Jetties

Jetties are long man-made structures extending perpendicular from the shoreline into the ocean. These structures are used to stabilize the sand and migration of inlets, and slow longshore drift processes. They collect sand on the updrift side and erode on the downstream side of the jetty. The cost of building jetties can exceed $50,00 per linear foot.

The Charleston Harbor Jetties were completed in 1898 to prevent sand from collecting in the entrance of the harbor that kept ships from entering (Neal, et al, 1984). Each of the two walls that make up the jetties, one extending from Sullivans Island and the other from Morris Island, extends for 2.5 to 3 miles in length. These were constructed to keep sand from accumulating and interfering with the ships' mobility in and out of the harbor.

For 25 - 30 years after construction, these structures did not cause erosion to Folly and Morris Islands. In fact, Folly's shoreline grew seaward more than 2000 feet (Lennon, 2000) as remnant sand bars moved onshore by wave action. Within 30 years of the jetties construction, the growth of Morris and Folly Islands worked to the demise of Charleston's ebb tidal delta (Lennon, 2000). This delta served as the primary location of sediment for Morris and Folly Islands. Sand from northern islands is delivered to this area by longshore currents and the Cooper and Ashley Rivers. By the 1930's, the islands' sediment supply from the delta was depleted, resulting in the return of the erosional state of both islands (Lennon, 2000). Since then, the shorelines retreat at approximately four to six feet per year.

Groins

Similar to jetties, yet smaller in size, groins are hard structures constructed perpendicular to the beach. They also cause erosion on the downdrift side of the structure. The cost of building groins is roughly $2,000 per linear foot. But construction of groins has been outlawed in most states. The first series of groin construction on Folly Island began in the 1940's. These were spaced 1000 feet apart and by the 1950's, a second series of groins was added between the original groins, resulting in a groin every 500 feet (Lennon, 2000). These remained in tack or partially functional until Hurricane Hugo struck the S.C. coast in 1989.

Photo provided by http://pbisotopes.ess.sunysb.edu/geo101_f00/ notes/11-1-00.htm

 

 

Folly Beach, SC

Picture courtesy of L.R. Sautter.

 

Both of these structures, if working properly, are successful in trapping sand. However, neighboring islands on the downdrift side are depleted of sediment accumulation. The construction of the Charleston Harbor Jetties and groins has modified the flow of sand along Morris and Folly Islands. Simply stated, if jetties or groins are useful in trapping sand on one island, they must be causing erosion on another!!!

For more information on the effects of groins on South Carolina's beaches, click here.

Like groins, these structures are made of wood, steel, rock, or concrete. Seawalls are constructed in front of buildings and parallel to the beach. They are designed to receive the full brunt of an incoming tide and halt the retreat of a shoreline. This is a useful tool in securing the property directly behind the seawall, however it is not without negative environmental impacts. One of the most serious is the partial reflection of the incoming wave energy. The sediment in front of the wall is scoured and removed, thus facilitating erosion. Larger and more costly seawalls must replace the former ones to combate the loss of beach in front of it.

Folly Beach provides a perfect example of the use of seawalls and their consequent destruction. The Holiday Inn (now called Charleston by the Sea) is the most dominant structure on the island and is built in a perilous location. The recreational beach would not exist, even at low tide, in front of this structure if it were not frequently renourished (see next section). Ironically, the seawall returns the sediment (brought from the inlet to renourish the beachfront) to its original location

The building of dunes and construction of the upper beach by depositing transported sand is commonly referred to as beach renourishment. This is the most nonevasive approach to beach reparation. Often, the sand is dredged from the back side of the island or from the continental shelf (Neal et al., 1984). The latter location is preferred, however it is the most costly. Often, the sand from the backside of the island tends to be too fine, thus facilitating erosion. If the sand is too coarse, the recreational quality of the beach is compromised.

Does it work? Yes, but on the short term. Renourshment is a costly process and must be continually implemented for optimal results. For example, in 1993 the Army Corps of Engineers completed the first stage of a fifty year project. This project was planned with the intention of renourishing every eight years. In the first stage in 1993, fifteen to eighteen million dollars was spent on Folly Island's renourishment. Approximately 2.5 million cubic yards of sand was pumped from the backside of the island and deposited over a length of 5.34 miles, from the south tip to the middle part of the shoreline (Lennon, 2000).

By 1995, approximately two thirds of the added sand had eroded. The Charleston County Park at the southern end of the island, in particular, suffered a significant loss of sand. The severe erosion at the park was linked to the Army Corps of Engineer’s 1993 renourishment project as the source of sand for the renourishment was the Stono Inlet in the Folly River. Removal of sand behind the island (in the river) upset the natural balance and sand from the park began to fill in the vacancies left by the dredging (Lennon, 2000).

An emergency renourishment was implemented in April 1998 at the southern end of Folly Island due to the advanced erosional state of the county park (coastalhazards.wcu.edu, 2001). The federal government deposited 90,000 cubic yards of sand. It was determined that the Army Corps of Engineer's 1993 renourishment project facilitated the erosion of the county park's beach. Therefore, the federal government was required to spend nearly $315,000 in retribution (Vettese, 2000).

Less than one year later, additional renourishment was required due to sand from the 1998 renourishment was gone and the park was continuing to erode. Only 130 of the original 400 parking spaces are left (as of December, 2001). To remedy the situation, 49,000 cubic yards of sand was recently dredged from the Folly River - enough for a 150 foot wide beach 1000ft long. The $228,000 project was again paid for by the Army Corps of Engineers (Vettese, 2000).

It is important to realize that this is a financially intensive effort to save our beaches. Therefore, it is common for only a part of a beach to be renourished at one time.

Even though this method of beach stabilization is preferred over jetty and groin construction, it is not without negative impacts. It can give uneducated investors, developers, and homeowners the impression that Folly Island has a healthy and stable beachfront, as well as a false sense of security. This can lead to the development on land that is highly unstable. It is important to remember erosion is a natural process of barrier islands. With the increase in population density on barrier islands, we must try to maintain the island's fragile equilbrium. If we attempt to abate erosion on one island, an adjacent island often will suffer.

To view images of Folly's beach renourishment history, click here.

Download the following activity: Creating Your Very Own Sand Dunes, courtesy of http//:askeric.com This document is written in PDF format and requires Adobe Acrobat Reader. To download this free software, click on the Adobe button. NOTE: Each activity is correlated to the S.C. Science Curriculum Standards