Stability of the Ashley River mouth, North Canterbury.
Thesis DisciplineEngineering Geology
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
During the Holocene, the position of the Ashley River mouth has varied between 8km south and 6.5km north of Waikuku. Northward migration of the river mouth is a recurring process indicated by two major phases of migration since European settlement: 1900-1923 (when the river mouth reached its northern limit); and 1979-1991. The pre-1923 migration phase was terminated by a major flood event of the Ashley River. Historical mouth migration rates exceed 200m per year. Survey results for the past 12 months indicate that the mouth migrated 150.6m. It is apparent that the rate of migration is lower than the mean rate for the past decade, perhaps indicating a slowing of river mouth migration. The principal cause of river mouth migration is erosion of the north bank caused by tidal current velocities in excess of 1.0ms·1. River mouth position does not significantly influence flood levels during major flood events of the Ashley River. A more important threat to Waikuku is erosion of the south bank near the township, as occurred in 1954. Passive management is the preferred strategy for the Ashley River-Saltwater Creek estuary and the lower reaches of the Ashley River system, but close monitoring of rainfall data, river flow data and the position of the Ashley River mouth should be continued.