Detecting ebb-tidal delta migration using Landsat imagery

Ebb-tidal deltas are important coastal sedimentary features that influence broad-scale coastal system behaviour. However, sediment transport within and around tidal deltas is complex, making it very difficult to understand the role they play within coastal sedimentary systems. Here we present details of a new method to examine ebb-tidal delta processes using long records of cloud-masked Landsat 7 and 8 satellite imagery. Our approach is similar to that adopted in nearshore process studies, in which time-averaged collections of digital photographs from terrestrial cameras are used to differentiate between broken and unbroken water based on image tone. Using the Google Earth Engine we are able to rapidly segment 100s of Landsat images into broken and unbroken water classes based on a threshold value. By then calculating the frequency of wave breaking at the Manukau ebb-tidal delta (METD) near Auckland, New Zealand, we can repeatedly map geomorphic features such as channels, linear bar formation, channel margin linear bars, swash bars, and the terminal lobe that marks the outer limit of wave breaking. Using a ~17-year long record of observations we observe >2 km of northward alongshore channel migration and the potential emergence of a new channel to the south. The approach is simple, quick and with the growing constellations of earth observation satellites, has easy application to a range of other coastal systems.