Sargassum detection and path estimation using neural networks

Sargassum has affected the Mexican Caribbean coasts since 2015 in atypical amounts, causing economic and ecological problems. Removal once it reaches the coast is complex since it is not easily separated from the sand, damaging dune vegetation, heavy transport compacts the sand and further deteriorates the coastline. Therefore, it is important to detect and estimate the sargassum mats path to optimize the collection efforts in the water. There have been some improvements in systems that rely on satellite images to determine areas and possible paths of sargassum, but these methods do not solve the problems near the coastline where the big mats observed in deep sea end up segregating in little mats which often do not show up in the satellite images. Besides, the temporal scales of nearshore sargassum dynamics are characterized by finer temporal resolution. This paper focuses on cameras located near the coast of Puerto Morelos reef lagoon where images are recorded of both beach and near-coastal sea. First, we apply preprocessing techniques based on time that allows us to discriminate the moving sargassum mats from the static sea bottom, then, using classic image processing techniques and neural networks we detect, trace, and estimate the path of the mat towards the place of arrival on the beach. We compared classic algorithms with neural networks. Some of the algorithms we tested are k-means and random forest for segmentation and dense optical flow to follow and estimate the path. This new methodology allows to supervise in real time the demeanor of sargassum close to shore without complex technical support.Environmental Fluid Mechanic

Assessing shoreline dynamics over multiple scales on the northern Yucatan Peninsula

Coastal erosion is critical in many locations along the northern Yucatan Peninsula. The area is characterized by a micro-tidal regime and low-energy wave conditions, with a high-incidence angle with respect to the shoreline. Port and harbor infrastructure for fisheries, commercial, and tourist activities has promoted the growth of coastal communities settled on barrier islands. However, the human settlements have degraded the coastal ecosystems and interrupted the littoral transport. Due to coastal development in the region, the land use of the remaining pristine coastal areas is expected to change in forthcoming years. Thus, understanding coastal changes occurring along the northern Yucatan Peninsula is fundamental for improving coastal planning. We employed open access remote sensing data sets and reanalysis information to investigate shoreline changes at different spatial and temporal scales. Shoreline position was obtained along a 150-km stretch of coast from satellite imagery using CoastSat. Firstly, reanalysis and satellite-derived information were validated with in situ measurements in the vicinity of coastal structures. A satisfactory agreement was found for characterizing the forcing conditions (waves and sea level) and shoreline evolution at different temporal scales. A dominant direction of alongshore sediment transport (50,000–80,000 m3/year) make the shoreline highly sensitive to any nearshore disturbance. We found that coastal erosion occurred in 50% of the analyzed transects, whereas beach accretion occurred in only 30%, suggesting net beach losses. Erosive trends are strongly correlated with the presence of coastal structures. The 6-km long Progreso pier induced significant beach erosion along O(10) km, while sheltered harbors induced downdrift erosion along O(1) km. Detached breakwaters and groins have an overall negative impact on downdrift areas (O(100) m). On the other hand, significant erosion was also observed in pristine areas located downdrift of a coastal lagoon due to the sediment impoundment associated with the growth of a sand spit. Moreover, shoreline sand waves drive 40-m shoreline oscillations and propagate (alongshore) at a rate of 300 m/year. The generation of sand waves seems to be related to both natural and anthropogenic perturbations, in combination with the high-incidence wave angle. Their propagation plays a key role in the shoreline dynamics of this region.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Environmental Fluid Mechanic

Beaching and Natural Removal Dynamics of Pelagic Sargassum in a Fringing-Reef Lagoon

Massive quantities of the pelagic brown macroalgae Sargassum spp. (sargassum) have been invading the Caribbean and West African shores since 2011, causing devastating effects on the coastal ecosystem and local economy. Little is known about sargassum beaching dynamics and the capacity of the coastal system to naturally remove beached sargassum. Here, we characterize the temporal variation in arriving and beached sargassum in a reef lagoon using a 5.2-year data set of hourly optical imagery, and identify the governing hydrometeorological conditions. Image classification reveals interannual variability in the start, duration, and intensity of the sargassum arrival season. Arrivals are associated with relatively low energy onshore directed winds and waves, and offshore abundance of sargassum. Furthermore, nearshore sargassum mat size is found to decrease with decreasing wave/wind energy. Once sargassum beaches, a berm of wrack is formed. Natural wrack removal was observed under elevated water levels and increased wave action. Three types of wrack removal were distinguished, depending on the water level (Formula presented.) with respect to the berm crest height (Formula presented.) and berm crest toe (Formula presented.) : gradual berm destruction with gaps developing in the seaward berm edge that grow larger with time (Type I; (Formula presented.)) and abrupt berm destruction with part of the wrack depositing on the upper beach (Type II; (Formula presented.)) or in the dunes (Type III; (Formula presented.)). Higher energy waves activate the reef circulation, which is suspected to flush part of the wrack out of the reef lagoon. We propose a conceptual model of nearshore sargassum dynamics in a reef lagoon system.Environmental Fluid Mechanic

Improving satellite monitoring of coastal inundations of pelagic Sargassum algae with wind and citizen science data

Massive blooms of pelagic Sargassum algae have caused serious problems to coastal communities and ecosystems throughout the tropical Atlantic, Caribbean Sea, and Gulf of Mexico since 2011. Efforts to monitor and predict these occurrences are challenging owing to the vast area impacted and the complexities associated with the proliferation and movement of Sargassum. Sargassum Inundation Reports (SIRs) were first produced in 2019 to estimate the potential risk to coastlines throughout the Intra-American Sea at weekly intervals at 10 km resolution. SIRs use satellite-based data products to estimate beaching risk from the amount of offshore Sargassum (quantified by a Floating Algal density index). Here we examine whether including wind metrics improves the correspondence between the offshore Floating Algal density index and observations of Sargassum along the coastline. For coastal observations, we quantified the percent coverage of Sargassum in photos obtained from the citizen science project “Sargassum Watch” that collects time-stamped, georeferenced photos at beaches throughout the region. Region-wide analyses indicate that including shoreward wind velocity with SIR risk indices greatly improves the correspondence with coastal observations of Sargassum beaching compared to SIR risk indices alone. Site-specific analyses of photos from southeast Florida, USA, and data from a continuous video monitoring study at Puerto Morelos, Mexico, suggest potential uncertainties in the suite of factors controlling Sargassum beaching. Nonetheless, the inclusion of wind velocity in the SIR algorithm appears to be a promising avenue for improving regional risk indices.Environmental Fluid Mechanic

Hazard assessment and hydrodynamic, morphodynamic, and hydrological response to Hurricane Gamma and Hurricane Delta on the northern Yucatán Peninsula

Barrier islands in tropical regions are prone to coastal flooding and erosion during hurricane events. The Yucatán coast, characterized by karstic geology and the presence of barrier islands, was impacted by Hurricane Gamma and Hurricane Delta in October 2020. Inner shelf, coastal, and inland observations were acquired simultaneously near a coastal community (Sisal, Yucatán) located within 150 km of the hurricanes' tracks. In the study area, Gamma moved slowly and induced heavy rain, mixing in the shelf sea, and strong winds (>20 m s−1). Similar wind and wave conditions were observed during the passage of Hurricane Delta; however, a higher storm surge was measured due to wind setup and the drop (<1000 mbar) in atmospheric pressure. Beach morphology changes, based on GPS measurements conducted before and after the passage of the storms, show alongshore gradients ascribed to the presence of coastal structures and macrophyte wracks on the beach face. Urban flooding occurred mainly on the back barrier associated with heavy inland rain and the coastal aquifer's confinement, preventing rapid infiltration. Two different modeling systems, aimed at providing coastal flooding early warning and coastal hazard assessment, presented difficulties in forecasting the coastal hydrodynamic response during these seaward-traveling events, regardless of the grid resolution, which might be ascribed to a lack of terrestrial processes and uncertainties in the bathymetry and boundary conditions. Compound flooding plays an important role in this region and must be incorporated in future modeling efforts.Environmental Fluid Mechanic