River Mouth Processes and Morphodynamics on a Mixed Sand-Gravel Beach

Where the Ashburton river meets the sea it is impounded by a mixed sand-gravel barrier, formed through littoral transport of sediment, causing a lagoon to form, locally referred to as hapua. From the hapua, discharge to the sea is maintained through small, ephemeral channels, which are unstable and are subject to morphological change in response to fluctuations in longshore transport of gravel and variations in river flow rate. There are few descriptions of the morphodynamics of these highly changeable features, particularly changes that occur over short time scales of hours to weeks. To investigate the patterns and processes involved in the migration of drainage channels, several new instruments were trialed. A methodology of was developed to provide estimates of longshore transport on a gravel beach using a Gravel Transport Sensor. Video camera technology was the primary tool used to study river mouth morphodynamics. The camera provided hourly images of the environment, enabling qualitative assessment using movies of the images to observe morphologic changes, and quantitative measurement of the migration of the channel. Measurements of river flow, wave climate and lagoon water levels were also gathered to investigate the relationships between the morphological response and the forcing factors. Results of the study showed that the ends of the channel behave differently, with the lagoon end remaining more stable than the seaward end. The seaward end is more exposed to the high wave energy prevalent along this coast. The wave climate, especially wave period and direction, were found to be predictors for the migration rate. The location of the seaward end is more variable due to the fluctuations in wave climate, differing from the lagoon end which is influenced predominantly by river flow rates. It has been found that the migration of the lagoon end occurs in 'steps', which are separated by raised gravel banks, the single persistent feature throughout the study. This stepping migration is driven primarily through episodic events such as high river flow or large wave events