Lower Low Water (LLW)

Lower Low Water (LLW) is the lower of the two low waters that occur during a tidal day where diurnal inequality is present. It is the deepest drawdown of the day and therefore the limiting level for under-keel clearance and the value averaged over the epoch to form Mean Lower Low Water, the US chart datum. LLW results from the diurnal constituents K1 and O1 reinforcing one semidiurnal low. In a pure semidiurnal regime the two lows are nearly equal, so lower and higher low water differ little.