Estuaries represent specific environments, mixing continental fresh and marine salty waters, associated with rich biodiversity and ecological services. Moreover, they are favored places for social and economic developments. Estuarine systems are complex and composed of different interconnected areas, usually including a main channel, ebb tidal delta banks, and lateral mudflats. Intertidal mudflats are a key feature within the complex estuarine ecosystem. The lateral, mixed sediment (clays, silts, sands) banks of the main channel, which are successively flooded and uncovered by tides and/or river discharge variations, play an important role in estuary sediment dynamics as a temporary sediment source or sink (e.g., [1
]). Mudflats are the interface between the flood plain and the main river channel, and are characterized by a cross-shore slope (either steep or smooth), usually separated into two specific environments: the shore, located in the upper part of the mudflat and flooded during high spring tides; and the tidal flat, the lower part of the mudflat, with an upper limit corresponding to the highest sea level reached during the mean tidal ranges. Shores are mainly colonized with halophile (salt-tolerant) vegetation, when vegetation is present, while the tidal flat can either be bare or colonized by vegetation in its upper part at the transition to the shore. Ridge/runnel and meandering tidal creeks perpendicular to the main channel are also a typical feature of intertidal mudflats, identified as preferential exchange pathways for sediment and water between the flood plain, shore, tidal flat, and main channel. Hydrodynamic conditions (currents, bed shear stress) in all parts of the system affect the development of vegetation, but, conversely, the presence of vegetation can also affect the hydrodynamics (e.g., reduction of current velocity, turbulence decrease [3
]), leading to changes in sediment dynamics (e.g., erosion/deposition [4
Hydrological (river discharge and continental suspended solid inputs), meteorological (wind, wave), and hydrodynamic (tide) forcing factors control estuarine sediment dynamics, and hence mudflat morphodynamics, at different time scales: from hours (tides), days (storm events), fortnightly cycles (neap/spring tides), and seasons (river discharges) to decades (changes in land use, anthropogenic stream alteration) and centuries (climate change). Morphological changes are associated with either erosion/deposition events leading to mudflat bed elevation changes, or tidal creek development and/or migration. Tidal forcing and storms result in millimeters to centimeters of bed elevation changes [6
] and the formation of bedforms (i.e.
, ripples). Changes occurring at moderate temporal scales are associated with centimeter to decimeter variations, while slow, long-term variability is potentially associated with mudflat retreat or progradation, i.e.
, decimeter to meter changes. These changes are not generally applied homogeneously across the mudflat but are expressed with strong spatial variability. Finally, the aforementioned physical processes driving mudflat morphodynamics interact directly with vegetation ground cover.
Reflecting such a complex environment, the macrotidal Seine Estuary (maximum tidal range of 8.0 m at its mouth) is located in the northwestern part of France (Figure 1
). It is one of the largest estuaries on the Northwestern European continental shelf, with a catchment area of more than 79,000 km2
. The mean annual Seine River flow, computed over the last 50 years, is 450 m3
. During the last two centuries, the Seine Estuary has been vastly altered by human activity [7
]. As a result, the lower Seine River was changed from a dominantly natural system to an anthropogenically-controlled system [8
]. Despite the highly dynamic nature of the system, tidal flats and salt marshes are still developed in the lower estuary; however, the intertidal surface area has drastically decreased during the last 30 years. The funnel-shaped estuary is exposed to the prevailing SSW winds, so that the intertidal regions at the mouth are subject to erosion under the combined effects of waves and currents [2
]. The lower estuary is characterized by a distinct turbidity [7
], which has a pronounced control on the sedimentation patterns on intertidal mudflats at the estuary mouth.
In the Seine Estuary, among others, monitoring of mudflat morphodynamics on different time scales has been developed for decades. The most basic method is the pole height manual measurement, which consists of putting a pole in the ground during a given period and regularly measuring (e.g., every day, week, or month) the bed level changes at the base of the pole. It represents a point-wise, low frequency method associated with a centimetric accuracy, which has been used to quantify accretion or erosion. Over the last 20 years, autonomous acoustic altimeters were developed to monitor high-resolution (sub-millimetric) and high-frequency (in the order of 1 Hz) bed elevation changes over long periods (years) [9
]. However, the survey area is localized and measurements are not necessarily representative of the morphological changes of the entire mudflat, hence leading to large uncertainties when sediment budgets are evaluated. More recently, airborne LIDAR surveys have been used to provide mudflat topography over large areas (on the order of 1 to 10 km2
). However, with vertical resolutions of decimetric order, infrequent survey return intervals, and their high deployment cost, airborne LiDAR surveys only allow us to capture long-term morphological changes. Terrestrial Laser Scanning (TLS) is a possible alternative [11
], even though the ground coverage is spatially limited around the TLS and the presence of surficial residual water is a major constraint preventing backscattering of the emitted light, thus potentially masking large areas in TLS surveys. As a consequence, there is a methodological gap for monitoring fast and moderate spatial mudflat morphodynamic evolution that could be bridged by photogrammetric surveys from Unmanned Aerial Vehicles (UAVs).
In a broader context, as mentioned in recent reviews on the subject [13
], remote-sensing approaches using UAVs have been applied in a variety of research and operational domains, such as geomatics, navigation, surveying, engineering, robotics, and data processing. A wide range of platforms are now available, offering varying possibilities in terms of payload and flight-time autonomy. Thanks to the miniaturization of sensors and onboard electronics for data logging and control systems, a growing number of sensor types (e.g., optical camera, LiDAR, hyperspectral camera) can operate from a UAV, offering extensive possibilities to fully exploit the entire electromagnetic spectrum for remote sensing purposes [14
]. Photogrammetric surveys from UAVs are particularly useful for environmental monitoring, in mountainous [15
], agricultural [16
], riverine, and coastal contexts [17
]. Drones can provide surveys at higher temporal frequencies than airborne photogrammetry or airborne topographic LIDAR systems. They are also more flexible to use and more maneuverable. As they are not subject to the same regulations, they can be flown at low altitude, which is crucial in order to improve the resolution and accuracy of the data. With the recent developments in small UAVs and the use of Structure from Motion (SfM) algorithms, the rapid acquisition of topographic data at high spatial and temporal resolution is now possible at low cost [19
]. The SfM approach is based on the SIFT (Scale-Invariant Feature Transform) image-to-image registration method [23
]. In comparison with classic digital photogrammetry, the SfM workflow allows more automation and is therefore more straightforward for users [21
UAV surveys have already been carried out above mudflat areas for imagery acquisitions [25
], but rarely for photogrammetric purposes [29
]. UAV photogrammetric surveys in mudflat areas are challenging. Mudflats are areas with difficult ground access, which is problematic for topographic surveys but also for deploying and surveying Ground Control Points (GCPs) for UAV surveys. Moreover, the duration of the time available for survey operations is controlled by low tide periods. Another difficulty is related to the stereo-photogrammetric method itself. Mudflats are generally characterized by surficial sediment with high water content, and even puddles and films of residual tidal water. In such environments, the stereo restitution method generally reaches its limits because of image correlation difficulties due to low textural pattern or sun-glint effects.
This study aims to demonstrate the potential and the limitations of high-resolution aerial photography and stereo restitution from a light UAV over mudflats. The results are interpreted in relation to the different morphological scales characterizing mudflat dynamics. Finally, these results are discussed in terms of the quality of the resulting imagery products, possible uses for mudflat monitoring, and future methodological improvements.
2. Study Area
This study is conducted within the Seine-Aval HYMOSED project (modeling of the HYdro-MOrpho-SEDimentary functioning of the Seine Estuary), with a focus on the “Vasière Nord” mudflat, the largest mudflat in the Seine Estuary (3.2 km2
), located in the northern part of the estuary mouth (Figure 1
). The Seine river flow ranged from 100 m3
to 2000 m3
over the last 20 years, with a mean annual discharge around 500 m3
. Previous studies show that deposition occurs in bursts during periods of low fluvial discharge, and is closely related to the highest spring tides and the presence of the turbidity maximum zone in the estuary mouth [2
]. These deposits are dominated by fine sediments (<50 μm), which represent 70%–90% of the material. As in the upstream fluvial mudflat, erosion events of several centimeters are observed, mostly following deposition periods [30
During the time frame of the study, three ALTUS altimeters [9
] were deployed on the Vasière Nord mudflat along a transect perpendicular to the main channel. The ALTUS provided bed elevation measurements with sub-millimetric resolution and a sampling frequency of one measurement every four minutes between April 2014 and October 2015. The three ALTUS are located along a single transect, one near the vegetation on the top of the tidal flat, one in the central (“flat”) portion, and one on or near the slope break. This distribution is to capture distinct processes, because the portions of the tidal flat may behave differently, both globally (i.e.
, on a seasonal scale: erosion/deposition depending on flow strength at different moments of the tidal cycle, so with different depth of the overlying water-column) and at short time scales (because their immersion time differs in relation to tidal amplitude).
UAV photographs and the resulting DEM, orthophotographs, and DoD provide copious information for mudflat studies. Given the ground texture of such environments, the stereo restitution process is made possible thanks to the high spatial resolution of the UAV photographs. For each UAV mission, computed orthophotographs and DEMs with about 2 cm and 4 cm of resolution, respectively, have been realized. The horizontal and vertical RMSEs of the DEM, computed on eight GR points, are less than 5 cm.
Providing a synoptic view, aerial photogrammetric surveys add significant value when monitoring large areas. The spatial extent of the information is a key asset in the study of mudflat dynamics. Both long-shore and cross-shore variability are captured with high resolution and accuracy. UAV photogrammetry also enables multiscale studies by characterizing and quantifying sedimentation dynamics to monitor both the general trend over large areas as well as the evolution of smaller structures, hence possibly inferring the role of the smaller structures on the larger-scale response.
In order to circumvent the effects of temporal interpolation of rates of change, this diachronic approach could be enriched by combining it with the pointwise, time-continuous ALTUS dataset in order to infer when and why the changes occur.