FIELD OBSERVATIONS OF BEACH-SHOREFACE
SEDIMENT TRANSFERS AT A MACROTIDAL
POCKET BEACH (PORSMILIN, FRANCE)

S Bertin, F Floc’H, N Le Dantec

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S Bertin, F Floc'H, N Le Dantec. FIELD OBSERVATIONS OF BEACH-SHOREFACE SEDIMENT TRANSFERS AT A MACROTIDAL POCKET BEACH (PORSMILIN, FRANCE). Coastal Sediment 2023, Apr 2023, Nouvelle Orléans (Louisiane), United States. hal-04974808

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# FIELD OBSERVATIONS OF BEACH-SHOREFACE SEDIMENT TRANSFERS AT A MACROTIDAL POCKET BEACH (PORSMILIN, FRANCE)

S. Bertin<fnref n="1" />,<fnref n="2" />, F. Floc’h<fnref n="1" />, and N. Le Dantec<fnref n="1" />,<fnref n="2" />

## 1. Introduction

The quantification, understanding and prediction of sediment exchanges between the beach and the shoreface is important in coastal management. Different morphological units have been defined in the past, depending on controlling variables and the degree of bed mobility. For embayed and pocket beaches, the initial consensus of a mostly closed sediment cell implies that the sediment eroded from the beach during storms is stored on the shoreface, before a progressive return to the beach during calmer periods. This general view is increasingly challenged with new research highlighting the eventuality of sediment by-passing (King et al. 2021), as well as sand inputs from deeper regions, during extreme events, which may affect the sediment budget overall, and hence the resilience of coastlines to future changes (Harley et al. 2022).

In this study, we analyze repeated surveys of the subaerial topography and nearshore bathymetry at the macrotidal beach of Porsmilin (Brittany, France), with the general objective to improve our understanding of the environmental conditions driving beach-shoreface sediment transfers. The study focuses on data collected over the period 2008-2019, which includes several energetic winters, and during which high-resolution bathymetry was measured bi-annually alongside subaerial profiles and DEMs. To help comprehend coastal behaviour, morphological surveys are assessed in relation to environmental conditions (waves and tide) at the study site, which were derived from numerical models and observations. Our results demonstrate a large variability in shoreface response to extreme events, whereby the 2013-2014 winter was responsible for the most notable sediment transfers between the beach and the shoreface.

## 2. Methods

Porsmilin is a macrotidal (2.7 – 5.7 m mean neap and spring tidal ranges, respectively) pocket beach located in the wider Bertheaume embayment (Figure 1). Despite an energetic wave environment, its sheltered location means that Porsmilin essentially pertains to the low-tide terrace beach state, with a mean wave height of approximately 0.7 m.

The data analyzed in this study are part of a long-term and multi-sensor dataset of beach topography and nearshore bathymetry collected at Porsmilin over the period 2003-2019. The morphological data are accompanied by continuous time-series of wave parameters and water levels. The complete dataset is open-access and presented in detail in Bertin et al. (2022).

The first bathymetric survey at Porsmilin was carried out in September 2008, followed by bi-annual surveys since. Bathymetric surveys are performed using a boat-mounted multibeam echosounder in conjunction with a GNSS, IMU and sound-celerity probe to account for boat, water-column and tide-induced effects on the measured bathymetry. The subaerial dataset consists of monthly measurements of a central cross-shore profile using RTK-GNSS and regular surveys of the whole intertidal beach topography with drone photogrammetry or terrestrial laser-scanning. Topographic (bathymetric) surveys are generally performed during spring low (high) tides to maximize survey coverage. To allow comparisons, all data are produced using a fixed horizontal resolution of 0.5 m and are referenced to the French geographic coordinate systems: Lambert-93 horizontal and NGF-IGN69 vertical. Using different benchmarks, topographic and bathymetric DEM quality was evaluated in terms of vertical registration error and precision, and was found to be essentially below 0.1 m. For the period spanning 2008-2019, we analyzed 24 bathymetric DEMs, 42 topographic DEMs and over 160 intertidal beach profiles.

<footnote n="1">¹UMR 6538 Geo-Ocean, Univ. Brest-CNRS-Ifremer, Institut Universitaire Européen de la Mer, Rue Dumont d'Urville, 29280 Plouzané, France. stephane.bertin@univ-brest.fr; france.floch@univ-brest.fr; nicolas.ledantec@univ-brest.fr.</footnote>

<footnote n="2">²UAR 3113 Institut Universitaire Européen de la Mer (IUEM), Univ. Brest-CNRS, Rue Dumont d'Urville, 29280 Plouzané, France.</footnote>
## 3. Results and conclusion

The study site at Porsmilin is presented in Figure 1. The maximum survey coverage extends to approximately 15 m water depth and encompasses the regions of significant morphological changes. The wave climate shows important seasonal fluctuations, with severe storms occurring over the winters interspaced by periods of calm during boreal summers. Superimposed on these fluctuations, some winters are more energetic than others, with the occurrence of clusters of storms. The intertidal sediment budget (black line) shows a relative stability over the period 2008-2019 at the exception of a large loss of sediment during the winter 2013-2014, which was amongst the most energetic along the European Atlantic seaboard since at least 1948. Our observations suggest that the sediment eroded from the intertidal beach deposited on the upper shoreface, before returning to the intertidal zone in a matter of a few months. Yet, despite an apparent symmetry between intertidal and subtidal sediment volumes, the sediment budget is not closed. We will discuss potential reasons behind this finding and explaining why the winter 2013-2014 was particular in terms of beach-shoreface sediment transfers at Porsmilin.

<figure><img src="image_2.png" /><figcaption>Figure 1. Location of Porsmilin in Brittany (northwest France) showing the maximum survey coverage and the location of the central cross-shore profile; hourly significant wave height (black) and 28-day running mean (green), with bubbles proportional to storm duration; evolution of subaerial (black) and subtidal (blue) sediment volumes per alongshore meter of beach length</figcaption></figure>

## Acknowledgements

Since its beginning in 2003, the continuing Porsmilin beach survey program has received funding by the Institut Universitaire Européen de la Mer (IUEM/Univ. Brest), as well as the national coastal observation service SNO-DYNALIT (https://www.dynalit.fr/). DYNALIT acknowledges funding from the National Centre for Scientific Research (CNRS-INSU), the French coastal research infrastructure (IR-ILICO), the ministry for higher education and research, and the agency for research on the environment. Tide measurements for Le Conquet are provided by the French naval hydrographic and oceanographic service (Shom). Ifremer is acknowledged for undertaking and providing wave hindcast databases. The authors wish to extend their gratitude and thanks to everyone who has contributed to the Porsmilin beach survey program.

## References

* Bertin, S., Floc'h, F., Le Dantec, N., Jaud, M., Cancouët, R., Franzetti, M., Cuq, V., Prunier, C., Ammann, J., Augereau, E., Lamarche, S., Belleney, D., Rouan, M., David, L., Deschamps, A., Delacourt, C., and Suanez, S. (2022). A long-term dataset of topography and nearshore bathymetry at the macrotidal pocket beach of Porsmilin, France. *Scientific Data*, 9(1), 79.

* Harley, M. D., Masselink, G., Ruiz de Alegria-Arzaburu, A., Valiente, N. G., & Scott, T. (2022). Single extreme storm sequence can offset decades of shoreline retreat projected to result from sea-level rise. *Communications Earth & Environment*, 3(1), 1–11.

* King, E., Conley, D., Masselink, G., Leonardi, N., Mccarroll, R., Scott, T., and Valiente, N. (2021). Wave, Tide and Topographical Controls on Headland Sand Bypassing. *Journal of Geophysical Research: Oceans*, 126.