Coastal Engineering

Processes, Theory and Design Practice

Dominic Reeve, Andrew Chadwick, Christopher Fleming

• Verlag: CRC Press
• Jahr: 2018
• ISBN:
  9781351165501
• Auflage: 3
• Thema: Technology & Engineering
• Thema: Civil Engineering

Effective coastal engineering is expensive, but it is not as costly as neglect or ineffective intervention. Good practice needs to be based on sound principles, but theoretical work and modelling also need to be well grounded in practice, which is continuously evolving. Conceptual and detailed design has been advanced by new industry publications since the publication of the second edition.

This third edition provides a number of updates: the sections on wave overtopping have been updated to reflect changes brought in with the recently issued EurOtop II manual; a detailed worked example is given of the calculation of extreme wave conditions for design; additional examples have been included on the reliability of structures and probabilistic design; the method for tidal analysis and calculation of amplitudes and phases of harmonic constituents from water level time series has been introduced in a new appendix together with a worked example of harmonic analysis; and a real-life example is included of a design adapting to climate change.

This book is especially useful as an information source for undergraduates and engineering MSc students specializing in coastal engineering and management. Readers require a good grounding in basic fluid mechanics or engineering hydraulics, and some familiarity with elementary statistical concepts.

Chapter 1

Introduction

1.1The historical context

The unsuspecting observer of the coast might consider that much of the shoreline reflects a natural response to the prevailing waves and tides. Further, they might have thought that the man-made structures such as harbour breakwaters have a relatively local impact. However, much of the coast we see today has been ‘engineered’ in some sense, and the effects of our interventions can have quite remote impacts due to the way that physical processes can connect apparently separate sections of the shoreline through the transport of sediment. Indeed, the coastline has been ‘engineered’ for many centuries, in the first place for the development of ports and maritime trade as well as fishing harbours to support local communities. Examples of such engineering works are the Port of A-ur built on the Nile prior to 3000 bc and, nearby on the open coast, the Port of Pharos which was constructed around 2000 bc. The latter had a massive breakwater more than 2.5 km long. The Romans invented a hydraulic cement and developed the practice of pile driving for cofferdam foundations, a technique that was used for the construction of concrete sea walls. Whilst these structures were no doubt built on the basis of trial and error procedures, there is no evidence that there was any real appreciation of coastal processes with respect to the siting of maritime infrastructure.

Many early sea defences comprised embankments, but when dealing with coastal erosion problems the hard edge approach dominated, at least in the United Kingdom. In particular, the Victorians were active in their desire to construct promenades in seaside resorts which were usually vertically faced. Coastal processes were not only poorly understood, but there was some confusion as to what the driving forces were. There have been several periods of development of coastal works in the UK over the past century. There was an extensive wall building programme during the 1930s as part of the unemployment relief schemes. These were based on dock wall designs with near vertical profiles. The consequences of ‘bad design’ by building a hard edge structure on a shoreline were, however, appreciated at about this time. An article written by T B Keay in 1941 notes that “… the efforts of man to prevent erosion are sometimes the cause of its increase, either at the site of his works or elsewhere along the coast.” This he explained with an example of a sea wall built at Scarborough in 1887. In just three years it was necessary to add an apron and in a further six years an additional toe structure and timber groynes. He went on to say that an essential preliminary of all coast protection works is to study the local natural conditions.

It was not until the post Second World War period that the theoretical models and ideas that underlie the basic processes began to be developed, save for basic wave and tidal motion. The development of the Mulberry Harbours in the Second World War led to the concept of determining wave climate, using wind data and design parameters such as wave height and wave period. Thus, contemporary coastal engineering effectively began at that time witnessed by the First Conference on Coastal Engineering at Berkeley, California, sponsored by The Engineering Foundation Council on Wave Research (USA). This was closely followed in 1954 with the publication and widespread acceptance of “Shore Protection, Planning and Design –Technical Report No.4” (TR4) by the US Army Corps of Engineers, Beach Erosion Board. The ‘Planning’ part of the title was later dropped, and it became the well-known “Shore Protection Manual,” and more recently has been updated to become the “Coastal Engineering Manual.”

History books are full of accounts of major storms that caused destruction and devastation to various sections of the coast. In more recent times, one of the most significant dates in coastal engineering in England is January 31, 1953, when an extreme storm surge travelled down the North Sea coincidentally with extreme storm waves. The effect was devastating and serves as a poignant reminder as to how vulnerable the low-lying areas of the East Coast are. The post-1953 period saw great activity in the construction of sea defences along that coastline at a time when sea walls and groyne systems were the norm and the overriding criterion was to provide a secure safety barrier against any such event occurring again.

The value of attempting to retain beach material, whether for sea defence, coast protection or recreational use has been recognised for some time. This is to some extent demonstrated by the extensive lengths of coastline that have been groyned in the past. However, it has been suggested that, prior to the 1970s, many responsible authorities, quite naturally, dealt with these matters on a parochial basis with little regard for, or appreciation of, the impact of their actions on neighbouring territory.

This has allegedly lead to some rather undesirable consequences in both conservation and planning terms, and the engineer has been criticised for being insensitive and not paying heed to these issues. There are a number of other factors that should be taken in account before coming to this conclusion. These include the constraints that have, in effect, been imposed by interpretation of government legislation and the nature of the responsibilities that fall upon the various authorities involved in implementing coastal works. These have primarily been to protect people and property from the effects of erosion or flooding in situations where economic justification can be established. In this regard they have generally been demonstrably successful.

It is also evident that, in the past, the planning system has not generally taken the question of long-term coastal evolution into account when in many instances planning permission has been granted for development on sites that have been well-known to be vulnerable to long-term erosion. At the same time conservation issues have developed alongside our appreciation of natural processes and the complex interactions involved.

The major influences that coastal works have had on the shoreline are centred on the degree of interference that is taking place with the natural processes. Harbours and their approach channels have had a significant impact on alongshore drift as have coastal defences themselves through the use of groynes or other similar structures. It is also evident that protection of some types of coast from erosion must deprive the local and adjacent beach system of some of its natural sediment supply. Given that nature will always try to reestablish some form of dynamic equilibrium, any shortfall in sediment supply is redressed by removing material from elsewhere. Such a situation can also be exacerbated by introducing structures that, instead of absorbing energy as a natural beach does, reflect the incident waves to do more damage on the beach in front of the wall.

By the 1960s a much greater understanding of coastal processes emerged as the theoretical development coupled with physical and numerical modelling developed. This led to a gradual reappraisal of coastal engineering techniques in such a way that the design process began to consider studies of the coastal regime and its interaction with the proposed works. By the early 1970s this led to the application of relatively novel solutions to coastal problems such as beach nourishment, artificial ...