16.4: Human Alteration and Mitigation

Human Alteration and Mitigation

With all of the potential loss of homes and land along the California coastline, it stands to reason that efforts would be made to protect the shore, to build structures to protect and preserve people’s property. Unfortunately, such structures, like the mythological Hydra, which regrew two heads for every head Hercules severed, often create new secondary problems without solving the original issue.

One obvious way to protect an eroding coastline is to “armor” it using large rocks. The term for such rocks is rip-rap. A related structure is a seawall (see Figure \(\PageIndex{1}\)), where a concrete wall is erected to absorb the energy of incoming waves and protect areas behind it. A problem with both of these protections, however, is what happens to the fierce energy of incoming waves.

On an unaltered beach, powerful waves roll up, carrying with them sand and debris, until the point where the waves lose enough energy that they pull back, leaving some of the sand behind. That’s nature’s way of preserving a beach.

If there is rip-rap or a seawall, however, then the incoming waves hit these structures before the waves have dissipated all their energy. The waves reflect back off these rigid surfaces. And as the outgoing waves reflect, they carry away sand on the beach. The unintended net effect of this hardening of the shoreline is not to preserve the beach, but rather to destroy it by reducing the very sand which made the beach desirable in the first place. In some cases, sand loss is so deep that the seawall itself is undermined and collapses.

Figure \(\PageIndex{1}\): A diagram of a seawall, protecting the house behind the wall, but reflecting wave energy in a way that removes beach sand. "Seawall" by Steven Newton is licensed under CC-BY-NC 4.0. Access a detailed description.

While seawalls are built parallel to the beach, another common human alteration is to construct a wall perpendicular to the beach, jutting out into the water. A single wall constructed out into the water perpendicular to the beach is called by the unfortunate term groin. When such walls protect the entrance to a harbor or a river, they are termed jetties; they are often found in pairs. These structures are composed of a variety of materials, including rock, concrete, metal, and wood.

Groins can be a useful means of docking a boat when there is no nearby harbor. But the primary use of groins/jetties involves altering the longshore drift of sand that continuously moves along a coast, driven by the longshore current. In a normal, unaltered coastline, this sand can move freely; however, in an altered situation, the sand piles up on the “upstream” side of a groin/jetty (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): The top diagram shows the “before” case, without any human alteration to the shoreline. The bottom diagram shows the “after” scenario, with considerable erosion/deposition created by jetties and groins interacting with the longshore drift. "Levee and groins" by Steven Newton is licensed under CC-BY-NC 4.0. Access a detailed description.

This accumulation of sand may be desirable for a homeowner who wishes to increase the size of that property’s beach. However, it comes at a cost. There is no way to change the net amount of sand flowing in a longshore drift. If you accumulate sand in front of your beach by installing a groin, this will remove sand from your neighbor’s beach. Your neighbor may then install a groin too, spreading the problem further down the coast, in a littoral arms race. The end result is a defacement of the shape of the beach, into a series of step-like shapes, without any actual change in the amount of sand on that beach. While California has few instances of this mutilation of the coastline, other areas of the country, in particular the East Coast barrier islands, are pock-marked with these ugly, and ultimately futile, attempts to seize a little more beach in front of one’s property. Worse still, the beach areas depleted of sand now have less protection from wave erosion during storms.

Another way to interact with longshore drift is a breakwater. A breakwater is a structure built in the water parallel to the beach, but some distance away from the beach. Figure \(\PageIndex{3}\) the before/after effects of the installation of a breakwater.

Figure \(\PageIndex{3}\): The top diagram shows the “before” case, without any human alteration to the shoreline. The bottom diagram shows the “after” scenario, with considerable erosion/deposition created by a breakwater interacting with the longshore drift. "Breakwater" by Steven Newton is licensed under CC-BY-NC 4.0. Access a detailed description.

The bottom line is this: human intervention at the shoreline always has consequences. The natural forces of waves, longshore current, and longshore drift do not yield to negotiation or bargaining. What brings sand to one place steals it from another, with mutilation of the shoreline the only lasting effect.

Query \(\PageIndex{1}\)