Coastal dynamics : Rarangi to Cape Campbell.

The coastline of Marlborough bordering Cook Strait was studied in order to assess present day processes of coastal development. Processes and characteristics revealed by this analysis have been applied to the post glacial evolution of the marine depositional landforms in the area. The Cook Strait wave environment differs from that of the exposed east and west coast of New Zealand. There are two principal components, a short period, steep-crested locally generated northerly wave and a long period externally generated shallow swell wave from the south. The resultant longshore distributions of beach morphologies and sediments are dependent on the degree of exposure to these two modes of wave advance. Two cells of longshore drift are present in the area, each dominated by the degree of exposure to both modes of wave advance. North of White Bluffs material is moved northwards by southerly swell and storm waves. South of the Awatere River material is moved southwards under locally generated northerly waves. In between these two cells is a zone of overlap and mixed movements. The differing characteristics of these two compartments have led to different sized materials being moved in each of the two drift cells. A further consequence of these features has been to the deposition of marine sequences at the downdrift end of each cell. In the south Lake Grassmere has been enclosed by a barrier beach while in the north of the study area the lower Wairau Valley has been enclosed by spit growth. The major source of materials for this latter development is argued to be White Bluffs rather than the Awatere River as proposed by previous workers. Spit growth and subsequent coastal progradation in the lower Wairau has been dated to parallel the slackening in the rate of sea level rise following the last glacial era