Mean Higher High Water (MHHW)

Mean Higher High Water (MHHW) is the average of the higher of the two daily high waters over the 19-year National Tidal Datum Epoch. It is the diurnal-inequality counterpart to Mean High Water and is the standard upper datum on US nautical charts for coastline delineation and bridge clearance. In a strongly mixed or diurnal regime MHHW stands well above MHW because one high each tidal day dominates. With Mean Lower Low Water it defines the great diurnal range Gt, the key measure of tidal range in US tidal datum work.