## 2 Materials and Methods  
### Field Sites  
The two coastal field sites presented in this study are located in Brittany in northwest France (Figure 1a,b). The field site at La Palue and Lostmarc'h beaches, hereafter generally referred to as \"La Palue\" for concision, is situated at the western end of Crozon Peninsula. Facing due west, La Palue and Lostmarc'h are adjacent beaches totaling \(\sim\)2.5 km along-shore separated by a rocky promontory (Kerdra point) at high tide, backed by granitic cliffs (20-50 m high) at each end point and a mostly consolidated and vegetated dune elsewhere. It is a macrotidal environment with mean heap and spring tidal ranges of 2.25 and 5.60 m [57], respectively. Waves originate from Atlantic depressions generating SW through NW swells, but also shorter period seas from locally generated wind [58]. Based on samples collected on the intertidal beach and processed using traditional size-sieving, sediment belongs to fine to medium sand with a D\({}_{50}\) (the median sediment size for which 50% of the sediment distribution is finer) of \(\sim\)0.25 mm and a sorting coefficient (\(\sqrt{\text{D}_{84}/\text{D}_{16}}\)) of 1.35 (i.e., well-sorted). The upper beach abutting the dune is composed of cobbles, especially on the northern parts (Figure 2b). The morphodynamics are akin to intermediate beaches according to the Masselink and Short classification [59], covering a range of beach states depending on ongoing hydrodynamic conditions. Generally, a flatter profile is observed over winter, contrasting with marked bar and rip channel morphologies developing across calmer months. During spring tides, the intertidal beach is approximately 300-400 m wide (Figure 1c). Large patches of ripples and megaripples can often be observed. Bedrock may locally appear near the low-tide line, depending on sand levels. Outside a few of channelized access points and small blockhouses remnant of World War 2, human encroachments are limited. Man-made features, at the condition they are relatively flat, were useful for preparing fixed ground targets for photogrammetry. Depending on survey date, up to six red-painted crosses approximately 40 cm in size were used (Table 1). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Field Site Date**} & \multirow{2}{*}{**Tidal Coefficient (\(\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\bmbm{ }}}} \bmbmbmbm{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{ }}}}}}}}}}}}}\)}}} & & ****************************(****(**)**)** & **********(**)** & **Survey Area (\(\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{ }}}}}}}}}}}}}}\)**)** & **GSD (\(\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{ }}}}}}}}}}}}}}}\)**)** & **Target Count (Fixed Targets)** \\ \hline \hline Sillon de Talbert 17 November 2020 & 103 & 2/734 & 58 & 80/70 & 125,000 & 0.014 & 21 (0) \\ \hline La Palue 17 September 2020 & 101 & 5/1407 & 106 & 80/70 & 1,320,000 & 0.026 & 30 (5) \\ \hline La Palue 17 December 2020 & 90 & 6/1548 & 105 & 80/70 & 1,280,000 & 0.026 & 22 (3) \\ \hline La Palue 2 March 2021 & 103 & 5/1411 & 108 & 80/70 & 872,000 & 0.025 & 19 (1) \\ \hline La Palue 29 April 2021 & 106 & 5/1386 & 107 & 80/70 & 1,080,000 & 0.026 & 28 (6) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of the field surveys and data collection for UAV–SfM photogrammetry. The tidal coefficient (20–120) represents the tide magnitude in relation to its mean. The second field site corresponds to the proximal section of the Sillon de Talbert gravel spit. This 3.5-km-long swash-aligned barrier is located north of Brittany. The sediment volume is estimated at 1.23 \(\times\) 10\({}^{6}\) m\({}^{3}\)[60]. According to the barrier morphology and sedimentology, the gravel accumulation can be classed as a \"composite gravel beach\" [61; 62].