HYDROGRAPHIC AND TIDAL INFORMATION FOR DEEP DRAUGHT SHIPS IN A TIDAL ESTUARY by Lt. Commander J. C. E. W h i t e, R.N., (Retd.) Port o f London A u thority This paper was presented at the XHIth International Congress of Surveyors (F.I.G.) in Wiesbaden, September 1971 SUM M ARY The principal oil refineries in the Port of London are situated at Tham eshaven and Coryton, over 50 miles from the true entrance to the port at Long Sand Head. Here deep draught tankers first enter areas where charted depths are less than their draught and so require the tide to give sufficient w ater to navigate. This 50 mile passage is through a com plex estuary, where drying sandbanks alternate w ith deeps; and where two tidal systems (that from the North Sea and that from the Straits of D over) meet and interact w ith resultant instability o f the sea bed in certain areas; some 12 m iles o f this passage are dredged. T o bring into the Port ships drawing up to 48 feet (100 000 ton tankers fu lly laden and 200 000 ton tankers part laden) w ith an underkeel clearance of only 3-4 feet requires great skill on the part of the Master and P ilo t and this skill must have a sound basis o f up-to-date data. Th ey must have accurate hydrographic surveys and actual tidal heights, as w ell as com prehensive tidal predictions to enable them to plan and execute the passage w ith safety. This paper discusses the hydrographic surveys required fo r this purpose in term s of accuracy of measured depth, tidal reduction of soundings, and accuracy o f position fixin g and how this is applied in the P o rt o f L o n don. It also describes the tidal predictions used, and how, by means of radio-linked tide gauges and the port s comm unication system actual tidal inform ation is passed to the ships. Th e means o f interpreting this in form ation co-tidally along the 50 m ile passage is also described. The paper then b riefly discusses future requirem ents in this field.
1. INTRODUCTION Fom the point o f view of a ship coming to the Port of London the Tham es Estuary starts at the N orth Foreland in the South and the Sunk in the North and extends to the ship s berth which m ay be at the oil jetties of Tham eshaven, the container port at Tilbury or riverside berths and enclosed docks further up the river. Th e distance steamed could be from 30 to 80 m iles through the complex outer estuarial area (illustrated in figure 1) and the inner riverine estuary. In this paper we w ill consider the most difficu lt problem posed by this estuary, that of getting the largest vessels from sea to the refineries in the Thameshaven area. These ships, tankers o f 115 000 tons fu lly laden and 230 000 tons part laden norm ally draw about 45 feet (13.7 m etres); the deepest that has come to the Port drew just under 49 feet (15 m etres); the largest are 1100 feet (335 m ) overall by 160 feel (49 m) beam : their entry route is through the Black Deep, the Knock John and Sea Reach dredged
channels to Tham eshaven a distance of 50 nautical miles. The depth at the start o f their passage is 42 feet (12.8 m ) below datum reducing to 31¾ feet (9.6 m ) off the berths, so the vessels have to come in w ith the flood tide. 2. WHAT DEPTH OF WATER IS REQUIRED? The depth o f water required by a ship on passage is the still water draught o f the vessel plus an allowance for Under Keel Clearance. This passage depth, as we shall call it, is the charted depth at a point (referred to local chart datum which is Low est Astronom ical Tide in the Tham es Estuary) plus the tidal height above datum at the instant o f passing a point. It is beyond the scope of this paper to discuss Under Keel Clearance in detail but one must have some idea o f its magnitude when considering the passage o f large vessels. Underkeel clearance can be defined as the d iffe r ence between the vessel s draught (w hen the ship is stopped and in w ater of the general ambient specific gravity) and the passage depth defined above. Thus it must allow fo r : (a ) Increase of draught due to hydrodynam ic effects (squat, roll, pitch, heave) [1 ]. (b ) A n y inaccuracy of charted depth. ( c ) A n y inaccuracy o f tidal height. (d ) A n y inaccuracy in ship s draught. In the Port o f London ships generally come into the Port w ith about 3 to 4 feet (1 m etre) underkeel clearance in the shoal areas. It should be noted that this refers to a rising tide, but nonetheless it points to a high accuracy requirem ent in assessing the factors listed above, not to say skill on the part o f Master and Pilot in handling the ship and keeping not only to the correct geographical track but also to the tim e schedule dictated by the tide. Having decided the draught of ship and underkeel clearance required, the passage depth is thus established. The design depths o f the approach channel can then be found by using a Ship Passage Diagram (see figure 2) [2 ]. This shows in Section ( A ) Tim e, the passage of the ship from sea to the berth in terms of time, measured relative to the time of high w ater at the berth. Before drawing this graph one has first to determine the optimum time of arrival at the berth and the speeds that the vessel w ill make oa the various stages o f the passage. In this case we are considering a 200 000 ton tanker arriving in the berth 15 minutes before local high w ater at speeds over the ground reducing from 12 knots in the outer approaches to knots in Sea Reach, an average passage for this class of vessel. From this inform ation, the local tidal heights along the ship s passage can be deduced here a typical mean neap tide has been used. The increase o f high water height as one progresses into the estuary is also shown in this section; the graph of tidal height against distance is shown in Section (B ) Tide. Section (C) Depth shows the resultant depth below
SHIP PASSAGE DIAGRAM - THAMES ESTUARY T H ^ N A E S H /W E N S E fc
datum needed to give the passage depth required : this is sim ply the passage depth minus the local tidal height from (B). Section (C ) also shows the existing bed profile and the sea bed depths required to give both 48 feet (14.6 m ) and 49.2 feet (15.0 m ) on passage. It w ill be seen that the existing depths w ill allow a ship to berth at Tham eshaven 15 minutes before local high w ater on mean neap tides and to carry a depth o f 48 feet (14.6 m ) from sea to the berth. T o increase this depth by just over one foot (0.4 m ) w ould mean dredging the sea bed area lying between the two lines. The cost of doing this w ork w ould be o f the order of 1 000 000 so the case for providing accurate hydrographic surveys and tidal inform ation rather than increasing underkeel clearance by dredging has sound econom ic backing. Th e Ship Passage Diagram considered covers one particular set of circumstances, nam ely a 200 000 ton tanker arrivin g at the berth just before H igh W a te r at mean neap tides and proceeding at certain speeds, and the channel is designed to those particular circumstances. Some ships on some occasions w ill not meet those conditions the ship m ay be too deep draughted fo r the tide, or not capable of m aking the speed required from some extraneous cause such as fo g in w hich case entry is made to the port by 1staging the passage, that is the ship anchors over the ebb tide and low w ater period in one o f the deep w ater anchorages and proceeds to the berth on the next, or subsequent, tide. 3. HYDROGRAPHIC SURVEYS Th e P ort o f London A uthority m aintains a hydrographic service whose area o f responsibility covers the Tham es estuary from Teddington to the outer Port L im its (see figure 1). The estuary is divided into survey areas which are surveyed at the frequency and on the scale necessitated by their usage, stability and significance. Thus the Knock John and Sea Reach Channels are surveyed at 2-month intervals, w hilst the Barrow Deep is surveyed every three years and other m ore significant areas are surveyed either 6 m onthly or annually. Less im portant areas w ait up to nine years before resurvey. Accuracy requirements of a position fix The surveyor s requirem ent is that a position fix must be accurate on the scale on w hich he is plotting his survey. It is not easy to define this requirem ent num erically but it could be said that the Standard D eviation of a fix should not exceed a certain m axim um plottable increm ent on the scale o f the survey : if this increm ent w ere 1/2 m illim etre, then for coastal surveys on a scale of 1/25 000 the Standard D eviation o f a fix w ould be 124 metres. This is the scale used for most surveys in the outer part o f the Tham es
estuary, but in the dredged channels and other critical areas surveys are made on a much larger scale (1/2 500 or 1/5 000) and the proportional fix in g accuracies (o f l i and 24 m etres respectively) are d ifficu lt to attain. Decca H i-fix, in the hyperbolic mode, is used for position fix in g by the Port of London A uthority; the dredged channels and other critical areas are in general w ithin the theoretical 3-metre repeatability accuracy contour o f the chains for a Standard D eviation of l/100th of a lane. F or the same areas, calibration checks carried out indicate that the Standard Deviation o f a fix is likely to be 3/100ths of a lane in terms of absolute accuracy (as distinct from repeatability) if m onitor corrections are applied from some point w ithin the coverage : 3/100ths lane w ill be equivalent to about 9 metres w ithin this area. Thus the Port of London A u th ority finds itself w ith a fixin g system having an accuracy o f say ± 10 metres, but apparently needing to fix w ith an accuracy 4 times better than this. In fact the repeatability accuracy o f H i-f ix is sufficient to ensure that closely spaced sounding lines are accurately positioned relative to one another, and thus that the survey area is being system atically covered. Th at the eventual plotted position of a shoal sounding is incorrect by say 10 m etres is not of great im portance; the dredger that comes to dredge it w ill be using the H i-fix and the 200 000 ton tanker w ill either treat that shoal depth as the ruling depth in the channel or ensure that it can be avoided by a m argin fa r larger than 10 metres. In practice, then, H i-f ix w ith its probable repeatability accuracy o f better than 5 metres and absolute accuracy o f 10 metres is used for sounding on scales o f between 1/2 500 and 1/25 000 in the outer Thames Estuary. It is normal practice for the surveyors to check the H i-fix against sextant fixes whenever these are obtainable, and the Estuary chain is continuously m onitored from a receiver at Sheerness. Calibration checks are still being made, particularly in the fringe areas. Accuracy of Sounding The accuracy of a sounding, given that the position fix is correct, is a com bination o f the accuracies o f the depth measurement and of the tidal reduction. It is standard practice in the P ort o f London Authority to bar check the echo sounder before com m encing and after completing the day s w ork; the echo sounder transmission setting and stylus speed are checked from tim e to tim e throughout the day. A depth read o ff the echo sounder trace should be accurate to w ithin 3 inches (one decim etre). W ith tidal reductions, the aim is to apply corrections in such a manner that the tidal reduction is correct to w ithin 3 inches (one decim etre). Th ere are difficu lties in achieving this; tidal propagation in the Tham es Estuary is com plex and, w hilst very useful co-tidal charts (in terms o f high and low water tim e differences and range, for mean spring and mean neap tides) have been drawn up by the H ydrographic Department of the Royal Navy, these are really a consensus derived from comprehensive past observations,
and errors can occur when they are used for individual tides. This particularly applies at times between high and low water, since the data on which they are based can take no account of the shape of the tidal curve in d iffe r ent parts o f the estuary. Nometheless the co-tidal charts can be used w ith confidence in transferring tidal heights over short distances in settled m eteorological conditions. There is no doubt that the most accurate means of obtaining tidal reductions is to use a tide gauge very close to the area being sounded. The Port of London Authority operates a number of permanently installed float-operated automatic recording tide gauges in the Thames Estuary, five of which (at W alton, Margate, Shivering Sands, Southend and T ilb u ry) are linked by U H F telem etry to remote recorders situated in the Thames Navigation Centre at Gravesend (see figure 1) [3 ]. For most purposes the tidal height at any point at any instant can be deduced by using gauges in conjunction w ith the co-tidal charts, preferably using tw o gauges as a cross check. However, when greater precision is required portable gauges or tide boards can be placed on navigational beacons near the survey area to augment the permanent gauges; these of course are not radio linked and their installation and maintenance is time consuming, and we have yet to find a truly portable, reliable and accurate recording gauge for this purpose. The other factor closely affecting sounding accuracy is the motion of the craft in a seaway; there is at present only one w ay o f reducing errors due to this, and that is to survey only when the sea is sufficiently calm. In the future it m ay well be possible to rem ove some o f the ship-motion from the echo-sounder record electronically [4 ], enabling w ork to be continued in worse conditions than is possible at present. 4. DISSEMINATION OF INFORMATION TO SHIPPING There are two classes o f inform ation to be disseminated; long and medium term as represented by the hydrographic surveys and tidal predictions, and short term as represented by actual tidal heights and immediate hydrographic inform ation, such as the existence o f shoals or wrecks. Long and medium term information The long term hydrographic inform ation is published by the Hydrographer o f the N avy as A dm iralty Charts, using the Port o f London Authority surveys as the basis fo r correction. T h e Port o f London Authority also publishes charts of the inner estuary; copies of all surveys (w hich can be considered medium term ) are sent to the Pilot stations at Folkestone, Harwich and Gravesend so that the T rin ity House Pilots are appraised o f the latest situation regarding depths in the dredged channels and approaches generally.
Tidal predictions of the times and heights of high and low water at W alton, Margate, Sheerness and London Bridge are also published in the Adm iralty Tide Tables; these are also published in the Port of London Authority s own tide tables, with Southend replacing Sheerness, together with predictions for Tilbury and North W oolwich. In addition, the Port of London Authority publishes hourly height predictions for W alton, Margate, Southend, Tilbury and North W oolwich : Shivering Sands w ill be added in 1972. The tables arc prepared by the Institute of Coastal Oceanography and Tides, who also carry out the harmonic analyses on which the predictions are based. Short term information The Thames Navigation Service, situated at Gravesend, makes routine broadcasts to shipping each half-hour on International VH F channels : these broadcasts contain general information for shipping and include any specific hydrographic information in the form of serially numbered P.L.A. Navigational W arnings; the latter are also sent to the Pilot stations by Telex. The half hourly broadcasts also include the tidal height at Southend as read from the radio linked gauge. The display o f this gauge is an ordinary graph recorder (see figure 3) on which the predicted tide curve is drawn; the broadcast can thus include reference to any marked divergence of the actual tide from the predicted tide. Tidal information from the other gauges (Margate, W alton, Shivering Sands and Tilbury) is passed to shipping on demand. F ig. 3. Rem ote T ide Recorders at the Tham es N avigation Centre, Gravesend.
5. INTERPRETATION OF THE HYDROGRAPHIC AND TIDAL INFORMATION From the point of view of the Master and Pilot of a deep draughted tanker coming into the Port there are a very large number of questions to be asked, answers obtained and decisions taken. A number of these are beyond the scope of this paper (Is the visibility good enough? Is the berth ready? W h at other ships are there in the Channel? are examples), but the principal questions from the hydrographic point o f view are : (a ) W h at are the critical depths in the channel, at chart datum? (b) At what points along the track do these critical depths occur? (c ) At what times w ill the tidal heights at these points be sufficient to give the ship the passage depth which is required? (d ) W hat are the latest times along the route that the passage depths are available? The first two questions can be answered from the sounding surveys which are available to the Pilot before he boards at the P ilot Station. The point along the route is defined in sea m iles from London Bridge a somewhat false datum point for the largest vessel but a convenient one for general purposes since but few seagoing vessels proceed above the bridge. The third question can be answered by using the tide tables in conjunction with the co-tidal charts and this is the method largely used at present for the m ajority of vessels where small underkeel clearances are not necessary and only occur at one or two points along the passage. Thus a vessel might require to know the time that the passage depth is available in the Edinburgh Channels, at Sea Reach No. 7 buoy and in Gravesend Reach. W here underkeel clearances are small and affect a larger proportion of the passage, then the number of such calculations makes this process cumbersome, and some useful conversion tables have been compiled by Pilots to ease this problem. Passage Planning Diagrams W ith the number of 100 000 ton tankers coming to the Port increasing and with the onset o f the 200 000 ton tanker it became apparent that the existing methods did not meet present needs. It was now necessary to be able to pre-plan the tanker s passage according to the predicted tides and to amend this planned passage in the light of the actual tides which developed as the passage proceeds. A promising w ay of achieving this was at first thought to be the production of an inversion of the Ship Passage Diagram (described in Section 2) in which the sea bed profile would be drawn and to which the tidal profile could be added. A fter discussion w ith the Pilots and ship owners concerned the Passage Planning Diagram (figu re 4) (* ) was produced. (*) Ed. note : In the reduced version reproduced herewith, on ly a general idea is provided ; readers desiring to follo w a specific analysis in detail should address a request to the author fo r a full-size example fo r study.
F ig. 4. This consists, essentially, of three parts. The top part has space for entering the details of the ship, the tidal predictions and any other notes the Pilot or Master may wish to make. The second, middle, portion consists on a tidal surface diagram showing the height o f the tide at hourly intervals along the passage from Thameshaven to the Trin ity Buoy (in the Sunk/ Long Sand Head vicinity see figure 1). The times are referred to High W ater, Southend the tidal station for which tidal heights are broadcast, and extend from (> hours before to 2 hours after High W ater there. The third, lower, part of the diagram is a tabulation o f the depths, tidal heights and times along the passage. The Pilot can enter depths at chart datum for the critical points (obtained from the latest surveys), the passage depth required (draught plus underkeel clearance) and hence the tidal height at each critical point to give this passage depth. The times at which the required tidal height is attained at each critical point can be interpolated from the Tidal Surface Diagram and entered into the Tabular part of the form. Then, from the expected performance of the vessel, the Pilot can deduce the earliest possible passage and making any allowances for delays etc. can list his Planned Passage Times. He can also enter the times at which he loses the tide for the latter part o f the passage. The lim itation of this method of passage planning is the small number
of different tides that can conveniently be expressed in this form and still give the prediction accuracy that is required (within about 6 inches or 2 decimetres in good conditions). To cover all tides at Southend, as defined by their high and low water height would require some 80 versions of the tidal surface diagrams if the high and low w ater heights w ere considered in 6-inch steps. It was decided that it was not practicable to produce more than 8 different Passage Planning Diagrams and tidal surface diagrams were therefore produced for tides having the follow ing High and Low W ater heights at Southend (heights in feet) : High W ater 19.0 19.0 18.0 18.0 17.0 16.0 15.0 14.0 Low- W ater 1.0 2.0 2.5 4.0 3.5 4.5 5.5 6.0 This limitation can be partly overcome by using the hourly height predictions for Southend to test the validity of the tidal surface diagram in use for a particular passage : the times at which heights of integral feet are attained are interpolated from the hourly height tables and these times marked on the tidal surface diagram where they can be compared with the individual graphs at hourly intervals. This can be done before the ship approaches the outer estuary; as the passage develops actual tidal heights from the gauges can be obtained by V H F radio and the passage times amended to accord with the actual tide. Some of these difficidties could be overcome by presenting the tide predictions along the route in graphical form on a daily basis (or in continuous roll form ) as shown in figure 5 a. H ere the vertical axis shows time (in terms of British Standard Tim e) and the horizontal axis shows distance in sea miles (again measured from London B ridge); the contours are lines of equal (predicted) tidal height, whilst the occurrence o f high and low water is shown by the slightly sloping dashed and dotted line w ith the times and heights m arked on it at the tidal stations. This diagram could be based on the predictions at Tilbury, Southend, Shivering Sands and W alton, w ith intermediate points interpolated, using the co-tidal charts to provide the interpolation coefficients : the method could be handled by a computer and the diagram be plotted by a drum plotter. Alternatively, the presentation could be in the form of a numerical print out from the com puter as shown in figure 5 b. Both of these presentations are being investigated by the Institute o f Coastal Oceanography and Tides. It is thought that this proposed tidal prediction diagram w ill be simpler to use in practice than the present Passage Planning Diagram. The presentation is clear and as accurate as are the predictions; the only correction required is for the difference between the predicted and actual tides. The passage inform ation can be drawn on the diagram and a separate Passage Planning form completed if required. The divergencies between predicted and actual tides can be significant and are sometimes considerable. In settled weather conditions one can expect the predictions to be w ithin 6 inches in height and 10 minutes in time from the actual tide. However, weather conditions severely affect tidal propagation in the southern North Sea and storm surges o f several hours duration and several feet in amplitude can raise the tide to about 8
F i g. 5a. Distance in Sea Miles ( f r o m London Bridge). Predicted tidal heights (m etres). feet (2.4 m etres) above prediction or lower it an equivalent amount below. Such large amplitudes are rare, but surges of ± 4 feet (1.2 m etre) occur one or twice a year and ± 2 feet (0.6 m etre) are not uncommon. 6. THE FUTURE The Port of London A u thority is considering the deepening of the approaches to the port so that the deepest draught vessels w ill have access to a possible new development in the Maplin Sands area (see figure 1). If this w ere to be so then a very much greater length o f the approach channel would be at critical depths and the need for accurate and up-to-date hydrographic and tidal inform ation w ill be accentuated.
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ACKNOWLEDGEMENTS I acknowledge w ith thanks the assistance o f m y colleagues in the preparation o f this paper, w hich is presented w ith the permission of Captain G. R. R e e s, the Manager o f the River Departm ent o f the Port of London Authority. REFERENCES [1 ] Captain A.F. D i c k s o n : Institute of Navigation, 19 A p ril 1967. A paper on Underkeel Clearance. [ 2 ] P o t t e r J. H. : X X IIn d International Congress, 1969, Ocean Navigation. Section II, Subject 3 : Tham es Approach Characteristics. [ 3 ] W h i t e J. C. E. : V U Ith International Conference on Lighthouses, 1970. Radio L in k ed Tid e Gauges in the Tham es Estuary. [4 ] S o t h c o t t J. E. L. and B e n n i n g N. H. : Conference on Electronic Engineering in Ocean Technology, 1970. Heave Correction in Echo Sounding.