Relationship Between Coastal Erosion and Inland Geophysical Data – receipt

The ongoing global climate change, responsible for the increasing ocean activity, exacerbates the natural phenomenon of coastal erosion. This results in significant material damage in densely populated coastal areas. One of the obstacles to coastal management for greater resilience is the difficulty in reliably measuring the intensity of extreme climatic events and the lack of understanding of their impact in terms of erosion. A large-scale phenomenon, such as a storm, can have vastly different consequences on a coastline depending on its nature (sandy or rocky), its exposure, and/or its morphology.
In addition to currently available data for measuring ocean movements at sea (such as instrumented buoys), there are numerous continuous measurements available on land. Meteorological data (wind force, pressure, rainfall, temperature) are the most well-known, but geophysical instruments (geodesy, gravimetry, seismology) can also provide reliable and highly relevant informations. Although rarely used for such purposes, the continuous seismic signal, for instance, is heavily influenced by ocean swell, whether far from in the deep ocean and near the coastlines, hence across a wide range of frequencies. Often described as seismic noise—in the sense of being a disturbance for earthquake studies—continuous seismic signals are, for example, used to draw global conclusions, such as the increase in ocean energy over recent decades. However, in the most cases, to avoid contamination of the seismic signal by spurious vibrations caused by ocean swell and wave breaking, the permanent seismic stations are installed far inland, which limits the understanding of the phenomenon, due to attenuation and to the presence of scatters in the crust. Conversely, when seismometers are installed just a few kilometres from the coast, all ground vibrations caused by water agitation are recorded as diffuse energy. This means that, with innovative and well-suited signal-processing methods, it would be possible to measure the seismic energy received on land at various points in a perfectly calibrated manner in order to locate the sources and to track the spatio-temporal evolution of the phenomenon.
The receipt project proposes to use geophysical sensors from various permanent networks, and to temporarily make them denser in targeted coastal areas. This will enable the comparison of land-based measurements with those from buoys and tide gauges, as well as with in-situ erosion measurements directly on the coast and through remote sensing. This project is divided into two parts: 1) a retrospective study to better understand the series of storms in autumn 2023 (Céline, Ciaran, Domingos, etc.), as observed by all the sensors in place at that time, alongside direct measurements and Lidar surveys, and 2) a analysis for the next two years thanks to a pioneering data acquisition campaign along the Pays-de-la-Loire and Normandy coastlines. Over two winters and one summer, seismic energy caused by ocean swell across a broad range of frequencies, and associated ground deformations (measured by geodetic and gravimetric sensors), will provide unique data set to quantify and monitor the climatic events that occur. The need to develop new approach to take into the complexity of the phenomenon will be done by a two-year Engineer position. In-situ erosion measurements in different areas (three in the Pays-de-la-Loire region and two in Normandy) will complement Lidar and aerial photogrammetric surveys, aiming to determine the transfer function between geophysical signals recorded during an extreme weather event (from its formation to its dissipation) and erosion. The goal is to document a complex and evolving phenomenon that is however highly repeatable, to better understand the links between wave/swell energy and erosion, and thereby, year after year, potentially anticipate the damages in areas most likely to be affected by future extreme weather events.