# Long-term monitoring of beach topography and nearshore bathymetry at Porsmilin, France: challenges and opportunities

S Bertin, France Floc’H, N Le Dantec, M Jaud, C Delacourt

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S Bertin, France Floc’H, N Le Dantec, M Jaud, C Delacourt. Long-term monitoring of beach topography and nearshore bathymetry at Porsmilin, France: challenges and opportunities. Coastal Dynamics 2021, Jun 2021, Delft (Virtual Event), Netherlands. hal-04974759

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# Long-term monitoring of beach topography and nearshore bathymetry at Porsmilin, France: challenges and opportunities

S.Bertin<fnref n="1" />, F.Floc'h<fnref n="1" />, N.Le Dantec<fnref n="1" />, M.Jaud<fnref n="2" /> and C. Delacourt<fnref n="1" />

Stephane.Bertin@univ-brest.fr; France.Floc'h@univ-brest.fr;

Nicolas.LeDantec@univ-brest.fr; Christophe.Delacourt@univ-brest.fr

Marion.Jaud@univ-brest.fr

## 1. Introduction

Beaches are very dynamic and varied environments that provide essential natural services. Perhaps most importantly to coastal inhabitants, they are able to adjust to changes in hydrodynamics and sediment availability, buffering storm waves and protecting the hinterland from submersion and shoreline recession. Looking at coastal morphological changes requires not only monitoring coastline evolution, but also sediment fluxes over the entire shoreface. The seaward depth at which morphological changes become insignificant is called the Depth of Closure (DoC) and has important implications regarding shoreface delineation, long-term stability of beaches and sediment budgeting at the scale of the beach compartment (Hamon-Kerivel et al. 2020, Valiente et al. 2019). DoC is neither fixed in time nor alongshore-uniform, which make its direct determination complex in terms of field instrumentation required and rare.

Embayed beaches, with reasonably scaled and confined geometries, represent ideal field laboratories to investigate the continuum of sediment exchanges from supratidal to subtidal regions. In this communication, we present for the first time the complete, archived and accessible dataset of topography and nearshore bathymetry obtained over the period 2003-2019 at Porsmilin, a macrotidal pocket beach situated in northwest France. The availability of a long-term, multi-method dataset provides a unique opportunity to identify and discuss challenges in terms of management of measurement uncertainty relevant for change detection studies. It shows that despite high data precision is regularly achieved, systematic errors (bias) can be present and need to be minimized. We then present preliminary findings concerning subaqueous sediment exchanges and closure depth and discuss future research avenues using the dataset.

## 2. Methods

Porsmilin is a macrotidal, moderately indented, embayed beach situated in northwest France (Figure 1). The sandy beach is surrounded by cliffs and is backed by small dunes. It is also bounded by extensive bedrock reef, exposed at low tide and extending offshore. With a semi-enclosed configuration facing south-southwest, away from dominant storm tracks, Porsmilin tends toward the lower-energy low-tide terrace beach state. The beach sediment consists in medium-grained quartz sand ($D_{50}$ = 0.37 mm) with some variability due to bedforms.

The Porsmilin beach survey program is approaching 20 years of existence (Table 1). Launched in January 2003 by the European Institute of Marine Sciences (IUEM/Univ. Brest), the dataset contains monthly (nominal) surveys of a central cross-shore beach profile, and since 2008 recurring (approx. bi-annual) high-resolution (0.5 m) topographic and bathymetric digital elevation models (DEMs) obtained using a combination of remote sensing techniques, including terrestrial laser scanning, drone photogrammetry and boat-mounted multi-beam echo-sounding.

Accompanying the field surveys, continuous hourly hindcasted waves directly offshore of Porsmilin in approximately 15 m water depth are available from a third-party organisation.

## 3. Results and discussion

While modern beach monitoring techniques generally provide means for some sort of quality

<footnote n="1">¹Institut Universitaire Européen de la Mer (IUEM), Univ. Brest, Laboratoire Géosciences Océan - UMR 6538, Technopôle Brest-Iroise, Rue Dumont d'Urville, F-29280 Plouzané, France.</footnote>

<footnote n="2">²Institut Universitaire Européen de la Mer (IUEM), Univ. Brest, CNRS—UMS 3113, Technopôle Brest-Iroise, Rue Dumont d'Urville, F-29280 Plouzané, France.</footnote>
<figure><img src="image_2.png" /><figcaption>Figure 1. Survey site, Porsmilin beach. (a) Orthophotograph showing the maximum survey coverage (yellow), central cross-shore profile line with graduations every 100 m, contour levels corresponding to MSL (black), intertidal beach (MLWS, MHWS, pink) and closure depth (-8 m NGF, estimated using classic wave-based parameterization, red) as well as hindcast point and temporary wave buoy (green circle and square). The wave rose is based on hourly hindcasted waves (1994-2019). (b) Composite DEM at 0.5 m resolution overlaid with ground truth elevations (light grey) and vertical variability using the complete dataset</figcaption></figure>

control, they are often survey or method specific, and thus they cannot be used confidently to ascertain the overall reliability of a long-term dataset. Facilitated by the wealth of data at our disposition, we tested and implemented different strategies in unison to assess data quality in terms of accuracy, precision and reliability. In particular, reference DEMs (ground truths) were prepared through iteratively averaging independent datasets, such that only the most consistent DEMs and DEM cells were retained to form composite elevations of the bedrock reef. In application to Porsmilin, ground truths counted over 100,000 known elevations in the subtidal zone for over 10,000 on the foreshore (Fig. 1b). Ground truths were subsequently used to test DEMs individually, providing each survey with realistic error estimates and eventual corrections to minimize inconsistencies (bias), both of which are critical for reliable change detection.

Sediment exchanges across the beach and upper shoreface are clearly apparent using common differentiation techniques (Fig. 1b), showing, at first order, large fluctuations in sand level at depths lower than LAT (lowest astronomical tide), and in relatively good agreement with common wave-based parameterizations of closure depth.

<table><caption>Table 1. Overview of available topographic and bathymetric surveys</caption><thead><tr><td>Survey coverage</td><td>Time-series / Frequency (nominal)</td><td>Number of surveys</td></tr></thead><tbody><tr><td>Central cross-shore profile</td><td>2003-2019 / Monthly</td><td>253</td></tr><tr><td>Subaerial DEM</td><td>2004-2019 / Bi-annual since 2008</td><td>48</td></tr><tr><td>Subtidal DEM</td><td>2008-2019 / Bi-annual</td><td>24</td></tr><tr><td>Seamless subaerial and subtidal DEMs</td><td>2008-2019 / Annual</td><td>11</td></tr></tbody></table>

## Acknowledgements

Since 2003, the Porsmilin beach survey program has received funding from IUEM/Univ. Brest and the French Coastal Observatory SNO-DYNALIT (https://www.dynalit.fr).

## References

* Hamon-Kerivel, K., Cooper, A., Jackson, D., Sedrati, M., Guisado Pintado, E. 2020. "Shoreface mesoscale morphodynamics: A review." *Earth-Science Reviews*, 209.

* Valiente, N. G., McCarroll, R. J., Masselink, G., Scott, T., and Wiggins, M. 2019. "Multi-annual embayment sediment dynamics involving headland bypassing and sediment exchange across the depth of closure." *Geomorphology*, vol. 343, pp. 48–64