NCI Manual Part 3 Station Operations & Training. Section 3 Training Guidance Notes. G Tides

Transcription

1 National Coastwatch NCI Manual Part 3 Station Operations & Training Section 3 Training Guidance Notes G Tides Station Administration Group NCI Manual 303.2G-1 National Coastwatch Institution Registered Charity No

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3 CONTENTS Page Introduction 8.1 Tides and Moon Phases - Springs and Neaps Tidal Definitions Tidal Streams Basic Competency G1: Obtain Information on Tides 8.5 Tide Tables Tidal Curves. Tidal Atlases. Judging Tidal Streams from Visual Observations. Tidal Diamonds Tidal Diamonds to establish the Direction & Rate of the Tide Additional Guidance: 8.9 Calculating Drift due to Tide and Wind Calculating Drift using Tidal Diamonds Drift Ready Reckoner SA Group Issue date: Nov 12

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5 G. TIDES Introduction Tides and Moon Phases - Springs and Neaps 1.1. For watchkeepers an understanding of what the tide is doing is as important as having up-to-date weather information. Around the UK the tide can rise or fall anything between 1.5 and 14 metres twice a day. That rise and fall gives rise to tide-induced currents, known as tidal streams, which commonly run along the coast at between 2 or 3 knots, but in some places can exceed 5 knots Tides are due to gravitational attraction of the moon and sun on the earth. In North-West Europe this effect produces two complete tidal cycles in a lunar day (about 24 hours 50 minutes). The gravitational effect of the moon causes the sea level to 'bulge' producing a high tide at the nearest and further sides of the earth. High water, at any given place, occurs about 12 hours 25 minutes after the preceding high water. High water gets progressively later by almost an hour a day. Low water also occurs at intervals of about 12 hours 25 minutes but not necessarily mid-way between high waters When the moon and sun line up with the earth their combined gravitational effect produces greater 'bulges' and higher tides than usual, these are called spring tides. Spring tides have higher than average high waters and lower than average low waters: they occur about once every days, 2-3 days after full and new moon. Particularly large spring tides occur around the equinoxes in March and September When the sun and moon are not in line but at 90 then their gravitational effect is least and high tides are at their lowest. These are known as neap tides. Neap tides have lower than average high waters and higher than average low waters. They occur about once every days, 4-5 days before full and new moon. SA Group Page 8.1 Issue date: Nov 12

6 Tidal Definitions 1.5. Chart Datum This is the reference level used for soundings, drying heights and tidal predictions that are found in tide tables and on charts. Chart datum is approximately the level of the Lowest Astronomical Tide (LAT). Tide Tables In the UK these are based on the tidal predictions for Standard Ports e.g. Plymouth, Dover, and for some Secondary Ports. Height of Tide The vertical distance between chart datum and sea level at a given time. Charted depths The vertical distance from chart datum to the seabed - metres and tenths of metres. Actual Depth The sum of the charted depth and the height of the tide at a time and place. In effect the depth that would be obtained by using a lead line. SA Group Page 8.2 Issue date: Nov 12

7 Set The direction in which a tidal stream flows measured in degrees true (expressed in three figures e.g. 060 degrees true). Note: the wind's direction is FROM where it blows. However, a tidal set indicates the direction the stream flows TOWARDS. Thus if the wind is SW and the tidal set is 225 T (SW ly) then wind is against tide. Rate The speed in knots at which a tidal stream flows. Drift The distance the stream carries in a period of time. Flood The rising or incoming tide. Ebb The falling or receding tide. Slack Tide The interval at the turn of the tide when little or no stream flows Tidal Streams 1.6. In tidal waters, all floating vessels are affected by tidal streams the horizontal movement of the sea caused by the rise and fall of the tide. In the English Channel for instance, tidal streams flow up-channel i.e. eastward on a rising or flood tide and down-channel on a falling or ebb tide. SA Group Page 8.3 Issue date: Nov 12

8 1.7. Tidal streams run more strongly past headlands and less so in the intervening bays where counter currents or back eddies may form in the tidal lee of the headland A tidal race often occurs where a strong tidal stream passes through a narrow passage, or off a headland. They can be dangerous, especially if wind is against tide. Further comments on the significance of tidal races can be found in Section 2H: Vessel Recognition & Local Knowledge Overfalls are caused by tide flowing strongly over an uneven seabed These effects will be less marked during neap tides when the rate of stream is about half the spring rate. Another factor to note is that the rate of the surface current of tidal streams will generally increase when tide and wind are from the same direction and decrease when wind is against tide. SA Group Page 8.4 Issue date: Nov 12

9 1.11. High tides can cover hazards, such as rocks and wrecks that may uncover at low tide. It is important that watchkeepers are able to recognise these on the local chart and appreciate their danger to inshore vessels. Basic Competency G1: Obtain Information on Tides 2.1. Watchkeepers must be able to establish the current state of the tide including tidal flows (rate and direction) using local tidal information, and have a good understanding of local conditions. The main sources of information on local tides are Tide Tables, Tidal Atlases and the Tidal Diamonds on Admiralty Charts. Tide Tables 2.2. Tide tables are published annually for most ports and can also be found in nautical almanacs. In the UK the times and the heights of tide at high and low water are based on the tidal predictions for Standard Ports e.g. Plymouth, Dover, and for some Secondary Ports. Watchkeepers should familiarise themselves with the tables applicable to their watch sector. The times and heights of high and low water for the day should be entered in the log at the commencement of the morning watch. Note that most tide tables require an hour to be added to the stated time during BST (British Summer Time) The diagram opposite shows the method of finding the depth of water from the chart and the tide tables. For those stations where it is important that watchkeepers know the depth of water at any one time e.g. where they are located on or near harbours and estuaries, they will need to know how to establish the height of the tide in the hours SA Group Page 8.5 Issue date: Nov 12

10 between the HW/LW times given in tide tables. When the tide begins to rise from LW, it starts off quite slowly for an hour and then gains height more rapidly during the middle hours of its six-hour rise, it then slows down again for the hour before HW. Where there is silting and mud flats over a large area, the rise may be much slower as the mud is being covered. Tidal Curves 2.4. Tidal curves provide the best method for establishing the rise and fall of the tide. They are published by the Admiralty for all standard and main ports. The curves or graphs show the rate of tidal rise and fall for the hours either side of HW and can be found in nautical almanacs such as Reeds. In addition the Admiralty Easy-Tide website - gives (free) daily tidal curves. The one-day predictions provide a convenient guide to the approximate height of the tide at any state between high & low. This can then be added to the charted depth to obtain the approximate depth of water. Tidal Atlases 2.5. Tide tables and tidal curves provide information on the vertical movement of the tide. Tidal atlases and tidal diamonds printed on charts provide information about the horizontal movement of the tide - the tidal stream. The data from the latter sources is based on the times of HW at standard ports and so adjustments may need to be made to relate them to the local time. Tidal atlases are published in separate books or are reduced in size and included in nautical almanacs. For each hour before and after High Water the direction or set of the tidal stream is shown on a separate chartlet. Arrows show the direction of the tidal stream and their length and thickness indicate its rate. The rate is also generally shown in figures which are often arranged in pairs, as in the chartlet below. Here the rate for first neaps and then springs is given in tenths of a knot: 24,48 for instance means the neap rate is 2.4 knots increasing to a very fast 4.8 knots at springs If a tidal atlas is available for your watch sector then it is a good idea to laminate the hourly chartlets so that actual times can be written on them, as on the example opposite, using a non-permanent marker. Watchkeepers can then see at a glance how the tide is setting at a particular time, in this case 1030hrs.This is the midpoint time so the watchkeeper can use this chartlet between 1000 and 1100hrs before SA Group Page 8.6 Issue date: Nov 12

11 turning to the next one. Note: in this example the HW-3 chartlet (3 hours before HW Plymouth) has been amended to HW-4 (4 hours before HW Portland) as local high water at Portland is an hour later than the standard port time. This makes it a lot easier to enter actual times based on the time of local HW. However, it may not be practical to do this where the time difference does not come close to a full hour. Judging Tidal Streams from Visual Observations 2.7. Visual clues and local knowledge should be used alongside tidal atlases and other sources of published information when assessing local tidal streams. The water swirling around a buoy, for instance, will indicate the turn of the tide more accurately than a tidal atlas. Boats attached to moorings also give a good indication of stream direction, as they will lay head to stream unless there is a strong cross-wind. Other visual clues to local tidal streams are pick-up buoys floating downstream of a mooring and lines of foam on the surface of the water trailing away from rocks (often a good indicator of back eddies). Tidal Diamonds 2.8. Positions where tidal streams have been measured are marked on Admiralty charts by a letter within a diamond. An associated table shows the direction or set and the spring and neap rate of the streams at these positions for each hour before and after HW at the standard port specified above the table. Tidal diamonds, particularly those which are close inshore, should be used with care, as the tidal stream may alter significantly within a short distance of the diamond if it is affected by back eddies. At many stations the locations of tidal diamonds are too peripheral to the watch sector (perhaps too far offshore) to be of any relevance, or too dispersed to cover all the local effects that watchkeepers need to be aware of. For these reasons tidal diamonds are best used in conjunction with tidal atlases, visual clues and a general understanding of local tidal conditions. SA Group Page 8.7 Issue date: Nov 12

12 2.9. Using Tidal Diamonds to establish the Direction & Rate of the Tide In the example above, at 3 hours before high water at the specified standard port, the tidal stream at the position of tidal diamond B will set in a direction of 244 T, at a rate of 1.3 knots at springs and 0.7 knots at neaps. If the tide is between springs and neaps it is necessary to interpolate between the two i.e. estimate a value between the two depending on how close you are to springs or neaps Guidance on calculating the drift of a casualty due to the effects of tide and wind is given in the Additional Guidance at the end of this section Note: stations equipped with a combined radar/plotter can access tidal information from the chart plotter. On most electronic charts coloured arrows replace tidal diamonds and these point in the direction that the tide is currently setting. The direction of the tidal stream and its rate SA Group Page 8.8 Issue date: Nov 12

13 in knots can be found by placing the cursor over the arrow. Tidal height information, which allows easy calculations of depth of water, is also available for many ports and harbours on electronic charts. Additional Guidance Calculating Drift due to Tide and Wind 3.1. When visibility is poor or when a casualty is a small object, such as a diver on the surface, visual contact may easily be lost. To plot the drift of a casualty the Coastguard use a program called SARIS which defines the search area for rescue craft. However, their calculations are not always available to NCI watchkeepers, nor are they always sufficiently sensitive to local tidal effects, particularly within one mile of the shore (as in the example opposite). If a search is being made for a casualty in or near their watch sector, watch-keepers will want to know where to scan. Their trained eyes, combined with an elevated viewpoint and powerful optics may make all the difference Even when the casualty s position is known, it is sometimes useful to be able to anticipate roughly where it may have drifted to by a certain time - perhaps the time when the lifeboat is expected on scene. The ability to assess drift is, therefore, a useful skill and it is recommended that it is included in the training syllabus, albeit at an advanced stage. It is important to emphasise, however, that the priority will always be to keep the casualty in visual contact. If the watch is single-manned, the watchkeeper should be keeping the casualty under close observation rather than spending time head down at the chart table. If visual contact is lost, he or she should make a mental calculation of the likely drift - based on an awareness of the current state of tide and wind and local tide and wind effects - whilst continuing to carry out systematic scanning sweeps from the last known position When two watchkeepers are on duty it may be possible for one to plot the drift on the chart whilst the other continues to scan. If the casualty remains visual then its likely position when the SAR vehicle SA Group Page 8.9 Issue date: Nov 12

14 is due on scene can be established by plotting its track from regular bearing/distance plots, say every 2 minutes, and then extrapolating or extending the track line to the position at the end of the elapsed time. This is illustrated in the diagram below If visual contact is lost, it will then be necessary to establish an area of search by calculating the likely drift from the last known position (LKP) based on the current speed and direction of wind and tide. Calculating Drift using Tidal Diamonds 3.5. An example of drift calculation using tidal diamonds is given in the accompanying diagrams. Note that the pitfalls associated with the use of tidal diamonds are set out earlier in this section. In many cases it will be more appropriate to use tidal atlas information backed up by knowledge of local tidal conditions In diagram A the casualty is at A and the watchkeeper wants to know where it is likely to be in 30 minutes. It is HW-3 springs and, from the tidal diamond table on the chart, it has been established that the tide SA Group Page 8.10 Issue date: Nov 12

15 is set in a direction of 244 T, at a rate of 1.3 knots i.e. roughly 0.6nm in 30 minutes. Diagram B shows a plot of the tidal drift based on this information. Note: In this example there is little difference between the HW-3 time at the standard port upon which the tidal diamond data is based, and the local HW-3 time. This may not be the case elsewhere In addition to tidal drift, boats and liferafts will also be affected by the wind, drifting down-wind by ± 60 at about 5% of wind speed (note: divergence of up to 60 either side of wind direction may occur because of the casualty sailing across the wind). In diagram C the wind is SW, i.e. blowing towards 045 T, at 40 knots. The rule of thumb figure of 5% of 40 knots = 2 knots, or 1 nautical mile in 30 minutes, has been added to the calculation to establish the centre of the search area Watchkeepers are bound to struggle with mathematical calculations of this sort when under pressure during an incident. Use of the ready reckoners on the following pages, perhaps as laminated quick reference sheets, could speed up the process and make it simpler. SA Group Page 8.11 Issue date: Nov 12

16 Drift Ready Reckoner SA Group Page 8.12 Issue date: Nov 12

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