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Technical Report: Shoreline Changes at Roches Beach, South-Eastern Tasmania 1957-2010

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Sharples, C (2010) Technical Report: Shoreline Changes at Roches Beach, South-Eastern Tasmania 1957-2010. Technical Report. Antarctic Climate and Ecosystems Co-operative Research Centre, Hobart.

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Abstract

Roches Beach is the eastern edge of a broad low-lying isthmus or neck of soft unconsolidated
sediments connecting the South Arm Peninsula to south-eastern Tasmania. The neck is occupied
by the residential township of Lauderdale, and a large number of residences have been constructed
within metres of the single low foredune backing the beach. Roches Beach is an open coast sandy
beach backed by a low relief beach-ridge plain of soft erodible sandy sediments extending to below
present sea level. The beach is exposed to a refracted northerly-directed swell, and to locally
generated wind-waves which may develop considerable erosive power when driven by easterly or
north-easterly winds over the long fetch of Frederick Henry Bay.
Persistent shoreline erosion at Roches Beach has been of concern to residents and others for several
decades. The present study has measured shoreline change at Roches Beach during the period
1957 to 2005, through the use of ortho-rectified air photos from 1957, 1977, 1987 & 2001 and a
high resolution QuickBird satellite image from 2005. The seawards vegetation limit on the
foredune front was used as a measurable indicator of shoreline position, which moves as a
shoreline moves and is readily detectable on imagery. Position error margins for features on each
image were determined by comparing apparent relative displacements of fixed reference features
visible on all images. Shoreline positions were digitised over the imagery for each epoch, and then
buffered with envelopes representing position error margins. Using this technique, any apparent
movement of shoreline positions between two images that is greater than the error envelope widths
must represent a minimum real shoreline movement between the times represented by the images.
Use of this method has shown that net (long term) shoreline recession has occurred along much of
Roches Beach between 1957 and 2005, by minimum (demonstrable) distances of 4 – 9 metres (and
probably 5 to 12.5 metres actual recession). Although several large cut-and-fill (erosion and
natural swell-driven recovery of the beach) cycles are detectable super-imposed on this progressive
recession trend, long stretches of the beach have never returned to their former 1957 position, but
rather have exhibited net progressive recession from prior to 1977 until 2005. Subsequent field
observations show the beach has remained in an erosive state up to the present (2010). The data
from this study together with an earlier study by Cromer (2006) indicates the onset of progressive
recession probably occurred around 1975 to 1977, representing a transition to a new style of beach
behaviour at that time. Whereas the 1957 shoreline (dune front) had a notably ‘ragged’ planform
indicative of little active erosion, subsequent to 1977 all air photo images show notably straight
dune front planforms indicative of more frequent active shoreline wave erosion. Nonetheless, over
the 1970’s to 1990’s period, several large ‘cut-and-fill’ cycles super-imposed on the underlying
recession trend are evident in the air photo record, which are indicative of some natural shoreline
rebuilding between erosion events. However from prior to 2001 until 2010 there has been no
incipient dune formation or dune-front rebuilding along most of the beach, which has instead been
in a persistently erosive condition over this period, suggesting more frequent wave erosion of the
beach and dune front in the last decade than during the earlier period of initial shoreline recession
from the 1970’s to 1990’s.
The explanation best fitting the observed conditions and changed behaviour of Roches Beach since
the 1970’s is that the shoreline is responding to global sea-level rise by receding in a fashion
commensurate with the approximately 14 centimetres of sea-level rise which has been
demonstrated to have occurred in south-east Tasmania since the 1800’s. Other explanations,
including local artificial interferences, long-term oceanographic cycles or other long term natural
progressive shoreline change trends, do not appear capable of explaining the observed changes at
Roches Beach. However it is noteworthy that most other swell-exposed open coast sandy beaches
in south-east Tasmania (and in south-east Australia generally) are not yet showing as clear a
progressive recession in response to sea-level rise, because swell-driven return of sand to the shore face following erosion events (the ‘cut-and-fill’ cycle) is still compensating for (or ‘masking’) the
recessional effects of sea-level rise, and will continue to do so until sea level rises to a threshold
beyond which wave erosion events are too frequent for shores to fully rebuild in-between erosion
events. That this threshold has evidently already been passed at Roches Beach is interpreted to be
due to locally unusual conditions. The most likely such conditions which could explain its early
response to sea-level rise is that Roches Beach is located between rocky headlands and oriented in
such as way as to be subject to a persistent northerly swell-driven longshore drift. As a result the
beach had some millennia ago settled into an equilibrium “zeta-form” plan shape, and indeed is the
only true zeta-form beach in the South Arm – Frederick Henry Bay region.
In this light, the interpretation which appears best able to explain the observed changes in beach
behaviour is that renewed sea-level rise over the last century reached a threshold around the 1970’s
at which wave erosion of the upper shoreface (beach and dune-front) was occurring sufficiently
frequently that the persistent longshore drift was beginning to remove eroded sand northwards
faster than it could be returned to the beach by the swell or replenished by longshore drift from the
south. As a result the beach system ‘flipped’ from having a balanced sand budget to having a losing
budget, resulting in an onset of progressive shoreline erosion which has been most marked in the
deepest (southwestern) part of the zeta-form. In other words, Roches Beach is (unusually for its
region) a zeta-form beach which had settled into a planform in equilibrium with (the stable Late
Holocene) sea-level some millennia ago, however the process of zeta-form deepening (shoreline
recession) has now been re-started by the observed recent onset of renewed rise in sea-level. If this
interpretation is correct the beach will have an ongoing tendency towards persistent shoreline
recession (deepening of the zeta-form) until sea-level stabilises at some future time and the beach
reaches a new zeta-planform in equilibrium with that future sea-level.
Additional future work which could further test this interpretation is described in this report.
However the shoreline change detection method used in this project has proven to be a simple
technique yielding reliable measures of minimum (demonstrable) shoreline changes over time, and
will have utility for both assessing shoreline change at other beaches, and for future monitoring of
rates of shoreline change at Roches Beach. Ideally, the remote-sensing (imagery) techniques used
in this study should be tied into ground-based monitoring work such as the TASMARC shoreline
monitoring program, in order to provide ground-truthing and calibration of shoreline change trends
detected in aerial or satellite imagery. Monitoring of changes and trends in shoreline behaviour will
be critical information for planning coastal adaptation responses to sea-level rise in residential
areas such as Lauderdale.

Item Type: Report (Technical Report)
Publisher: Antarctic Climate and Ecosystems Co-operative Research Centre
Additional Information:

Copyright © 2010 The Antarctic Climate & Ecosystems Cooperative
Research Centre.

Date Deposited: 30 Oct 2011 23:20
Last Modified: 07 Nov 2011 05:40
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