The influence of infragravity waves on the safety of coastal defences: a case study of the Dutch Wadden Sea

Many coastlines around the world are protected by dikes with shallow foreshores (e.g. salt marshes and mudflats) that attenuate storm waves and are expected to reduce the likelihood and volume of waves overtopping the dikes behind them. However, most of the studies to date that assessed their effectiveness have excluded the influence of infragravity (IG) waves, which often dominate in shallow water. Here, we propose a modular and adaptable framework to estimate the probability of coastal dike failure by overtopping waves (Pf). The influence of IG waves on overtopping is included using an empirical approach, which is first validated against observations made during two recent storms (2015 and 2017). The framework is then applied to compare the Pf values of the dikes along the Dutch Wadden Sea coast with and without the influence of IG waves. Findings show that including IG waves results in 1.1 to 1.6 times higher Pf values, suggesting that safety is overestimated when they are neglected. This increase is attributed to the influence of the IG waves on the design wave period and, to a lesser extent, the wave height at the dike toe. The spatial variation in this effect, observed for the case considered, highlights its dependence on local conditions – with IG waves showing greater influence at locations with larger offshore waves, such as those behind tidal inlets, and shallower water depths. Finally, the change in Pf due to the IG waves varied significantly depending on the empirical wave overtopping model selected, emphasizing the importance of tools developed specifically for shallow foreshore environments.

Climate Change Induced Coastline Change Adjacent to Small Tidal Inlets

The many thousands of small tidal inlets (STIs), and their adjacent coastlines, are almost certain to be affected by climate change in multiple ways, due to their behaviour being closely linked to both oceanic and terrestrial drivers such as riverflow, sea level, and ocean waves, all which are projected to change over the 21st century. Development of risk informed adaptation strategies for these highly utilized and inhabited inlet-interrupted coast zones requires projections of both alongshore average coastline recession and alongshore variability in coastline position along the coast under future forcing conditions, the latter being an aspect that has not received much attention to date. Here, a combination of a process-based morphodynamic model (Delft3D) and the reduced complexity coastline model (SMIC), concurrently forced with tides, waves, riverflows, and sea level rise, is used to investigate both of these phenomena at STI-interrupted coasts. The models are here applied to schematised conditions representing two systems in Sri Lanka, representing two of the three main Types of STIs: Negombo lagoon – permanently open, locationally stable inlet (Type 1), and Kalutara lagoon – permanently open, alongshore migrating inlet (Type 2). Results indicate that, under a high emissions climate scenario following RCP 8.5, by end-century, the coastline adjacent to the Type 1 STI may experience an alongshore average recession as large as 200 m, and that the alongshore variability in coastline position may be up to twice that at present. The Type 2 STI is projected to experience an alongshore average coastline recession of about 120 m, and up to a 75% increase in alongshore variability in coastline position by end-century, relative to the present. Thus, both the alongshore average coastline recession and the increase in the alongshore variability in coastline position are greater at the Type 1 STI, compared to at the Type 2 STI. These findings highlight the importance of accounting for both alongshore average coastline recession and future changes in alongshore variability in coastline position when assessing coastal hazards and risk on inlet-interrupted coasts to adequately inform climate adaptation strategies.

Probabilistic projections of the stability of small tidal inlets at century time scale using a reduced complexity approach

Climate change is widely expected to affect the thousands of small tidal inlets (STIs) dotting the global coastline. To properly inform effective adaptation strategies for the coastal areas in the vicinity of these inlets, it is necessary to know the temporal evolution of inlet stability over climate change time scales (50–100 years). As available numerical models are unable to perform continuous morphodynamic simulations at such time scales, here we develop and pilot a fast, probabilistic, reduced complexity model (RAPSTA – RAPid assessment tool of inlet STAbility) that can also quantify forcing uncertainties. RAPSTA accounts for the key physical processes governing STI stability and for climate change driven variations in system forcing. The model is very fast, providing a 100 year projection in less than 3 seconds. RAPSTA is demonstrated here at 3 STIs, representing the 3 main Types of STIs; Permanently open, locationally stable inlet (Type 1); Permanently open, alongshore migrating inlet (Type 2); Seasonally/Intermittently open, locationally stable inlet (Type 3). Model applications under a high greenhouse gas emissions scenario (RCP 8.5), accounting for forcing uncertainties, show that while the Type 1 STI will not change type over the twenty-first century, the Type 2 inlet may change into a more unstable Type 3 system around mid-century, and the Type 3 STI may change into a less unstable Type 2 system in about 20 years from now, further changing into a stable Type 1 STI around mid-century. These projections underscore the need for future adaptation strategies to remain flexible.

Twenty-first-century projections of shoreline change along inlet-interrupted coastlines

Sandy coastlines adjacent to tidal inlets are highly dynamic and widespread landforms, where large changes are expected due to climatic and anthropogenic influences. To adequately assess these important changes, both oceanic (e.g., sea-level rise) and terrestrial (e.g., fluvial sediment supply) processes that govern the local sediment budget must be considered. Here, we present novel projections of shoreline change adjacent to 41 tidal inlets around the world, using a probabilistic, reduced complexity, system-based model that considers catchment-estuary-coastal systems in a holistic way. Under the RCP 8.5 scenario, retreat dominates (90% of cases) over the twenty-first century, with projections exceeding 100 m of retreat in two-thirds of cases. However, the remaining systems are projected to accrete under the same scenario, reflecting fluvial influence. This diverse range of response compared to earlier methods implies that erosion hazards at inlet-interrupted coasts have been inadequately characterised to date. The methods used here need to be applied widely to support evidence-based coastal adaptation.

Anthropocene Physiography and Morphology of Chilika; India

Introduction: Based on stratigraphy, events, ecology and climatology, the present time is assigned Anthropocene epoch due to dominance Homosapiens over geo- bio-hydro-aero spheres of the mother earth during its accepted Anthropocene epoch succeeding the official 11700 years old Holocene epoch from 1950. Asia’s largest shallow brackish water lagoon, the Chilika housed over about 1000km2, behind 64.3km barrier spit with multiple tidal inlets to Bay of Bengal. It is sinking and shrinking due to rapid dimensional diminution, environmental degradation, sedimentation, salinity depletion, phytoplankton invasion. Present study envisages the elementary morphology, formation mechanism, sediment transport, and dynamic performance of tidal inlets and the lagoon by applying GIS methodology between the year 1930 and 2017 including its local catchment land use changes within the lagoon and associated south Mahanadi delta. Under vulnerability, the brackish water lagoon is constantly deteriorating its ecosystem, it is required for wise use of the wetland that can alleviate the poverty, uplift lagoon users economy, and disallowing the stakeholders living standard during present Anthropocene epoch

A trail of crumbs: ancient geomorphological indicators of past sea-level rise in the Northern Adriatic Sea (Italy)

<p>The reconstruction of timing and modes of the last marine transgression is often hampered by the scarceness of available indicators, which is caused by bad preservation, lack of formation or difficult accessibility.</p><p>This is particularly true for the first period of the Holocene, between 7 ka and 11 ka cal, when the rate of transgression was high (hence little to absent formation of possible indicators) and the sea level was placed below ca. -20 m MSL (hence scarce accessibility).</p><p>Shoreline deposits and erosional landforms have long been recognized as geomorphological indicators of past sea levels. Such indicators (e.g. beach ridges, tidal notches) can be both submerged or exposed due to RSL variations of coastal progradation.</p><p>A major group of potential indicators which, up to date, is largely underrepresented, is constituted by paleo tidal inlets. Being excavated up to several meters below the surrounding lagoon and filled during the migration or deactivation of the inlet, such landforms may represent outstanding archives with a potentially high chance of preservation from erosion. Paleo tidal inlets can be easily recognized and cataloged through shallow sub-bottom profiling methods.</p><p>The analysis of almost 7000 km of high resolution seismic profiles collected in the northern Adriatic Sea allowed to recognized almost 100 paleo tidal inlets dating to the early Holocene, which constitute the only widespread witnesses of the post-LGM marine transgression in the area. Paleo tidal inlets are essential features to the paleo-geographic and -environmental reconstruction and provide new data to constrain the position of the transgressive coastlines. The presence of widespread lagoon environments during a phase of strong RSL rise comes from the interplay between sediment dispersal operated by the main fluvial actors of the area and phases of slowdown of the RSL rise. This study sheds light on the phenomena affecting coastal plains in response to RSL rise and constitutes the first report of an extensive distribution of paleo tidal inlets on a regional scale.</p>

Sediments and bedform mapping of the Lower Saxony Wadden Sea and North Sea (Germany)

<p>Seafloor mapping is the subject of several worldwide research programs dealing with the growing awareness that changes of the marine environmental conditions have to be accurately monitored. The monitoring requirements strongly stimulate the scientific interest in innovative mapping methods and tools, which should be exploitable within the extensive mapping programs carried out by governmental agencies and institutes.</p><p>The Coastal Research Station within the NLWKN is carrying out a long-term program to map subtidal areas of the Lower Saxony coastal and marine waters, adopting a methodological approach aimed to increase objectivity and repeatability of results.</p><p>The study area is one of the world’s largest tidal system encompassing a multitude of transitional zones between land, marine, and estuarine environments. The geological and geomorphological setting is closely related to the Late Quaternary evolution of the North Sea and the actual morphodynamic processes. The seabed is made of Holocene sand to silt deposits and peat layers. They overlay Pleistocene fluvioglacial deposits, made of sands, rocks, and boulders, which locally outcrop in small areas of the North Sea and in the deepest sectors of the Wadden Sea tidal inlets.</p><p>Even though existing maps provide a good broad-scaled representation of the sediments distribution, they were produced by the interpolation of grab-samples therefore lacking of spatial resolution and bedforms characterization. The ongoing mapping program provides full-coverage detailed sedimentological and geomorphological data, by means of swath-bathymetrical systems, subbottom profiler, and validation samples. The methodological approach integrates bathymetric, backscatter, and stratigraphic information to characterize bedforms and substrates. Bathymetry and seabed images are interpreted using geomorphometric as well as object-based image analysis, to increase the objectivity and generate reproducible results.</p><p>Maps outline common sedimentological and geomorphological features across all the observed Wadden Sea tidal inlets, which are made of fine sandy sediments and narrow outcrops of peat layers on the main tidal channels slopes. Both erosive and depositional geomorphological processes are present, represented by several orders of scarps, mainly connected to alternations of hard-substrates and unconsolidated sands, and medium to very large sand waves. Moreover, data reveal high-resolution information about hard-substrate outcrops in the North Sea area.</p><p>The mapping program provides new detailed geological-geomorphological features of a very dynamic coastal area, using repeatable and objective methods. The combination of different datasets and tools allows the quantitative analysis of the complex subtidal morphology, the correlation of bedforms and substrates. Resulting products will be further developed for habitat mapping purposes and morphological and hydro-dynamical modelling.</p>