The bight runs for approximately 135 kilometres (84 mi) from the southern end of Banks Peninsula to the settlement of Timaru and faces southeast, exposing it to high-energy storm waves originating in the Pacific Ocean.

[2] Sediment from these rivers, predominantly Greywacke, is deposited along the coast and extends up to 50 kilometres (31 mi) out to sea from the current shoreline.

[3] The Southern Zone runs from Dashing Rocks at Timaru in the south to the Rangitata River mouth in the north.

The Central Zone is the largest and runs between the Rangitata River mouth and Taumutu on the southern end of Kaitorete Spit.

Continued cliff erosion contributes around 70% of the coarse material supplied to the mixed sand and gravel beaches of the Canterbury Bight.

[1] In the Canterbury Bight system, wind transport and biogenous and hydrogenous deposition can be excluded as agents of sediment inputs.

Biogenous deposition can be excluded as the high-energy environment and coarse sediment deter shelled animals from occupying the area.

Erosion of the alluvial cliffs (and subsequent longshore transport) through the Central Zone of the Canterbury Bight is believed to provide the majority of coarse material to the beach system.

[1] This eroded material is then subjected to longshore transport, which in the case of the Canterbury Bight is predominantly from south to north.

[1] The marine processes include swash and backwash, with the larger storm induced waves creating stronger swash/backwash, which removes more eroded material.

In the offshore zone, sediment movement is unimpeded as the local bathymetry of the continental shelf is relatively flat with no major obstructions.

[1] Sediment removed from the backshore is predominantly moved offshore rather than along shore as only large southerly storm waves and their subsequent swash, which usually flow perpendicular to the beach are able to reach this area.

The most frequent velocities reached by winds on the Canterbury Bight are capable of moving medium to coarse sized sand particles.

[1] Given that swash is a component of longshore drift on the Canterbury Bight, it is easy to assume that these waves will inherently cause an increase in sediment transport, particularly from south to north.

In the long term, most mixed sand and gravel beaches are in an erosional state due to the lack of available coarse sediment needed to withstand the high-energy environments they reside in.

Ecan believes that in many instances, the increased risk of erosion and seawater inundation are caused by the inappropriate location of assets and activities and by a reliance on inadequate works to protect from the ocean.

To date, erosion has led to the loss of agricultural land, threatened valuable infrastructure and some holiday settlements, and reduced coastal lagoons and wetlands.

[6] This process has created a significant hazard, as the Washdyke barrier is the only line of protection between the high-energy ocean and valuable infrastructure including State Highway 1, an important railway and a large industrial area.

Along the Central Zone of the Canterbury Bight, different mitigation methods are needed in order to decrease erosion risks.