Episodic, seasonal, and long term morphological changes of Coromandel beaches

Abstract

The Coromandel Peninsula was subject to subdivision and development primarily since the 1960's. Much of the development that has occurred now renders protection from the existing beach systems which have typically been altered by development. Coupled with huge populations during summer, the region is of national significance therefore an understanding of coastal impacts is paramount. Analysis of the spatial and temporal variation of beaches along the eastern Coromandel Peninsula from Whangapoua in the north to Whiritoa in the south provided results ranging from single storm events to decadal scale oscillations. Beach similarity was determined by measuring parameters such as beach length, beach connectivity to neighbouring beaches, aspect, and beach slope. The analysis of variability in beach face volumes was undertaken using an extensive beach profile database collected by R. Keith Smith, Ron Ovenden and a monitoring program maintained by Environment Waikato since 1978. The database had a higher-resolution sampling interval from 1996 until present (a maximum sampling frequency of approximately bimonthly). Results showed that short term beach volume changes were explained by the beach classification devised from the Wright and Short (1984) model and available planform morphology data. Intermediate beaches overall had a greater range of variation, but had a higher frequency of low magnitude of change events. Reflective beaches had a higher frequency of large magnitude of change events and subsequently greater short term volume changes. Beaches adjacent to harbours and two outliers were identified which did not accord to the classification. The classification model maintained its applicability for seasonal scale beach response. Embayed beaches on the Coromandel Peninsula also exhibited beach rotation to varying degrees. Beaches with similar planform morphology showed similar long term beach rotation characteristics. A biennial oscillation related to the El Ni o Southern Oscillation (ENSO) was evident as well as an interdecadal oscillation related to the Interdecadal Pacific Oscillation (IPO) was evident across the Peninsula. In particular, no beaches north of the Kuaotunu Peninsula showed a strong ENSO signal, and the strongest IPO response was on beaches north of the Kuaotunu Peninsula. The IPO appeared to enter a long term negative phase indicating decadal scale persistence of La Ni a events, therefore Coromandel beaches are likely to exhibit erosion dominant trends for the next 20 to 35 years. Based on these results, 3 sediment transport and behavioural cells were defined, they were: beaches located north of the Kuaotunu Peninsula from Whangapoua to Otama with northerly orientations; Mercury Bay beaches including Opito Bay; and, easterly orientated beaches south of Mercury Bay.