Oceanographical Engineering

Robert L. Wiegel

• Publisher: Dover Publications
• Year: 2013
• ISBN:
  9780486160191
• Topic: Naturwissenschaften
• Subtopic: Meeresforschung

As is the case with many modern fields of study, oceanographical engineering cuts across the boundaries of several disciplines. Like other scientific endeavors, it aims to understand the nature of the ocean and to make use of this understanding for the benefit of humanity through better ports, safer and more economical operations at sea, and greater use of the oceans' natural resources--food, raw materials, and recreation.
This graduate-level text requires a knowledge of fluid mechanics; a background in the motions of sediments in fluids is advisable, as is a concurrent course in structural dynamics. Topics include the theory of periodic waves; tsunamis, storm surges, and harbor oscillations; the effect of structures on waves; waves in shoaling water; tides and sea level changes; currents; shores and shore processes; some characteristics of the oceans' waters; moorings; and other related subjects. Certain portions of the book pertaining to the distribution of temperatures and salinities in the ocean are largely descriptive; other portions, such as the sections on waves, are mathematical. Numerous drawings and photographs supplement the text.

CHAPTER ONE

Introduction

1. INTRODUCTION

Physical oceanography can be defined as that science treating of the oceans, their forms, physical features and phenomena. This book deals largely with the applications of physical oceanography to civil engineering. It is not intended to be a design manual. To the contrary, considerable emphasis is given to the lack of precise quantitative design information and procedures. Before World War II, the designs of most civil engineering structures built in the ocean were based upon past experience. Because of the stimulus of amphibious operations during World War II, and more recently, because of offshore oil operations there has been a great increase in the knowledge of oceanographical engineering.

Some aspects of oceanographical engineering are new, whereas other aspects go back to antiquity. The design and construction of harbors is one of the oldest branches of engineering. The port of A-ur was located on the Canopic branch of the Nile prior to 3000 B.C., and it is well established that a harbor was built nearby on the open coast of Egypt (a forerunner of Alexandria) about 2000 B.C., called the Port of Pharos (Savile, 1940). This port, apparently built by the Minoan Cretans, had an area of about 300 acres. Among other structures it had a breakwater 8500 ft long which consisted in the main part of two rubble mound structures between 130 and 200 ft apart, each with an upper width of 26 to 40 ft and 20 to 30 ft high. These were made of very large blocks and the space between them was filled with smaller stone. The top width was between 180 and 250 ft.

Another famous harbor of antiquity was Tyre, which was built about 900 to 1000 B.C. Hewn rectangular blocks weighing as much as 15 tons each were used, with many of them being tied together with iron dowels run in with lead (Savile, 1940).

Great harbors were constructed by the Greeks and Romans, the last of the large construction jobs being done by the Romans. The Romans invented a hydraulic cement (pozzuolana) in the third century B.C., and developed pile driving for foundations and cofferdams (Savile, 1940). Using a combination of these, they constructed seawalls of concrete. They had considerable trouble with sanding up of harbors in some locations, such as in the vicinity of the mouth of the Tiber. For example, at Ostia the shoreline advanced about 3300 ft between 634 B.C. and 82 B.C., and about 27,500 ft since 634 B.C. The port of Ostia was built about 43 A.D., and it apparently became inoperable within 75 years because of its sanding up.

2. WAVES AND THEIR EFFECTS

A large portion of this book is devoted to waves, the theory of waves, their generation, propagation, and effects upon sediments and structures. This is due to three reasons: To begin with, waves are important in almost every phase of oceanographical engineering; secondly, the author’s main field of research has been on waves and their effects; finally, not as much is known of most of the remaining fields.

There are many examples in the literature on the combined effects of waves, storm tides, and astronomical tides. The great winter coastal storm of 5-9 March, 1962, off the east coast of the United States was responsible for 34 dead and more than 300 injured, together with damage in excess of $200 million (U.S. Congress, 1962). About 1800 houses were destroyed and another 2200 were severely damaged. The storm surge in the North Sea of January 31-February 1, 1953, drowned more than 300 people in England and 1800 people in the Netherlands; 1800 houses were destroyed in the Netherlands along with a total damage to dikes, buildings, agriculture, and livestock of the order of $250 million (Wemelsfelder, 1954). Tsunamis have also caused tremendous damage. The June 15, 1896, tsunami killed more than 27,000 people and destroyed over 10,000 houses in Japan (Leet, 1948); the April 1, 1946, tsunami was responsible for the deaths of 163 people and $25 million damage in the Hawaiian Islands (Shepard, MacDonald, and Cox, 1950).

Waves are also of interest from the standpoint of understanding the physics of fluids. It is well established that, for one fluid motion to be similar to another, the two situations must not only be geometrically and kinematically similar, but also dynamically similar. Dynamical similarity can exist only if the ratio of one prototype force to model force, say, has the same numerical value as the ratio of each other pair of forces; i.e., the ratio of the prototype to model inertia force must have the same numerical value as the ratio of the prototype to model gravitation force, the ratio of the prototype to model viscous force, etc. These can also be transposed so that the ratio of the inertia force to gravity force of the prototype must be equal to the ratio of the inertia to gravity force of the model, etc. These ratios form the well-known dimensionless numbers of fluid mechanics: Froude number (ratio of inertia to gravity forces), Mach number (ratio of inertia to elastic forces), Weber number (ratio of inertia to surface tension forces), and Reynolds number (ratio of inertia to viscous forces). These ratios can be expressed in a different form. Froude number becomes the ratio of the speed of a disturbance to the speed of a surface gravity wave; Mach number becomes the ratio of the speed of a disturbance to the speed of an acoustic wave; Weber’s number becomes the ratio of the speed of a disturbance to the speed of a surface tension wave; Reynolds number (VD/ν = VD/λ, where ν is the kinematic viscosity) becomes the product of the ratio of the speed of the disturbances (V) to the mean molecular speed (c) and the ratio of representative dimension of the disturbance (D) to the mean free path of the molecules (λ). This last relationship was pointed out by Von Karman in 1923 (see Von Karman, 1956, p. 164) for compressible flow.

The theory of periodic waves is covered rather thoroughly, beginning with the linear theory of irrotational waves, both standing and progressive. It is surprising how well linear theory predicts many of the characteristics of uniform periodic waves. Where experimental data were available, they are given, as it is mandatory for an engineer to know the range of validity of the linear theory. Curves and tables are also presented to facilitate the use of theory by practicing engineers. Linear theory can be used, by the principle of linear superposition, to develop equations that express irregular waves which, in some ways, are a better approximation of the sea surface than is a set of regular waves. One example in which linear superposition is used to a considerable extent is the treatment of the two-dimensional power spectra of wind waves and swell. It is the author’s opinion that this method must be used with considerable caution. The waves in the generating area are highly nonlinear, and the techniques used for calculating power spectra assume that they are a linear phenomenon. This results in ascribing energy to high frequencies as linear component waves, whereas these frequencies are higher harmonics of the lower-frequency nonlinear waves. Because of simplifications of this sort, wrong impressions are created, and the essential nonlinear processes which govern many aspects of wave generation are neglected.

A considerable literature has been developed in which linear superposition has been used to describe ocean waves. This approach appears to be useful in some engineering problems in deep water, such as the response of ships to waves. Most of the work along these lines has been done by statistical means, relating the power spectrum of the waves to the “power spectrum” of heave, pitch, or roll of the ship. Some work has been done on the co-and quadrature spectra, but the physical significance of the results is not always clear. In deep water, the results are probably more likely to be valid for swell than for seas. In shallow water, some of the nonlinear characteristics of waves start to predominate even for swell. The second, third, and higher harmonics become important in calculating water particle velocities and accelerations which are necessary to calculate wave-induced forces on structures. Under these conditions, use of linear superposition is probably a poorer representation of the waves than is the approximation of replacing a single wave of a group by a nonlinear theoretical wave train of the height and period of the single wave. This is especially true, considering the characteristic of ocean waves for the largest waves to be in groups of three, four five, etc., waves which are nearly periodic, as will be shown in Chapter 9.

Because of the importance of the nonlinear waves to engineering problems, the section on linear theory is followed by the theory for Stokes’ waves of finite amplitude, including a section on the highest possible waves of this type. The theory of Stokes’ waves for a ratio of wave length to water depth of greater than about 10 is not satisfactory in many respects. In the more shallow water region, the theory of cnoidal waves should be more useful, and a section on this type of wave is presented.

In nature, wave trains are not of infinite extent, rather they consist of groups of waves. There are many facets of wave group theory that are still not clear; but the theory as it exists, making use of the principle of linear superposition, is a useful tool in predicting, for example, the time it will take waves to travel from the storm area to the section of coast, or other ocean areas, in question. The phenomenon of wave groups is connected with the dispersive characteristic of waves.

“Water seeks its own level” is a well-known saying. How it does so is a gravity water wave phenomenon. If by some means water is heaped up over an area of the water surface and then released, the distrubance disperses, the water seeks its own level. The heaped-up water may be expressed in Fourier integral form consisting of a combination of all possible wave lengths. If each wave component traveled out at the same velocity, the disturbance wouldn’t disperse, it would travel as a concentrated disturbance. If each wave length travels at a different velocity, say, a function of the wave length, then the disturbance will disperse. The longer the wave, the faster it will travel, for a given depth of water. A group of irregular waves will sort itself as it travels, with the longer components gradually moving ahead, and the shorter waves dropping behind.

This is the case, theoretically, except for the peculiar nonlinear phenomenon known as a solitary wave. The solitary wave may be considered as a limiting case of wave motion. It is a purely positive wave, i.e., there is no trough. The theory, and observations of this wave are treated in a separate chapter.

Waves are generated impulsively, whether by a gust of wind blowing over the water surface, a ship moving through the water, a ground effect machine moving over the water surface, the low-pressure area of a hurricane moving over the continental shelf, or by a submarine seismic disturbance. The theory of impulsively generated waves is developed and compared with observations. At one limit are waves which behave as predicted by linear theory. As will be shown in a section describing experiments of waves generated impulsively by suddenly adding a volume of water at one end of a wave tank, the other limit is not simply the solitary wave of theory. A series of crests and troughs, all above the initial water level, has been observed, with the group being amplitude-dispersive. Under extreme conditions, bores have been generated.

As has been pointed out, the effects of tsunamis and storm surges on man are great, even devastating at times. Many of the things that are known of these catastrophic phenomena are described, together with some information on predicting possible maximum conditions in certain areas. Because of the close relationship between the storm surges and tsunamis as forcing functions and the response characteristics of shelfs, bays, and harbors, the problem of this type of oscillation is covered in the same chapter.

Beaches, currents, breakwaters, drilling platforms—how do they affect waves? How high do waves run up on a beach or breakwater? How much energy is dissipated, and how much is reflected? Here we must leave theory, except for the simplest of cases, because we can tell the mathematician so little on how wave energy is dissipated at a structure. Results of much laboratory work must be used to answer problems of the type just posed.

Waves moving in shoaling water transform. The speed of a wave depends upon the water depth as well as upon the wave length, with the more shallow the water, the slower the wave speed. Waves moving in shoaling water at an angle to the bottom contours must bend because of this dependence of speed upon water depth. This process is known as wave refraction. Graphical methods are presented for the determination of the pattern, and the resulting spatial distribution of wave energy, in complicated coastal regions. Finally, these waves either break along shores, or transform into multiple crests. Methods of predicting wave heights on beaches and in harbors are given. In calculating breaker heights, it is necessary to leave theory and make use of empirical results.

Waves reach areas in the geometric shadow of a breakwater. This phenomenon is known as wave diffraction. The theory of this phenomenon and the comparison of model studies with theory are given. Graphical means of predicting diffracted wave heights in the lee of breakwaters, and similar structures or natural obstructions, are given. Connected with diffraction is the Mach-stem type of reflection (or nonreflection). Because of this, wave energy is concentrated in some circumstances, and waves can even be swung around by a curving breakwater. This can lead to conditions that could not be predicted by linear theory. The theory that is available is for blast waves for the simplest of conditions. It is in areas of this sort that the engineer must resort to model studies. Laboratory studies are presented for several problems of this sort in order to develop in the engineer a feeling for some of the more complex phenomena that he may encounter.

Great detail is given on the initial generation of waves by winds and their growth under the action of continuing winds. Statistical...