Sea bed ramblings

Saturday dawned windy yet again with the promise of substantial showers around in the afternoon so I decided to head out towards Seymour Tower, off the south east corner of the Island.  Due to the large tidal range in Jersey (almost 12 metres on the larger tides) a virtually unique coastal environment is exposed twice a day, particularly on Spring Tides.  For several hours, each day, the opportunity of walking around on the sea bed presents itself and I took full advantage of the opportunity yesterday.  Amazingly I was the only person who seemed to want to experience the delights of this corner of the Island on blustery Saturday in January. 

 Looking towards Seymour Tower, my aim was to try and reach a navigation mark, Karame, which is beyond the obvious tower.  As I headed out across the exposed foreshore I was the only person in the area.

 The rescue beacon.  In the past people have lost their lives when they have been cut off by the rapidly rising tide.  Between Christmas and New Year two German tourists were fortunate to be able to climb the tower when they underestimated the speed at which the tide rises.  The inshore lifeboat collected them from the tower which prevented them having to speed a cold and dark night 6 hours perched above the swirling waters.

 Viewed up close the superb construction of the tower is clearly visible.  The concrete additions are as a result of the German occupation.  It is possible to hire the tower from Jersey Heritage, a very unique place to spend a night.

 A quick lunch stop coincided with almost the only sunshine of the day.  The tower casting its shadow over the sandbank.

 Looking north towards Gorey, with the magnificent Mont Orgueil Castle towering above the harbour.  The shiny slopes to the left of the castle are some of the earliest fields to be planted with Jersey Royal potatoes.

 Seymour Tower seen from the south.  Painted white as a navigation mark.

 I had been hoping to get as far as Karame, it was going to be touch and go but as I stood here the atmospheric pressure was 987mb.  As many of you know, tidal heights are based on mean atmospheric pressure of 1013mb and as a general rule a change of one millibar results in a change of one centimetre in sea level.  26mb lower than  average pressure meant that the tide was 26 cm higher than predicted.

 Heading towards Little Seymour, my final navigation mark before heading back to the shore.  The clouds were obviously building to the south and west, it was time to up the pace.

Looking towards La Rocque Harbour, the end of the pier clearly visible, being painted white.  It was at La Rocque that Baron de Rullecourt landed on the 6th January 1781, an invasion which resulted in the Battle of Jersey being fought in the Royal Square in Jersey, followed by a rapid expansion of the building of coastal fortifications.  Many of the towers survive today, part of the distinctive coastal architecture.

The predicted heavy showers approaching from the south west.  A significant increase in walking speed could not prevent major soaking as the downpour hit.  A very wet end to a thoroughly enjoyable 3 hours, and the only people I saw were on the slip at the beginning and end.  There is still isolation in Jersey, waiting for those who seek it out.

How far can you see?

How far can you see whilst sitting in your kayak?  Knowing how far away you can see an object whilst paddling is a useful technique and a valuable aid to navigation.  As a simple rule the higher up you are the further you can see.  Standing on top of the Empire State Building, with good visibility it is amazing how far away the horizon is.  Also the taller the object you are looking at the further away that it can be seen.

As paddlers it is not that easy to raise our eye level, our eyes are generally just less than 1 metre above sea level.  At times in rough weather or when there is a swell running it is possible to take advantage of the extra elevation that results from being on top of the swell to increase how far we can actually see.    Due to the movement up and down of the kayak, on the water, the distance off an object which is obtained should be seen as an approximation.

Clearly a further problem is caused by the rise and fall of the tide, which may well be significant in certain areas.  For example on a chart a lighthouse’s charted height is given above MHWS.  In certain areas of the world with a large tidal range the height above water of the light may vary by more than 10 metres, considerably affecting the distance away that the light may be seen from.  If you want to be really accurate it is necessary to add the estimated height that the tide is below MHWS to the height of the land or the light before referring to the table.

An example of the effect of this from Saturday, when we went to the Ecrehous on a large spring tide, is as follows:

Maitre Ile at the Ecrehous has a height of 8 metres at MHWS, which in Jersey is 11.1 metres, but the tide on Saturday was 11.8 metres, it was bigger than a mean spring.  This meant that at high water the maximum height of Maitre Ile was not 8 metres but 7.3 metres

When the eye of the observer is 1 metre above water level an object 8 metres high is visible from 7.8 nm away but when the height of the object drops to 7 metres  it is not visible until you are within 7.4 nm.  When we left La Rocque, Maitre Ile was 8.3 nm away.  This meant that even in excellent visibility we would have not been able to see our destination when we left.

On a spring tide when the water level may drop by as much as 11 metres the highest point on Maitre Ile is now 19 metres above the water level.  This means that the island is now visible from 10.9 miles away for a paddler whose eye is 1 metre above the water. 

The lesson is that objects will be visible from much further away when you approach them at low water, particularly in an area with a large tidal range, such as the Channel Islands.  

Approaching Maitre Ile, early on Saturday morning.  We had been paddling for some time before our destination came into view.

The following table gives an approximate relationship between the distance from which an object is visible and its height above sea level.  It is assumed that the height of the paddlers eyes above water is 1 metre.

Height of the object in metres	Distance visible in nautical miles
0 1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 22 24 26 28 30 40 50 60 70 80 90	2.0 4.1 4.9 5.5 6.1 6.6 7.0 7.4 7.8 8.1 8.5 9.1 9.6 10.2 10.6 11.1 11.6 12.0 12.4 12.8 13.5 15.3 16.8 18.2 19.5 20.7 21.8

Clearly there are number of variables which impact upon the accuracy of the above table such as sea state, the exact height of the paddlers eye above sea level and the height of the tide but it is a useful tool in helping the sea paddler to locate their position.  For example 12 nautical miles to the south of my nearest beach is the superb reef of the Minquiers.  The tallest rock on the northern edge is only 3 metres high, which according to the table means that they only become visible when they are 5.5 nautical miles away.  Therefore there is no point in even starting to look for the reef until you have paddled for over 6 nautical miles or have been underway for over an hour and a half.

Using this technique as a way of assisting navigation is particularly satisfying but it is a method which is gradually slipping into obscurity.  Today the vast majority of us simply turn to the switch on the GPS to receive far more accurate information about our position than we could ever obtain by using the above method.  That said there is a degree of satisfaction from being able to navigate using the more traditional methods and you never know if the batteries are going to run out!

The photo is of La Conchiere Beacon.  At  MHWS it is 2 metres high and so is visible from 4.9 nautical miles away from an observer whose eye is 1 metre above sea level.  Clearly an object this narrow would be more difficult to sea that something more substantial, such as a reef but of similar height.

A Spring Tide Ecrehous

Saturday's high tide was predicted to be 11.8 metres or 38.8 feet, with a low tide of 0.5 metres or 1.7 feet. This is quite a large tide even by Jersey's standards.  With light winds forecast it seemed to good an opportunity to miss, the Ecrehous was our favoured destination but with 11.3 metres of water moving and tidal streams in excess of 5 knots at times the navigation needed to be more accurate than normal.

So at just gone 06.00 we were packing the sea kayaks on the rapidly disappearing beach at La Rocque.  The aim was to leave an hour before HW, in fact we were ready a few minutes early, so 06.30 saw us heading away from the south east corner of Jersey.  

Selecting La Rocque as opposed to St Catherine's our normal departure point gave us considerably more leeway as we approached the reef.  As our track from yesterday shows we did return to St Catherine's, if we had arrived back at the south east corner of the island we would have ended up having to carry the kayaks well over a mile as it was low water.

It was interesting to observe the impact that the pressure had on the height of the tide.  Tidal predictions in the UK are based on average atmospheric air pressure, which is 1013 mb.  The actual air pressure whilst we were paddling was 1040 mb.  This probably explains why when we were at the Ecrehous the tide wasn't as quite as high as we expected.  Then on arrival back at St Catherine's the tide was as low as I can remember for a long time.  The sea level drops 1 cm for every mb that the pressure is above 1013 mb, so the height of the tide would have been 27 cm lower than predicted.  

 Approaching the Ecrehous, they are the slight bumps to the left of the bow.  The crossing was just over 9 nm and it took just under two hours.  The last 3 nm passed rapidly as we experienced the full flow of the tide.

Alex approaching Maitre Ile, the largest of the islands on the reef.  We rarely land on this island choosing instead to head straight to Marmotier.

 On an 11.8 metre tide there is not much to see.  Marmotier is where the majority of the small houses are located.  This photograph was taken an hour after HW.

Looking north towards Blanche Ile.  The tide is flowing strongly north at this point.  A couple of hours later it was a simple walk along the shingle bank to the other houses.

When we arrived there was only one other person on the reef, with a few hours there were plenty of other visitors from both Jersey and France.

 Alex and John having a very early lunch or a late breakfast.  It was delightfully warm when sheltered from the NW breeze.

 

 The obligatory picture looking north from the bench.

 A 12.00 departure enabled us to gain maximum advantage from the low water slack in the main channel between Jersey and the Ecrehous.  We did head south along the reef though to ensure that we were at the optimum point for departure.

 Jersey ahead.  This view shows part of both the north and east coasts.  Using a hand held VHF we often find that there is a blind spot in close so we have to radio in when we are still about 0.5 nm offshore.

 Arriving at St Catherine's and it is still not low water.  It was not that easy getting the kayaks up the steps though.

A great day out and due to the alternative departure point what could have been a really challenging paddle became something which was relatively straight forward.