (with a grain size of 2 mm) the mean beach slope is 11 deg., but if it is composed of very fine sand (with a grain size of 0.063 mm) the mean slope is only 1 deg. Beach slope equilibrium is established when the sediment moved up-slope in the swash establishes an equilibrium with the sediment moved down-slope in the backwash (i.e., the backwash returns as much sediment as that moved toward shore by swash). The force of the breaking wave moves sediment upslope (regardless of grain size), but once the water comes to a stop near the top of the slope, downslope gravity is the force that causes the backwash.
Beaches made up of large-grain sands have lots of void-spaces under the surface of the beach, so some of the water in the backwash percolates beneath, and runs back toward the ocean under the surface of the beach. This means that some of the sand moved up the beach by the swash stays there -- it is not moved back in the backwash -- and a larger equilibrium slope must be established before an equal amount of sediment can be returned in the backwash. Very fine sand, on the other hand, creates no appreciable void-space, so all the water and the vast majority of sand that runs up the beach in the swash, runs back down the beach in the backwash on top of the surface, and the equilibrium of the beach establishes a very small slope.
 |
Equilibrium Beach Slope |
The level of the shoreline can change by the movement of continents or by an increase in the volume of water in the ocean.
Sea level may change as a result of the movement of the continents, either by
Sea level also may change due to the increase and decrease of water volume of the ocean. Though it is impossible to assign exact numbers, this change can occur in two ways:
It is not clear if global warming is a reality or that the earth's temperature will rise the 2-4 C predicted by some global circulation models. If it does warm even a few degrees, and if the seawater warms and the glaciers melt, you can see that the sea level would rise dramatically.
As humans have built more and more on our coasts (and particularly on our more fragile barrier islands and other locations near the dynamic zone of the beach), the effects of intense storms (with high winds and storm surge), wave erosion and the rise in water levels, have become more critical. The availability of flood insurance to rebuild damaged structures and infrastructure (roads, bridges, etc.) has greatly accelerated this construction, and encouraged the construction of artificial barriers to mitigate the loss of property.
Artificial
barriers attached ("anchored") to, and extending out into the
longshore drift from land are called
jetties. Some jetties are
breakwaters made of rock
rick-rack (or other surfaces with large void spaces and surface
areas) that cause incoming waves to break and dissipate their energy.
A breakwater interrupts the longshore drift and prevents the movement
of any sand directly along the beach at its location. Some other
jetties are groins (shown
in figure on the right) that are low-lying structures (may be long
cylindrical sandbags) that stick out into the ocean and interrupt,
but do not prevent the movement of all sediment -- they usually are
designed so that some of the sand moves over its top. A more
promising type of breakwater is the
tethered-float breakwater.
Designed at Scripps Institution of Oceanography, it dissipates wave
energy without interrupting the flow of sand.
Seawalls are rigid and solid structures generally built parallel with the shoreline, and are used as a last act of desperation when everything else has failed ( see Fig. 12.34 above). Seawalls require constant effort and a great deal of money to maintain, because the turbulence from breaking waves causes erosion that will eventually undermine the seawall and cause it to fall into the sea.
The importing of sand is nearly useless. This 'new' sand will not withstand strong waves and will only temporarily 'preserve' a beach. In the mid 1970's Virginia Beach spent more than $25 million to replenish sand on its main strand, but within the period of one week, two back-to-back Nor'easters that lingered off the coast of Virginia for a couple of day, completely stripped the beach of the new sand.
You must understand that beaches are "rivers of sand", so that the sand in the dynamic zone that you walk on today in front of your cottage will not be the sand that you will walk on tomorrow -- it will have been moved downstream. Therefore, any structure in the dynamic zone that interrupts this natural flow will create problems downstream. Note that as the longshore drift is interrupted, sand accumulates on the upstream side of the jetties, but erodes on the downstream side. The erosion on the downstream side results because the sand moved away from that part of the beach by a wave's action is not replaced by sand from the normal upstream flow. Once one property owner puts in a jetty, it will force the next property owner downstream to do the same, and you will end up with an entire length of beach that looks from the air like the teeth of a saw blade.
There are other examples of how the poor design of artificial barriers can bring unwanted consequences. A breakwater jetty was installed to protect the Santa Barbara Harbor. But it interfered with the eastward-moving longshore drift and created a broad beach to the west of the breakwater. The sand that deposited against the breakwater, and that turned the corner of the breakwater and was deposited into the harbor, was no longer available to nourish the beaches further to the east, so they began to erode. The less than ideal solution was to move a permanent dredge into the harbor to move the accumulating sand and send it downstream to continue the river of sand.
A final example of poor design is the dual, mile-long breakwater jetties proposed by the Army Corps of Engineers for Oregon Inlet. Oregon Inlet is the northern-most inlet between Pamlico Sound and the Atlantic Ocean. Fisherman on Roanoke Island want a safer access to the Atlantic and complain that Oregon Inlet keeps shoaling up with sand and is sometimes impassible (true statements all). The main direction of longshore drift is from the north.
The Corps says that it is too costly to dredge the inlet as often as is needed to keep it free of sediment and say that the breakwater jetties proposed will solve everyone's problems. Many, many scientists do not agree and have fought these jetties for more than 20 years. Unless the Corps is willing to keep a dredge permanently stationed in the inlet (remember that they said they couldn't afford to dredge tow to three times a year) to pump the accumulated sand north of the inlet to replenish the deficit of sand south of inlet, severe beach erosion south will result. Not only that, but after a few years, the space between the jetties will very likely fill up with sand as it spills around the end of the northernmost jetty and is brought into the inlet by flooding tides. Of equal importance, however, is the fear that larvae of flounder and other marine fishes that lay their eggs offshore, and depend upon the flooding tides to carry these eggs into Pamlico Sound where they can hatch and the juvenile fish live before returning to the ocean, will be kept out of the sound by the jetties.
We would all be so much better off if we understood and respected the natural beach processes and the dynamic zone and build accordingly.
|
|
|
