Airborne Remote sensing of surface currents on the Cape Fear River

Temporal sequences of optical imagery allow for the observation of fine-scale structure in the surface current field and has great practical application to increasing our knowledge of dynamic circulation patterns encountered in complex coastal, estuarine and riverine environments. The Airborne Remote Optical Spotlight System (AROSS) is a high resolution optical imaging system consisting of a large format, 11-megapixel panchromatic camera outfitted with a red color filter combined with a dynamical positioning and pointing system. Flying at an altitude of 10,500 ft, AROSS captures high resolution (~1-2 meter ground) surface images at a frequency of 2 Hz. It has recently been shown that it is possible to derive water current vector fields from the AROSS imagery by tracking the advection of modulations in the radiance from the surface. This paper reports on AROSS imagery collected in November 2009 over the Cape Fear River estuary in North Carolina. This is a partially mixed tidal estuary with visible sediment loading that provides a good signature for current retrievals. Imagery was collected at various stages of the tide over a period of six days. During flood tide, a high intensity streak is visible in the imagery and stretches for several kilometers parallel to the channel of the river. Several smaller, less intense features stretching perpendicular to the channel are also visible. One side of the streak is lighter in intensity than the other, indicating it is a frontal boundary between water masses. AROSS current retrievals show a distinct convergence pattern on either side of the visible front, which consists of foam, surfactants, and other debris that have collected at the convergence boundary between the two water masses. We believe this tidal front to be an example of axial convergence. As the flood tide propagates up the river, the fastest moving salt water travels in the middle of the channel at the surface. However, being denser than the surrounding fresh water, it sinks towards the riverbed. This draws in water from the banks and creates two opposite rotating, cross-river circular flows. Concurrent in situ measurements consisting of conductivity, temperature, depth (CTD) measurements along with horizontal and vertical velocity measurements using an Acoustic Current Doppler Profiler (ADCP), give further support to the hypothesis of axial flow convergence and provide additional insight into the characterization of the river at these locations. These results highlight the potential of imaging technology to improve our understanding of river and estuarine dynamics over a range of spatial scales.