Northern Adelaide Beaches Sand Harvesting Impacts 15/9/2020 Water - PowerPoint PPT Presentation

Northern Adelaide Beaches – Sand Harvesting Impacts 15/9/2020 Water Technology

Phase 1: Interim sand management • Additional sand carting from the northern beaches to West Beach • Match current rates of sand loss from West Beach • Proposed new sand pumping system • Proposed to be constructed from West Beach to the northern beaches • Phase 1 – trucking in advance of pumping system

Current sand movement • Adelaide - one long beach running 28 kilometres from Kingston Park to Outer Harbor • The beaches are managed in six sections or ‘cells’ • Phase 1 – northern beaches to West Beach

New sand movements • Sand to be collected by scraper from the beach between low water mark and 5m from the toe of dune • Sand moved by truck along the beach to the existing beach access point at the Semaphore Surf Life Saving Club • Loaded onto road haulage trucks and transported to West Beach • Sand to be moved under the Semaphore jetty using existing mobile conveyor belt system

• Astrid Stuer – effects of sand movement • Jamie Kaye – flora and fauna • Mel Lutton – truck movements and water quality

Introduction Average of 100,000m 3 with a wide range of • 50,000 to 150,000m 3 /year Beaches are constantly changing and exact • 50,000 – 150,000 amount will vary every year m 3 /year

Harvest areas Current harvest rules: • Collection as far seaward as tide permit • 6m from toe of dune • 1:10 batter to avoid sharp drops Limitations: • Beach wrack • Sand availability • Dune width and condition

Planned 2020/21 campaign - scenarios

Methodology 1. Data analysis from 2010 onwards (after rock breakwater construction): • Previous reports • Record of previous campaigns • Profile analysis • Aerial surveys from 2 campaigns at breakwater and 1 campaign north of Largs Jetty • Aerial photography 2. Storm impacts

Methodology • Previous reports

Methodology • Record of previous campaigns 120,000 100,000 80,000 60,000 40,000 20,000 - 14/15 15/16 16/17 17/18 18/19 19/20 Yearly harvesting quantities behind Semaphore breakwater

Methodology • Profile analysis

Methodology Aerial surveys from 2 campaigns at breakwater and 1 campaign north of Largs Jetty

Methodology • Aerial photography

Methodology 2. Storm impacts

Record of previous campaigns Harvesting quantities by area 120,000 100,710 100,000 77,947 80,000 58,229 56,883 60,000 40,000 33,864 21,242 19,987 20,000 9,710 9,389 9,042 3,761 - 14/15 15/16 16/17 17/18 18/19 19/20 Sem BW Sem Jetty Betw jetties Largs north

Profile analysis 2020 profile comparison 8 6 4 2 0 0 50 100 150 200 250 300 350 400 450 500 -2 -4 -6 -8 200129A05022020 200008A17022020

Profile analysis – example in between jetties Profile 200004 200004A20012010 5 4 200004A14022012 3 200004A07022014 AHD Height (Metres) 2 200004A05022016 1 200004A14022017 0 50 75 100 125 150 175 200 200004A05022019 -1 200004A05022020 -2 Chainage (Metres)

Profile analysis – 2016 storm No erosion in 2016 2016 erosion 2017 beaches growing again 2018 beaches exceeded past pre-2016storm profiles. Except north of breakwater.

+35 Profile analysis +30 2020: 35,000 m 3 • Figure showing erosion and accretion from 2010 +9 to 2020 in metres 2014-2020: 43,000 m 3 • Most foredunes are +14/-7=+7 growing seawards (despite harvesting operations and storm -27 (-40) impact in 2016) +10 • Erosion north of 2014-2020: Semaphore breakwater 325,000 m 3 +8

Aerial surveys – Largs North • Details about where sand has been taken and how deep 04/2020-06/2020: 35,000 m 3

Aerial surveys – 10/2018-10/2019: ~100,000 m 3 Semaphore 4 different campaigns, spread breakwater over one year 10/2019-01/2020: 70,000 m 3

Yearly harvesting quantities behind Semaphore Surveys - conclusion breakwater 120,000 100,000 80,000 60,000 40,000 20,000 - 14/15 15/16 16/17 17/18 18/19 19/20 Yearly harvesting quantities behind Semaphore breakwater • Large quantities have been harvested at Semaphore breakwater, especially in 2018/19 and 19/20. Harvesting quantities exceed natural replenishment as experienced during 19/20 campaign (no sufficient sand available behind breakwater). • It is likely that the intense harvesting at Semaphore breakwater caused the ongoing erosion trend at Semaphore Jetty. In other words, if no or reduced harvesting would have occurred at Semaphore Breakwater (equal or less than natural rate of replenishment) it is possible that the profile would be in a state of accretion rather than ongoing erosion. • Future harvesting campaigns at the breakwater should therefore be reduced to be no more than the natural rate of replenishment to limit further downdrift impacts.

Aerial imagery analysis breakwater Limitations: - Cloud cover - Timing of aerials vs. timing of campaigns - Varying tidal levels Findings: • Salient most years

Aerial imagery analysis breakwater Findings: • Salient most years • No salient in Aug 2016 due to harvesting campaign • No salient 12/18 to 03/20 due to harvesting campaign Yearly harvesting quantities behind Semaphore breakwater 120,000 100,000 80,000 60,000 40,000 20,000 - 14/15 15/16 16/17 17/18 18/19 19/20 Yearly harvesting quantities behind Semaphore breakwater

Aerial imagery analysis breakwater • Latest aerial 07/20 shows salient starting to form again • Short term beach recovery quick to smooth out bumps • Long term beach recovery dependent on longshore sediment transport

Aerial imagery analysis breakwater • Short term beach recovery quick to smooth out bumps • Long term beach recovery dependent on longshore sediment transport

Yearly harvesting quantities behind Semaphore Aerial imagery analysis breakwater breakwater 120,000 100,000 80,000 Limitations: 60,000 - Cloud cover 40,000 20,000 - Timing of aerials vs. timing of campaigns - 14/15 15/16 16/17 17/18 18/19 19/20 - Varying tidal levels Yearly harvesting quantities behind Semaphore breakwater Findings: • Salient most years • No salient in Aug 2016 due to harvesting campaign • No salient 12/18 to 03/20 due to harvesting campaign • Latest aerial 07/20 shows salient starting to form again • Short term beach recovery quick to smooth out bumps • Long term beach recovery dependent on longshore sediment transport

Storm impacts – Semaphore – 1 year storm 20008 6 5 4 3m 5m 3 2 1 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 -1 -2 Initial 1yr Final INITIAL 1yr FINAL

Storm impacts – Semaphore – 10 year storm 20008 6 3m 5 4 3 2 1 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 -1 -2 Initial 10yr Final INITIAL 10yr FINAL

Storm impacts – Largs – 1 year storm 200129 POST (-25,000) 4 3.5 0m additional erosion 3 2.5 2 1.5 1 0.5 0 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 -0.5 -1 Initial 1yr Final INITIAL 1yr FINAL

Storm impacts – Largs – 10 year storm 200129 POST (-25,000) 4 5m 3.5 3 2.5 2 1.5 1 0.5 0 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 -0.5 -1 Initial 10yr Final INITIAL 10yr FINAL

Storm impacts - conclusion • Amount of additional erosion on the ‘harvested’ profile is relatively small, especially in comparison to the beach accretion rates. • The additional erosion will only be experienced if a storm event occurs right after the harvesting campaign. If the storm occurs a few month after the campaign the beach would have already had time to re-fill. • Erosion after a storm event is a short term impact. In the long-term beaches will start growing again after a few month. • Amount of erosion or beach response is highly dependant on water levels during the storm and the height, shape and slope of the dune.