Coastal and Continental Shelf Oceanography

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OBJECTIVES: The long-term objective is to predict the dynamic adjustment of the mobile seabed to combined wave-current forcing. The focus of this proposal is the role of bottom stress, and non-linear wave-wave and wave-current interactions, in the adjustment process. Primary objectives are improved knowledge and understanding of: (1) the relationships between wave energy, near-bed fluid turbulence, bottom stress and bed state occurrence; (2) the vertical sediment flux balance and sediment entrainment; (3) the role of higher-order velocity moments in the horizontal sediment flux; and (4) bedform genesis, growth, migration and decay. APPROACH: Acoustic and optical remote sensing technologies are used to obtain comprehensive seabed imagery and velocity, turbulence and suspended sediment concentration profiles spanning the bottom boundary layer. Central to the approach is a recently-developed multi-frequency acoustic Doppler system for remotely measuring velocity, turbulence and sediment flux profiles within the thin wave bottom boundary layer; a laboratory facility for oscillatory-flow boundary layer studies above both fixed-grain and mobile sediment beds; and the use of these sensors in deep-ocean observing systems. SIGNIFICANCE: The adjustment of mobile seabed sediments to fluid forcing variability is one of the important challenges in modern ocean science. The importance resides in the applied science and societal needs for more accurate predictions of seabed change, in the effects of seabed adjustment on the predictability of ocean dynamics, and in the potential for improved interpretation of paleo-environmental conditions through the sedimentary record. The challenge resides in the observational difficulties, in the responses of the mobile sediments and the fluid motions being both mutual and non-linear, and in the fundamental connections to fluid turbulence, one of the great unsolved problems in modern physics. Particular to the coastal and shelf regions of the ocean, relative to other mobile bed environments (rivers, deserts, snow fields), is the central role of forcing by surface gravity waves.