Climatic Signals in Beach Volume Measurements from 19 Intermediate Embayed Beaches

Abstract:

The northeast coast of New Zealand has a range of embayed intermediate beaches whose cross-shore profiles can vary on time scales extending from individual storms to inter-decadal. Some of these beaches erode and accrete a lot whereas others can remain extraordinarily stable despite exposure to comparable wave climates. Here a profile dataset collected intermittently since 1980, and 6-weekly since 1995, on 19 northeast coast beaches (62 profiles) is used to study variability between sites. Spectral analysis of the profile data showed annual, biannual and interannual energy, with the relative magnitude varying surprisingly between sites. Wave climate information for the sites was provided by a regional SWAN model which had been forced by data extracted from the NOAA WWIII wave database. Waves were modelled from 1980-2008. The wave climate was used to cluster the beaches into those that were likely to behave in a similar way, based on the relative proportions of variability in these time scales in the significant wave height, mean period and alongshore wave energy flux extracted from model output. Beaches with a northward aspect had generally low biannual variability but higher interannual and seasonal variability. In contrast, sites exposed to easterly conditions had a greater biannual contribution. Analysis of climate indicators show that the southern oscillation index and the Pacific decadal oscillation also have variability in seasonal, biannual and interannual scales to different degrees and these are weakly but significantly correlated to beach volume variations. Overall, the degree of correlation appears directly related to beach orientation.