Average Overtopping Rate Research Articles

Hydrodynamic Loads and Overtopping Processes of a Coastal Seawall under the Coupled Impact of Extreme Waves and Wind

Driven by strong winds, huge ocean waves can cause devastating destruction to coastal regions during harsh weather events. There is growing evidence showing that extreme waves can occur in both shallow and deep waters. To protect the coast against the destructive power of huge waves, coastal protection facilities, such as seawalls, are often built along the coast. The integrity and stability of these coastal protection facilities are essential to the safety of coastal regions. Since huge waves are often accompanied by strong winds in real ocean environments, to fill the knowledge gap left by previous relevant studies, this study numerically investigates the hydrodynamic loads and overtopping of a coastal seawall model on a sloped beach under the coupled impact of an extreme wave group and wind. The influences of several main factors are considered, such as water depth, wind speed, and significant wave height. The research results reveal that strong wind can greatly increase the average overtopping rate and enhance the hydrodynamic loads exerted by the extreme wave group on the seawall.

Numerical simulation of waves overtopping over impermeable sloped seadikes

In this study, a numerical model of white water overtopping of regular waves over an impermeable seadike is developed using the Volume of Fluid (VOF) method based on Reynolds-Averaged Navier-Stokes (RANS). The numerical model is favored due to the challenges in experimentally quantifying the hydrodynamic features of overtopping flow, particularly at the beginning of the crest, as well as the uncertainty in various design formulae estimating wave overtopping rates. Such uncertainties are caused by insufficient wave height statistics in irregular wave trains, which may be addressed by taking into account overtopping rates of regular wave tests as well as robust statistical distributions of wave heights and periods. For this purpose, first, by ensuring the numerical model's validity, effective hydraulic and structural parameters on regular waves overtopping are investigated and their dimensionless forms are determined using the dimensional analysis method. Based upon the developed model and in-depth estimation of dimensionless parameters' effect on overtopped layer thickness, the overtopping velocity at the beginning of the crest, and the average overtopping rate, the wave breaker type is determined to be highly effective at reducing overtopping parameters. By analyzing the numerical model run results, formulae are proposed to predict the layer thickness, the wave overtopping velocity at the beginning of the crest, and the average wave overtopping rate independent of the flow characteristics on the slope seaward. Comparing the results of the proposed formulae with other researchers highlights the appositeness and reliability of the numerical simulation and the proposed formulae. Using the obtained formulae of layer thickness and overtopping velocity, along with consideration of sliding, toppling, and floating criteria, appropriate criteria are proposed to control the overtopping safety for pedestrians and vehicles. Moreover, compared to the average overtopping rate, the overtopping velocity and layer thickness parameters are found to be more appropriate in investigating the overtopping risk for pedestrians and vehicles.

SPATIAL DISTRIBUTION OF WAVE-BY-WAVE OVERTOPPING AT VERTICAL SEAWALLS

Over the years, many physical and numerical modelling research has been carried out to investigate the wave-structure interactions and the resulting mean overtopping characteristics at sea defences. The most reliable empirical predication formulae for prediction of mean overtopping rates have been reported in the overtopping manual, EurOtop (2018). In addition to average overtopping rates, in recent years, the spatial distribution of overtopped water has become an important topic of research to understand the safe zone behind coastal defences. The existing empirical formulae for spatial distribution of overtopping provide conservative predictions, as it has been derived from the mean overtopping volumes. The extreme wave overtopping hazards in generally originate from individual overtopping events rather than the mean overtopping volumes. This study presents comprehensive laboratory investigations on the spatial distribution of wave-by-wave overtopping at vertical seawalls.

A new average wave overtopping prediction formula with improved accuracy for smooth steep low-crested structures

Wave overtopping is a key process in coastal protection and its assessment defines the design of the sea defence structures. An existing knowledge gap in wave overtopping prediction is identified for steep low-crested structures, i.e., structures with steep slopes up to the limit case of vertical structures, with small relative freeboards down to the limit case of zero freeboards. This type of structure is increasingly relevant in a sea level rise context due to climate change. Additionally, steep low-crested structures are also of interest when used as overtopping wave energy converters. To cover the identified knowledge gap, more than 900 2D hydraulic model tests have been performed in the wave flume of the Department of Civil Engineering at Ghent University. Wave conditions and the overtopping performance have been measured. After analysing the average overtopping rates of the new tests, we found that there is a lack of accuracy of the recommended EurOtop 2018 manual overtopping prediction formulae for steep low-crested structures. Based on the new tests, a new average overtopping prediction formula for steep low-crested structures is obtained. This formula improves the prediction accuracy of the average overtopping rates for steep low-crested structures with respect to the recommended predictions in the EurOtop 2018 manual by reducing the RMSE by 35% for zero freeboards, by 16% for very small relative freeboards, by 31% for very steep slopes and by 24% for vertical structures. The accuracy of the EurOtop 2018 manual predictions for other structural types is maintained.

INFLUENCE OF CROWN-WALL ON WAVE OVERTOPPING PROBABILITY AND PROBABILITY DISTRIBUTION OVER A SEAWALL

In the past decades, the crest height of the seawall was determined by the allowable average wave overtopping rate q. Franco (1994) concluded that individual wave overtopping，particularly the maximum individual volume, provided a better design method than q, considering the stability of the seawall and the safety of vehicles and people along the coastal area protected by the coastal defense structures. Shanghai, located on the west bank of the Pacific Ocean, is quite sensitive to the risk of storm surges and violent wave overtopping. The crown-wall serves as an engineering measure to reduce the wave overtopping effectively.

A Critical Analysis and Validation of the Accuracy of Wave Overtopping Prediction Formulae for OWECs

The development of wave energy devices is growing in recent years. One type of device is the overtopping wave energy converter (OWEC), for which the knowledge of the wave overtopping rates is a basic and crucial aspect in their design. In particular, the most interesting range to study is for OWECs with steep slopes to vertical walls, and with very small freeboards and zero freeboards where the overtopping rate is maximized, and which can be generalized as steep low-crested structures. Recently, wave overtopping prediction formulae have been published for this type of structures, although their accuracy has not been fully assessed, as the overtopping data available in this range is scarce. We performed a critical analysis of the overtopping prediction formulae for steep low-crested structures and the validation of the accuracy of these formulae, based on new overtopping data for steep low-crested structures obtained at Ghent University. This paper summarizes the existing knowledge about average wave overtopping, describes the physical model tests performed, analyses the results and compares them to existing prediction formulae. The new dataset extends the wave overtopping data towards vertical walls and zero freeboard structures. In general, the new dataset validated the more recent overtopping formulae focused on steep slopes with small freeboards, although the formulae are underpredicting the average overtopping rates for very small and zero relative crest freeboards.

EXPERIMENTAL STUDY ON THE OVERTOPPING BEHAVIOUR OF STEEP SLOPES - TRANSITION BETWEEN MILD SLOPES AND VERTICAL WALLS

Extensive knowledge is available on the overtopping behaviour of traditional smooth impermeable sea defence structures, such as mildly sloping dikes and vertical walls, both typically featuring a high crest freeboard to reduce wave overtopping. A particular design application emerges in the development of wave energy converters of the overtopping type, where maximisation of wave overtopping is required, i.e. smooth impermeable steep sloping structures with low crest freeboards subjected to non-breaking waves. To date, only relatively limited knowledge is available on the overtopping behaviour of those structures. In this study, the average overtopping rate obtained from new experiments has been analysed and compared with existing prediction methods. This study contributes to a better knowledge on the overtopping behaviour of the steep low-crested structures, which is positioned in between that of mildly sloping dikes and vertical walls on the one hand, and in between that of structures with zero crest freeboards and relatively large crest freeboards on the other hand. The existing prediction methods seem unable to predict the significant effects of the slope angle and small relative crest freeboards on the average overtopping rate accurately. Therefore, a new set of prediction formulae is proposed based on the new experiments combined with existing data from literature. These formulae include wave overtopping at vertical walls subjected to non-impacting waves and at structures with zero crest freeboard.

Wave Overtopping at Smooth Impermeable Steep Slopes with Low Crest Freeboards

AbstractExtensive knowledge is available on the overtopping behavior of traditional smooth impermeable sea defense structures, such as mildly sloping dikes and vertical walls, both typically featuring a high crest freeboard to reduce wave overtopping. A particular design application emerges in the development of wave energy converters of the overtopping type, where maximization of wave overtopping is required, i.e., smooth impermeable steep sloping structures with low crest freeboards subjected to nonbreaking waves. To date, relatively limited knowledge is available on the overtopping behavior of those structures. In this study, the average overtopping rate obtained from new experiments has been analyzed and compared with existing prediction methods. This study contributes to a better knowledge on the overtopping behavior of the steep low-crested structures, which is positioned in between that of mildly sloping dikes and vertical walls and that of structures with zero crest freeboards and relatively large...

Layer thicknesses and velocities of wave overtopping flow at seadikes

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.

Full-scale wave-overtopping measurements on the Zeebrugge rubble mound breakwater

Field measurements of wave overtopping over a rubble mound breakwater have been carried out at Zeebrugge, Belgium, during the period 1999–2003. The overtopping volumes of water are collected in an overtopping tank located on the breakwater crest behind the access road. A short side of the tank is made of a weir, allowing the collected water to flow out continuously, and allowing measurements to be made for long duration. Calculation of the inflow discharge is based on the outflow discharge over the weir and the volume in the tank. This calculation requires an accurate measurement of the water depth inside the tank. The water depth is measured indirectly using a pressure sensor near the bottom. The measurement setup for wave overtopping at the breakwater is described in detail. Eight-wave records with wave propagation normal to the breakwater have been used in the analysis of the field data. Wave heights range between 2.6 and 3.9 m (i.e., up to 63% of the design wave height) for wave records with a typical duration of 1–2 h. The measured average overtopping rate q has been determined for all records using three methods ( q<1 l/s m). Average overtopping rates from the field data have been compared with predicted values from prediction formulae by van der Meer et al. [van der Meer, J.W., Tönjes, P., de Waal, H., 1998. A code for dike height design and examination. In: Proceedings International Conference on Coastlines, Structures and Breakwaters. Institution of Civil Engineers, London, Thomas Telford, London, pp. 5–19], Owen [Owen, M.W., 1980. Design of seawalls allowing for wave overtopping. Hydraulics Research, Wallingford, Report No. EX 924, UK], and Besley [Besley, P., 1999. Wave overtopping of Seawalls. Design and Assessment Manual. Hydraulics Research Wallingford. R&D Technical Report W178, ISBN 1 85705 069 X]. Three crest freeboard values have been used in the analysis. The calculated reduction factor for surface roughness used in the formulae is 0.51. Best agreement is observed for van der Meer et al.'s and Owen modified by Besley's prediction formulae using the actual average crest level. In this case, measured average overtopping rates are within the 95% confidence intervals of these prediction formulae.

Numerical Model of Wave Run-Up, Overtopping, and Regeneration

A numerical model of wave run-up, overtopping, and regeneration is presented. The model (called OTT) is based on the one-dimensional nonlinear shallow water equations on a sloping bed, including the effects of bed shear stress. These equation are solved using a finite-volume technique incorporating a Roe-type Riemann solver. The main advantage of this approach over previously used finite difference solvers is that no special shoreline-tracking algorithm is required, so that noncontiguous flows can easily be simulated. Hence, this model can be used to simulate the transmission of waves over water surface-piercing obstacles. The numerical scheme and boundary conditions are described, and several existing data sets used to test the ability of the model to simulate wave transformation, run-up, and overtopping. Experiments of random wave (unimodal and bimodal) overtopping, presented here for the first time, indicate that the model performs much better than empirical formulas in predicting average overtopping rates, and that it provides good estimates for the number of overtopping events. Experiments of overtopping of a sea wall by solitary wave are also presented, including measurements of wave regeneration in lee of the dike. The model does a reasonable job of reproducing the water depths on top of the dike, and performs well in simulating the initial height of the regenerated waves.

Irregular Wave Overtopping of Revetments in Surf Zones

Twelve test runs were conducted to examine the overtopping processes of normally incident irregular waves over a revetment of a 1:2 rough impermeable slope fronted by a gentle, smooth slope and situated well inside the surf zone. The measured overtopping probability and average overtopping rate are affected noticeably by the spectral shape and wave grouping of the incident irregular waves measured immediately outside the surf zone. The empirical procedure for estimating these overtopping quantities is found to yield only order‐of‐magnitude estimates for this experiment. The measurements made for the 12 runs are used to calibrate and evaluate the existing one‐dimensional, time‐dependent numerical model, which is modified slightly to allow the spatial variation of the bottom friction factor. The calibrated numerical model is shown to be fairly capable of predicting the average overtopping rate and the depth of overtopping flow on the crest of the revetment. The measured and predicted time series and spectra of the flow depth including entrained air show the grouped nature of irregular wave overtopping events.

Wave Overtopping on Coastal Structures

Monochromatic wave overtopping over the crest of an impermeable coastal structure located on a sloping beach is predicted numerically by expanding the numerical model developed previously for predicting wave run-up on such a structure located on the horizontal seabed. The expanded numerical model predicts the temporal variations of the velocity and depth of the flow over the crest of the structure from which the average overtopping rate per unit width is computed. The model accounts for the effect of wave shoaling on the sloping beach in front of the structure located in relatively shallow water. The computed average overtopping rates are shown to be in agreement with available small-scale test data for which smooth impermeable structures were fronted by a 1:10 slope. The numerical model also predicts the decrease of wave reflection due to the increase of wave overtopping. However, more detailed measurements will be required to further calibrate and evaluate the numerical model which may be extended to examine the armor stability of overtopped breakwaters.