Lagoon inlets and river entrances on sandy coasts are shaped by waves, tides and freshwater outflows interacting subject to geological constraints. In dimensionless terms the relative importance of fresh water discharge Qf versus peak tidal discharge is quantified simply by , where the tidal peak discharge may be taken either as the actual corresponding to the actual bay tidal amplitude aB or the potential , which is based on the ocean tide amplitude aO. ωtide is the tidal angular frequency and

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... AB is the bay surface area at mid tide. The quantification of the relative strength of waves versus tides is less obvious and has not previously been clearly resolved. The case is made here for it being quantified by 5 / s Q g H , where tidal dominance resulting in the canonical funnel shaped estuary occurs for 5 / 75 s Q g H >  and, at the other end of the spectrum, intermittently open/closed systems (ICOLLS) occur for 5 / 20 s Q g H <  . denotes the average offshore significant wave height and g is acceleration due to gravity. More comprehensive data may lead to the inclusion of the wave period as well as the wave height in future formulations.