September 24, 2017, 1:23 am
Process and Management Issues for Accreting Barrier Beaches - Southeast USA

1. Introduction

A number of barrier islands in the southeast USA have accreted more than 300 meters (m) over the past several decades. Segments of Kiawah Island, Seabrook Island, Fripp Island and Sullivan’s Island in South Carolina, and St. Simmons Island in Georgia are much wider today than 30-50 years ago. The primary source of sand in each case has been large scale bypassing at the up-coast inlet.

 


Episodic releases of sand from large ebb tidal deltas have produced new beach ridges before the former dune lines have gained volume and elevation. As incipient dunes form with each bypassing event, the prior shoreline becomes vegetated, effectively preserving low elevations and relief. Sheltered back beach areas then follow a normal vegetation sequence transforming from grasslands to shrubs to maritime forest over time. Low areas between incipient dune ridges may transition from intertidal sand flats to brackish wetlands. The resulting landscape is one of low hummocky dunes and swales having maximum elevations only a meter or so above the normal dry sand beach. Shrubs such as Myrica cerifera (southern waxed myrtle) tend to dominate by 10-20 years after formation of each new beach ridge. Climax maritime forest species, including Quercus virginiana, Juniperus virginiana and Pinus taeda (live oak, cedar, and loblolly pine), require another decade or two to penetrate the shrub cover and become dominant.

 

Accreted land and forest growth along some developed southeast barrier islands is leading to unplanned management issues. First, the views, on which many coastal land purchases are driven, have been lost. Second, the degree of storm surge protection has not increased proportionally with the volume accretion rate. Third, vegetation pruning (for view preservation) has led to unforeseen environmental problems, such as rodent infestation.

 


2. Case Study – Sullivans Island, SC
The authors evaluated accreted land along Sullivans Island (SC – Figure 1) and determined through modeling (using WHAFIS and a digital elevation model from detailed field surveys):

  • Accreted land reduces surge levels from sea to shore generally in proportion to backbeach width.
  • Removing or pruning vegetation incrementally increases flood depths (up to ~0.3 m) and waves (up to 0.5 m) during a 10-year or 100-year storm event.
  • Preservation of existing topography with dense climax forest cover offers only incrementally reduced property damage compared with grassland and shrub-cover conditions.
  • Addition of a small dune constructed ~1.3 m above typical backshore elevations potentially reduces flood damage by ~50% compared with existing conditions. Flood damage reduction increases much more with larger dunes.

3. Management Recommendations
New land is actively forming along some developed barrier islands. In the southeast USA, much of this land will become vegetated and stabilized with view-blocking forest before substantial dunes can build height and volume. This will leave backshore properties vulnerable to storm surges regardless of the density of vegetation. The authors recommend coastal zone management regulations be amended to allow some landscape manipulation in such areas for purposes of elevating the topography and increasing the level of storm protection along the oceanfront.

Figure 1. Infrared aerial image of Sullivans Island (SC – 2006). Episodic bypassing at Breach Inlet has produced a succession of low beach ridges and swales (area in green) since the 1940s leaving oceanfront homes set back 300-600 m from the ocean. Over the past 30 to 50 years, the accreted land has retained low relief topography averaging less than 1.3 m above the normal dry sand beach level for the area. Much of the land is now heavily forested with only incrementally improved storm surge protection relative to barrier islands with high foredunes.

T. Kana1, C. Mack2, and J. Sabine3
1Coastal Science & Engineering, PO Box 8056, Columiba, South Carolina 29202, USA. This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
2AECOM Corp, 3820 Faber Place Drive, Suite 300, North Charleston, SC 29405, USA. This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
3Sabine & Waters, Inc, PO Box 1072, Summerville, SC 29484, USA. This e-mail address is being protected from spambots. You need JavaScript enabled to view it .