Mean Lower Low Water comes from picking the lower of the two lows in every tidal day and averaging those heights across the tidal datum epoch. Where only one low occurs in a day, that low counts as the lower low. It is the chart datum for US nautical charts and the zero of US tide predictions, so a predicted tide of minus 1.2 ft means 1.2 ft less water than the charted depth.
Design depth is minus 45 feet MLLW, plus two feet of overdredge allowance.
Because MLLW is an average of the daily lower lows, roughly half of those lows fall below it, so negative predictions are routine rather than rare.
Also heard as
- mean lower low water
Do not mix it up with
- LAT: LAT is the lowest predicted astronomical tide over many years and sits lower than MLLW; MLLW is only an average of daily lower lows.
Related terms
MHHW
Mean Higher High Water: the average height of the higher of each day's two high waters over the datum epoch.
Chart datum
The low-water zero that charted depths and tide table heights are measured from.
Tidal datum
A local zero for heights, computed by averaging one stage of the tide, such as high or low water, across many years.
Minus tide
A low tide predicted to fall below the chart datum zero, shown as a negative number in the tide table.
Low-low
Shorthand for the lower low water of the day in places with mixed tides.