Abstract:
Recent studies have shown that the presence of sand ripples on the seabed
improves sonar detection of buried mines at sub-critical angles. Sidescan sonar data of
ripples on the west Florida shelf were collected as part of ONR's Ripples
Departmental Research Initiative (DRI) September 26-29th and November 7-9th, 2004.
Hurricane Ivan, the strongest storm of the 2004 hurricane season, passed over the
experiment site a week before the first data collection. This study focuses on the ripples
created by Ivan. Average relict ripple wavelengths left after the storm were found to
increase with water depth (50 cm, 62 cm, and 83 cm in 20, 30, and 50 meter water
depths) despite the fact that orbital diameter decreases with water depth.
Ripple prediction requires information about surface gravity waves and sediment
grain size. The most reliable offshore wave field available was created with Wavewatch
III by Naval Postgraduate School scientists. These waves were inputted into Delft3D
WAVE, incorporating the nearshore wave model SWAN to predict waves at the locations
where ripples were measured. Orbital motions at the seabed and grain size were inputted
into a time-dependent ripple model with varying dissipation parameters to estimate sand
ripples created by Hurricane Ivan. Ripple wavelength was found to be more strongly
dependent on grain size than wave dissipation.
