The Abrasion of "Greywacke" on a Mixed Sand and Gravel Coast

The coastline of the Canterbury Bight between Timaru and the southern end of Kaitorete Barrier at Taumutu is in long term erosion. Paradoxically, several large, braided rivers drain from the rapidly eroding Southern Alps into the Canterbury Bight transporting indurated sandstone of the Torlesse Super Group or greywacke to the coast. Griffiths (1981) states that catchment-specific sediment yields average 1856± 261t km²yr⁻¹ compared with the world average of 182t km²yr⁻¹ and are among the highest known specific sediment yields. However, the large sediment loads from these Canterbury rivers are comprised of predominantly fine sediments in suspended load (more than 90%) and only a small amount of coarse bedload (less than 10%). This is reflected in the nature of the mixed sand and gravel beaches. The Canterbury Bight coastline is then apparently abundantly supplied with sediment which is subsequently transported northward ultimately forming Kaitorete Barrier. Since the 1950's there has been very little accumulation of sediment against Banks Peninsula at the downdrift end. Preious research has suggested that sediment is lost from the beaches due to abrasion. The work presented here is a step towards deriving a more precise measure of abrasion for input to the sediment budget of the mixed sand and gravel beaches of the Canterbury Bight. Tumbler experiments were carried out using a concrete mixer. Sediment samples were collect from Ashburton and Washdyke sites within the Canterbury Bight. Results showed a consistency of method in a laboratory situation. However, the results also highlighted differences between sediments from different sites. The results clearly showed that no single value can adequately represent the abrasional behaviour of greywacke sediments from mixed sand and gravel beaches.