Processes controlling river-mouth lagoon dynamics on high-energy mixed sand and gravel coasts

The river mouth lagoons of New Zealand's gravel-bed braided rivers are highly dynamic, responding to changes in waves and river flows at timescales of hours and days to years. Unlike other freshwater-dominated coastal wetlands and lagoons, they exist along open coasts that are typically retreating with cliffed hinterlands. These lagoons are common on New Zealand's high energy, coarse sediment coastlines and are known locally as 'hapua'. This paper employs field observations from the Hurunui hapua, to investigate the main processes controlling the dynamics of hapua lagoons. Based on two years of time-lapse imagery, plus concurrent wave, river flow and sea level data an improved conceptual model of lagoon dynamics was developed. The model describes how onshore bar migration of river sourced gravel and sand drives initial constriction/offsetting of the river mouth following floods. It also explains how gravel deposition in the lagoon reduces the threshold flow required to cause primary breach of the barrier opposite the point the river enters the lagoon. Three processes are identified which affect lagoon width: wave overwashing of the gravel barrier narrows the lagoon; migration of the highly dynamic lagoon outlet channel 'resets' the barrier position to seaward; and river floods occurring while the outlet channel is offset from the river flush sediment from the lagoon system, eroding its bed and banks. Both processes affecting lagoon widening rely on lagoon outlet channel migration, indicating that outlet channel dynamics are an important control on lagoon size.