Tidal datum

[Auzzee]

I have just bought a pilot book for a specific part of the Caribbean and notice tide heights are given above MLLW (Mean Lower Low Water) rather than the more familiar to me, MLWS (Mean Low Water Springs).

Given the definition of both (MLLW a mean of the lowest two daily tides over a 19 year period and: MLWS, an average of spring lows over the same period), is it safe to assume that MLLW as a value, is not quite so low as MLWS?

I expect the difference would be marginal in any location where the tidal range is 2 or 3 feet. But in areas where the range is over 18 feet, as in Darwin, I imagine the difference would be significant.

Anyone have any practical knowledge of this? I am soon to skipper a 57 footer with a 6'5" draft through the Abacos and I have a distinct aversion to hitting the bricks.

[delatbabel]

I would say that by reading the definitions that MLLW is lower than MLWS.

To calculate MLLW you pick the two lowest tides over a 19 year period. The choice of 19 years is significant because it tracks the moon through the closest and furthest perigee points. So in that 19 year cycle you're going to get at least two very low tides. Pick the lowest two and find the half way point.

MLWS picks all of the tides over a period that isn't defined to be 19 years but may as well be. Then just filter out the springs lows, which will be the lowest tides that appear at spring tides, roughly 12 times per year. Over 19 years you'll get 19 x 12 = 228 low tides, and you pick the average (or mean to be exact) of all of those.

By definition, the two tides selected for MLLW will be part of that 228 low tide set and will in fact be the lowest two of that set.

The average of a set of tides will be higher than the average of the lowest two of that set of tides.

[Cowboy Cruiser]

I think Auzzee has it correct, MLWS is on average a lower data set. An average of the lowest of the lows over 19 years is going to be lower than the average of the day's lowest for 365 days x 19 years.

Coastal Zone Management Handbook - Google Books

Check out page 441

Mike

[piclarke]

Originally Posted by Auzzee

I have just bought a pilot book for a specific part of the Caribbean and notice tide heights are given above MLLW (Mean Lower Low Water) rather than the more familiar to me, MLWS (Mean Low Water Springs).

Given the definition of both (MLLW a mean of the lowest two daily tides over a 19 year period and: MLWS, an average of spring lows over the same period), is it safe to assume that MLLW as a value, is not quite so low as MLWS?

I expect the difference would be marginal in any location where the tidal range is 2 or 3 feet. But in areas where the range is over 18 feet, as in Darwin, I imagine the difference would be significant.

Anyone have any practical knowledge of this? I am soon to skipper a 57 footer with a 6'5" draft through the Abacos and I have a distinct aversion to hitting the bricks.

MLLW is only a mean, so some tidal levels may be negative relative to MLLW
Lower tides may occur in practice due to other factors like meteorological effects such as high pressure systems and strong of shore winds blowing with the tide direction for long periods such as the entire out going tide plus approx. 46mins to 1 hour against on turning incoming tides.

Using charts and tables that are not based on the same chart datum that you are not accustom to can result in incorrect calculation of water depths and should be avoided. Suggest you stick to your accustom charts types or give bricks a wider berth like a further 50 meters.

I assume you are used to referencing British admiralty charts.


Anchor at a min depth 30 meters MLLW to avoid being dragged impacted by Tsunamis flows.

[piclarke]

Also an astute captain always gives charted bricks a wide berth because re earthquakes and the changing sea bed floor that are cause by rising datum levels that are not considered unless a full new survey has been completed even then there is no guarantee the sea bed floor is as the chart states between official surveys.

Sea level is known to vary around all coasts territorial waters or shallow waters . This means that the level of MSL determined each datum's tide-gauge will be different and that offsets will occur between adjacent datum's and all local mean sea level datum's.

Avoid all bricks to the same extent as you do to avoiding dengue fever.