Hydraulic Behaviour of Estuaries PDF
Preview Hydraulic Behaviour of Estuaries
Hydraulic Behaviour of Estuaries Civil Engineering Hydraulics Series General Editor: E. M. Wilson Professor of Hydraulic Engineering, University of Salford Hydraulic Behaviour of Estuaries D. M. McDowell and B. A. O'Connor Simon Engineering Laboratories. University of Manchester M C D. M. McDowell and B. A. O'Connor 1977 Softcover reprint of the hardcover 1st edition 1977 All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, without permission First published 1977 by THE MACMILLAN PRESS LTD London and Basingstoke Associated companies in New York Dublin Melbourne Johannesburg and Madras ISBN 978-1-349-01120-9 ISBN 978-1-349-01118-6 (eBook) DOI 10.1007/978-1-349-01118-6 This book is sold subject to the standard conditions of the Net Book Agreement Contents PREFACE vu 1. A GENERAL DESCRIPTION OF ESTUARINE BEHAVIOUR 1 2. HYDRODYNAMICS OF ESTUARIES 30 3. MIXING PROCESSES 48 4. SEDIMENT MOVEMENTS 83 5. THE STUDY OF TIDAL SYSTEMS: FIELD MEASUREMENTS 124 6. THE STUDY OF TIDAL SYSTEMS: MATHEMATICAL TIDAL MODELS 146 7. THE STUDY OF TIDAL SYSTEMS: WATER QUALITY MODELS 177 8. THE STUDY OF TIDAL SYSTEMS: HYDRAULIC MODELS 197 9. CONTROL OF ESTUARIES 225 10. DISCUSSION OF CASE HISTORIES 250 INDEX 219 [vi Preface Estuaries are meeting places-of salt water and fresh water, of fresh-water and salt-water flora and fauna, of land-borne and sea-borne sediments, of sea farers. This complexity accounts for much of their fascination for people like the authors of this book, but it makes it difficult to study estuaries and to describe them in a single introductory volume. We have confined the book to the flow of water, salts and water-borne solids; that is, to the rather narrow view of physical behaviour that excludes flora and fauna. We realise that plant and animal life have an essential role in the physical behaviour of most estuaries, in colonising mud fiats, in changing the character of fine sediments and, in warm climates, in determining the topography by growth of coral and cementation of shell debris. We have concentrated on the movement of water and its immediate effects. Our purpose has been to expose the mechanics of estuarine systems and to indicate the principal methods used to control them. There have been many descriptions of estuarine behaviour, but all, including this book, have been limited by the power of words and the poverty of our scientific formulations. In most of them, an 'if only ... ' approach has been used. The estuaries have been straightened and their cross-sections deformed into neat shapes while the water has been homogenised or, as a concession to reality, stratified in an attempt to make the physical processes manageable. An optimistic view has been taken of such complexities as sediment transport, use being made of equations that do not work at all well even for the uni directional flow situations for which they were designed. In this book, we have tried to give an honest account of how real estuaries behave and to show how they might be managed. Underlying the whole book is a description of the interaction of the many physical factors. Much of this can be read and understood without any knowledge of advanced mathematics. Inevitably some chapters are based on mathematical equations. Summaries of these have been provided so that the non-mathematician can see what can be achieved by mathematical tools. Chapters 1, 5, 9 and 10 give an account of estuarine behaviour; of the care that must be taken in trying to measure it quantitatively; of methods of estuarine management by engineering work; and of the response of several actual estuaries to control work. These can all be read without much [vii] PREFACE viii specialised knowledge. The other chapters are concerned with mathematical formulation of the behaviour of real estuaries and with aids to solution of real problems using physical and mathematical modelling techniques. We have tried to describe the physical basis of each technique and to demonstrate its use and limitations in dealing with real situations. We consider that, in an introductory book such as this, it is more important to present a global view of estuarine situations than to explore techniques or specific items of estuarine behaviour in great depth. We also believe that a sound understanding of estuarine physics is a necessary prerequisite for all concerned with their management and control. There is nothing more frightening to a beginner than an exhaustive reference list and bibliography. The selection that we have made should open the way to the tremendous amount of published work that is now available. Exclusion or inclusion of your favourite book should not be construed as our judgement of its merit! We are indebted to many people, publishing houses and institutions whose work we have quoted and gratefully acknowledge. We have included much that has not been previously published, mostly of work done by us or with which we have been closely concerned. We would particularly like to thank the following organisations for permission to reproduce their material: Department of Civil Engineering, University of Liverpool and the Mersey Docks and Harbour Company for data concerning the Mersey Estuary; British Transport Docks Board for data concerning the Severn and Humber estuaries; the Chairman and Chief Hydraulic Engineer of Calcutta Port Trust for data concerning the River Hooghly; the Hydraulics Research Station, Wallingford and the Water Research Centre, whose work is acknowledged under the all embracing title of Her Majesty's Stationery Office. Manchester, 1977 D.M.McD. B.A.O'C. 1 A general description of estuarine behaviour The hydrological cycle begins and ends in the sea. The first stage in the cycle is evaporation of water from the surface of the sea and the last stage is the return of water to the sea through rivers. Most rivers enter the sea where there is enough tidal rise and fall to modify flow near their mouths. The part of the river system in which the river widens under the influence of tidal action is the estuary, but the behaviour of the estuary is influenced by the circulation of water and solids in the sea as well as by the whole tidal part of the river system. The estuary cannot be considered in isolation; the whole system has many inter-dependent parts, extending from the landward limit of the tidal rivers forming it to a point offshore beyond which the effect of an individual estuary on water circulation and sediment movement can no longer be discerned. Much detailed work has been done on the behaviour of estuaries and some of this will be described in later chapters. Before any particular study can be begun, however, it is essential to understand the mechanism of the estuary; the way in which the individual parts of the system react with each other and how each affects, or is affected by, the others. Movement of water under the action of tides and river flow is closely inter-related with movement of sedi ment; both are affected by wave action and tidal currents in the sea outside the estuary. It is convenient to consider the various components separately, but it must never be forgotten that they cannot behave independently. For this reason, this first chapter is devoted to a general discussion of the behaviour of typical estuaries. Estuaries are governed by tidal action at the sea face and by river flow. These are the main independent variables. The boundary shape of the estuarine system is determined by the geomorphology of the land and the properties of all alluvial materials that form the bed and banks of the channels. Usually, the overall boundary shape changes only slowly, though there may be rapid local or short term adjustments. In some cases, tectonic movements have caused changes but these are not considered here. Gradual changes take place due to accumulation and re-distribution of river-borne solids, but their importance varies very greatly in different estuarine systems. The geomorphology of an estuary basin is, essentially, a fixed boundary [ 1 ] 2 HYDRAULIC BEHAVIOUR OF ESTUARIES condition but the channels as modified by flow can be regarded as a variable boundary. Sea-borne sediment stirred up by tidal currents and wave action can enter an estuarine system from beyond any immediate zone influenced by the estuary. Where this happens, the influx of sea-borne sediment becomes another independent variable that must be reckoned with in any analysis. 1.1 TIDES AND MEAN TIDAL CURRENTS 1.1 .1 Tides at coastal sites Tides in the sea result from the gravitational pull of the moon, the sun and the planets and from local meteorological disturbances. The effect of varying gravitational pull can be predicted with quite high accuracy. The meteoro logical effects are random in their occurrence and, apart from some general seasonal trends, can only be predicted a short time in advance. Rise and fall of sea-level is essentially independent of conditions within an estuary, the only exception being that very large discharges of fresh water may occasionally result in a slight increase in water level for a few kilometres inland of the mouth of an estuary. The moon orbits the Earth once in 28 days and the whole system orbits round the sun once in 365·2 days. The paths of the moon round the Earth and the Earth round the sun are both elliptical, so the gravitational force of attraction passes through a maximum and a minimum during each orbit. The axis of the Earth is inclined to the plane of its orbit round the sun, and the orbital plane of revolution of the moon around the Earth is also inclined at an angle to the Earth's axis. Consequently, the gravitational tide-producing force at a given point on the earth varies in a complex but predictable manner. The largest component of the force is due to the moon, and has a period of about 12 h 25 min. The lunar force itself reaches a maximum once in 28 days, when the moon is nearest to the Earth, or at perigee. When the moon is at apogee, furthest from the Earth, the lunar tidal force is only about ! of its maximum value. The total tidal force due to the combined action of sun and moon is greatest when they act together; that is, when sun and moon are as nearly in line with the Earth as possible. This occurs twice a month, when the moon is on the same side of the Earth as the sun and when they are on opposite sides, i.e. at new moon and full moon. When this happens, spring tides occur, having a range of movement greater than average. When sun and moon are in quadrature with the Earth, their effects give rise to smaller-than average neap tides which also occur twice a month. It may not be immediately obvious that two tides will occur during each rotation of the Earth, and that spring tides will occur when the forces due to the sun and the moon appear to be in opposition to each other. The explana tion follows from the law of gravity: that the gravitational force varies inversely as the square of the distance between two objects. Consider the Earth and the moon as two bodies moving around a common centre. They are kept in orbit by gravitational pull, which is just equal to the centrifugal force due to their rotation. The Earth, however, is large enough for the forces
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