BURIED ­­­­­­­SCARS: GEORADAR IMAGES OF OVERWASH AND OVERTOP STORM-SURGE CHANNELS ON SOUTHWEST FLORIDA BARRIER ISLANDS

Storm-surge channels are conduits in coastal lithosomes that result in overtop or overwash deposits, as well as become foci for ebb-surge and inlet formation (ephemeral or long-term). They may serve as geoindicators of erosional hot spots, however, many of these longshore paleochannel cut-and-fill anomalies are masked by subsequent overtopping, aeolian deposition, or human-induced development. We present high-resolution (500 MHz) georadar signatures of multiple paleochannels from three mixed bio-clastic barrier islands along the microtidal SW Florida coast (North Captiva, Sanibel, and Lovers Key). Along the undeveloped southern part of North Captiva, two paleochannels (apparent maximum width: 12-16 m; depth: 1.2-1.8 m), imaged in shore-parallel survey sections near the middle and landward sections of the barrier, coincide with Hurricane Charley (August 2004) breaches. Prior to closure, these channels generated overwash and incipient flood-tidal deltas. At Silver Key Beach of Sanibel Island, a 300-m-long, 70-m-wide section across a mega-swale (Clam Bayou) reveals two buried channels that exceed 20 m in width and contain laterally prograded fill. At Lovers Key State Park, three recent 50-80-m-wide overtop patches reveal 5-m to >15 m-wide buried channels. Active drainage gulleys may follow recent surge channels, which are incised into foredune and backdune lithosomes. These channels contain conformable infill through post-storm retrogradation and aggradation. In contrast to North Captiva, which has inter- and subtidal paleochannels, the Lovers Key breaches were supratidal and ephemeral resulting in overtop fans with landward-dipping clinoform architecture. Without backdune ridges (onlap surfaces), these overtop fan packages downlap onto landward-sloping backdune platforms and may extend into mangrove wetlands (incipient overwash). Overtop results in barrier aggradation and provides new environments for burrowing coastal species. Such sites may also become legal boundaries or hazard zones, as a legacy for implementing setbacks during future development. It is likely that many older parts of prograded barriers contain similar incised overtop segments. Georadar and aerial drone LiDAR, when integrated with traditional field investigation, can help discern these relict features.