THE BIRTH AND DEATH OF EBB TIDAL DELTAS FOLLOWING HURRICANE IRWIN: SAPELO ISLAND, GA

Prior to Hurricane Irwin, which struck the Georgia coast in 2017, a well-developed ebb tidal delta occurred seaward of the tidal inlet separating two transgressive Holocene barrier islands (Blackbeard and Cabretta Islands) which front Sapelo Island, an undeveloped Pleistocene barrier island on the Georgia coast. The lagoon behind Blackbeard Island is dominated by tidal marsh (vegetated tidal flats), and tidal water is exchanged via ‘Blackbeard Creek’, a tidal creek that cuts through the extensive tidal marshes. Prior to Hurricane Irwin, Blackbeard Creek was highly sinuous and one of its meander bends was impinging on a narrow strip of beach on Blackbeard Island, that separated the lagoon from the ocean. This narrow strip was breached during Irwin, forming a new tidal inlet, and isolating the southern tip of Blackbeard Island to form a new barrier island (‘Little Blackbeard Island’). Almost immediately, the newly formed inlet became the dominant means of tidal exchange between the ocean and the lagoon behind Blackbeard Island, with minimal tidal exchange occurring at the old inlet. This caused the former ebb tidal delta to become sediment starved. Within 3 years of Hurricane Irwin, sand has filled that part of ‘Little Blackbeard Island’ lagoon which was formerly subjected to high-velocity tidal currents, effectively cutting off the ‘Little Blackbeard Island’ lagoon and the new tidal inlet. This turned the ‘Little Blackbeard Island’ lagoon an into isolated tongue of marine water, effectively killing the former ebb tidal delta.

The former ebb tidal delta was a complex, extensive tidal flat, characterized by areas of megaripples, interference ripples and wave ripples, often separated by small topographic lows, which flooded quickly during the flood stage of the tidal cycle. It is now evolving into an intertidal zone with ridge and runnel development. A new ebb tidal delta is forming at the mouth of the newly formed tidal inlet. The new ebb tidal delta is an extensive, continuous tidal flat that lacks the complexity of the former ebb tidal delta. However, sediment in the new ebb tidal delta is reworked by upper flow regime processes (standing waves and antidunes) as flood currents flowing toward the new tidal inlet interact with wave approach during the rising tide.