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The sedimentary facies architecture of a Plio‐Pleistocene diamond‐bearing gravel barrier spit within the palaeo‐Orange River mouth in south‐western Namibia has been revealed during onshore mining activities. Four sedimentary facies associations build this ancient coastal landform and have been interpreted, from a seaward to landward direction, as sediments in: (i) the foreshore, represented by an intertidal beach; (ii) a subtidal spit platform on which the spit developed; (iii) storm‐constructed spit head recurves on the landward side of the spit; and (iv) an estuarine back‐barrier. This sedimentary record reflects an evolving spit that grew in response to a combination of marine and fluvial processes in a wave‐dominated, high‐energy coastal regime. Here, the southern Atlantic Ocean aggressively dispersed sediment delivered by a Plio‐Pleistocene Orange River, ensuring that the fine‐grained sediments were removed to allow only the gravel fraction to build up along the coastline. This extreme dispersal of sediment also precluded the construction of a classic delta; hence sediment storage offshore of the river mouth was limited and spit growth was dependent on the direct supply of river sediment. In response to the intermittent supply of sediment, spit growth and decay alternated during periods of sediment abundance and starvation, respectively. During positive sediment budget, the spit extended northward and cut off parts of the mainland to form an estuarine back‐barrier. Foreshore and spit head progradation was sustained under these conditions, and formed a beach‐ridge plain with a topographic expression of several coast‐parallel ridges that curved at the spit head into the back‐barrier. Spit extension and recurve development were controlled by a combination of high sediment supply, a vigorous longshore drift and powerful waves approaching from various directions. This Plio‐Pleistocene coastal landform in Namibia offers a depositional model relevant to gravel‐dominated barrier spits near river mouths along wave‐dominated clastic coasts. [ABSTRACT FROM AUTHOR]

Coastal barriers are a natural defence against the marine submersion of low-lying back-barrier areas. In March 2018, a breach opened up in the proximal section of the Sillon de Talbert spit barrier (North Brittany, France), causing great concern among local residents and elected representatives. A study to assess the risk of submersion of the low-lying areas of the Laneros Peninsula, located behind the barrier, was carried out in 2020. Hydrodynamic conditions (waves, currents, and water levels) were modeled using Telemac-2D coupled with the phase-averaged wave model TOMAWAC. Simulations were compiled for the storm of February 1, 2014, corresponding to the most morphogenic event to occur in the last two decades. Five topomorphological configurations were considered for the numerical simulations: configurations #1 and #2 correspond to the ante-breach and post-breach morphological settings, while configurations #3, #4, and #5 correspond to hypothetical scenarios, from the enlargement of the breach (from 300 to 500 m width), to complete crest lowering of the barrier after a complete removing of the crest due to catastrophic overwash events. The results show that the breach accelerates the draining of the back-barrier sandflat, thereby reducing the height of extreme water levels at the coastline. This is mainly due to different tidal impacts on either side of the barrier, with a higher water level and the tidal peak occurring 15 min later and the on the back-barrier than on the ocean-side zone. As a result, ebb currents begin to flow before the high tide level is reached, inducing a significant draining with a lowering of the extreme water level at the back-barrier zone. However, these simulations do not take into account the effects of climate change over the coming years, in particular the future rise in sea level and/or the acceleration of erosion processes already underway. [Display omitted] • The breach opening on gravel spit barrier through cannibalization and rollover processes is described. • A method of coastal flooding risk assessment is proposed through modelling of hydrodynamic forcings. • An example of nature-based coastal management is presented as a model for a new regional strategy. [ABSTRACT FROM AUTHOR]

Advances in image-based remote sensing using unmanned aerial vehicles (UAV) and structure-from-motion (SfM) photogrammetry continue to improve our ability to monitor complex landforms over representative spatial and temporal scales. As with other water-worked environments, coastal sediments respond to shaping processes through the formation of multi-scale topographic roughness. Although this topographic complexity can be an important marker of hydrodynamic forces and sediment transport, it is seldom characterized in typical beach surveys due to environmental and technical constraints. In this study, we explore the feasibility of using SfM photogrammetry augmented with an RTK quadcopter for monitoring the coastal topographic complexity at the beach-scale in a macrotidal environment. The method had to respond to resolution and time constraints for a realistic representation of the topo-morphological features from submeter dimensions and survey completion in two hours around low tide to fully cover the intertidal zone. Different tests were performed at two coastal field sites with varied dimensions and morphologies to assess the photogrammetric performance and eventual means for optimization. Our results show that, with precise image positioning, the addition of a single ground control point (GCP) enabled a global precision (RMSE) equivalent to that of traditional GCP-based photogrammetry using numerous and well-distributed GCPs. The optimal model quality that minimized vertical bias and random errors was achieved from 5 GCPs, with a two-fold reduction in RMSE. The image resolution for tie point detection was found to be an important control on the measurement quality, with the best results obtained using images at their original scale. Using these findings enabled designing an efficient and effective workflow for monitoring coastal topographic complexity at a large scale. [ABSTRACT FROM AUTHOR]

Advances in image-based remote sensing using unmanned aerial vehicles (UAV) and structure-from-motion (SfM) photogrammetry continue to improve our ability to monitor complex landforms over representative spatial and temporal scales. As with other water-worked environments, coastal sediments respond to shaping processes through the formation of multi-scale topographic roughness. Although this topographic complexity can be an important marker of hydrodynamic forces and sediment transport, it is seldom characterized in typical beach surveys due to environmental and technical constraints. In this study, we explore the feasibility of using SfM photogrammetry augmented with an RTK quadcopter for monitoring the coastal topographic complexity at the beach-scale in a macrotidal environment. The method had to respond to resolution and time constraints for a realistic representation of the topo-morphological features from submeter dimensions and survey completion in two hours around low tide to fully cover the intertidal zone. Different tests were performed at two coastal field sites with varied dimensions and morphologies to assess the photogrammetric performance and eventual means for optimization. Our results show that, with precise image positioning, the addition of a single ground control point (GCP) enabled a global precision (RMSE) equivalent to that of traditional GCP-based photogrammetry using numerous and well-distributed GCPs. The optimal model quality that minimized vertical bias and random errors was achieved from 5 GCPs, with a two-fold reduction in RMSE. The image resolution for tie point detection was found to be an important control on the measurement quality, with the best results obtained using images at their original scale. Using these findings enabled designing an efficient and effective workflow for monitoring coastal topographic complexity at a large scale.

Suanez, S.; Stéphan, P.; Fichaut, B.; Ammann, J., and Accensi, M., 2020. Medium-term morphological changes of a gravel spit driving by storm events (Sillon de Talbert, Brittany, France). In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 669-673. Coconut Creek (Florida), ISSN 0749-0208. The Sillon de Talbert (Northern coast of Brittany) is a large 3.5 km-long swash-aligned gravel spit comprising a volume of sediment of 1.23x106 m3. Since 2002, a morphodynamic survey based on annual DEMs, and waves and water level measurements and/or modeling, has been carried out. The 17-year (2002-2019) monitoring program shows that cross-shore sediment transfers reaching 430,000 m3 are dominant, while the longshore sediment transfer -through cannibalization process- is about 52,000 m3. The maximum landward displacement of the spit due to rollover processes reaches –4 m.yr-1. Storm events control more than 95% of this retreat due to catastrophic overwash/inundation processes that led to the opening of a breach in March 2018. The morphological evolution of the Sillon de Talbert is driven by anthropogenic forcing (i.e., impact of coastal defence structures, cutting off of longshore sediment transport), and natural forcing such as the depletion of the supply of sediment from the platform. [ABSTRACT FROM AUTHOR]