Christopher Daly

Hurricane Ian impacted the Southwest Florida barrier island coast with devastating storm surge flooding. In its wake, numerous ebb surge channels cut across extensive stretches of shoreline on Sanibel Island, while Lover’s Key was overwashed and breached. The process-based model, XBeach, was used to predict the formation of these ebb surge channels and breaching events, and model results were compared to post-storm LiDAR DEMs. Results show that XBeach accurately predicts the number and locations of ebb surge channels; however, the results were limited by the ability to represent highly varied land cover within the model. Results suggest that non-erodible surfaces such as roads and parking lots prevented the extension of ebb surge channels deep inland on Sanibel during the surge. XBeach predicted substantial deflation of Lover’s Key compared to measurements, highlighting the importance of high resolution land cover data and accurate hydrodynamic boundary conditions.

Hurricane Ian made landfall in Southwest Florida, USA, on 28 September 2022 as a Category 4 storm exhibiting many of the characteristics associated with recently intensified storminess (e.g., rapid fueling, slow forward motion, large diameter). The extreme storm surge severely impacted the coastal geomorphology of the barrier islands and mainland beaches, resulting in wholesale destruction and loss of life in many communities. To assess the morphosedimentary impact of the storm, our research included: 1) high-resolution topographic mapping using UAV-flown LiDAR pre- and post-event; 2) post-storm bathymetric surveys using fixed-wing LiDAR flown by U.S. Army Corps of Engineers, and 3) imaging of partially buried surge channel structures using ground-penetrating radar (500/750 MHz GPR) for comparison with pre-Ian conditions. Pre- and post-event digital elevation models were used to assess sediment volume changes and to extract topographic profiles. Incoming storm surge, because of its extreme height, resulted in "overwash and inundation regime" impacts, placing the erosional capacity of wave energy well above the substrate. As a result, incoming surge caused sand transport via sheet overtopping, overwash and overtop fan deposition, and foredune deflation and retrogradation. Ebb-surge return, however, caused channelized erosion principally on the foredune and upper berm, sites that may not necessarily anneal through fairweather aggradation. It also induced substantial damage to infrastructure just behind the coastal setback boundary. Narrower barrier segments that experienced ebb-surge incision from Ian were breached by approximately 1.5-2.0-m-deep scour channels, similar to those imaged by GPR prior to the event, thereby allowing for identification of chronic erosional hot spots. In contrast, strandplains exhibited greatest resilience, with limited overtopping and lack of channels. The qualitative and quantitative observations are being shared with all coastal residents, but particularly with the municipalities of Sanibel, Fort Myers Beach, and Naples, three of the most seriously impacted coastal communities. Ultimately, these databases will inform regional hydrodynamic (Delft3D) and morphodynamic (XBEACH) modeling efforts for anticipating and adapting to future storm impacts.

Exposed to extreme cyclonic episodes and the progressive rise of sea level in relation with climate change, the coastlines of the Caribbean are subject to natural hazards such as coastal erosion and marine inundation. The associated risks relate mainly to the safety of goods and populations, but also to the tourism economy linked to the maintenance of beaches and the natural heritage of these interface environments where biodiversity is particularly rich (mangroves, coral reefs, meadows). The CARIB-COAST project (2018-2022, https://www.carib-coast.com/en/), was set up to develop a modeling pltform for hydrodynamics, to monitor the coastal erosion and mitigation using natural ecosystems so as to assist decision-making, exchange, training and sensitization of Caribbean stakeholders to manage the risk and adaptation. The use of operational tools, the dissemination and availability of the results via a web portal and the actions of training and awarness. All the results of the project are available on the carib-coast website (www.carib-coast.com), including the modeling platform that provides with a insight on all the modelling effort of the project on present-day and future oceanic circulation and climate, tsunami and hurricanes hydrodynamics.

Large-scale coastal bathymetry is paramount to understand natural and human-induced coastal behaviour and plays a vital role in coastal research and governance. Here, we use a recently developed algorithm, S2Shores (Satellite to Shores), to invert coastal bathymetry from wave kinematics, based on the linear dispersion relation. Wave numbers and celerity are extracted from optical Sentinel-2 imagery, by exploiting the small temporal offset between the image bands of its Multi-Spectral Instrument. Inverted depths are output at 200 m resolution, and individual depth points are merged to create a composite bathymetry using a weighted average of images from 10 different dates. The resulting bathymetry mosaic spans 4000 km along the West African coast. S2Shores is able to detect depths up to 35 m, depending on mean incident wave conditions and cloud cover, which varies by location. Underwater features are well reproduced by S2Shores, such as flow channels in Guinea, the St. Ann's Shoal in Sierra Leone, and ebb delta lobes at several outlets along the Niger River Delta. S2Shores results match well (r2 = 0.76, RMSE = 4.9 m) with a bathymetry survey along the Senegalese coast. As a difference with traditional satellite-derived bathymetry methods based on water colour, a wave-based approach allows estimations in turbid areas and relatively deep waters, which suggest that the two approaches are complementary and should be used in combination to cover coastal environments in their diversity. The new possibility offered by this regional coastal atlas opens the door to increased research and planning capabilities and sets an example that can be applied to the rest of the world.
•S2Shores SDB algorithm used to estimate bathymetry from Sentinel-2 MSI image bands.•Depth estimates spanning 4000 km along the West African coast compared to GEBCO.•Maximum detecable depth of 35 m is 2.5 times deeper than colour-based SDB methods.•Shallow features are well reproduced, such as underwater dunes and ebb delta lobes.•S2Shores opens the door to increased research and planning capabilities in the region.

Sea-level rise represents a severe hazard for populations living within low-elevation coastal zones and is already largely affecting coastal communities worldwide. As sea level continues to rise following unabated greenhouse gas emissions, the exposure of coastal communities to inundation and erosion will increase exponentially. These impacts will be further magnified under extreme storm conditions. In this paper, we focus on one of the most valuable coastal real estate markets globally (Palm Beach, FL). We use XBeach, an open-source hydro and morphodynamic model, to assess the impact of a major tropical cyclone (Hurricane Matthew, 2016) under three different sea-level scenarios. The first scenario (modern sea level) serves as a baseline against which other model runs are evaluated. The other two runs use different 2100 sea-level projections, localized to the study site: (i) IPCC RCP 8.5 (0.83 m by 2100) and (ii) same as (i), but including enhanced Antarctic ice loss (1.62 m by 2100). Our results show that the effective doubling of future sea level under heightened Antarctic ice loss amplifies flow velocity and wave height, leading to a 46% increase in eroded beach volume and the overtopping of coastal protection structures. This further exacerbates the vulnerability of coastal properties on the island, leading to significant increases in parcel inundation.

A field of beach cusps formed during a field experiment at Nha Trang Beach, Vietnam, under accretive conditions. The measured data was used to set-up morphodynamic simulations in XBeach, which was able to simulate cusp formation from an initially long-shore uniform beach profile. Several types of simulations were run in order to observe the resulting variation in mean cusp dimensions (length, depth and height), swash flow patterns, and sediment sorting. Both time-constant (JONSWAP) and time-varying (measured) wave forcing conditions were superimposed on the measured tide. In the former, four wave parameters were varied (wave height, period, direction, and spreading), while in the latter, the median sediment size and sediment composition were varied. The wave period was found to primarily influence long-shore length scales, the wave height cross-shore length scales, and obliquely incident waves enhance all these dimensions particularly under narrow-banded conditions. Cusps are not prominent if the wave energy is too low to effect significant onshore transport, if the wave angle of incidence and spreading are too large (effectively smoothing out swash perturbations), or if the sediment is too fine in relation to the wave conditions (dissipative beaches or highly erosive wave conditions). Coarse sediment generally tends to be located on cusp horns above the waterline, but is otherwise variable depending on cross-shore location and tide levels. As the XBeach model results show large agreement with well-established norms, it may therefore be used to more rigorously study processes that help to initiate cusps in future work.
•Beach cusps were simulated based on conditions at Nha Trang Beach, Vietnam.•Cusps form under accretive or mildly erosive reflective wave conditions.•Cusps are prominent under moderate wave heights and long wave periods.•Small angles of incidence/spreading enhance cusp spacing, large angles smooth them out.•Coarser sediment generally tends to be located on the horn of cusps.

A field of beach cusps formed during a field experiment at Nha Trang Beach, Vietnam, under accretive conditions. The measured data was used to set-up morphodynamic simulations in XBeach, which was able to simulate cusp formation from an initially long-shore uniform beach profile. Several types of simulations were run in order to observe the resulting variation in mean cusp dimensions (length, depth and height), swash flow patterns, and sediment sorting. Both time-constant (JONSWAP) and time-varying (measured) wave forcing conditions were superimposed on the measured tide. In the former, four wave parameters were varied (wave height, period, direction, and spreading), while in the latter, the median sediment size and sediment composition were varied. The wave period was found to primarily influence long-shore length scales, the wave height cross-shore length scales, and obliquely incident waves enhance all these dimensions particularly under narrow-banded conditions. Cusps are not prominent if the wave energy is too low to effect significant onshore transport, if the wave angle of incidence and spreading are too large (effectively smoothing out swash perturbations), or if the sediment is too fine in relation to the wave conditions (dissipative beaches or highly erosive wave conditions). Coarse sediment generally tends to be located on cusp horns above the waterline, but is otherwise variable depending on cross shore location and tide levels. As the XBeach model results show large agreement with well-established norms, it may therefore be used to more rigorously study processes that help to initiate cusps in future work. (c) 2021 Elsevier B.V. All rights reserved.

A principal component analysis (PCA) is used to decompose data on the coupled morphodynamics of the shoreline and nearshore sandbar of a typical single‐barred embayed beach (Tairua Beach, New Zealand). Dynamic patterns are classified into simultaneous modes, where the bar and shoreline move at the same time, and nonsimultaneous modes, where the shore moves independently from the bar, and vice versa. Two simultaneous modes accounting for 65% of the variance of the shoreline and barline dominate the system. One mode describes inverse shoreline and sandbar cross‐shore migrations (alongshore averaged), occurring with simultaneous rotations in the same direction. The other mode accounts for migration in the same direction accompanied by variations of the barline curvature (similar to “breathing modes” previously described in embayed beach shoreline modeling studies). Two nonsimultaneous modes of lesser importance account separately for independent shoreline and barline rotations (10 to 15% of the variance explained). A PCA applied to the shore and sandbar behaviors modeled by four standard equilibrium models simulating shore and sandbar cross‐shore migrations and rotations show that these are interrelated because of a correlation between wave energy and direction. Shore and bar rotations are coupled partially because the shape of the bay induces a correlation of their respective drivers, the wave angle of incidence and the alongshore gradient of wave energy. However, this correlation depends on the wave energy. This, in combination with different shore and sandbar response times (quantified using the models), also explains the independent rotations reflected by the nonsimultaneous modes. Key Points Shoreline and bar migrations, mainly simultaneous, dominate the 2‐D variations of morphology Shoreline and bar rotations do not respond to the same environmental drivers but often occur simultaneously Barline breathing occurs with an overall retreat or progradation of the shoreline and barline

Spatial and temporal scales of beach morphodynamics were assessed for the island of Sylt, German Wadden Sea, based on continuous video camera monitoring data from 2011 to 2014 along a 1.3 km stretch of sandy beach. They served to quantify, at this location, the amount of shoreline variability covered by beach monitoring schemes, depending on the time interval and alongshore resolution of the surveys. Correlation methods, used to quantify the alongshore spatial scales of shoreline undulations, were combined with semi-empirical modelling and spectral analyses of shoreline temporal fluctuations. The data demonstrate that an alongshore resolution of 150 m and a monthly survey time interval capture 70% of the kilometre-scale shoreline variability over the 2011-2014 study period. An alongshore spacing of 10 m and a survey time interval of 5 days would be required to monitor 95% variance of the shoreline temporal fluctuations with steps of 5% changes in variance over space. Although monitoring strategies such as land or airborne surveying are reliable methods of data collection, video camera deployment remains the cheapest technique providing the high spatiotemporal resolution required to monitor subkilometre-scale morphodynamic processes involving, for example, small- to middle-sized beach nourishment.

The location of a shore-parallel nearshore sandbar derived from 7 years of video imagery data at the single-barred embayed Tairua Beach (NZ) is investigated to assess the contribution of barline rotation to the overall morphodynamics of sandbars in embayed environments and to characterize the process of rotation in relation to external conditions. Rotation induces cross-shore barline variations at the embayment extremities on the order of magnitude of those induced by alongshore uniform cross-shore migration of the bar. Two semiempirical models have been developed to relate the barline cross-shore migration and rotation to external wave forcing conditions. The rotation model is directly derived from the cross-shore migration model. Therefore, its formulation advocates for a primary role of cross-shore processes in the rotation of sandbars at embayed beaches. The orientation evolves toward an equilibrium angle directly related to the alongshore wave energy gradient due to two different mechanisms. Either the bar extremities migrate in opposite directions with no overall cross-shore bar migration (pivotal rotation) or the rotation relates to an overall migration of the barline which is not uniform along the beach (migration-driven rotation). Migration and rotation characteristic response times are similar, ranging from 10 to 30 days for mild and energetic wave conditions and above 200 days during very calm conditions or when the bar is located far offshore. Abstract Copyright (2016), . American Geophysical Union. All Rights Reserved.