Spatial Patterns in Groundwater Seepage and Surf Zone Morphology: Muriwai Beach, New Zealand

Groundwater seeping from the beach face can induce erosion and so may play a role in controlling the development of beach morphology. A study at Muriwai Beach, New Zealand, was conducted to measure the relationship between surf zone morphology, groundwater, and intertidal beachface volume. Time exposure images from a video camera were used to study the surf zone morphology. Statistical analysis showed that the volume variation correlated with alongshore variation of the seepage line and surf zone morphology. Higher groundwater was always associated with lower volume. The surf zone morphology also correlated with changes in volume. This correlation was generally positive in winter (volume decreased with reduced pixel intensity) and negative in summer, suggesting a seasonal switch in the processes responsible for the correlation. To provide insight into the processes leading to this correlation, the sensitivity of the seepage line to hydraulic conductivity, beach morphology, tide, and wave setup was modeled using an exploratory model based on the Boussinesq formula. Results indicated that changes in setup played an important role in changing the seepage line location across the beach face. Spatial patterns in setup are likely to be driven by changes to wave breaking patterns caused by morphodynamic patterns. The results also showed that changes in conductivity and slope had a significant influence on seepage line variation. These findings indicated that alongshore variations in groundwater dynamics, which could in part be driven by offshore and onshore changes to water level, may play a stronger role in pattern development on beaches than previously expected.