Assessing beach width dynamics through earth observations

Abstract

Coasts are constantly under the pressure of hydrodynamic conditions such as waves, tides, and storms. In the Netherlands, sand nourishments are executed every few years in order to maintain the country’s sandy beaches for purposes of safety, recreation, and ecology. In order to determine where to carry out these nourishments, the whole sandy coastline of the Netherlands is measured annually by Rijkswaterstaat. This annual survey, called JAarlijkse KUStmetingen (JARKUS), monitors where (increasing) erosive trends appear or persist and, due to its timespan (dating back to 1843), is a valuable dataset to understand the evolution of the Dutch coast. However, this measurement survey is restricted to its annual frequency and is costly. The use of optical satellite imagery for measuring land cover types and geographic features is rapidly becoming more popular due to their high temporal frequency and relatively low costs (due to the public availability of some satellite missions, such as NASA’s Landsat and ESA’s Sentinel-2 missions). This research studied the possibilities of using optical satellite imagery for measuring beach width dynamics in addition to the existing measurement campaigns.

In this study, we derived the Satellite-Derived Beach Width (SDBW) as the cross-shore distance between the Satellite-Derived Shoreline (SDS) and the Satellite-Derived Vegetation line (SDV). We adopted a widely used and validated method for SDS detection and adapted this method to establish the SDV detection method. The SDS and SDV are derived from optical satellites by deriving vectors from the border between two contrasting land cover types that are identified by differences in (sun)light reflectance values. The SDS and is derived from the contrast between water and land, the SDV from the contrast between and sand/sediment and vegetation. The SDBW data was measured from both composite and individual satellite images. The different techniques are suited for different applications, and both have their advantages and disadvantages. A composite image is an image that is composed of a sequence of individual satellite images available within a set window. E.g., a composite is the average image of all those images. Recent research showed that, at the cost of temporal resolution, composite images are suited for analysis of long-term (structural) shoreline trends since they mitigate certain factors influencing image quality (such as clouds and cloud shadows, waves and tides, and satellite instrument errors). Individual images are better suited for analyses of short-term dynamics since they provide instantaneous measurement data. However, they are hampered more by the factors mentioned above, and hence need to be screened before use...