Tuesday, November 15, 2005

Sea Walls: Retention Comprehension

by Charlie Bartlett (Class of '05)

With so many people wanting to live near the water, it is no surprise that many attempts have been made to control the sea and rivers, and to increase inhabitable coastline. The most common technique in both residential and public construction is to build a seawall or a retaining wall. Seawalls come in many forms, but they all have the same basic purpose: to create more livable and safer areas near the coast. Retaining walls do the same along riverbanks, channels, etc. But do these attempts succeed? Can the forces of something as powerful as an ocean really be controlled by human construction?

A seawall is something that tries to slow the forces of erosion. In the short term, seawalls can be very successful at accomplishing this. If the material used to construct the seawall is durable enough, it can provide a shielding effect for the part of the beach behind the wall. It does this by absorbing and deflecting the forces of the waves, forces that could eat away at the shore.

However, the problem with a seawall arises in the long term. While the seawall does in fact deflect the force of a wave, the force is not lost. This force of a wave creates a backwash off of the wall. The backwash in turn, along with crosscurrents, up currents and other factors, creates what is known as erosion (erosion is also increased by such influences as wind, water level, etc.). This generates a problem when all of the sediment in front of the sea wall has been taken out to sea.

When the beach in front of the wall has been eroded, the property owner is in the most danger. At this point, the ocean will proceed to erode the seawall, and the wall will ultimately crumple. When this happens, not only is a smaller and more vulnerable section of the beach exposed, but also the pieces of the seawall will now aid in the erosion. The pieces of the wall will be moved and churned with the waves, and as a result, will cut away more at the sand underwater, and will quicken erosion.

A retaining wall is similar to a seawall, but it is usually meant to control a river instead of an ocean. A retaining wall does not have to deal with the effects of erosion to the extent of a seawall, as the water does not undercut the structure. However, the problem with a retaining wall is presented when the force and height of a river increase.

When the river height rises, there is the possibility of flooding, as the river could rise above the wall. When the force is increased, the wall could break, causing a similar problem. The only seemingly certain way to stop this threat would be to create a massive, extremely thick wall. This, however, is very impractical and very unsightly, and will probably not be instituted in most areas.
This is all well and good, but who cares? This is applicable in one way or another to many areas of the country, but it become immeasurably important to an area such as New Orleans. When an area such as this is below sea level, one the coast, and next to a major river, all of these factors come into play. Hopefully, the area will learn from its mistakes and correct the problem in the future.


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