It is human nature to seek out boundaries in nature. Coastlines are one of the most classic examples. After all, there are few things quite like standing on the edge of the continent (or close to it) and gazing out on the expanse of the Pacific. (My family has owned one of these beachfront homes in California since the 1910s)
Heavy armament is required, however, to live here. Concrete, rip rap, steel and wood are assembled into bunker-like forms that bear evidence of how tenuous beachfront property really is. Battered by storms and salt, the sea walls and homes are in a near constant state of decline. Jeffrey Manson, a friend of mine, has a good song about the "armored coast" - you can hear his work on his My Space music page.
Sea walls represent one of human being's relationships with nature. They are fortification against the elements, and in many ways they diminish the aesthetic and ecological function of coastlines, but they also allow us to sit on the edge and look out on something that will always be bigger than us.