ABSTRACT

During 20-31 October 1982, we observed the formation of megaripples in en echelon runnel systems 0.2-2 km long and 8-15 m wide on Nanny Goat and Cabretta Island Beaches, Sapelo Island, Georgia. The runnels contained a complex assemblage of lower-flow-regime bedforms, which were dominated by large, sinuous to lunate megaripples with heights of 15 to 50 cm and wave-lengths of 4 to 14 m. At times, smaller megaripples 80 to 100 cm long and 5 to 10 cm high formed on the backs of the large megaripples. Small, linguoid-lunate ripples were superimposed on the megaripples.

The magaripples form in the fine sand (0.125-0.25 mm) during rising high tide associated with higher than "normal" wave energies and northeasterly waves and wind. As the tide rises, wave-generated surges override the ridge and are translated into unidirectional, pulsating flow along the runnel (to the south on Sapelo Island), thus producing the megaripples. As the tide lowers, waves no longer override the ridge and small, linguoid-lunate ripples form on top of the megaripples as water drains from the runnel. Because our measured velocity values are lower than those required by experimental work to produce megaripples in fine sand, they may represent minimum velocities due to averaging pulses over time and distance.

These large, lower-flow-regime bedforms produce sets of high-angle cross laminae oriented along shore within upper-flow-regime plane beds and should be allowed for during environmental interpretation of nearshore deposits in the rock record.