Showing papers on "Wave flume published in 1996"

Simulation of a surf zone with a barred beach. Report 1. Wave heights and wave breaking

Abstract: The present report describes some of the results obtained during experiments in the Large Wave Flume of the Fluid Mechanics Laboratory of the Delft University of Technology. The experiments are part of the PhD-work of M. Boers. They follow on the LIP llD-experiments, carried out in the Delta Flume of Delft Hydraulics in spring 1993. During the LIP 11D-experiments much information was obtained about physical parameters in the surf zone such as wave heights, wave set-up and velocities. The experiments have the following objectives: To add measuring data to the LIP llD-data To obtain data which can reconstruct the mass, momentum and energy balances To obtain detailed information about regions with steep gradients of wave heights and wave set-up (onshore slope of breaker bar and toe of foreshore) To obtain information about the breaking behaviour of waves To measure bed shear stresses To measure turbulence motions The objective of this report is to distribute the results of the measurements among researchers working in the field of coastal engineering. Further, it gives information about the accuracy of the measuring data. The results of the experiments are described in two reports. Report 1 (the present report) describes the experimental set-up [Chapter 2], wave height measurements and the video recordings of the wave breaking [Chapter 3]. The results of velocities and shear stress measurements are described in Report 2 [Boers, 1996]. Some of the results are already published by Boers and Van de Graaff [1995]. The results of the analysis of the measurements is presented in many figures. Data are also available in files [Enclosure A].

Model of Beach Profile Change Under Random Waves

Abstract: A numerical model is developed to calculate the average net cross-shore transport rate and beach profile evolution under random waves. Cross-shore transport formulas for random waves are derived by superimposing the transport from individual waves, which belong to an ensemble that represents the random wave field. The transport relationships for individual waves are based on experiments with monochromatic waves in large wave tanks. The model is validated using beach profile data from the SUPERTANK Laboratory Data Collection Project. Three different types of profile evolution events are studied, namely equilibrium erosion with bar formation, berm flooding, and the impact of breaking waves on an offshore mound. The berm flooding tests include the erosion of a well-developed summer berm and the erosion of an artificially constructed foredune, and the offshore mound tests encompass narrow- and broad-crested mounds. The new model represents an attempt to consistently treat random waves in all components of a beach profile change numerical model. Previous approaches to model the profile evolution under random waves have typically involved using statistical wave measures in equations primarily developed for monochromatic waves.

Wave Forces On a Horizontal Plate

Abstract: An experimental study of the vertical force due to regular nonbreaking waves acting on a rigid horizontal plate located near the water surface is described. Time histories of the force and corresponding water surface elevations are measured for various wave heights, wave periods and plate elevations. These are analyzed to provide results relating to the maximum upward and downward forces, their times of occurrence, and their points of application. The results are used to relate the force characteristics to suitable parameters governing the problem. A numerical model of the force time history is considered and compared with the experimental results. THEORETICAL CONSIDERATIONS A rigid horizontal plate of length A, width b, and negligible thickness is located at an elevation h above the still water level, and is subjected to a unidirectional, regular, nonbreaking wave train propagating in the direction of the plate's longitudinal axis (Fig. 1). On the basis of a dimensional analysis, the vertical force F on the plate may be expressed in the form:

Bedform patterns in nearshore sands

Abstract: In this thesis a self-organization mechanism is shown to reproduce the observed patterns of ripples in nearshore sands under waves. When viewed from above, these ripples assume a variety of patterns, including linear, brick pattern, oblique crossed sets, and lunate forms. Experimental and modeling evidence combine to demonstrate that the factors controlling bedform pattern include wave height and wave asymmetry, and the mechanisms of formation include self-organization in which the shape of the bed modifies the trajectories of those sand particles which are in motion such that a specific bed pattern is reinforced. In particular, the formation of crossed sets of ripples oblique to the incident wave direction is difficult to explain except by the self-organization mechanism described in this work, hence these crossed ripple sets are the principal focus here. -- Previous field observations have associated these different patterns with depth and hence distance from shore, but little quantitative work has been done due to difficulties in making observations. In this study difficulties in making quantitative field observations were overcome by using a remotely operated, bottom mounted rotating head sidescan sonar system to image the bottom bedforms, and electromagnetic current meters and pressure sensors mounted on the same frame to quantify the wave forcing. Experimental results from a field experiment at Burley Beach, Ontario and wave tank results from two experiments at the National Research Council wave flume in Ottawa are presented. A simple computer simulation is developed to establish mechanisms of formation of the ripple pattern types observed. -- Cross-ripples appeared in the field study at the rise and decline of each storm under similar wave conditions each time. In the wave flume cross-ripples were observed for two different sand grain sizes under regular waves, grouping waves, and irregular waves. Thus cross-ripples are formed under a range of sand sizes and wave conditions. The computer model produces cross-ripples when there is asymmetry in the forcing under sufficiently energetic wave conditions. This is consistent with the field and laboratory results. There are no length scales of forcing in the model which correspond to the length scales of the bedforms produced. The sum of these observations supports the theory that cross-ripples form as a result of the self-organization mechanism.

The overtopping of seawallsA comparison between prototype and physical model data

Abstract: Significant sections of the United Kingdom coastline are protected from flooding by sea walls. These sea walls, which are commonly fronted by sand or shingle beaches, have a wide range of cross-sections ranging from vertical faces to relatively shallow sloping structures with gradients approaching 1:5. Whatever the sea wall cross-section, the selection of the crest elevation is of primary importance in determining the overtopping discharge performance of the structure and hence the susceptibility of the hinterland to flooding. Traditionally the overtopping performance of simple sea wall cross-sections has been determined from empirical equations whilst complicated cross-sections have been assessed using site specific physical models. The empirical equations employed to estimate overtopping have generally been derived from physical model data obtained during wave flume tests at scales ranging from 1:15 -1:30. A concern with using empirical equations derived from wave flume tests is that the physical model does not reproduce all the physical effects present at prototype sea walls. The most obvious deficiency of physical models is the omission of onshore winds which generally accompany storm events. This comission has two major influences. Firstly the onshore wind raises the still water level at the structure (called wind set-up) and secondly often causes water thrown into the air to be blown over the sea wall. This latter influence may be particularly important for vertical or near vertical walls and slopes topped with a recurve where water reflected from the structure is commonly thrown up into the air. A research project was therefore undertaken to measure overtopping at prototype sites. The aim of the study was to compare prototype discharges with those obtained using physical model techniques. Two sites were subsequently selected on the North Wales coast to complete the fieldwork exercise. The first was a vertical wall whilst the second was a 1:4 simply sloping sea wall. This report discusses the section of the sites, the measurements made and how whey compare with existing prediction methods. The study forms part of a continuing programme of research into the behaviour of sea walls being carried out at HR Wallingford with support from the Ministry of Agriculture, Fisheries and Food under Commission FD0201, Marine Flood Protection, Sea Defence Structures.