Coastal Inundation Research Articles

Linking geomorphological and demographic movements: The case of Southern Albania

Albania has experienced one of the Europe's highest population migration rates in recent decades. At the same time, the landscape of this country has very dynamic geomorphological behaviour (erosion and ground subsidence), as an effect of various drivers. This paper examines the correlation between the population migrations and geomorphological dynamics in the wider Vlora area in Southern Albania between 1979 and 2016. In social studies and the humanities this interplay is largely unrepresented, whereas in the environmental sciences these two issues have been conceptualized as having a causal relationship. The article develops a tripartite analytic-synthetic model consisting of (A) remote sensing analysis based on automated land/water detection from satellite data; (B) demographic analysis focusing on population changes using the methods of statistical analysis; and (C) extensive anthropological fieldwork grounded on participant observation, archival work and gathering previously inaccessible statistical and other relevant data. The results of this study show that the coastal erosion or water inundation along with other geomorphological (site-specific relief, place-bound ground subsidence and water bodies distribution) and geotectonic characteristics of the wider Vlora area are inextricably bound up with the population migrations. The paper foregrounds new methodological approaches that correlate coastal or peri-coastal erosion with geomorphological, geotectonic and population movements, and provides a holistic framework that can be applied to examine this issue in similar areas elsewhere.

Breeding sites of a narrowly distributed amphibian, a key element in its conservation in the face of global change

Abstract Global change processes (including climate change, sea‐level rise, and land‐use change, among others) represent significant threats to biodiversity. These changes are usually studied across broad areas, and understanding their effects on narrowly distributed endangered species is challenging. Peltophryne florentinoi is a critically endangered Cuban toad, restricted to two coastal localities and potentially threatened by these phenomena. Despite its endangered status, few analyses have focused on threats to its survival in the face of global change. Current and future threats from global change to this species were evaluated using ecological niche modelling and multiscale geographic information system (GIS) approaches. Climate change effects were assessed by comparing the species niche with six future (2050) climate scenarios across its distribution area. Risks from sea‐level rise were evaluated by simulating potential coastal inundation under three scenarios of sea‐level rise. Recent land‐use changes were quantified using information on the loss of forest cover between 2000 and 2015. Within the species distribution area, warmer and drier conditions can be expected in the future. Sea‐level rise will not have direct impacts on the species range; however, occasional inundations and saltwater intrusion may present additional stressors. Although only ~2% of the forest cover has been lost in the species range and its surroundings during 2000–2015, deforestation trends were variable during this period, and future losses in forest cover can be expected. Considering these threats, and that the colonization of new areas seems unlikely, indirect and additive adverse effects on the species can be anticipated. These impacts will most probably be observed during the larval stage of this toad; hence, breeding sites constitute a key element in the conservation of this species. Long‐term studies and experimental manipulations may be the next steps towards a better understanding of the responses of P. florentinoi to expected changes. Initial conservation actions are proposed based on these results and the ecology of this species.

Modeling wave effects on storm surge and coastal inundation

We present a parametric study of surface wave effects on storm surge and coastal inundation. Hurricane wind forcing terms, atmosphere pressure gradient terms, and radiation open boundary conditions are implemented into an existing quasi-3D nearshore model, NearCoM-TVD, which uses a shock-capturing TVD scheme and can better simulate the wetting-drying process during inundation than a conventional finite difference model. Systematic numerical experiments are carried out in an idealized continental shelf-beach-land system to identify the role of waves in modeling storm surge and inundation under different storm characteristics. Four storm parameters, including storm intensity, storm size, translation speed and incident angle are investigated. Modeling results reveal that the presence of waves can increase the maximum storm surge heights significantly through wave setup, and the contribution of waves varies considerably depending on the storm characteristics. In addition to direct wave forcing, the wave-enhanced bottom stress in the surfzone also promotes higher wave setup and hence higher surge heights. Waves also have a major influence on the maximum inland inundation distance, however, we find that waves do not always favor more inundation as expected. For storms traveling at very slow translation speeds or nearly parallel to the coast, waves surprisingly exert a negative effect on the maximum inundation distance. We argue that wave-enhanced bottom stress combined with longer forcing duration in such storms counteract the positive effect of wave setup, and thus less inundation is predicted in wave-current coupled simulations. Our results highlight the necessity of fully-incorporating wave effects in storm surge and inundation modeling.

A peak-over-threshold approach for the numerical modeling of 26 December 2004 Indian Ocean Tsunami at the Kalpakkam coast, Tamil Nadu, India

ABSTRACT The Boxing Day tsunami of 26 December 2004 wreaked substantial havoc along the east coast of India. Repetition of an event with similar or greater magnitude will undoubtedly affect the coastal regions of India with much more severity. Precise estimation of tsunami wave heights along the coastal belts would be very important for a Risk Based Decision Making (RBDM) system. Kalpakkam located in the east coast of India, a region of strategic national interest is chosen for this study. In this study, the relevant earthquake data was extracted from NOAA-NCEI for the geographical region bounded between 92.96° E, 14.14° N to 105.75° E, 5.89°N covering regions such as Sumatra Fault, Java Trench, and Spreading Ridge in the Andaman Sea. This study examined 89 earthquake events from 1770 to 2013. The preliminary data on significant earthquake parameters viz; moderate damage, death toll exceeding a total of 10, earthquake magnitude of 7.5 or greater, modified Mercalli intensity of 10 or greater, earthquakes that generated a tsunami were subjected to Peak-over-Threshold (POT) analysis using Generalized Pareto Distribution (GPD) for varying thresholds. Thereafter, the study performed computations of five different earthquake magnitudes corresponding to return periods of 10, 102, 103, 104 and 105 years. Using Mansinha deformation model and SWAN code, tsunami simulations were carried out for all the five different scenarios. In addition five different locations were identified to estimate the tsunami heights considering Kalpakkam as the nodal point. With availability of accurate near-shore bathymetry, a more detailed study can be attempted for coastal inundation characteristics arising from future tsunamigenic events.

Development Control And Vulnerable Coastal Lands: Examples Of Australian Practice

ABSTRACTA range of regulatory instruments can be used to modify development-control frameworks for the purposes of adapting urban areas to climate change-induced coastal erosion and inundation. This article investigates the approach of three local governments across Australia. It finds that local governments are modifying development-control frameworks to ensure the appropriate development of vulnerable coastal lands. However, the article also demonstrates the limitations in relying on development control to achieve adaptation objectives such as preserving public beach amenity, and highlights the need for legislative reform or the emergence of incentive-based instruments to complement development control.

Resolving Surface Displacements in Shenzhen of China from Time Series InSAR

Over the past few decades, the coastal city of Shenzhen has been transformed from a small fishing village to a mega city as China’s first Special Economic Zone. The rapid economic development was matched by a sharp increase in the demand for usable land and coastal reclamation has been undertaken to create new land from the sea. However, it has been reported that subsidence occurred in land reclamation area and around subway tunnel area. Subsidence and the additional threat of coastal inundation from sea-level rise highlight the necessity of displacement monitoring in Shenzhen. The time Series InSAR technique is capable of detecting sub-centimeter displacement of the Earth’s surface over large areas. This study uses Envisat, COSMO-SkyMed, and Sentinel-1 datasets to determine the surface movements in Shenzhen from 2004 to 2010 and from 2013 to 2017. Subsidence observed can be attributable to both land reclamation and subway construction. Seasonal displacements are likely to be associated with precipitation. The influence of ocean tidal level changes on seasonal displacement is not strongly evident from the results and requires further investigations. In general, InSAR has proven its ability to provide accurate measurements of ground stability for the city of Shenzhen.

Feature extraction of coastal surface inundation via water index algorithms using multispectral satellite on North Jakarta

The study aims to characterize water index algorithms using Sentinel 2A-MSI and Landsat 8-OLI sensors to generate the performance of CSI and its proper uses in science. Feature extraction of the 35 combinations of CSI specifically uses information deriving from a number of water index spectral bands. Supervised classification is used to categorize water and non-water features using Spectral Angle Mapper (SAM). Furthermore, Dynamic Surface Water Extent (DSWE) algorithm was taken to mapping inundation and non-inundation. Result of this research shows that the best water index algorithms is exhibited by the MNDWI algorithm where it can enhance the water reflectance to be positive value. Re-classification on all water index algorithms indicates a similar pattern to the feature extraction of water and non-water. It is determined by overall accuracy (99.56-99.99%), Kappa coefficient (0.97-1.00), producer accuracy (99.05-99.99%) and user accuracy (93.43-99.94%). Substituting all water index algorithms into DSWE algorithm points out that the MNDWI is still the good performance and other substituted algorithms generate low performances. Overall feature extractions of CSI in North Jakarta at this research manifests that the MNDWI algorithm using multi-spectral and satellites is the best algorithm.

Near real-time coastal flood inundation simulation with uncertainty analysis and GPU acceleration in a web environment

A proof of concept is presented on how to produce uncertainty-aware near real-time coastal flood inundation Web maps from water-level observations and predictions, which have been computed for tide gauge sites and made publicly accessible. The stochastic inundation simulation takes into account several sources of uncertainty, which have until now not been employed in either bathtub models or hydrodynamic models. The simulation is based on the Monte Carlo method. The feasibility of the proposed approach is demonstrated by an implementation using general-purpose computing on graphics processing units. The outcome of the research is that the current technologies enable the building of a novel system that connects to official data sources and that takes into account sources of uncertainty whose inclusion in the past has been avoided by being either weakly known or computationally too expensive.

Assessment of coastal inundation of low lying areas due to sea level rise

Sea level rise due to climate change have a profound impact on low lying coastal zones. The objective of this study is to identify the potential of coastal inundation area due to Sea Level Rise (SLR) along of Cherating to Pekan coast. The shoreline of Pahang has been undergoing severe erosion and inundated by sea water in some locations, hence affects the socio-economy and the livelihood of the coastal communities. Numerical modelling using the MIKE 21 FM software was done to predict coastal inundation for the years 2020 and 2080 along the Cherating to Pekan shoreline by using the condition for the year 2017 as the baseline. The results of the statistical analysis of this numerical model is congruous with the measured data, such as tide, current, and wave direction. The results show that about 17 to 22% of the Cherating to Pekan shoreline is at risk of being inundated due to the projected SLR for the years 2020 and 2080. The map for projected inundation shows that the infrastructure located in the 1km buffers zone from the shoreline will be affected by the sea level rise. This information could be of benefit to all parties involved in ensuring effective coastal management and making preparation and plan to and protect the coastal areas from the potential impacts of climate change and future disasters.

Climate change impacts on critical international transportation assets of Caribbean Small Island Developing States (SIDS): the case of Jamaica and Saint Lucia

This contribution presents an assessment of the potential vulnerabilities to climate variability and change (CV & C) of the critical transportation infrastructure of Caribbean Small Island Developing States (SIDS). It focuses on potential operational disruptions and coastal inundation forced by CV & C on four coastal international airports and four seaports in Jamaica and Saint Lucia which are critical facilitators of international connectivity and socioeconomic development. Impact assessments have been carried out under climatic conditions forced by a 1.5 °C specific warming level (SWL) above pre-industrial levels, as well as for different emission scenarios and time periods in the twenty-first century. Disruptions and increasing costs due to, e.g., more frequent exceedance of high temperature thresholds that could impede transport operations are predicted, even under the 1.5 °C SWL, advocated by the Alliance of Small Island States (AOSIS) and reflected as an aspirational goal in the Paris Climate Agreement. Dynamic modeling of the coastal inundation under different return periods of projected extreme sea levels (ESLs) indicates that the examined airports and seaports will face increasing coastal inundation during the century. Inundation is projected for the airport runways of some of the examined international airports and most of the seaports, even from the 100-year extreme sea level under 1.5 °C SWL. In the absence of effective technical adaptation measures, both operational disruptions and coastal inundation are projected to increasingly affect all examined assets over the course of the century.

Economic impacts of storm surge and the cost-benefit analysis of a coastal spine as the surge mitigation strategy in Houston-Galveston area in the USA

Rapid population growth, urbanization, and concentration of valuable assets and strategic infrastructure in coastal regions make coastal inundation, flooding, and storm surge national problems for many countries, including the United States of America (USA). Enhancing coastal resilience is a complex problem and involves an integrated risk management approach, entailing both structural protection as well as other risk reduction strategies (e.g., building codes and ecosystem preservation). The former is an increasingly recognized mitigation option for densely populated areas and industrial hubs. Fully justifying benefits of costly flood defense structures is crucial, particularly when lack of funding and other institutional barriers make such projects easy targets for omission from or cuts to a budget. Justification usually requires a comprehensive cost-benefit analysis. This paper explores the economic feasibility of a coastal barrier, i.e., coastal spine, as a potential storm surge mitigation strategy to protect the Houston-Galveston metropolitan area of Texas, one of the most flood-prone and economically important regions in the USA. We provide an assessment of residential and chemical manufacturing plant and refinery exposure to multiple synthetic hurricane storm surge events by comparing losses with and without a coastal spine. While under all scenarios, benefits exceed engineering costs of a spine, our results indicate that the project feasibility largely hinges on accounting for industrial losses and resultant indirect and induced effects. As many regions and industrial hubs globally are designing adaptation and mitigation strategies to combat the consequences of extreme events, structural solution to surge mitigation maybe one of the few mitigation options for them. Unlike population and residential structures that can retreat and insure, these options are not viable for industrial plants that are resource-based. However, expertise and knowledge pertinent to surge barrier systems are relatively scarce as there are only handful of barriers around the world and they are all unique in engineering designs. As storm surge is becoming a threat for many coastal urban centers, one of the recommendations is to consolidate knowledge base and research across countries in order to foster knowledge exchange internationally. This will help identify concerns associated with existing barrier systems, pragmatic ways to improve them and will also aid the investment decision, engineering designs, and operational aspects of barriers in other parts of the world. Furthermore, forming regional research collaborations with developing countries at risk of storm surge and the sea level rise is vital to further facilitate knowledge spillover and exchange of expertise.

Geomorphologic changes in the lower Pearl River Delta, 1850–2015, largely due to human activity

We use 165 years of navigational and bathymetric data and 60 years of sediment discharge data to document and explain geomorphic changes in the Pearl River Delta (PRD) and its estuary, which in recent years has experienced rapid urbanization and explosive economic growth. A series of British and Chinese navigation charts dating back to 1850 show that since then the PRD has gained 1160 km2 of new land, most of it reclaimed land derived from disposal of dredge spoils. Accordingly, the PRD estuary has decreased by 35% in ocean area and by 39% in water volume. Changes were most dramatic in the western part of the delta, where the Modaomen and Huangmao seas decreased in ocean area by 62% and 39%, respectively. To the east, upper Lingding Bay (LDB) decreased in ocean area by 36%; in contrast, the lower part of LDB decreased by only 3% in area, although shoaling more by than 2 m, some of which may come via influx of sediment from the South China Sea. Since 1995 the rate of land accretion in the PRD has declined to 40% of what it was over the previous 145 years, and to <30% of what it was the previous 25 years, probably in response to less fluvial input because of river damming and river sand mining, as well as increased use of dredged sediment for construction. With decreased sediment supply and accelerated sea-level rise, the Pearl River delta may well begin to experience land loss and coastal inundation in the not-too-distant future.

Modelling of Storm-induced Seawater Flooding in the Suyeong River Area, South Korea: A Case Study due to the Storm Surge and Waves during Typhoon Sanba

Yuk, J.-H.; Park, J.H., and Joh, M., 2018. Modelling of Storm-induced Seawater Flooding in the Suyeong River Area, South Korea: A Case Study due to the Storm Surge and Waves during Typhoon Sanba. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 746–750. Coconut Creek (Florida), ISSN 0749-0208.The Suyeong River Area (SRA), located in Busan, a major city on the southeastern coast of South Korea, is frequently damaged by storm-induced seawater flooding. One of the main reasons for seawater flooding is the propagation of storm surges and wind waves through the Suyeong River in the SRA. In this study, a series of prediction systems of storm tides, surges, waves and resultant seawater flooding for the SRA based on unstructured meshes with a high resolution of approximately 10 m covering the entire flood plain and a HPC-based parallel tide-surge-wave model were developed. Meteorological forcing variables such as wind and air pressure calculated by the parametric tropical cyclone model based on typhoon information were used in the storm-surge and wave models. The modeled meteorological data were in good agreement with observations of both the wind and air pressure. The simulated surge heights and wave properties showed good agreement with the observations as well. Waves reached the mid-stream area of the SRA, likely affecting the duration of seawater flooding. The prediction system in this study can reasonably simulate tide-surge-wave and resultant seawater flooding (coastal inundation) during extreme conditions, e.g., storms in the SRA of Busan, South Korea.

The Need for Coastal Hazard Prevention and its Valuation Methodologies in Europe

ABSTRACT Malvarez, G.; Navas, F.; Parker, D.J., and Penning-Rowsell, E., 2018. The Need for Coastal Hazard Prevention and its Valuation Methodologies in Europe. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 926–930. Coconut Creek (Florida), ISSN 0749-0208. The artificialisation of the coast has significantly increased the potential for hazard affecting greater populations and still methodologies for valuation of coastal protection are lacking a multidimentisional approach taking into account both hydrometeorological as well as human induced risks in a way that swift decision making can be achieved. Environmental and economic appraisal is needed for managing coastal erosion and inundation as well as oil spills. In depth analyses from the UK, Poland, Italy, Greece and Spain legislative and planning frameworks show that methodologies vary significantly and that...

Flood Damage Assessment in Building Scale Caused by the Coastal Inundation Height at Haeundae Beach, Busan

ABSTRACT Jeon, H.; Eem, S.H., and Park, J., 2018. Flood Damage Assessment in Building Scale Caused by the Coastal Inundation Height at Haeundae Beach, Busan. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1561–1565. Coconut Creek (Florida), ISSN 0749-0208. Damages of coastal areas due to natural disasters such as storms and floods are expected to occur along with rise in sea level owing to climate change. To reduce the flood damage, it is important to identify the expected inundation areas and estimate its loss. Therefore, this study evaluates the assessment of the flood damage in a building scale of Busan around the Haeundae beach in various scenarios. The scenarios vary with the inundation height from the approximate highest high water level announced by the Korea Hydrographic and Oceanographic Agency. The geographic information system (GIS) is established ...

Vulnerability assessment and adaptation to sea level rise in high-wave environments: A case study on O'ahu, Hawai'i

High energy waves intensify the impacts of sea level rise and threaten coastal communities, requiring measures to prevent harmful consequences. Vulnerability assessments using NOAA's assessment guide were performed in a high wave environment, accounting for the impacts of sea level rise on coastal inundation, longshore transport, benthic habitat as well as on beach users and homeowners. This case study of the North Shore region of Oahu, Hawai'i’ uses multi-criteria evaluation and cost-benefit analysis based on vulnerabilities, impacts, and protective actions such as shoreline hardening, beach nourishment and dunes, vegetative cover, and elevation or relocation of structures. Geomorphology and natural habitats dramatically affect coastal exposure in high energy wave environments. The most viable adaptation strategies include expansion of artificial reefs in benthic habitat dominated areas while the vegetative cover and beach nourishment are most appropriate in developed locations and surf tourism areas. Suitable adaptation strategies were determined based on differences in the biophysical environment and community perceptions. The value of this approach lies in both a generalized method for compiling and analyzing large amounts of data as well as customizing the proposed framework to identify crucial elements necessary for effective adaptation to sea level rise in high-energy wave environments.

The use of genetic programming to develop a predictor of swash excursion on sandy beaches

Abstract. We use genetic programming (GP), a type of machine learning (ML) approach, to predict the total and infragravity swash excursion using previously published data sets that have been used extensively in swash prediction studies. Three previously published works with a range of new conditions are added to this data set to extend the range of measured swash conditions. Using this newly compiled data set we demonstrate that a ML approach can reduce the prediction errors compared to well-established parameterizations and therefore it may improve coastal hazards assessment (e.g. coastal inundation). Predictors obtained using GP can also be physically sound and replicate the functionality and dependencies of previous published formulas. Overall, we show that ML techniques are capable of both improving predictability (compared to classical regression approaches) and providing physical insight into coastal processes.

High resolution topobathymetry using a Pleiades-1 triplet: Moorea Island in 3D

Worldwide the coastal land-sea interface is increasingly subject to natural and anthropogenic hazards. Monitoring this crucial interface may be addressed with satellite imagery as a cost-efficient mapping solution. Topography and bathymetry, defining the structural complexity of the coast, are commonly studied separately given specific thematic and methodological contexts, yet many science questions and societal challenges require an integrated approach (e.g., coastal inundation). In such cases, triplet multispectral imagery represents an affordable solution based on a single satellite product. Here we examine how Pleiades-1 triplet imagery may be used to retrieve a seamless and accurate topobathymetry digital surface model (DSM, from −20 to 1207 m) over an entire, socio-ecologically complex island: Moorea, French Polynesia. Creation of the topography DSMs was based on stereo and tri-stereo photogrammetry, and the bathymetry DSMs relied on quasi-nadiral multispectral data subject to light/water interaction modelling. Results were compared with over 3.9 million airborne LiDAR topobathymetry measurements to quantify how the spatio-spectral mode, the third imagery, and the level of radiometric correction act on two- and three-dimensional accuracy. Horizontal accuracy of the tri-stereo panchromatic dataset (ERMSE = 4.22 and NRMSE = 7.59 m) was better than the multispectral (ERMSE = 5.06 and NRMSE = 7.51 m) dataset. Topography point cloud density increased by a factor of three in the tri-stereo panchromatic (9.11 points/m2) or multispectral (0.23 points/m2) datasets. Topography vertical accuracy enhanced with tri-stereo + level-1 radiometric correction (RMSE = 7.40 m) for panchromatic and with stereo + level-1/-2 radiometric correction (RMSE = 7.77/7.77 m) as well as tri-stereo without radiometric correction (RMSE = 7.85 m) for multispectral datasets. Bathymetry vertical accuracy improved with level-2 (atmospheric) correction (RMSE = 0.83 m). Topobathymetry DSM derived from this optimized method has a wide spectrum of applications along coastal margins.

Appraising Variations in Climate Change Parameters Along the Lower West African Region

Narrow minded skeptics in the twenty first century deny the existence of climate change and the resultant risks threatening the welfare of cities through the destruction of properties, displacement and environmental degradation. The global climate change concern is now more important than ever in the sub Saharan African region of West Africa where the vulnerabilities remain obvious. Accordingly, in the last several years, the lower West African region continues to be an epicenter of inclement weather hazards. For that, there exists mounting evidence of risks based on recurrent challenges triggered by surge in extreme climate parameters in the savannahs, the tropical moist areas and the coastal landscapes in Nigeria, Benin, Togo, Ghana, Ivory Coast and Senegal. In these areas of the West Africa region, climate change parameters such as droughts, flooding, storm surge, and elevated temperature and greenhouse gas emissions continue to threaten the surrounding ecology. Just as climate hazards have captured the attention of various scholars in other areas, very little has been done to study the area despite its vulnerability to sea level rise, torrential rainfalls, coastal inundation and storm surge. In all these, the projected climate change trends point to continual dispersion of elevated temperature and evidence of harsh weather conditions all through the region. With the gravity of the hazards now obvious, this research fills that void in the literature, by analyzing climate change parameters threatening the study area with the use of mix scale methods of GIS, secondary data, and descriptive statistics with focus on the trends, factors, impacts and efforts of the institutions. Taking a cue from the analysis, the results show the prevalence of climate change risks and impacts in the form of flooding hazards, droughts, greenhouse gas emission, sea level rise, pollution, environmental degradation and displacement of citizens. With the impacts attributed to socio-economic and physical elements, the GIS mapping reveals a gradual spreading of climate change hazards under different scales to various areas across the region from Nigeria to Senegal. To remedy the issue, the paper proffers some suggestions such as the need for adaptation, the design of efficient planning and policy in the region.

Geological record of marine tsunami backwash: The role of the hydraulic jump

AbstractTsunamis are marked by distinct phases of uprush during coastal inundation and backwash when tsunami water recedes. Especially in the case of a steep coastal profile, the return flow may operate in a Froude‐supercritical regime, eroding the flooded area and transporting large volumes of sediment seawards. Important sediment accumulation occurs when the supercritical flow goes through a hydraulic jump where it becomes subcritical upon deceleration. An inferred example in coarse‐grained, mixed carbonates from the Lower Pleistocene on Rhodes (Greece) is described, with offshore bars up to 10 m long with scour‐and‐fill structures and steep antidune stratification. In finer‐grained sandy depositional systems such structures may be much longer, up to hundreds of metres. It is suggested here that, analogous to some turbidite beds, the apparent lack of structures or the presence of faint stratification that is common for graded sand layers within marine tsunamiites may in fact consist of extremely low‐angle, landward‐dipping backset‐strata that formed under a landward‐migrating hydraulic jump during the basinward retreat of tsunami water. Numerical simulations that focus on the internal stratification of backwash‐generated offshore bars support this hypothesis. The recognition of such deposits in the sedimentary record enlarges the toolbox for assessing the past frequency of tsunamis in coastal areas.