River-mouth Lagoons On Mixed Sand and Gravel Beaches
River-mouth lagoons on mixed sand and gravel (MSG) beaches form at the river-coast interface where a typically braided, athough sometimes meandering, river interacts with a coastal environment that is significantly affected by longshore drift. The lagoons which form on the MSG coastlines are common on the east coast of the South Island, New Zealand and have long been referred to as 'hapua' by the Māori. This classification differentiates hapua from similar lagoons located on the New Zealand coast termed 'waituna'. Hapua are often located on paraglacial coastal areas where there is a low level of coastal development and minimal population density. Hapua form as the river carves out an elongated coast-parallel area, blocked from the sea by a MSG barrier which constantly alters its shape and volume due to longshore drift. Longshore drift continually extends the barrier behind which the hapua forms by transporting sediment along the coast. Hapua are defined as a narrow shore-parallel extensions of the coastal riverbed. They discharge the majority of stored water to the ocean via an ephemeral and highly mobile drainage channel or outlet. The remainder percolates through the MSG barrier due to its high levels of permeability. Hapua systems are driven by a wide range of dynamic processes that are generally classified as fluvial or marine; changes in the balance between these processes as well as the antecedent barrier conditions can cause shifts in the morphology of the hapua, in particular the barrier. New Zealand examples include the Rakaia, Ashburton and Hurunui river-mouths.
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