Erosion of the Washdyke-Seadown lowland coast : past, present and future.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
This thesis examines the coastal erosion phenomenon of the Washdyke-Seadown lowland coast. The area consists of 12.25 km of mixed sand-gravel beach between Dashing Rocks and the Opihi River Mouth. The coast is backed by a low-lying hinterland of fluvial origin. Erosion and sea flooding pose a hazard and are threatening many valuable assets. These include a substantial wildlife habitat, farmland, the Seadown drain, State Highway One, the main trunk railway line, and the Washdyke Industrial Estate. A combination of historical, field and laboratory data were used in determining the morphological process and sedimentary characteristics of the area. The beach is dominated by pebbly, moderately-poorly to very poorly sorted greywackes. Grain size was found to have decreased by 0.8 Φ since 1978. Coarsest sediments are associated with construction works on the beach. Most sediment has been lost from the mid section of the beach since 1977 (-247 142 m³ ). In this period the southern end of the Washdyke Barrier has gained 40 194 m³ of sediment. The hinterland is composed of typical lowland swamp deposits. It was found to contain gravel of sufficient size to be used on the beach. However, because of the gravels' oxidised character, its long term value to the beach may be limited. Maximum coastal retreat recorded was -440 m at the southern end of the beach. Erosion decreased towards the Opihi River Mouth. This was due to the presence of stopbanks and net northerly drift of sediment feeding that end of the beach. The highest long term erosion rate found was -3.6 -1 m.yr This was considerably less than previous studies have indicated. If current rates of erosion persist the Washdyke Lagoon, Seadown Drain and remaining beach sediment will be lost in about 89 years, 36 years and 5 1 years respectively. These predictions were considered optimistic because they were based on linear extrapolation. It was found that the unconfined Washdyke Barrier, and the stopbank controlled Seadown coast, behaved in different ways. The Washdyke Barrier has rotated and become very broad while the Seadown Coast has retreated parallel, confining the backshore against the stopbanks.