Tom Bruce

New Physical Insights and Design Formulas on Wave Overtopping at Sloping and Vertical Structures

AbstractMean wave overtopping discharge is a key design parameter for many coastal structures, typically designed to limit overtopping discharge to below a chosen admissible value. Dutch, German, and British design guidance from the 1990s was updated using results of research projects supported by the European Commission, and subsequently unified with the publication of the European Manual for the Assessment of Wave Overtopping, or EurOtop [EurOtop. (2007). European Manual for the Assessment of Wave Overtopping, T. Pullen, et al., eds.], now used all over the world. This paper explores five technical issues that were not well covered in the unified manual. (1) For sloping structures, overtopping at low and zero freeboard conditions: new analysis brings together the conventional exponential formulas with the few reliable datasets including very low and zero freeboard. In doing so, early Dutch work from the 1970s was revisited. Weibull-type formulas are proposed, describing wave overtopping at slopes for th...

Flow field visualization of sediment-laden flow using ultrasonic imaging

This investigation concerns the application of particle image velocimetry (PIV) to ultrasonic images of sediment-laden flow in order to obtain accurate, quantitative, two dimensional vector maps of the flow field within the sediment suspension. Experiments were carried out using a medical ultrasound scanner to obtain ultrasonic images in a kaolin/salt water suspension of mean concentration 16 g/l and to compare results obtained with the ultrasound scanner in clear water with those from established measurement techniques. Measurements of mud were obtained, and the comparison with established methods indicated that the application of PIV to the analysis of ultrasound images yields velocity information accurate to within ±15%. This measurement technique will be of great value in furthering the understanding of the behavior of sediment-laden flows.

Impulsive seaward loads induced by wave overtopping on caisson breakwaters

Abstract An investigation of seaward impulse loads on caisson breakwaters caused by wave overtopping is described. Attention is focused on mechanisms that may cause such loads, and consideration given to implications for the design of vertical breakwaters. A review of literature shows that there have been several reported cases of seaward tilting of breakwater caissons. A description of the seaward failure of a prototype structure, apparently resulting from overtopping, was also found. The measurements of seaward impulsive loads reported in the literature do not show any case in which they are higher than associated landward forces. Physical model tests that are described show overtopping of a low crest breakwater in which the maximum seaward force is greater than the maximum force acting towards the land. A theoretical model for the pressure impulse generated by the re-entry of the overtopping plume is given. Comparison with physical model data shows that, for these experiments, pressure in the air pocket trapped during overtopping contributes significantly to the impulse. When this effect is included, good agreement is obtained between experiment and theory. It is concluded that seaward overturning should be considered as a failure mode of caisson breakwater designs that allow overtopping, particularly for structures of relatively low mass.

Violent Wave Overtopping: CLASH Field Measurements at Samphire Hoe

As part of a major European research project into wave overtopping at coastal structures, overtopping discharges have been measured at full-scale on a vertical seawall in Southern England. The measurement site, Samphire Hoe, is an area of reclaimed land just west of Dover on the English channel coast, and is an ideal location for monitoring overtopping. The site is described in detail and the design and operation of the measurement equipment are also outlined in the paper. Overtopping was measured on three occasions and the storms and their results are discussed. It is shown that the field measurements compare will with empirical prediction methods. Additional discussion interprets the hazardous nature of each of the storms.

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.

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.

Heat transfer coefficient distributions in an experimental kettle reboiler thin slice

Pressure drop and heat transfer coefficient measurements have been made in a 241 tube bundle kettle reboiler thin slice rig, boiling pentane at atmospheric pressure. The effects of liquid/vapour separation problems in the shell are described and discussed. A conventional 1-D recirculation model predicted the data taken at a uniform heat flux of l0 kW m −2 . Another recirculation model was developed and compared to the data at 50 kW m −2 where the flow in the bundle was markedly two dimensional. This model matched the pressure drop data in the bundle apart from that in the outside columns which were judged to have been affected by adjacent rapidly rising 2-phase flow in the shell. The model was adjusted to eliminate this defect and making use of two flow boiling models reproduced the heat transfer data mainly within experimental error. The consequence of these findings for the application of data from thin sliced rigs to design of full scale kettle reboilers and horizontal recirculation shellside evaporators is discussed.

EFFECTIVENESS OF RECURVE WALLS IN REDUCING WAVE OVERTOPPING ON SEAWALLS AND BREAKWATERS

Designers of vertical seawalls and breakwaters have often included some form of seaward overhang (recurve / parapet / wave return wall / bullnose) as part of the structure with the design motivation of reducing wave overtopping by deflecting back seaward uprushing water. Despite a lengthy track record in the field and relevance to current design issues, very little generic guidance is available for their incorporation into seawall / breakwater design. This paper reports a study whose aim is the formulation of generic guidance for recurve structure design. Particular attention is given to high freeboard and / or wave breaking conditions under which the recurve / parapet gives very large reductions (recurve k-factor < 0.05). The paper presents tentative guidance in the form of a decision chart. Finally, overtopping and loading results from a case study into a wall of particularly complex geometry are presented and compared with earlier studies. Forces on the vertical wall are found to be highly impulsive in nature and approximately double the magnitude of those expected on a simple wall, with additional forces of a similar magnitude measured on the underside of the parapet.

Wave effects on blockwork structures: model tests

Up to the middle of this century many coastal structures were built from blockwork, using either natural stone blocks or concrete blocks. Those blockwork structures subjected to breaking wave impacts often show a particular damage type, whereby individual blocks are shifted out of their position and moved into the sea. Engineers have suspected for a long time that wave impact pressures can travel into the water filled cracks and joints of such structures, building up pressures inside of the structure and thus destroying the structure from within. In order to verify the damage mechanism, and to investigate the characteristics of impact induced pressure pulses, model tests on the propagation of wave impact pressures into water filled cracks were conducted. It was found that impact generated pressure pulses can enter water filled cracks and that they exhibit wave-like characteristics such as finite propagation speed, reflection, superposition and attenuation. Changes in cross section were found not to affect pressure pulse magnitude or duration. The possibility of wave impact pressures to damage or destroy cracked or fissured structures from within the structure could thus be verified.

PIV MEASUREMENTS OF OSCILLATORY FLOW OVER A RIPPLE BED

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.