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deltas can also become stabilized by

marsh vegetation. This situation and

that of the vegetated

washover flat can

lead to the presence of very thin coal

lenses overlying organic-rich sheet

sandstones in the rock record (Fig.

14). Subaqueous shale and siltstone

facies in Cretaceous lagoon deposits

are often characterized by brackish

water bivalve shells, (Kirschbaum,

1989) and coquinid oyster beds (Land,

1972). Disseminated carbonaceous

material, imprints of plant remains,

and root and reed fragments are

common in some shale beds, indi-

cating the interfingering of proximal

marsh and subaqueous lagoonal envi-

ronments.

Estuarine sequences from partially

closed and open-ended estuaries will

also be characterized by stacked

facies representative of several depo-

sitional subenvironments, although

vertical successions will tend to be

less variable than those generated in

shallow lagoons. Examples of such

modern estuaries include the Mira-

michi, New Brunswick (Reinson,

197713); Georgia and South Carolina

estuaries (Frey and Howard,

1986),

and those situated on the New South

Wales coast, Australia (Roy, 1984).

Typical generalized successions are

described by Clifton

(1982), and Frey

and Howard (1 986).

Ancient estuarine facies are charac-

terized by a set of sedimentary struc-

tures which indicate tidal influence and

biotic stress. Physical sedimentary

structures diagnostic of diurnal and

lunar tidal-flow responses include tidal

bundles, mud-layer couplets, sigmoidal

bedding and inclined heterolithic strati-

fication (Chapter 11; Nio and Yang,

1991

;

Thomas et al., 1987; Kreisa and

Moiola, 1986). Examples of these struc-

tures in ancient successions are docu-

mented also by Rahmani

(1989),

Reinson et al. (1988), Reinson (1989)

and Kuecher et al. (1990). Charac-

teristic trace fossil assemblages reflect

brackish to stressed conditions (Chap-

ter

4),

and have been illustrated in

several rock studies

(Reinson et al.,

1988;

Wightman et a/., 1987).

Because the occurrence of estuarine

deposits is usually related to submer-

gence or transgression, complete axial

facies successions should reflect the tri-

padite depositional realms, from fluvial

at the base through mixed fluvial-ma-

rine, to marine at the top (Nichols and

Biggs, 1985). The Gironde estuary

model (Fig. 8) depicts marine estuary-

mouth deposits overlying estuary muds,

and the rock sequences illustrated in

Figure 13 depict similar superposition of

progressively more seaward deposits

over landward inner estuarine and

fluvial

bounding unconformities must also be

identified in order that sequence strati-

graphic ideas can be applied; they

include ravinement surfaces (trans-

gressive surfaces of erosion, Chapter

I), marine flooding surfaces, surfaces

onlap and downlap, channel bases,

and regressive surfaces of erosion

deposits, within the continuous valley fill

(chapter 1). Most important of all is

succession. Wave-dominated coastal the recognition of incised valleys

(SE,

plain estuaries (whether partially closed

,daerial erosion in Figure 8 of

or open-ended) will be characterized by

Chapter 1

in which the estuaries

a set of vertical successions that reflect form. A vertical facies progression

position within the estuarine depositional

from

fluvial to estuarine in some valley

system. In ancient sequences, for fill successions can be expected

example, the recognition of

fluvial de- (Howard and Whittaker, 1990;

posits grading upwards to estuarine

Wheeler et al., 1990). In other succes-

facies should reflect location in the upper

sions, reworking of the

fluvial sedi-

reaches or

fluvial realm. Accretionary

ments and modification of the original

channel fill and tidal delta facies should river valley topography early in the

indicate littoral deposits situated at the transgressive phase have obscured or

estuary mouth. The tidal range and geo-

erased the evidence of initial

fluvial in-

morphologital configuration of ancient

cision. A

fluvial origin can commonly

estuaries could be determined if laterally

be inferred from the valley orientation

accreting estuarine point-bar or coars-

with respect to regional dip and strike

ening-upward bay fill facies successions directions, and the stratigraphic

were encountered.

context of the valley fill deposits.

The placement of estuary fill facies

Estuary bounding discontinuities

in the lowstand systems tract by the

Vertical facies successions containing Exxon sequence stratigraphic school

stressed trace fossil assemblages and

(Van Wagoner et

a/., 1990) should

tidal

bedform structures do not un-

probably be revised. The river valley is

equivocally identify ancient estuarine

incised during falling stage and low-

deposits. Recognition of the overall

stand, but is only transformed into an

geometry and bounding discontinuities

estuary during rising stage of sea

is important for identifying

allostrati-

level. The deposits are therefore better

graphic units, and for defining the in-

placed in the transgressive systems

ternally complex but genetically

tract. The base of the incised valley,

related estuarine rock bodies. The

as seen in the geological record, may

S~LTSTONE

WITH

QUARTZOSE

SANDSTONE

FLASERS

TIDAL FLAT

CLAY SHALE WlTH SIDERITE BANDS,

LAGOON

BURROWED FOSSILIFEROUS

AL SEATROCK,CLAYEY

NDSTONE, QUARTZOSE PLANAR ACCRETIONS BED

WASH

SHALE AND SILTSTONE COARSENING UPWARD BURROWED

LAGOON

CLAY SHALE, SIDERITE BANDS,

SWAMP

LIMESTONE BURROWED. FOSSILIFEROUS

TIDAL

CHANNEL

SANDSTONE.

QUARTZOSE

FINING

UPWARD,

RIPPLED AND CROSS- BEDDED

TIDAL FLAT

SILTSTONE WITH SANDSTONE FLASERS

FLOOD TIDAL DE

BURROWED SIDERITIC SANDSTONE

SANDSTONE

QUARTZOSE CROSS- BEDDED

SHALE AND

SILTSTONE

COARSENING UPWARD, BURROWED

LAGOON

CLAY SHALE SIDERITE BANDS.

BURROWED,

F'OSSILIFEROUS

Figure

Generalized lagoonal sequence through back barrier deposits in the

Carboniferous of eastern Kentucky and southern West Virginia. Such sequences range from

7.5

m thick. From Horne and Ferm

978).

_________

10. BARRIERS AND ESTUARIES

I) the original fluvial incision surface,

or 2) an original

fluvial incision totally

modified by the initial stages of marine

transgression. In the first case,

fluvial

facies may immediately overlie the inci-

sion and pass upward into estuarine

facies.

the second case, no fluvial

facies will be preserved, and the valley

fill will begin with brackish to estuarine

facies, as in the estuary fills in the Viking

WATER

DEPTH

(rn.)

DEPOSITIONAL

ENVIRONMENT

OUTER

SHELF

------------

INNER

SHELF

---------

SHOREFACE

RAVINEMENT

DISTAL

LAGOON

~~I~~A

SEQUENCE BOUNDARY

OVERWASH-DOMINATED COAST

OUTER

SHELF

INNER

SHELF

RAVINEMENT

TIDAL

SHOAL

CHANNEL

ESTUARINE

LAGOON

Formation, Alberta (Reinson et

a/.,

1988; Boreen and Walker, 1991).

The correct identification of se-

quence-bounding unconformities and

ravinement surfaces on transgressed

barrier island and estuary coasts may

be difficult and controversial. The

problem has been discussed, with

several useful examples (Fig.

15), by

Demarest and Kraft (1 987).

WATER

DEPTH

(m.)

DEPOSITIONAL

0 100

ENVIRONMENT

OUTER

SHELF

-------------

INNER

SHELF

RAVINEMENT-

TIDAL

CHANNEL

BASE

INLET-

ESTUARINE

LAGOON

INLET/SPIT-DOMINATED COAST

WATER

DEPTH

(m.)

DEPOSITIONAL

loo

ENVIRONMENT

OUTER

SHELF

------------

INNER

SHELF

I@@//@

SEQUENCE

BOUNDARY

HIGHLAND BEACH

Figure

Vertical sequences produced through transgression of different types of coastal

environments. Modified from Demarest and Kraft

(1987).

TRANSGRESSIVE

STRATIGRAPHIC

MODELS

Estuarine valley fill deposits rarely

consist of a single fill cycle formed

during one rising stage of sea level

(Clifton, 1982). Rather, several suc-

cessions may be represented in the

valley fill deposits because the estuary

would probably occupy the same

valley during several minor sea level

fluctuations. The recognition of estu-

arine deposits within incised valleys is

important in understanding transgres-

sive systems tracts, bounding disconti-

nuities (ravinements, flooding sur-

faces), and overall transgressive strati-

graphic models.

Transgressive

depositional systems

Barrier island and estuarine depositional

systems commonly form part of the

transgressive systems tract. Barrier

islands can also

prograde (normally in

the highstand systems tract), but com-

monly are transformed into

strand-

plains (Chapter 12). Estuaries have a

uniquely transgressive origin, and

rarely occur as significant components

of the highstand systems tract. Trans-

gressive and regressive barrier shore-

line migrations produce very different

facies successions, related to trans-

gression, regression and inlet migra-

tion (Fig. 9;

Reinson, 1984).

Beaches, barrier islands and estu-

arine shorelines retreat landward in re-

sponse to sea level rise or subsidence

or a reduction in sediment supply.

Most modern barrier island systems

have migrated landward in response

to the Holocene transgression, and the

trend is continuing today (Kraft and

Chrzastowski, 1985). Modern coastal

plain estuaries also formed during the

Holocene transgression, and major

estuary tracts usually are juxtaposed

or associated with barrier shoreline

coasts (Meade, 1969; Hayes, 1975,

1979; Frey and Howard, 1986).

Transgressive systems;

contlnuous shoreface

retreat versus

In sltu drowning

The idealized transgressive barrier

island facies succession (Fig. 9) will be

extremely modified and attenuated as

the depositional system migrates land-

ward in response to sea level rise. This

is evident from the two proposed

mechanisms of barrier island shoreline

behaviour during marine transgression

(Fig. 16). The mechanism of

erosional

shoreface retreat

is widely accepted

because of strong evidence from de-

tailed studies of the U.S. Atlantic

shoreface and inner continental shelf

(Niedoroda

et al.,

1985). This involves

sediment being eroded from the upper

shoreface and transported

1) to the

lower shoreface or offshore as storm

beds, or

to the lagoonal estuary as

washover deposits. As the upper

shoreface zone moves landward and

upward during continued transgres-

sion, it erodes the barrier,

washover

and lagoonal facies to form a planar

erosional surface (ravinement surface),

upon which the redistributed lower

shoreface-inner shelf sands are de-

posited (Fig. 16A; Nummedal and

Swift, 1987). This model predicts that

the thickness of transgressive facies

successions will depend on the rate of

relative rise of sea level. If sea level

rise is slow compared with the rate of

landward erosion, almost all of the

barrier system may be destroyed. The

only stratigraphic record will be a thin

layer of lagoonal, tidal channel and

washover facies resting unconformably

on older sediments (Pleistocene in Fig.

16A). This thin layer is truncated by the

ravinement surface, which in turn is

overlain by offshore storm deposits in

an overall transgressive, fining-upward

succession.

Under conditions of continuous but

slow erosional shoreface retreat, cer-

tain architectural elements of trans-

gressive depositional systems may be

prese~ed. Transgression of a coastal

plain previously dissected by an epi-

sode of significant sea level fall will

produce shore-normal estuaries and

shore-parallel lagoons (both with asso-

ciated tidal channels). The resultant

stratigraphic succession would be

characterized by an inner-shelf sand

sheet that overlies

fluvial and estuarine

sediments which fill former subaerial

valleys (Nummedal and Swift, 1987;

Reinson

et al.,

1988; Boreen and

Walker, 1991). Indeed, relict tidal inlets

and channels have been interpreted for

the numerous incised channels found to

be cutting the middle-inner continental

shelf off North Carolina (Hine and

Snyder, 1985).

The

in place drowning

model (Fig.

166) has been proposed as an alterna-

tive to erosional shoreface retreat. It is

suggested that the barrier remains in

place as sea level rises, until the wave

zone reaches the top of the barrier.

The wave zone then oversteps land-

ward and a new sand barrier forms on

the inner side of the lagoonal estuary.

The old barrier system is essentially

drowned in place

(Rampino and

Sanders, 1980). This in place drowning

model predicts successions with a high

preservation potential,

such that with

rapid relative rise of sea level, almost

complete transgressive sequences

could be preserved (Fischer, 1961;

Swift, 1975).

Transgressive vertical successions

Specific vertical successions can indi-

cate the type of shoreline being trans-

gressed, whether it is tide or storm

SHOREFACE RETREAT

WASHOVER

ADVANCING

BARRIER

SANDS

SHOREFACEISHELF

SAND

IN-PLACE DROWNING

NEW

BARRIER

SHOREFACE

DROWNED

BARRIER

SEDIMENTS

Figure

Mechanisms

landward barrier migration during a transgression by shoreface

retreat

(A)

and by in place drowning

(B).

Modified after Rampino and Sanders (1980) and

Elliott

986).

TRANSGRESSION THROUGH EROSIONAL SHOREFACE RETREAT

NON-DISSECTED

BARRIER-ISLAND

HIGHLY-DISSECTED

COASTAL

PLAIN

COASTAL

PLAIN

COASTAL

PLAIN

SHELF

ESTUARY BAY

SEQUENCEBOUNDARY

VERTICAL

SCALE

metre8

Figure

Generalized bnd-member" transgressive facies successions for nondissected,

barrier-island, and highly dissected coastal plain settings.

10. BARRIERS AND ESTUARIES 191

wave dominated, whether there may be

thick shore-parallel or shore-normal

sand bodies along depositional strike, or

whether extensive estuarine channel or

valley-fill deposits are likely to be

present. For example, the transgressive

vertical succession expected on an

overwash-dominated coast (Fig. 15A)

will indicate a high wave energy,

mi-

crotidal coast with abundant shore-par-

allel lagoons and a paucity of large

active tidal inlets. This is in contrast to

the facies succession indicative of a

mesotidal coast dominated by tidal

channels and spits. Here, thick channel

sands occur below the ravinement

surface (Fig.

158). The vertical facies

succession generated in a large estuary

(Fig. 15C) should exhibit an overall

coarsening-upward succession that rep-

resents the superposition of distal over

proximal deposits as the estuary mouth

transgresses landward. Note the simi-

larity of this succession to the estuary

bay sequences illustrated in Figure 13.

The thinnest transgressive vertical suc-

cession (Fig. 15D) is produced when

shoreface retreat erodes below the

sequence boundary; for practical

purposes, the sequence boundary and

ravinement surface are coincident. The

thin transgressive shoreface deposits of

Figure 15D are similar to many of the

transgressive coarse-grained deposits

in the Cardium Formation (Chapter 12,

and Figure 8 in Chapter 1). Careful

study of Figure 15 shows that many

bounding discontinuities are difficult to

identify, and may be even more difficult

to interpret. They raise many problems

in the use of sequence stratigraphic

concepts (Chapter 1).

Transgression due to erosional

shore-

face retreat will tend to create thin facies

successions, with only partial preser-

vation or nonpreservation of many of

the diagnostic depositional facies. In

place drowning would result in preser-

vation of more complete barrier suc-

cessions, as in the idealized end

member models of Figure 9. However,

recognition of the partially preserved

successions may be difficult. Erosional

shoreface retreat may occur in dif-

ferent coastal plain settings (Fig.

17),

and different transgressive vertical

facies successions would be gener-

ated. Figure 17 is generalized, but en-

compasses several variations which

have been discussed and illustrated in

Figures 13 and 15.

TRANSGRESSIVE BARRIER AND

ESTUARY MODELS

Vertical facies successions formed

during transgression of barrier island

and estuarine systems can be highly

variable in terms of thickness and

facies complexity. Their degree of pre-

servation in the rock record will also be

varied and dependent upon the factors

that govern the mechanism and rate of

transgression (erosional shoreface

retreat versus rapid inundation).

Intense shoreface erosion during

transgression will tend to favour pre-

servation of only partial sequences

whereas rapid inundation should result

in the preservation of almost complete

successions. If complete successions

are present in the geological record,

they should be similar to the "end

membern barrier model described by

Reinson (1 984).

Transgressions commonly affect

coastal plains that have recently been

dissected during a significant base level

drop. This may lead to the preservation

of characteristic incised valley fill and

estuary sequences overlain by inner

shelf deposits (Fig. 17). Estuaries are

by definition transgressive depositional

systems; therefore estuarine succes-

sions should be associated with other

transgressive deposits as part of the

transgressive systems tract. On

wave-

dominated and microtidal barrier island

coasts which have not been highly dis-

sected by a preceding base level drop,

intense transgressive erosion of the

shoreface will lead to preservation of a

thin back-barrier lagoon deposit capped

by an extremely thin coarse lag which

recards the ravinement surface. This, in

turn, will be overlain by offshore de-

posits (partly bioturbated mudstones,

partly storm-emplaced sandstones). In

the extreme case of a nondissected

mainland coast the only evidence of

transgression by erosional shoreface

retreat will be the thin ravinement lag

deposit encased in shelf sediments

(Fig. 17).

Barrier island and estuarine systems

clearly respond rapidly and sensitively

to relative fluctuations of sea level. It is

therefore very important to identify cor-

rectly the various bounding discontinu-

ities, in order that the discrete units

(allostratigraphic units) can be recog-

nized, and interpreted in vertical suc-

cession. A fall in sea level may result in

valley downcutting (incision) with corre-

sponding erosion in the marine realm

(regressive surface of erosion; Chap-

ters 1 and 12). During rising sea level,

the surface of initial transgression

very important in estuarine deposits. It

may lie above a preserved lag of

fluvial

deposits in the estuary, or it may coin-

cide with the basal (initially

fluvial)

erosion surface if all of the fluvial de-

posits have been reworked during initial

transgression. The estuary fill may be

truncated by a second transgressive

erosion surface (ravinement surface) or,

in open-ended estuaries, the estuary fill

may grade vertically into nearshore

marine deposits. Elucidation of estuary

type and the history of sea level fluctua-

tion is therefore dependent largely on

the identification and correct interpreta-

tion of the bounding discontinuities.

ACKNOWLEDGEMENTS

thank Roger Walker and Douglas

Cant for critical review of the prelimi-

nary manuscript.

am grateful to

Eleanor Gordon for typing the various

manuscripts and Peter Gubitz who

drafted the figures.

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1985,

Coastal

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es-

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---

--_____

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INTRODUCTION

Most books, including former editions of

Facies Models, subdivide coastal and

shallow marine environments on the

basis of geomorphology

(e.g

deltas,

barrier islands, shelves). As a result,

tide-dominated environments are dis-

cussed in several different chapters.

However, the tidal sand ridges that

characterize tide-dominated deltas

have more in common with tidal shelf

and estuarine sand ridges than with the

distributary mouth bars of fluvially domi-

nated deltas, or with the shoreface of

wave-dominated deltas and barrier

coastlines.

This chapter takes a new approach,

examining all tide-dominated deposi-

tional systems together (tidal flats, estu-

aries, deltas and shelves), and dis-

cussing their response to changes of

relative sea level. Thus it is recognized

that the relative intensity of the pro-

cesses (tidal currents, waves,

storm-

generated currents, and fluvial cur-

rents) determines the character of the

deposits as much, if not more, than the

geomorphological setting (Galloway,

1975; Hayes, 1979; Johnson and

Baldwin, 1986).

Tidal dominance over other proces-

ses is most common in areas where the

tidal range is large, because this results

in strong tidal currents. Consequently,

most microtidal (0-2 m tidal range) and

mesotidal (2-4 m range) areas are

wave (or storm) dominated, whereas

some mesotidal and most macrotidal

(>4

m range) areas are tide dominated.

However, if wave action is limited due

to topographic sheltering,

the tidal

current speeds are increased by a top-

ographic constriction, tidal dominance

can even occur in microtidal areas.

TIDES AND TIDAL CURRENTS

Origin

tides

tide is any periodic (regularly re-

peating) fluctuation in water level

(Fig. 1) that is generated by the

gravi-

lST

QUARTER FULL

3RD

QUARTER NEW

NEAP

SPRING NEAP

SPRING

-'3

1~111

l,l~l~sl

JANUARY 1982

Figure

Predicted tidal variations in water level at Saint John, New Brunswick. Following

standard practice, the datum is approximately the spring low-tide elevation. The tides are

semidiurnal with a pronounced variation in range between neap and spring tides.

Tidal-

current speeds mimic the variations in tidal range because the range determines the tidal

discharge which passes a point each half tidal cycle. After Forrester

(1983).

Centre

mass,

Earth

Moon system

Tidal bulge

FORCES

Tide -generating

Gravitational

Centripetal

Figure

Origin of the tides. On the side of the Earth facing the moon, the tide-generating

force is gravitational-minus-centripetal. On the opposite side of the Earth, the gravitational

force from the moon is weaker, and the tide-generating force is

centripetal-minus-gravita-

tional. At the top and bottom of the Earth, as drawn, the tide-generating force is directed

inward, as the resultant of a gravitational force directed toward the moon, and the centripetal

force. The centripetal forces affecting the ocean all operate in the same direction, and result

from movement of the Earth around the centre of mass of the Earth-moon system. This point

is about one-quarter of the distance from the surface to the centre of the Earth. The effects

of the gravitational field of the sun have been omitted. Diagram courtesy Roger Walker.

196 DALRYMPLE

tational attraction of the moon and

sun. The moon, because it is closer to

the Earth, exerts a tide-generating

force which is twice as large as that of

the sun. One of the best nonmathe-

matical discussions of the origin of

tides is given by the Open University

Course Team (1 989). More mathemat-

ical descriptions are given by Howarth

(1 982) and Forrester (1 983).

Tides represent the vector sum. of

two forces: 1) the gravitational attrac-

tion of the moon, which is strongest on

the side of the Earth facing the moon,

and 2) the centripetal force caused by

the revolution of the Earth-moon sys-

tem about its common centre of mass.

This centripetal force is of uniform in-

tensity all over the Earth (Fig.

2).

the side of the Earth facing the moon,

the gravitational attraction of the moon

is greater than the oppositely directed

centripetal force, while the reverse is

true on the other side of the Earth. The

water in the oceans therefore piles up

in two bulges, one underneath the

moon, and the other on the opposite

side of the Earth. Because the Earth

Figure

Arnphidrornic tidal system in the North Sea. The tidal range (dotted corange

lines) increases outward from each of the three amphidromic points, with the highest ranges

occurring in coastal ernbayrnents such as the German Bight (3

m) and those on the east

coast of England

rn). The solid cotidal lines show the times of high water in lunar hours,

and indicate the direction of propagation of the tidal wave (anticlockwise). After Houbolt

968).

rotates, the bulges appear to travel

around the Earth as two ''tidal waves",

causing water levels to rise and fall

regularly. Rising water levels are

known as the

flood

tide, whereas the

fall is the

ebb

tide. The most com-

monly observed period for one com-

plete

tidal

cycle

is the

semidiurnal

(twice a day) period of 12.42 hours

(Fig. 1). This is longer than 12 hours

because the moon orbits in the same

Figure

Rotary tidal currents and the

tidal ellipse for a site on Georges Bank.

The tidal ellipse is constructed by joining

the tips of the successive current-velocity

vectors which have a length scaled to the

current speed. Note that current speeds

never fall below 0.3

rnls. After Stewart and

Jordan

964).

'3%

'5'E

General

Flow

Direction

Estuariesllnlets

im/sec

\\QQOOB~Q

Figure

Tidal current ellipses for corn-

puted velocities

rn above the bed in the

southern North Sea. Note the extreme

elongation of ellipses in the south due to

the coastal constriction, and the more cir-

cular shapes in the north. Current direc-

tions in the mouth of the Thames River and

Dutch estuaries are approximately perpen-

dicular to those on the shelf. After

McCave

(1971).

11.

TIDAL SYSTEMS

direction as the Earth rotates, and any

point on the surface of the Earth re-

quires a little more than one revolution

to return to the peak of the tidal bulge.

Because the rotational axis of the

Earth is inclined with respect to the

orbital plane of the moon most of the

time, any given point on the Earth's

surface passes closer to the crest of

one tidal bulge than the other, thereby

adding a

diurnal

(once a day) compo-

nent to the tidal spectrum. The diurnal

component is generally small relative

to the semidiurnal tide, but it is the

main tidal period in areas where the

semidiurnal tide is damped out by de-

structive interference.

The interaction of the moon and sun

produces a still longer periodic varia-

tion in

tidal range

(the difference in

water level between successive

high-

tide and low-tide levels). When the sun

and moon lie in a straight line relative

to the Earth, their effects add to pro-

duce greater than average tidal ranges

(spring tides;

Fig.

1).

When the sun

and moon are at right angles, their

forces counteract each other and the

tidal range is smaller than average

(neap tides).

For semidiurnal tides, the

spring-neap cycle has a period of

14.77

days, and contains

tidal

cycles. Diurnal tides have a neap-

spring period of

13.66

days that con-

tains

tidal cycles.

Tidal range

Because the tide-generating forces

are small, only the open oceans

develop significant tides, and even

these typically have a range of less

than

m. Smaller bodies of water (in-

cluding enclosed seas such as the

Mediterranean and the water overlying

the present-day continental shelves)

cannot develop an appreciable tide of

their own. The tides which are ob-

served on continental shelves are due

to the

forcing

action of the oceanic

tide. As the tidal wave moves from the

Figure

Maximum near-surface current speeds (cmls) during spring tides on the conti-

nental shelf around the British Isles. After

Howarth

(1982).

open ocean onto a continental shelf,

shoaling, and convergence associated

with coastal embayments concentrate

the energy within the tidal wave into a

smaller cross sectional area, and the

tidal range increases. Particularly

large ranges occur if the

natural period

of the water body (i.e., the period for

the water to "slosh" back and forth in

the absence of any forcing action) is

close to one of the astronomically de-

termined tidal periods, in which case

resonant amplification

takes place.

This is shown dramatically by the Bay

of Fundy-Gulf of Maine system where

tidal ranges reach

16.3

Tidal currents

Tidal current speeds and flow directions

vary systematically over a single tidal

cycle. In open shelf settings, far from

the confining influence of a coastline,

the

Coriolis

force causes the tidal wave

to rotate about a fixed

(amphidromic)

point (Fig.

3).

Consequently, the tidal

current speed and direction are con-

stantly changing, and the tip of the

current velocity vector traces out a path

termed a

tidal ellipse

over a single tidal

cycle (Fig.

4).

In the northern hemi-

sphere, the tidal wave typically moves

in an anticlockwise direction about an

amphidromic point, while the current

vectors rotate in a clockwise sense.

These

rotary

tides have no

slack-water

period when current speeds decrease

to zero. In the ideal case, the tidal

ellipse is a circle, but in nearshore

areas, the presence of the shoreline

causes current speeds to be higher

parallel to the coast, and weaker in an

onshore-offshore direction (Fig.

5).

the shore, these

rectilinear

currents

simply reverse by

180"

at each slack

water period. In estuaries and coastal

embayments, the currents tend'to flow

into and out of the basin, with flow di-

rections that are at a high angle to the

general trend of the shoreline (Fig.

5).

The maximum current speed at any

location is

controlled by the volume of

water that must pass that point in each

half tidal cycle. The volume of water is

a function of the tidal range

and

the

size of the area being drained or

flooded. Strong tidal currents are com-

monly associated with large tidal

ranges. However, they can also occur

in areas with small tidal ranges if the

estuary or embayment is large relative

to the cross sectional area of the mouth