Geol 33 Environmental Geomorphology

J Bret Bennington

Coastal Engineering - Hazards and Problems

Erosion control structures

A variety of engineering structures have been devised to stabilize beaches and prevent their erosion. Unfortunately, most of these structures are ultimately futile because they disrupt the dynamic equilibrium established by coastal processes. The sand that makes up beaches is continuously in motion and our attempts to make the sand stay put often serve only to make a bad situation worse.

Breakwater: This is a constructed barrier to waves that functions to create an artificial harbor by projecting out from the shoreline and encircling a portion of the coastline.

Seawalls: These are massive, very expensive walls and barriers built to prevent the direct erosion of the shoreline by waves. They work, but only for a few years. In fact, seawalls have a tendency to accelerate erosion. This occurs because as waves strike the seawall their energy is reflected downward instead of being dissipated over a wide area of beach. This causes rapid erosion of the beach in front of the seawall, which quickly becomes narrow or non-existent. Eventually, erosion will undermine the seawall and topple it.

Revetments: Similar to seawalls, these are erosion barriers erected along the shore from boulders, masses of concrete, or other large objects. Revetments are designed to absorb the impact of waves on the shoreline, rather than deflect the impact as seawalls do.

Retaining walls: These are similar to seawalls, but are not as securely anchored, nor are they designed to absorb wave energy or prevent foundation scour.

Groins: These are long piles of rock, concrete, or wood built perpendicular to the shore to trap sand and make the beach grow wider. What they do is they stop the movement of sand by longshore drift. The drift sand is stopped by the groin and deposited on its updrift side. However, downdrift of the groin sand is still being moved by longshore currents, but no new sand is moving in to replace it. Thus, beaches downdrift from the groin erode and become narrow. This neccesitates the construction of another groin downdrift, which in turn neccesitates the construction of yet another groin, etc, etc.

Jetties: Similar to groins, jetties are built on the updrift side of channels and inlets to prevent sand deposits from closing the channels and cutting off access from the ocean to the bay. As with groins, sand will accumulate on the updrift side of the jetty and beaches will become starved and eroded on the downdrift side. As sand is accumulates around the jetty it becomes washed into the channel by tidal currents rather than down the beach on the other side. This sand must be dredged out of the channel, creating the same problem that the jetty was originally built to solve.

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Sand Starvation due to Dams

Many shoreline areas experience coastal erosion because of large dams and reservoirs upstream that trap sand and other sediments that would normally be transported to and added to the coast. This problem has been documented in California, along the Gulf coast of Texas, and near the mouth of the Mississippi River in Louisiana.

How can we protect our beaches?

The answer to this question is simple - we donât have to. The beaches can take care of themselves - they will widen, narrow, and migrate as they need to in response to storms, waves, and sealevel rise. What needs protecting are the structures that we have built on beaches and barrier islands. Unfortunately, these cannot be protected except at prohibitive cost (and then only temporarily) because the beachs and barrier islands are inherently unstable features.

Currently, our solution to shoreline property damage is to insure the properties and then rebuild them each time they are destroyed and to rebuild the beaches where they were as they erode. To make coastal insurance affordable, it must be heavily subsidized by the government, which means that your tax dollars are being used to maintain private structures on unstable beaches.

The only solution to this expensive problem is to prevent people from developing beaches and barrier islands. Beaches and barrier islands should be maintained as public properties with as little development as possible. Roads, pavilions, bathhouses, and concession stands can be rebuilt much more inexpensively than summer homes and condominiums, and would allow maximum access to these national treasures without disturbing the natural processes that maintain them.

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