Paper No. 4-1
Presentation Time: 8:00 AM
DISTRIBUTION AND MORPHOLOGY OF SORTED BEDFORMS ALONG THE RHODE ISLAND SOUTH SHORE WITH IMPLICATIONS ON PROCESSES AND PERSISTENCE
Sorted bedforms (SBs), shore oblique to perpendicular depressions floored by ripples and small dunes are found on shorefaces around the world, although the exact mechanism for forming and maintaining these features (i.e. cross-shore or alongshore processes) remains a subject of debate. Various projects over the past two decades mapping the depositional environments of the shoreface of the Rhode Island South Shore (RISS) using side-scan sonar coupled with underwater video imagery, high-resolution seismic reflection profiles, surface sediment grab samples and limited sediment cores have identified SBs along the entire RISS. The form of the SBs here varies here from narrow (10’s of meters wide) swaths with negative relief of >1 m, to broad areas (>1,000 m) of coarse sand with small dunes. Transgression of the glacial deposits here produced a heterogeneous shoreface, ranging from very fine sand to boulders, with limited bedrock outcrop. SBs on the RISS and other glaciated shorefaces are relatively unique, as many studies from the Mid-Atlantic and elsewhere report the Ss are coarser than the surrounding sandy shoreface. The RISS SBs are often finer grained (containing coarse sand and pebble gravel) than the adjacent pebble to cobble gravel shoreface, and some swaths extend through areas of abundant boulder outcrop. The side-scan signature of SBs on gravelly shorefaces show distinct edges (on both sides) while those on sandy portions of the shoreface have more diffuse edges, particularly on the western edge. The various forms suggest different formation processes, and could be related to the antecedent (glacial) topography or differing wave conditions, and point to the complexity of shoreface processes. Acoustic Doppler Current Profilers (deployed 2010-2011) recorded wave heights and depth-averaged currents, notably during Tropical Storm Irene (August 2011) off the RISS. Depth averaged cross-shore currents near the center of the RISS recorded persistent offshore flows >0.1 m s-1 during the storm. While not exceeding the threshold velocity for sand, combined with asymmetric wave orbital motion, this could result in net offshore sediment transport. While not a focus of this talk, these features largely persistent in form and location at time-scales from weeks to decades.