Observations of the Spatial and Temporal Variability of Wave Formed Ripples from the 2007 Martha's Vineyard RipplesDRI Experiment

The long-term goal of this work is to increase our understanding of bedform geometry and processes associated with bedforms on the continental shelf. The primary objectives of the work are as follows: (1) measure the temporal and spatial variability of ripple geometry in wave-dominated shelf environments using both stationary tripod mounted sonar systems and autonomous underwater vehicle (AUV) side scan and multibeam systems, and (2) develop models to predict ripple geometry as a function of hydrodynamic forcing and seafloor sediment characteristics. The models are intended to increase our skill in predicting the ripples that remain after storms. We deployed tripods with rotary side scan and pencil beam sonars and ADVs on two tripods in fine and coarse sand on the 8 m isobaths. These tripods were part of a larger array that contained ripple measurement tripods in fine and coarse sand on the 12 m and 16 m isobaths and a large array of wave sensors. We recently recovered and redeployed the tripod instrumentation and appear to have a high quality data set from the first half of the deployment. We also conducted weekly surveys with a REMUS-100 equipped with multibeam and side scan sonars to examine the spatial variability of ripples left after wave events. In previous work (RipplesDRI04 in Florida), we used a single beam echosounder to attempt to estimate ripple heights. This required the use of an expensive and delicate inertial navigation system to estimate the vertical motion of the vehicle and still produced a noisy estimate of RMS roughness. In this experiment, the vehicle was equipped with an Imagenex delta T multibeam, which provides much better data than the echosounder. The tripod data have been processed into imagery and made into movies to allow viewing of the ripples' temporal evolution. Processing to calculate parameters such as wavelength, height, and spectra are underway.