Angele
Well-Known Member
I have planned a preferred itinerary for a "delivery" trip to Jersey that takes in Cherbourg and Diélette. Please, no suggestions like "Diélette - don't bother". There is a reason behind my planning (mostly related to allowing me enough time in the Cherbourg supermarkets to stock up on booze in the morning and then only having a limited time between the tidal gate round Cap de la Hague closing and my crew demanding yet another meal ashore). /forums/images/graemlins/smile.gif
So, Diélette it is. Trouble is that I have never been there and the write up in Reeds (suggesting limited access for deep draught yachts) has tended to put me off. Then, I sat down and did some calculations and I wondered whether Reeds had got it wrong. I penned an email to the editor and the response .... not a word. Not even an acknowledgement. /forums/images/graemlins/frown.gif
Apologies for the length of the post, but if one of you forumites has the time and inclination to be 'rsed to check my logic I would be grateful.
To the editor of Reeds Almanac
I believe there is an error in the almanac under the heading of “Navigation” for Diélette. The “Navigation” section appears to cover access through the outer harbour (above chart datum) to the “Bassin de Commerce” (below chart datum) and not to the inner marina, access to which is quite restricted as is discussed under the heading “Facilities”. The particular phrase that I believe is inaccurate is “with Coefficient >55 accessible for 2.5m draft”. With the use of the “greater than” sign I read this as saying that a deep draft vessel can only enter the port on a day when the tidal coefficient is more than 55. Conversely, on any day when the tidal coefficient is more than less than 55 (eg. an average neap tide), a deep draft vessel cannot access the harbour at all. Looking back at previous editions it would appear that the same description has been used for several years.
The tidal coefficient is, so far as I understand, a relative measure of the tidal range on a given day (i.e. springs, neaps, or somewhere in between) and not a comment about a particular height of tide at any given moment. The greater the tidal coefficient the larger the tidal range on a given day and so the higher the height of tide at high water and the lower the height of tide at low water.
Since the outer harbour is, according to the description under “Navigation”, dredged to 0.5m above chart datum, a 2.5m draft vessel would require a height of tide of at least 3.0m to enter the Bassin de Commerce. From the tidal information for the port it is clear that this would not be the case at low water springs (MLWS St Malo 1.5m less 0.3m = 1.2m). However, at average neap tides, the height of tide varies from 3.5m at low water to 7.4m at high water and so this would allow 24 hour access to the Bassin de Commerce for the 2.5m draft vessel referred to above. So, this immediately suggests that the phrase “with Coefficient >55 accessible for 2.5m draft” is misleading.
On closer investigation, I calculate that a tidal coefficient of 55 implies a tidal range that is 20% along the path from mean neaps (coefficient 45) to mean springs (coefficient 95). Spring range is 8.5m (9.7m less 1.2m) and neap range is 3.9m (7.4m less 3.5m). So, the range on a day when the tidal coefficient is 55 is 3.9m plus 0.2 x (8.5 – 3.9) = 4.8m. If we assume that the additional 0.9m of tidal range over mean neaps is split equally between a 0.45m increase in the height of tide at high water and a similar reduction in the height of tide at low water, then the height of tide varies between 3.0m and 7.9m. At low water, therefore, there would be a clearance of 2.5m across the channel in the outer harbour dredged to 0.5m. This would allow your 2.5m draft yacht 24 hour access on that day (although not if the channel had silted up at all, nor if the yacht strayed from the recommended track, and nor when pressure is materially higher than 1013 Mbar). Perhaps a margin for error of 0.5m would be appropriate.
Once in the Bassin de Commerce, which appears from the chart to be at 2m below datum, the tidal data suggests that a yacht drawing 2.5m could happily stay afloat on all but the lowest of spring low waters (when even the dredged channel in the outer harbour would have totally dried).
All this leads me to suggest that a better description for access would be “with Coefficient LESS THAN 55 accessible 24 hrs for 2.0m draft.”
I have to admit that I have never visited the port (largely because your description of what appears to be rather limited accessibility for my 2.1m draft yacht has so far put me off) and so I cannot confirm any of my conjecture against actual observations. However, I have observed that your information regarding the dredged depth of the outer harbour access channel is consistent with that on the port’s own website (http://www.cc-lespieux.fr/fr/port-dielette/presentation-et-plan-du-p/default.asp).
One small additional observation is that Reeds Almanac states that the sill into the marina is at 4m above CD, whereas the above website states it is at 3.5m above CD. Mind you, I would never argue against inclusion of a 0.5m margin for safety!
I would be interested to hear whether you agree with my conclusions.
So, Diélette it is. Trouble is that I have never been there and the write up in Reeds (suggesting limited access for deep draught yachts) has tended to put me off. Then, I sat down and did some calculations and I wondered whether Reeds had got it wrong. I penned an email to the editor and the response .... not a word. Not even an acknowledgement. /forums/images/graemlins/frown.gif
Apologies for the length of the post, but if one of you forumites has the time and inclination to be 'rsed to check my logic I would be grateful.
To the editor of Reeds Almanac
I believe there is an error in the almanac under the heading of “Navigation” for Diélette. The “Navigation” section appears to cover access through the outer harbour (above chart datum) to the “Bassin de Commerce” (below chart datum) and not to the inner marina, access to which is quite restricted as is discussed under the heading “Facilities”. The particular phrase that I believe is inaccurate is “with Coefficient >55 accessible for 2.5m draft”. With the use of the “greater than” sign I read this as saying that a deep draft vessel can only enter the port on a day when the tidal coefficient is more than 55. Conversely, on any day when the tidal coefficient is more than less than 55 (eg. an average neap tide), a deep draft vessel cannot access the harbour at all. Looking back at previous editions it would appear that the same description has been used for several years.
The tidal coefficient is, so far as I understand, a relative measure of the tidal range on a given day (i.e. springs, neaps, or somewhere in between) and not a comment about a particular height of tide at any given moment. The greater the tidal coefficient the larger the tidal range on a given day and so the higher the height of tide at high water and the lower the height of tide at low water.
Since the outer harbour is, according to the description under “Navigation”, dredged to 0.5m above chart datum, a 2.5m draft vessel would require a height of tide of at least 3.0m to enter the Bassin de Commerce. From the tidal information for the port it is clear that this would not be the case at low water springs (MLWS St Malo 1.5m less 0.3m = 1.2m). However, at average neap tides, the height of tide varies from 3.5m at low water to 7.4m at high water and so this would allow 24 hour access to the Bassin de Commerce for the 2.5m draft vessel referred to above. So, this immediately suggests that the phrase “with Coefficient >55 accessible for 2.5m draft” is misleading.
On closer investigation, I calculate that a tidal coefficient of 55 implies a tidal range that is 20% along the path from mean neaps (coefficient 45) to mean springs (coefficient 95). Spring range is 8.5m (9.7m less 1.2m) and neap range is 3.9m (7.4m less 3.5m). So, the range on a day when the tidal coefficient is 55 is 3.9m plus 0.2 x (8.5 – 3.9) = 4.8m. If we assume that the additional 0.9m of tidal range over mean neaps is split equally between a 0.45m increase in the height of tide at high water and a similar reduction in the height of tide at low water, then the height of tide varies between 3.0m and 7.9m. At low water, therefore, there would be a clearance of 2.5m across the channel in the outer harbour dredged to 0.5m. This would allow your 2.5m draft yacht 24 hour access on that day (although not if the channel had silted up at all, nor if the yacht strayed from the recommended track, and nor when pressure is materially higher than 1013 Mbar). Perhaps a margin for error of 0.5m would be appropriate.
Once in the Bassin de Commerce, which appears from the chart to be at 2m below datum, the tidal data suggests that a yacht drawing 2.5m could happily stay afloat on all but the lowest of spring low waters (when even the dredged channel in the outer harbour would have totally dried).
All this leads me to suggest that a better description for access would be “with Coefficient LESS THAN 55 accessible 24 hrs for 2.0m draft.”
I have to admit that I have never visited the port (largely because your description of what appears to be rather limited accessibility for my 2.1m draft yacht has so far put me off) and so I cannot confirm any of my conjecture against actual observations. However, I have observed that your information regarding the dredged depth of the outer harbour access channel is consistent with that on the port’s own website (http://www.cc-lespieux.fr/fr/port-dielette/presentation-et-plan-du-p/default.asp).
One small additional observation is that Reeds Almanac states that the sill into the marina is at 4m above CD, whereas the above website states it is at 3.5m above CD. Mind you, I would never argue against inclusion of a 0.5m margin for safety!
I would be interested to hear whether you agree with my conclusions.
