Tides in Estuaries
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Why are tides important in Estuaries ?
Some Definitions of Tides
Response of estuaries
Measurement of Tides
Tidal Waves
• Regular rise and fall in sea level
– Quasi-periodic but does not repeat exactly.
Predictable
– Tidal frequencies in N.W. European estuaries.
– (1) Semi-diurnal period (12 hours 25 minutes)- the
major tide !
– (2) Diurnal period (24-25 hours)
– (3) Spring-Neap of the semi-diurnal tidal range
every 2 weeks
– (4) Longer period modulations and fluctuations in
sea level.
Main tidal periods
Tides produced by the moon
M2 (semidiurnal lunar) 1/2 lunar day = 12h 25min
O1 (diurnal lunar) 1 lunar day = 24h 50min
Tides produced by the sun
S2 (semidiurnal solar) 1/2 solar day = 12h
K1 (diurnal solar) 1 solar day = 24h
The tides can be represented as the sum of harmonic oscillations with these
periods, plus harmonic oscillations of all the other combination periods.
Each oscillation (a tidal constituent) has its amplitude, period and phase.
Hundreds of such oscillations have been identified, but in most situations and
for predictions over a year it is sufficient to include only M2, S2, K1 and O1.
Tidal Classification
The form factor F = ( K1 + O1 ) / ( M2 + S2 ) is used to classify tides.
where the symbols of the constituents indicate their respective amplitudes.
Four categories are distinguished :
value of F 0 - 0.25 semidiurnal; 0.25 - 1.5 mixed mainly semidiurnal
1.5 – 3 mixed, mainly diurnal and F> 3 diurnal
Immingham: semidiurnal; two
high and low waters each day.
San Francisco: mixed, mainly
semidiurnal; two high and low
waters each day during most of the
time, only one high and low water
at neap tides.
Manila: mixed, mainly diurnal, one
dominant high and low water each
day, two high and low waters during
spring tide.
Do San: diurnal; one high and low
water each day.
• Definitions of different
tidal levels
– MHWS = Mean High
Water Springs
– MSL= Mean Sea Level
– MLWS= Mean Low
Water Springs
– MHWN= Mean High
Water Neaps
– MLWN= Mean Low
Water Neaps
– LAT = Lowest
Astronomical Tides =
CHART DATUM
Estuarine classification by tides
• Macrotidal estuaries
Tides have a dominant effect on all other
processes ( Range > 4 metres)
• Mesotidal estuaries
Tides have a strong but not dominant role
(Range 2-4 metres).
• Microtidal estuaries
Tides have too small an amplitude to alter
physical conditions in estuary. (Range < 2 metres)
Importance of tides for estuaries
• Sea level movement exposes beaches
and mudflats
– Physical effects: Temperature
Drying
Consolidation
Salinity
Effects on the biota: Dessication
Importance of tides for estuaries
• Tidal movement in and out every 12.42 hours
promotes flushing of estuary
– Salt water brought in and fresh water taken out
– Potential cleansing effect
– Carries plant and animal life in and out
• Tidal water movement causes sediment transport,
erosion and deposition
• Tidal movement causes stirring and mixing
Breaks down stratification
Tidal forcing
Tidal propagation
High water
Tidal energy dissipation
Low water
• Tides in the sea adjacent to the estuary
The tidal rise and fall in the sea
outside controls water movement in estuary.
• Tidal propagation into the estuary
How does the tide penetrate the estuary ?
How is tidal elevation distributed in time & space ?
• Tidal dissipation
Where does the tidal energy go ?
How is tidal energy dissipated ?
sloping surface
ζ
current u
As the tide level rises at the mouth of the
estuary, the surface slopes down into the
estuary. This generates a pressure force
into the estuary, which causes an
acceleration to drive a current that flows
into the estuary.
This is expressed in the motion
equation:
current
x
surface height
du
dζ
= −g
dt
dx
(1)
horizontal dimension
time
acceleration due to gravity
The current carries water into the estuary, causing the surface
height to increase with time. This is expressed in the continuity
(conservation of mass) equation:
mean depth of water
h
du
dζ
= −
dx
dt
(2)
As the surface rises, it forces the position of steepest surface
slope to move into the estuary, and thus the disturbance caused
by the tidal rise at the mouth propagates into the estuary. The
same occurs in reverse when the tidal level outside the estuary
falls. The surface slopes upwards into the estuary, causing water
to flow out, lowering the level inside the estuary. The rate at which
a surface height change propagates into the estuary is the wave
speed,
c = gh
Eg h=10m g=9.81 m/s2 gives c= 9.9 m/s
Measurement of Tides
• Tidal Pole
• Tide Gauge
• Pressure device on sea bed
shallow waters
ocean pressure gauges
• Satellite altimeter
Tides in Shelf Seas and Estuaries
• Tidal elevation changes propagate around
shallow seas.
• Represented on a COTIDAL MAP
• COTIDAL LINES join all places having high
tide at the same time.
• CO-RANGE LINES join all places having the
same tidal range
• AMPHIDROMIC POINT- where COTIDAL
LINES MEET – ZERO RANGE
Co-tidal= solid lines
Co-range =dashed
Tides in the North Sea.
Red lines - co-phase lines of the M2 tide, labelled
in hours after the moon's transit through the
meridian of Greenwich.
Blue lines - the mean tidal range at spring tide
(co-range lines of the sum of M2 and S2).
The progress of the tidal wave from the Atlantic
Ocean into the North Sea is demonstrated by the
co-phase lines.
The wave enters from the north and moves along
the British coast; it then proceeds around two
amphidromic points along the Dutch, German
and Danish coastline. Another wave enters from
the south west, through the English Channel. In
the Irish Sea the wave enters from the south.
The influence of the Coriolis force is showed by
the co-range lines, which show large tidal range
along the British coast and small tidal range
along the German, Danish and Norwegian coast.
The same effect (amplification on the right side of the wave) is seen in the English
Channel, where the tidal range along the French coast is as high as 11 m compared with
3 m on the English coast, and in the Irish Sea, where 8 m on the English coast compare
with 2 m on the Irish coast.
Bore: a tidal wave which
propagates as a solitary
wave with a steep
leading edge up certain
rivers. Formation is
favoured in wedgeshaped shoaling
estuaries at times of
spring tides.
Hangzhou,
China.
8th Sept
2002
Summary
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Tidal Definitions
Importance of tides for estuaries.
Tides in coastal seas
Tides in estuaries
Tidal Propagation
Tides are the most important
contribution to mixing in UK estuaries.
Tides in Estuaries
• Tides in the sea adjacent to the estuary
– Tidal rise and fall in the sea outside controls the
movement within estuary
• Tidal Propagation
– How does the tide penetrate into the estuary
– How is the tidal elevation distributed within estuary
in time and space.
• Tidal Dissipation
– How and Where is tidal energy dissipated ?
Equilibrium Theory of Tides
• Water covered globe
• Ocean is in equilibrium with TGF at every
point on surface.
• The observed tides have the same
frequencies as those predicted by the
equilibrium theory
• Basis for the harmonic analysis of the tides
which is used for tidal prediction many years
ahead.
TGF=tide generating force
The effect is greatly exaggerated; the moon would generate a tidal height of
about 55 cm only!