point lonsdale groyne investigations

Point Lonsdale Groyne Investigations Long term options to maintain a sandy beach

22 August 2018 Cardno i

Point Lonsdale Groyne Investigations

Long term options to maintain a sandy beach

59918185

Prepared for

The Department of Environment, Land, Water and Planning

22 August 2018


© Cardno. Copyright in the whole and every part of this document belongs to Cardno and may not be used, sold, transferred, copied or reproduced in whole or in part in any manner or form or in or on any media to any person other than by agreement with Cardno.

This document is produced by Cardno solely for the benefit and use by the client in accordance with the terms of the engagement. Cardno does not and shall not assume any responsibility or liability whatsoever to any third party arising out of any use or reliance by any third party on the content of this document.

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Contact Information Document Information

Cardno Victoria Pty Ltd

ABN 47 106 610 913

Level 4

501 Swanston Street

Melbourne 3000

Australia

www.cardno.com

Phone +61 3 8415 7777

Fax +61 3 8415 7788

Prepared for The Department of

Environment, Land, Water

and Planning

Project Name Long term options to

maintain a sandy beach

File Reference 59918185_PtLonsdale_Groy

neIvestigation_Rev0.docx

Job Reference 59918185

Date 22 August 2018

Version Number Rev 0

Author(s):

Phebe Bicknell

Senior Engineer

Effective Date 20/08/2018

Approved By:

Chris Scraggs

Principal Coastal Engineer

Date Approved 20/08/2018

Document History

Version Effective Date Description of Revision Prepared by Reviewed by

V1 Internal Draft Phebe Bicknell David Provis

Rev A Draft for Client Review Phebe Bicknell Chris Scraggs

Rev B Draft for Client Review Phebe Bicknell Chris Scraggs

Rev 0 Final Phebe Bicknell Chris Scraggs


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Table of Contents

1 Introduction 6

1.1 Overview 6

1.2 Scope of Work 7

1.3 Methodology 8

2 Study Area 9

2.1 Background 9

2.2 Site Inspection 12

2.3 How groynes work 14

3 Existing Conditions 15

3.1 Currents 15

3.2 Waves 15

3.3 Sediment Transport 17

4 Concept Option Development 20

4.1 Preliminary Options 20

4.2 Concept Options 22

4.3 Options Summary – Layout, Efficacy & Considerations 23

5 Option Evaluation 30

5.1 Evaluation Criteria 30

5.2 Evaluation of options 31

6 Conclusions & Recommendations 38

7 References 39

Tables

Table 3-1 Mean wave conditions 16

Table 3-2 Summary of wave model cases 16

Table 4-1 Summary of Groyne Variables 20

Table 4-2 Groyne material options for Point Lonsdale 21

Table 4-3 Option: Maintain existing rock groynes 24

Table 4-4 Option: Extend existing groynes - Rock 25

Table 4-5 Option: Add additional groynes – Rock 26

Table 4-6 Option: Add additional groynes – Timber/Polymer 27

Table 4-7 Option: Combination – Rock & Timber Groynes 28

Table 4-8 Option: Combination – Rock & Timber Groynes with Small Breakwater 29

Table 5-1 Criteria Definitions 30

Table 5-2 Evaluation of Options – Summary 31

Table 5-3 Evaluation of Options – Detailed analysis 32


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Figures

Figure 1-1 Study Area (Nov 2017, Nearmap image) 6

Figure 1-2 Methodology Flow Chart 8

Figure 2-1 Annotated site map (Nearmap Image, Nov 2017) 13

Figure 2-2 Diagram of sediment movement along a beach and trapping by a barrier such as a groyne. 14

Figure 3-1 Distribution of wave direction and significant wave height (Hs) for waves approaching the Entrance from Bass Strait (data from PoMC buoys south east of Pt. Nepean) 16

Figure 3-2 Average wave energy vectors, with arrow length representing energy level; and direction of the arrow the direction of the wave energy at the origin of each arrow. (Nearmap Nov 2017) 17

Figure 3-3 Modelled shoreline at site of interest with existing groyne structures from an average 1-year wave time series. (Nearmap Nov 2017) 19


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Glossary

Symbol Name Definition

AHD Australian Height Datum

The Australian Height Datum is a geodetic datum for altitude measurement in Australia, "In 1971 the mean sea level for 1966-1968 was assigned the value of zero on the Australian Height Datum at thirty tide gauges around the coast of the Australian continent”

BoM Bureau of Meteorology

CD Chart Datum The datum to which soundings on a chart are referenced. It is usually taken to correspond to a low-water elevation, typically the lowest astronomical tide.

Crest Height The height of the crest of the structure

Ebb tide Outgoing or receding tidal current, (that is, when water is flowing away from the shore line) leading to low tide.

Fetch The maximum distance over water that winds of a given direction can generate waves. Areas such as Western Port Bay are defined as fetch-limited meaning that wave heights will always be restricted by the area over which wind can blow.

Groyne A rigid structure on a shoreline that interrupts water flow and limits the movement of sediment. These structures are effective at trapping material as it is moved along the coast by longshore drift, creating beaches or preventing them being washed away.

Hs Significant wave height The significant wave height is originally defined as the average height of the largest one third of the waves in a given record. With the advent of digital processing techniques and spectral analysis of wave records, the significant wave height is now commonly defined as Hs=4 √m0, where m0 is the variance of the wave spectrum or the “zero order moment”. For the purposes of this study the definition based on variance is used. Wave heights referred to in this report are the significant wave height.

For practical purposes, the significant wave height is close to the value reported by an experienced observer making visual observations of the wave height.

Tp Spectral peak wave period

The period associated with the peak of the wave energy spectrum

Pdir Peak wave direction The wave direction associated with the peak of the wave energy spectrum. Directions are given as the direction the waves are coming from.

SLR Sea-level rise

Swell

Waves which have travelled away from the area where they were generated such as a remote storm system. Often appearing as a series of regular spaced waves of unbroken appearance. The waves may have travelled many 100’s of kilometres from their point of origin.

Tidal current Movement of water associated with the rise and fall of the tides

Wave direction

Direction from which the waves are coming from and are given as the bearing, in degrees, clockwise from true north.


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1 Introduction

1.1 Overview

The Department of Environment, Land, Water and Planning (DELWP) has commissioned Cardno to investigate long-term beach management options in the form of groynes to maintain sand at the Point Lonsdale Front Beach, starting from in front of the shops, up to the commencement of the rock revetment. This investigation looks to build on the existing efforts towards maintaining a long-term sandy beach.

Figure 1-1 Study Area (Nov 2017, Nearmap image)

The natural processes for this dynamic section of coast at Point Lonsdale have been significantly changed through intervention from manmade infrastructure including masonry seawalls, rock revetments and timber and rock groynes. The state of the beach fluctuates following exposure to certain storm events and at various times of the year.


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Modelling and analyses are required to understand the effectiveness the existing and alternate configuration opportunities for using groynes to capture and hold a beach at this location. This project aims to undertake a technical investigation to explore and present possible options, providing a scientifically informed analysis. The potential solutions from this assessment are to be presented in an Open House Community session to inform an outcome that reflects the feedback and opinions of this community.

1.2 Scope of Work

DELWP’s key objectives as part of this project are to:

> Understand the coastal processes at Point Lonsdale Front Beach and existing groyne configurations actions in influencing sand supply;

> Identify and model options with the objective of maintaining a sandy beach

> Compare options

> Present technical report to major stakeholders

Deliverables:

Report – Presenting long-term options to maintain a sandy beach

Posters – “butcher paper” style poster for each option conveying both the technical aspects and development criteria of each option. Intended to promote discussion and feedback from the community.


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1.3 Methodology

Cardno’s methodology was designed to align with DELWP’s scope of works (Figure 1-2). This method was informed by our experience and knowledge of the Port Phillip Heads (PPH) and Point Lonsdale foreshore and similar groyne investigations within Port Phillip Bay. Each of the components to be undertaken as part of the Project is presented in more detail the sections below.

Figure 1-2 Methodology Flow Chart

> Data Inputs – Site data (survey, wave measurements, available site literature) was combined with experience and knowledge from Cardno’s previous work was used as the basis of the project’s analytical approach.

> Analysis & Design – Existing wave models will be used to establish wave climate to informed the development of groyne concepts. The most suitable groyne options were selected for detailed analysis through sediment transport modelling. Modelling results were used to update and refine options.

> Outputs –

- Technical Report – This document presents the results of the investigation and includes sections on site context, coastal processes overview, options development, modelling and analysis details, evaluation of options and recommended solution.

- Community Materials Posters – While conducting modelling, analysis and design, focus on was also placed on generating deliverables that will be used to convey this information to the wider community. through was mapping, figures and tailored content.


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2 Study Area

2.1 Background

2.1.1 Overview

Point Lonsdale foreshore is a highly modified section of coast with a combination of seawall, revetments and groynes. The seawall acts to increase wave reflection which scours the beach in front. High volumes of sediment transport through the system in a north eastwards direction which lends itself to the possibility of sand capture through the use of groynes. The beach is underlain by a rock platform, which is intermittently exposed and buried by sand.

Seasonal conditions see the beach fluctuate at different times of the year. As sand movement is most dynamic during storm events with increased waves and water levels, the beach can become eroded during these periods (typically during winter), with the beach rebuilding again over the calmer periods. As a result, the beach is often still in a state of rebuilding during high foreshore usage periods, December and January.

Various groynes have been present at Point Lonsdale, and over the years, there have been changes to the number, location, lengths and material of these structures. Currently three groynes, a combination of rock and timber, are in place and appear to be effectively holding sand on the beach, although the spacing of these structures means the beach is not always continuous. However, the functionality of these structures may change in the future.

Various options have been identified in previous studies that may be effective in keeping a beach at Point Lonsdale. These options are to be considered in further detail, to determine the feasibility and likely success of such a solution. The recent BMT WBM investigation (BMT WBM, 2017), states that the existing groynes provide some sand retention during low sand supply and that continued maintenance and ‘do nothing’/status quo is a valid short and long-term option. While it may not provide a continuous beach for the entire foreshore, it does act hold enough sand such that there is some beach present at all times of the year.

2.1.2 Foreshore management

The management of the Point Lonsdale foreshore and its assets is primarily the responsibility of the Borough of Queenscliffe. However, it is noted that DELWP also play an important role in coastal protection assets, here and throughout the state.

Currently the foreshore assets, including groynes, seawalls and revetments undergo minor maintenance on an “as needs” basis. In recent years’, significant maintenance undertaken to the groynes has included rock armouring of the timber groynes and removal of relic/remnant groynes. There is no manual redistribution of sand undertaken on the beach, meaning all sand placement is driven by the natural processes and their interaction with the existing groyne structures. The MNP jurisdiction for the area means that any major works are done in close consultation with Parks Victoria

2.1.3 Site significance

Highly renown for the environmental, recreational, cultural heritage and economic values that it provides for the community and its visitors, Point Lonsdale is recognised as a site of significance. Part of the beach is protected, with portions of the Point Lonsdale foreshore and nearshore areas of Lonsdale Bight located within the Port Phillip Heads Marine National Park (PPHMNP), as park boundaries extend to High Water Mark, the Point Lonsdale foreshore and its surrounds are highly protected. An overview of the Marine Park values is shown here, as outlined in the BMT WBM review (BMT WBM, 2017).


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The dynamic nature of the sediment transport processes means that nearshore habitats and reefs are intermittently exposed because of changing transport and sand coverage. Human intervention, such as seawalls and groynes, has already significantly impacted the natural processes. The potential environmental, cultural and recreational impacts of making additional changes to the foreshore and its structures will be an important consideration in determining the feasibility of options. A critical aspect in evaluating the potential options is ensuring that these values are not compromised.

The approvals process associated with constructing or modifying groynes at this site will also require consideration.

2.1.4 Approval Requirements

Point Lonsdale is one of six marine areas or sections that make up the Port Phillip Heads Marine National Park (PPHMNP). As such, the area is protected under the National Parks Act 1975, National Parks Regulations 2013. The management objectives for marine national parks and sanctuaries are set out in Section 17D National Parks Act 1975. Which states:

(3)(a) The Secretary (DELWP), subject to this Act will ensure that each marine national park and marine sanctuary is controlled and managed in accordance with the objects of this Act in a manner that will –

(i) preserve and protect the natural

environment and indigenous flora

and fauna of the park and any

features of the park which are of

geological, geomorphological,

ecological, scenic, archaeological,

historic or other scientific interest;

and

(ii) promote the prevention of the

introduction of exotic flora and fauna

into the park; and

(iii) provide for the eradication or control

of exotic flora and fauna found in the

park; and

NATURAL VALUES

> Intertidal reef platforms, supporting diverse invertebrate

> Diverse and abundant algal assemblages on Lighthouse Reef;

> Deep undercuts in the Lonsdale Reef, supporting algae communities more typical of deeper waters;

> Diverse fish and invertebrate assemblages on the Lonsdale Wall, including extensive encrusting communities such as ascidians, bryozoans and sponges;

> Calcarenite shore and reef platforms- significant for shorebird feeding; and

> Threatened marine mammals such as Australian Fur Seal, Southern Right Humpback Whales

CULTURAL VALUES

> Heritage-listed shipwrecks and shipping history;

> Botanical and scientific survey sites of research and historical significance;

> Historical significance - One of the first marine reserves protected in Victoria;

RECREATIONAL VALUES

> Swimming, surfing, walking, birdwatching, nature observation and passive recreation leisure activities

> Internationally recognised subtidal reef dive sites

> Snorkelling sites between Lightning Reef and the Point Lonsdale Pier

> Recreational boating and sailing

> Opportunities for guided marine education and nature-based tourism

> Scenic landscapes.


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(b) subject to paragraph (a) –

(i) provide for the use, enjoyment and

understanding of Marine National

Parks and Marine Sanctuaries by the

public; and

(ii) promote and understanding of the

purpose and significance of Marine

National Parks and Marine

Sanctuaries; and

(c) prepare a plan of management in

respect of each marine national park

and each marine sanctuary.

The Point Lonsdale Section of the section of the PPHMNP is recognised for its ecological values and protected as one of the first marine reserves in Victoria since 1979. In summary, this section includes:

Intertidal reef platforms that contain the highest invertebrate diversity of any calcarenite reef in Victoria

Diverse and abundant algal assemblages on Lighthouse Reef

Deep undercuts in the Lonsdale Reef with algae communities more typical of deeper waters — a geological feature that is seldom found along Victoria’s open coast

Part of the area occupied by the Port Phillip Bay entrance deep canyon marine community falls within the boundary of the Point Lonsdale section of the Port Phillip Heads Marine National Park. The community is dominated by sessile invertebrates, predominantly sponges, with ascidians, bryozoans, hydroids and corals. The biota is diverse in terms of the variety of sponges and other invertebrate forms present (including species listed as protected under the Victorian Fisheries Act 1995, and the EPBC Act 1999 - weedy seadragon Phyllopteryx taeniolatus. And is one of two marine communities in Victoria listed under the FFG Act 1988.

Calcarenite shore and reef platforms that are of regional significance and state significance for shorebird feeding habitat (including species listed under the FFG Act 1988 or the Victorian Rare or Threatened Species (VROTS) list.

The relevant plan for the Point Lonsdale section is the Port Phillip Heads Marine National Park Management Plan (2006) (the Management Plan). The Management Plan provides a strategic guide for the management of the park. The Management Plan states that a number of uses and activities may be permitted in the park, subject to specified conditions to minimise impacts. Section 7 of the Management Plan (Strategies for Authorised and Adjacent Uses) includes the following strategies:

Review all uses of the park that do not conform with the objectives of the National Parks Act.

Allow uses to continue only in accordance with authorisations that are consistent with legislation and include conditions that effectively minimise the impacts of uses on the park.

Permit authorised uses with appropriate conditions, and monitor authorised activities to ensure conditions are met. Assess the effectiveness of conditions of authorisations in protecting the park and seek review of authorisations if necessary to mitigate impacts.

In summary, it would appear possible for a consent under the Marine and Coastal Act 2018 to be issued by the Secretary or their delegate. However, the development of the groynes would need to be considered against the objectives of the National Parks Act 1975. Any impacts on the recognised ecological values would need to be minimised so as not to affect the natural environment.

A review of the project against these objectives should be completed as part of the next phase of the study.


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2.2 Site Inspection

A site visit of the study area was undertaken by Cardno on June 1, 2018. Arriving on site at 9:30 am, the tide was in flood phase with low tide occurring at 7:19 am and high tide at 2:23 pm. The study area comprises of three groynes delineating four beach compartments, starting from the end of the seawall in the south to the start of the rock revetment to the north.

Inspection of the Point Lonsdale front beach delivered the following key observations:

Compartment 1 – 120 m in length from southern seawall to southern groyne (Groyne 1). The compartment was full to groyne height in the northwest corner, at least 1 to 1.5 m higher than in the southwest corner due to angle of the incoming waves. The beach slope is relatively steep in this corner.

Groyne 1 – Southernmost, 70 m in length. The original timber structure has been armoured with rock material. Some of this rock material had been displaced on the south side toward the landward end. The north side of the timber had solid rock interlocking which maintained structure integrity.

Compartment 2 –140 m in length. Much higher sand build-up in the north west corner than the south west corner, with a mound approximately 10-15 m south of Groyne 2, reaching almost to the top of the seawall.

Groyne 2 – Middle groyne, 55 m in length. Approximately 1 m diameter rock armour, with no timber in this structure. Some rock movement had occurred, not all being interlocked, exposing the geotextile layer.

Compartment 3 – 230 m in length. The beach was scoured around the outlet of a storm water drain outflowing from the seawall. Half way along, around the bend of the cliffs, the exposed seawall was 2 m in height, showing a low beach level. Water level was at the toe of the seawall, with the base exposed. Beach builds back up from the access point to Groyne 3 at the northern end of the compartment and reaches the top of the sea wall.

Groyne 3 – Northern groyne 50 m in length. Similar structure to Groyne 1 with timber running through the middle of rock armour, though armour is a little less organised and interlocked, and shorter in length.

Compartment 4 – Running from Groyne 3 to the start of the rock revetment at 270 m. Very big compartment with similar scour/exposed seawall footing halfway along.

The breaking waves were visually turbid, having high amount of sand (brown/discolour) in the water.

Given the highly modified nature of this coastline, complex interactions between the waves, the headland and the masonry seawall, particularly toward the southern end, scouring sand away for this section of beach in front of the shops.

Winds was light, meaning waves on the day were mostly swell incoming at an angle 45 degrees to the general beach alignment, going toward the north west. This wave direction may explain the battered condition of the groynes on the south sides, with the angled waves impacting on the groynes which run approximately east west). Angled groynes better aligned with the dominant wave direction may be a design option.

Three sediment samples were taken along the beach and particle size distribution (PSD) was carried out for sediment sizes for use in modelling.

Sample Location D50 (mm) D90 (mm)

SAMPLE 1: 10 m south of Groyne 1 and 35 m seaward of seawall 0.25 0.47

SAMPLE 2: Within Compartment 3, Beach Access Point toward north of compartment, near seawall

0.27 0.57

SAMPLE 3: Within Compartment 2, 10-15 m south of Groyne 2 and 20 m seaward of seawall

0.30 0.59


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Figure 2-1 Annotated site map (Nearmap Image, Nov 2017)

Locations known to have sand over seawall

Sand build up in North West corner to

height of seawall

Exposed footing of seawall where beach

had eroded

Long Compartments

Cut back in southern end of each compartment

Short interlocked rock armour groynes allowing sand to build up beyond length and transport

to next compartment

COMPARTMENT 2 COMPARTMENT 1

COMPARTMENT 3

COMPARTMENT 4 Groyne 1

Groyne 3

Groyne 2

N

Southern edge of Groyne 1 is damaged

Sediment Sample

Beach is regularly cut back, exposing rock


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2.3 How groynes work

In order to demonstrate possible groyne solutions for the Point Lonsdale Foreshore, it is important to understand how groynes work in shaping and holding the beach. When waves approach a beach at an angle, they tend to move sediment along the beach. When there is a barrier in the beach, such as a groyne, this captures sand which is moving along a coast and thus builds up a beach. This is represented diagrammatically in Figure 2-2.

Figure 2-2 Diagram of sediment movement along a beach and trapping by a barrier such as a groyne.

When sediment builds up sufficiently, it will be moved around the end of the barrier or groyne into the next beach compartment or cell. The sediment on a beach will move until the beach is aligned at right angles to the direction of the incoming wave energy. Thus in a situation such as Point Lonsdale, where the wave direction is relatively constant (or at least predominantly from one direction) (see Section 3.2), groynes are often used to maintain a beach, or series of beaches, along a general alignment which would otherwise not allow the formation of a beach other than a narrow strip of sand.

Barrier or groyne


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3 Existing Conditions

Cardno have undertaken various projects at this site, initially as part of the Vantree (1998) investigation when initial modelling of waves and potential groyne configurations was included and estimates of the longshore transport undertaken. Subsequent investigations are included in the studies as part of the Channel Deepening Project (Cardno Lawson Treloar, 2007 and Cardno, 2011) and include modelling of waves, currents, water levels and sediment transport. Existing hydrodynamic models (DELFT3D) and wave models (SWAN) have been tailored for this analysis, to determine the nearshore metocean conditions at Point Lonsdale. Previously calibrated models were updated using the bathymetric data available from LADS surveys undertaken as part of the Future Coasts Project by in 2007 and 2008/9. Offshore areas, not covered by LADS and Lidar surveys, using bathymetry derived from the Australian Bathymetry and Topography Grid, Geoscience Australia (2009) which has a resolution of 9 arc seconds. (~ 250 m at the equator).

3.1 Currents

3.1.1 Model

The hydrodynamic model (Delft3D) was used to define the astronomical tidal-currents at the site. Currents and water levels at Point Lonsdale were extracted from a validated hydrodynamic model for Port Phillip, including Port Phillip Heads, the entrance and adjacent waters of Bass Strait. This numerical model was developed through the FLOW module of the Delft3D modelling system developed by Deltares.

For swell penetration, swell in Bass Strait imposed at the relevant boundary of a SWAN wave model (Ris et al. 1999) based on the results of analysis of waves from the Port of Melbourne wave data at Port Phillip Heads. The current fields from this modelling (ebb, flood and slack scenarios) were used in the swell-wave modelling below.

3.2 Waves

Offshore wave measurements are available from Triaxys wave buoys located approximately 6 km south east of Point Nepean. These measurements are available from 2003 and a five-year period 2011-2015 inclusive has been selected for use in this study (Cardno, 2016). The analysis of the wave climate shows the dominance of swell waves from the south west with some contribution from locally-generated wind waves. Figure 3-1 shows the distribution of significant wave height against incoming wave direction and demonstrates the very narrow range of directions, especially for larger wave heights.


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Figure 3-1 Distribution of wave direction and significant wave height (Hs) for waves approaching the Entrance from Bass Strait (data from PoMC buoys south east of Pt. Nepean)

An analysis of five years of wave measurements within Port Phillip Heads, from 2011 to 2015 established a mean wave climate.

Table 3-1 Mean wave conditions

Hs (m) 1.52

Tp (s) 11.5

Dir (from) 207°T

Frequency of occurrence tables were used to determine suitable scenarios for modelling in order to reproduce a representative wave climate at Point Lonsdale. The cases selected for the major variables are shown in Table 3-2. Combining each of these gives a total of 54 scenarios. This combination of scenarios covers the majority of the actual wave conditions for the 5-year measurement period.

Table 3-2 Summary of wave model cases

Variable Cases for each variable

Currents (3 cases) Ebb Flood Slack

Hs (m) (3 cases) 0.5 1.5 2.5

Tp (s) (3 cases) 11 13 15

Directions (2 cases) SSW (202.5°T) S (180°T)

Using a year of measured wave data (2014) combined with all the model runs, wave time-series were synthesised at various nearshore locations along the Point Lonsdale foreshore. For each measured condition, the relevant model run was selected and the model results for that case selected for each of the output locations selected in the study area. The resulting time series were used to calculate an annual average wave-energy for each location. In order to determine suitable groyne options, the likely stable shoreline alignment must be determined. The direction at which the average wave-energy approaches the shoreline provides a robust measure of the potential coastline alignment. The coastline is generally stable at an alignment approximately perpendicular to this wave-energy direction. Note that this direction is an average and in reality will vary at different times of the year with the seasonality of the local wind climate, however the offshore wave directions at this location do not vary markedly with the seasons.


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Figure 3-2 Average wave energy vectors, with arrow length representing energy level; and direction of the arrow the direction of the wave energy at the origin of each arrow. (Nearmap Nov 2017)

The average wave energy direction seen here shows excellent alignment with the shoreline and breaking waves in the Nearmap image from November 2017. This was consistent with the wave direction noted during the site inspection. It clearly demonstrates the shoreline equilibrating approximately perpendicular to the wave energy.

Several observations can be made from these results. The average wave direction was reasonably consistent, while the wave energy is reducing to the north along this coastline. The directionality confirms that, for the existing groyne configuration, the spacing between the groynes is too long for groynes of this length, or conversely, groynes are too short for compartments of this distance. Therefore, a sandy beach cannot be held within the entire length of the compartment.

It is understood that there is some variability around the average alignment of the shoreline. This can lead to more or less beach width in each compartment. There was no clear seasonality in the average wave energies. However, the measured waves from offshore (Figure 3-1) show that waves from the easterly quadrant do occur and these may be more significant at Point Lonsdale, which is more exposed to this direction than the offshore wave buoy location. Waves from the east would cause a temporary realignment of the shoreline. Similarly, large waves occurring with a high sea-level, such as during a storm, are likely to result in sand movement seawards off the beach and into the general longshore currents experienced during such events. This can result in a significant loss of sand and require some time for the beach to rebuild during less severe weather conditions.

3.3 Sediment Transport

There is a significant amount of longshore sediment transport occurring along this section of coast, coming from the open coast to the west, moving and in a northerly direction around the headland at Point Lonsdale, up towards Dog Beach and eventually around to Queenscliff. Volumes transported are estimated to be 80 to 100,000 m3/year (e.g. Vantree, 1998). The amount of sediment movement is such that you can regularly see suspended sediment in the nearshore zone when looking down on the water from cliffs and pathways above.

Using the wave time-series synthesised at various nearshore locations along the Point Lonsdale foreshore, a one-line coastal processes model has been developed using the LITLINE module of the LITPACK coastal processes modelling system.


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This modelling system has been developed by the Danish Hydraulics Institute. It is used internationally for the assessment of coastal processes. LITPACK includes a number of modules. One of these, LITDRIFT, computes longshore sediment transport from a time-series of wave parameters. Natural beach profiles, graded sediments, currents, wind and local roughness are included. Generally, the highest transport rate occurs in the breaking wave zone.

LITLINE is a module of LITPACK and is used to determine changes to a shoreline over time using spatially and temporally varying longshore transport. It includes coastal structures such as groynes and revetments (seawalls). In this case, the existing groynes were included in a model of the Point Lonsdale shoreline. Further structures, such as groynes and offshore breakwaters, were later added, with the difference being offshore breakwaters are not modelled to trap sand where groynes are, though both still allow for wave diffraction effects. Note that groynes in LITLINE can be specified to have an apparent length; that is, sand bypassing can commence before sand builds-up fully on the updrift side to the full length of a groyne. This process is important because it more closely describes what happens naturally than the alternative of delayed bypassing. The extent of bypassing depends on this apparent length and the shore normal profile of longshore sediment transport in different wave and water level conditions. The operating length of a groyne is also dynamic in terms of the shore normal profile and reduces as a beach builds-out against the updrift side of a groyne. LITLINE includes realistic shore normal beach profiles, the active depth limitation and dune height in computation of shoreline changes.

LITDRIFT and LITLINE use the basic Engelund and Fredsoe (1976) transport formulation which includes combined wave and current motion as well as bed and suspended sediment loads. It takes account of the threshold shear stress for initiation of sediment transport through the Shields Parameter and includes graded sediments.

For this study, basic beach profile survey data has been used to develop existing shoreline profiles using a simplified straight coastline. The coastline is defined as the 0 m AHD contour line and alongshore grid points were set at 10m intervals. In the shore normal direction, the resolution was 2.5 m so that a good description of nearshore wave breaking could be achieved. A median sediment particle size (D50) of 0.27mm was adopted, based on sediment sizing.

Using the data described in Section 3.2, a wave parameter time series spanning 1-year was used to assess the longshore transport. Shown in Figure 3-3, the shoreline has been modelled using this wave parameter time series along with the existing shoreline groyne structures placed into the model.

While the aerial image represents a snapshot in time, the modelled shoreline with the inclusion existing groyne structures closely relates to the actual shoreline. These results give context to the dynamic coastline and validate the model.


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Figure 3-3 Modelled shoreline at site of interest with existing groyne structures from an average 1-year wave time series. (Nearmap Nov 2017)


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4 Concept Option Development

4.1 Preliminary Options

Based on the existing conditions at Point Lonsdale, three options have been identified as feasible ways to hold the beach at the Point Lonsdale Foreshore. Three preliminary options have been considered:

- Do nothing

- Construct new groynes

- Modify existing groynes

Within these options there are various configurations and design variables that will influence the feasibility of the options. Some of these influence the functionality and effectiveness of the structure in retaining a long term beach. Some also have secondary effects, such as safety, maintenance requirements, costs, environmental effects. Key variables include alignment, crest height, material and aesthetics (Table 4-1).

Table 4-1 Summary of Groyne Variables

Variable Description

Alignment This refers to the angle of the structure. Groynes function most effectively at retaining a beach when they are aligned with the mean wave direction.

The existing groynes at Point Lonsdale are at an angle approximately perpendicular to the seawall, which is different to the average wave direction (by ~30° to 40°). This alignment exposes the groynes to larger wave forces. In this situation, the structures act more like a breakwater, dissipating wave energy rather than just holding sand. The damage evident on the most southern groyne is evidence of this exposure.

Maintain existing alignment

Adjust the alignment to mean wave direction

Crest Height

This refers to the height of the structure. The existing height is effective in capturing sand, raising the height could capture more sand.

With the existing configuration and alignment, increasing the height will not change the shoreline orientation, but it may raise the height and extent of the beach. Given the existing compartments are quite full at the northern end of each compartment, it is likely to result in overtopping of the groyne and the seawall, with sand encroaching on the footpath as the compartment becomes “overfull”. This is already occurring intermittently under existing conditions, and clear sand build up was noted during the site inspection.

An increased crest height can also result in safety risk as a result of drop off between the compartments.

Maintain existing crest heights

Increase crest height

Material This refers to the material used to build the structure. Materials have varying lifespans, design constraints, costs, maintenance requirements and constructability implications.

Further considerations regarding the material options are presented in Table 4-2

Rock

Timber

Polymer

Concrete

Geotextile

Aesthetics This refers to the appearance of the structure conserving how it looks and fits in with its surrounds. Factors that can influence the look and feel of the design include the size and crest height, alignment and material selection


59918185 | 22 August 2018 21

Table 4-2 Groyne material options for Point Lonsdale

Material Approximate Cost

Advantages Disadvantages

RO

CK

$5000/m Low Maintenance

Resilient during a storm

Sustainable material

In keeping with surrounds

Large footprint (20 times wider than a vertical structure)

Most expensive

Relatively high demolition cost

TIM

BE

R

$3500/m Small footprint

Relatively cheap and easy to build

Timber piles are durable

Sustainable material (if exotic timbers are avoided)

Relatively high maintenance – Timber planks are vulnerable to rot and marine borer

High reflection from vertical wall can lead to greater scour during storm events

Protruding elements can be a safety hazard if not maintained.

Vertical drop on the downdrift side may be a safety hazard.

PO

LY

ME

R C

OM

PO

SIT

E

$4000/m Small footprint

Very low maintenance

Can be made to look like timber or sand colour

Can be used in combination with durable timber piles

UV degradation can cause colour to fade

Subject to vandalism

Relatively high upfront cost for a vertical structure

Longer ‘lead times’ for material generally

Vertical drop on the downdrift side may be a safety hazard.


59918185 | 22 August 2018 22

Considering each of these variables in conjunction with various combinations available for the preliminary options, the list of feasible options can be reduced. Key exclusions for the more detailed analysis are:

> Usage of timber and polymer for extended/longer groynes - larger wave energies further offshore, not able to withstand these loads. A timber structure of this length would also create a significant drop-off hazard on the northern side of the groyne.

> Usage of timber and polymer on the existing alignment – history has shown that timber groynes are not durable enough to withstand wave energy on the existing alignment. Increased wave loads experienced in this scenario

> Raising of groyne heights is likely to result in increased sand accumulation in against the seawall. Levels are likely to overtop and encroach of the footpath, limiting access and creating hazards on the pathways. Maintenance required to manage this. Additionally, increased levels on the southern side will increase the drop-off hazard on the northern side of the groyne.

> Geotextile and concrete were excluded as material options, predominately due to the associate costs. Geotextile bags would end up being a similar price to rock but are unlikely to offer the same durability, likely needing more regular maintenance and repair.

The preferred structure orientation should be similar alignment to the mean wave direction. In determining modelled shoreline resulting from different groyne configurations, LITLINE modelling assumes the groyne is positioned perpendicular to the coast. Therefore, as part detailed design, consideration should be given to aligning the structures closer to the wave direction, as it will likely result in an improved shoreline response and lessen the structure exposure (and hence structure longevity). It has not been considered further as part of modelling or options analysis.

4.2 Concept Options

Six concept options have been developed, based existing configuration, metocean conditions and the likely coastline alignment as per the calculated average weighted wave direction.

Option – Maintain existing rock groynes

Option – Extend groynes – Rock

Option – Add additional groynes – Rock

Option – Add additional groynes – Timber/Polymer

Option – Combination: Rock & Timber Groynes

Option – Combination: Rock & Timber Groynes with Small Breakwater

The layout of each option are presented (Table 4-3 to Table 4-8).

For the purposes of comparison, each option has been applied to the entire study area extent, compartments 1 to 4. However, given the sediment transport mechanism and beach response is compartmentalised, there is the opportunity to construct the groynes in a staged manner, likely prioritising the southern and middle compartments (Compartment 1 and 2 as per Figure 2-1).

The numbers, positions, alignment and lengths of the structures are indicative of what is required to maintain a beach at this location and have sufficient level of specification detail for option evaluation. Precise dimensions, would need to be confirmed at detailed design.

The nominated locations of new structures have been selected based on the distance between compartment and the shoreline alignment response due to the wave angle. The existing beach access points have also been considered.

An additional feature that can be incorporated into any new rock structures is to have a lowered crest height, (potentially buried) at the shoreward end, to improve the connectivity and beach access and enable a clearer passage for beach users along the beach between compartments. This will reduce the likelihood of overtopping, allowing some bypassing of sand over into the next compartment, as well as adding additional material to the, at times, sand depleted south west corners of the groyne compartments. This approach is consistent with response of the existing structures, having been later lowered at the shoreward end, and recommendations in the BMT WBM report, (BMT WBM 2017).


59918185 | 22 August 2018 23

4.3 Options Summary – Layout, Efficacy & Considerations

The concept options have been modelled though LITPACK to give a conceptual understanding of the resultant shoreline and efficacy of the option. A 1-year wave time series was applied to the region, generating a resultant shoreline. Note that the altering alignment of groyne structures is a more complex exercise, not something that can be easily applied within this simplified model. Given the time and scope of this project, groynes on wave directional alignment were not simulated. All options involving the addition of groynes have simulated the structure running perpendicular to the coastline.

The modelled shoreline response under average yearly wave climate has been presented for each option in the following tables (Table 4-3 to Table 4-8). The initial existing coastline has also been shown to demonstrate the possible change that can be made that option. Note that simulations have been undertaken using an average wave climate, and the shoreline position will still fluctuate at different times of year. However, this modelling does provide a reasonable indication of likely capture of sand and possibility for widening and lengthening of the beach through groynes.

Considerations, impacts and secondary effects have been highlighted for each option and have been incorporated in the option evaluation in Section 5.


59918185 | 22 August 2018 24

Table 4-3 Option: Maintain existing rock groynes

Option – Maintain existing rock groynes

CONSIDERATIONS

> No capital costs on new structures

> No further environmental impact

> No guarantee of beach all year around, beach will fluctuate

> The alignment of the existing groynes means they are exposed to higher wave loads, resulting in increased repair and maintenance requirements

> There may be an opportunity to undertake some redistribution of sand. However, it must be carefully timed to limit the possibility of loss of sediment to storms (suggest during the calm near the beginning of summer). The beach will still realign and will not provide a constant beach

MODELLED OUTCOMES

Shoreline dynamics and alignment are expected to continue behaving in the same way

Shoreline will continue to intermittently accrete and erode with changing metocean conditions

Seawall of northern compartments (three and four) will continue being vulnerable to wave action

Does not create a beach in front of shops


59918185 | 22 August 2018 25

Table 4-4 Option: Extend existing groynes - Rock

Option – Extend existing groynes – Rock

CONSIDERATIONS

> No increase of structure footprint on beach.

> Increased structure footprint offshore, near to MNP

> Increased beach width, but may not run full compartment length

> Very little change on visual aesthetics

> Asymmetric beach compartments, similar to existing alignment, with beach building out on the same angle, but positioned further offshore (wider beach)

> Constructability, working in water increasing cost

> Overtopping of sand at onshore end of structure likely to increase as more sand captured by longer groyne. Sand will accrete in north west corners, where sand may be blown onto pedestrian footpath backing seawall

> Encroaching on intertidal zone, MNP boundaries

> Safety concerns by having a structure extend further offshore, enabling people climbing on the structure to reach deeper water and bigger waves.

> Proposed extension length (~25-30% longer) is unlikely to create a continuous beach for the entire compartment, given the spacing between the existing groynes, in particular compartment 3.

> The alignment of the existing groynes means they are exposed to higher wave loads, resulting in increased repair and maintenance requirements

MODELLED OUTCOMES

Increased sediment trapping on the lee side of the extended Groyne 2

Little to no change to shoreline elsewhere


59918185 | 22 August 2018 26

Table 4-5 Option: Add additional groynes – Rock

Option – Add additional groynes – Rock

CONSIDERATIONS

> Reduced compartment size yielding a more even spread of sandy beach

> Increased control of beach erosion

> Maintain current beach and groyne extents, limiting incursion on MNP

> Increased structural footprint on beach

> Increased safety concerns with more structures on the beach

> Increased visual intrusion

> Modified beach connectivity with increased number of structures. There is an opportunity to lower shoreward end of groyne.

> Wave load on structures on this alignment

> Constructability working predominantly on land

> The alignment of the existing groynes means they are exposed to higher wave loads, resulting in increased repair and maintenance requirements, particularly the more southern groynes

> As wave height and energy varies throughout site, rock sizing will need to consider wave climate relevant to each individual location

MODELLED OUTCOMES

Greater coverage and spread of sandy beach across entire Front Beach

Beach width does not extend as far offshore, due to altered compartment size and shape

Extended beach to the south of Compartment 1


59918185 | 22 August 2018 27

Table 4-6 Option: Add additional groynes – Timber/Polymer

Option – Add additional groynes – Timber/Polymer

CONSIDERATIONS

> Easier constructability working, predominantly on land


> Likely to be shorter in length than existing groynes, as timber groyne can’t withstand wave loads in the deeper water. Requires more individual, smaller groynes

> Safety concerns from shear drop-off created by a vertical structure on the down drift side of the groyne and also due to more with more structures on the beach

> Maintains existing beach and groyne compartment extents, limiting incursion on MNP

> Increased structural footprint on beach - much smaller than rock, but more individual structures

> Modified beach connectivity

> Increased visual intrusion with more structures – but smaller impact than rock

> Could use polymer material instead of timber is a longer lasting option

> The timber/polymer structure would be continuous. Permeable timber groynes are not suitable for this site

If structure orientation be adjusted to align with average wave direction it would mean:

> Lower wave loads on structures on this alignment. Less maintenance required

MODELLED OUTCOMES

Greater coverage and spread of sandy beach across original Compartment 3.

Beach width is slightly narrower towards north of original Compartment 3

Little to no change to shoreline across original Compartments 1 and 2 (southern sections), suggesting less suitable option in compartments 1 & 2


59918185 | 22 August 2018 28

Table 4-7 Option: Combination – Rock & Timber Groynes

Option –Combination: Rock & Timber Groynes

CONSIDERATIONS


> Requires more individual, smaller groynes

> Increased control of beach erosion

> Safety concerns from shear drop-off created by a vertical structure on the down drift side of the groyne and also due to more with more structures on the beach

> Maintains existing beach and groyne compartment extents, limiting incursion on MNP


> Modified beach connectivity

> Increased visual intrusion with more structures

> Use of the polymer material instead of timber is a longer lasting option

> The timber/polymer structure would be continuous. Permeable timber groynes are not suitable for this site

With alignment as

> Lower wave loads on structures on this alignment. Less maintenance required

> Inconsistency with existing structures, may be less visually appealing. However, this is less noticeable at beach level.

MODELLED OUTCOMES

Greater coverage and spread of sandy beach across entire Front Beach, particularly in original Compartments 1 and 2

Beach width is slightly narrower towards north of original Compartment 3

Extended beach to the south of Compartment 1


59918185 | 22 August 2018 29

Table 4-8 Option: Combination – Rock & Timber Groynes with Small Breakwater


CONSIDERATIONS

As above in ‘Combination: Rock & Timber Groynes’

Design Considerations with respect to the Small Offshore Breakwater:


> Requires larger rock sizing, and structure dimensions to act as a breakwater

> Increased visual intrusion

> Increased safety concerns with more structures on the beach

> Modified beach connectivity with increased number of structures.

MODELLED OUTCOMES

As above in ‘Combination: Rock & Timber Groynes’

Modelled Outcomes with respect to the Small Offshore Breakwater:

Extended beach to the south of Compartment 1.

The breakwater at the southern end shelters this section of beach from the high wave energy and its scouring effects against the seawall, allowing the beach to build up.


59918185 | 22 August 2018 30

5 Option Evaluation

5.1 Evaluation Criteria

Evaluation criteria were developed to compare and evaluate the six options against defined evaluation criteria, tailored for the needs of this site and project. The following list has been developed in conjunction with the Project Control Group (DELWP, Parks Victoria, Borough of Queenscliffe), (Table 5-1)

Table 5-1 Criteria Definitions

Criteria Definition Scale

Technical Effectiveness

Likelihood of achieving a sandy beach under existing conditions. Considers how effective the structure at capturing and retaining sand.

VERY LOW (Poor Performance) to VERY HIGH (Excellent Performance)

Technical Effectiveness in the longer term (changing climate)

Likelihood of achieving a sandy beach under climate change conditions. Considers how effective the structure at capturing and retaining sand with increased waves heights and water levels.

VERY LOW (Poor Performance) to VERY HIGH (Excellent Performance)

Cost (Design, construct, maintain)

The upfront cost of building the structure - labour, materials, equipment.

The amount of construction required to build the structure. Considers complexities of on land/in water, material type, required equipment.

The ongoing costs associated with maintaining the structure - labour, materials, equipment.

VERY LOW (Low Cost) to VERY HIGH (High Cost)

Design Life

Longevity of the structure. Likelihood that the structure will experience failure under existing and future conditions, based on the material lifespan the location and the structure alignment.

VERY LOW (Low Maintenance/Failure) to VERY HIGH (High Maintenance/Failure)

Amenity

The space the structure will take up. Considers overall footprint - length, width, height, and the how this may effect visual amenity.

Impact on beach connectivity. Does it divide the beach into isolated sections?

VERY LOW (Low Amenity Impact) to VERY HIGH (High Amenity Impact)

Key Risks

Public safety - Potential hazard the structure presents. Considers drop-off in beach elevation between the two sides of the groyne (difference 1-2 m), beach users climbing on the structure, trip hazards, cuts/abrasions

VERY LOW (Low Impacts) to VERY HIGH (High Risk)

Environment/ Effects on Intertidal Reef Communities in PPH MNP

Effects on the intertidal reef, rockpools, biodiversity

Construction, footprint within the MNP

Modification to sediment transport

VERY LOW (Low Environmental Impacts) to VERY HIGH (High Environmental Impacts)

Further work required (Design, analysis monitoring)

Amount of extra work required for detailed design phase Include analysis, further data collect (survey)

VERY LOW (Low Cost) to VERY HIGH (High Cost)


59918185 | 22 August 2018 31

5.2 Evaluation of options

The evaluation criteria have been applied each option, where the comparison is in relation to the other groyne options. It does not consider other protection solutions / coastal structures or scenarios. Each criterion rated on a scale from Very Low to Very High.

As each criterion had a varying scale, rating have been coloured coded to convey well performing options, where green shading is a good result and red shading is a poor result.

Table 5-2 provides a summary of the evaluation while Table 5-3 presents further context around each of the ratings provided, consider for each individual option.

Table 5-2 Evaluation of Options – Summary

Criterion

Tech

nic

al

Effe

ctiv

en

ess

Tech

nic

al

Effe

ctiv

en

ess

-

lon

g te

rm (

chan

gin

g cl

imat

e)

Co

st (

De

sign

,

con

stru

ct,

mai

nta

in)

De

sign

Lif

e

Am

en

ity

Ke

y R

isks

Envi

ron

me

nt

/Eff

ect

s o

n In

tert

idal

Re

ef

Co

mm

un

itie

s in

P

PH

MN

P

Re

qu

ire

d

de

sign

/an

alys

is

/wo

rk

Option –

Do Nothing LOW LOW LOW LOW LOW LOW N/A MODERATE

Option – Extend existing groynes

BELOW AVERAGE

BELOW AVERAGE

MODERATE HIGH LOW MODERATE HIGH MODERATE

Option –

Add additional groynes – Rock

VERY HIGH

HIGH VERY HIGH

VERY HIGH

VERY HIGH

ABOVE AVERAGE

ABOVE AVERAGE

HIGH

Option –

Add additional groynes – Timber/Polymer

MODERATE MODERATE BELOW

AVERAGE BELOW

AVERAGE MODERATE MODERATE MODERATE

ABOVE AVERAGE

Option – Combination: Rock & Timber Groynes

VERY HIGH

ABOVE AVERAGE

HIGH HIGH HIGH ABOVE

AVERAGE HIGH HIGH


VERY HIGH

HIGH VERY HIGH

HIGH VERY HIGH

ABOVE AVERAGE

VERY HIGH

VERY HIGH

NB. The colour coding assigned to the rating changes due to varying rating scales for each criteria. In general, green is considered a GOOD rating, while red is considered a POOR rating.


59918185 | 22 August 2018 32

Table 5-3 Evaluation of Options – Detailed analysis

Criterion Technical Effectiveness

Technical Effectiveness in the longer term

(changing climate)

Cost

(Design, construct, maintain)

Design Life Amenity Key Risks Environment /Effects on

Intertidal Reef Communities in

PPH MNP

Further work Required

(Design, analysis monitoring)

Op

tio

n –

Do

No

thin

g

Some sand is being held on beach, but not continuous.

Strong dependence on seasonal influence.

Conditions are likely to remain similar, if not worsen overtime.

Similar to existing, though likely to erosion effects. Likely to be exacerbated with SLR and increases in strength and frequency of storm events

Design & Construction - N/A

Continual maintenance is likely required to structures due to existing design (poor alignment, undersized, no core and filter layers). Associated costs will be minor ongoing repairs to relay and replace displace rocks

Existing structures currently damaged. Already failing.

Alignment and size appear to be ill-designed, but are still functional. Likely to worsen with time unless significant repairs are undertaken.

Beach connectivity same as existing.

Footprint same as existing. Noting that structures are undersized.

Public Safety -

Large height offset between compartments from being too full in corners. Creates a drop off between compartments.

Sand blown over footpath.

No further influence.

No change to current effects experienced by the intertidal reef communities

Run a controlled trial of the redistribution sand within the compartment at calmer times of year.

RANKING LOW LOW LOW LOW LOW LOW N/A MODERATE


59918185 | 22 August 2018 33

Criterion Technical

Effectiveness


(changing climate)

Cost


Design Life Amenity Key Risks

Environment/ Effects on


PPH MNP



Op

tio

n –

Ext

en

d e

xist

ing

gro

yne

s

Some minor improvement, though not across entire beachfront

Small increases in beach width at select spots along coastline.

Average beach width increase -

Compartment 1: 0 m Compartment 2: Up to +15 m Compartment 3: Up to +5 m

Minor improvement, but not an effective long term solution

Will require some upgrade of existing structures to improve structure integrity.

Rock is more expensive material. Approximately $5000/m of structure. Less new materials required only extending existing and upgrading structures.

Construction effort increased working further offshore (deeper, larger waves)

Structure will be exposed to bigger waves being deeper.

Retaining existing orientation means structure is exposed to higher wave energies on this alignment

Design will need to consider conditions in sizing material and structure design.

Rock is long lasting and structure will last if suitably designed

Connectivity same as existing.

Increased offshore footprint.

Large height offset between compartments from being too full in corners. Sand blown over footpath.

Structure extends further offshore exposing public to larger waves and deeper water

Extends into

More sensitive zones of the MNP, closer to reefs.

Small change to sediment transport through Marine National Park.

Construction efforts entering Marine National Park.

Detailed design of groyne extension

Site Survey

RANKING BELOW AVERAGE

BELOW AVERAGE

MODERATE HIGH LOW MODERATE HIGH ABOVE

AVERAGE


59918185 | 22 August 2018 34



(changing climate)

Cost


Design Life Amenity Key Risks Environment/ Effects on


PPH MNP



Op

tio

n –

Ad

d a

dd

itio

nal

gro

yne

s –

Ro

ck

Shortened compartments improves beach holding capacity

Improved width across entire study area.


Compartment 1: Up to +25 m Compartment 2: Up to +30 m Compartment 3: Up to +30 m

Advise that structure is designed on an orientation that aligns with average wave direction.

Large Improvement across entire beach front

Large Improvement across entire beach front with more spread and symmetrical compartments

Rock is expensive material. Approximately $5000/m of structure

Works will be onshore and offshore.

Rock is long lasting and structure will last with site specific design.

If kept on same alignment as existing groynes, experiencing higher wave loads. Advise angling structure more in to average wave direction

More groyne structures on the beach will alter connectivity.

More large groyne structures extending offshore.

Increased number of beach structures.

Large rocks on rock flats.

Moderate change to region sediment transport.

Construction efforts within Marine National Park.

Works onshore and offshore

Detailed design of timber groynes

Site Survey

RANKING VERY HIGH HIGH VERY HIGH VERY HIGH VERY HIGH

ABOVE AVERAGE

ABOVE AVERAGE

HIGH


59918185 | 22 August 2018 35



(changing climate)

Cost


Design Life Amenity Key Risks Environment/ Effects on


PPH MNP



Op

tio

n –

Ad

d a

dd

itio

nal

gro

yne

s –

Tim

be

r/P

oly

me

r

Shorter compartments improves beach holding capacity, though timer structure unable to extend as far offshore

Notable increase in Compartment 3. Little change elsewhere. Average beach width increase -

Compartment 1: 0 m Compartment 2: 0 m Compartment 3: Up to +10 m

Timber structure should be designed on an orientation that aligns with average wave direction

Large improvement through Compartment 3; Little to no effect in Compartments 1 and 2

Timber is cheaper than rock.

Smaller structures. Less material.

Works largely on shore.

Approximately $3500/m of structure

Timber is less durable and not a strong as rock. Improved alignment will reduce repeat wave impact structure.

Increased number of smaller groyne structures on the beach will alter connectivity.


Timber structures create vertical drop-off between compartments

Climbing, trip hazards.

Smaller structure. Piling into beach. Small to moderate change to region sediment transport.

Detailed design of timber groynes

Site Survey

RANKING MODERATE MODERATE

BELOW AVERAGE

BELOW AVERAGE

MODERATE MODERATE MODERATE ABOVE

AVERAGE


59918185 | 22 August 2018 36



(changing climate)

Cost


Design Life Amenity Key Risks Environment /

Effects on Intertidal Reef

Communities in PPH MNP

Required design/analysis

/work

Op

tio

n –

Co

mb

ina

tio

n:

Ro

ck &

Tim

be

r G

royn

es

Improved width across entire study area. Even spread of sand across compartments


Compartment 1: Up to +25-30 m Compartment 2: Up to +10 m

Compartment 3: Up to +10 m


Combination of timber and rock proves nearly as effective, but at cheaper cost

Timber Approximately $3500/m of structure

Rock Approximately $5000/m of structure


Timber is less durable and not a strong as rock. Improved alignment will reduce repeat wave impact structure. But shorter structures to limit exposure to wave energy


Advise angling structure more in to average wave direction

Less impact on beach connectivity with less large rock structures








Detailed design:

- Rock groynes

- Timber groynes

Site Survey

RANKING VERY HIGH

ABOVE AVERAGE

HIGH HIGH HIGH ABOVE

AVERAGE HIGH HIGH


59918185 | 22 August 2018 37



(changing climate)

Cost


Design Life Amenity Key Risks Environment /

Effects on Intertidal Reef

Communities in PPH MNP



Op

tio

n –

Co

mb

ina

tio

n:

Ro

ck &

Tim

be

r G

royn

es

wit

h S

mal

l Bre

akw

ate

r



Compartment 1: Up to +25-30 m Compartment 2: Up to +10 m

Compartment 3: Up to +10 m

Limits wave energy reaching the location where beach most desired, enabling sand build up near shops.


As waves and storm get bigger limits wave energy reaching the location where beach most desired, enabling sand build up near shops.

Combination of timber and rock proves nearly as effective, but at cheaper cost

Timber Approximately $3500/m of structure

Rock Approximately $5000/m of structure


Timber is less durable and not a strong as rock. Improved alignment will reduce repeat wave impact structure. But shorter structures to limit exposure to wave energy


Advise angling structure more in to average wave direction

Less impact on beach connectivity with less large rock structures but large rock structure within vicinity of key location on the site.




Structure extends further offshore exposing public to larger waves and deeper water





More environmental impacts with structure located nearer to headland – rock pools

Detailed design:

- Rock groynes

- Timber groynes

- Breakwater

Site Survey

Requires further environmental assessment and monitoring at and around headland

RANKING VERY HIGH HIGH VERY HIGH HIGH VERY HIGH ABOVE AVERAGE VERY HIGH VERY HIGH


59918185 | 22 August 2018 38

6 Conclusions & Recommendations

Cardno’s investigation looked at long-term beach management options in the form of groynes, to maintain sand at the Point Lonsdale Front Beach.

The assessment determined the wave climate at the site to inform possible options to maintain a beach along the point Lonsdale foreshore. The average wave climate was found to be consistent in directionality along the coast, with higher wave energy in the south (1.91 kN/m2) than in the north (~0.55 kN/m2).

The average wave direction was found to align well with the typical shoreline position. In contrast, the groynes have been constructed at right angles to the seawall. It is suggested as part detailed design, consideration should be given to aligning and new groyne structures closer to the wave direction, as it will likely result in an improved shoreline response and lessen the structures exposure (and hence structure longevity).

Modelling results showed that various groyne types could help to create more beach along the point Lonsdale foreshore. A variety of combinations were considered, extending groynes, adding groynes and a groyne/breakwater structure. Various materials were also considered – rock, timber and polymer, which influenced structure specification such as the length, height and alignment.

A sediment transport model, LITPACK, was used to assess the effectivity each option. The larger rock structures were shown to capture sand across the study area, while smaller timber structures were able to effectively hold sand further north, with a minimal structure footprint. A small breakwater, south of the shops, was shown to help limit bigger waves reaching the seawall and beach. This, along with wave diffraction effect, should reduce scour from wave reflection on the southern section of the seawall, and enable the beach to build up in this area. Options were assessed against evaluation criteria tailored for the needs of this site, and considered key aspirations of this project including desired targeted locations for beach improvement, costs and environmental sensitivities.

To enable a robust and in-depth comparison between each of the options, a more detailed multi-criteria analysis is recommended. This analysis requires key stakeholders to determine clear objectives to be achieved by this project, highlighting the priorities. This will allow associated weightings to be incorporated into the analysis, aiding in the differentiation between options and the decision making.

Cardno suggest that an option that uses a combination of the suggested structures could be effective in creating more beach at the Point Lonsdale foreshore, allowing for a balance between effectivity, cost, amenity and environmental considerations.

In terms of functionality, the option Combination: Rock & Timber Groyne with a small breakwater has been nominated as a suitable option to maintain a beach along this foreshore, particularly at the beach in front of the shops. However, this option does have additional considerations that must be taken into account, particularly environmental concerns, potential impacts of wave interaction with the adjacent cliffs and possible bypassing effects of the southern-most groyne. Therefore, it will need to be investigated in further detail.

Cardno also recommend that DELWP look to undertake works in a staged approach, working from south to north. This will manage funding and allowing the beach to respond and adjust as well as targeting priority areas, nearer to the shops.

All options have been presented at a concept level and require detailed design to be undertaken to further validate and appropriately design the nominated option.

It is noted due to the sensitivity of this site, any development/construction will present significant challenges in regards to approval processes.


59918185 | 22 August 2018 39

7 References

BMT WBM, 2017, Lonsdale Bight Investigations Review and Options Overview, prepared for Department of Environment, Land, Water and Planning

Cardno ,2011. The Great Sands and Adjacent Coast and Beaches. Report prepared for the Port of Melbourne Corporation by Cardno. Report No. RM2289_LJ5518, Ver 1.0

Cardno, 2016. Wave Data in the Entrance of Port Phillip. Unpublished report prepared for the Port of Melbourne Corporation by Cardno, report 59916509_R01.

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