Vertical Seawall Research Articles

Intertidal seawall communities in Singapore: Are they all one and the same?

Ecological rehabilitation of seawalls requires a good understanding of the underlying community structure, which is still poorly known in the tropics where coastal urbanization is rapidly taking place. In this study, we compared intertidal sessile biota on sloping (20°) and vertical (90°) seawalls on a small, reclaimed offshore island in the Singapore Strait and examined the pattern of heterogeneity of sessile assemblages within each seawall type. Cover of sessile organisms, abundance of hard coral species as well as their growth forms across two tidal height ranges in the low littoral region (−0.2 to +0.3 m Chart Datum (CD) and + 0.3 to +0.8 m CD) were assessed. Seawalls had high biotic cover (mean cover of 91 ± 15%), comprising a variety of sessile organisms ranging from algae and non-colonial organisms including oysters and barnacles to colonial organisms such as cnidarians and sponges. Turf algae comprising coralline and non-coralline rhodophytes dominated the intertidal zone below +0.8 m CD. A moderate negative correlation was detected between crustose coralline and non-coralline algae (Spearman coefficient = −0.56). Coralline algae were relatively conspicuous on several vertical seawall sites. Seawall type was a significant factor differentiating sessile cover composition (p < 0.05) even though within-type site variability was significant (p < 0.005). Coral species diversity and growth form abundance were also significantly differentiated by seawall type (respectively p < 0.05). Tidal height (+0.3 m CD) was another significant factor for sessile cover composition and growth form abundance (p < 0.05). In general, sloping seawalls showed higher within-site variability and less obvious tidal stratification of sessile cover composition compared to vertical seawalls. In contrast, vertical seawalls showed lower within-site variability but higher site-to-site variation, as well as significant tidal stratification of coral growth form. These differences in community structure on apparently uniform seawall surfaces suggest the need for nuanced approaches to rehabilitate and enhance biodiversity in the intertidal zone.

Numerical Analysis of Wind Effect on Wave Overtopping on a Vertical Seawall

Onshore wind significantly affects wave run-up and overtopping, thereby representing a major variable to account for in the design process. The aim of this study is to analyze the ability of numerical models to properly reproduce the wind effect on the overtopping at vertical seawall and to use them to understand how the wind influences the overtopping process as well. We use the RANS model, FLOW-3D, and the NLSW model, SWASH; both model the action of wind through the shear stress that it exerts on the sea surface. Although a simplified modelling of wind has been adopted, the CFD model has led to physically consistent results. On the other hand, SWASH seems to be unsuitable for reproducing the enhancement of the overtopping rate due to wind. CFD numerical results show that the wind affects only the lower overtopping regime (i.e., q &lt; 1 l/s/m); as the mean overtopping discharge decreases, the influence of wind increases. Specifically, wind plays a key role in pure “white overtopping”, where the advection of spray is the predominant mechanism of the overtopping process. The effects of wind thus seem to be important in the design of seawalls, but are difficult to quantify through hydraulic models because of the unknown scaling laws. Therefore, we have found that a full-scale numerical approach is suitable to study the influence of wind on wave overtopping and to evaluate the wind factor to take into account in the design process.

Local overshoot and wind effects on wave overtopping at vertical coastal structures

Wind effects on wave overtopping over a fully impermeable vertical sea wall were studied numerically using the open-source computational fluid dynamics library OpenFoam. A pressure gradient correction term was incorporated in the momentum equations. In recent studies, it was found that, in the absence of wind, an increase in wave steepness results in a reduction of wave overtopping. This is related to the instability of the standing wave formed at the front of a vertical structure. Such instability was noticed in the range of steepness 0.285–0.443 from previous physical experiments for a regular wave interacting with a vertical structure. The existence of this regime was confirmed in the current study. It was also found that the stability of the standing wave determines the shape and volume of the overshooting jet, which has a strong effect on wave overtopping. When the wind is relatively weak (e.g. a wind speed of 10 m/s) it is unable to alter the overshooting jet very much, meaning a weak wind effect on wave overtopping. When the wind is strong (e.g. a wind speed of 30 m/s) it completely deforms the overshooting jet resulting in overtopping discharge almost three times that without wind.

Formula of Overtopping Discharge of Vertical Seawall with A Form Similar to Solitary Wave Function

Formula of Overtopping Discharge of Vertical Seawall with A Form Similar to Solitary Wave Function

New insights in the probability distributions of wave-by-wave overtopping volumes at vertical breakwaters

Advances in the development of prediction tools for wave overtopping allow now for overtopping volumes to be estimated with good accuracy, with the combined use of mean overtopping rates and maximum wave by wave overtopping volumes in a sequence of wave overtopping events. While previous literature has tended to focus on mean overtopping rates at coastal structures, limited studies have investigated the wave by wave overtopping volumes at coastal sea defences; in particular, a paucity of studies have focussed on the prediction of the shape parameter in the Weibull distribution (i.e., Weibull b) of overtopping volumes. This study provides new insights on the probability distribution of individual wave overtopping volumes at plain vertical seawalls by analysing the measured Weibull b values derived from a series of laboratory experiments on seawalls performed on a wide range of wave conditions and crest freeboards. The influence of wave conditions (wave steepness, significant wave height), structural parameters (crest freeboard, toe water depth), impulsiveness, probability of overtopping waves, and overtopping discharge on Weibull b parameter were examined, and then compared with the well-established empirical formulae. For the conditions covered within this study, it was found that the probability distribution of wave-by-wave overtopping volumes follow a 2-parameter Weibull distribution. No apparent differences in Weibull b values were reported with the variation of incident wave steepness and impulsiveness parameter. Results of this study revealed that Weibull b values at vertical walls, subjected to non-impulsive wave conditions, can be predicted reasonably well using relative freeboard and relative overtopping rates. A new unified formula is proposed for the estimation of Weibull b values at vertical walls under impulsive and non-impulsive wave attack.

Numerical modelling of breaking wave impact loads on a vertical seawall retrofitted with different geometrical configurations of recurve parapets

Abstract Experiments are the traditional techniques used in coastal engineering to study complex wave structure interactions. However, with the advent of high-performance computing, even performing 1:1 scale numerical simulations has become a reality. The progress aids in extending the parametric investigation or repeating the procedure for comparable structures. In this study, a numerical model in OpenFOAM® with waves2Foam wave boundary conditions is used to simulate wave structure interactions at seawalls with varied geometrical configurations of recurved parapets. The numerical model is validated by employing ForschungsZentrum Küste (FZK)'s large-scale (1:1) experiments. The validated model is then applied to the plain parapet and vertical wall to understand better overtopping behaviour, pressure distribution, and structural loads. Numerical modelling is used in this study to visualise and assess intrinsic parameters such as the velocity profile, vorticity, air entrapment, and entrainment better to understand the dissipation characteristics of seawalls with recurved parapets.

The Impact of Eco-retrofitting on Coastal Resilience Enhancement – A Physical Modelling Study

Recent climate change studies highlight that the sea-level rise and increase in intensity and frequency of extreme climatic events and storm surges will result in catastrophic wave overtopping events from coastal defences. Retrofitting of the existing seawalls provides great potentials for enhancement of the climatic resilience in coastal region through overtopping attenuation. With increasing attention towards sustainable and low emission solutions for improving the resilience of critical infrastructures to natural hazards, providing coastal protection service is no longer the only concern of scientists, but the environmental impacts of such interventions also started to be considered. This paper presents a laboratory-scale investigation of ‘eco-retrofitting’ approaches including vertipools and reef breakwater for their impact on mitigating overtopping from seawall. The laboratory tests were conducted on a vertical seawall with 1(V):20(H) smooth foreshore. Each test was consisted of approximately 1000 pseudo-random waves based on JONSWAP spectrum. Both impulsive and non-impulsive wave conditions were tested. The plain vertical seawall was taken as the reference case, that exhibited an overall good agreement with empirical predictions, when compared to EurOtop. The analysis of data highlights the significance of the tested eco-interventions in mitigating wave overtopping volume, with approximately 70% reduction of mean the overtopping rate.

Can port concrete infrastructures be optimized to promote algal and macrofaunal colonisation in the marine intertidal zone? Case study of Port Haliguen (Brittany, France)

Various studies highlight the efficiency of working on the nature of port infrastructures (materials, structure design for water retention and creation of microhabitats) in order to improve ecological functions in and around port areas. The aim of this work which was carried out at Port Haliguen (Brittany, France) was to study the colonisation of different types of concrete formulations, including 3D-printed concrete, assumed as more favourable for sessile marine life due to their intrinsic characteristics (surface pH, roughness, porosity). During the first year, no differences were shown between the settlement of algae and macrozoobenthos on each of the formulations regardless of the water level. Percentages of coverage were extremely high after only 5 months and mean diversities were not different. However, the finer grain porous concrete showed a great potential to promote the settlement of the engineer species Fucus vesiculosus on vertical seawalls. The 3D-printed concrete formulation appears to date to be compatible with marine life colonisation. The advantages of this new technologies to create complex habitats could be of great interest applied to the integration of large scale eco-engineered strategies in urbanized shoreline areas such as ports in Brittany.

Overtopping Scour due to Tsunami Bore: Laboratory Study

Tsunami wave is one of the most serious threats to lives and activities, particularly in coastal cities within 100 m height from the mean sea level. Construction of coastal defense structures, such as seawalls and breakwaters, is the most popular measure adopted by engineers for protection from sea waves, including tsunamis, and mitigation of their effects. Many researchers determined through their post-tsunamis field studies that the scour at the landward toe of coastal defense structures induced by tsunami overtopping is the predominant cause of structural failures. İn the present study, a series of laboratory experiments was conducted at National Hydraulic Research Institute of Malaysia to investigate the scour profile at the landward toe of a vertical seawall induced by tsunami bore overtopping vertical seawall and examine the influence of breakwater and bore characteristics on the induced scour depth and length. The experiments showed that the breakwater in low initial depth conditions was more effective in reducing bore velocity and scour depth at the landward toe of the seawall than that in high initial depth conditions. Moreover, a strong positive relationship was confirmed between the Froude number of the overtopping flow and the induced scour depth and length.&#x0D; HIGHLIGHTS&#x0D; &#x0D; The most common cause of the coastal structural failure is scour at the landward toe of coastal defense structures caused by tsunami overtopping&#x0D; A series of laboratory tests were carried out to explore the scour profile at the landward toe of a vertical seawall caused by a tsunami bore overtopping it, as well as the impact of breakwater and bore features on the induced scour depth and length&#x0D; The Froude number of the overtopping flow and the resultant scour depth and length were shown to have a strong positive relationship&#x0D;

Tsunami-like Flow-Induced Forces on the Landward Structure behind a Vertical Seawall with and without Recurve Using OpenFOAM

It is more common to introduce the parapet/recurve/wave return wall over the existing structure, such as a vertical seawall or composite structure, to reduce the overtopping efficiently. The advantage of a recurve wall on top of the sea wall has been studied in the past in regards to wave interaction and overtopping. However, their efficiency in protecting the inland structure during extreme events such as flooding during a tsunami is unexplored. The present study addresses the effect of a vertical seawall with recurve in reducing the dam break surge simulating tsunami-induced forces on an inland structure. The study compares the momentum transferred on the landward structure behind a Vertical seaWall (VW) and a vertical wall with the Large ReCurve on the top (LRC) during overtopped conditions. The outcome from the numerical simulation shows an insignificant contribution due to the LRC in reducing the force on the inland structure compared to the VW, albeit delaying the impact time. However, the LRC performed slightly better in the case of a low-rise wall located near the inland structure than the VW. Furthermore, a low-rise VW increases the force and overturning moment on the inland structure compared to no-wall conditions. Both the LRC and the VW reduced the horizontal force on the structure linearly with the increase in height. An exponential decrease in the overturning moment was observed on the landward structure with the increase in the height of the VW or the LRC. Design equations are proposed for the forces and overturning moment reduction based on the height of the VW or the LRC.

Adaptation measures for seawalls to withstand sea-level rise

Sea level rise necessitates adaptation measures for coastal protection structures like seawalls as changes in the design conditions will generate higher wave overtopping discharges and coastal flooding. Although increasing crest height is a common measure, the recreational function of urban seawalls limits the applicability. In this paper, performance on overtopping control of crest modifications such as storm walls, parapets, promenade, and stilling wave basin (SWB), are studied for simple and composite vertical seawalls. Two independent physical model studies from Turkey and Italy that cover a wide range of hydrodynamic conditions focusing on low relative freeboard are presented. Reduction factors that can be integrated into EurOtop prediction formulae (2018) are proposed within the experiment boundaries. The results show that a simple promenade, extending landward of a vertical seawall, provides very little reduction, whereas a seaward storm wall, under low freeboard conditions, is not effective as a similar storm wall once located on the landward edge of the promenade. Parapets decrease the overtopping further, however, the increase in relative freeboard influences the effect of parapets. Basin width and storm wall heights are important design parameters for SWB. Although the performance of different SWB configurations converges to lower reduction factors as the relative freeboard decreases, they perform better overall. Further analysis showed that the multiplication of the two individual reduction factors, one for the parapet effects and one for the promenade effects could provide an accurate representation of the composite reduction factor to determine the total effect. However, for complex geometries, it is seen that the composite reduction factors should reflect the interdependency of components when different elements with different mechanisms that change the overtopping discharge exist such as an overtopping bore on the promenade overtopping a storm wall. However, for developing future design guidelines, it is also important to consider the influence of individual components on the composite reduction factors such as the influence of storm wall height for a storm wall at the end of a promenade.

Benefits of vegetation for mitigating wave impacts on vertical seawalls

This study quantifies the benefits of vegetation for reducing wave loads on seawalls and the probability of failure due to sliding and overturning. A computational fluid dynamic model is used to quantify wave forces and moments on a laboratory-scale vertical seawall in the presence and absence of vegetation. The model is first calibrated using existing laboratory measurements and then implemented to simulate wave impacts under 200 various wave scenarios. The simulated forces are analyzed to calculate the sliding force and overturning moment and quantify the benefit of vegetation for reducing the seawall failure incidents and increasing the factor of safety. Under the laboratory-scale conditions considered in this study, vegetation causes a reduction of up to 89 percent in the wave force, leading to a substantial decrease in sliding and overturning failure scenarios and increase in the scenarios with acceptable factors of safety. It is found that a smaller seawall combined with vegetation results in an overturning and sliding performance, in terms of failure and factor of safety, similar to the performance of a larger seawall without vegetation. The results suggest that vegetation has a potential to protect coastal structures and allow engineers to design efficient hybrid nature-based and engineering defenses.

Breaking Solitary Wave Impact on a Vertical Seawall

Wave interactions with vertical and sloping seawalls are indeed complicated, especially for the impacts due to breaking waves, which are unsteady, turbulent and multi-phase. Available studies successfully measured the impact pressure due to waves acting on seawalls, whereas the associated flow velocity and turbulence characteristic received limited attention, indicating that the momentum of such violent free-surface flows cannot be determined. In this study, new experiments were carried out in a laboratory-scale wave flume using a non-intrusive image-based measuring technique (bubble image velocimetry, BIV) to measure the flow velocities due to a shoaling solitary wave impinging on and overtopping a vertical seawall. By varying the wave height of solitary waves, the breaking point of a shoaling wave can be changed. As such, the impact point of a breaking wave in relation to the seawall can be thereby adjusted. Considering the same still-water depth, two wave height conditions are studied so as to produce different levels of aerated flows. Effects of high- and low-aerated cases on free surface elevations, flow velocities and turbulence characteristics are presented so as to develop a better understanding of wave-structure interactions. More specifically, the maximum velocities and turbulence intensities at different evolutionary phases are identified for these two cases.

Two-phase experimental and numerical studies on scouring at the toe of vertical seawall

This paper presents experimental and numerical modeling of scouring erosion at the toe due to wave impact on a highly reflective vertical seawall. For the experimental study, new experiments and analyses are provided in the light of advanced measurement techniques. They show the complex nature of the triple wave–structure–sediment interactions with their non-linearities, asymmetries, and hysteresis. For the numerical study, a two-dimensional x/z two-phase Eulerian model is introduced using a new fluid–solid particle interaction. The numerical model is also based on a projection technique, σ-transformation, and a second-order hybrid finite-difference and finite-volume method. A good agreement is obtained between the numerical and experimental results. Both models show that the wave run-up or down on vertical wall conducts to the formation of ascending or impinging vertical jets, which leads to the seawall scour.

Unstructured Finite-Volume Model of Sediment Scouring Due to Wave Impact on Vertical Seawalls

The numerical modeling of sediment transport under wave impact is challenging because of the complex nature of the triple wave–structure–sediment interaction. This study presents three-dimensional numerical modeling of sediment scouring due to non-breaking wave impact on a vertical seawall. The Navier–Stokes–Exner equations are approximated to calculate the full evolution of flow fields and morphodynamic responses. The bed erosion model is based on the van Rijn formulation with a mass-conservative sand-slide algorithm. The numerical solution is obtained by using a projection method and a fully implicit second-order unstructured finite-volume method in a σ-coordinate computational domain. This coordinate system is employed to accurately represent the free-surface elevation and sediment/water interface evolution. Experimental results of the velocity field, surface wave motion, and scour hole formation hole are used to compare and demonstrate the proposed numerical method’s capabilities to model the seawall scour.

Wave Impact Loads on Vertical Seawalls: Effects of the Geometrical Properties of Recurve Retrofitting

This study investigates the variation of wave impact loads with the geometrical configurations of recurve retrofits mounted on the crest of a vertical seawall. Physical model tests were undertaken in a wave flume at the University of Warwick to investigate the effects of the geometrical properties of recurve on the pressure distribution, overall force, and overturning moment at the seawall, subject to both impulsive and non-impulsive waves. Additionally, the wave impact and quasi-static loads on the recurve portion of the retrofitted seawalls are investigated to understand the role of retrofitting on the structural integrity of the vertical seawall. Detailed analysis of laboratory measurements is conducted to understand the effects of overhang length and height of the recurve wall on the wave loading. It is found that the increase in both recurve height and overhang length lead to the increase of horizontal impact force at an average ratio of 1.15 and 1.1 times larger the reference case of a plain vertical wall for the tested configurations. The results also show that the geometrical shape changes in recurve retrofits, increasing the overturning moment enacted by the wave impact force. A relatively significant increase in wave loading (both impact and quasi-static loads) are observed for the higher recurve retrofits, while changes in the overturning moment are limited for the retrofits with longer overhang length. The data generated from the physical modelling measurements presented in this study will be particularly helpful for a range of relevant stakeholders, including coastal engineers, infrastructure designers, and the local authorities in coastal regions. The results of this study can also enable scientists to design and develop robust decision support tools to evaluate the performance of vertical seawalls with recurve retrofitting.

Wave Overtopping Rate on Vertical Seawall with Deformed Wave Dissipating Work

Araki, S.; Kotake, Y.; Kubota, S.-I.; Shibutani, Y.; Kawauchi, Y., and Yamamoto, F., 2021. Wave overtopping rate on vertical seawall with deformed wave dissipating work. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 176–180. Coconut Creek (Florida), ISSN 0749-0208. This paper describes the variation in wave overtopping rate with deformation of wave dissipating work. Slight deformation of wave dissipating work is expected to cause no significant increase in wave overtopping rate. A series of hydraulic experiment was conducted to measure the wave overtopping rate at a vertical seawall with deformed wave dissipating work. The experiment shows that the deformation of wave dissipating work does not necessarily cause a drastic increase in the wave overtopping rate. Although a procedure for estimating the wave overtopping rate is proposed, the results show that the procedure needs to be improved.

Investigation on the Reflection Coefficient for Seawalls Protected by a Rubble Mound Structure

Sea wave reflection from coastal protection structures is one of the main issues in the coastal design process. Several empirical formulas have been proposed so far to predict reflection coefficient from rubble mound breakwaters and smooth slopes. The aim of this study is to investigate wave reflection from a rubble mound structure placed in front of a vertical concrete seawall. Several experimental tests were performed on a two-dimensional wave flume by reproducing on a rubble mound structure with a steep single primary layer armored with a novel artificial unit. A new approach for the prediction of the reflection coefficient based on dimensional analysis is also proposed, and a new empirical equation is derived. The performance of the proposed equation was compared with widespread existing formulas, and a good accuracy was found.

Spatial distribution of wave-by-wave overtopping behind vertical seawall with recurve retrofitting

Understanding the spatial distribution of wave overtopping water behind coastal protection structures is essential for assessing the safety level of coastal regions behind the coastal defences. The spatial distribution of wave overtopping volume determines the planning and arrangements of critical infrastructures (e.g., railways, roads and buildings) in coastal regions. This study presents a series of laboratory-scale physical modelling experiments to investigate the spatial distribution of wave overtopping volume behind plain vertical seawall and seawall with recurve retrofitting. The hydrodynamic conditions are designed to mimic both swell and storm sea conditions. Tests were conducted for a range of relative freeboard and empirical-based predictive equations were derived for both mean and extreme overtopping events under impulsive and non-impulsive wave conditions. Test results showed that overtopping impact hazard zone reduces up to 87% with the increase of relative freeboard (Rc/Hm0). The hazard zone for extreme overtopping events was found to be up to 3 times of the mean overtopping volumes. The effectiveness of recurve retrofitting on mitigating spatial distribution of wave overtopping were investigated with three recurve prototypes of varying overhang length and recurve's height. It was found that increase in overhang length provides higher reductions in the spatial distribution of hazard zone behind the wall and improve the climate resilience of the seawall. The results highlight that recurve retrofitting is more effective in reducing the length of hazard zone under impulsive wave conditions. The findings of this study provide key data and predictive formulae for robust assessments of the hazard zones behind the key coastal defence infrastructures during mild and extreme climatic events.

Nature-Based Coastal Protection by Large Woody Debris as Compared to Seawalls: A Physical Model Study of Beach Morphology and Wave Reflection

Anchored Large Woody Debris (LWD) is increasingly being used as one of several nature-based coastal protection strategies along the north-western coasts of Canada and the US. As an alternative to conventional hard armoring (e.g., seawalls), its usage is widely considered to be less harmful to the coastal ecosystem while maintaining the ability to protect the beaches against wave attack and erosion. The effects of seawalls on beaches have been extensively studied; however, the performance and efficacy of LWD and its potential as a suitable alternative to seawalls (and other shoreline protection structures) are still understudied in current research. This paper presents and compares the effects of a conventional vertical seawall with two different LWD structures on beach morphology and wave reflection through large-scale physical modeling in a wave flume at a 1:5 scale. An assessment of techniques used to measure beach morphology and an assessment of model effects were included in the study. It was found that the wave reflection could be reduced by using a single log instead of a wall structure, while changes in the beach morphology response largely depended on the type of the LWD structure. A stacked log wall showed near-identical behavior as a conventional seawall. Visible model effects from the experiments, including the effect of the flume sidewalls on the beach morphology, were quantified and analyzed to inform future research.