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

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

Aug 31, 2023 352 likes •916 views

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

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SLIDE 1

Northern Adelaide Beaches – Sand Harvesting Impacts

15/9/2020 Water Technology

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SLIDE 2

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
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SLIDE 3

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

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SLIDE 4

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

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SLIDE 5
  • Astrid Stuer – effects of sand movement
  • Jamie Kaye – flora and fauna
  • Mel Lutton – truck movements and water quality
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SLIDE 6

Introduction

50,000 – 150,000 m3/year

  • Average of 100,000m3 with a wide range of

50,000 to 150,000m3/year

  • Beaches are constantly changing and exact

amount will vary every year

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SLIDE 7

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
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SLIDE 8

Planned 2020/21 campaign - scenarios

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SLIDE 9
  • 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

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SLIDE 10
  • Previous reports

Methodology

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SLIDE 11
  • Record of previous campaigns

Methodology

  • 20,000

40,000 60,000 80,000 100,000 120,000 14/15 15/16 16/17 17/18 18/19 19/20 Yearly harvesting quantities behind Semaphore breakwater

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SLIDE 12
  • Profile analysis

Methodology

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SLIDE 13

Aerial surveys from 2 campaigns at breakwater and 1 campaign north

  • f Largs Jetty

Methodology

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SLIDE 14
  • Aerial photography

Methodology

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SLIDE 15
  • 2. Storm impacts

Methodology

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SLIDE 16

Record of previous campaigns

21,242 58,229 56,883 9,042 77,947 100,710 9,710 9,389 3,761 19,987 33,864

  • 20,000

40,000 60,000 80,000 100,000 120,000 14/15 15/16 16/17 17/18 18/19 19/20

Harvesting quantities by area

Sem BW Sem Jetty Betw jetties Largs north

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SLIDE 17

Profile analysis

  • 8
  • 6
  • 4
  • 2

2 4 6 8 50 100 150 200 250 300 350 400 450 500

2020 profile comparison

200129A05022020 200008A17022020

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SLIDE 18

Profile analysis – example in between jetties

  • 2
  • 1

1 2 3 4 5 50 75 100 125 150 175 200 AHD Height (Metres) Chainage (Metres)

Profile 200004

200004A20012010 200004A14022012 200004A07022014 200004A05022016 200004A14022017 200004A05022019 200004A05022020

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SLIDE 19

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.

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SLIDE 20

Profile analysis

  • Figure showing erosion

and accretion from 2010 to 2020 in metres

  • Most foredunes are

growing seawards (despite harvesting

  • perations and storm

impact in 2016)

  • Erosion north of

Semaphore breakwater

+8 +10

  • 27 (-40)

+14/-7=+7 +9 +30 +35 2014-2020: 325,000 m3 2014-2020: 43,000 m3 2020: 35,000 m3

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SLIDE 21

Aerial surveys – Largs North

  • Details about where sand has

been taken and how deep

04/2020-06/2020: 35,000 m3

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SLIDE 22

Aerial surveys – Semaphore breakwater

10/2018-10/2019: ~100,000 m3 4 different campaigns, spread

  • ver one year

10/2019-01/2020: 70,000 m3

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SLIDE 23

Surveys - conclusion

  • 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.

  • 20,000

40,000 60,000 80,000 100,000 120,000 14/15 15/16 16/17 17/18 18/19 19/20

Yearly harvesting quantities behind Semaphore breakwater

Yearly harvesting quantities behind Semaphore breakwater

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SLIDE 24

Aerial imagery analysis breakwater

Limitations:

  • Cloud cover
  • Timing of aerials vs. timing of campaigns
  • Varying tidal levels

Findings:

  • Salient most years
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SLIDE 25
  • 20,000

40,000 60,000 80,000 100,000 120,000 14/15 15/16 16/17 17/18 18/19 19/20

Yearly harvesting quantities behind Semaphore breakwater

Yearly harvesting quantities behind Semaphore breakwater

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
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SLIDE 26

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

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SLIDE 27

Aerial imagery analysis breakwater

  • Short term beach

recovery quick to smooth out bumps

  • Long term beach

recovery dependent on longshore sediment transport

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SLIDE 28
  • 20,000

40,000 60,000 80,000 100,000 120,000 14/15 15/16 16/17 17/18 18/19 19/20

Yearly harvesting quantities behind Semaphore breakwater

Yearly harvesting quantities behind Semaphore breakwater

Aerial imagery analysis breakwater

Limitations:

  • Cloud cover
  • Timing of aerials vs. timing of campaigns
  • Varying tidal levels

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
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SLIDE 29

Storm impacts – Semaphore – 1 year storm

  • 2
  • 1

1 2 3 4 5 6 10 20 30 40 50 60 70 80 90 100 110 120 130

20008

Initial 1yr Final INITIAL 1yr FINAL

5m 3m

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SLIDE 30

Storm impacts – Semaphore – 10 year storm

  • 2
  • 1

1 2 3 4 5 6 10 20 30 40 50 60 70 80 90 100 110 120 130

20008

Initial 10yr Final INITIAL 10yr FINAL

3m

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SLIDE 31

Storm impacts – Largs – 1 year storm

  • 1
  • 0.5

0.5 1 1.5 2 2.5 3 3.5 4 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300

200129 POST (-25,000)

Initial 1yr Final INITIAL 1yr FINAL

0m additional erosion

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SLIDE 32

Storm impacts – Largs – 10 year storm

  • 1
  • 0.5

0.5 1 1.5 2 2.5 3 3.5 4 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300

200129 POST (-25,000)

Initial 10yr Final INITIAL 10yr FINAL

5m

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SLIDE 33

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
  • ccurs 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.

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SLIDE 34

Summary and Conclusion

  • Most northern beaches are accreting and sand harvesting is therefore possible.
  • Erosion is experienced north of the Breakwater which will need to be managed actively. This requires
  • ngoing beach nourishment rather than sand harvesting.
  • All other beaches will continue to grow despite short term storm impacts and harvesting works.
  • Semaphore breakwater has been intensely harvested in 18/19 and 19/20, resulting in increased extent of

downstream erosion at Semaphore Jetty.

  • Future sand harvesting campaigns at Semaphore breakwater should be no more than the natural rate of

replenishment (e.g. harvesting should be focused on the salient. If no salient has formed sand harvesting should be avoided or heavily limited in quantity).

  • Amount of additional storm erosion on the ‘harvested’ beach profiles is relatively small, especially in

comparison to the beach accretion rates and are experienced over a short time only.

  • From a coastal engineering perspective, sand harvesting has therefore very limited impacts on the harvest

area in terms of impacts on the dune/available erosion buffer.

  • Amount of erosion or beach response is highly dependant on water levels during the storm and the height,

shape and slope of the dune.

  • Beach conditions shall always be assessed prior to each campaign to ensure that sufficient volumes are
  • available. In other words, if there was a stormy season, harvesting depth shall be minimised further than

what is recommended in the following slides.

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SLIDE 35

Recommendations – Semaphore Breakwater

  • Focus harvesting campaigns on the salient. If no salient has formed it

means that the natural replenishment rate was low and the beach did not have sufficient time to rebuild. Sand should only be harvested if a salient has formed. Harvesting quantities shall be dependent on the amount of sand replenishment that has occurred in that year. Breakwater design report suggests that 28,000 to 45,000m3/year is a sustainable harvesting rate for the breakwater area.

  • Due to intense harvesting in 18/19 and 19/20 it is recommended to not

exceed 40,000m3 during the 20/21 campaign to limit further downdrift

  • impacts. If the salient (and beach just south of the breakwater) does not

contain that quantity, the 20/21 harvesting campaign should be limited further to the sand quantity that is actually available.

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SLIDE 36

Max 40,000 m3 in 2020/21 Continuous back-passing 0 – 10,000 m3/year

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SLIDE 37

Recommendations – South of Semaphore Breakwater and in between Jetties

  • Difficulty in determining toe of dune
  • Min distance of 5m recommended (some short term impacts will be
  • bserved, however, due to the natural replenishment and existing dune

buffer and growth rate, this is acceptable)

  • Increase of ‘transition slope’ from 1:10 to 1:20
  • Harvesting depth shall be kept above MLWS
  • Harvesting depth shall not exceed 0.5m
  • Natural beach features such as ~MSL berm shall be maintained
  • Majority of sand harvesting to be undertaken in the intertidal area
  • 2
  • 1

1 2 3 4 5 6 7 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200

20008

Pre harvest profile New suggested post harvest profile

Toe of dune 5m MHWS MLWS

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SLIDE 38

Recommendations – North of Largs Jetty

  • Same, but harvesting depth shall not exceed 0.6m
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SLIDE 39

What does this mean for the 2020/21 campaign

Max 40,000 Max 40,000 Max 40,000 Max 60,000 Max 85,000

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SLIDE 40

Environmental Considerations

Jamie Kaye Water Technology

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SLIDE 41

Environmental Considerations

Key Environmental Assets for consideration

  • Dune Flora – native vegetation
  • Dune Fauna – beach nesting

birds

  • Wrack
  • Beach Infauna
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SLIDE 42

Potential impacts from sand harvesting

  • Physical removal or exposure of

infauna

  • Compaction of sands and

infauna

  • Crushing /loss of wrack/fauna
  • Scraper and truck movement

keeping fauna from swash zone

  • Noise and activity disturbance
  • Steepening dune
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SLIDE 43

Flora

  • T&M Ecologists March 2020
  • 20 Mapping Units described
  • Traffic light classification
  • Potential impacts – physical loss

with dune retreat

  • Mitigation – no particular

requirements if fauna/wrack mitigation practices followed

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SLIDE 44

Fauna

  • Key species – Red-capped Plover
  • Potential impacts – dune slope,

wrack loss, noise, crushing

  • Mitigation – identify nesting

birds, timing of works, minimise traffic, timing of traffic, keep away from toe of dune, minimise disturbance to wrack

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SLIDE 45

Wrack

  • What and why important?
  • Potential impact – removed or

crushed

  • Mitigation – relocate if likely to

be disturbed, if abundant don’t remove but consolidate traffic path

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SLIDE 46

Infauna

  • Knowledge gaps and

inconsistent research results? Many variables affecting infauna

  • Potential impacts – removal,

compaction

  • Mitigation – ‘Compromises’

large shallow or small deep, harvest in strips, minimise carting track footprints

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SLIDE 47

Sand Movement

Mel Lutton Water Technology

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SLIDE 48

Removal of sand

  • Access to beach
  • Truck movements
  • Methodologies to

minimise disruption

  • Water quality SW filtration

through dunes

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SLIDE 49

Access to beach

  • Semaphore SLSC at Point

Malcolm

SLSC Semaphore Jetty Largs Bay Jetty Strathfield Terrace 1.5 km 2 km 2 km

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SLIDE 50

Trucking movements

  • Volume of sand to be removed annually up to 120,000m3
  • 20-25 m3 per truck = 17.5-22 m3 actual sand
  • Number of yearly truck movements around 6,000 (between spring

and autumn)

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SLIDE 51

Minimising disruption

  • Single access point
  • Timing of removal to avoid
  • School holidays
  • Early morning and after work
  • Balance between truck movements

and size of truck

  • Larger vehicles vs more journeys
  • Noise
  • Vibration
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SLIDE 52

Minimising disruption

  • Have considered single winter

campaign

  • Weather issues
  • Fewer opportunities for effective

working

  • Potentially more disruptive short term
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SLIDE 53

Water Quality

  • Filtration through dunes
  • Not a typically recognised treatment method
  • Clearly rubbish collected that would otherwise go to the beach
  • Dunes will not be eroded to the point they don’t provide same level
  • f filtration
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SLIDE 54

Sand Dunes as Treatment

  • Untreated stormwater often

contains high levels of bacteria

  • North Carolina innovative Dune

Infiltration System

  • Works by diverting stormwater

beneath the sand dunes

  • Have been highly successful in

reducing stormwater discharge to the recreational beach areas

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SLIDE 55

Thanks for your attention, any questions?

Water Technology