Morpho-sedimentary indicators monitored to assess the coastal system erosion

The diversity of the coastal erosion indicators measurable from EO data, the monitoring frequency adapted to every end user expectation, and the accuracy achieved offer valuable and fundamental tools to support decision-makers in the mitigation of coastal hazards. Earth Observation also provide support for communicating on the effects of coastal protection measures and to anticipate consequences of planned actions, one-off events such as storms, or the effects of climate change.
For the first time in Europe, a consortium of multidisciplinary experts with adequate background in the use of satellite remote sensing for coastal erosion proposes an EO-based packaged service suited for every kind of coastal areas.
After proving that EO products provide relevant complementarity to field observations by providing new, reliable and crucial information lacking along many coastal areas around the world, the ESA Coastal Erosion initiative has probably definitely changed the game in the way to consider coastal erosion monitoring.

Shoreline evolution

Hundreds of satellite images over the 1995-2020 period of time were processed to get information about the change in the waterline and shoreline positions over time. These are the main coastal erosion indicators, the acknowledgement of shoreline change computed over a long period of time (decades) serving basically as a coastal erosion assessment.

Over microtidal areas, the waterline, i.e. the boundary between sea and land, is preferably monitored or extracted during low wave agitation conditions, in order to be relevant from one date to another and not to depend on sea level fluctuations due to wind/storm. This is the indicator that we usually favor for the study of trends in the evolution of the coastline. 

Changes in the waterline position are computed and estimated along regularly spaced profiles perpendicular to the coast (but also in terms of area surface changes) highlighting hotspots of shoreline erosion which experience strong coastal dynamics or being particularly vulnerable to coastal erosion hazard.

The upper swash limit is a hybrid indicator, developed to be assimilated to the monitoring shoreline usually instrumented by many coastal managers and scientists, notably in contexts of strong instantaneous fluctuation of the waterline over reflective beaches (steep). To overcome this bias, the dry/wet sand limit is preferred in the field, and can be easily extracted on a single very high resolution image (eg Pléiades, 0.5 m). In the use of high resolution spatial data (eg Sentinel, 10 m), the result is the product of a combination of waterlines extracted on images acquired over a very short period.

Over macrotidal sandy coasts where a coastal dune system is present, we monitor the dune foot position and its cross-shore dynamics as the main indicator of shoreline variations.

Even if the dynamics of the dunes are usually slower than the waterlines in the intertidal area, it is sometimes more impressive with retreats or even breaches in the dune itself which can reach several tens of meters during a storm, which represents movements of impressive sediment volumes.

Both long-term trends and extreme episodes can be spatialized and quantified using our tools and satellite images. 

Intertidal sedimentary dynamics

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Nearshore dynamics

Satellite-derived bathymetry was also systematically computed over stretches of european coastlines. This approach which is known to produce reasonnably accurate (below 50 cm of vertical accuracy) Digital Elevation Models in depths of 0-20m (depending on coastal water transparency) has been deployed in several locations in France, Portugal, Greece and Romania. Here is an example of satellite-derived bathymetry DEMs along the Camargue Rhone Delta region (80 km) over the last 25 years computed using only archives of Spot-4/5, Landsat-7/8 and Sentinel-2 data (no need of ground truth).

So then, in many coastal areas in Europe, changes in the nearshore bathymetry can be observed at relatively high frequency (in theory weekly using Sentinel-2, more realistically a couple of times a year) providing frequent observations of bottom changes at the scale of the coastal system or that of the sediment cell enabling the assessment of sediment stocks and their variation over the relevant timescales of coastal dynamics. Coastal scientists and beach managers are all of one mind with regards to the benefits of satellite-derived bathymetry which provides routine easy-to-update information about the beach morphology and bathymetry at an  accuracy level almost equivalent to aerial Lidar techniques. And, in consequence, evidence of the natural fluctuations affecting the underwater sediment stocks at seasonal and annual time scales which is crucial information for the coastal manager in charge of the mitigation of beach and shoreline vulnerability.

Extraction of features from the underwater morphology as sand bank and sandbar migration over time is easily done from each individual satellite image. This combined information over time gives the envelope of sand bar variations across the beach profile, from inland to the underwater, and alongshore over hundreds of kilometers. Done on a routinely basis, this can be used as early warning for scientists or coastal managers highlighting any vulnerable position of underwater sandbars, or can help understand large-scale coastal erosion processes that may impact

When the waves propagate from deeper to shallower water their wavelength decreases and their direction changes, and as a result these properties allow to infer sea bottom features. To complete the shallow-water bathymetry, the SAR use for swell inversion allows to derive the bathymetry in the higher enegetic areas, more exposed to swell.

Thus, we can provide a continuum of bathymetric reconstruction, from shallow water (<1 m) to sea (> 50 m).


Rocky coasts

Rocky coastal areas are particularly tricky to deal with because coastal cliffs are exposed to very few changes on a yearly basis, but in the meanwhile spectacular landslides at the cliff face may occur occasionally. Efforts were made to raise the awareness and use of satellite remote sensing for cliff retreat monitoring. Sentinel-1 SAR dataset has been tested for cliff DEM reconstruction with rather poor results with regards to the coastal end user expectations, while SAR interferometry has given valuable insight on vertical ground movements occurring on-top of the cliff. The use of Pleiades stereo pairs for high resolution cliff DEM reconstruction has shown evidence of past and recent coastal landslides, which can then be coupled with the presence of rocky scree at the cliff foot which was found to be detectable from Sentinel-2 timeseries.