Higher Low Water (HLW)

Higher Low Water (HLW) is the higher of the two low waters that occur during a tidal day where diurnal inequality is present. It is the shallower of the day’s two lows, set by the phasing of the diurnal constituents K1 and O1 against the semidiurnal M2 and S2. HLW matters for under-keel planning because the more limiting depth comes from the lower low water, not this one. In a pure semidiurnal regime the two lows are nearly equal and the higher and lower low water distinction nearly vanishes.