Final Report Point Lonsdale Groyne Feasibility Study DELWP
22 September 2020
Document Status
Version Doc type Reviewed by Approved by Date issued V01 Interim GXC GXC 04/06/2020 V02 DRAFT GXC GXC 1/09/2020 V03 FINAL GXC GXC 11/09/2020 V04 FINAL GXC GXC 22/09/2020 V05 FINAL GXC GXC 22/09/2020
Project Details
Project Name Point Lonsdale Groyne Feasibility Study Client DELWP Client Project Manager Dianne Moore Water Technology Project Manager Michael Miloshis Water Technology Project Director Gildas Colleter Authors Michael Miloshis, John Gater Document Number 19010148_R01_V05_FINAL.docx
COPYRIGHT
Water Technology Pty Ltd has produced this document in accordance with instructions from DELWP for their use only. The concepts and information contained in this document are the copyright of Water Technology Pty Ltd. Use or copying of this document in whole or in part without written permission of Water Technology Pty Ltd constitutes an infringement of copyright.
Water Technology Pty Ltd does not warrant this document is definitive nor free from error and does not accept liability for any loss caused, or arising from, reliance upon the information provided herein.
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Notting Hill VIC 3168 Telephone (03) 8526 0800 Fax (03) 9558 9365 ACN 093 377 283 ABN 60 093 377 283
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CONTENTS
EXECUTIVE SUMMARY 4
1 INTRODUCTION 5 1.1 Community Design Proposal 5
2 BACKGROUND 7 2.1 History of Shoreline Management 7 2.2 Local Coastal processes 10 2.3 Previous studies 11
3 METHODOLOGY 13 3.1 Numerical Modelling 13 3.2 Model Development 13 3.2.1 Bathymetry and Model Mesh 14 3.2.2 Water Level Boundary 14 3.2.3 Wave Boundary 14 3.3 Wave and tide data 16 3.4 Model Calibration 17 3.4.1 Water Level 17 3.4.2 Waves 18 3.5 Model Simulations 20 3.5.1 Design wave parameters 20 3.5.2 Beach morphology 20 3.6 Groyne field effectiveness 21
4 CONSTRUCTABILITY 23 4.1 Concept design standard and process 23 4.2 Concept design 24 4.2.1 Armour sizing 24 4.2.2 Geometry 24 4.2.3 Construction access 27 4.2.4 Construction Sequence 28 4.2.5 Construction program 28 4.2.6 Safety 28 4.2.7 Material Quantities and Cost Estimate 29
5 COMMUNITY DESIGN OPTION REVIEW 30 5.1 Stakeholder engagement 30 5.2 Multicriteria assessment 30 5.3 Limitations – model and groyne design limitations 31
6 CONCLUSION 32
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LIST OF FIGURES Figure 1-1 Existing and proposed groyne plan 5 Figure 2-1 Point Lonsdale Foreshore, 1920s 7 Figure 2-2 Point Lonsdale Point, 1962 9 Figure 2-3 Point Lonsdale front beach before and after rock groyne A installation, 1990-2019 9 Figure 2-4 Existing groyne locations with moderate wave action, 14 January 2014 10 Figure 2-5 Groyne designs and likely accretion zones, BMT WBM Study, 2017 11 Figure 2-6 Existing and modelled groyne designs, Cardno, 2018 12 Figure 3-1 Port Phillip Bay model mesh with high resolution around Port Phillip Heads 15 Figure 3-2 Proposed groyne field as captured by the model mesh 15 Figure 3-3 Wave and tide data recorded at Point Lonsdale and at the Point Nepean wave buoy 16 Figure 3-4 Tide calibration – Point Lonsdale tide gauge 17 Figure 3-5 Tide calibration – Queenscliff tide gauge 17 Figure 3-6 Tide calibration – Point Lonsdale Bight 18 Figure 3-7 Wave calibration – offshore, Point Nepean wave buoy 18 Figure 3-8 Wave calibration – Point Lonsdale Bight 19 Figure 4-1 Beach Groyne typical section 24 Figure 4-2 Point Groyne 1 and 2 typical section 25 Figure 4-3 Point Groyne 4 26 Figure 4-4 Point Groyne 1, 2 and 3 26 Figure 4-5 Construction site plan 27
LIST OF TABLES Table 3-1 Storm events wave during deployment 16 Table 3-2 Offshore wave height and water level at Point Lonsdale Bight 20 Table 3-3 2014 to 2015 storm waves 20 Table 4-1 Project Standards and Guidelines 23 Table 4-2 Groyne armour mass 24 Table 4-3 Cost estimate of each group of groynes 29
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EXECUTIVE SUMMARY
The Department of environment, Land, Water and Planning engaged Water Technology to review a “Community Design Option” (CDO) for Point Lonsdale Front Beach to improve the beach amenity.
The Community Design Option, prepared by three members of the Point Lonsdale community, proposes a denser groyne field. The aim is to improve the volume and consistency of sand on the Point Lonsdale Front Beach.
The CDO Groyne field incorporates eight additional groynes along the Front Beach. Four Point Groyne structures are spread around the southern rocky headland, in the same location as historic timber barriers. Four additional Beach Groynes are added north of the existing Cheshunt Street groyne.
Three existing groynes (rock type) and a seawall have been built to manage coastal erosion along the Front Beach. These groynes improve beach amenity for beach users and protect the seawall from beach scour.
The beach evolution at Point Lonsdale Front Beach is influenced by transient natural processes as well as the seawall and the groynes. The existing groynes increase the longevity of intermittent sand build-up and widen the beach profile.
A range of numerical modelling and engineering techniques were used to investigate the performance, functionality, feasibility and construction of the CDO, including of:
◼ Collection of local wave and tide data, including storm waves
◼ Coastal processes assessment for the existing and proposed groyne fields
◼ Extreme value analysis to understand the likely design conditions affecting the groynes
◼ Numerical modelling of tide, currents and waves at the site for a range of scenario
◼ Conceptual engineering design review of the CDO groynes and high-level cost estimation
◼ Engagement and consultation, including 3 meetings, prepared in association with the CDO stakeholders, Victoria Parks and Borough of Queenscliff
◼ Multi-criteria analysis and ranking of groynes
The CDO Beach Groynes are likely to increase the average beach width at the Front Beach, however the last groyne may exacerbate beach erosion to the north. The CDO Point Groynes would require significant strengthening to meet the necessary level of performance required for structural safety.
On balance, Beach Groyne 5 provides the most effective outcome for improving beach amenity. Point Groyne 4 and Beach Groyne 6 would also be good at improving beach amenity. Point Groynes 1 & 2 are not suitably located for a consistent improvement to sand volume. They generally pose challenging construction and maintenance issues as well as a significant impact to visual amenities.
The estimated budget required for the resulting CDO groyne field modification is approximately $4.5 million.
Construction may take 5 months considering the tidal limitation, particularly for the Point Groynes, which would need to be constructed during spring and summer to maximise the most suitable weather conditions for construction.
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1 INTRODUCTION
The Department of environment, Land, Water and Planning engaged Water Technology to assess a proposed community-based groyne field modification in terms of technical functionality, feasibility, and construction.
Members of the CDO aim to increase the volume and consistency of sand on the Point Lonsdale Front Beach. Additional rock groynes are proposed at locations of historic timber barriers, along the point, and at foreshore beach locations that have a typically low volume of sand.
1.1 Community Design Proposal
Figure 1-1 shows the layout proposed by the CDO with the 3-existing groynes (A, B and C).
8
7
6
5
4
3 2
1
Figure 1-1 Existing and proposed groyne plan
A denser groyne field of eight additional groynes is proposed. It is suggested that the existing groynes may be
spaced too far for sand build-up to overlap, which reduces the available beach surface for beach goers. 19010148_R01_V05_FINAL.docx
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There are four additional groynes proposed north of the existing groyne at Cheshunt Street, along the beach. These four new Beach Groynes would be typically 50 to 60m long and start 1m below the level of the seawall, sloping 1:30 down towards the sea - over 30m, with the remaining length 2m below the seawall crest. The seawall crest is located at 2.6m AHD.
Four additional rock groyne structures would be positioned at the southern rocky headland, in the same location as historic timber barriers. The rock groyne structure would be typically shorter, about 30m long and would be built as low as possible.
The Department of Environment, Land, Water and Planning (DELWP) conducted a community-centred process, with direct consultation and involvement of the CDO representatives to study the Point Lonsdale Front Beach Groyne field modification.
The current coastal management arrangements originate from practical knowledge and site experience with more recently scientific and engineering design used to manage the site amenities. The current investigation aims to identify if the CDO proposal is technically feasible and can achieve increased sand on the Point
Lonsdale Front Beach.
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2 BACKGROUND
2.1 History of Shoreline Management
The coastline at Point Lonsdale Front Beach is exposed to strong currents due to its proximity to Port Phillip Heads and to moderate waves refracting in “The Rip” from Bass Straight. A combination of seawalls, rock revetments and groynes have been built and tested overtime to manage the dynamic beach environment. Several structures have been implemented to manage the coast along Point Lonsdale Front Beach.
Figure 2-1 shows timber barriers have been constructed and maintained since the 1920s, in an attempt to stabilise the beach. Such timber barriers did not prevent the shoreline from retreating.
Figure 2-1 Point Lonsdale Foreshore, 1920s
A bluestone seawall was built in the 1930s to protect the foreshore area and bathing boxes near the headland during the great depression. The seawall would have been considered essential to maintain these assets along the foreshore. This addition fixed the shoreline in position and further reduced available sand.
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Figure 2-2 shows additional rock revetment and timber barrier built along the Point in the 1960s and 1970s to prevent flanking of the bluestone seawall to the south and protect the vegetated dunes to the north of the Bight.
Figure 2-3 shows how older timber barriers were further stabilised with rock in 1999-2000 at Cheshunt St (Groyne A) from 2005 to 2019. The groyne successfully trapped sand on the southern side around 2005. However, the 2015 picture shows a shoreline position similar to the 1990 beach configuration, despite the presence of the Groyne A.
The Nicholas St groyne (Groyne C) was stabilised with rock in a similar fashion in 1999-2000. The Loch St rock groyne (Groyne B) was re-constructed between the Cheshunt and Nicholas St groynes in 2004 with rock rather than timber barrier. Finally, small section of rock revetment was built in 2016 to protect the base of the southern cliff at the end of the seawall as the bluestone seawall continues to be flanked by erosion.
Several groyne and seawall types and configurations have been trialled and tested “in-situ” over the past 100 years. The on-going evolution of the coastal management arrangements demonstrates that it is challenging to stabilise the Point Lonsdale Front Beach and that it is unlikely that there is a permanent “ideal” configuration to the coastal management works.
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Figure 2-2 Point Lonsdale Point, 1962
Figure 2-3 Point Lonsdale front beach before and after rock groyne A installation, 1990-2019 19010148_R01_V05_FINAL.docx
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2.2 Local Coastal processes
The longshore transport moves sand along the coast under the predominant action of waves and tides. Longshore transport is colloquially referred to as a “river of sand” which flows along the coast towards the North along Point Lonsdale beach. However, this sand flow is not continuous. It is restricted by sand supply across coastal headlands and temporary reversal of longshore transport during strong wind-wave events in Port Phillip Bay, moving sand towards the south.
The resulting “net” sand movement through Point Lonsdale Bight towards the Point Lonsdale Point comes in heaps, or “sand slugs”, this was understood as early as the 1980’s (P. Riedel). These sand slugs are intermittent large volumes of sand (typically over 50,000m3 or more). They move slowly, in the order of hundreds of metres along the coast per year in the longshore transport direction. In between sand slugs the beach is typically narrow, increasing the risk of erosion of primary dunes, this can persist for many years.
Groynes along Point Lonsdale beach have captured sand from these sand slugs as they pass through the area. The groynes are not attracting sand, but merely form a temporary obstruction to sand movement. When the beach sand volume is low across the beach profile, the groynes become less and less effective.
As waves approach the shoreline they refract and turn parallel to the shore. The groynes provide an opportunity to trap sand upstream of the longshore transport direction. However, downstream of the last groyne, the waves refract energy downstream and this typically scours the beach. At Point Lonsdale, the existing groynes are close enough to one another to mitigate some of this beach scouring effect. Only the last groyne (to the north) has a significant erosion shadow with the shoreline attached to the seawall.
Figure 2-4 shows the wave crest alignment, the sand trapping upstream of Beach Groynes A, B and C as well
as the erosion shadow and shoreline attachment to the seawall in the lee of Groyne C.
Figure 2-4 Existing groyne locations with moderate wave action, 14 January 2014
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2.3 Previous studies
Two recent prior studies have envisaged modifications to the Groyne field, the BMT study of 2017 and the Cardno study of 2018. Figure 2-5 shows the likely accretion zones for an optimised Groyne field proposed by
BMT WBM study in 2017 which investigated a wide range of options to improve the local beach amenities.
Figure 2-5 Groyne designs and likely accretion zones, BMT WBM Study, 2017
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Figure 2-6 shows the result of sand accretion patterns in the Cardno study of 2018.
Figure 2-6 Existing and modelled groyne designs, Cardno, 2018
While this report provided a description of a modelled shoreline this modelling was not based on data immediately offshore of Point Lonsdale front beach. Hydrodynamics were modelled with an existing calibrated model while wave modelling was based on assumptions. The aim of the study was to investigate various management options, without addressing design and construction feasibility. The study recommended further work to truly understand the local coastal processes.
The community design has been inspired by these studies and seek to complement these to improve the local
beach amenities. 19010148_R01_V05_FINAL.docx
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3 METHODOLOGY
3.1 Numerical Modelling
A model was developed to simulate the currents and waves in the Point Lonsdale area. Large storms lead to extreme forces on groyne structures so water levels, wave height and wave period are a major consideration to groyne design. This section outlines the model framework used for this project including input data and parameters.
3.2 Model Development
An existing Water Technology hydrodynamic model of Port Philip Bay was adapted for this assessment. Several modifications were made to the model mesh and boundaries in order to accurately replicate oceanographic processes at the site. DHI Water and Environment’s coupled Hydrodynamic (HD), and spectral wave (SW) model has been utilised for this study. The MIKE 21 HDSW flow model is based on an unstructured flexible mesh and uses a finite volume solution technique. The mesh is comprised of quadrilateral and triangular elements. This approach enables a variation of the horizontal resolution of the model mesh within the model area, and therefore for a finer resolution in selected sub-areas. The computational triangular mesh of the model is made with sufficiently small cells to resolve the complexities of the Port Phillip Bay Heads and Point Lonsdale area.
The modelling system is based on the numerical solution of the two-dimensional shallow water equations - the depth-integrated incompressible Reynolds averaged Navier-Stokes equations. Thus, the model consists of continuity and momentum equations.
The hydrodynamic (HD) module simulates water level variations and flows in response to a variety of forcing functions such as:
◼ Momentum dispersion
◼ Bottom shear stress
◼ Coriolis force
◼ Wind shear stress
◼ Wave radiation stresses
The spectral wave (SW) module simulates the growth, decay and transformation of wind-generated waves. The SW model includes the following phenomena:
◼ Wave growth by action of wind
◼ Non-linear wave-wave interaction
◼ Dissipation due to white capping
◼
Dissipation due to bottom friction
◼ Dissipation due to depth-induced wave breaking
◼ Refraction and shoaling due to depth variations
◼ Wave-current interaction
◼ Effect of time-varying water depth and flooding and drying
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3.2.1 Bathymetry and Model Mesh
A Port Phillip Bay bathymetric model was developed using available data from the GeoScience Australia (GA) Ausbath Digital Elevation Model (DEM) and the Victorian Coastal DEM, details below:
◼ 2017 Victorian Coastal DEM – Comprising high resolution data from DELWP, PoM, and Deakin University (10 metre grid),
◼ 2009 Australian Bathymetry and Topography Grid, 250 metre grid.
The resolution of the model mesh was refined to capture sufficient details within Port Phillip Bay to resolve the necessary processes of the study area.
The high-resolution model mesh has 9,335 elements and is presented in Figure 3-1. The location of the model tide and wave boundary within Bass Strait is also shown. The detail of the groyne field captured by this mesh can be seen in Figure 3-2.
3.2.2 Water Level Boundary
A predicted tidal boundary is combined with measured storm tide data for calibration to measured water level data within at Point Lonsdale and Queenscliff tidal gauges.
The water levels are driven by tidal data from temporally and spatially varying tidal elevations at the southern boundary shown in Figure 3-1. These tidal boundaries were extracted from the global tide model developed by DTU Space. The model utilises the latest 17 years of multi-mission measurements from TOPEX/Poseidon, Jason-1 and Jason-2 satellite altimetry for sea level residual analysis. The model is available on a 0.125° x 0.125° resolution grid for the 10 major tidal constituents.
The generated tidal boundary does not include any seasonal tidal constituents or local forcing that creates residual water level elevations. To account for these the tidal data records from Lorne tide gauge (managed by Bureau of Meteorology) were used to find the residual water levels over the period of interest. These residuals were then smoothed over a 1.5-hour period to remove rapid fluctuations that would affect model stability and added to the generated points along the southern tide boundary to create the final boundary for use in the model calibration.
3.2.3 Wave Boundary
The Point Nepean wave buoy managed by Port of Melbourne provides a good approximation of the wave conditions at the model boundary as it is exposed to prevailing swell refracted around Cape Otway and is in deep enough water where energy loss is insignificant. Therefore, that data was used to apply wave forcing parameters at the model boundary in Figure 3-1.
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Tidal Boundary
Figure 3-1 Port Phillip Bay model mesh with high resolution around Port Phillip Heads
Figure 3-2 Proposed groyne field as captured by the model mesh
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3.3 Wave and tide data
A coastal monitoring station was deployed to calibrate the coastal processes at the Point Lonsdale Front Beach. The deployed device (RBR solo3 D | Wave) recorded waves and water level at 10-minute intervals with a 512-sample burst. It was deployed on the 29th of January 2020 in approximately 5 metres of water chart datum (CD) at Lat -38.285018° Lon 144.621116°. The device stopped recording on the 7th of February due to battery exhaustion. Figure 3-3 shows the resulting wave height at Point Lonsdale, Nepean and the tidal cycles with the ebb and flood of the tide.
Figure 3-3 Wave and tide data recorded at Point Lonsdale and at the Point Nepean wave buoy
Three storm events were captured and are summarised in Table 3-1.
Table 3-1 Storm events wave during deployment
Storm Event Wave Height Bass Strait (m) Wave Height in Point Lonsdale Bight (m) 29th to the 1st of February 2020 1.5 1.2
23rd of February 2020 3.5 1.4
6th February 2020 2.5 1.4
These peak storm waves appear to coincide with high tide and were affected by tidal currents through the Heads.
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3.4 Model Calibration
3.4.1 Water Level
The model was calibrated to the water levels recorded throughout the deployment period. Sites included:
◼ Tides, Point Lonsdale tide gauge (BoM)
◼ Tides, Queenscliff tide gauge (BoM)
◼ Tides, Point Lonsdale Bight (RBR deployment)
Figure 3-4, Figure 3-5 and Figure 3-6 shows the modelled and measured tidal levels well. The model accurately reproduced the water level fluctuation with R2 correlation ranging between 97% and 98.5%.
Figure 3-4 Tide calibration – Point Lonsdale tide gauge
Figure 3-5 Tide calibration – Queenscliff tide gauge 19010148_R01_V05_FINAL.docx
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Figure 3-6 Tide calibration – Point Lonsdale Bight
3.4.2 Waves
The model was calibrated to the wave conditions recorded throughout the deployment period. Sites included:
◼ Waves, Offshore - Point Nepean Wave Buoy (PoM)
◼ Waves, Point Lonsdale Bight (RBR deployment)
Figure 3-7 compares the observed and modelled waves during the measurement campaign at the Point Nepean Buoy, in deep water. Calibration to the offshore wave buoy was satisfactory, meaning that the model transfers wave energy from the boundary to the exposed coastline without significant attenuation.
Figure 3-7 Wave calibration – offshore, Point Nepean wave buoy
Figure 3-8 shows the modelled waves at the Point Lonsdale deployment site.
The model does provide good calibration to peaks in wave height, successfully dissipating 3.5 m Hs offshore
waves to 1.4 m Hs at the deployment site. 19010148_R01_V05_FINAL.docx
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Figure 3-8 Wave calibration – Point Lonsdale Bight
Simulation of complex wave-current interactions and variation in bathymetric influence on waves throughout the tidal cycle was challenging. There are some limitations to the model in terms of capturing the cyclic nature of the wave height which is influenced by tidal currents in the Port Phillip Bay entrance.
Overall, the model performed well and can be utilised for assessments of large storm/swell events at the site as well as future modelling work associated with coastal management works.
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3.5 Model Simulations
3.5.1 Design wave parameters
Two simulations were assessed to investigate waves during storm events for the groyne concept design. These simulations were as follows:
◼ 100-year average recurrence interval (ARI) wave height with 20-year ARI water levels and
◼ 20-year ARI wave height with 100-year ARI water levels
The water level and wave datasets spanned approximately 20 years so ARIs were calculated using extreme value analysis (EVA) of events over a threshold and analysed with a Weibull probability distribution. The best fit for data extrapolated to the 100-year ARI was in the order of 99% and is presented in Table 3-2.
Table 3-2 Offshore wave height and water level at Point Lonsdale Bight
ARI (years) Wave Height (m Hsig) Water Level (m, AHD) 20 6.8 1.2 100 7.6 1.3
3.5.2 Beach morphology The model was also used to investigate the 2015 eroded beach configuration shown on Figure 2-3. These series of unusual storm events were associated with beach erosion.
Table 3-3 shows the resulting storm conditions experienced at the site during that period of time.
Table 3-3 2014 to 2015 storm waves
Event Wave Height (m Hsig) ARI (years) 24th June 2014 6.70 approx. 20 4th March 2015 5.72 range 2 to 5 12th May 2015 5.44 2 2nd August 2015 5.36 range 1 to 2
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3.6 Groyne field effectiveness
The model was used to investigate the effect of the groynes on the nearshore zone processes. This simulation modelled both tide and wave coastal processes. The assessment considers how wave processes interact with the existing groyne field and with the CDO groyne field.
Figure 3-9 shows wave height distribution across the site for the existing and proposed groyne locations during a 100 year ARI storm.
P1
Figure 3-9 100yr ARI averaged significant wave height - existing (left) and CDO (right) groyne fields
The minor discrepancies in offshore wave height are a numerical artifact due to the difference in computational mesh’s between the two scenarios and minor adjustments in term of time step and courant numbers to produce an efficient computational model.
On average, the waves onshore dissipate quickly, from 2.5 m offshore, down to 1.0 m nearshore. All groynes are in the “blue” zone, where the significant wave height is typically lower than 1.0 m. At the peak of the storm tides the waves can grow larger nearshore for a limited time, prior to the tide ebbing away. This resulted in higher design wave conditions at the Point, intermittently during the peak of the storm.
The maximum waves may become higher than 2m along the tip of the Point Groyne. This is because maximum
waves are typically twice the height of significant wave. The largest waves break offshore of the Point and are
affected strongly by the underwater reefs.
The main impact from the groyne field is for Beach Groynes 6, 7 and 8 which interrupt and break waves earlier than in the present case. The Point Groynes are too short and too sheltered by the Point to change the local wave field.
Wave refraction was modelled to produce direction maps for the existing and proposed groyne fields. Figure
3-10 and Figure 3-11 show the nearshore wave direction from the model during storms for the Point Groyne. 19010148_R01_V05_FINAL.docx
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A A
P4
P3
P2
P1
Figure 3-10 Modelled wave direction - existing (left) and CDO (right) Point Groyne fields
Wave directions immediately offshore of the groyne compartments were analysed for sand trapping potential.
Although waves are influenced by the proposed groyne field, the wave approach angle remain acute, reducing the potential of those groynes to trap sand effectively.
This may lead to small sand deposits in the protected corners at the shoreline, but no large volume of sand accretion because of back wash and wave reflection on the beach. A sand bar would form offshore the Point Groynes 1, 2 and 3 on which the sediment transport will flow towards Point Groyne 4 or Groyne A. The high wave energy impacting the cliffs around the Point will bounce and scour the sand deposits leaving an exposed reef platform along the toe of Point Groyne 1 and 2.
Point Groyne 4 is more favourable for sand build-up and retention relative to Point Groynes 1,2 and 3 due to the following:
◼ The coastline angle begins to turn to the north, facing the swell more directly
◼ The incoming waves are lower energy as they are located further within the bay
◼ Incoming waves refract more as they approach the foreshore, further improving their approach angle.
Proposed Beach Groynes 5, 6, 7 and 8 appear able to capture longshore transport and to intercept sand within the groyne compartments. The end effect of Groyne 8 is however very prevalent with larger waves nearshore in the lee of Groyne 8 and this may be detrimental to beach stability towards the north. These last groynes
would increase the scour at the end of the groyne field. 19010148_R01_V05_FINAL.docx
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4 CONSTRUCTABILITY
4.1 Concept design standard and process
Table 4-1 details the State directives, design documents and Australian Standards relevant to the project.
Table 4-1 Project Standards and Guidelines
Designation Title Typical Usage AS 1141 Methods for sampling and testing aggregates Soil laboratory testing AS 1170 (Parts Minimum design loads on structures (SAA Loading General loadings including wind 0 to 4) Code – Australian Standard) load and earthquake loads; Load combinations AS2758.6 Guidelines for the specification of armourstones Armourstone durability BS 6349 (Part 1 & 7) British Standard Code of Practice for Design wave loadings; Design of Maritime Structures revetment ASTM C295 Standard guide for petrographic examination of Armourstone durability aggregates for concrete CIRIA C683 The Rock Manual, the Use of Rock in Hydraulic Rock stability and revetment Engineering (2nd Edition), 2007. works CIRIA, 2013 International Levee Handbook Levee works Eurotop (II) EurOtop II – Manual on wave overtopping of sea Overtopping flow from wave Manual defences and related structures. 2016 actions DEP 8419 Levee Management Guidelines. DELWP Guideline Operations AS 1726 Geotechnical site investigations Site investigation AS 1657:2018 Fixed Platforms, Walkways, Stairways & Ladders: Stairs and access Design, Construction & Installation AS 1428.1:2009 Design for Access & Mobility Part 1: General All access ramp Requirements for Access – New Building Work
Other relevant documentation used in compiling the schematic design includes:
◼ Victorian Tide Tables, 2020
◼ Planning for Sea Level Rise Guideline, Melbourne Water
Determining the appropriate design life for infrastructure has an important effect on maintenance and operational costs. A longer design life typically provides better life cycle costs. However, a design life that is
too long may translate to uneconomical, obsolete infrastructure which is difficult to adapt to future use.
The proposed design life is 50 years for the groynes. Individual groynes will need to be monitored and maintained within a suitable asset management framework to remain functional.
Engineering design of the groynes should consist of a concept design followed by a detailed design for tender. This detailed design would be based on further engagement (information, monitoring) and scientific analysis (modelling). which would be subsequently amended “for construction” following construction contractor
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4.2 Concept design
4.2.1 Armour sizing
The Table 3-2 modelled wave height and water levels were used to estimate wave and water level conditions at the toe of each of the 8 proposed groynes (Goda, 2000).
The appropriate armour rock mass for each groyne was calculated using the Van Der Meer equation (Van Der Meer, 1988) for each storm conditions. No damage were allowed for the 20 year event and minor displacement allowed for in the 100 year ARI storm.
Table 4-2 summarised the necessary armour rock mass for each groyne.
Table 4-2 Groyne armour mass
Location (Figure 1-1) Primary Armour (Type Secondary Armour Distance from Access A) Size (tonnes) (Type B) Size (kg) Point (m) Point 1 3 - 6 300 - 600 390 Point 2 3 - 6 300 - 600 330 Point 3 2 - 4 200 - 400 275 Point 4 1 - 3 100 - 300 205 Beach 5 1 - 3 100 – 300 165 Beach 6 1 - 3 100 – 300 400 Beach 7 1 - 3 100 – 300 475 Beach 8 1 - 3 100 – 300 555
The rock armour sizes are preliminary and would need to be refined during detailed design.
4.2.2 Geometry
The geometry of the groynes will be influenced by the armour size, as well as the objective of the community
design objectives, as outlined in section 1.1. Figure 4-1 shows a typical cross section for the Beach Groynes.
Figure 4-1 Beach Groyne typical section 19010148_R01_V05_FINAL.docx
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The concept design for the Beach Groyne is as follows: ◼ 50m long
◼ 1:1.5 side slopes
◼ Landward elevation = 1.6 m AHD, 1m below top of existing seawall along the beach
◼ Groyne crest falls by 1m over 30m
◼ Crest level then continues at 0.6 m AHD, 2.0m below top of seawall, for the last 20 m The general arrangement is similar to the Community Design Option which generally had similar length and heights. Figure 4-2 shows a typical cross section for the larger Point Groynes 1 and 2.
Figure 4-2 Point Groyne 1 and 2 typical section
The concept design for the Point Groynes is as follow:
◼ 30m long
◼ 1:1.5 side slopes
◼ Elevation = 2.3 m AHD
The Point Groyne geometry is based on a standard groyne design aiming for a crest level matching that of the landward elevation of the Beach Groynes.
While the length and position of these groynes are similar to the CDO, the height of the Point Groynes however, is approximately 1m higher than envisaged. A larger groyne is necessary to use armourstones which will be stable under wave actions. Larger armourstones allow high interlocking and allow to form of a core structure
within the groyne, which is important to capture longshore sand.
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Figure 4-3 Point Groyne 4
Figure 4-4 Point Groyne 1, 2 and 3
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4.2.3 Construction access
Access to the beach is via the car park adjacent to the existing groyne B. Construction plant would track along the beach to the point and Beach Groyne locations. Figure 4-5 shows a likely arrangement for the construction laydown area and beach access.
Figure 4-5 Construction site plan
Table 4-2 details the distance between the access point and proposed groyne locations.
Overtopping of the construction platforms is very likely at high tide, particularly during storms. An elevated construction platform would be safe at mid-tide when the waves are mild (see below construction methodology).
A calculation based on an extended working hour scenario demonstrated that the tidal working window for the proposed Point Groynes would be about 3 hours per day. While the tidal window for the proposed Beach Groyne locations a tidal working window of 6 hours per day.
Temporary works may be required due to the small tidal working window. This is especially relevant for the
proposed Point Groynes. A sand berm may be constructed to form a raised access track adjacent to the seawall which would increase the amount of time for safe access to the groyne locations. While this would result in increased material this may reduce construction costs due to increased productivity. The imported sand may be used for beach nourishment following completion of the works.
This highlights that the construction will be heavily dependent on beach levels. Topographic survey prior to works commencing will be necessary to determine beach levels and appropriate access.
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4.2.4 Construction Sequence
It is anticipated that construction materials will be transported to a stockpile landward of the seawall at the access location. Rock armour will then be transported along the beach to form stockpiles for each groyne at the landward end. This may be a tidally dependant activity to be planned in accordance with the available tidal information.
Temporary works are assumed to be necessary to form a ramp access between the proposed access point and the beach. Depending on beach levels at the time of construction, additional access tracks may also be required to gain access over the existing groyne structures. Following the setting up of the sufficient material stockpiles, groyne construction can proceed seaward.
Typically, a heavy-duty long arm excavator would be necessary for armour trim and placement, a second excavator for material loading and onloading as well as two trucks will also be needed onshore to unload and load trucks.
The Point Groynes would be launched in stages from nearshore to offshore. The long arm excavator can reach the toe and place rock at distance along the structure to form a construction stage, typically over approximately 5 to 10m distance (depending on tidal window and equipment) to place material along the groyne.
The long arm excavator would trim and adjust the filter material prior to placement of primary armourstones on the sides of the groynes.
During construction, it may be envisaged to elevate the structure higher than its final level, to allow safe access all the way towards the tip of the groyne. If this method is used, after the tip of the groyne is placed, the excavator can trim excess core into trucks and place the final filer and armour rock on-top of the groyne, recovering the access track.
4.2.5 Construction program
Weather is typically more favourable in spring and summer. Autumn and winter works will be more expensive due to bad weather interruption.
Approximately 3 months would be required to complete the construction of Point Groyne 1. This is necessary to source material, establish stockpile and onshore works and to gain familiarity with the construction in such a tidally constrained environment. The actual work on the beach will be of shorter duration and may last one month. Another 3 weeks construction period may be necessary for each additional groyne. The actual timing will depend on the nature of the construction contract, fast tracking is possible but will be risky and possibly expensive.
A reduced construction crew is likely to be more successful than a large construction crew, particularly for the Point Groynes as we recommend building only one groyne at a time, to mitigate the effect of storm events on the construction costs.
Construction of the CDO groyne field modification may take 5 months considering the tidal limitation,
particularly for the Point Groynes.
4.2.6 Safety
Installation hazards include tide, currents, wave, visibility and wind. Public interaction with the works will need to be considered, in particular with managing an exclusion zone around equipment and stockpile/construction compound. A “turtle shell”, neat and interlocked finish would be beneficial to reduce voids and manage public
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Following the works the structures will be subject to overtopping, which is particularly hazardous for gaining access for fishing or access over the groynes. Management of these risks need to include signage, marking and possibly fence or railing.
4.2.7 Material Quantities and Cost Estimate
An estimated linear meter of cost to procure each group of groynes is provided in Table 4-3.
Table 4-3 Cost estimate of each group of groynes
Groyne Location Total Groyne Length Cost per metre Total Cost (m) Point Groynes 120 $18,000.00 $2,160,000.00 Beach Groynes 200 $12,875.00 $2,575,000.00
This linear estimate considers direct costs as well as indirect costs, risks and includes some construction contract contingencies and design contingencies.
It is useful to consider that these costs are likely to be higher than the final value of the construction contract as detailed design and engineering optimisation as well as risk management and safety in design processes could reduce the scope of work associated with each groyne.
The total budget necessary to build the CDO would be approximately $4.5 million.
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5 COMMUNITY DESIGN OPTION REVIEW
5.1 Stakeholder engagement
The Department of Environment, Land, Water and Planning (DELWP) conducted a stakeholder engagement process, with direct consultation and involvement of the CDO representatives, to study the Point Lonsdale Front Beach Groyne field modification. Three stakeholder interactions have been carried out during the project with the following stakeholders:
◼ Point Lonsdale Community Design Option (CDO) representatives
◼ Parks Victoria
◼ Borough of Queenscliff
Three meetings have been held throughout the project to provide progress on the study:
◼ Meeting 1 – Inception meeting to discuss aims and deliverables – 4 March 2020
◼ Meeting 2 – Presentation of feasibility – 18 June 2020
◼ Meeting 3 – Presentation of cost estimate and multi-criteria assessment – 16 July 2020
A brief summary of feedback from the stakeholders was as follows:
◼ Parks Victoria - the proposed Point Groynes would be a safety, maintenance and environmental risk with little benefit in terms of beach amenity. The coastal landscape will be negatively affected by the Point Groynes. Based on the findings, proposed Beach Groyne 5 would provide the best outcome for increased sand on the beach.
◼ CDO – The CDO was supportive during the first and second presentation, when the groyne concepts were presented. However, following the third meeting the feedback from the CDO demonstrated that the Point Groynes 1 and 2 were of high value to the group. The CDO proposed further modification to these Point Groynes, in particular a size reduction and reconfiguration of the Point Groynes. The proposed modifications from the CDO following meeting 3 are not feasible due the need for larger rock sizes to withstand the higher wave energy at The Point. The design rock sizes and subsequent groyne size presented in Sections 4.2.1 and 4.2.2 are large enough to meet the recommended level of performance for structural safety. Constructing with smaller rock size could lead to degradation of the structure and high maintenance costs.
5.2 Multicriteria assessment
The proposed groynes have been assessed for their effectiveness, constructability, cost and beach amenity value. The criteria considerations were:
◼ Effectiveness – ability to trap sand and hold it
◼ Constructability – feasibility in construction, time and resources
◼ Cost – relative cost of each groyne
◼ Beach amenity value – how it improves the foreshore for beach users including how close the area is to existing infrastructure and main access points.
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Table 5-1 Multicriteria assessment of each groyne in the community design option.
Constructability Cost Capacity to Beach Amenity
Increase Sand Value
Point 1 Difficult Higher Low Poor Point 2 Difficult Higher Low Poor Point 3 Reasonable Lower Unlikely Reasonable Point 4 Good Lower Moderate Good Beach 5 Good Lower Good Excellent Beach 6 Good Medium Good Good Beach 7 Reasonable Medium Good Reasonable Beach 8 Reasonable Medium Scour Poor
Beach Groyne 5 is on balance the best option. At any given time, sand trapped at that groyne will be similar volume to the existing groynes. The area is in close proximity to the access point, meaning that any benefits are of higher value to beach users and it would be easier to access for construction.
Beach Groyne 6 and Point Groyne 4 perform similarly in the MCA, albeit presenting different compromises. Point Groyne 4 has a reduced effectiveness to trap sand, although it is estimated that it will cost less to build than Groyne 6, it is also ideally located in terms of beach access.
Beach Groyne 7 could trap similar amounts of sand to the existing groynes. Due to their distance from the access point, would be more difficult to construct and less valuable to beach users.
Beach Groyne 8 could need to be modified to mitigate and smooth the downstream scour effect. Point Groyne 3 could be positioned in a favourable location to trap sand due to the change in orientation of the coastline to a more northerly angle, while waves are of reduced intensity. However, due to its short length the sand trapped there would be limited. Any maintenance work on this groyne will need to consider limited access issues related to the position of Point Groyne 4, blocking access for mechanical equipment such as excavators. Point Groynes 1 & 2 perform poorly in the MCA as they are not suitably located and generally pose challenging construction and maintenance issues as well as a significant impact to visual amenities. The orientation of the coastline relative to the dominant wave direction would mean that only small amounts of sand could be trapped at any one time behind these groynes. Point Groynes 1 & 2 would need to be larger (similar to breakwaters) to withstand the wave and storm conditions and will have little no effect on sand accumulation. Structures of this size are not required for protection and sand build-up at other existing and proposed groyne locations further within the Bight.
5.3 Limitations – model and groyne design limitations
There are limitations in simulation of sediment and wave processes in surf zones due to the complex processes involved. The nature of the intermittent sediment supply at the site in combination with complex wave and current interactions means that conceptual models, and empirical calculations should also be used to provide sound engineering judgement. Such methods have been employed where possible, to avoid uncertainty in our recommendations.
This project did not address hazard identification and risks associated post-construction. These are significant as the beach is in the public realm, nor does it consider the environmental implications of groynes within the
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6 CONCLUSION
This study investigated the sand trapping efficiency and beach amenity improvements of eight additional groynes proposed by the CDO for Point Lonsdale Front Beach. Four Groynes are proposed at the Point and four groynes along the northern section of Front Beach.
The proposed concept was reviewed and engineered to meet the necessary level of performance required for structural safety. The four Beach Groynes are technically feasible, however the Point Groynes require substantial strengthening to meet stability requirements.
The estimated cost of the resulting Groyne field modification is approximately $4.5 million and would take at typically 5 months to be built considering the tidal limitation, particularly for the Point Groynes.
Performance, construction and logistical considerations were investigated using a range of numerical modelling and engineering techniques. On balance, the CDO Beach Groyne 5 provides the most effective outcome for improving beach amenity. Point Groyne 4 and Beach Groyne 6 would also be good at improving beach amenity.
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