Hydrodynamic variability and bedform dynamics at an inner shelf artificial reef

摘要

The hydrodynamics and seabed morphodynamics on the inner continental shelf are having 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 objects will inform best practices for structural design, seabed mine and unexploded ordnance detection, and archaeological and benthic habitat studies. Previous studies have 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), but our understanding of bedform morphodynamics is still incomplete. Consequently, this study examines the specific hydrodynamic and bedform morphodynamics occurring over an annual cycle at an inner continental shelf artificial reef site. Using the Redbird artificial reef off Delaware, this project instituted a combination of synoptic mapping techniques, through the use of an autonomous underwater vehicle and surface survey vessel, and time-series hydrodynamic data collection from acoustic Doppler current profiling and local NOAA buoy 44009. Site-wide sorted bedform evolution and sediment transport volumes were investigated through a combination of backscatter imagery segmentation and bathymetric analysis. Focusing within sorted bedforms, ripple bedform geometry was characterized using the Fingerprint Algorithm imagery analysis technique (Skarke and Trembanis, 2011). Using the Fingerprint Algorithm as a baseline, predictive ripple geometric models were compared, tested and applied to best characterize intra-annual ripple evolution. Further, this study considered the impacts of seabed objects on ripple geometry and ripple defects (e.g. bifurcations and terminations). Lastly, it tested automated imagery segmentation methods and bathymetric comparisons to quantify sorted bedform evolution at the site. Bedform morphodynamics were found to both dynamic, often in response to storm events, and persistent, regarding storm signatures preserved in relict seabed ripples and sorted bedform evolution in the periods following large storm events.