COSM

The Coastal Georgia Habitat

St. Catherines Island is one of 12 barrier islands, The Golden Isles, fringing the Georgia coast at the head of the Georgia Bight. The coastline has a 2.0 m tidal range with moderate wind generated waves averaging 0.25 m in height. The Golden Isles are elongate barrier islands broken by numerous sounds into relatively short, discrete segments which act as semi-independent sedimentary cells with a net southward transference of sediment (Oertel 1974). Parts of the coast actively receiving fluvial sediment have built discernible deltas seaward that are rapidly modified and redistributed forming chenier-like deltas built as wedges subparallel to the strand.

The Georgia Coast and its Golden Isles. (With permission from SherpaGuides).

The Georgia Coast and its Golden Isles. (With permission from SherpaGuides).

Parts of the coast not receiving significant amounts of fluvial sediment are more erosional. St. Catherines Island is bounded on the north by St. Catherines Sound and on the south by Sapelo Sound; both of which are tidal estuaries with no significant input of fresh water or fluvial sediment from the mainland. Damming of rivers to the north and dredging of the Savannah Ship Channel have significantly decreased sediment influx and led to highly erosional conditions on St. Catherines Island. Global warming is now causing significant sea level rise, exacerbating already significant erosion by inundation of the already transgressing sea.

 

St. Catherines Island is exceptionally erosional because 1) Sea level is rising at about 2.8 mm per year, 2) The Jerico and Newport Rivers are not through-flowing sediment-contributers on the Georgia Coast, 3) Through flowing, sediment-bearing rivers to the north have been dammed, trapping the sediment load before reaching the modern coast, and 4) Dredging of the Savannah Ship Channel removes sediment from the southward transport system. Note the chenier-like delta wedges south of the Savannah River and south of the Altamaha River.

Beaches on Georgia’s Barrier Islands are dynamic systems dominated by semidiurnal tides,which directly affect the intensity of storms, waves, and winds. Tidal energy is generated by an average tidal range of ~2.7 m semidiurnal tides (two highs and two lows each lunar day) (Thurman, 1993), with two sets of high-range Spring Tides and two sets of lower-range Neap Tides per lunar month. The Georgia coast is characterized by a low wave energy (normal summer wave heights = 0.25 m) regime that is dissipated on barrier island beaches. Transfer of sand by wind is a significant process on some islands, particularly because of the wide beaches and extensive ebb deltas exposed during low tides. Major modifications of the shore are made during short but intense storms that may include occasional hurricanes, but typically include storms systems which generate moderate to intense northeasterly winds, called Nor’easters.


 

tide_chart

Semidiurnal tides with a range of ~2.7 m dominate the Georgia coast. Highest Spring Tides occurring on new and full moons average about 0.6 m higher than highest Neap Tides occurring on half-moons. (Diagram from HarborMaster 5.5 courtesy of Zihua Software).


 

Nor’easters occur throughout the year and, particularly during the Fall and Winter; and when combined with high spring tides, they can rapidly rework the sediment of the beach and erode a prominent scarp along the back of the beach. This may erase the physical and biological sedimentary structures comprising the beaches regenerated during intervals between storms [i.e. during “normal” sedimentation].

These processes give rise to beaches that are typically 0-10 m wide at high tide and 100-150 m wide at low tide comprised of an upper beach with a 2o seaward slope and a lower beach with a 1o degree seaward slope, separated by runnel and ripple systems. The substrate of Georgia beaches is comprised of firmly packed, fine grained sand.

 

Yellow Bank Bluff, St. Catherines Island

 

 The picture at left shows Yellow Bank Bluff undergoing erosion at high tide during nor’easter. This photograph taken looking north along the Bluff shows ~ 3 m swells breaking on upper backbeach through skeletal trees of this beach “boneyard,” undercutting Yellow Bank Bluff, and sorting quartz sand out of a backbeach heavy mineral placer.

 

 

 

 

 

 

The beach can be subdivided by the line of demarcation of the latest high tide line onto the back-shore lying above high tide line, and the fore-shore, lying below the high tide line. The area of the backshore can be termed the back-beach and that of the foreshore, the fore-beach. Normally, the boundary between the back-beach and the Island is marked by a storm-wrack, storm berm, or scarp. The high tide line moves higher onto the beach with Spring Tides and lower with Neap Tides. The mid-level on the beach is marked by the highest position of burrow openings of Carolinian Ghost Shrimp.

Shoreline of North Beach, St. Catherines Island

Shoreline of North Beach, St. Catherines Island

 

This picture depicts onshore winds transporting sand shoreward off the exposed beach building a linear dune ridge as part of progradation of shoreline on North Beach, Yellow Bank Bluff in distance.

 

 

 

Lying behind the beach may be accretionary dune ridge systems, deposits vegetated by maritime forest, wash-over fans, or marsh meadows. Bluffs are commonly developed along high standing sections of beach, spits, or berms along low areas fronting the marsh meadows. Abundant wash-over and wash-in fans are deposited in low areas by storms occurring during spring tides. Erosive areas that are forested develop prominent areas of skeletal trees on the beach, which are called boneyards.

Last updated: 7/27/2016

Sea Turtle Program at St. Catherines Island • (912) 478-1744 • jgaskin@georgiasouthern.edu