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Breakwaters & Training Walls | The good, the bad and the ugly

How are breakwaters and training walls impacting our coastline? Lex Nielsen explains the good, the bad and the ugly at the NSW Coastal Conference

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Breakwaters & Training Walls | The good, the bad and the ugly

  1. 1. Breakwaters & Training Walls The good, the bad and the ugly Lex Nielsen, Principal Consultant, Advisian Angus Gordon, Principal Consultant, Coastal Zone Management & Planning November 2016
  2. 2. Across Australia, breakwaters and training walls have instigated fundamental perturbations to coastal and estuary processes, with significant consequences.
  3. 3. Why build breakwaters and training walls? … to improve hydraulic conveyance of entrance channels to: • Increase discharge capacity for major floods • Assist in minimising nuance flooding of low lying areas • Increase estuary/lower river flushing thereby “improving” water quality • Improve navigability by increasing depths, particularly over entrance bars • Stabilise entrance location
  4. 4. Impacts on coastal processes • Trap of littoral drift causing down-drift erosion (e.g. Tweed River breakwaters starving sand to Gold Coast beaches, Coffs Harbour etc.) • Change in nearshore wave patterns causing coastal re-alignment - a sleeper that occurs somewhat frequently
  5. 5. Mobile bed scale modelling has demonstrated conclusively that breakwaters can change beach alignments by creating artificial headlands forming crenulated bays on what previously were linear shorelines.
  6. 6. Crenulate shaped bays • Crenulate shaped bays have been used to stabilize coasts (Richard Silvester, Siew-Koon Ho) • Crenulate shaped bays exist between headlands • They have been scale modelled successfully to predict future shorelines accurately
  7. 7. The red line shows the coastline before artificial breakwater headlands were developed for the river, and an artificial headland and reef to the right. Mobile bed scale modelling is used to predict future stable shoreline accurately (e.g. Brunei scale modelling by Angus Gordon)
  8. 8. Coastal re-alignments Mobile bed scale models study the impact of breakwaters on beach alignment
  9. 9. Coastal re-alignments Computer model study of Stockton Beach Newcastle The northern Hunter River breakwater had a significant impact on the shoreline at Stockton: • Cut off the flood tide channel that had run along the beach and into the entrance - removing the entrance bar and accretion of sand • Induced erosion to the north as the shoreline responded to the changed tidal flow and wave conditions
  10. 10. Coastal re-alignments Similar could occur if breakwater are constructed at The Entrance on the NSW Central Coast
  11. 11. Impacts on estuary processes Breakwaters enhance hydraulic efficiency and tidal conveyance
  12. 12. Long-term impacts on tidal planes The construction of entrance breakwaters on Wallis Lake, Lake Macquarie, Lake Illawarra and Lake Wagonga has triggered unstable scouring modes at these estuary entrance channels. High water levels are increasing while low water levels are decreasing (note various scales).
  13. 13. Long-term impacts on channels • Channels scoured to greater depths with removal of seagrasses • Undermining and collapse of banks and retaining structures • Undermining and collapse of infrastructure (e.g. bridges) • Deposition on flood tide and ebb tide deltas including smothering of seagrasses
  14. 14. Training walls enhance hydraulic efficiency and tidal conveyance 100% 116% 120%
  15. 15. Long-term impacts of training walls Training walls stabilise channel and increase hydraulic conveyance A rock training wall was constructed on the northern bank of the Coffs Creek entrance changed an ICOLL (Intermittently Closed and Open Lakes and Lagoons) to a permanently open estuary.
  16. 16. The Good • Improve hydraulic conveyance of entrance channels to increase discharge capacity, eliminating nuisance flooding of low-lying areas and improve water quality • Stabilise channel thalwegs and increase depths, improving navigation • Safer entrance navigation • Marine sands added to the littoral system and nourished beaches where entrance scour has jetted sand out into the littoral system • Improved surfing opportunities due to “new breaks” created by breakwaters
  17. 17. Channel scour has removed seagrass and sand transport into the lakes as flood tide delta growth has smothered seagrass The Bad
  18. 18. The Bad Wetland areas have been eroded (e.g. Lake Macquarie)
  19. 19. The Bad 0.11 ha/a 0.31 ha/a 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 1950 1960 1970 1980 1990 2000 2010 Area(ha) Year Pre-Breakwater Construction Post-Breakwater Construction -0.035 ha/a -0.11 ha/a 10 11 12 13 14 15 16 17 18 19 20 1950 1960 1970 1980 1990 2000 2010 Area(ha) Year Pre-Breakwater Construction PostBreakwater Construction Mangrove Increase Saltmarsh Decrease Changing tidal planes have increased the rates of mangrove colonisation and salt marsh loss (e.g. Lake Wagonga).
  20. 20. The Bad Channel scour has necessitated rock works along the banks (e.g. Lake Macquarie, Lake Wagonga).
  21. 21. The Bad Channel scour has necessitated expensive bridge foundation works (e.g. Wallis Lake)
  22. 22. The Bad Coastal littoral drift has been interrupted (e.g. Coffs Harbour), harbour siltation.
  23. 23. The Bad Expensive capital works have been necessary to reinstate beaches that have eroded as a consequence of breakwater construction (e.g. Tweed River)
  24. 24. The Ugly
  25. 25. The Ugly Channel scour has resulted in the total collapse of some foreshore buildings and undermined bridge foundations as well as undermining river training works (e.g. Swansea Channel Lake Macquarie)
  26. 26. The Ugly Breakwaters have re-aligned the foreshore causing beach erosion, loss of residential development and other infrastructure (e.g. loss of the entire village of Sheltering Palms at Brunswick Heads)
  27. 27. The Ugly Breakwaters causing erosion have sometimes necessitated revetment construction to limit erosion thereby destroying beach amenity (e.g. Stockton Beach at Hunter River Newcastle).
  28. 28. The Ugly Training walls in tidal channels can create dangerous conditions that may cause serious injury to swimmers (e.g. Coffs Creek). High ebb tide velocities on the ocean bars have presented challenges to recreational boating and have seen boating fatalities (e.g. Foster and Lake Wagonga, Narooma).
  29. 29. DISCLAIMER This presentation has been prepared by a representative of Advisian. The presentation contains the professional and personal opinions of the presenter, which are given in good faith. As such, opinions presented herein may not always necessarily reflect the position of Advisian as a whole, its officers or executive. Any forward-looking statements included in this presentation will involve subjective judgment and analysis and are subject to uncertainties, risks and contingencies—many of which are outside the control of, and may be unknown to, Advisian. Advisian and all associated entities and representatives make no representation or warranty as to the accuracy, reliability or completeness of information in this document and do not take responsibility for updating any information or correcting any error or omission that may become apparent after this document has been issued. To the extent permitted by law, Advisian and its officers, employees, related bodies and agents disclaim all liability—direct, indirect or consequential (and whether or not arising out of the negligence, default or lack of care of Advisian and/or any of its agents)—for any loss or damage suffered by a recipient or other persons arising out of, or in connection with, any use or reliance on this presentation or information.

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How are breakwaters and training walls impacting our coastline? Lex Nielsen explains the good, the bad and the ugly at the NSW Coastal Conference


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