Microplastics (MPs) are emerging contaminants, with wastewater treatment plants (WWTPs) as principal hotspots for their release into downstream systems, including constructed wetlands (CWs), a nature-based solution for water treatment. While non-buoyant MPs readily settle, buoyant MPs risk bypassing CWs and entering aquatic environments. Biofilm formation could influence MP transport by altering buoyancy, promoting sinking, and enhancing MP retention, yet its role in CWs remains unknown. This study, for the first time, quantifies the effects of MP polymer type, particle characteristics, exposure time, and seasonality on biofilm colonisation and its impact on terminal rising velocities of initially buoyant MPs in a UK-based CW receiving partially treated wastewater. Polypropylene (PP), expanded polystyrene (PS), and low-density polyethylene (LDPE) particles (3-5 mm) in spherical, beaded, and film shapes were incubated in situ over 12 months. Sampling followed two approaches: (1) a rolling bi-monthly schedule to capture seasonal variation, and (2) a long-term deployment with subsets retrieved every two months. Biofilm biomass was quantified by crystal violet staining, surface characteristics were captured by scanning electron microscopy (SEM), and terminal rising velocity experiments measured buoyancy changes. Biofilm growth showed strong seasonality, with peak biomass in late spring showing up to a 1972 % increase compared to winter. Despite widespread colonisation, changes in terminal rising velocity were minimal and largely non-significant (p < 0.05), indicating that biofilm formation alone is insufficient to retain initially buoyant MPs in CWs. These findings are crucial for deriving MP transport models and challenge assumptions that biofilm-induced density changes drive MP retention in CWs.

The material quantities and associated costs of studied in full, and this paper aims to provide an insight adaptation infrastructure for climate change is expected based on experimental findings. to rise, in the coming century (Jonkman et al., 2013). Therefore, using nature-based soft coastal defences to safeguard coastlines will result in economic advantages in addition to environmental benefits. Coastal vegetation (mangroves, salt marshes, sea grass, etc.) is a source of drag, and hence waves propagating over them lose energy. By damping near bed velocities, seagrass reduce local resuspension and promote the retention of sediment, stabilizing the seabed (Fonseca and Cahalan, 1992). Coastal vegetation can be used alone to minimize disturbances caused to day-to-day activities by rough sea conditions, and sometimes soft coastal defences can be coupled with hard coastal defences (Jackson, 2014) to reduce the wave loads induced on hard defences (e.g., a salt marsh in front of a seawall). Seagrasses grow mainly in physically dynamic regions which are exposed to both tide and wave generated currents, such as shallow regions along coastal margins and within estuaries (Thomas and Cornelisen, 2003).

Surface flow constructed wetlands (CWs) are a nature-based wastewater treatment technology designed to serve as a buffer between wastewater treatment plants (WWTPs) and the receiving environment. While the treatment efficacy of CWs has been investigated, surface flow systems are susceptible to hydraulic inefficiencies, and a comprehensive understanding of the factors influencing pollutant transport remains limited, hindering their optimisation and predictability. The hydraulic performance of a CW, determined by the efficiency of flow hydrodynamics, dictates the residence time and spatial interactions between pollutant-laden water and purification mechanisms, such as those provided by vegetation and substrate. However, unstandardised designs and a limited understanding of water-sediment-plant interactions often result in sub-optimal hydraulic conditions, such as short-circuiting and dead zones, which impair treatment efficiency. This study investigates the influence of inter-seasonal climate variability, vegetation growth cycles, and operational conditions on the interplay between hydraulic performance, and subsequent pollution removal efficacy, in a full-scale integrated surface flow CW located in Norfolk, UK. Five tracer test campaigns were conducted during 2022-2023 using Rhodamine WT dye and fluorometric sensors to evaluate seasonal variations in hydraulic behaviour across four interconnected vegetated Cells. Hydraulic performance was characterised using indices such as mean residence time, tank-in-series model, hydraulic efficiency, effective volume ratio, short-circuiting, mixing, and dispersion coefficients. To understand the roles of CW design, operation, vegetation, and climate on hydraulic performance, high-resolution LiDAR vegetation scans, nutrient concentrations, and climate monitoring data were collected concurrently with the tracer tests. The combined mean residence time ranged from 30.03 h in autumn to 47.67 h in summer. Individual Cell hydraulic indexes revealed significant non-uniform flow patterns, with 80 % of tracer tests indicating poor (λ < 0.5) hydraulic efficiency and 55 % exhibiting dead zones occupying >50 % of the Cell volume. These inefficiencies were predominantly driven by smaller Cell geometries, sub-optimal inlet-outlet configurations, high influent hydraulic loading rates (0.47 to 0.66 m3/day/m2), and high emergent vegetation cover. Despite these hydraulic deviations, Cell nutrient removal performance was more strongly influenced by vegetation growth stage and seasonal water physicochemical conditions. These findings provide novel field-scale data into how design, seasonal, and operational factors influence CW performance, highlighting the critical need for enhanced design and management strategies to optimise hydraulic and treatment performance in CWs, particularly in climate variability.

Free-water surface constructed wetlands (CWs) are sustainable, low emission, nature-based solutions for water and wastewater treatment. However, the discharge of nutrient-rich effluents from CWs treating wastewater can adversely impact freshwater ecosystems and exacerbate eutrophication. Despite their ecological benefits, limited research exists on the treatment efficiency and pollutant dynamics of CWs under varying seasonal and environmental pressures. This study investigates the treatment efficiency of an integrated CW (ICW) serving as a nature-based solution for treating partially treated wastewater before release into the environment. Located in Ingoldisthorpe, Norfolk, near the East coast of the UK, the ICW receives 1014 +/- 538 m3/day of effluent from a wastewater treatment plant (WWTP). The system comprises four interconnected vegetated ponds (i.e. Cell) with a mean effective volume of 2697 m3, operating at an average depth of 0.19 m. Seasonal variations in vegetation density and coverage range from sparse in winter and spring to dense in summer and autumn. Bi-monthly field investigations were conducted over one year (August 2022-June 2023) to examine the impacts of inter-seasonal climate variability on the ICW's treatment performance. Removal rates of solute and solid pollutants, including nitrate (NH3-), nitrate-nitrogen (NH3- N), ammonium (NH4+), total nitrogen (TN) orthophosphate (PO43- ), sulphate (SO42- ), non-purgeable total organic carbon (NPOC), total inorganic carbon (TIC), and total solids (TS), were quantified. Significant seasonal variations were observed in Concentration Removal Rates (CRR) and Mass Removal Rates (MRR) for all nutrients. Nitrate CRRs ranged from -39.1 % to +51.64 %, corresponding to reductions of up to 14.57 mg/L and increases of 26.71 mg/L in effluent concentrations, while MMRs varied between -77.13 % to +84.25 %, reflecting changes of -38.93 kg/day to +26.69 kg/day. For phosphate, CRRs ranged from -22.79 % to +2.57 %, and MMRs ranged from -71 % to +93.22 %, equivalent to -0.57 kg/day to +0.26 kg/day. These findings highlight the dynamic and sensitive mechanisms influencing nutrient removal in CWs, driven by seasonal hydraulic conditions, vegetation phenology, and climatic factors. The study provides critical insights for optimizing CW design and management under fluctuating environmental conditions to enhance their resilience, ensure regulatory compliance, and maintain long-term treatment efficiency. This understanding is essential for guiding future regulatory policies and ensuring that CWs meet water quality standards in response to climate pressures.

The transport of microplastics in the hyporheic zone remains poorly understood with few studies attempting to quantify microplastic hyporheic exchange processes. A laboratory scale erosimeter was utilized in combination with fluorometric techniques to experimentally quantify the dispersion of 3D pore-scale microplastics across the hyporheic zone. Rhodamine WT dye, Polypropylene (PP), polyethylene (PE), and polymethyl methacrylate (PMMA) were well-mixed within the riverbed and individually tested using solute transport theory for three sediment diameters and five bed shear velocities (u∗) common in the natural environment. Effective dispersion coefficients for solutes significantly differed from that of PE and PMMA in most cases, where their critical sinking velocity within sediment pore water was observed and a method for predicting polymer dispersion was proposed. When u∗ ≥ 0.0304 m/s, PMMA followed similar pathways to solutes and the effective dispersion scaling model was successfully implemented to predict its fate. PP near the riverbed interface ascended to the surface but was immobilized deeper in the riverbed, likely due to aggregation and flocculation processes. When polymer buoyancy became the dominant process, high concentrations of lighter than water microplastics ascended into the water column and high concentrations of denser than water microplastics descended through pore water, which is concerning for real-world groundwater systems. These findings provide valuable insights to guide future policy and mitigation strategies of microplastic contamination in fluvial systems by advancing our understanding of microplastic transport. Further data collection will enhance our ability to accurately quantify these transport processes and strengthen mitigation efforts, especially within high permeability sediments.

Permeable baffles can play a significant role in enhancing the pollution removal efficiency of retention and treatment ponds. Understanding and quantifying the impact of permeable baffles on hydraulic performance and solute transport characteristics is crucial for determining the optimal number and configuration of baffles to ensure robust treatment performance in retention ponds. A three-dimensional numerical model has been developed in a Cartesian coordinate system, incorporating both the Reynolds-averaged Navier-Stokes (RANS) hydrodynamic model and the k - omega turbulence closure model. In this study, a non-reactive tracer model based on the advection-diffusion equation is implemented to evaluate contaminant transport and mixing within the retention pond system. The modified Darcy equation is utilized to model the interaction between permeable baffles and the tracers. The proposed numerical model is successfully validated against physical modelling measurements of solute characteristics in retention ponds with permeable baffle retrofitting. The developed model is then used to run ten scenario-based simulations with varying baffle porosity, position, and number, achieving a root mean square error (RMSE) of less than 0.04, showcasing the robustness of the proposed model. The effects of permeable baffles on the flow hydrodynamic characteristics in the pond are comprehensively analyzed using velocity fields and turbulent kinetic energy (TKE). Subsequently, the tracer transport pathways, residence time distributions (RTDs), and the associated hydraulic indices are determined to assess the treatment efficiency of the system affected by baffle characteristics. Analysis of the numerical results highlights the significant role of permeable baffles in homogenizing flow distribution, dampening inflow momentum, and dissipating turbulent kinetic energy. The resulting flow modifications contribute to an augmentation of the pond's effective volume, thereby leading to elevated treatment performance. The porosity of baffles is the key underlying parameter that influences the overall hydrodynamics and hydraulic performance of the retention system, leading to increases in the momentum index MI ranging from 3.15 % to 14.6 % across various cases tested in this study. Baffles with finer porosity are more effective in preventing short-circuiting and ensuring a more uniform distribution of tracers. The positioning of the first baffle markedly affects the spatial distribution and turbulence intensity of the inflow, exhibiting varying mitigating effects on surface short-circuiting with alterations in baffle porosity, with these effects ranging from 2 % to 25 %. Increasing the number of baffles from 2 to 3 within the pond is found to intensify viscous energy loss and optimize hydraulic efficiency by 9.3 %. The proposed numerical model serves as a robust tool for optimizing treatment pond design, offering detailed insights into the critical role of baffle characteristics in enhancing pollution removal processes in retention pond systems.

An experimental study was conducted to identify the behaviour of neutrally buoyant microplastics (specific density, 0.94) in different hydrodynamic conditions while focusing on combined wave–current conditions and the mixing across the hyporheic zone. For in-water-column microplastics, it was observed that the streamwise dispersion of neutrally buoyant microplastics is comparable to solute dye in both slow open-channel flow conditions and combined wave–current conditions. However, for in-bed microplastics, when compared to soluble tracers, the longer timespans associated with the hyporheic exchange process allowed the density effects to enhance the vertical exchange when compared to solutes.

This study investigates the reduction in overtopping discharge along a vertical seawall through the implementation of a recurve retrofitting. A comprehensive set of physical modelling experiments were undertaken in a laboratory-scale wave flume at the University of Warwick, to investigate the wave overtopping processes under both swell and storm wave conditions. The tests measured overtopping discharges for impulsive and non-impulsive wave conditions. The effects of geometrical design of recurve retrofitting on overtopping reduction are examined by four configurations with varying overhang length and recurve hight. The study revealed that the reduction in overtopping is primarily determined by the length of the overhang in the recurve wall, while the influence of the recurve height is limited. A longer overhang length results in a more substantial decrease in overtopping discharges on the seawall crest. The results also highlight the role of incident wave steepness and the crest freeboard on the overtopping mitigation performance of the recurve walls. A new enhanced methodology is proposed to predict the wave overtopping from vertical seawalls with recurve retrofitting., considering the effects of freeboard and wave steepness. The findings of this study provide new important insight in the role of retrofitting as a robust intervention to improve the wave overtopping mitigation performance of seawalls. The predictive empirical formulae proposed by this study facilitate readily and accurate estimation of overtopping rates as a function of retrofitting geometrical design, allowing for wider application of retrofitting solutions.

An experimental study was conducted on how polymer density affects the transport and fate of microplastics in aquatic flows. For the first time, polypropylene (PP), polyethylene (PE), polymethyl methacrylate (PMMA), polyetheretherketone (PEEK), and polyvinyl chloride (PVC) were chemically stained and tested using solute transport techniques and velocities found among rivers in the natural environment (0.016 - 0.361 m/s). The movement of 3D-polymers with densities ranging from 0.9 - 1.4 g/cm(3) was quantified in a laboratory flume scaled to simulate open-channel flows in fluvial systems. Except for PP, in most conditions microplastics exhibited similar transport characteristics to solutes regardless of density and established solute transport models were successfully implemented to predict their transport and fate. Mass recoveries and ADE routing model demonstrated microplastic deposition and resuspension was associated with polymer density below critical velocity thresholds < 0.1 m/s. When density becomes the dominant force at these slower velocities, concentrations of denser than water microplastics will be momentarily or permanently deposited in channel beds and microplastics follow the classical Shields sediment transport methodology. This data is the first to provide microplastic suspension and deposition thresholds based on river velocity and polymer density, making a key contribution to research predicting microplastic fate and organismal exposure.

Physical interactions of microplastics within vegetation and turbulent flows of freshwater systems are poorly understood. An experimental study was conducted to investigate the underlying physical transport mechanisms of microplastics over submerged canopies across a range of flow conditions common in the natural environment. The effects of changing canopy heights were investigated by testing two model canopies of varying stem heights, simulating seasonal variation. This study determined and compared the mixing and dispersion processes for microplastics and solutes utilizing fluorometric tracing techniques. A hydrodynamic model was developed based on the advection‐dispersion equation for quantifying microplastic mixing in submerged canopies. Longitudinal dispersion coefficients for neutrally buoyant microplastics (polyethylene) and solutes were significantly correlated within submerged model vegetation irrespective of the complexity of the flow regime. Hydrodynamic and solute transport models were shown to be capable of robust predictions of mixing for neutrally buoyant microplastics in environmental flows over a canopy, facilitating a new approach to quantify microplastic transport and fate. We compare the mixing processes for microplastics and solutes then propose a hydrodynamic model for quantifying the mixing in submerged canopies.

The transport of microplastics within urban water systems remains poorly understood, with little prior research on their behaviour within manhole configurations. This study represents the first to measure and model the transport dynamics of microplastics within circular and square manholes under different hydraulic scenarios. The transport and fate of polyethylene (PE) was quantified and compared to solutes (Rhodamine WT dye) using energy losses, residence time distributions (RTDs), and mixing models within surcharging and overflowing manholes. The bulk mass of solute and PE concentrations followed similar flow paths across all conditions except for 17.3 ± 7.9 % of PE mass that was immobilized in a dead zone above the inlet pipe for manholes with a surcharge to pipe diameter ratio ≥2. Consequently, these microplastics only exit after a significant change in hydraulic regime occurs, causing microplastics to be at risk of being contaminated over a prolonged duration. No significant mixing differences for PE and solutes were found between manhole geometries. The deconvolution method outperformed the ADZ model with goodness of fit (Rt2) values of 0.99 (0.60) and 1.00 (0.89) for PE and solute mixing, respectively. This establishes the deconvolution method as the most accurate and appropriate model to accurately predict microplastic mixing in manholes and urban drainage systems.

The presence of dense submerged vegetation alters mixing characteristics in open channel flows as they cause differential velocities inside and above canopies. The prediction models for longitudinal mixing in the presence of submerged canopies often use the drag coefficient to represent the canopy, which limits the usability of the models when the canopy properties are not fully understood. Here, attempts were made to present a methodology which can be used for deriving the coefficient of longitudinal dispersion in the presence of submerged vegetation based on velocity measurements, using a mixing length approach to model turbulence. An experimental study was conducted in a large-scale laboratory facility to investigate the longitudinal dispersion characteristics in open channel flow with submerged aquatic vegetation canopies. Detailed velocity and solute tracer measurements were undertaken for a representative range of flow velocities. The velocity measurements were used for deriving turbulent shear stress, mixing length, and diffusivity using established theoretical and empirical relationships to derive the longitudinal dispersion. The longitudinal dispersion measured in two locations in the water column for the two canopy submergences was discussed based on the amount of vertical mixing and differential advection. The canopy with a smaller stem length (i.e., higher submergence ratio) has a higher vertical diffusivity, resulting in increased vertical mixing in the water column. The canopy with the higher stem length (i.e., lower submergence ratio) consists of minimal vertical diffusivity, causing the longitudinal dispersion measured above the canopy to be significantly high, even though the longitudinal dispersion measured inside the canopy is much lower. The mathematical model which was adapted for calculating the coefficient of longitudinal dispersion and the tracer results show good agreement, indicating that the N-zone model can accurately predict the longitudinal dispersion in submerged aquatic canopies when used with the presented methodology.

Before agrochemicals can be registered and sold, the chemical industry is required to perform regulatory tests to assess their environmental persistence, using defined guidelines. Aquatic fate tests (e.g. OECD 308) lack environmental realism as they are conducted under dark conditions and in small-scale static systems, which can affect microbial diversity and functionality. In this study, water-sediment microflumes were used to investigate the impact of these deficiencies in environmental realism on the fate of the fungicide, isopyrazam. Although on a large-scale, these systems aimed to retain the key aspects of OECD 308 tests. Tests were carried out under both a non-UV light-dark cycle and continuous darkness and under both static and flowing water conditions, to investigate how light and water flow affect isopyrazam biodegradation pathways. In static systems, light treatment played a significant role, with faster dissipation in illuminated compared to dark microflumes (DT50s = 20.6 vs. 47.7 days). In flowing systems (DT50s = 16.8 and 15.3 days), light did not play a significant role in dissipation, which was comparable between the two light treatments, and faster than in dark static microflumes. Microbial phototroph biomass was significantly reduced by water flow in the illuminated systems, thereby reducing their contribution to dissipation. Comprehensive analysis of bacterial and eukaryotic community composition identified treatment specific changes following incubation, with light promoting relative abundance of Cyanobacteria and eukaryotic algae, and flow increasing relative abundance of fungi. We conclude that both water velocity and non-UV light increased isopyrazam dissipation, but the contribution of light depended on the flow conditions. These differences may have resulted from impacts on microbial communities and via mixing processes, particularly hyporheic exchange. Inclusion of both light and flow in studies could improve the extent they mimic natural environments and predict chemical environmental persistence, thus bridging the gap between laboratory and field studies.

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.

This paper investigates the hydraulic performance and solute transport processes in waste stabilization ponds (WSPs). A numerical model comprised of Reynolds-averaged Navier-Stokes (RANS) flow hydrodynamic model with the standard k - epsilon turbulence closure coupled with the advection-diffusion solute transport model, is developed in a three-dimensional Cartesian coordinate system. The proposed numerical model is successfully validated against laboratory-scale physical modelling measurements of flow hydrodynamics and solute characteristics across trapezoidal pond geometry. The developed numerical model is adopted to run series of scenario-based simulations to investigate the effects of WSP's geometrical features and implementation of an island retrofitting on the hydraulic performance and treatment efficiency of the WSPs. Fifteen pond configurations with varying side-walls slope and island configurations are simulated. Vertical and horizontal structures of flow hydrodynamics across the pond are investigated. Solute transport processes are studied through determining residence time distribution (RTDs) curves based on numerical tracer simulations. Two deflector island configurations (parallel and rotated) are simulated to investigate their influence on enhancing the hydraulic performance of the WSP. The analysis of the numerical results indicates an overall positive impact of deflector island retrofitting on the hydraulic performance of the WSP. The side-walls slope are shown to play a key role in determining the overall performance of the WSP. For the cases with side-walls slope of 1:1, 0.5:1 and 0:1, the hydraulic efficiency of the WSP was enhanced by adding both parallel and rotated islands. However, for the cases with side-walls slope of 2:1 and 1.5:1, addition of island deflector is shown to have negative impacts on the hydraulic performance of the waste stabilization pond.

ABSTRACT With increasing urbanization, wastewater and industrial effluents are being discharged into watercourses. Due to high-density development, it is difficult to construct conventional flow measurement structures like weirs. Hence, to measure the flow, dye tracing studies are an appropriate method for urban drainage channels. Dye tracing studies along with water quality monitoring can be analyzed to understand the behavior of pollutants in specific sections of the study area. This paper presents a state-of-the-art review for employing dye tracing studies to ascertain the pathways of pollutants and a case study. A dye tracing study was carried out at the Kolshet drain, Thane, India, to conceptualize contaminant transport using rhodamine WT (water tracer) dye. The study was carried out jointly by the Indian Institute of Technology Bombay, India and the University of Warwick, UK. Rhodamine WT dye was injected at the injection point and the distribution of dye concentration with time was logged with four portable fluorometers installed at measurement points. The data were analyzed and the longitudinal dispersion coefficient (‘DL’) for solute transport in the drain was computed.

The computational limitations of complex numerical models have led to adoption of statistical emulators across a variety of problems in science and engineering disciplines to circumvent the high computational costs associated with numerical simulations. In flood modelling, many hydraulic and hydrodynamic numerical models, especially when operating at high spatiotemporal resolutions, have prohibitively high computational costs for tasks requiring the instantaneous generation of very large numbers of simulation results. This study examines the appropriateness and robustness of Gaussian Process (GP) models to emulate the results from a hydraulic inundation model. The developed GPs produce real-time predictions based on the simulation output from LISFLOOD-FP numerical model. An efficient dimensionality reduction scheme is developed to tackle the high dimensionality of the output space and is combined with the GPs to investigate the predictive performance of the proposed emulator for estimation of the inundation depth. The developed GP-based framework is capable of robust and straightforward quantification of the uncertainty associated with the predictions, without requiring additional model evaluations and simulations. Further, this study explores the computational advantages of using a GP-based emulator over alternative methodologies such as neural networks, by undertaking a comparative analysis. For the case study data presented in this paper, the GP model was found to accurately reproduce water depths and inundation extent by classification and produce computational speedups of approximately 10,000 times compared with the original simulator, and 80 times for a neural network-based emulator.

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.

This study develops a numerical model for investigating the hydraulic characteristics of a retention pond with porous baffles. The numerical model is developed using the Reynolds-averaged Navier-Stokes equations (RANS) with k-εturbulence closure model. The model is successfully validated using physical modelling measurements. The proposed model is used to investigate the key mechanisms that govern and influence the hydraulic efficiency of retention ponds with porous baffles. Three configurations with varying numbers and locations of baffles are simulated. The numerical results are analyzed by comparison of velocity fields, tracer transport patterns, and associated residence time distributions (RTDs) across all the simulation scenarios. It was found that the porous baffles effectively improve hydraulic performance by creating uniform flow distribution and dissipating the flow energy, thereby avoiding dead zones and mitigating short-circuiting. Results show that the location of the first baffle plays a critical role in the flow momentum dissipation. Carefully considerations are required to determine the optimal number and positions of baffles in a specific system. The numerical RTDs are in good agreement with the physical modelling data, confirming the positive contribution of porous baffles to the overall hydraulic performance of the pond by extending the average tracer residence time.