Coastal wetlands and rivermouth lagoons are dynamic systems, which respond rapidly to sea-level, tectonic, meteorological, anthropogenic and other synergistic drivers. This research used a multi-disciplinary approach to investigate two representative West Coast lagoon systems (Totara Lagoon and the Shearer Swamp-Waikoriri Lagoon Complex) in order to document their present-day geomorphology and determine the development and processes acting on these systems over historical time. This information was then used to predict their future under varying climate, development and management pressures. In addition to adding to the West Coast knowledge base, the findings of this research are applicable to similar systems elsewhere in New Zealand and internationally. This investigation used a multidisciplinary approach to investigate the dynamics, structure, development and active processes in the two study systems. Techniques to document current hydrology and topography included hydrological records of water level, temperature and conductivity, and Global Navigation Satellite Surveys (GNSS). Outlet dynamics over a decadal scale were investigated through temporal aerial photograph analysis, and sediment core analyses showed changes occurring over longer timescales. Significant differences in morphology and dynamics were observed between Totara Lagoon and Waikoriri Lagoon, with the former being much larger, more stable, and less dynamic in terms of dune morphology and outlet migratory patterns. Hydrologically, Totara Lagoon is currently in an estuarine phase, and experiences significant tidal inflows, which demonstrates the connectivity between definitions of coastal lagoons and estuaries. Waikoriri Lagoon is freshwater, and can be described as a hapua-type system, but exhibits very different river flow and barrier composition to East Coast examples. Sediment core analyses from Shearer Swamp and northern Totara Lagoon showed little change over a decadal to centennial scale, but evidence of a change in margin dynamics in response to farming and stabilisation of adjacent dune ridges was observed in Shearer Swamp. Results suggest landward migration of the southern end of Totara Lagoon occurred over this timeframe. The future of these systems depends on the interaction between climate and anthropogenic (including management) factors. A conceptual model of process and response suggests three possible resultant scenarios: lagoon loss, natural lagoon, or artificially modified lagoon. A significant finding of this research is the recognition that some systems exist on a continuum between a hapua and an estuary, switching hydrological states through time while maintaining consistent morphology. In addition, the importance of barrier permeability in hapua formation is highlighted, and the term ‘sandy hapua’ introduced to distinguish these low-flow systems with low barrier permeability from the typical mixed sand and gravel examples documented on the East Coast. These findings enhance scientific understanding of rivermouth lagoon systems, and demonstrate the wide spectrum of conditions under which they may form. This process-based understanding is important from a coastal management perspective as concerns of human induced climate change and accelerated sea level rise grow.

Hapua are complex and dynamic systems, and are especially vulnerable due of their location at the end of river catchments. The Hurunui River hapua is currently under pressure from the intensification of irrigation and agriculture, and a number of dam proposals in its catchment. The purpose of this research was to investigate the current conditions in the Hurunui River hapua, how they respond to the observed range of contemporary catchment and coastal processes, and to examine of the longer-term behaviour and vulnerability of the hapua. This information was then used to make predictions on how the hapua could be impacted if dams were to be built in the catchment, or if significant changes in the catchment occur. A multidisciplinary approach was used to investigate the short-term baseline conditions, and the long-term geomorphology of the Hurunui River hapua. Water characteristics were investigated over a falling tide, in different areas of the hapua, and in different energy conditions. The short-term behaviour of the hapua was investigated using hourly images from a time-lapse camera. The long-term vulnerability over decadal time scales was analysed using aerial photographs. This study showed that the flow of the river, the shape of the hapua, and the position of the outlet has a major control over the characteristics of the water. The surface area, the position of the barrier, and the width of the barrier of the Hurunui River hapua have been variable historically. From this research, it is predicted that the greatest impact on the Hurunui hapua would result if there is a dam related change the shape and outlet of the hapua to a state that reduces water residence time and decreases water quality. It is also predicted that if the outlet is maintained at the northern end of the hapua, and no ponded areas are present, that there would be the least problems with water quality. The findings of this research have improved the understanding of the water characteristics and processes of the Hurunui River hapua, and how they respond to change.