Observation of Rip Currents by Synthetic Aperture Radar
Jose da Silva(1) , Luis Quaresma dos Santos(2) , and Francisco Sancho(3)
Institute of Oceanography,
University of Lisbon,
(2) Instituto Hidrografico, Rua das Trinas, 49, 1249-093 Lisboa , Portugal
(3) Laboratorio Nacional de Engenharia Civil (LNEC), Av. do Brasil, 101 , 1700-066, Lisbon , Portugal
Rip currents are near-shore cellular circulations that can be described as narrow, jet-like and seaward directed flows. These flows originate close to the shoreline and may be a result of alongshore variations in the surface wave field. The onshore mass transport produced by surface waves leads to a slight increase of the mean water surface level (set-up) toward the shoreline. When this set-up is spatially non-uniform alongshore (due, for example, to non-uniform wave breaking field), a pressure gradient is produced and rip currents are formed by converging alongshore flows with offshore flows concentrated in regions of low set-up and onshore flows in between. Observation of rip currents is important in coastal engineering studies because they can cause a seaward transport of beach sand and thus change beach morphology. Since rip currents are an efficient mechanism for exchange of near-shore and offshore water, they are important for across shore mixing of heat, nutrients, pollutants and biological species. So far however, studies of rip currents have mainly relied on numerical modelling and video camera observations. We show an ENVISAT ASAR observation in Precision Image mode of bright near-shore cell-like signatures on a dark background that are interpreted as surface signatures of rip currents. These signatures are typical of very low wind conditions, and were observed off the Portuguese West coast. A mechanism is proposed to explain the feature high contrast. It is suggested that, at such low wind speed conditions, wind direction relative to the beach may play a dominant role on the observability mechanism, enhancing the effective Bragg wave wind generation threshold within the rip cells. Because new SAR missions (such as Terrasar-X) are equipped with finer resolution modes, SAR observations could significantly improve our capabilities to observe rip currents from satellite. The work was carried out in the frame of ESA project AOPT-2423, and Portuguese FCT project “Amazing – A Multi-sensor Analysis and Interpretation System for the Coastal Zone Remote Sensing”.