Ken Endean explains how datums are stepped up certain rivers

 In the April 2007 issue of Yachting Monthly (on sale now) Ken Endean explains how chart datum is stepped up the Tamar River. Here is the full text of the original question, reply and consequent response.

Dear YM

In his article ‘The Majestic Tamar’ (YM May 06) Ken Endean included the following :
“The cartographers have chosen to step up the local chart datum to follow the general rise in the channel bed (see diagram of bed profile) and this complicates depth calculations. For example, at Plymouth sea level at MLWS in 0.8m above Chart Datum. Off Cotehele Quay the Chart Datum is 1.1m. higher than off Plymouth and, at MLWS, the water level is 0.4m above that raised datum.”

I keep my boat on the Tamar and I need to calculate accurate depths for the purposes of taking the boat up to its winter quarters far up the Lyner River and launching again in the spring.

The above puzzles me somewhat – particularly the bit about the Chart Datum stepping up which is quite new to me, and I wondered if you could give me Ken’s email address so that I can ask him about it?

Robert Shepard

Ken Endean replies:

The cartographers often ‘step up’ the chart datum on drying estuaries, although this topic is rarely, if ever, mentioned in sailing manuals and ‘Yachtmaster’ courses!

Where the bed of a river channel is higher than the chart datum at the river entrance, some charts show a drying height in the channel but on other charts the datum is adjusted upwards, in steps, to follow the general rise of the channel bed. If you look at the Fowey River, on BA chart 31, you will see that it includes a little diagram showing the stepped datum.

Now for a worked example: at Lostwithiel, which is the head of navigation on the Fowey, the chart shows a LW depth of 0.5m in the river (ie. the bed is 0.5m below CD). Therefore, on a 5.4m spring tide, one might expect a HW depth of 5.9m. However, the chart datum at Lostwithiel is raised by 3.9m, so the actual HW depth will be about 2.0m. Does that make sense?

For the Tamar, BA Chart 871 does not have a tidal diagram but its table of tidal levels shows that there is at least one datum step. At most of the tidal stations, the chart datum is 3.22m below Ordnance Datum (map datum). However, at Cotehele, it is only 2.13m below Ordnance Datum – ie. it has been stepped up by about 1.1m.

Further up the Tamar, the chart gives no additional information on the chart datum but the charted depths only make sense if the datum is stepped up in several places! (For instance, close to Weir Head, the chart shows a LW depth of 0.2m below CD, but I know that the HW depth at that point, on a 5.5m spring tide, is only about 2.0m.) Using depths recorded at HW, I worked out approximate values for the height of chart datum and then checked my conclusions with the UK Hydrographic Office before writing the article.

For you, the good news is that the Lynher may not have a stepped chart datum. BA 871 shows that the chart datum at St Germans is the same as at Devonport. I once took our boat right up to within half a mile of Notter Bridge and found depths of around 3m on a high spring tide of about 5.7m, which suggests that the channel’s charted drying height, of 2.7m at that point, is related to the same datum. I don’t know where you will be laying up but a spring tide should give you somewhat more than 3m to the boatyard at Treluggan.

Does this help? The subject would be less complicated if the cartographers were consistent, but I guess the Navy is not overly concerned about depths at Weir Head or Notter Bridge!

Robert Shepard responds:

Thank you very much indeed for taking the time to let me have such a comprehensive reply!

I think the problem is that I have almost always used Imray charts! I rang the chart production mangager at Imrays and he told me that their C14 chart (Plymouth and Tamar) did not have any upward steps of CD.
So the mystery is solved – HO and Imrays don’t step up the CD on the Lyner, but the HO does on the Tamar! How very confusing!

My boat is a Hallbery Rassy 36 drawing 1.7m and this year I’ve asked the yard (good guess of yours, yes it’s Treluggan Marina) to put me in the water on Easter Saturday, early tide. The yard expressed caution over the depth and I noted that tides were taking off too – not a recipe for a happy easter to be stuck at 45 degrees under the viaduct for a fortnight!

With the help of your information I’ll now go for it, on the basis of 5.0m ht (Reeds), less 2.1m drying ht (Imray chart), less 0.3m tidal difference for St Germans (Reeds), less 1.7m draught, gives hopefuly a margin of 0.9m.

I also checked with my Saltash mooring servicing chap (Alan at Diving Belle who knows the rivers very well) and he reassured me by saying that a SW F6 would increase heights by 0.5m and that strong winds from ANY direction increased heights, never decreased them (on the Lyner River that is), and fascinated me by reporting the record increase over predicted – this was 3.0m above a predicted MHWNeaps (yes Neaps) in a Force 11 gale many years ago!

Many thanks again.