Abstract
Abstract Mixed sand and gravel beaches form a wedge of protective sediment at the base of eroding cliffs. In profile these beaches are typically steep with a prominent storm berm. If the volume of beach sediment is insufficient, storms strip beach sediments seaward, exposing the cliff toe to wave attack. The beach volume is thus crucial to the protection of sea cliffs. In this article we describe a method of calculating alongshore variation in the volume of mixed sand and gravel beaches using ground penetrating radar (GPR). Eighteen sites were studied along 50 km of the east coast of South Island, New Zealand. The method was underpinned by an ability to map the boundary between beach sediments and underlying Pleistocene alluvial-fan sediments. This was achieved by studying the radar facies, particularly landward-dipping overwash deposits and seaward-dipping beach erosion surfaces. The method was ground-truthed in three ways: (1) a stream provided a clean section through one site that was imaged by radar; ...
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