Inferring surface currents within submerged, vegetated deltaic islands and wetlands from multi-pass airborne SAR

Water flow patterns across coastal and deltaic wetlands affect biogeochemical cycling, denitrification, organic carbon burial, and coastal landscape evolution. Our understanding of such patterns across these important landscapes is incomplete, however, because of the inherent difficulty of conducting spatially and temporally dense ground- or boat-based surveys in shallow, vegetated terrain. We conducted an airborne L-band synthetic aperture radar (SAR) acquisition campaign on Wax Lake Delta, Louisiana, USA, in May 2015, to investigate whether water velocities and flow patterns over kilometer scales can be determined from remote sensing. Thirteen SAR flight lines over the delta region were acquired in 3 h with six different flight directions, concurrently with a small boat campaign. We show that SAR azimuth displacement due to Doppler shift can be used to estimate the surface water flow relative to the static and submerged vegetation interspersed on delta islands, using a simple Bragg wave scattering model and accounting for the Bragg wave's free velocity and wind drift. At Wax Lake Delta, we find that ~0.40 m/s water velocities within the main deltaic channels slow to 0.1-0.2 m/s as flow spreads laterally across, and converges within, the vegetated islands, coinciding with shallow (<0.5 m) depths and heightened flow resistance. This SAR-based technique opens up new avenues for understanding shallow submerged, vegetated coastal wetlands and deltas.