Observations of morphological change at an ebb-tidal delta

Abstract

Video observations of depth-limited wave-breaking patterns at an ebb-tidal delta on the energetic west coast of New Zealand at Raglan were used to identify geomorphic features over a 5-year period. The terminal lobe, mouth bar, channel margin linear bars, and swash bars were identified and tracked over the duration. Morphodynamic response was related to environmental conditions by correlating observed movements with concurrent wave and tidal conditions. Movements occurred throughout the record with a slight tendency to occur more during the transition between seasonal forcing trends. Winter deltas were generally broader and extended further seaward than the summer deltas which were more cuspate. The formation of a double-barred ebb-shoal was observed, with the cross-shore position of the outer bar influenced by wave conditions while the inner bar was influenced by ebb-jet strength. Furthermore, swash bars were observed to constrict the seaward extent of the main channel during large wave events, which was subsequently eroded by tidal currents. These observations are consistent with the present consensus that ebb-shoal features are dependent upon competition between ebb-jet strength and opposing waves. Interannual morphological changes were not significantly correlated to any particular environmental forcing, suggesting that either some process or combination of processes not considered were influential, or that the system might be showing signs of emergent behaviour.