Jonathan Pearson

Influence of Parapets and Recurves on Wave Overtopping and Wave Loading of Complex Vertical Walls

Increasing sea water levels and storminess has intensified the need for structural measures to reduce wave overtopping without significantly raising the height of the wall. The use of recurves, wave return walls, and parapets on vertical walls has been shown capable of significantly reducing wave overtopping, but may increase wave loading. Many parapet and recurve solutions have been used in practice, but no general guidance on their design are yet available. In this paper a significant amount of data have been gathered together under the EC CLASH project (EU project no. EVK3-CT-2001-00058) and studied more systematically for the first time. This paper discusses problems with systematic approaches to both overtopping and wave loading. It concludes with a simple reduction factor for wave overtopping depending on geometrical dimensions of the parapets and some guidance on wave loading for these cases.

Vertical variation of mixing within porous sediment beds below turbulent flows

Abstract River ecosystems are influenced by contaminants in the water column, in the pore water and adsorbed to sediment particles. When exchange across the sediment‐water interface (hyporheic exchange) is included in modeling, the mixing coefficient is often assumed to be constant with depth below the interface. Novel fiber‐optic fluorometers have been developed and combined with a modified EROSIMESS system to quantify the vertical variation in mixing coefficient with depth below the sediment‐water interface. The study considered a range of particle diameters and bed shear velocities, with the permeability Péclet number, PeK between 1000 and 77,000 and the shear Reynolds number, Re*, between 5 and 600. Different parameterization of both an interface exchange coefficient and a spatially variable in‐sediment mixing coefficient are explored. The variation of in‐sediment mixing is described by an exponential function applicable over the full range of parameter combinations tested. The empirical relationship enables estimates of the depth to which concentrations of pollutants will penetrate into the bed sediment, allowing the region where exchange will occur faster than molecular diffusion to be determined.

Hazards at coast and harbour seawalls - velocities and trajectories of violent overtopping jets

This paper describes new research under the VOWS (Violent Overtopping of Waves at Seawalls) project. Established guidance on admissible overtopping volumes is based upon values of mean discharge. In cases where hazard to pedestrians / vehicles are concerned, it is clear that an admissible level of overtopping would be more appropriately based upon the volume of an individual overtopping event. Further, the hazard presented is not only a function of the volume of that overtopping wave, but also of the speed and trajectory of the jet. Methods exist to predict maximum individual overtopping volumes. This paper presents new data and a first predictive tool for overtopping “throw velocities”, and first data from a device designed to measure directly the trajectories of overtopping jets.

Violent Wave Overtopping -- Extension of Prediction Method to Broken Waves

Well-verified guidance exists for the prediction of wave overtopping over vertical walls, including breaking/impulsive and non-breaking/pulsating wave attack. For broken waves only Goda’s (1975) design charts give guidance, and only for sea steepnesses. This paper presents the results of tests to quantify overtopping of vertical walls under predominantly broken wave attack. Tentative guidance is suggested. For cases where the toe of the wall is submerged, and adjustment to the method of Besley (1999) is suggested. For cases where the water level falls below the toe of the wall, adjustment to the method for sloping structures of van der Meer & Janssen (1995) is suggested.

Violent Overtopping of Vertical Seawalls under Oblique Wave Conditions

This paper describes how most prediction methods for wave overtopping are based on physical model tests under simple 2-D conditions. This paper also describes experiments to measure mean and wave-by-wave overtopping discharge under oblique wave attack. Results suggest that overtopping discharges reduce significantly only for angles of wave attach that were greater than 30 degrees. With increasing obliquity, impulsive events transform to “impact-like” events and then eventually to reflecting waves (60 degrees). Tentative guidance is given for appropriate formulae for each obliquity.

A COMPARISON OF OVERTOPPING PERFORMANCE OF DIFFERENT RUBBLE MOUND BREAKWATER ARMOUR

This paper describes a major programme of tests to establish better the influence of armour roughness and permeability on overtopping. Specifically, the tests determined the relative difference in overtopping behaviour for various types of armour units. Roughness factors γf for the database and for use in the neural network prediction of overtopping were determined for rock (two layers), cubes (single-layer and two-layers), Tetrapod, Antifer, Haro, Accropode, Core-Loc and Xbloc.

Numerical and Experimental Predictions of Overtopping Volumes for Violent Overtopping Events

This paper describes how storm waves breaking in the surf zone and crashing over seawalls are a natural phenomenon with the potential for causing enormous damage. Much of the existing design guidance for seawalls considers the mean overtopping discharge on the structure. However, under impacting conditions the overtopping volumes associated with individual events can be significant. Under the UK Engineering and Physical Sciences and Research Council (EPSRC) funded VOWS project and the European Commission funded CLASH project work has been undertaken using both laboratory investigation and computer simulation in order to quantify these wave by wave overtopping volumes. This paper discusses the application of the AMAZON-SC multiphase numerical wave flume developed by Manchester Metropolitan University to simulate experiments of violent wave overtopping conducted in the Edinburgh University 20m wave flume.

Overtopping of Seawalls under Oblique and 3-D Wave Conditions

The majority of prediction methods for overtopping of seawalls are based on physical model tests under simple 2-dimensional conditions. There is some evidence (not unambiguous) that overtopping may increase at small degrees of obliquity, and that corners (in plan) may give local concentrations of overtopping. This paper, produced as part of the VOWS project on impulsive (violent) overtopping of vertical seawalls, describes experiments to measure mean and wave-by-wave overtopping discharge under conditions of oblique wave attack and at 3-d corners. Results analysed in the first phase of this 3-d study suggest that mean overtopping discharges reduce significantly with increasing angle of wave attack and that the occurrence of impulsive overtopping diminishes rapidly with obliquity of wave attack > 30°. It is also observed that overtopping may not increase in corners with an approach beach or berm.

Hydraulic and Design Parameters in Full-Scale Constructed Wetlands and Treatment Units: Six Case Studies

The efficiency of pond and constructed wetland (CW) treatment systems is influenced by the internal hydrodynamics and mixing interactions between water and aquatic vegetation. In order to contribute to current knowledge of how emergent real vegetation affects solute mixing, and on what the shape and size effects are on the mixing characteristics, an understanding and quantification of those physical processes and interactions were made. This paper presents results from tracer tests conducted during 2015–2016 in six full-scale systems in the UK under different flow regimes, operational depths, shapes and sizes, and inlet/outlet configurations. The aim was to quantify the hydraulic performance and mixing characteristics of the treatment units, and to investigate the effect of size and shape on the mixing processes. Relative comparison of outlet configuration, inflow conditions, and internal features between the six different treatment units showed variations in residence times of up to a factor of 3. A key outcome of this study was that the width is a more important dimension for the efficiency of the unit compared to the depth. Results underlined the importance of investigating hydrodynamics and physics of flow in full-size units to enhance treatment efficiency and predictions of water quality models.

Solute dispersion in the nearshore: laboratory and numerical results

The nearshore zone experiences pollutant loading through both the seaward and shoreline boundary. At the seaward limit this occurs through impacts from marine outfalls. However, more importantly close to or at the shoreline, pollutants enter through storm overflow discharges from overloaded sewerage systems during rainfall events. The nearshore zone is an area of high amenity value and there is therefore a strong need to manage efficiently the conflicting demands of waste management, recreation and fisheries. Through a series of hydrodynamic and tracer measurements and their comparisons with numerical modeling results, this study quantifies the physical processes and their integrated effects on a solute tracer in the nearshore zone subject to combined waves and longshore currents. The results improve the estimate of surfzone dispersion coefficients.