Cliff erosion and coastal change, mid Canterbury.

The mid Canterbury coast has been largely neglected in the coastal research of the South Island's East Coast. This thesis investigates cliff erosion and coastal change in mid Canterbury. The mid Canterbury coast is comprised of mixed sand and gravel beaches with unconsolidated alluvium cliffs landward of the beaches. The average erosion rate of the cliffs is 0.43m.y( 1 • This rate masks spatial and temporal variations in cliff erosion rates. Erosion at the northern section of the study area is 0.7m.y( 1 faster than erosion at the southern end. In the long-term (15 years) cliff height is the major controlling factor on the spatial variations of cliff erosion (r = 0.733). Beach volume controls short-term (1 year) spatial variations of cliff erosion (r = -0.774). Coastal storm frequency significantly controls temporal variations of cliff erosion (r = 0.635). Erosion of the mid Canterbury cliffs yields 228,339m3 .y( 1 to the coast. Longshore sediment transport is predominantly in a northward direction and provides a net sediment quantity of 40,645m3 .y( 1 • The mid Canterbury coast is bisected by two major rivers, the Ashburton and Rangitata. Their specific sediment yields are among the largest in the world. Sediment yields of beach forming material (coarse sands and gravels) are much lower supplying 25,000 and 28,000m3 .yr- 1 of sediment to the coast. The mid Canterbury coast has a sediment budget deficit of 27,500m3 .yr- 1 • Major sources of sediment to the mid Canterbury coast are cliff erosion (70 per cent), river transport (17 per cent) and longshore sediment transport (13 per cent). Major sediment sinks include offshore transport through abrasion (76.8 per cent) and longshore sediment transport (23.2 per cent). The large amount of sediment lost through abrasion suggests that sediment, once it arrives on the coast has a short 'life span' before it is ground up. Total beach sediment volume varies significantly from year to year but is losing 27,500m3 .yr- 1 of sediment on average