Coastal Dynamics of the Ganges-Brahmaputra Delta: 1988-2014

Abstract:

In this study we quantify erosional and depositional processes in the coastal zone (including tidal flats and river channels) of the lower Ganges Brahmaputra delta (GBD). Recent availability of accurately coregistered, radiometrically intercalibrated, Landsat TM, ETM+ & OLI collected since 1988 allows for spatiotemporal (ST) analyses of both natural and anthropogenic processes in the coastal zone on seasonal to interannual time scales. We quantify changes in the coastal zone using 106 cloud-free acquisitions in the area of the 3 Landsat scenes spanning the lower delta. Changes are quantified using multitemporal spectral mixture analysis of exoatmospheric reflectance to represent land cover and water bodies as continuous fields of soil and sediment substrates (S), vegetation (V), and dark surfaces (D; water & shadow). We also use MODIS 16-day EVI composite time series and high spatial resolution (2-4 m) imagery post-2000 to extend and vicariously validate the Landsat-derived observations. Because water levels on the lower delta change by several meters on time scales of hours (tides), months (discharge) and years (relative sea level rise), we use a network of 11 tide gauges to distinguish the effects of these changes in the coastal zone imaged by Landsat. Cross spectral analysis of this network of tide gauge records quantifies the dominant periods and relative magnitudes as well as phase of water level variations across these time scales. Tide gauge records are used to identify Landsat scenes acquired at similar water levels as well as the effects of water level on variations in tidal flats. Water level and water leaving radiance are used to map spatiotemporal variations in suspended sediment. Tri-temporal change maps of SVD fractions show progressive changes of coastlines throughout the study period. We find significant change in tidal flats in acquisitions from different tidal heights, alluding to the importance of tidal phase in coastal analyses. Erosion of forested coastlines is distributed along channels of all sizes, while deposition occurs primarily in more localized regions. Expansion of mangrove forest is observed in several larger areas of deposition. These results suggest that the balance between erosion and deposition is more complex than often assumed in studies of sea level rise on the GBD.