Table Of ContentHydraulic 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
