Sediment Patterns on a Mixed Sand and Gravel Beach, Kaikoura, New Zealand.

An unusual longshore grain size 'variation series' is examined on North Beach, Kaikoura, for two related reasons: To identify the processes that have been responsible for creating the sediment pattern and; to establish whether there is a relationship between these processes and an erosion problem on the Kaikoura township foreshore. A study of North Beach over 30 years ago revealed an unusual curvilinear grain size pattern that phased in a regular fashion from gravel to sand alongshore, but how such a pattern formed was unknown. Research conducted by the author over three decades later reveals that this same pattern still exists in the beach. Sixty-four surface sediment samples were analysed and modeled by a range of techniques that enabled a thorough examination of the sediment characteristics of the beach. The distinctive longshore variation series is explained fundamentally as two linear grain size series occurring one after the other, and is a combination of modern and relict features. Previously it has been assumed that the Hapuku River has supplied all the sediments to North Beach by a simple southward drift of gravels and sands. Whilst this is partly true, it was determined that a second source, the Kowhai River was also active in the past, contributing materials to the southern part of the shoreline. Sediments from this old source are still present in the beach, preserved by a unique combination of local geomorphological features, and are in effect relict. The Kowhai River has since migrated further south, and this in combination with a recent period of tectonic uplift has caused sediment starvation and subsequent erosion of the township foreshore. Essentially, this links the processes responsible for the longshore sediment grain size pattern with the erosion problem. These findings highlight the importance of source area characteristics and challenge a common assumption that because beaches are dynamic systems, sediment patterns on them must reflect contemporary hydrodynamic processes. [ABSTRACT FROM AUTHOR]