Short-Term Changes in a Coastal Zone

Review of Sedimentation Processes

Before proceeding, we will review some aspects of water transport.
1. A moving body of water may transport solids ________

   in suspension - within the column of water
   on the bottom - bed load
   both of the above
   

2. Solids in the suspended load tend to be ______ than those in the bed load.

   smaller
   larger
   could be larger or smaller
   

3. Solids in the suspended load are moved at ________ velocity than those in the bed load.

   a lower
   a higher
   the same
   

4. If the velocity of the moving water is lowered, grains leave the bed load and enter the suspended load.

   True
   False
   

Shoreline Processes

A long shore current flows parallel to the shore and is created when waves approach the shoreline at an angle. The waves begin to bend as they approach the shore and slow down in shallow water (wave refraction). The long short current can transport sand grains parallel to the shoreline. In addition, sand grains canbe transported in a series of "zigs and zags" down the beach. Thus, you may think of a beach as a "river of sand". Processes that interrupt this down current migration of sand can lead to beach erosion or beach replenishment. [U.S. Geological Survey]

Dr. McConnel at the University of Akron provides a good introduction to Shorelines and Sediment Budget.

A groin is a barrier built out frmo the shore to prevent sand movement and beach erosion. Watch this Groin Animation and be prepared to answer some questions.

1. Sand is deposited on the ________ side of the groin.

   up current
   down current
   neither
   

2. Sand is deposited when the velocity of the current transporting it ________.

   increases
   decreases
   

3. Sand is removed from the _________ side of the groin.

   up current
   down current
   neither
   

4. Water passing the groin is carrying little sediment and more easily transport sands from the beach behind the groin.

   True
   False
   

5. Long term changes in the predominate wind direction could change the direction of the long shore current.

   True
   False
   

A breakwater is a barrier built offshore to protect a portion of a beach. Watch an animation showing the effect of Constructing a Breakwater and be prepared to answer some questions.
1. The beach behind the breakwater ________ at the expense of the adjacent, unprotected beaches.

   enlarges
   declines
   stays the same
   

2. A change in the direction of the long shore current would change the response to the breakwater.

   True
   False
   

3. A seawall is a barrier built along a section of a beach to prevent storm surges from moving inland. Would you expect a beach to enlarge in front of the seawall?

   Yes
   No
   

Barrier Islands

A barrier island (such as Isles Dernieres barrier island chain in central Louisiana - to the right) is a long, narrow island parallel to the shore, composed of sand and built by wave action. A barrier island can act in much the same way as a breakwater.

The U.S. Geological Survey notes that "the barrier islands of Louisiana are eroding at an extreme rate. In places up to 100 feet of shoreline are disappearing every year. Though it has long been assumed that this erosion was due to the area's rapid rate of relative sea level rise, recent studies by the U.S. Geological Survey show that other coastal processes, such as the longshore redistribution of sediments and the general absence of sand-sized sediment, are responsible for this erosion. In addition, long-shore currents redistribute the available sand from headland areas to embayments, depriving shorelines of much needed sand.

.... As the islands disintegrate, the vast system of sheltered wetlands along Louisiana's delta plain are exposed to increasingly open Gulf conditions. Through the processes of increasing wave attack, salinity intrusion, storm surge, tidal range, and sediment transport, removal of the barrier islands may significantly accelerate deterioration of wetlands that have already experienced the greatest areal losses in the U.S."

For example, historical data over the past 100 years indicate that the shoreline at Bayou Lafourche has eroded back about 3 kilometers for an average rate of 30 meters per year.

Experience suggests that the most cost-effective means for the preservation of Louisiana's barrier islands may be to renourish them while permitting their landward migration. Placement of hard, fixed structures appears to be a less effective strategy for preserving barrier islands in the face of historically high rates of shoreline and bathymetric change.

Nantucket's beaches and dunes serve the important function of protecting the Island from the surrounding sea. Look at the shapes of sand bodies along a coast line and see if you can predict the direction of sand transport.

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