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• W1710543924 abstract "Roches Beach is the eastern edge of a broad low-lying isthmus or neck of soft unconsolidatedsediments connecting the South Arm Peninsula to south-eastern Tasmania. The neck is occupiedby the residential township of Lauderdale, and a large number of residences have been constructedwithin metres of the single low foredune backing the beach. Roches Beach is an open coast sandybeach backed by a low relief beach-ridge plain of soft erodible sandy sediments extending to belowpresent sea level. The beach is exposed to a refracted northerly-directed swell, and to locallygenerated wind-waves which may develop considerable erosive power when driven by easterly ornorth-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 severaldecades. The present study has measured shoreline change at Roches Beach during the period1957 to 2005, through the use of ortho-rectified air photos from 1957, 1977, 1987 & 2001 and ahigh resolution QuickBird satellite image from 2005. The seawards vegetation limit on theforedune front was used as a measurable indicator of shoreline position, which moves as ashoreline moves and is readily detectable on imagery. Position error margins for features on eachimage were determined by comparing apparent relative displacements of fixed reference featuresvisible on all images. Shoreline positions were digitised over the imagery for each epoch, and thenbuffered with envelopes representing position error margins. Using this technique, any apparentmovement of shoreline positions between two images that is greater than the error envelope widthsmust 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 ofRoches Beach between 1957 and 2005, by minimum (demonstrable) distances of 4 – 9 metres (andprobably 5 to 12.5 metres actual recession). Although several large cut-and-fill (erosion andnatural swell-driven recovery of the beach) cycles are detectable super-imposed on this progressiverecession trend, long stretches of the beach have never returned to their former 1957 position, butrather have exhibited net progressive recession from prior to 1977 until 2005. Subsequent fieldobservations show the beach has remained in an erosive state up to the present (2010). The datafrom this study together with an earlier study by Cromer (2006) indicates the onset of progressiverecession probably occurred around 1975 to 1977, representing a transition to a new style of beachbehaviour at that time. Whereas the 1957 shoreline (dune front) had a notably ‘ragged’ planformindicative of little active erosion, subsequent to 1977 all air photo images show notably straightdune front planforms indicative of more frequent active shoreline wave erosion. Nonetheless, overthe 1970’s to 1990’s period, several large ‘cut-and-fill’ cycles super-imposed on the underlyingrecession trend are evident in the air photo record, which are indicative of some natural shorelinerebuilding between erosion events. However from prior to 2001 until 2010 there has been noincipient dune formation or dune-front rebuilding along most of the beach, which has instead beenin a persistently erosive condition over this period, suggesting more frequent wave erosion of thebeach and dune front in the last decade than during the earlier period of initial shoreline recessionfrom the 1970’s to 1990’s.The explanation best fitting the observed conditions and changed behaviour of Roches Beach sincethe 1970’s is that the shoreline is responding to global sea-level rise by receding in a fashioncommensurate with the approximately 14 centimetres of sea-level rise which has beendemonstrated 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 naturalprogressive shoreline change trends, do not appear capable of explaining the observed changes atRoches Beach. However it is noteworthy that most other swell-exposed open coast sandy beachesin south-east Tasmania (and in south-east Australia generally) are not yet showing as clear aprogressive 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’) therecessional effects of sea-level rise, and will continue to do so until sea level rises to a thresholdbeyond which wave erosion events are too frequent for shores to fully rebuild in-between erosionevents. That this threshold has evidently already been passed at Roches Beach is interpreted to bedue to locally unusual conditions. The most likely such conditions which could explain its earlyresponse to sea-level rise is that Roches Beach is located between rocky headlands and oriented insuch as way as to be subject to a persistent northerly swell-driven longshore drift. As a result thebeach had some millennia ago settled into an equilibrium “zeta-form” plan shape, and indeed is theonly 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 beachbehaviour is that renewed sea-level rise over the last century reached a threshold around the 1970’sat which wave erosion of the upper shoreface (beach and dune-front) was occurring sufficientlyfrequently that the persistent longshore drift was beginning to remove eroded sand northwardsfaster than it could be returned to the beach by the swell or replenished by longshore drift from thesouth. As a result the beach system ‘flipped’ from having a balanced sand budget to having a losingbudget, resulting in an onset of progressive shoreline erosion which has been most marked in thedeepest (southwestern) part of the zeta-form. In other words, Roches Beach is (unusually for itsregion) a zeta-form beach which had settled into a planform in equilibrium with (the stable LateHolocene) sea-level some millennia ago, however the process of zeta-form deepening (shorelinerecession) has now been re-started by the observed recent onset of renewed rise in sea-level. If thisinterpretation is correct the beach will have an ongoing tendency towards persistent shorelinerecession (deepening of the zeta-form) until sea-level stabilises at some future time and the beachreaches 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 simpletechnique yielding reliable measures of minimum (demonstrable) shoreline changes over time, andwill have utility for both assessing shoreline change at other beaches, and for future monitoring ofrates of shoreline change at Roches Beach. Ideally, the remote-sensing (imagery) techniques usedin this study should be tied into ground-based monitoring work such as the TASMARC shorelinemonitoring program, in order to provide ground-truthing and calibration of shoreline change trendsdetected in aerial or satellite imagery. Monitoring of changes and trends in shoreline behaviour willbe critical information for planning coastal adaptation responses to sea-level rise in residentialareas such as Lauderdale." @default.
• W1710543924 created "2016-06-24" @default.
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• W1710543924 date "2010-01-01" @default.
• W1710543924 modified "2023-09-27" @default.
• W1710543924 title "Technical Report: Shoreline Changes at Roches Beach, South-Eastern Tasmania 1957-2010" @default.
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