Wednesday, January 21, 2015

Interesting tidal flows - Tidal Diamonds

In the early 2000's Chris Jones and myself ran the Sea Paddler website.  At the time it was very popular although nowhere as technically advanced as present sites.  We updated it regularly over the years, at least weekly, so there were literally hundreds of articles and thousands of pictures.  I may post some of the more relevant ones over the coming months.

Tidal diamonds are invaluable sources of information in relation to the speed and direction of tidal streams.  Whilst away on a paddling trip recently I came across, what must be an almost unique diamond?  It is diamond C on Admiralty Chart 808, East Guernsey, Herm and Sark.

49°27.5 N
2°31.4 W
Rate (kn)
  Sp           Np




5.1            2.2
4.1            1.8
2.7            1.2
1.2            0.5
1.2            0.5
3.9            1.7

5.2                  2.2

 4.9            2.1
 3.3            1.4
 1.5            0.5
 1.3            0.5
 4.0            1.7
 5.2            2.2

The first observation is that the streams only flow in two directions, exactly opposite each other.  The tide is flowing approximately SSW and then it changes abruptly and goes NNE.  An exact 180° change.
Something else to note is that maximum rate occurs at high and low water, with slack, if it can be called that, occurring at mid tide.  Many a sea paddler or other water user has been caught unawares because they assume that maximum rate must occur at mid tide and have set out on what they thought was high water slack.
The advice is to always double check your data and to keep your eyes open for interesting and possibly unique items of information as shown by this tidal diamond.
The location of the diamond is in line with the two towers on the photograph and to the right of Brehon Tower, which is in the middle of the channel between Guernsey and Herm, the Little Russel.

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