Coastal and especially estuarine environments are characterized by high sediment dynamics. The distribution of sediments over an area will change depending on discrete events (high river run-off, spring-tides) or gradual events (such as sea level rise) resulting in changes in coastline. Erosion will occur in high flow conditions and accretion in the opposite situation. A typical example of a dynamical coastal system with continuous coastline changes is a bird foot delta. Monitoring changes is important for local risk assessment and shoreline management.

What does CoastObs offer?

CoastObs can map the changes in land-water boundary over time. By using archived imagery, it is also possible to go back in time with a high resolution. Mapping of the land-water boundary is based on the Normalized Difference Water Index (NDWI; Mc.Feeters, 1996).

How was the data validated?

Satellite-retrieved Chl-a concentrations were validated against ground data collected close in time to the satellite overpass. The Chl-a samples collected by UVIGO, CNR, USTIR and HZ were analysed at University of Stirling using High Performance Liquid Chromatography (HPLC).

Case study example: Wadden Sea, the Netherlands

Figure 1. Morphology change over the Ems estuary 1987-2016 showing the contour lines in a sub-region (Lin, 2016).

For the highly dynamic Ems estuary in the Wadden Sea, images from different optical satellites (Landsat 5 and 8 and Sentinel-2) were used to visualise long-term changes. Water and land surfaces are classified using image-specific thresholds to extract the water surface area for each year from de NDWI images. Then, contour line of the water surface was generated for further investigation of changes.

Limitations

• The output is the instantaneous shoreline without considering tides. Tidal level and/or beach profile are needed to compare changes over time.
• Spatial resolution and accuracy depend on the satellite data used in the analysis (30 m Landsat; 10 m Sentinel-2; less than 2 m with very high-resolution images).