West Beach Coastal Processes Modelling (The DHI Study) • James Guy, Project Manager Securing the Future of Our Coastline, DEW
West Beach Study (2017 – 2018) Funded by: • DEW • Coast Protection Board • City of Charles Sturt • Adelaide Shores
Study Objectives • Establish and validate a coastal sediment transport model of West Beach. • Using the model, evaluate alternative coastal management scenarios. • Assess possible design modifications to the Adelaide Shores boat harbor breakwater to minimise sand and seagrass wrack ingress and wave penetration
DHI (Danish Hydraulics Institute) • Independent, private and not-for-profit organisation. • 80% of 1,200 employees hold an MSc or a PhD degree. • Knowledge represents 50 years of dedicated research. • 21% of resources are allocated to R&D to enhance knowledge and innovation.
DHI Study – Sediment Transport Used sophisticated computer modelling to calculate sediment transport rates. Also used DEW survey data to calculate sediment transport rates. Conclusion: • The littoral transport rate at West Beach is between 50,000 and 150,000m 3 per year, with an average rate of around 100,000m 3 per year.
DHI Study – Sediment Volumes DHI analysed historic survey data to determine total sand volumes within the West Beach cell. Conclusions: • The West Beach sediment cell has lost in the order of 500,000m 3 of sand since the late 1990s and the rate of decline has accelerated since approximately 2011. • No significant volumes of sand are being lost from West Beach to off-shore areas. Significant volumes are being transported and accumulating in the northern sediment cells.
What Options were Considered?
Hard Engineering Structures? • Offshore breakwaters and “headland control” structures were considered, together with “hybrid” combinations of hard engineering structures and replenishment. • Not supported due to: • Significant cost • Do not resolve underlying sediment deficit problem • Require initial and ongoing sand nourishment • Complexity of assessing impacts, potential for downdrift erosion. • Public safety and amenity issues.
Geotextile groynes or submerged offshore breakwaters? One of the existing Somerton groynes
Geotextile groynes or submerged breakwaters • Provide limited protection unless of sufficiently large size to break waves or interrupt sand movement during storm events (i.e. at very high water levels). • Unless the structure causes waves to break during storm events, it can cause increased erosion due to wave refraction and focussing of energy. • Can create rip currents during stormy conditions, which increase rate of sediment transport. • Durability issues. • Not used successfully anywhere else.
Modelled Management Options “The most feasible, sustainable and cost effective management option identified for West Beach was one that incorporated some variation on a large scale nourishment option, to restore the littoral volume in the West Beach cell and provide a large source of sediment to meet the downdrift transport capacity of the Henley Beach cell into the future.” Pg 22 West Beach Coastal Processes Modelling (DHI, 2018)
Advantages of Large Scale Nourishment • Cost efficiencies in sourcing large nourishment volumes. • Ongoing supply to downdrift beaches • Shoreline response can be more reliably predicted. • Nourishment can be constantly adjusted based on shoreline response. • Public safety and amenity risks are reduced.
Modelled Management Options Scenarios: 1 – Do nothing. 2 – Mass renourishment (“sand engine”) 3 – Interim management. 4 – Backpassing from northern beaches.
Modelled Management Options 2 – Mass renourishment approx. every 10 years (1.8 million m 3 ) • 500,000m 3 added to full length of West Beach cell • plus 1,000,000m 3 added to West Beach (south of Torrens outlet) • 300,000m 3 added to Henley Beach south • Backpassing using pipeline from Torrens outlet to West Beach Parks after 5+ years.
Modelled Management Options 3 – Interim management. • Small scale initial nourishment around Torrens outlet (100,000m 3 ). • New seawall from WBSLSC to West Beach Harbour (along West Beach Parks dunes). • Backpassing 30,000m 3 prior to each summer to create a beach in front of seawall.
Modelled Management Options 4 – Backpassing from northern beaches: • First 4 years 150,000m 3 /year to West Beach; 20,000m 3 /year to Henley Beach South • Then 100,000m 3 /year to West Beach and nothing to Henley Beach South
Modelling results 1 – do nothing 2 – mass renourishment, 1.8 million m 3 3 – Interim management, seawall and minor replenishment 4 – ongoing backpassing from northern areas
Adopted Strategy Hybrid of mass nourishment and backpassing options: • Mass nourishment of West Beach with 500,000m 3 of sand from outside the metropolitan beach system. • Backpass sand from northern beaches to West Beach to match alongshore sand transport rate using sand pumping system (approx. 115,000m 3 /year). • Stabilise and revegetate dunes. • Interim replenishment of West Beach with 115,000m 3 /year for two years during planning and delivery.
Rationale for Adopted Strategy • Availability of (and to maximise efficiency of using) external sand: 500,000m 3 vs 1.8 million m 3 . • Minimises impacts on northern beaches through use of external sand (compared with mass replenishment from northern beaches) • Successful operation of the pumping system from Glenelg to Kingston Park. • Flexibility: to deal with seasonal and long term variability of coastal processes. • Adaptability: to deal with rising sea levels and a changing climate.
Impact Assessment Interim replenishment (2019-20 and 2020-21) from northern beaches. • DEW assessed likely impacts associated with moving sand from the northern beaches to include maximum dune recession of approximately 4.0m to 6.0m. • Dune vegetation assessment by DEW found no high value native vegetation at risk (since confirmed by March 2020 independent assessment).
Impact Assessment Longer term impacts following completion of the pipeline. • Movement of sand back to West Beach from areas of accumulation will be based on monitoring. • Amount of sand to be moved back to West Beach = Amount of sand leaving West Beach. • No net impact on volumes of sand in the northern beaches.
Volume changes 1994 to 2016 Cell 3 (West Beach) Cell 6 (Semaphore, Largs, Taperoo).
Independent Expert Comments Professor Andrew Short OAM School of Geosciences, University of Sydney
Why manage? • Adelaide sand movement is part of a natural system extending from Seacliff to North Haven & operating for the past 7,000 years • It is a natural system that is slowly running out of sand • Since 1800s it has also been hindered in part by human impacts • Unless it is managed the southern beaches & dunes will erode • Property and infrastructure will be at risk • Public beach amenity will deteriorate
Some other similar issues and solutions • Mackay Harbour, Qld : northward transport of sand is stopped by the harbor walls. Sand is pumped from south side to northside of harbor Presented by independent coastal expert, Professor Andrew Short, 25 June 2020
Some other similar issues and solutions • Collaroy Beach, Sydney: sand is dredged and trucked from the northern tidal inlet. Pumping has been proposed as a better solution but not initiated as yet.
Some other similar issues and solutions • Port Geographe , WA: northward transport of sand & seagrass stopped by entrance wall. Sand is pumped from south side to northern side. Seagrass debris removed by equipment.
Some other similar issues and solutions • Port Bouvard , WA: When entrance walls were built a hook groyne and pumping system was incorporated and now regularly pumps sand under the entrance from south to north
Some other similar issues and solutions • Gold Coast: most heavily managed and best managed beaches in the world. Includes: major sand pumping operations under Tweed River since 2000 and Seaway since 1986 (~500,000 m 3/ yr)
Recommend
More recommend
