Hurricane Sandy’s Fingerprint: Ripple Bedforms at an Inner Continental Shelf Sorted Bedform Field Site

The hydrodynamics and seabed morphodynamics on the inner continental shelf and near shore environments have increasing relevance with continued development of near shore structures, offshore energy technologies and artificial reef construction. Characterizing the stresses on and response of the seabed near and around seabed objects will inform best practices for structural design, seabed mine and unexploded ordnance detection, and archaeological and benthic habitat studies. As part of an ONR funded project, Delaware’s Redbird Reef is being studied for object scour and sorted bedform morphodynamics (Trembanis et al., in press). Central to this study are the effects of large storm events, such as Hurricane Sandy, which have had significant impact on the seafloor. Previous studies of inner shelf bedform dynamics have typically focused on near bed currents and bed stressors (e.g. Trembanis et al., 2004), sorted bedforms (e.g. Green et al., 2004) and object scour (e.g. Quinn, 2006; Trembanis et al., 2007; Mayer et al., 2007), but our understanding of the direct effects of objects and object scour on bedform morphodynamics is still incomplete. With prominent sorted bedform ripple fields, the Delaware Redbird artificial reef site, composed of 997 former New York City subway cars, as well as various military vehicles, tugboats, barges and ballasted tires, has made an ideal study location (Raineault et al., 2013 and 2011). Acoustic mapping of the Redbird reef three days prior to Sandy and two days after the following nor’easter, captured the extensive effects of the storms to the site, while acoustic Doppler current profilers characterized both the waves and bottom currents generated by the storm events. Results of the post-Sandy survey support the theory of sorted bedform evolution proposed by Murray and Thieler (2004). Acoustic imagery analysis indicates a highly energized and mobile bed during the storms, leading to self-organization of bedforms and creation of large orbital ripples. Using the Fingerprint Algorithm technique developed by Skarke and Trembanis (2011), sonar images have been analyzed to quantify ripple orientation, wavelength and defects (e.g. bifurcation and terminations). Correlation to time-series current and wave data shows strong agreement between peak-storm ripple wavelength scaling predictions and Fingerprint Algorithm wavelength measurements of relict ripples, indicating a non-equilibrated response of ripple bedforms to near bed orbital currents. Preliminary results further indicate an increase of ripple bedform defects near seabed objects, and deviations in ripple orientation and wavelength possibly related to current steering and vortices shed from nearby objects. Subsequent surveys and instrument deployments at the site have recorded the burial of these ripple bedforms during low-energy conditions, typical with the cyclical evolution of sorted bedform sites