Seabed Spectra Predictions Using a Time-Dependent Seafloor Boundary Layer Model

Thursday, 18 December 2014
Allison Penko1, Kyle O'donnell Olejniczak2, Joseph Calantoni1, Margaret L Palmsten3, Alexandru Sheremet4, James Michael Kaihatu5 and Robert Weiss6, (1)Naval Research Laboratory, Stennis Space Center, MS, United States, (2)California State University Monterey Bay, Seaside, CA, United States, (3)Naval Research Lab Stennis Space Center, Stennis Space Center, MS, United States, (4)University of Florida, Engineering School for Sustainable Infrastructure and Environment, Gainesville, FL, United States, (5)Texas A&M University, College Station, TX, United States, (6)Virginia Tech, Blacksburg, VA, United States
Waves and currents on the continental shelf interact to produce time-varying complex ripple patterns on the seafloor. While high-resolution, two-phase models can provide details on the physics of sediment transport in the bottom boundary layer, time-varying ripple models can predict the seafloor topography spectrum providing estimates of ripple height, length, orientation and ultimately, seafloor roughness. Roughness is an important characteristic of the bottom boundary layer that affects waves and currents as well as acoustic scattering and penetration into the seabed. A one-dimensional spectral ripple model is used to predict the time-dependent seafloor spectra given a time series of observed or forecasted wave conditions. The model allows each wave number component of the seafloor spectra to evolve independently and treats the temporal evolution of the components as a relaxation process. The approach allows for an adjustment timescale that is dependent on the previous bed state, includes a wash out criteria for strong wave conditions, and is forced with robust equilibrium ripple predictors. We compare the spatial and temporal seafloor spectra predictions from the model to ripples observed during an experiment at the O.H. Hinsdale Wave Research Laboratory at Oregon State University. Ripple lengths were estimated from data collected by a high-frequency sector scanning sonar throughout the 6-day experiment. Wave heights and periods ranged from 0.25 m to 1 m and 2 s to 5 s, respectively. The observed data is used to validate the timescale of ripple evolution and ripple lengths predicted by the model.