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.
