As a progressive wave enters shallow waters its speed decreases. Since the trough is shallower than the crest retardation is greater resulting in the steepening of wave front so in estuaries advance of trough is so-much retarded that the crest of the rising tide overtakes it and advances upstream in a churning wall of water.. Dangerous for moored ships which can SURGE.
Chart Datum:
Lowest astronomical tide below which the level of water doesn’t usually fall.
Negative Surges:
Fall in low water or predicted water due to meteorological reasons.
Seiches:
Occurs in a confined body of water. A long wave having crest at one end and trough at other end of confined space. Abrupt changes in meteorological conditions like passage of an intense depression or line of squall may cause oscillations of sea level, so the period between successive waves can be unpredictable, few minutes to two hours.
