Tidal phase and amplitude, NW Andros Island, The Bahamas

In shallow coastal regions, tides often control the water flux, which in turn directs sediment transport, nutrient delivery, and geochemical gradients. However, tides in shallow areas are spatially heterogeneous, making it challenging to constrain the geographic structure of tidal phase and amplitude without extensive networks of tide gauges. We present a simple remote sensing method for deriving tidal structure from satellite time series. Our method is based on two observations: (1) tidally-driven variations in water depth can be detected as changes in pixel intensity in optical satellite imagery, and (2) repeating passes by an orbiting satellite capture a region at different phases of the tidal cycle. By stacking multiple satellite acquisitions of a shallow bank, we can compute the relative tidal phase and amplitude for each pixel location, thereby resolving a detailed map of tidal propagation and attenuation. Although our method requires a set of local water-depth measurements to calibrate the color-to-depth relationship and compute tidal amplitude (in meters), our method can produce spatial estimates of tidal phase and relative amplitude without any site-specific calibration data. As an illustration of the method, we use Landsat imagery to derive the spatial structure of tides on the Great Bahama Bank, estimating tidal phase and amplitude with mean absolute errors of 15 minutes and 0.15 meters, respectively.