Wave Boundary Layer Processes Over an Irregular Bottom
LONG-TERM GOALS The broad long-term goal of this research is to extend understanding of wave processes over a very rough boundary, specifically that presented by a coral reef. From this understanding we aim to develop models that account for the effects of roughness on wave dissipation, sediment transport and biophysical interactions. OBJECTIVES The objective of this project is to relate measurements of roughness over a highly irregular bottom to observations of the wave flow at various scales,
... with the goal of developing a relation between roughness and wave energy dissipation and shear stress. The specific objectives include three elements: 1) observations of the small-scale turbulent processes over a wave orbital excursion; 2) a broad scale characterization of the wave field and its response to roughness; 3) high-resolution spatial surveys of the roughness over the study region. These observations will be further extended using a numerical model of the wave field in the nearshore region. Concurrent observations of sediment load and optical properties will explore the connection between shear stress and sediment suspension and transport over the complex reef topology. APPROACH A series of field observations are being carried out to address the objectives outlined above. Measurements of the wave and current field over a broad region of a coral reef are used to determine energy dissipation in an integral sense. Near-bed observations at smaller scales using acoustic profilers deployed on the Rough Boundary Profiler (RBP) then characterize the spatial structure of the boundary flow over a wave orbital excursion. The RBP moves an instrument package along a horizontal track over a distance of up to 3 meters (see figure 1) , collecting data over a set time period at each location (typically 0.5 -2 hours) thus allowing a spatial view of the near-bed flow. The 3 m track can be oriented in the swell direction within a window of +/-10 degrees. In parallel to the wave dissipation and boundary layer observations we are carrying out high-resolution roughness surveys over the observational domain. Boat-based surveys examine the roughness at scales down to around half a meter and diver-based surveys determine variability at higher wavenumbers. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.