Canterbury Bight

Canterbury Bight is a 135 km stretch of coastline between Dashing Rocks (north Timaru) and the southern side of Banks Peninsula (Birdlings Flat) on the eastern side of the South Island, New Zealand. The bight faces southeast, which exposes it to high-energy storm waves originating in the Pacific Ocean (Kirk, 1967). The most frequent wave approach direction for the Canterbury Bight is from the southeast and the most dominant the south with wave heights of over 2m common (Kirk, 1967). The bight is a large, gently curving bend of shoreline of primarily mixed sand and gravel (MSG) beaches. The MSG beaches are steep, highly reflective (of wave energy) and composed of alluvial gravel deposits. The alluvial gravels are the outwash products of multiple glaciations that occurred in the Southern Alps during the (Kirk, 1967). Large braided rivers transported this material to the edge of the current continental shelf, which, due to sea level rise is 50 km seaward of the coasts current position (Kirk, 1967). The MSG beaches of the Canterbury Bight therefore occur where the alluvial fans of the Canterbury Plains rivers are exposed to high-energy ocean swells (Hart et al., 2008). The dominant rock ‘greywacke’ in the Southern Alps is consequently the primary constituent of the MSG beaches (and Canterbury Plains), which is partially indurated sandstone of the Torlesse Supergroup (Hart et al., 2008). River-mouth lagoons are a relatively common occurrence on the MSG beaches of the Canterbury Bight.

The Canterbury Bight can be split into three distinct regions; Southern Zone, Central Zone and the Northern Zone (Hemmingsen, 2004).

The Southern Zone represents the southernmost part of the Canterbury Bight, which runs from Dashing Rocks to the Rangitata River mouth (Hemmingsen, 2004). Features of note along this section of the bight include the Washdyke Barrier and Washdyke Lagoon.

The Central Zone is the largest of the three zones and runs between the Rangitata River mouth and Taumutu on the Southern end of Kaitorete Spit. Unconsolidated alluvial cliffs interbedded with sands and silts, which back steep, narrow MSG beaches unify this zone (Hemmingsen, 2004). The cliffs are the result of erosion of the Rangitata River, Ashburton River and Rakaia River alluvial fans whose mouths are all encompassed by this region (Single, 2006). Continued cliff erosion contributes around 70% of the coarse material supplied to the MSG beaches of the Canterbury Bight (Hart et al., 2008).

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