The geomorphic significance of hurricanes on coral-fringed calcium carbonate coastlines: Hurricane Wilma 15-25 October 2005, northeastern Yucatan Peninsula, Mexico

<p>Hurricanes and tropical storms can cause large scale morphological changes to barrier beach systems in tropical environments. Many such systems are fronted by coral reefs; however, unlike siliciclastic barrier beaches, little is known about the significance of hurricanes to barrier beach evolution on coral-fringed calcium carbonate coastlines. This study provides a detailed assessment of the impacts of Hurricane Wilma, a major hurricane, on the reef-protected and exposed barrier beaches of northeastern Yucatan Peninsula, Mexico. The study considers both the short (0-8 months) and medium term (8-56 months) response, and postulates the significance of major storm events over the longer term. Hurricane Wilma made landfall in late October 2005 as a Category 4 hurricane, bringing sustained wind speeds of 67 ms-1, and storm waves with significant wave heights (HS) ≈ 13 m. The storm persisted for over 20 hours, while storm waves inundated the low lying barrier beaches and rainfall flooded inland wetlands and lagoons. To determine the impacts of Hurricane Wilma and quantify post-storm recovery of reef-protected and unprotected barrier beaches, geomorphic mapping and post-storm surveying (2006 and 2010) was completed at 49 locations between Punta Nizuc and Punta Maroma. In addition, 220 sediment samples were collected from across barrier beaches and the backreef lagoon for textural and petrographic analysis. Satellite imagery was also used to quantify immediate storm impacts and recovery of the shoreline. Barrier beaches were found to have responded to storm waves in two broadly different ways: reef-protected beaches accreted by between 2.1 and 24.6 m, as the beach and foredunes were reworked. In contrast, unprotected beaches underwent erosion of over 10 m. By 2006, reef-protected beaches had undergone rapid shoreface and beachface adjustment. Over the next four years, these beaches gradually transgressed landwards and aggraded subaerially as they readjusted to their pre-storm equilibrium beach profile. Exposed beaches responded much more rapidly than those protected by reefs, with shoreline adjustment occurring within eight months of the storm. Subaerial beach development was, however, much slower, requiring extended calm conditions to infill the eroded beach. The storm and post storm geomorphic responses were found to be highly variable alongshore, and influenced by several factors, including dune height, beach width, and wave exposure. The results indicate that under the contemporary climatic conditions hurricanes are key drivers of barrier beach evolution over the short (0-8 months) to medium terms (8-56 months), but are not so influential over longer time scales. However, an expected increase in the number of major storms (category 3-5) in the future may increase the significance of hurricanes to longer term barrier evolution, with the storm impacts likely to be greater and the recovery times longer. Understanding these responses is particularly critical as many areas continue to be developed, and as the coral reef protecting the coastline becomes threatened by the implications of climatic change.</p>

The geomorphic significance of hurricanes on coral-fringed calcium carbonate coastlines: Hurricane Wilma 15-25 October 2005, northeastern Yucatan Peninsula, Mexico

<p>Hurricanes and tropical storms can cause large scale morphological changes to barrier beach systems in tropical environments. Many such systems are fronted by coral reefs; however, unlike siliciclastic barrier beaches, little is known about the significance of hurricanes to barrier beach evolution on coral-fringed calcium carbonate coastlines. This study provides a detailed assessment of the impacts of Hurricane Wilma, a major hurricane, on the reef-protected and exposed barrier beaches of northeastern Yucatan Peninsula, Mexico. The study considers both the short (0-8 months) and medium term (8-56 months) response, and postulates the significance of major storm events over the longer term. Hurricane Wilma made landfall in late October 2005 as a Category 4 hurricane, bringing sustained wind speeds of 67 ms-1, and storm waves with significant wave heights (HS) ≈ 13 m. The storm persisted for over 20 hours, while storm waves inundated the low lying barrier beaches and rainfall flooded inland wetlands and lagoons. To determine the impacts of Hurricane Wilma and quantify post-storm recovery of reef-protected and unprotected barrier beaches, geomorphic mapping and post-storm surveying (2006 and 2010) was completed at 49 locations between Punta Nizuc and Punta Maroma. In addition, 220 sediment samples were collected from across barrier beaches and the backreef lagoon for textural and petrographic analysis. Satellite imagery was also used to quantify immediate storm impacts and recovery of the shoreline. Barrier beaches were found to have responded to storm waves in two broadly different ways: reef-protected beaches accreted by between 2.1 and 24.6 m, as the beach and foredunes were reworked. In contrast, unprotected beaches underwent erosion of over 10 m. By 2006, reef-protected beaches had undergone rapid shoreface and beachface adjustment. Over the next four years, these beaches gradually transgressed landwards and aggraded subaerially as they readjusted to their pre-storm equilibrium beach profile. Exposed beaches responded much more rapidly than those protected by reefs, with shoreline adjustment occurring within eight months of the storm. Subaerial beach development was, however, much slower, requiring extended calm conditions to infill the eroded beach. The storm and post storm geomorphic responses were found to be highly variable alongshore, and influenced by several factors, including dune height, beach width, and wave exposure. The results indicate that under the contemporary climatic conditions hurricanes are key drivers of barrier beach evolution over the short (0-8 months) to medium terms (8-56 months), but are not so influential over longer time scales. However, an expected increase in the number of major storms (category 3-5) in the future may increase the significance of hurricanes to longer term barrier evolution, with the storm impacts likely to be greater and the recovery times longer. Understanding these responses is particularly critical as many areas continue to be developed, and as the coral reef protecting the coastline becomes threatened by the implications of climatic change.</p>

Destabilisation and Accelerated Roll-Back of a Mixed Sediment Barrier in Response to a Managed Breach

Sea level rise increases the pressure on many coastlines to retreat landwards which will lead to coastlines previously held in position through management, being allowed to retreat where this is no longer affordable or sustainable. Barrier beaches have historically rolled back in response to different hydrodynamic events and sea level rise, but very little is known as to how quickly and how far roll-back is going to occur once management has ceased. Data from more than 40 topographical surveys collected over 7 years along the 1.5 km long, almost swash-aligned shingle barrier at Medmerry (southern England) are used together with hydrodynamic data in a wide-ranging assessment of barrier roll-back. This study shows that roll-back is progressing through time along the barrier in downdrift direction in response to a gradual reduction in cross-sectional area through longshore transport. The Barrier Inertia concept provides a practical means to assess stability/instability for events experienced, but also a tool to assess the short- to medium term risk to the coast downdrift of the immediate study area where flood risk still needs to be managed. Roll-back is influenced particularly by the creation of an artificial tidal breach and removal of its sediment, the elevation of the underlying marsh and clay sediments, the number and severity of storms experienced and the presence of legacy groynes; roll-back has exceeded modelled predictions and expert judgement by an order of magnitude.

Geometrical Analysis of the Inland Topography to Assess the Likely Response of Wave-Dominated Coastline to Sea Level: Application to Great Britain

The need for quantitative assessments at a large spatial scale (103 km) and over time horizons of the order 101 to 102 years have been reinforced by the 2019 Special Report on the Ocean and Cryosphere in a Changing Climate, which concluded that adaptation to a sea-level rise will be needed no matter what emission scenario is followed. Here, we used a simple geometrical analysis of the backshore topography to assess the likely response of any wave-dominated coastline to a sea-level rise, and we applied it along the entire Great Britain (GB) coastline, which is ca. 17,820 km long. We illustrated how the backshore geometry can be linked to the shoreline response (rate of change and net response: erosion or accretion) to a sea-level rise by using a generalized shoreline Exner equation, which includes the effect of the backshore slope and differences in sediment fractions within the nearshore. To apply this to the whole of GB, we developed an automated delineation approach to extract the main geometrical attributes. Our analysis suggests that 71% of the coast of GB is best described as gentle coast, including estuarine coastline or open coasts where back-barrier beaches can form. The remaining 39% is best described as cliff-type coastlines, for which the majority (57%) of the backshore slope values are negative, suggesting that a non-equilibrium trajectory will most likely be followed as a response to a rise in sea level. For the remaining 43% of the cliffed coast, we have provided regional statistics showing where the potential sinks and sources of sediment are likely to be.

IMPLICATIONS OF CONSOLIDATION ON BARRIER BEACH STABILITY

Barrier beaches often overlie backbarrier deposits composed of poorly consolidated sediments. Hence, they can consolidate significantly if loaded. A retreating barrier beach provides such a load. In the static situation of beach nourishment, the increased load of the raised beach volume will also cause increased consolidation. These can lower beach elevation promoting wave overtopping, overwashing and retreat. However, there is limited research concerning the role of consolidation on the stability of barrier beaches worldwide. This paper focuses on this issue using Hurst Spit on the UK south coast as a study site where consolidation is a known significant process (Nicholls, 1985; Burt et al., 2018). It is a storm beach composed of shingle (pebble and cobble) sediments and formerly retreated at 2 to 3 m/yr, Since the later 1990s it has been more stabilized by a major nourishment (Bradbury and Kidd, 1998), but continues to retreat slowly (Figure 1). A second nourishment phase is now being actively assessed following major damage in the large storm of 14 February 2014. In this context, the role of consolidation has been analyzed via new data collection, consolidation modelling and morphodynamic modelling. This paper presents these results and their implications.