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Gibraltar Point Erosion Control

Nearshore Reef Design Report

March 12, 2018 | 11503.102.R1.Rev0

Gibraltar Point Erosion Control

Nearshore Reef Design Report

© 2018 W.F. Baird & Associates Coastal Engineers Ltd. (Baird) All Rights Reserved. Copyright in the whole and every part of this document, including any data sets or outputs that accompany this report, belongs to Baird 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 without the prior written consent of Baird.

This document was prepared by W.F. Baird & Associates Coastal Engineers Ltd. for Toronto and Region Conservation Authority (TRCA). The outputs from this document are designated only for application to the intended purpose, as specified in the document, and should not be used for any other site or project. The material in it reflects the judgment of Baird in light of the information available to them at the time of preparation. Any use that a Third Party makes of this document, or any reliance on decisions to be made based on it, are the responsibility of such Third Parties. Baird accepts no responsibility for damages, if any, suffered by any Third Party as a result of decisions made or actions based on this document.

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Prepared for: Prepared by:

Toronto and Region Conservation Authority (TRCA)

101 Exchange Avenue

Vaughan, Ontario

L4K 5R6

Canada

W.F. Baird & Associates Coastal Engineers Ltd.

For further information, please contact

Mohammad Dibajnia at +1 905 845 5385

[email protected]

www.baird.com

11503.102.R1.Rev0

Z:\Shared With Me\QMS\2018\Reports_2018\11503.102.R1.Rev0_GP Nearshore Reef Design Report.docx

Revision Date Status Comments Prepared Reviewed Approved

R1 9/02/2018 Draft MD MOK MD

R1 12/03/2018 Final MD MOK MD

Gibraltar Point Erosion Control Nearshore Reef Design Report

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Table of Contents

1. Design Summary .................................................................................................................... 1

1.1 Introduction 1

1.2 Nearshore Reef Concept 1

1.3 Groyne 3

1.4 Sand Management 4

2. Numerical Modelling of the Nearshore Reef ....................................................................... 5

2.1 Offshore Waves 5

2.2 HYDROSED Modeling of the Nearshore Reef 6

2.3 HYDROSED Modelling of Nearshore Wave Transformation 10

2.4 Profile Modelling of the Nearshore Reef Concept 11

2.5 Boussinesq Modelling of the Nearshore Reef Concept 13

2.5.1 Model Grid 13

2.5.2 Model Scenarios 16

2.5.3 Model Results 16

3. Calculation of Stone Size .................................................................................................... 20

3.1 Empirical Method 20

3.2 Shear Stress Method 21

3.3 Physical Model Method 22

3.4 Stone Size Summary 23

3.5 Ice 25

3.6 Navigation 25

4. Nearshore Reef Monitoring and Maintenance ................................................................. 26

4.1 Nearshore Reef Monitoring 26

4.1.1 Introduction 26

4.1.2 Frequency of Reef Monitoring 26


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4.1.3 Routine Wading Inspection 26

4.1.4 Detailed Inspections and Surveys 27

4.1.5 Profile Survey Procedures 28

4.1.6 Safety Considerations 28

4.2 Maintenance and Repairs 28

5. Beach Monitoring and Maintenance ................................................................................. 29

5.1 Beach Monitoring 29


5.1.2 Frequency of Beach Monitoring 29

5.1.3 Waterline Survey 29

5.1.4 Beach Profile Survey Procedures 29


6. Rubble Mound Groyne Monitoring and Maintenance ..................................................... 31

6.1 Groyne Monitoring 31


6.1.2 Frequency of Groyne Monitoring 31

6.1.3 Visual Inspection 31

6.1.4 Cross-section Surveys 32

6.1.5 Cross-section Survey Procedures 32

6.1.6 Monitoring Armour Stone Degradation 33

6.1.7 Safety Considerations 33


7. References ............................................................................................................................ 34

Tables

Table 3-1 List of parameters used for stone size calculations ...................................................................... 20


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Table 3-2 Calculated stone size for the outer perimeter of the nearshore reef ............................................ 21

Table 3-3 Comparison of calculated stone sizes using the empirical and shear stress methods ............... 22

Table 3-4 Test parameters for the EB physical model compared with conditions at Gibraltar Point .......... 23

Table 3-5 Required stone size for toe protection .......................................................................................... 24

Table 4-1 Breakwater monitoring frequency ................................................................................................. 26

Table 5-1 Beach monitoring frequency .......................................................................................................... 30

Table 6-1 Groyne monitoring frequency ........................................................................................................ 31

Figures

Figure 1-1 Nearshore reef and groyne design layout for shoreline erosion control at Gibraltar Point ...........2

Figure 1-2 Stone sizes for the proposed nearshore reef .................................................................................3

Figure 2-1 Wave rose offshore Toronto Island .................................................................................................5

Figure 2-2 Wave point rose offshore Toronto Island ........................................................................................6

Figure 2-3 Model bathymetry around Gibraltar Point with nearshore reef in place (WL=+0.6 m) ..................7

Figure 2-4 Predicted wave height (colour contouring) and direction (arrows) for southwesterly storm .........8

Figure 2-5 Predicted nearshore currents for southwesterly storm ...................................................................8

Figure 2-6 Predicted wave height (colour contouring) and direction (arrows) for easterly storm ...................9

Figure 2-7 Predicted nearshore currents for easterly storm ............................................................................9

Figure 2-8 Predicted 20-year wave heights at various water levels and nearshore locations ..................... 10

Figure 2-9 Time series of wave height during January 1996 storm ............................................................. 11

Figure 2-10 Transformation of various wave heights over nearshore reef during January 1996 storm simulated by COSMOS .................................................................................................................................................... 12

Figure 2-11 Predicted shoreline response to the selected storm for different beach material (grain sizes) with the nearshore reef in place .............................................................................................................................. 13

Figure 2-12 Model bathymetry – reef conditions ............................................................................................ 14

Figure 2-13 Model bathymetry existing conditions – zoomed to Gibraltar Point ........................................... 15


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Figure 2-14 Model bathymetry reef conditions – zoomed to Gibraltar Point ................................................. 15

Figure 2-15: Instantaneous water surface elevation for existing (top) and reef (bottom) conditions – H=1.5 m, T=4.0 s and WL=0.6 m. ................................................................................................................................... 17

Figure 2-16: Instantaneous water surface elevation for existing (top) and reef (bottom) conditions – H=3 m, T=8.0 s and WL=1.0 m. ................................................................................................................................... 18

Figure 2-17: Predicted significant wave height distributions for existing (top) and reef (bottom) conditions – H=3.0 m, T=8.0 s and WL=1.0 m.................................................................................................................... 19

Figure 3-1 Calculated stone size using shear stress and Shields criterion for initiation of motion .............. 21

Figure 3-2 Physical model results adapted from by Baird & Associates (2000) .......................................... 24

Figure 4-1 Recommended profile surveys .................................................................................................... 27

Figure 6-1 Recommended cross-section surveys for the groyne ................................................................. 32


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1. Design Summary

1.1 Introduction

The Toronto and Region Conservation Authority (TRCA) completed an Environmental Study Report (ESR), in accordance with Conservation Ontario’s Class Environmental Assessment for Remedial Flood and Erosion Control Projects (Class EA) to develop a long-term solution to address the shoreline erosion around Gibraltar Point (TRCA, 2008). Based on the outcome of the Class EA process for the project, the proposed remedial action was a sand management plan, which recognized that some form of offshore structure would likely be required to make the sand management plan technically and economically feasible. W.F. Baird & Associates Coastal Engineers Limited (Baird) was retained by TRCA to complete the coastal engineering analysis and final design of the preferred alternative for the Gibraltar Point Erosion Control Project. Baird completed the final design of the preferred alternative in 2015 which consisted of an offshore breakwater, groyne and sand management. Funding constraints put the project on hold.

In 2016 partial funding was secured from the City of Toronto to implement the Gibraltar Point Erosion Control Project. As erosion control measures were not implemented within five years of the 2008 approval of the Class EA, an addendum under Section 6.0 of the process is required. TRCA decided to use this opportunity to reconfirm their understanding of the existing conditions of the shoreline and to ensure that the underlying conditions and results of the Class EA remain valid.

Baird were thus tasked by TRCA to investigate potential innovative improvements in the design within the allocated budget. A revised preferred concept using a softer engineering approach to erosion protection was developed. The new design is a nearshore reef concept that mimics natural coastal features and has built within it improvements in aquatic habitat. From a functionality perspective, the proposed reef possesses an exterior perimeter submerged breakwater much closer to shore than the original preferred alternative. The reef is then formed by filling of rock on the shoreward side of the exterior perimeter of progressively smaller materials as one approaches the shoreline. The design will also include a sand management program similar to the original preferred concept, where sand will be placed strategically to protect Gibraltar Point and nourish Hanlan’s Beach. The new preferred design is described in this report.

1.2 Nearshore Reef Concept

Naturally occurring nearshore reefs and cobble/boulder lag deposits can mitigate wave transmission and sediment transport. Coastal engineers have mimicked the idea by designing artificial reef-type breakwaters, or the so-called Low Crested Structures (LCS), to provide protection against storm waves and to avoid the often-undesired visual obstruction of emerged structures. These types of structures also have the potential to improve the local ecology and recreational amenity (i.e., snorkeling). The main physical function of a reef breakwater is to cause wave energy dissipation in the nearshore during storm events and mitigate storm-induced beach erosion. Unlike emergent structures, low crested breakwaters do not stop all wave transmission but reduce the incoming wave height by forcing the larger waves to break. The percentage of energy reduction increases with incident wave energy. In other words, they work most efficiently when required, i.e., during storm events.

The shoreline at Gibraltar Point is facing towards the southwest. As a result, most of the shoreline erosion occurs during southwesterly storm events that generate waves arriving directly at this shoreline. Furthermore, the alongshore transport system at Gibraltar Point typically flows towards the north (i.e. towards Hanlan’s Beach) particularly during easterly storm events. The erosion process at Gibraltar Point may thus be summarized as material being taken away from the shoreline during southwesterly wave attack and


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subsequently carried away towards Hanlan’s Beach during easterly storm wave events. Shoreline erosion control at Gibraltar Point thus requires mitigation measures that can effectively reduce both the impact of southwesterly waves and the transport potential of the northward alongshore currents.

Based on the above discussion, a shore-connected semi-circular shallow flat reef (called “nearshore reef” hereafter) is proposed, as shown in Figure 1-1, to control erosion of the Gibraltar Point shoreline. The crest of the nearshore reef extends to the present location of the -2.0 m CD contour and is approximately 130 m wide at its widest section. Crest elevation would be constant at +0.0 m CD (74.2 m IGLD’85). The average crest width of the proposed reef is approximately 90 m. The overall outline of the lakeward perimeter of the proposed reef roughly follows the historic shoreline of 1980. In this manner, the proposed nearshore reef would roughly occupy the historic land that has been eroded and lost from Gibraltar Point since 1980. The perimeter of the proposed reef would have an approximately 1V:5H slope and be built with large stones to protect it against incoming waves. Inshore of the perimeter the stone size decreases as one moves closer to the shoreline as shown in Figure 1-2. Using the bathymetry from 2017, the proposed nearshore reef covers an area of approximately 37,000 m2 requiring approximately 35,000 m3 or 65,000 to 70,000 tonnes of stone. Approximately an additional 10,000 m3 to 15,000 m3 of material (i.e., sand) will then be used to nourish the beach once the nearshore reef is in place.

Figure 1-1 Nearshore reef and groyne design layout for shoreline erosion control at Gibraltar Point


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1.3 Groyne

The shoreline to the east of Gibraltar Point (i.e., the west end of Manitou Beach) has an approximately east-west orientation exposing it to both easterly and southwesterly waves. As a result, sand typically moves back and forth along this shoreline under alternating incoming wave directions. In other words, easterly waves tend to carry sand towards the west (i.e., towards Gibraltar Point) while southwesterly waves carry it back towards the east. During an easterly event, however, the sand moving towards the west may continue its journey around Gibraltar Point and beyond towards Hanlan’s Beach resulting in a net loss of sediment for Manitou Beach. A groyne is thus proposed to be constructed as shown in Figure 1-1 to reduce sand loss at the west end of Manitou Beach. The groyne will extend out from the shore to ‐1.5 m CD and will be about 55 m long. The groyne will be constructed from armour stone and core stone material with its crest elevation at about +2.5 m CD (approximately the same elevation as the ground around the washroom building).

Figure 1-2 Stone sizes for the proposed nearshore reef


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1.4 Sand Management

Using a comparison of historic shorelines, Baird (2015) estimated that the shoreline at Gibraltar Point has been eroding at an approximate average rate of 4 m/year since 1980. This is equivalent to approximately 15,000 to 20,000 m3/year (on average) being eroded from this shoreline. Most of the eroded material moves northward towards Hanlan’s Beach. On the other hand, approximately 3,000 m3/year is bypassing the structure at the north end of Hanlan’s Beach (i.e., the seawall at the end of airport runway) and deposited in and around the Western Gap. As a result, Hanlan’s Beach has been accreting at an approximate average rate of 15,000 m3/year over past few decades (i.e., since 1980). Construction of the proposed nearshore reef will mitigate shoreline erosion at Gibraltar Point. In this case, it was estimated (Baird, 2015) that Hanlan’s Beach would start to erode at a rate of approximately 3,000 m3/year if not compensated by artificial nourishment. Baird (2015) recommended placement of approximately 15,000 m3 of sand every 5 years at a location close to the south end of Hanlan’s Beach (i.e., immediately north of Hanlan’s Point) so the placed sand is distributed naturally to the north along the beach by the action of waves. The material can possibly be sourced from the existing deposition of sand at the Western Gap Channel or other sand sources as described by Baird (2011).

Currently TRCA is adding a beach restoration component to the Gibraltar Point Erosion Control Project. Their proposed dune beach restoration plan is to replace some of the rare and sensitive dune beach habitats lost due to erosion over the last few decades, providing ecosystem enhancements and improving recreational opportunities on the Island. According to this plan, approximately 10,000 m3 to 15,000 m3 of beach sand will be used to create a beach-dune system once the nearshore reef is in place.

It is estimated that the placed beach sand (D50 ≥ 0.2 mm) may be eroded at an approximate rate of 3,000 to 6,000 m3/year depending on future lake levels. This is similar to the required volume for sand management at Hanlan’s Beach (i.e., 15,000 m3 of sand every 5 years as described above). Therefore, the two beach nourishment tasks (i.e., beach restoration at Gibraltar Point and supply of sand to Hanlan’s Beach) may be combined pending future monitoring results. Future lake levels would be one of the key factors defining the beach berm and dune height. More beach loss to Hanlan’s Beach and less dune development are expected during higher lake level years. On the other hand, dune building is promoted if low lake level conditions persist for several years. An adaptive sand management scheme based on monitoring is required to balance between nourishment volumes, beach erosion and supply to Hanlan’s Beach, and dune building processes.


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2. Numerical Modelling of the Nearshore Reef

2.1 Offshore Waves

The offshore wave rose for Toronto Islands based on Baird’s WAVAD hindcast is shown in Figure 2-1. The hindcast data shown here is for 1961 to 2010. Figure 2-2 shows the corresponding scatter plot (point rose) of wave height by direction. In general, the wave climate off Toronto Islands is dominated by waves arriving from east and southwest. The hindcast indicates that waves arrive from south (S) to west (W) window for about 44% of the time with SW being the predominant direction. Offshore significant wave heights up to 5 m may occur during severe storms from SW direction. Waves arrive from east-southeast (ESE) to east-northeast (ENE) window for about 43% of the time with E being the predominant direction. Wave heights up to 6 m may occur during severe storms from E direction. Offshore significant wave height is less than 1 m and 2 m for 86% and 98% of the time, respectively. Alternatively, wave height is larger than 1 m and 2 m for 14% and 2% of the time, respectively. Shoreline erosion events are expected to occur mostly during major storm events. Predicted wave periods range between 2 to 10 s from east and 2 to 8 s from southwest. Ice cover on the lake was neglected in the above hindcast. Under most climate change scenarios, it is expected that there may be less ice cover in the future. Wave transformation to the nearshore is discussed in Section 2.3.

Figure 2-1 Wave rose offshore Toronto Island


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Figure 2-2 Wave point rose offshore Toronto Island

2.2 HYDROSED Modeling of the Nearshore Reef

Numerical modeling using the HYDROSED model was undertaken to calculate nearshore wave transformation and nearshore currents in the area of the proposed nearshore reef. The objective of the simulation was to 1) calculate wave parameters under different lake level conditions at various locations on the proposed nearshore reef required to determine the stone size, and 2) examine resulting waves and nearshore current patterns under more frequent and extreme storm conditions to assess the nearshore reef performance in redefining local littoral processes and mitigation of erosion of Gibraltar Point.

Model bathymetry was composed of the 2009 and 2017 hydrographic surveys completed by TRCA, 2017 topographic survey of Gibraltar Point by TRCA, 2015 topographic LiDAR data, as well as Geodas bathymetry outside of the above survey datasets. As the waves arrive at this site from two completely different directions, two separate grids were considered to handle easterly and southwesterly waves. Figure 2-3 shows model bathymetry around Gibraltar Point with the proposed nearshore reef in place.


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Figure 2-3 Model bathymetry around Gibraltar Point with nearshore reef in place (WL=+0.6 m)

Example results corresponding to 3.5 m wave height, 8 s wave period from SW direction are presented in Figure 2-4 and Figure 2-5. This corresponds to a severe southwesterly storm condition occurring approximately once every 10 years. Calculations were completed for a lake level of 1.0 m above CD to incorporate storm surge effect on water level. Figure 2-4 shows the distribution of wave height (colour contouring) and wave direction (arrows). Upon arriving at the lakeside edge of the nearshore reef, waves undergo further shoaling, breaking, refraction and focusing resulting in dissipation of wave energy and reduction in wave height over the proposed reef. Figure 2-5 shows the corresponding nearshore currents. Colour contouring in this figure shows predicted current speed, while arrows indicate current direction. Strong alongshore currents are predicted to form along the lakeside edge of the reef away from the shoreline. Through effectively reducing wave heights and current speeds at the shoreline, the proposed reef is thus expected to mitigate shoreline erosion at Gibraltar Point.

Example results corresponding to 5 m wave height, 10 s wave period from E direction are presented in Figure 2-6 and Figure 2-7. This corresponds to a severe easterly storm condition occurring once every 5 to 10 years. Figure 2-6 shows the distribution of wave height (colour contouring) and direction (arrows). Offshore easterly waves undergo considerable refraction before reaching the nearshore reef and continue to dissipate over the reef. Wave height at the south end of the reef is around 2 m. Figure 2-7 shows the corresponding nearshore currents around Gibraltar Point for the above wave condition. Colour contouring in this figure shows predicted current speed, while arrows indicate current direction. There is a well-established alongshore current system around Gibraltar Point that flows towards Hanlan’s Beach. Stronger currents, however, are predicted to form along the lakeside edge of the reef away from the shoreline (except for a small area at the south end of the reef). Model results thus confirm that the proposed nearshore reef can effectively reduce wave heights and current speeds at the shoreline and mitigate shoreline erosion at Gibraltar Point.


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Figure 2-4 Predicted wave height (colour contouring) and direction (arrows) for southwesterly storm

Figure 2-5 Predicted nearshore currents for southwesterly storm


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Figure 2-6 Predicted wave height (colour contouring) and direction (arrows) for easterly storm

Figure 2-7 Predicted nearshore currents for easterly storm


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2.3 HYDROSED Modelling of Nearshore Wave Transformation

The spectral wave transformation module of HYDROSED was utilized to transform the offshore wave hindcast to 29 points in the nearshore reef area. The 50-year hindcast period was represented by a selection of wave conditions and the model was run for those representative wave conditions (i.e., seven different wave heights, wave periods every 2 s, and directions every 15°). In total, 248 easterly wave conditions and 168 westerly wave conditions were simulated. Calculations were completed for three lake levels: 0.6 m (average), 1.0 m, and 1.4 m above CD (10-year water level). Model results were then tabulated as lookup tables for a numerical interpolation program that stepped through the 50-year time series of hourly offshore wave data to transform and create hourly time series at each of the 29 nearshore locations.

An Extreme Value Analysis (EVA) was subsequently performed at each of the 29 locations to determine wave heights with various return periods. The analysis was conducted separately for the selected three lake levels. Figure 2-8 shows the results for nearshore waves with 20-year return period at the three lake levels. These results were subsequently used to determine the required stone size.

Figure 2-8 Predicted 20-year wave heights at various water levels and nearshore locations


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2.4 Profile Modelling of the Nearshore Reef Concept

The degree of shoreline protection provided by the proposed nearshore reef is mainly a function of the reef crest width and the reef crest height relative to the water level. Greater dissipation of wave energy can be achieved with a higher crest and/or a wider crest. The crest height is usually determined through the competing objectives of minimizing the undesired visual impairment of the seascape (requiring a lower structure) while maximizing wave dissipation functionality (requiring a higher crest elevation). Also, lower, submerged crests can utilize smaller stone material than structures with higher, emergent crests. The crest of the proposed nearshore reef at Gibraltar Point is at low water datum (+0.0 m, CD) so that it is submerged and out of sight for most of the time but may be emergent during very low lake level periods. The reef width should be wide enough to cause sufficient wave energy dissipation in the nearshore during storm events and mitigate storm-induced beach erosion. The performance of the reef crest width was investigated using the COSMOS profile numerical model.

The COSMOS model was used to examine wave energy dissipation over the proposed nearshore reef. COSMOS is a two‐dimensional (2D) profile model that consists of several predictive modules for simulation of nearshore processes. The COSMOS model requires beach profile, waves, and water levels as input. A beach profile was developed from the bathymetry and topographic data as described in Section 2.2. The profile extends from the dune, offshore to a depth of 10 m. The model was run for the storm of January 27, 1996 at +1.0 m CD water level. The selected event represents a storm that occurs approximately every 10 to 15 years at Gibraltar Point. Figure 2-9 shows time series of this storm at 10 m water depth off Gibraltar Point. At 10 m depth, the event had a significant wave height of 2.9 m with peak wave period of 6.5 s from 220 degrees (SW).

Figure 2-9 Time series of wave height during January 1996 storm


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Figure 2-10 shows simulated wave height transformation across the profile (orange line), including the nearshore reef, at various stages of the January 1996 storm. The horizontal axis in this figure represents distance from the model offshore boundary. Different coloured lines indicate wave height transformation at various stages during the storm. The figure indicates that waves undergo a rapid decrease in their energy/height upon arriving at the lakeside slope/edge of the proposed reef (at approximately 700 m distance on horizontal axis). The waves continue to lose their energy over the reef surface as they proceed towards the shoreline for approximately 80 to 90 m. At this point, the wave height has been reduced to approximately 0.3 m regardless of incoming wave height. These small waves continue their journey over the reef surface towards the beach without any further dissipation until they finally break very close to the shoreline (i.e., around the 810 m distance on the horizontal axis). In other words, wave energy dissipation is minimal after approximately 90 m shoreward from the outer perimeter of the reef. Based on these findings, a nearshore reef with an average width of approximately 90 m was proposed to control shoreline erosion at Gibraltar Point. The proposed nearshore reef is approximately 130 m wide at its widest section.

Figure 2-10 Transformation of various wave heights over nearshore reef during January 1996 storm simulated by COSMOS

The COSMOS model was subsequently used to assess stability of the beach at Gibraltar Point after construction of the proposed nearshore reef. Different beach material (i.e., grain size) were examined. The results are provided in Figure 2-11 and indicate that beach material with median grain size (D50) of 1.0 mm or larger would be able to survive waves similar to the January 1996 storm. For example, using sand with median grain size (D50) of 1.0 mm, substantial beach maintenance may be required every 15 to 20 years following the occurrence of more severe storms than the 10 to 15 year storm investigated here.


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Figure 2-11 Predicted shoreline response to the selected storm for different beach material (grain sizes) with the nearshore reef in place

2.5 Boussinesq Modelling of the Nearshore Reef Concept

Another useful tool to further investigate the impact of the proposed nearshore reef on waves approaching Gibraltar Point and confirm its performance in dissipating wave energy is the Boussinesq Wave (BW) model. The BW module of DHI’s MIKE21 modelling package was used. It is a state-of-the-art nonlinear wave modelling system that solves various nearshore wave transformation processes in the time domain. As such, evolution of individual waves over the proposed reef can be simulated with accuracy.

2.5.1 Model Grid

The BW model is computationally intensive requiring careful selection of the computational domain. For this study, a rectangular model grid was developed to model the proposed reef scenario. The 2017 bathymetry and topographic data provided by TRCA were utilized for setting up the numerical model grid. The numerical grid extents as well as the overall model bathymetry with the nearshore reef in place are shown in Figure 2-12.


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The model grid covers a surface area of 5.6 km2 around Gibraltar Point and has a grid size of 1.5 m. A constant bed elevation of -20 m was set in the offshore to avoid numerical instabilities. This assumption would have no impact on the model results. The zoomed in model bathymetry for existing conditions and with the nearshore reef can be observed in Figure 2-13 and Figure 2-14, respectively.

Figure 2-12 Model bathymetry – reef conditions


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Figure 2-13 Model bathymetry existing conditions – zoomed to Gibraltar Point

Figure 2-14 Model bathymetry reef conditions – zoomed to Gibraltar Point


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2.5.2 Model Scenarios

The BW model uses a virtual offshore wave paddle to simulate incoming waves. The incoming waves were generated with the Random Wave Generation Tool of MIKE Zero. Two wave conditions were selected to represent severe and more frequent storm wave conditions:

Wave condition with Hm0 = 3.0 m and Tp = 8.0 s coming from 225o. Lake level was set to +1.0 m CD.

Wave condition with Hm0 = 1.5 m and Tp = 4.0 s coming from 225o. Lake level was set to +0.6 m CD (i.e., approximately average Lake Ontario level).

Simulations were completed for the existing (2017) bathymetry conditions as well as for the nearshore reef conditions (i.e., total of 4 model runs).

2.5.3 Model Results

Figure 2-15 shows snapshots of instantaneous water surface elevations for the existing and reef conditions under the 1.5 m, 4 s more frequent storm waves at +0.6 m CD water level. The colour shading in this figure highlights the wave crests while wave troughs are represented in white. In addition to reducing wave crest elevations, the model predicts an interesting wave crossing pattern formed by waves refracted over the perimeter of the reef and subsequently travelling in opposite directions. Note that a unidirectional longshore current does not form under crossing waves. Therefore, construction of the proposed nearshore reef is expected to result in reduced sediment loss from Gibraltar Point in the alongshore direction.

Creation of crossing waves is more pronounced for waves with shorter wave periods (e.g., 2 to 6 s) as these waves go under substantial refraction upon arriving at the nearshore reef. Figure 2-16 shows snapshots of instantaneous water surface elevations for the existing and reef conditions under the sever 3 m, 8 s storm waves at +1.0 m CD water level. An 8 s wave has a longer wavelength, approximately 4 times that of a 4 s wave, hence the wave crests are well separated from each other in this figure. While crest elevations are significantly reduced over the nearshore reef, the crossing wave pattern is less noticeable in this case. Therefore, alongshore movement of sediment towards Hanlan’s Beach is expected to occur during storm events (although at a much smaller rate compared to the existing conditions).

The colour shading in Figure 2-17 shows predicted distributions of significant wave height by the BW model for the existing and reef conditions under the 3 m, 8 s storm waves at +1.0 m CD water level. The model predicts significant reduction of the wave height (and wave energy) over the proposed reef. These results are similar to and support those predicted by the HYDROSED model (Figure 2-4).


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Figure 2-15: Instantaneous water surface elevation for existing (top) and reef (bottom) conditions – H=1.5 m, T=4.0 s and WL=0.6 m.


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Figure 2-16: Instantaneous water surface elevation for existing (top) and reef (bottom) conditions – H=3 m, T=8.0 s and WL=1.0 m.


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Figure 2-17: Predicted significant wave height distributions for existing (top) and reef (bottom) conditions – H=3.0 m, T=8.0 s and WL=1.0 m.


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3. Calculation of Stone Size

The proposed nearshore reef is not a conventional rubblemound structure and therefore there are no specific design guidelines. The required stone size was thus determined using three different methods as described in the following.

3.1 Empirical Method

The outer perimeter of the proposed nearshore reef may be considered as a low crested rubblemound structure. The widely used Van de Meer (1988) and Van Gent (2003) formulations were used to calculate the size of required stone (double layer). Calculations were conducted for 10-year water level (~1.4 m) combined with 20-year wave height (2.4 m, at the south end of the proposed reef, see Figure 2-8). Table 3-1 provides values of various parameters used in stone size calculations.

Table 3-1 List of parameters used for stone size calculations

Parameter Value Comments

Significant wave height (H1/3) 2.4 m 2.3 m and 1.9 m at the middle and north end of the reef, respectively

Peak wave period 9 s

Water depth 3.4, 3, 2.6, 2 m 2.9, 2.5, 2.1, 1.5 m at the north end

Density of rock armour units 2,640 kg/m3

Width of armour crest 5 m

Total width of crest 100 m

Seaward slope of armour layer (tan α) 0.2

Roughness factor of armour 0.4

Permeability factor of armour slope (P) 0.4

Damage level (Sd) 2

Safety factor stone sizes in double layer 1.2

The resulting maximum calculated stone sizes (Dn50) required for the outer perimeter of the reef are summarised in Table 3-2 .


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Table 3-2 Calculated stone size for the outer perimeter of the nearshore reef

Location Dn50 (m) - Van der Meer Dn50 (m) - Van Gent

South end 0.54 0.48

Middle 0.49 0.43

North end 0.39 0.32

3.2 Shear Stress Method

The required stone size was also calculated using wave-induced bottom shear stress and Shields criteria for initiation of motion. In this method, maximum bottom shear stress occurring during the 50-year hindcast was calculated at several locations on and around the reef. A bed roughness equal to 2.5 times the stone size (2.5×D50) was used to be on the conservative side. The results were compared with Shields parameter for initiation of motion (=0.05) to determine the size of the stone that would not move during the 50-year period. Calculations were completed for three water levels (+0.6, +1.0, and +1.4 m LWD) and the largest of the three stone sizes was selected. Calculated stone sizes (D50, mm) are shown in Figure 3-1.

Figure 3-1 Calculated stone size using shear stress and Shields criterion for initiation of motion


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The results in Figure 3-1 correspond to spherical/rounded stone. The corresponding nominal diameter (having the same weight) near the perimeter of the reef was calculated and compared with the results from Van der Meer formula in Table 3-3.

Table 3-3 Comparison of calculated stone sizes using the empirical and shear stress methods

Location Dn50 (m) - Van der Meer Dn50 (m) – Shear Stress

South end 0.54 0.72

Middle 0.49 0.48

North end 0.39 0.16

Note that shear stress is a function of wave height, wave period, and depth/water level. Therefore, maximum shear stress may occur under a wave condition different from the design wave and water level conditions determined through EVA analysis. Nevertheless, calculated stone size by the two methods are similar. Nearshore currents were not considered in calculation of shear stress as they are relatively weak during predominant SW wave conditions.

3.3 Physical Model Method

In a feasibility study conducted by Baird & Associates, 2000 (Illinois Shoreline Interim 1V, Lakebed Armoring Plan), three plans of lakebed armouring were developed and investigated including: 1) a thin lakebed pavement consisting of one to two layers of cobbles spread between the 1 m and 3 m CD depth contours; 2) a submerged stable berm running parallel to the shore placed near the 3 m depth contour, and 3) a reshaping berm, also placed near the 3 m depth contour, that can be reshaped by wave action, naturally spreading the cobble-sized stone over the nearshore profile towards the shore. Figure 3-2 below shows the results for an 8 m wide submerged stable berm running parallel to the shore placed inshore of the 3 m depth contour.

Water depth at the toe was approximately 3 m, similar to the design conditions for the cobble reef at Gibraltar Point. The offshore/lakeward slope of the berm was approximately 1V:1.5H and 12”-24” (300-600 mm) graded riprap (in prototype scale) was used to build the berm. Under the selected severe storm conditions at WL=0.0 m CD, the figure indicates reshaping of the lakeward slope of the berm to an approximately 1V:4H slope. For the lakeward slope of the nearshore reef at Gibraltar Point, 600-900 mm graded riprap with 1V:5H slope is proposed. The proposed combination of stone size and slope is thus expected to be stable under the design conditions.

Additionally, Baird completed a Physical Model Study of the low-crested breakwaters for Toronto Eastern Beaches (EB) in 2001. Test parameters for the EB model were similar to the conditions at the southerly outer perimeter of the proposed Gibraltar Point nearshore reef (see Table 3-4).


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Table 3-4 Test parameters for the EB physical model compared with conditions at Gibraltar Point

Parameter Eastern Beaches Physical Model (prototype)

Gibraltar Point

Toe Elevation -1.9 m, CD -2.0 m, CD

Crest Elevation +0.5 m 0.0 m

Front Slope 1V:1.5H 1V:5H

Crest Width 15 m >15 m

Dn50 550 mm 750 mm (based on 600-900 mm)

Water Level +1.5 m; +1.0 m; +0.5 m, CD +1.4 m; +1.0 m; +0.6 m, CD

Rc (freeboard) -1.0 m; -0.5 m; 0.0 m -1.4 m; -1.0 m; -0.6 m

Hs at toe 1.98 m; 1.70 m; 1.35 m 2.41 m; 2.30 m; 2.13 m

Following the tests for the EB breakwater, it was observed that although some stone movement had occurred, there seemed to be no significant change in the breakwater cross-section. It was concluded that the structure would still be functional. The EB model results for “start of damage” with respect to stability of the stone are consistent with the stability results obtained from empirical methods used here for low-crested structures. It should be noted that where the highest waves are depth-limited, then slightly submerged conditions are the most critical (i.e., Rc values approaching 0.0). For the EB model case, with Rc equal to 0, stones with Dn50 = 550 mm were adequate, for the front slope (at 1V:1.5H), crest (width 15 m) and rear slope. At Gibraltar Point, 600-900 mm diameter stone is proposed for the southern half of the reef perimeter at a flatter (1V:5H) slope; flatter slopes are more stable. The crest elevation of Gibraltar Point will be more submerged, more often. Also, the empirical formula for stability of low-crested structures are generally applicable for breakwaters with relatively narrow crest widths, while the Gibraltar Point crest is wider, hence less prone to damage. Gibraltar Point has no rear slope that could be damaged.

3.4 Stone Size Summary

The final stone size and its gradation across the nearshore reef are shown in Figure 1-2.


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Figure 3-2 Physical model results adapted from by Baird & Associates (2000)

As for the toe protection, a 3 m wide toe berm with two layers of stone is recommended. The required stone size was calculated, and the results are summarised in Table 3-5.

Table 3-5 Required stone size for toe protection

Location Dn50 (m) - Van der Meer (1998) Dn50 (m) – Baart (2010) Dn50 (m) – Van Gent (2014)

South end 0.43 0.40 0.43

Middle 0.38 0.36 0.39

North end 0.32 0.28 0.34


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3.5 Ice

The formation of ice during winter months affects the coastal climate at the project location in two ways. First, the formation of shorefast ice, in combination with an “ice foot”, protects the shoreline area from wave action even when the main body of the lake is relatively ice-free. The second factor is that ice formed within the greater water body has the effect of reducing wave generation during winter months, thereby limiting the wave climate at Gibraltar Point.

On Lake Ontario ice usually originates (and is most prevalent) at the east end of the lake next to the entrance to the St. Lawrence River. However, in cold winters it is not uncommon for ice cover to extend west along the north shore of the Lake, where it may occasionally affect Gibraltar Point. Typical winter coverage in Lake Ontario peaks around 17%. Ice coverage data is the concentration of ice, that is, the fraction of a unit of lake surface area that is completely covered with ice. The Lake is divided into grid cells and each grid cell is coded with a number between 0 and 100, representing the percentage of that cell that is covered by ice. Mild winters may only have 10% coverage while severe winters reach 65% coverage. For example, four such extreme events occurred in the winters of 1973, 1979, 1994 and 2015; in 1979 there was near-total ice coverage on the lake. However, even when it does occasionally develop, the ice cover is not very thick, and the ice foot is usually less than 2 m deep.

In case of emerged structures, ice piled on shore by wind and wave action does not, in general, cause serious damage to sloped rubble-mound structures. Typically, the net effects of ice formation are beneficial, as spray from wind and waves freezes on the structures and covers them with a protective layer of ice. Accepted practice for exposed shorelines of the Great Lakes is to size the primary armour layer for emerged structures to resist wave forces and accept some level of risk that ice damage could occur and that repairs may be required. The performance of the existing breakwater at Toronto Island since 1930 suggests that armour stone can reasonably be expected to perform adequately under ice action.

At present there are no guidelines for design of submerged structures against ice damage. During low lake level winters, it is expected that shorefast ice may cover the reef crest thus protecting it against further ice pile up. During high lake level winters, the proposed nearshore reef is expected to remain under the water below the ice sheet for majority of the time thus suffer less to no damage compared to emerged revetments.

It is also likely that ice cover will have a higher frequency of a “no ice” condition under future climate change scenarios (Lofgren et al., 2002).

3.6 Navigation

The nearshore reef should be appropriately marked with signs and/or warnings regarding shallow depths and other potential navigation hazards. The Canadian Coast Guard should be notified so that navigation charts can be updated.


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4. Nearshore Reef Monitoring and Maintenance

4.1 Nearshore Reef Monitoring

4.1.1 Introduction

The nearshore reef has been designed to function as a natural feature and part of the lakebed. It connects to the shore and has a relatively mild 1V:5H slope on its lakeside slopes to reduce potential for significant future reshaping or adjustments. Nevertheless, post construction monitoring of the completed nearshore reef is strongly recommended because stone structures respond to wave and ice action, water level changes and cycles of freeze-thaw and wetting and drying by changes in the profile of the structure and to the size and shape of its component parts. Stones may be displaced, cracked/fractured and/or abraded. Significant damage by storm action needs to be identified by survey as the reef is under the water most of the time. Gradual degradation caused by settlement, fracture or abrasion is more difficult to detect.

The monitoring program should include the establishment of an as-built database of a minimum of five reef cross sections, stone gradations, and underwater photographs (close-ups of specific areas), all documented by location to allow duplication in the future. Reef crest elevation is the most important parameter to its function and warrants special attention during inspection. A qualified coastal engineer with experience in nearshore reef design and performance should complete the monitoring. Subsequently, these cross-sections, stone gradations and photographs should be repeated and compared to the as-built information, thereby allowing quantification of changes and identifying the requirement, if any, for maintenance and repair work.

4.1.2 Frequency of Reef Monitoring

The frequency of reef monitoring is presented in Table 4-1.

Table 4-1 Breakwater monitoring frequency

Timing Monitoring Action Follow-up Action as Required

Comments

Immediately after construction

- Profile survey - Photographs

After significant storms during the first year

- Routine wading inspection - Photographs

- Profile survey Hs>2 m (waves at deepest lakeward toe of the reef)

After the first year - Profile survey - Photographs

Annual (spring) - Routine wading inspection - Photographs

- Profile survey

Every 5 years - Profile survey - Photographs

4.1.3 Routine Wading Inspection

A routine wading inspection of the reef crest should be carried out every year in the late spring and following any major storm during the first year. The routine inspection would consist of a visual reconnaissance of the reef crest along with measurement of water depth (over the reef flat/crest) using a graded survey rod at a minimum of 20 points along five profiles (see Figure 4-1, 4 equally-spaced points on each profile).


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Measurement locations should be recorded by a DGPS (or RTK) device. Measured depths should be converted to represent elevation relative to Chart Datum (CD) using measured Lake Ontario levels at the time of inspection. The following changes should be noted and photographed:

Settlement of the crest below the Chart Datum;

Large gaps between the stones;

Dislodged, displaced, or piled stones;

Stone degradation (deterioration, cracking, fracture, spalling, rounding of corners).

The annual survey should be compiled in digital format for future reference.

Figure 4-1 Recommended profile surveys

4.1.4 Detailed Inspections and Surveys

If the routine wading inspection uncovers significant deficiencies in the nearshore reef, additional detailed inspections should be undertaken as required, including profile surveys and snorkeling inspection surveys.

Profile surveys are recommended every 5 years.


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4.1.5 Profile Survey Procedures

A conventional profile survey is conducted using a rod and total station at regular intervals along the cross-section profiles. Recognized survey procedures are also presented in EM 1110-2-1003 Engineering & Design - Hydrographic Surveying (U.S. Army Corps of Engineers 2002). A sonar equipment may be used if/where the water is deep enough.

Survey data should be collected along 5 profiles. The location and extent of the profiles is shown in Figure 4-1. Survey lines should start from a survey baseline that will be established well inland. Survey lines should then extend out to the -2.5 m CD contour in the lake. Data should be collected at a minimum frequency of every meter.

4.1.6 Safety Considerations

Stone structures are potentially hazardous areas on which to operate. The stone is uneven with gaps and the sloping front face pose an increased risk. Individual stones can be smooth and can be particularly slippery when wet. Near the waterline, water level variation exposes weed and/or algal growth making the surface slippery and movement can be dangerous and requires great care. Monitoring should not be undertaken when ice is present on the reef.

A team of at least two people should carry out the monitoring. All personnel should wear personal floatation devices. The monitoring team should be familiar with the nature of the lake including storm waves and should check the marine weather forecasts on a regular basis. Special care shall be exercised on the slope of the structure, near the water’s edge and on wet or slippery surfaces. When surveying in the water from a boat, all applicable boating safety regulations shall be observed. Snorkeling should only be carried out by qualified personnel and in strict accordance with all applicable worker safety rules and regulations.

4.2 Maintenance and Repairs

The shoreline of Lake Ontario is a harsh environment (e.g., wave action, abrasion by suspended sediment, ice forces, freezing and thawing) and maintenance of the nearshore stone reef in response to the deterioration or “wear and tear” of the feature can be expected. In addition to the replacement costs due to long-term deterioration of the stone, there should be an allowance for repairs to the reef required as a result of storms exceeding the design event.

Repairs might include replacing or backfilling of potential displaced stone areas such that the reef crest elevation remains at +0.0 m CD everywhere across the reef. Due to its innovative nature, data on maintenance costs of nearshore reefs of this kind is not available. Typically for coastal structures, an estimate for an annual maintenance and repair budget of between 0.5% and 1.0% of the initial capital cost would be reasonable. It is unlikely that actual maintenance and repair work would be necessary every year, but the annual allowance should be budgeted on an ongoing basis to have sufficient funds accumulated when repair work is required. It is expected that repairs can be carried out by creating temporary access extending from land across the reef and using a crane and/or backhoe.

Stone material should be sound, durable and meeting the original specifications. The material should be clean, inert rock or concrete rubble meeting the lakefilling requirements of the Ministry of Environment.

The navigational lights should be inspected and maintained on an annual basis. The lamp, battery and solar panel could be removed for the winter and reinstalled each spring prior to the boating season.


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5. Beach Monitoring and Maintenance

5.1 Beach Monitoring

5.1.1 Introduction

Toronto Islands are part of a complex, naturally evolving recurved sand spit that has been subject to large‐scale human interventions for more than 100 years. Gibraltar Point has been virtually starved of a supply of sediment and thus experiencing erosion at least for the past 50 years. As an example, the shoreline at Gibraltar Point was eroding at an average rate of 4.5 m/year between 1980 and 2009. Most of the shoreline/beach erosion is believed to occur under southwesterly storms. The eroded material is subsequently transported towards the north by the alongshore currents generated during easterly events thus supplying sand to Hanlan’s Beach.

The proposed nearshore reef is expected to control the erosion at Gibraltar Point by absorbing the energy of incoming waves. Nevertheless, the beach may still erode by storm events particularly during higher lake level conditions. The rate of beach erosion would depend on future lake levels and the grain size of beach material (i.e. sand). Average annual beach volume loss due to shoreline erosion for a beach made of fine sand (D50≥0.2 mm) is expected to be in the range of 3,000 to 6,000 m3/year once the nearshore reef is in place. Winds from the southwest are expected to contribute to dune development during lower lake level conditions. On the other hand, more beach loss to the downdrift/north shore and less dune development are expected during higher lake level years. Therefore, an adaptive sand management scheme based on monitoring is required to balance between nourishment volumes, beach erosion and supply to Hanlan’s Beach, and dune building processes. Post construction monitoring of the beach is thus strongly recommended.

The monitoring program should be based on surveying of the 5 profiles shown in Figure 4-1, survey of the waterline, and photographs of the beach vegetation and dune conditions, all documented by location to allow duplication in the future. Survey data collected immediately after construction will serve as the baseline to measure beach erosion rates.

5.1.2 Frequency of Beach Monitoring

The frequency of beach monitoring is presented in Table 5-1.

5.1.3 Waterline Survey

A survey of the waterline should be carried out every year in the spring and following any major storm during the first year. The survey would consist walking along the waterline while recording the waterline position at least every 5 m using a DGPS (or RTK) device. Lake Ontario levels (recorded by the Canadian Hydrographic Services Toronto gauge) during the survey should be recorded. Beach vegetation and dune conditions should also be photographed and documented. All surveys should be compiled in digital format for future reference.

5.1.4 Beach Profile Survey Procedures

A conventional profile survey is conducted using a rod and total station at regular intervals along the cross-section profiles. Recognized survey procedures are also presented in EM 1110-2-1003 Engineering & Design - Hydrographic Surveying (U.S. Army Corps of Engineers 2002).

Beach profile data should be collected along the 5 profiles shown in Figure 4-1. Survey lines should start from the survey baseline that will be established well inland such that future inshore sand migration can be detected


11503.102.R1.Rev0 Page 30

and quantified. Survey lines should extend out to the lakeward edge of the reef flat/crest. Data should be collected at a minimum frequency of every meter.

Table 5-1 Beach monitoring frequency


Comments


- Profile survey - Waterline survey - Photographs


- Waterline survey - Photographs

- Profile survey Hs>2 m (waves at deepest lakeward toe of the reef)

Annual-first five years (spring)

- Profile survey - Waterline survey - Photographs

- Nourishment

Annual-after five years (spring)

- Waterline survey - Photographs

- Profile survey

Every 5 years - Profile survey - Waterline survey - Photographs

- Nourishment


The sand management scheme at Gibraltar Point requires placement of 15,000 m3 of sand every 5 years at Gibraltar Point to be naturally distributed along Hanlan’s Beach. It is recommended that 20,000 to 25,000 m3 of sand be initially placed at Gibraltar Point to ensure that enough sand is available for initial dune building processes as well as supply of sand to Hanlan’s Beach. The volume of sand remaining at Gibraltar Point should then be quantified after each beach profile survey. Nourishment should be planned for the following year once approximately 10,000 m3 of the sand is lost. On average, it is expected that approximately 15,000 m3 of sand will be required every 5 years.


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6. Rubble Mound Groyne Monitoring and Maintenance

6.1 Groyne Monitoring

6.1.1 Introduction

Rubble-mound structures respond to wave and ice action, water level changes and cycles of freeze-thaw and wetting and drying by changes in the profile of the structure and to the size and shape of its component parts. The armour stone may be displaced, cracked/fractured and/or abraded. Major failures by storm action are easily identified (e.g., displaced stone, gaps in the armour layer, slumping slopes, reduced crest width). Gradual degradation caused by settlement, fracture or abrasion may be much less apparent.

Post construction monitoring of the completed groyne is recommended. The monitoring program should include the establishment of an as-built database of cross-sections, stone gradations and photographs (general slope conditions as well as close-ups of specific areas), all documented by location to allow duplication in the future. The head of the groyne is typically where damage, if any, might be expected first and warrants particular attention during inspection. A qualified coastal engineer with experience in rubble mound structure design and performance should complete the monitoring. Subsequently, these cross-sections, stone gradations and photographs should be repeated, if necessary, and compared to the as-built information, thereby allowing quantification of changes and identifying the requirement, if any, for maintenance and repair work.

6.1.2 Frequency of Groyne Monitoring

The frequency of reef monitoring is presented in Error! Reference source not found..

Table 6-1 Groyne monitoring frequency


Comments


- Cross-section survey - Photographs


- Visual Inspection - Photographs

Hs>2 m (waves at the toe

of the head) Annual (spring) for the first five years

- Visual Inspection - Photographs

- Cross-section survey

Every 5 years - Visual Inspection - Photographs

- Cross-section survey

6.1.3 Visual Inspection

A visual inspection of the groyne should be carried out every year in the spring for 5 years and following any major storm during the first year. The routine inspection would consist of a visual reconnaissance of the structure. The following changes should be noted and photographed:

Slumping slopes;


11503.102.R1.Rev0 Page 32

Large gaps between the stones;

Dislodged, displaced, or piled stones;

Stone degradation (deterioration, cracking, fracture, spalling, rounding of corners).

The survey should be compiled in digital format for future reference.

6.1.4 Cross-section Surveys

If the visual inspection uncovers significant deficiencies in the rubble mound groyne, additional detailed inspections should be undertaken as required, including cross-section surveys and snorkeling inspection surveys.

6.1.5 Cross-section Survey Procedures

A conventional profile survey is conducted using a rod and total station at regular intervals along the cross-section profiles. Recognized survey procedures are also presented in EM 1110-2-1003 Engineering & Design - Hydrographic Surveying (U.S. Army Corps of Engineers 2002).

Survey data should be collected along 3 cross-sections. The location and extent of the cross-sections are shown in Figure 6-1. Data should be collected at a minimum frequency of every meter.

Figure 6-1 Recommended cross-section surveys for the groyne


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6.1.6 Monitoring Armour Stone Degradation

Monitoring procedures are intended to identify armour layer damage as given by: 1) cavities; 2) unstable armour; and 3) stone deterioration. Stone deterioration may arise from abrasion, breakage due to movement or impact, spalling and facture due to weathering (freeze-thaw; wetting and drying). Monitoring armour degradation should be included in the visual inspection. It would be preferable to complete the survey at a time when the water levels are low but prior to or after potential ice conditions.

6.1.7 Safety Considerations

Stone structures are potentially hazardous areas on which to operate. The stone is uneven with gaps and the sloping front face pose an increased risk. Individual stones can be smooth and can be particularly slippery when wet. Near the waterline, water level variation exposes weed and/or algal growth making the surface slippery and movement can be dangerous and requires great care. Monitoring should not be undertaken when ice is present on the groyne.

A team of at least two people should carry out the monitoring. All personnel should wear personal floatation devices. The monitoring team should be familiar with the nature of the lake including storm waves and should check the marine weather forecasts on a regular basis. Special care shall be exercised on the slope of the structure, near the water’s edge and on wet or slippery surfaces. Snorkeling should only be carried out by qualified personnel and in strict accordance with all applicable worker safety rules and regulations.


Maintenance of the rubble mound groyne in response to the deterioration or “wear and tear” of the feature can be expected. In addition to the replacement costs due to long-term deterioration of the stone, there should be an allowance for repairs to the reef required as a result of storms exceeding the design event.

Repairs might include replacing displaced or cracked armour stones and backfilling slumped core material at the toe of the slope. Available data on maintenance costs of marine structures is limited. However, an estimate for an annual maintenance and repair budget of between 0.5% and 1.0% of the initial capital cost would be reasonable. It is unlikely that actual maintenance and repair work would be necessary every year, but the annual allowance should be budgeted on an ongoing basis to have sufficient funds accumulated when repair work is required.

Marine plant will likely be required to carry out repairs. The plant will include a crane and/or backhoe mounted on spud barge, material delivery barges and tugs to move the barges to and from the site and to position the barges around the site. Stone material should be sound, durable and meeting the original specifications. The material should be clean, inert rock meeting the lakefilling requirements of the Ministry of Environment.


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7. References

Baird & Associates, 2011. Evaluation of Potential Offshore Sand Sources. Memo prepared for TRCA by W.F. Baird & Associates as part of Gibraltar Point Erosion Control Project, March 2011.

Baird & Associates, 2015. Gibraltar Point Erosion Control, Final Design. Report prepared for TRCA and the City of Toronto by W.F. Baird & Associates, August 2015.

Toronto and Region Conservation Authority (TRCA), 2008. Environmental Study Report, Gibraltar Point Erosion Control Project, City of Toronto. February 15, 2008.

Van der Meer, J.W., 1988. Rock slopes and gravel beaches under wave attack. Delft Hydraulics Communication, vol. 396.

Van Gent, M.R.A., A.J. Smale and C. Kuiper, 2003. Stability of rock slopes with shallow foreshores. ASCE, Proc. Coastal Structures, Portland, USA.

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