Plan I. Thames Estuary (depths in fathoms)
[Link to Estuary Chart 2008]
From the Nautical Magazine
In the estuary of the River Thames, the principal fairway for the general traffic leading up from the North
Foreland, is the Princes Channel, from which vessels
have either to turn off into the Alexandra Channel
or continue on through the Girdler passage, both courses
leading into the Oaze Deep, or Knob Channel, from whence,
owing to the complete system of guiding lights and buoys, navigation is comparatively safe and easy up the river.
But for ships of deep
draft, say over 20 feet, neither the Alexandra Channel nor the
Girdler passage is available at low water, the depth being then
insufficient, consequently such vessels consisting for the most part
of the largest and most valuable steamers coming to London,
have frequently to anchor for some hours off the North Foreland,
or in the Princes Channel, waiting, at some risk, if the weather is
bad, and certain inconvenience and delay under any circumstances,
until there is sufficient water for them to proceed in safety.
It is
with the object of affording such large maritime traders additional
facilities for making their passages into and out of our great port,
and to render unnecessary the inconvenient detention to which they
have hitherto been subject, that the Trinity Board have taken the
necessary steps for making another channel navigable for ships of
the deepest draught at all times of tide by day or by night.
This new channel has under various names existed for more
than 50 years.
Once it was known as the Smugglers' Swash,
subsequently it received the name of Thomas's Channel, afterwards its name was changed to that of the Bullock Channel,
and finally, on being buoyed in 1882 by the Trinity Board, it was
named the Duke of Edinburgh Channel, in honour of the Royal
Master of that ancient corporation.
Up to that date the channel had
steadily increased in capacity, until it was considered expedient to
mark it out by buoys for use in the day time, but of late years the
shipping interests have been very desirous that this channel should
be lighted, so that it might be navigable at night time.
Owing,
however, to the impecunious condition of the Mercantile Marine
Fund, upon which the cost would be chargeable, this admittedly
desirable undertaking has had to be postponed from year to year,
but, now financial matters being presumably in a more satisfactory
condition, the work has at last been carried through, and since the
18th December, the Duke of Edinburgh Channel has been open for
the passage of the largest ships at all times of tide by day or night.
The channel itself leads from the open sea into the Black Deep,
which communicates directly with the Oaze and Knob passages.
It has a least depth of 37 feet at low water, which is ample for
the largest ironclad or merchant ship afloat, and its narrowest
part is a little more than half-a-mile wide.
To give a good lead from seaward into the Duke of Edinburgh Channel, the Tongue Light-vessel, with a powerful light and fog
signal, has been moved out to the eastward two miles, to a position
close to that hitherto occupied by the Tongue Knoll Buoy, which
mark is superseded by the lightship, the latter in her new position
still affording an excellent guide into the Princes Channel.
To
compensate for this alteration, so far as the Princes Channel
is concerned, the Princes Channel Light-vessel has also been moved
two miles to the E., her new position being off the southern projection
of the South Shingles Sand at the narrowest part of the channel,
superseding the Shingles Spit Buoy, while higher up, the small
buoy marking the S.E. end of the Girdler Sand, is replaced by
a gas-lighted bell buoy.
By these changes, the general lighting of
the Princes Channel is greatly improved.
With the Tongue Light-vessel furnishing an excellent lead for the
entrance of the Duke of Edinburgh Channel, the chief guide
through the passage is a new lightship, called Edinburgh Channel, placed about midway on the N.E. side, the narrowest part of the
waterway being here situated between a shoal patch in the middle
of the channel, suitably indicated by buoys, and the lightship.
Where
the channel debouches into the Black Deep, another new lightship
is moored, called the Black Deep, which, in conjunction with a gas
lighted buoy off the N.E. projection of the N.Shingles Sand, affords
an effective indication of the necessary turning point into the
Black Deep.
To complete the lighting of the fairway to the Knob
Channel or into the Oaze Deep, the Knock John and the Knob Buoys
are altered to gas-lighted buoys, and thus the new night course is
simply and effectively marked out.
The practical advantages of lighting up these channels will be
that deep-draught steamers bound to the Thames can, in clear
weather, keep under way at full speed, and run right up the river
if the tide serves in the upper waters, or anchor in safety between
the Mouse and Nore lights; and outward bound steamers leaving
the docks at high water can proceed direct to sea without waiting
at all.
Her Majesty's ships will also be now enabled to pass out to
sea at any time of tide, a possibility which had recently become
endangered by the shoaling of the water in the North Channel.
The placing of four new gas-lighted buoys is a marked feature in
this development of the marking of the Thames entrances, and
bears out the anticipations of their extended employment made in
these columns last year.
In order to prevent the lights from these
buoys being mistaken for the anchor lights of small vessels or
fishermen's lights, they are all made occulting ; the ingenious
mechanism for automatically producing the occultations being the
patent of Pintsch's Lighting Company.
The combination of sound and light in the gas-lighted bell-buoy
is quite a novelty in our waters, and it will be interesting to learn
the opinions of shipmasters and pilots as to its practical
efficiency.
The lightships are illuminated by means of mineral oil having a
flashing point (close test) of 250° Fahrenheit ; the distinctive
characteristic for the Edinburgh Channel Lightship is one powerful
white flash every five seconds, and for the Black Deep vessel four
quick flashes every half-minute.
The effect of this great improvement will doubtless be to still
further augment the increasing number of deep-draught and large
tonnage steamers using the port of London, probably drawing
some of them from Continental ports, and enlarging the shipping trade of the port in respect of both passenger traffic and
conveyance of the heavier descriptions of merchandise.
ALTHOUGH it is not practicable to say precisely
where the river ends and the estuary commences,
it will be sufficient for general purposes if the westward,
or inner, boundary of the Thames estuary is assumed to be a line from Southend to Sheerness, the northern
boundary as the coast of Essex, and the southern the roast of Kent ; and it may be said to extend eastward to the meridian of the Kentish Knock light-vessel.
The
area inclosed between these lines is upwards of 800
square nautica] miles, and the whole of the space is encumbered with banks, between which are the several
channels leading to the river.
As the shores of Essex and Kent are low, and have no
natural features by which they may be distinguished at a
distance, and as a great part of the estuary is out of sight
of land, even in the clear weather so rare in this country,
it is evident that artificial marks in considerable number
are required to make navigation at all practicable between
the banks.
In early times, when vessels were small and
of light draught, few marks were necessary, but with increasing trade, necessitating vessels of heavy draught, new channels have to be marked farther from shore, and
the demand for additional security to navigation has especially increased of late years, so that now there are no
less than 3 lighthouses, 11 light-vessels, 8 gas buoys, 10
beacons, and 117 ordinary buoys marking the channels
at present in use ; and the demand for additional marks
is likely to increase rather than diminish, for the deepest
channels through the estuary have not yet been buoyed,
and the changes in progress seem to favour the opinion
that before many years some of them will have to be
opened up to facilitate traffic.
In endeavouring to give an account of the changes in the channels of the estuary, it is difficult to obtain any
authentic records earlier than the commencement of the
present century.
If such records exist, they are not at the Admiralty or Trinity House, the earliest surveys
worthy of notice being those of Mackenzie, Graeme
Spence, and Thomas, between 1790 and 1810; but no
thorough investigation appears to have been taken up
until Sir Francis Beaufort was Hydrographer, when, under
his instruction, Captain Bullock surveyed the whole estuary between 1835 and 1845.
Since then, Calver re-surveyed
the whole of the southern part in 1862-63, and examined
the northern banks in 1864, and lately the Triton has re-surveyed all important channels and delineated the
banks, and from these several surveys some idea can be
obtained of the condition of the estuary at different epochs, and of the changes that are taking place.
These changes seem to be of two kinds ; viz. permanent
changes and periodic changes.
Before, however, describing the changes in progress, it will be well to give a general description of the estuary ;
and, to render the description more intelligible, three plans
have been constructed,
the first showing the whole estuary on a small scale with the tracks followed by vessels ;
the
second being a diagram showing the state of an obstruction in a channel at different epochs, a characteristic
permanent change ;
whilst the third plan shows the state of the Duke of Edinburgh Channel from the time of its
first opening out to the present date, to illustrate what
seems to be a channel opening and closing periodically.
It is worthy of notice that all the banks of the estuary
are of sand intermixed with shells ; even the foreshore
consists mostly of sand, between high and low water
marks ; in two places only is it of shingle (viz. off Whitstable and at Garrison Point, Sheerness) ; and in a few
places, near the entrance of the rivers discharging into
the estuary, there is a little mud, whilst in the vicinity of
Margate there are some ledges of chalk.
The sand is very fine, and although, when dry, it possesses a tolerably
hard surface, directly it begins to be covered it is all alive.
When beacons are erected on any of the banks, or a ship
gets on shore, the tidal streams scour out the sand in the
immediate neighbourhood, and cause the wrecks to sink
and finally disappear.
Although without actual boring it is not possible to give the exact depth of these sands, it
is probable that they are upwards of 60 feet thick, for
channels of that depth have opened out across the sands
and again closed up, so that the bank has been dry at low water where 60 feet formerly existed ; and the Goodwin Sands, in the Downs, which have been bored, proved
to be 80 feet in thickness.
All the banks, and the channels
between them, trend in a north-east and south-west direction : this is doubtless due to the fact that the stream
outside the estuary is running to the northward whilst
the tide is ebbing from the river, and, consequently, the
ebb stream in the estuary is deflected to the north-eastward.
The channels into the estuary, therefore, must be classed
under two headings :
(a) those which follow the main line
of the flood and ebb streams, and
(b) those which do not
follow the general stream of the tide.
In the former category are the Warp, West Swin,
Middle Deep, East Swin, Barrow Deep, Oaze Deep, and
Black Deep ;
in the latter are the Middle Swin, Queen's
Channel, Prince's Channel, Alexandra Channel, Duke of
Edinburgh Channel, Gore Channel, &c.
which are all more or less of the nature of swatchways across the main
line of the sand-banks of the estuary.
In the Black and
Barrow Deeps, which are the deepest and straightest
channels through the estuary, the ebb stream runs 7 hours
and the flood 5 hours, and the ebb is much stronger than
the flood, the stream setting fairly through.
In the Duke
of Edinburgh Channel, the deepest swatchway of the
estuary, the streams at the north and south ends are of
a rotatory character, revolving with the hands of the
clock.
I would here explain that in a large space like the
Thames estuary the difficulty of buoying the various
channels increases very considerably with their distance
from the shore.
With permanent marks erected on the
shore, it is easy to place buoys in selected positions, not
far from land, in fairly clear weather.
But when the distance from the shore has increased so that the marks
erected on the land cannot be seen, we have either to
erect other marks on the sand-banks and carry out a triangulation, or we are dependent on floating bodies (fixed
by land objects) to fix other floating bodies farther off.
That this is an eminently unsatisfactory method will be
evident when it is stated that each time the Kentish
Knock light-vessel has been satisfactorily fixed, the position has been very different from that supposed.
When
fixed by Calver in 1864, she was found to be one mile
N.E. by N. of her charted position ; and when fixed by the
Triton last year, she was found to be one mile and a half
S.E. by E.of her supposed position.
The errors probably creep in somewhat in the following
way.
Something goes wrong with the light-vessel after she
has been satisfactorily fixed : a collision takes place, the
fog-siren gets out of order, or one of the many things happens which necessitates the vessel being taken into port.
A temporary light-vessel is substituted, and she is anchored
in almost precisely the same position as the other, but
probably before her mushroom bites the ground it has
dragged somewhat.
By the time the other vessel is repaired and brought out, the temporary one may be a
cable or so away from the original position.
As the
weather is usually thick, the permanent vessel has to be
anchored as nearly as practicable in the position of the
temporary craft, and her mushroom may drag somewhat
before biting the ground, &c.
Thus a series of errors creep in without there being adequate means of checking
the position of the light-vessel, and within the last few
years the Triton has found the Leman and Ower light-vessel one mile away from her charted position, the
Dudgeon light-vessel about one mile from her supposed
position, and the Outer Downing light-vessel nearly two
miles from the charted position.
All these light-vessels are either out of sight of land, or can only be seen from an elevated position on the shore on rare occasions.
It is therefore naturally the object of the Elder Brethren
of the Trinity House to utilize the channels closest to the shore, and, as these channels are also the most direct
into the Thames, the northern channel following the
general trend of the Essex coast, and the southern that
of the Kentish coast, no other channels would require
marking if the depth in these was sufficient for the traffic.
Hitherto the one northern channel has been enough, but
this is steadily shoaling, as will be described further on ; but the southern channels are mostly shoal, and one after another has had to be opened up as the size of the vessels
and their draught of water increased, until there are now
five buoyed channels off the Kentish coast, two of which
are lit ; but only one can be termed a deep-water channel,
and this would seem to be the very channel which opens
and closes periodically, as will be shown subsequently.
Should this prove to be the case, there will be intervals
during which there will be no deep-water channel into the
river on the south side of the estuary.
Plan I. Thames Estuary (depths in fathoms)
By a reference to Plan I., showing, on a small scale, the
whole estuary, it will be seen that the northernmost
channel, viz. that close to the coast of Essex, is named the
Wallet, and that this is separated by a series of banks,
termed Buxey and Gunfleet, from the channel next it.
These banks, which are collectively 18 miles long, are dry
for the most part at low water ; there are, however, two narrow passages across them, one separating the Buxey
from the Gunfleet, called the Spitway, and the other
separating the Buxey from the Dengie flat (extending
from the Essex coast).
The Spitway, which, when
sounded in 1800, had a depth of nine feet, has remained
at that depth until recently, but now has only a depth of
5 feet at low water ; the channel between the Buxey
sand and Dengie flat has about 12 feet, and is merely an
outlet for the River Crouch.
It will therefore be seen that
the Wallet is really only a channel to the Rivers Colne,
Blackwater, and Crouch, and is of no importance as a channel towards the Thames.
It was last surveyed by
Staff-Captain Parsons in 1877, and as its features have
not materially changed since 1800, it will probably not be
surveyed again for many years, unless the swatchways
across the Gunfleet should deepen or others open up of
sufficient importance to render the Wallet useful as a
traffic channel.
There were formerly other swatchways
across the Gunfleet, but these are now closed.
The channel next the Wallet is named the King's
Channel, or Swin ; the eastern part is named East
Swin,; the central part Middle Swin, and the inner
part West Swin.
This is the channel through which
all the traffic between London and the northern ports
of the Kingdom passes, and it is almost always
crowded with shipping.
The East Swin is bounded at
first by the Gunfleet sand to the north-westward and the
Sunk sand to the south-eastward, and is 3 miles wide ; but 8 miles within its entrance two other banks com- mence—one, the IJarrow, being very extensive, upwards
of 13 miles in length and 2 in breadth ; and the other,
the Middle or Hook sand, a narrow ridge about 6 miles
long, extending along the north-west face of the Barrow
sand, and leaving a channel nowhere less than r! of a mile
wide between them.
It will thus be seen that 8 miles
within the entrance of the East Swin it is split up into 3 channels ; the northernmost retaining the same name,
the channel between the Middle, or Hook sand, and the
Barrow being known as the Middle Deep, whilst the
channel between the Barrow and Sunk sands is known
as the Barrow Deep.
The Middle Deep rejoins the Middle
Swin, but the Barrow Deep and West Swin bothriin into
what is known as the Warp.
The Swin is well buoyed
and lighted throughout, but the Middle and Barrow Deeps
have not yet been buoyed.
In fact, it has hitherto not
been necessary to do so, as the least water in the main
channel of the Swin has, up to recently, been ample for
all that has been required ; but a steady shoaling has
been taking place in a critical part of this channel since
1800, and it now seems to be only a question of time
before the Middle Deep will have to be marked.
To illustrate the changes in progress here, Plan II. has
been constructed, showing the condition of the critical part of the navigation in the Swin each time it has been
thoroughly surveyed.
By this diagram it will be seen
that in 1800 the ruling depth in the channel between
Foulness sand and the Middle or Hook sand was 35 feet at low water.
Forty-three years later, a bar, on which the
depth at low water was 28 feet, had formed between the
Foulness sand and the Middle.
In 1864 the depth had
decreased to 24 feet, and, in 1889, to 21 feet, showing a steady decrease since 1800 of about one foot in every six years.
The deposit is of sand, shells, and mud.
This is the only shallow part of the Swin ; and as it is evident
that, so far as our knowledge extends, we may expect it to continue to decrease in depth, and as even now, with
strong south-west winds prevailing in the North Sea, it is by no means rare for the tide to fall 3 feet below the level of low water ordinary springs, so that the depth would be
reduced to 18 feet, it is clear that vessels of heavy draught
will either have to wait for tide or use another channel.
Already our small armoured vessels of war have to time
themselves to reach this obstruction by half-tide.
For-tunately, the Middle Deep is an alternative channel with
ample depth in it, which only requires to be buoyed, and
this can readily be done.
This Deep seems to be in a
better condition now than it has been for 50 years, for,
when surveyed by Bullock, in 1843, there was a bar of
25 feet at its east end.
This had disappeared when it was surveyed by Calver in 1864, and there was then a channel of two cables in width between the edges of the
30 feet contour lines of soundings surrounding the Middle
sand and Barrow.
There is now a channel four cables in width between those contour lines in the narrowest part
of the Deep.
The Barrow Deep, referred to as the third channel
branching away from the East Swin, is deep throughout,
and without obstruction.
It varies somewhat, as shown
by the different surveys, but is an excellent highway,
which only requires buoying to be available for traffic.
At
present the London County Council are allowed to empty
rubbish in this Deep, which seems rather a pity, as there
is no knowing what may be the result eventually, more
especially as we have at present no observations to show
to what depth the tidal scour is of service.
.
Any interference with the channels, likely to cause an obstruction,
should be avoided.
The Sunk sand, which is the south-eastern boundary
of the Barrow Deep and the north-western boundary of
the Black Deep, has undergone great alterations since
originally surveyed in 1800.
In that year it is shown as a long sand which really extended from the present north-east end in one continuous line of shallow water to the
inner end of the Oaze sand, a distance of 26 miles.
On
it were many dry patches, named Great Sunk, Little Sunk, Middle Sunk, Knock John, &c., and the only passage across was a three fathoms channel at low water at the eastern end of the Oaze.
When surveyed by Bullock,
1835-45, this chain of sands had altered very consider-ably, and had several channels or swatchways across it —a swatchway of 22 feet at low water between the Great
and Little Sunk sands : a swatchway of 60 feet at low
water between the South-West Sunk and the Knock John
sands ; a 35 feet channel 1 mile wide between the Knock
John and North Knob sands ; and a swatchway of 26
feet between the North Knob and the Oaze.
When
surveyed by Calver, 1862-64, this series of banks had
again altered : the swatchway between the Great and
Little Sunk sands had only 12 feet in it at low water;
the swatchway between the South-West Sunk and the
Knock John had shoaled to 40 feet ; but the channel be-tween the Knock John and North Knob had deepened
to 45 feet, and a narrow channel of 40 feet at low water
had opened out between the Oaze and North Knob.
In 1888-89, when surveyed by the Triton, the swatch-way between the Great and Little Sunk sands had en-tirely disappeared ; the swatchway between the South-West Sunk and the Knock John sands had narrowed and
shoaled to 29 feet ; the channel between the Knock John
and North Knob shoals had decreased to 24 feet, whilst
the channel between the North Knob and the Oaze had
increased its width to one mile, with about the same
depth (viz. 40 feet) at low water.
In fact, the chain of
sands known as the Sunk, Knock John, Knob, and Oaze,
which were, in 1800, one continuous bank, after breaking
up into separate patches, again show signs of resuming
the form they possessed when originaliy surveyed, the
only deep channel across them now being between the
Oaze and North Knob.
The Black Deep is the channel bounded to the north-westward by the chain of sands just described, and to the south-eastward by another chain of sands named
Long Sand : Shingles, Girdler, and the flats extending
from the Kentish shore.
It is a deep-water channel, the
inner part of which has been buoyed since 1882, and lighted
since December last, as it communicates by a deep-water
swatchway, named the Duke of Edinburgh Channel,
with the deep water off the North Foreland, and so forms
a convenient outlet for the heavy-draught vessels bound
southward from the Thames.
There seems to be some
tendency to shoal in the north-east end of the Black
Deep, but it has only once been sounded viz. by Bullock,
in 1843 ; and we have not yet quite completed our examination of it throughout, so that no thorough comparison is yet practicable.
PLAN II.
PLAN III.
The chain of sands which bound the south-east side of
the Black Deep formerly extended in one continuous line from the Kentish coast to the Long Sand Head, a distance
of over 30 miles.
Across this chain of sands there have
always been shallow swatchways which communicated
by somewhat circuitous channels with the deep water of
the estuary.
These are now 5 in number : (i) the Gore
Channel, which passes close to Margate and then across
the Kentish flats ; (2) the Queen's Channel, which, passing
between the Margate sand and Tongue sand, also leads across the Kentish flats ; (3) the Prince's Channel, which
leads between the Tongue sand on the south side, and
the Shingles and Girdler sands on the north side, into the Black Deep ; (4) the Alexandra Channel, which leads
from the Prince's Channel to the Black Deep; and (5) the Duke of Edinburgh Channel, which leads from the
deep water of the North Sea into the Black Deep.
All
these channels are buoyed.
In the Gore Channel (some-times called the South Channel), which has been in use from early times, the depth at low water is 10 feet.
The
shallow grounds shift backwards and forwards, but there seems to have been always as little as 10 feet at low
water in some parts of this channel.
In the Queen's
Channel, which was buoyed in the last century, the least depth in passing over the Kentish flats is 13 to 14 feet at low water.
In Prince's Channel, which was buoyed in 1846, and lighted in 1848, the least depth is 20 feet at low water, but theie is a patch of 17 feet at its western
end in the centre of the channel which seems to be
always in this channel though not always in the same
position.
It is shown by Bullock in 1839, by Calver
in 1862, and by the Triton in 1880.
The Alexandra
Channel, which is a swatchway between the Shingles and
Girdler sands, had no existence in 1800, the Girdler and
Shingles forming with the Long Sand a continuous chain
at that date.
In Bullock's survey of 1839, the Alexandra
is shown as a blind inlet on the north side of the Prince's
Channel, which was cut off from the Black Deep by a
ridge over which the depth was 7 feet at low water.
When surveyed by Calver in 1862, the least depth in the
channel was 20 feet ; and when.
surveyed by the Triton in 1888, the least depth was 23 feet.
It is, however, much
narrower now than in 1862, and if it continues to decrease
in width will not be available for traffic, as there is not now much more than room for two large vessels to pass
each other, and bad steerage might cause an accident.
Of the Duke of Edinburgh Channel, which is a broad
swatchway at present dividing the Long Sand from the
Shingles Sand, we have a tolerably complete history ; and
as this would seem to be a channel which opens and
closes periodically, Plan III. has been constructed to show
its condition each time it has been surveyed.
The first record we have of it is on an old chart of 1794, when it is shown as a 9-feet swatchway, and is named "Smugglers' swatch".
When surveyed by Thomas, in 1810, it was named "Thomas's New Channel," and there was then a narrow passage carrying 30 feet at low water between the
Long Sand and Shingles.
In 1839, when surveyed by
Bullock, and named "Bullock Channel," this 30-feet swatchway of Thomas's was obstructed by a bank in the
middle, which dried at its north end, leaving a passage of
15 feet on its east side, and a very narrow gat of 25 feet on its west side, but one mile farther west a new channel was opening out, the shoalest water in which was 16 feet.
This appears as an inlet into the sand-bank on Thomas's
chart.
The next time it was surveyed was by Calver, in 1862, at which date Thomas's Channel had closed completely, but
the channel west of it had opened out and become a wide
deep-water swatchway, the least depth in which was 42
feet at low water.
Early in 1882 it was thought advisable
to buoy this channel, and the Triton was ordered to examine it, when a 30-feet patch was discovered near its centre.
In the autumn of 1887, this patch was reported
to have shoaled ; and in 1 888, when examined again by the
Triton, it was found to be upwards of a mile in length
with 22 feet on it.
In October 1889, the channel was
again examined, when the least depth on the central
patch was found to be 21 feet, and it had a tendency
to shallow to the eastward.
The channel was buoyed in the summer of 1882, and re-named by the Elder Brethren
of the Trinity House "Duke of Edinburgh," after the
Master of the Trinity House.
It was lighted in December
1889.
The various surveys seem to show that the estuary
has a tendency for the most part to return to the condition it was in about 1800.
In that year there were no deep-water swatchways across the banks, and the
channels that opened up subsequently seem now to be
all closing again.
At any rate, those in use as ship
channels evidently will require constant watching.
Should the Duke of Edinburgh Channel close, and none
other open out, it will materially interfere with the heavy
traffic into the estuary from the southward, for it will necessitate either waiting for high water or passing round
outside into the Black or Barrow Deeps, which will have to be buoyed and lighted to make them readily accessible.
There is one other shoal, the "Kentish Knock," which
may be said to belong to the estuary.
This is a sand-bank about 6 miles in length and 2 in breadth, on the
south-east side of the outer part of the Long Sand.
Its shape and area, within the contour-line of five fathoms,
would appear to be fairly constant ; but it had a swatch-way across the north end, when surveyed by Calver in 1864, which has now entirely disappeared.
Between the
Kentish Knock and Long Sands is a channel, two miles- wide, named the Knock Deep.
At the north end of this channel the soundings arc much shoaler than when
surveyed by Bullock.
In some cases the difference is as much as 12 feet.
Although the general tendency of the banks in the
estuary seems to be to revert to the condition they were
in about the year 1800, it is not possible to predict that
this will certainly be the case.
If, as seems probable, the
condition of the estuary is due to the action of the sea
in casting up banks, and of the tidal flow in cutting
channels through the banks thus formed, it is evident that
much will depend on prevailing types of gales.
There
can, however, hardly be a doubt that any diminution of the volume of the water running into and out of the
estuary would diminish its power of making deep-water
channels, so that any action tending to decrease the flow
into and out of the various rivers should be avoided if possible ; as although it is conceivable that a given type
of strong winds, extending over a lengthened period,
might have the effect of closing the various swatchways
across the banks, it does not follow that a cessation of
these winds would cause the channels to be again opened
out if the volume of the tidal flow was seriously
diminished.
T.H.Tizard.