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Landward displacements of coastal boulders are geomorphological signatures of sea flooding and erosion processes. In this study, using open-access resources that do not require the integration of specialist software, the 2017 to 2021 annual mobility of medium, coarse, and very coarse boulders spread over about 100 km of the eastern coast of the Gulf of Taranto (Italy, Mediterranean Sea) was explored. The boulder displacement data obtained from remote sensing imagery were verified and refined by means of geomorphological field investigation. The main results are the following: (1) A large interannual variability in the boulder mobility was found; (2) storm Detlef, which crossed over the Mediterranean during 11–13 November 2019, was recognized as the cause of a massive displacement phenomenon; and (3) the marine weather conditions driving the investigated morphodynamic process were inferred. [ABSTRACT FROM AUTHOR]

Laboratory experiments were conducted on the breaching of homogeneous non-cohesive sandy fluvial dikes induced by flow overtopping. Tests were conducted using a main channel, an erodible lateral dike and a floodplain. The main channel width and Froude number prior to overtopping were systematically varied. Breach discharge was deduced from water level measurements and mass conservation. High-resolution 3D reconstructions of the evolving breach geometry were obtained using a non-intrusive laser profilometry technique. The main channel width and Froude number show significant influence on the breach expansion and hydrograph. Breach hydrographs are divided into three types, depending on the Froude number and a non-dimensional main channel width. An adapted fluvial dike breaching model based on the concept of "effective breach width" is proposed. Using the laboratory data, the computed breach discharge is found extremely satisfactory, although the breach downstream expansion is not accurately reproduced by the model. [ABSTRACT FROM AUTHOR]

Coastal embankments often collapse due to the tremendous destructive energy of an overtopping tsunami flow due to a deep scour by nappe flow. Hence, to clarify the nappe flow formation condition due to the overtopping, a series of tests were carried out within a laboratory flume with immobile settings by lowering the downstream surface angle of an embankment model while keeping the upstream surface slope constant (1:1) with five non-dimensional overtopping depths and six different crest conditions. The conditions imposed on the embankment crest in the flow direction were without vegetation; horizontal crest, (−)4% descending crest slope, (+)4% ascending crest slope, and adding vegetation model with three different densities across the horizontal crest to improve resistance to the flow. The increased resistance provided by the vegetation models were categorized based on the spacing ratio between cylinders to diameter: sparse, intermediate, and dense. Increased vegetation density above the crest results in a significant reduction of flow energy by approximately 30–50% at the downstream brink edge and 40–60% at the downstream plunge basin. In contrast, the maximum energy reduction was found to be by the dense vegetation model. Additionally, owing to the steep slope of the water surface profile and the increasing vegetation density, the impinging jet's impact point moved closer to the toe of an embankment. This implies that vegetation covers a smaller area while increasing density to mitigate the destructive intensity of flood/tsunami movement. Meanwhile, the descending crest scenario results in a faster nappe flow formation. In contrast, the ascending crest scenario delays the nappe formation while reducing the downstream slope angle. It maintains the sub-critical flow at the crest, except near the downstream brink edge. [ABSTRACT FROM AUTHOR]

Flow overtopping can lead to the initiation of breaching and failure of fluvial dikes, causing severe inundations and damage in the protected areas. For flood risk management and prevention, the accurate estimate of flow discharge across the fluvial dike breach is paramount, requiring precise understanding of the breach expansion. Laboratory experiments were conducted to analyse the effects of fine sand, inducing apparent cohesion in the dike material, on breach development and outflow. Tests were conducted under controlled inflow discharge and dike material composed of either homogeneous non-cohesive coarse sand or heterogeneous fine sand/coarse sand mixtures. Based on the non-intrusive laser profilometry technique, high temporal and spatial resolution of the three-dimensional breach geometry evolution was measured, indicating a small effect of the fine material on the overall breach dynamics. A detailed analysis revealed, however, that fine sand induces less frequent slope collapses but larger sliding/failing lumps compared to homogenous non-cohesive coarse sand. [ABSTRACT FROM AUTHOR]

Dike failures due to overtopping frequently occur during floods, causing serious damage to settlements and the environment. For risk management, it is of great importance to understand the mechanisms of dike failure and predict this process accurately. In this study, a three-dimensional numerical model is developed to simulate both overtopping and seepage flows during dike breaching. The governing equations describing the free water flow are extended and applied to simulate also the filtration through porous media. A non-equilibrium sediment transport model is utilized for simulation of the bed deformation. The numerical model is firstly applied to flows over a solid embankment in order to validate the flow part of the model, and then the dike breaching is simulated such that the bed deformation model is also incorporated. By comparison with the experimental results, it is shown that the developed model reproduces overflow and the dike failure processes reasonably well. [ABSTRACT FROM AUTHOR]

Rock riprap is commonly used to protect levees, embankment dam, steep channels, and other structures being vulnerable to deteriorative erosion caused by overtopping flow. A review of the literature in this context indicates that over 24 riprap design expressions exist to predict the stone size in the riprap layer. However, each equation was originally derived on the basis of limited data sets under certain experimental conditions. In this investigation, Gene-Expression Programming (GEP), Model Tree (MT), and Evolutionary Polynomial Regression (EPR) were evaluated to estimate riprap stone sizing by virtue of 102 experimental data sets. Efficiency and performance of GEP, MT, and EPR approaches for training and testing stages were analysed and discussed. Results analysis revealed that the EPR technique provided an accurate prediction of riprap sizing in testing stage compared with other selected data-driven models as well as the empirical equations. The robustness of the developed data-driven predictive techniques was verified through the external validation: the selected data-driven models are definitely valid, have the strong capability to predict the riprap stone size which was not achieved by a chance. The prediction uncertainties of the data-driven models were quantified and compared with the selected empirical equations. [ABSTRACT FROM AUTHOR]

Lateral berms are often constructed to protect highway bridges against debris flows in mountainous regions. Currently, different solutions for lateral berm design are debated. The lack of standardization results in the improper design of lateral berms, limiting the mitigating effect. In this paper, a monitoring case of the mitigating effect of a lateral berm is introduced. The lateral berm was constructed through a bridge culvert at the alluvial fan of a debris-flow gully. In September 2008, a debris flow in this gully completely buried the lateral berm. The proposed numerical integral method was used to back analyse the flowing velocity and mud depth in berm. Results supported the speculation that abrupt decreases in mud depth and flow velocity in the lateral berm caused deposits that compressed the effective berm depth and resulted in overtopping flow. Therefore, we suggested reducing the berm width in order to increase the flow velocity in the berm. In June 2011, another debris flow with a smaller magnitude occurred in the gully, and the reconstructed lateral berm reportedly performed well to protect the bridge of Yalu highway. The case studies highlighted that berm width should be one of the main considerations in the berm design. [ABSTRACT FROM AUTHOR]

In order to study the destructive mechanism of superstandard storm surge caused by typhoon, we studied the overtopping rate and overtopping flow of the sloped seadike with parapet under superstandard tide and wave through physical experiments. The empirical formulas between the relative crest ultra-height, relative wall-top ultra-height, relative parapet height and the overtopping rate were established based on experimental data of regular waves. In addition, the appropriate dimensionless parameter was proposed for the research on the relation between overtopping flow thickness and overtopping rate. The proper relative position parameter was also defined to study the overtopping flow thickness on the dike crest. [ABSTRACT FROM AUTHOR]

Wave overtopping can be a great threat to the pedestrians visiting the coastal structure's crest area, but a good tool for risk assessment is lacking. In this study, a quantitative risk assessment framework was developed, which is based on considering the overtopping impact force acting on a pedestrian's body. The centerpiece of the framework is a model for the likelihood of overtopping accident, which is defined as being mobilized by overtopping flow at least once during a visit. The largest single wave of a given sea state at the structure's toe is first evaluated, and the maximum overtopping force of this largest wave is then calculated using a force predictor (assuming non-breaking waves before the structure), in which the human body is approximated as a cylinder. A probabilistic instability model, which was calibrated based on real-human experiments in flood flows, was developed in this study. This model converts an instability number, i.e., the ratio of maximum impact force to ground friction, into the probability of instability, so the likelihood of overtopping accident can be quantified. By applying our model to an adult standing on a 1-on-3 sloped revetment, it was found that the model gives thresholds of tolerable overtopping conditions (in terms of discharge rate and maximum overtopping volume) very close to those suggested in EurOtop (2018). A risk assessment framework, which quantifies risk as the sum of likelihood and severity, was proposed. The merit of this framework is that it can account for different pedestrian's body size, location of the pedestrian, structure's slope and etc. As a demonstration, the risk assessment framework was applied to Singapore's coastal structures. Limitations of this framework are given at the end of the paper. [ABSTRACT FROM AUTHOR]

The objectives of the current study are to develop a two-phase flow solver and to investigate the two-dimensional regular wave overtopping flow field over a sea dike. The solver is based on the Navier-Stokes equations and the classical Smagorinsky eddy-viscosity model. A coupled approach of wave generation and absorption is accomplished in the numerical wave flume, and the wave reflection from the work zone can be eliminated efficiently. Simulations of the wave run-up and breaking were performed to validate the effectiveness of the numerical wave flume. The influences of turbulent eddy viscosity and grid refinement on the simulations of the breaking waves are discussed. The flow features of wave overtopping over a sea dike are investigated systematically for both nonbreaking and breaking waves. The numerical results show that the layer thicknesses along the dike crest decay following an exponential rule, whereas the maximum velocities along the dike crest exhibit a mild increasing trend. Furthermore, a relationship between the overtopping discharges and the maximum velocities on the dike crest is obtained by regression analysis. With this correlation, the maximum overtopping velocity can be estimated from the overtopping discharge, which provides a practical approach for the risk assessment of a sea dike. [ABSTRACT FROM AUTHOR]

Rock riprap is one of the most widely used erosion control methods for protecting embankments, levees, spillways, and instream structures subjected to overtopping flow conditions. At least 21 stone-sizing relationships exist to determine the median stone size of a protective riprap layer based on the results of 96 overtopping, laboratory experiments. Test parameters include median stone size, slope, unit discharge, coefficient of uniformity, and riprap layer thickness. A regression analysis was performed relating the observed median stone size to the predicted median stone size to each of the 21 relationships, yielding a coefficient of determination ( R2) and percent error for the full spectrum of data. Zonal (partial spectrum of rock sizes) and complexity analyses were also conducted for each relationship. It was resolved that the Khan and Ahmad, and Chang relationships best aligned with the composite dataset. The predictive expressions by Olivier, Hartung and Scheuerlein, Knauss, Maynord, Abt and Johnson, and Siebel yield a noteworthy second tier of stone-sizing relationships for overtopping conditions. [ABSTRACT FROM AUTHOR]

The tsunami caused by the Great East Japan Earthquake on 11 March 2011, with a magnitude of 9.0, broke most of the sea embankment and coastal vegetation belt and caused catastrophic damage to people and buildings in the Tohoku and Kanto regions of Japan. A field survey was conducted to elucidate the damage to river embankments and their hinterlands (residential area) by tsunami propagation in river channels and overtopping of embankments. Two, three, and four rivers in Iwate Pref., Miyagi Pref., and the Kanto region, respectively, were selected for the field investigation. In the hinterlands, the tsunami came from two directions, coast and river, and the situation, including the evacuation of people, became complex. Therefore, it is necessary to identify locations of river embankments that can be easily overtopped by a tsunami in different tsunami conditions. Tsunami inundation patterns were classified by the river capacity and whether a river or sea embankment was breached or not. This will provide useful information for making new hazard maps and planning new cities. [ABSTRACT FROM AUTHOR]

Super Typhoon Herb caused tremendous damages on the coastal defense structures and severe flooding near and at the National Taiwan Ocean University (NTOU) on the north coast of Taiwan. This article examined nearby structure failures, coastal waves, and water level conditions during this typhoon. It was found that the damaged sections of the coastal structures were related to coastal bathymetry, the severe seas, and great overtopping flows. A wave gauge, which was located 17 km east of NTOU and 30 km from the track of the typhoon center, recorded sea conditions at a mean depth of 11.5 m. The data showed that the recorded highest significant wave height during the storm was 7.8 m, which occurred during the passage of the typhoon center. The maximum wave height at that time was about 10.5 m, close to the breaking limit. It was also found that the time evolution of the wave conditions, including energy spectra, correlated well with the movement of the storm. Right after the typhoon center passed, the wave height quickly died down, much faster than the build-up rate of waves before the center passed. Water level analysis indicated that, historically, the highest storm surge at least in the past 40 years and high wave setups had increased the water level by more than 2 m in the shallow water. Together with high waves and over 5 hours of large overtopping flow rates (greater than 0.02 m³/s/m), superstructures of seawalls and scoured lee-side armor units of breakwaters were broken. [ABSTRACT FROM AUTHOR]

Abstract: Seadikes often fail due to wave overtopping and a failure of the landward slope. Therefore, these aspects have to be taken into account for the design of seadikes. In present design, the calculation of the crest height of seadikes is essentially based on using a design water level and the corresponding wave run-up height. An average overtopping rate is generally considered for wave overtopping which can not account for the stresses and other effects due to extreme individual overtopping events. Landward slope design is more or less based on experience. It can be concluded from failure analysis that dike failures on the landward slope are rather initiated by individual overtopping events, in particular by the related overtopping flow velocities and layer thicknesses which are relevant for the prediction of erosion, infiltration and slip failure. Therefore, overtopping flow velocities and layer thicknesses are required in addition to average overtopping rates as hydraulic boundary conditions for the geotechnical stability analysis of seadikes. The objective of the present paper is the theoretical and experimental determination of overtopping flow velocities and layer thicknesses on the seaward slope, the dike crest and the landward slope of a seadike. Overtopping parameters are derived on the basis of small scale model tests which are required for the design of the landward slope and to avoid dike failures by wave overtopping in the future. For the prediction of the layer thicknesses and the velocities of the overtopping flow on the seaward slope, the dike crest and the landward slope, a set of theoretical formulas is derived and validated by hydraulic scale model tests. [Copyright &y& Elsevier]

As a countermeasure against tsunami inundation, the present study conducted a series of laboratory experiments using a compound mitigation system in which a seaward embankment (E) followed by landward coastal vegetation (V) over a mound (M) (EMV) was investigated in supercritical flow conditions. The changes of flow around the mitigation system and energy reduction were clarified under varying conditions of mound height and vegetation density. Cases of an embankment followed by only a mound (EMNV) were also considered for comparison. Experimental results showed that three basic types of flow structures were observed within the mitigation system in EMV cases. A water cushion was created within the mitigation system mainly due to the combined effects of the mound and vegetation. It significantly reduced the maximum total energy in EMV cases by approximately 41–66%, whereas in EMNV cases, the maximum energy reduction was found to be 23–65%. Increments in both mound height and vegetation density increased the intensity of the water cushion within the mitigation system by offering more drag and reflecting the flow, and hence, significantly reduced the energy of the flow. [ABSTRACT FROM AUTHOR]

Extensive coastal engineering activities including channel dredging and constructions of large infrastructure have greatly altered hydrodynamics in the Changjiang Estuary. While the changes in longitudinal circulation have been well-documented, limited studies have focused on the alteration in lateral circulation and its impact on estuarine circulation in the Changjiang Estuary. Lateral circulation in an estuary plays an important role in modulating estuarine circulation, sediment transport, and morphological change. Different from many of previous numerical studies, we utilize an unstructured-grid numerical model configured in a way to allow inundation over dikes and tidal flats. Numerical model experiments show significant changes in lateral circulation and stratification along the North Passage in response to the altered geometry and bathymetry due to the channel deepening and the dike construction. Both the channel deepening and the dike construction led to an enhanced stratification. Further analysis suggests that the along-channel, the lateral, and the vertical (including up- and downwelling and vertical mixing) effects are all important in modulating the intra-tidal stratification-destratification process. The channel deepening enhances both the lateral and the vertical effects around the high-water slack. The construction of dikes that completely blocks the lateral water exchange between the North Passage and surrounding tidal flats, changes the system from a multiple-channel system into a single-channel system. With dike construction, the estuarine circulation tends to have a horizontally sheared pattern with weaker friction and a stronger stratification. • An unstructured grid model with the cross-scale capability and allowing inundation over dikes and tidal flats was applied. • A dramatic change of lateral circulation occurs in response to the construction of along-channel dikes. • The lateral effect is one of the key mechanisms in modulating the stratification process during tidal cycles. • The blocking of cross-channel exchange shifts the estuary from a laterally shear dominant to horizontally shear dominant. [ABSTRACT FROM AUTHOR]

Rock riprap is one of the most widely used erosion control techniques for protecting embankments, levees, spillways, and in-stream structures subjected to overtopping flow conditions. The literature indicates that over 21 riprap design expressions exist to appropriately size the stone. However, each of these relationships was developed from limited databases with narrow bands of boundary conditions. A power regression analysis was performed using 102 overtopping observations from 10 separate investigations; the analysis related median stone size to slope, unit discharge, coefficient of uniformity, rock layer thickness, and stone specific gravity. A unique, riprap-sizing expression was developed with a coefficient of determination () of 0.97 and a variance of 11%. The resulting design relationship expands the boundary conditions of the independent variables beyond earlier efforts, thereby reducing the need for extrapolating previous relationships. [ABSTRACT FROM AUTHOR]