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Cost Estimates for Dolos Revetment with Reclamation (Option 2) ... A conventional revetment with a design life of 50 years is required in order to safeguard ...
Granger Bay Rock Revetment Concept Study ______________________________________________________________________________________________________________

VICTORIAL & ALFRED WATERFRONT GRANGER BAY ROCK REVETMENT CONCEPT STUDY

TABLE OF CONTENTS PAGE NO.

EXECUTIVE SUMMARY

1.

INTRODUCTION ................................................................................................................................... 4

1.1

Background .............................................................................................................................................. 4

1.2

Scope of Work .......................................................................................................................................... 4

2.

DAMAGE ASSESSEMENT AND FUTURE RISKS ........................................................................... 5

2.1

Gravel Beach ............................................................................................................................................ 5

2.2

Unprotected Embankment ...................................................................................................................... 9

3.

REMEDIAL WORKS ............................................................................................................................. 10

3.1

Repair Options to Gravel Beach............................................................................................................. 10

3.1.1

Rock revetment - minimum capital cost .................................................................................................... 10

3.1.2

Dolos revetment with land reclamation ..................................................................................................... 10

3.1.3

Dolos revetment without land reclamation ................................................................................................ 10

3.2

Repair Work at Unprotected Embankment .......................................................................................... 11

4.

DESIGN BASIS ....................................................................................................................................... 12

4.1

Guidelines and Codes of Practice ........................................................................................................... 12

4.2

Design Life ................................................................................................................................................ 12

4.3

Topography and Bathymetry ................................................................................................................. 12

4.4

Water Levels ............................................................................................................................................ 12

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4.4.1

Astronomical Tides.................................................................................................................................... 12

4.4.2

Design Water Levels.................................................................................................................................. 13

4.5

Wave Climate ........................................................................................................................................... 13

5.

DESIGN PROCEDURES ....................................................................................................................... 15

5.1

Revetment Design .................................................................................................................................... 15

5.2

Revetment Sections .................................................................................................................................. 19

5.2.1

Rock Revetment at Gravel Beach .............................................................................................................. 19

5.2.2

Dolos Revetment with Reclamation .......................................................................................................... 19

5.2.3

Dolos Revetment without Reclamation ..................................................................................................... 19

5.2.4

Rock Revetment at Unprotected Embankment .......................................................................................... 19

5.3

Construction Materials ........................................................................................................................... 20

5.4

Setback Lines ........................................................................................................................................... 20

6.

COST ESTIMATE .................................................................................................................................. 22

7.

CONCLUSIONS ...................................................................................................................................... 26

APPENDICES

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LIST OF TABLES

Table 4.1

:

Tide Levels for Cape Town Harbour (SA Navy Tide Tables)

Table 4.2

:

Design Water Levels

Table 5.1

:

Limits for Overtopping from EurOtop (2007)

Table 5.2

:

Overtopping Discharges: Rock and Dolos Revetments

Table 5.3

:

Rock Quality Parameters

Table 6.1

:

Cost Estimates for Rock Revetment (Option 1)

Table 6.2

:

Cost Estimates for Dolos Revetment with Reclamation (Option 2)

Table 6.3

:

Cost Estimates for Dolos Revetment without Reclamation (Option 3)

LIST OF FIGURES

Figure 2.1

:

Extents of Gravel Beach and Unprotected Embankment

Figure 2.2

:

Abraded Rock Fragments on Gravel Beach

Figure 2.3

:

Existing Revetment on Gravel Beach

Figure 2.4

:

Overtopping Discharges for Existing Revetment

Figure 2.5

:

Unprotected Embankment Consisting of Random Fill

Figure 4.1

:

Effect of water depth on significant wave height at the structure toe

Figure 5.1

:

Effect of water depth on revetment rock size

Figure 5.2

:

Effect of water depth on armour unit size

Figure 5.3

:

Spatial distribution of overtopping discharges (for rock and dolos revetment without reclamation)

LIST OF APPENDICES

Appendix A1

:

Rock Revetment – Minimum Capital Cost

Appendix A2

:

Dolosse Revetment with Reclamation

Appendix A3

:

Dolosse Revetment without Reclamation

Appendix A4

:

Rock Revetment at Unprotected Embankment

Appendix B1

:

Plan Layout - Rock Revetment – Minimum Capital Cost

Appendix B2

:

Plan Layout - Dolos Revetment with Reclamation

Appendix B3

:

Plan Layout - Dolos Revetment without Reclamation

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EXECUTIVE SUMMARY

Existing Situation

The existing unprotected shoreline between Oceana Powerboat Club and the V&A dolos revetment is vulnerable to damage and poses a flooding risk to the adjacent areas. In the event of damage to the shoreline overtopping and flooding of infrastructure and services is inevitable.

Shoreline between Oceana Powerboat Club and V&A dolos revetment

The gravel beach just west of the dolos revetment was originally designed as a temporary dynamic rock revetment using a quarry yield with a grading of 50 to 1000 kg. This rock has been degraded to a gravel size due to the dynamic behaviour of the structure. A small strip of large rock on the upper profile has remained due to limited wave impact at these higher levels and partly because of a continuous supply of new armour rock. Due to the high mobility of the gravel in the lower profile, the structure is presently at risk of being breached under extreme storm conditions. Such a breach will result in flooding of the landward V& A property and in particular the underground parking basement.

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The embankment west of the gravel beach consists of uncontrolled random fill. Ad hoc repairs are done when damage is sustained and as there is no as-built information this may be considered as a high risk “structure” with significant uncertainty in terms of performance under extreme storm conditions. This was illustrated by the August 2012 storm that eroded more than 4 m of the bank over significant distances and required repair work of approximately R1 million.

Unprotected Embankment

Design Options and Preliminary Cost Estimates

A conventional revetment with a design life of 50 years is required in order to safeguard landward properties and future developments. For the unprotected embankment a rock revetment of 3 to 6t rock at a slope of 1:1.5 is proposed. For the more exposed gravel beach section the following three options were investigated (all cost estimates include the rock revetment cost for the unprotected embankment section): •

Option 1 (Appendix A1 and B1): A 3 to 6 t rock revetment with a toe level at -3.5m MSL and slope of 1:3.6 as a minimal capital cost option – R 69million.



Option 2 (Appendix A2 and B2): A dolos revetment with a toe level at -6m MSL at an average offset of 60m from the existing shoreline structure, including the reclamation of 2 ha of land and fill disposal potential of 135 000 m3 – R 181 million



Option 3 (Appendix A3 and B3): In case a detailed design indicates that rock is not an appropriate solution for Option 1 (for the gravel beach section), a dolos alternative could be considered – R 118 million

All the structures have been designed for a life of 50 years. Costs estimates include provision for the following: Preliminary and general: 30% Contingency:

25%

Professional fees :

10%

From an engineering and planning perspective Option 2 is preferred for the following reasons: •

Efficient use of surplus fill. Option 2 requires 145 000 m3 fill material which could be obtained from various V&A based sources which would otherwise have to be hauled to an approved disposal site.



Improved geometry. The discontinuity between the dolos and rock revetment is avoided (potential damage to dolos units due to rock displacements). Sharp bends along revetments leads to lower stability which is avoided with a straight alignment.

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Larger set-back distance. The straight line edge provides for a much larger set-back distance. This would ensure less impact of overtopping and spray during storm conditions.



Confirmed toe stability. Previous model studies have confirmed toe stability for the dolos revetment. The shallower rock revetment may experience toe stability problems that may require founding in deeper water at an increased cost.



Predictable wave reflection. The corner between the dolosse and rock has the potential to focus reflections towards the west (Oceana), which is avoided with a straight line. Potential larger reflections off a steeper dolos slope have been confirmed not to be a problem in previous model studies.



Stormwater run-off. By following the proposed straight line the stormwater outfall in the corner of the rock/dolos interface can be properly engineered and accumulation of debris avoided.



Landside planning. An abrupt end to the dolos revetment will integrate poorly with the existing road and storm water infrastructure and inhibit continuous public access to the shoreline.

Setback Lines for Development

The Integrated Coastal Management Act (DEA/SSI, 2009) stipulates a 100 m setback distance landward of the high water mark. WSP (2010) recommends that development setback distances be calculated by taking into account erosion, wave run-up and overtopping for a 100 year return period. Due to the lack of information regarding material types and gradings along the existing shoreline it would be difficult to define a setback line for the existing shoreline with any degree of certainty.

The new structure has been designed for a storm event with a return period of 100 years in order to protect the landward area in respect of overtopping, erosion and flooding. The proposed new rock revetments will limit overtopping to allow development to extend to within approximately 15 m of these structures. For Option 2 (dolos revetment at -6m MSL), the wave conditions on the structure will be more severe and development would have to be set back more or alternatively crest levels would have to be increased. A drainage system will have to be incorporated over a distance of approximately 15 m from the crest to manage the drainage of overtopped water.

Once conventional revetments have been constructed along the V&A shoreline west of the existing dolos revetment, the set-back line for development will be approximately 15 m for most of the area. This will enable development of a large area as indicated by the setback lines in Appendix B1 and B2.

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1.

INTRODUCTION

1.1

Background

The developments in the area west of Cape Town Harbour western breakwater some years ago made it necessary to reclaim a strip of land currently part of the V&A Waterfront. The new coastline was protected partly with a permanent dolos revetment and partly with a temporary rock revetment. The revetment was constructed on two terraces at +3.0m MSL and +7.5m MSL and has subsequently been reshaped by wave action to a natural s-profile. The revetment was constructed using the available quarry rubble from the old upper tank farm area. This temporary revetment (here referred to as gravel beach) has since reached the end of its design life and repair work is therefore required to minimize the risk of damage to adjacent property.

Further westward towards Granger Bay small craft harbour, there is an unprotected embankment of uncontrolled random fill consisting of a combination of sandy gravel, rock and concrete rubble. To ensure reliable protection against wave attack a conventional rock revetment is essential along this section. This was illustrated by the August 2012 storm that eroded more than 4 m of the bank over significant distances and required repair work of approximately R1 million.

1.2

Scope of Work

The scope of work entails a field inspection, assessment of damage and the associated risks. Different options for managing the gravel beach area are defined and this is aimed at identifying the most economical option as well as options that include obtaining additional reclaimed land together with preliminary cost assessments of each of these options. A standard rock revetment is proposed to manage the risk associated with the unprotected embankment area.

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2.

DAMAGE ASSESSEMENT AND FUTURE RISKS

An engineering inspection of the gravel beach and unprotected embankment was carried out on 14 September 2010. This inspection was land based and consisted of visually inspecting the rock and rock grading, wave structure interaction and general state of the revetment and embankment, as well as compiling a photographic record of the inspection. Figure 2.1 below shows the extent of the gravel beach and unprotected embankment.

FIGURE 2.1: EXTENTS OF GRAVEL BEACH AND UNPROTECTED EMBANKMENT 2.1

Gravel Beach

The revetment was originally designed as a dynamic rubble beach shore protection using a quarry yield with a grading of 50 kg to 1000kg (WPR, 1993). Rock on the revetment slope has been severely abraded resulting in much reduced stone sizes. An average rock mass is now estimated to be in the order of 1kg as shown in Figure 2.2 below:

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FIGURE 2.2: ABRADED ROCK FRAGMENTS ON GRAVEL BEACH

The revetment rock structure near and around the crest level has remained intact due to the limited wave attack at these levels of the shore protection and partly because of additional material that may have been placed as part of ongoing maintenance. This condition is indicated in Figure 2.2.

FIGURE 2.3: EXISTING REVETMENT ON GRAVEL BEACH

Coastal rock structures exposed to direct wave attack can be classified by means of the stability number. The stability number Ns is defined as:

Where: Hs

= significant wave height at toe of structure

Dn,50

= median stone size



= relative density

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With respect to static and dynamic stability, the structures can be classified as statically stable structures and dynamically stable (reshaping) structures:

Structure Type Statically stable structures

Ns

Description

1-4

No or minor damage to the armour layer is allowed under design conditions

Dynamic/reshaping structures

3-6

Structures are allowed to be reshaped by wave attack, resulting in a development of their profile. Individual pieces are displaced by wave action until the transport capacity along the profile is reduced to such a low level that an almost static profile is reached

Dynamic rock structures

6-20

Diameter of the armour stones is relatively small and cannot

withstand

severe

wave

attack

without

displacement Gravel beach

15-200

Gravel beaches will change continuously under varying wave conditions and water levels.

The original structure was designed as a dynamic reshaping breakwater with a stability number around 4. The stability number Ns of the actual structure has been increased to a value significantly above 20. This means that the lower section of the structure will continuously reshape as a gravel beach under wave action, vulnerable to material losses.

The size of the remaining rock fragments will continue to decrease over time due to the increase in abrasion. This will further reshape the structure and flatten the slope. More and more material in the upper profile will become dynamic and vulnerable to abrasion.

The picture shown in Figure 2.2 clearly indicates that the intertidal zone along the revetment has been the area most vulnerable to rock movement, abrasion and reshaping. The revetment therefore needs repair to:



Reduce risk of damage to property, operation and/or infrastructure landward of the revetment.



Reduce the risk of erosion of the lower profile of the revetment



Reduce the risk of damage or a breach in the crest of the revetment resulting in flooding of the land behind the revetment and flooding of the V&A parking basement.



Limit overtopping and its associated risks (inconvenience and danger for pedestrians, damage to parked cars, flooding of low laying areas) – this has a direct impact on the property insurance premiums since flood insurance rate is determined by the level of exposure of property relative to the hazard.



Minimize the possibility of rocks being thrown landward of the revetment under severe storms.

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In order to assess the risk to the existing structure, the average overtopping discharge has been estimated for design wave conditions with a return period of 100 years. These overtopping volumes have been compared with acceptable overtopping discharges for different crest levels. It is not possible to give unambiguous or precise limits to tolerable overtopping for all conditions. EurOtop Manual (Pullen et al, 2007) gives some guidance for different conditions:

Conditions

Discharge

Remark

[l/s.m] Pedestrians with a clear view

0.1

to the sea

This conditions has not been used in the assumption that no pedestrians will be present during extreme sea conditions

Building structure elements

1

Although buildings are not present in the directly behind the revetment, this condition is set as a safe limit, as the area behind the revetment runs down towards the shopping centre of the V&A waterfront.

Vehicles

driving

at

low

10-50

speed, vehicles not immersed

The lower limit is used as an absolute maximum to prevent flooding of the basement parking of the V&A property.

Damage

to

paved

or

200

armoured promenade behind seawall

Figure 4.2 below indicates that the existing rock revetment doesn’t meet the safe limit of 1l/s.m for extreme storm conditions with a return period of 100 years. For a wave height of 3m at the toe of the structure, the limit of 1l/s.m is met, but as soon as the crest level drops below 6.0m MSL the absolute maximum allowable overtopping discharge is exceeded. As the lower profile of the rock structure has been degraded to a gravel beach, this section of the revetment is highly vulnerable to erosion during extreme storm conditions. The risk of damage and breaching of the upper profile is high.

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FIGURE 2.4: OVERTOPPING DISCHARGES FOR EXISTING REVETMENT 2.2

Unprotected Embankment

This area constitutes a steep embankment of random fill consisting of sand, gravel, rock and concrete rubble, as shown in Figure 2.5. This part of the shoreline is vulnerable to wave attack and therefore a conventional revetment is required to ensure a reliable and safe coastal edge similar to that proposed for the gravel beach.

FIGURE 2.5: UNPROTECTED EMBANKMENT CONSISTING OF RANDOM FILL

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3.

REMEDIAL WORKS

3.1

Repair Options to Gravel Beach

3.1.1

Option 1. Rock revetment - minimum capital cost

This refers to the minimum repair work required to manage the risk of major damage during a single storm. It entails a standard rock revetment with a flat slope in order to ensure stability of available rock gradings. Limiting the toe level to -3.5 m MSL enables the use of rock (depth limited waves) compared to a requirement for dolos units where toe levels are deeper.

3.1.2

Option 2. Dolos revetment with land reclamation

This approach considers using dolosse as the primary armour and therefore allows the revetment to be positioned further seaward in order to reclaim additional land.

The cost of the revetment and

associated reclamation costs should be related to the estimated value of new land at this location in order to assess the viability of this option. A preliminary layout has been chosen to provide a rough cost estimate of this shoreline protection option. It is designed to ensure that there will not be any major repair costs during the design life of the structure.

3.1.3

Option 3. Dolos revetment without land reclamation

This option is similar to the minimum capital cost revetment but armoured with dolosse. It provides an alternative solution in case adequately sized rocks are not available. There is no land reclamation proposed for this option and therefore no new land income will be generated.

3.1.4

Maintenance Costs

PRDW (2011) provided guidelines for on-going maintenance to the existing revetment. This strategy is not recommended because it does not fully address the risk of breaching and flooding under extreme storm conditions. The option of simply maintaining the existing revetment can therefore not be compared with Options 1 to 3 which represent final solutions to the risk of breaching. However, the cost of on-going maintenance has been estimated to provide some form of reference in assessing the cost of final solutions. PRDW (2011) estimated that the type of repair work carried out to date could be expected to be significantly abraded over a period of approximately 5 years. Using this as a basis for the frequency of repair a cost of R1 million could potentially be spent every 5 years in order to maintain the revetment.

Options 1 to 3 will be designed for minimal to no maintenance. Typically maintenance for these designs would only be required under exceptional storm events.

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3.2

Repair Work at Unprotected Embankment

A rock revetment is proposed along this part of the shoreline to prevent undercutting of the slopes by wave action. This part of shore protection works will be in addition to any one selected option in section 3.1 above. To ensure a proper filter between the revetment and existing reclamation fill a geotextile is required. This implies removing large rocks and/or blocks prior to placing a geotextile and constructing the new revetment.

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4.

DESIGN BASIS

This section provides a technical basis for the design of the shoreline protection for the rock revetment.

4.1

Guidelines and Codes of Practice

The following publications will take precedence unless indicated to the contrary:

4.2



EurOtop Wave overtopping of Sea Defences and related structures: Assessment Manual



CUR/CIRIA/CETMEF Rock Manual



CEM Coastal Engineering Manual



BS 6349-7:1991 : Maritime structures. Guide to the design and construction of breakwaters

Design Life

It is proposed that both the dolos and rock revetment be designed for a 50 year design life.

4.3

Topography and Bathymetry

The source of the available topographic and bathymetric surveys cannot be confirmed at present. Information available within PRDW has been assumed to be sufficiently accurate for this concept study.

4.4

Water Levels

4.4.1

Astronomical Tides

Table 4.1 below shows the astronomical tide levels as recorded in the South African Navy Tide Table.

TABLE 4.1 TIDE LEVELS FOR CAPE TOWN (SA NAVY TIDE TABLES) Tide

Level (m MSL)

Highest Astronomical Tide

+1.20

Mean High Water Springs

+0.92

Mean Level

0.16

Mean Low Water Springs

-0.58

Lowest Astronomical Tide

-0.83

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4.4.2

Design Water Levels

The design water levels are presented in Table 4.2. These water levels are made up of the mean high water spring level, tidal residuals (which include the effects of wind, atmospheric gradients, density, currents and other similar phenomena) and the predicted effects of climate change, including long term sea level rise. Details of tidal residuals and effects of climate change are included in the PRDW’s Climate change position paper, (PRDW, 2010). The effects of climate change over the next 50 years have been considered for the design of revetments.

TABLE 4.2 DESIGN WATER LEVELS (m MSL)

Design Water Level (m MSL)

100 years (without climate change)

climate change projected to 2060)

MHWS

+0.92

+0.92

Storm Surge

0.74

0.74

Sea Level Rise (2060)

0

0.48

Increase in Storm Surge

0

0.08

+1.66m MSL

+2.22m MSL

Design Water Levels 4.5

100 years (with

Wave Climate

In June 1994 an exceptionally large sea storm occurred with a peak wave period of 18.3 seconds and significant wave heights recorded at Slangkop and Table Bay of 8.5m and 9.6m respectively (WPR, 1994). Therefore an offshore significant design wave height of 9.6m and 18 second peak wave period has been assumed for this study.

The offshore design waves have been transformed to design waves in front of the toe of the structures by means of the method of Goda (1975). This simplified method is known to be conservative and incorporates some safety in the revetment design. A foreshore slope of 1:20 was used to determine the shallow water wave conditions. The 1:20 slope is estimated to be representative of some of the steeper sections of coastline along the Granger bay revetment and is therefore probably on the conservative side.

The water depths in front of the dolos revetment with reclamation and the rock revetment are 8m and 6m respectively. Figure 4.1 shows the depth limited wave height as a function of water depth for an unrefracted deep sea wave as well as an assumed refracted wave condition corresponding to a 30% ______________________________________________________________________________________________________________ Prestedge Retief Dresner Wijnberg (Pty) Ltd 13

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reduction in deep sea height. This only serves to illustrate the possible effect of refraction. A full refraction study will be required for a detailed design.

FIGURE 4.1: EFFECT OF WATER DEPTH ON SIGNIFICANT WAVE HEIGHT AT THE STRUCTURE TOE

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5.

DESIGN PROCEDURES

5.1

Revetment Design

5.1.1

Rock revetment alternative

The stability of armour rock to the design waves has been calculated by using the stability equations according to van der Meer (CIRIA/CUR/CETMET, 2007). Van der Meer’s formulae are listed below:

Hs  S  = 6.2 ⋅ P 0.18 ⋅  d  ∆ ⋅ Dn 50  N

0.2

Hs  S  = 1.0 ⋅ P −0.13 ⋅  d  ∆ ⋅ Dn 50  N

⋅ ξ m−0.5 For plunging waves 0.2

⋅ cot α ⋅ ξ mP For surging waves

The parameters featuring in the above formulae are listed below: Hs

significant wave height [m]



relative density [-]

Dn50

nominal diameter of the rock [m]

P

permeability factor [-]

Sd

damage level [-]

N

number of waves [-]

ξm

surf similarity parameter [-], based on Tm-1,0

α

slope [°]

The following basic values were assumed for the above parameters: ρr

2650 kg/m3

ρw

1025 kg/m3

P

0.4 [CIRIA/CUR/CETMEF, 2007]

Sd

Sd can be seen as the number of cubic stones with a side of Dn50,armour eroded within a Dn50,armour wide strip of the structure . The actual number of stones eroded within this strip can be more or less than Sd, depending on the porosity, the grading of the armour stones and the shape of the stones. Generally the actual number of stones eroded in a Dn50, armour wide strip is equal to 0.7 to 1 times the damage level Sd

N

7500

Tm

Tm-1,0 was assumed

Figure 5.1 below indicates the median rock size of the armour layer (for a damage level Sd=6) for extreme offshore wave events (June 1994 conditions) as a function of water depth at the structure toe. From this figure it is clear that rock sizes are highly dependent on the seabed level in front of the structure. The proposed rock grading is given in section 5.1.1. ______________________________________________________________________________________________________________ Prestedge Retief Dresner Wijnberg (Pty) Ltd 15

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FIGURE 5.1: EFFECT OF WATER DEPTH ON REVETMENT ROCK SIZE 5.1.2

Dolos revetment alternative

The stability of dolos to the design waves has been calculated by using the stability equations according to Burcharth and Liu (CIRIA/CUR/CETMEF, 2007). Burcharth and Liu formulae are listed below:

The parameters featuring in the above formulae are listed below: Hs

significant wave height [m]



relative density [-]

Dn

nominal diameter of dolos units [m]

N

number of waves [-]

Nod

number of displaced armour units within a strip of breakwater slope of width Dn [-]

r

dolos waist ratio [-]

ф

packing density coefficient[-]

Figure 5.2 shows the results of average dolos size required relative to water depth for the various wave periods. The same conclusions drawn in section 5.1.1 above, that armour unit sizes are highly dependent on water depth at the structure toe, are made here as well. Sections 5.1.2 and 5.1.3 give the proposed armour unit sizes for respective revetment options.

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FIGURE 5.2: EFFECT OF WATER DEPTH ON ARMOUR UNIT SIZE 5.1.3

Crest Level Design

Wave overtopping is usually given as an average discharge per linear metre of structure length. Damage to seawalls, buildings or infrastructure has been defined as a function of the mean overtopping discharge. Table 5.1, taken from the EurOtop Manual (Pullen et al, 2007), provides limits for embankment and revetment seawalls.

TABLE 5.1: LIMITS FOR OVERTOPPING FROM EUROTOP (2007)

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The area behind the revetment is sloping down towards the buildings and there is a basement parking lot as well. Therefore a strict allowable overtopping discharge of 1 l/s/m is recommended. If this value is exceeded, a drainage system is proposed behind the revetment to convey overtopped discharges back into the sea to safeguard low lying areas from flooding.

Overtopping was calculated using formulae according to EurOtop Manual (Pullen et al, 2007) on Wave Overtopping of Sea Defences and Related Structures. Wave overtopping can be described in two formulae: one for breaking waves (breaking parameter 2).

Influence factors for berm and angle of wave attack have been set at 0.8 and1 respectively. Influence of roughness for a 2 layer rock slope is 0.55. This influence factor is only valid for breaking parameters lower than 1.8. For values in between 1.8 and 10, the effect of surface roughness on wave run-up and overtopping reduces linearly. For large values of the breaker parameter (larger than 10) run-up is similar for the armour rock and smooth slopes, and the influence factor is set to 1 in this case. For design conditions, the breaker parameter is 3.3 and 2.8 for rock and dolos revetments respectively.

The wave overtopping formulae are exponential functions with the general form:

q = a. exp(b.Rc ) , Where q

average wave overtopping discharge (m3/s per m)

Rc

free crest height above design still water level

a,b

functions of wave height, slope angle, breaker parameter and different influence parameters

The following Table shows overtopping discharges for dolosse and rock revetment options.

TABLE 5.2:

Rock Revetment Dolos Revetment with Reclamation Dolos Revetment Without Reclamation

[l/m/s]

discharge

Overtopping

wavePeriod

Peak

structure [m]

Height at toe of

Significant Wave

at toe [m MSL]

Min seabed level

[m MSL]

Water level

Return period

[m MSL]

TYPE

Level

REVETMENT

Revetment Crest

OVERTOPPING DISCHARGES: ROCK AND DOLOS REVETMENT

5.13

+7.5

100 yrs

+2.22

-3.5

5.28

18s

+8.0

100 yrs

+2.22

-6.0

7.03

18s

26.48

+7.5

100 yrs

+2.22

-3.5

5.28

18s

5.13

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Granger Bay Rock Revetment Concept Study ______________________________________________________________________________________________________________

Overtopping rates are very sensitive to small variations in seawall geometry, local bathymetry and wave climate. Furthermore it should be noted that overtopping rates, using empirically derived equations, should only be regarded as being within, at best, a factor of 3 of the actual overtopping rate (HR Wallingford, 1999). Overtopping discharges presented in Table 5.2 above are higher than a design value of 1 l/s/m, therefore provision of a drainage system is recommended.

5.2

Revetment Sections

5.2.1

Rock Revetment at Gravel Beach

This revetment option is designed as a stable shoreline protection expected to sustain intermediate damage under the design storm. It consists of 3 to 6 ton armour rock and 5 to 3000kg core, at a flat slope of 1:3.6 and crest level at +7.5m MSL as shown in Appendix A1. The structure toe level is at 3.5m MSL to reduce material volume, simplify the construction and to minimise the revetment costs.. The layout of this option is shown in Appendix B1. Surveys of the existing beach profile will have to be carried out during the detail design stage in order to accurately develop the cost estimates. 5.2.2

Dolos Revetment with Reclamation

A dolos revetment with a design life of 50 years is designed for a design water level with a return period of 100 years and therefore considers the effect of climate change. The structure toe is founded at a seabed level of -6m MSL and therefore gives potential for additional land reclamation. The structure as shown in Appendix A2 is designed as a 3 to 5000 kg rock bund with a crest width of 4m. The seaward side of the core bund is protected by a 2 to 4 ton rock underlayer and 20 ton dolosse primary protection. The landward side of the bund could be reclaimed with material obtained from various V&A based sources which would otherwise have to be carted away. Appendix B2 shows the plan layout of this option.

5.2.3

Dolos Revetment without Reclamation

A dolos revetment with a structure toe founded at a seabed level of -3.5m MSL as presented in Appendix A3 will consists of 5 to 300kg core and 2 to 4t rock mound toe acting as sacrificial scour protection. The 2 to 4t rock continues to act as an underlayer for the 20 ton dolos primary armour layer with a revetment crest level at +7.5m MSL. The plan layout is shown in Appendix B3.

5.2.4

Rock Revetment at Unprotected Embankment

A stable shore protection consisting of 3 to 6t armour rock and a 5kg to 3000kg core is proposed. Existing rock and concrete rubble will be cleared to prepare for placement of a Grade A10 geotextile, to prevent migration of fines from the embankment slopes. The revetment has a variable crest width and a crest level varying between +3m MSL and +7m MSL as shown in Appendix A4. The design crest level

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Granger Bay Rock Revetment Concept Study ______________________________________________________________________________________________________________

is low due to the perceived shelter from waves along this section. It will require checking once a wave refraction study has been completed as part of a detailed design. Crest levels can be designed to avoid wave overtopping in this sheltered area The layout of this rock revetment is shown in Appendices B1 to B3 since shore protection in this area has been proposed regardless of which revetment option has been selected for the gravel beach area.

5.3

Construction Materials

5.3.1

Rock

It is envisaged that rock will be sourced from Tygerberg quarry operated by Lafarge. This rock must meet the typical rock quality parameters as given by CIRIA/CUR/CETMET (2007) as shown in Table 5.3 below:

TABLE 5.3 ROCK QUALITY PARAMETERS

ARMOUR

UNDERLAYER

CORE

Weathering

Minimal

Minimal

Minimal

Discontinuity Spacing

1.0 m+

0.5 m+

0.2 m+

Porosity (percentage)

0-5

0 - 10

0 – 10

Water Absorption

< 2.0

< 2.5

< 3.0

> 100

> 100

> 50

Rock Density (kg/m )

> 2600

> 2600

> 2600

RQD* (percentage)

80 - 100

75 - 100

55 – 100

Uniaxial Compr. Strength (MPa) 3

*Rock Quality Designation: ratio of intact core sections greater than 100 mm length to total length drilled, expressed as a percentage.

The shape of the rock normally specified as a ratio of maximum axial length and minimum axial breadth, should ideally be less than 1.5 to 2.0.

5.4

Setback Lines

A setback line can be described as the distance that any development should be set back from the coastline or a buffer zone between the development and the sea. CSIR (2006) defined a setback line as a line landward of which fixed structures may be built with reasonable safety against the physical impact of the coastal processes. WSP (2010) recommends that development setback distances be calculated by taking into account erosion, wave run-up and overtopping for a 100 year return period. Due to the lack of information regarding material types and gradings along the existing shoreline it would be difficult to

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define a setback line for the existing shoreline with any degree of certainty. This was not attempted since it was beyond the scope of the present study.

Two setback lines are considered in this study namely, an offset distance of 100m landward from the high water mark as stipulated in the Integrated Coastal Management Act (DEA/SSI, 2009). The second setback line is defined as the distance from the crest where overtopping discharge drops below the limit of 1 l/s/m and is proposed to be located at 15m landward from the crest line provided that adequate drainage is provided for.

For option 2 (dolos revetment at -6m MSL), the wave conditions will be more severe and development would have to be set back more or alternatively crest levels would have to be increased. Figure 5.3 shows overtopping discharge spatial distribution from the crest line. From the Figure it is evident that there is limited or no overtopping at the location of the proposed setback line. However, since the area behind the revetment crest gradually slopes down towards the V&A waterfront shopping centre, there could be a risk of inundation during major storms therefore a drainage system is proposed in the area between the crest and the setback line to drain water back into the sea to safeguard low lying areas from flooding.

Once conventional revetments have been constructed along the V&A shoreline west of the existing dolos revetment, the setback line for development will be approximately 15m for most of the area. This will allow development of a large area as indicated by setback lines in Appendix B1 to B3.

FIGURE 5.3: SPATIAL DISTRIBUTION OF OVERTOPPING DISCHARGES (FOR ROCK AND DOLOS REVETMENT WITHOUT RECLAMATION)

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Granger Bay Rock Revetment Concept Study ______________________________________________________________________________________________________________

6.

ESTIMATE

Costs have been estimated for a rock revetment, dolos revetment with reclamation and dolos revetment without reclamation as summarised below:

TABLE 6.1: COST ESTIMATES FOR ROCK REVETMENT (OPTION 1)

ITEM DESCRIPTION

UNIT

QTY

RATE

AMOUNT

t

9 945

337

3 360 000

0 5 850

302 57

340 000

75 531 44 430

351 181

26 520 000 8 050 000

8 000

53

430 000

REVETMENT Rock Supply to stockpile core material by selection and sorting from suitable rock and concrete material from demoliiton stockpile Supply core rock (5 - 3000kg) from approved quarry Construct bund with core of grading 5 - 3000kg Supply rock of grading 3 - 6t from approved quarry Construct underlayer with rock of grading 3 - 6t

t 3

m t

3

m

Geotextile Supply and place Bidim A10 geotextile or equivalent

2

m SUB-TOTAL Preliminaries and Generals Design Risk Allowance Engineering TOTAL

30 25 10

38 670 000 11 600 000 12 570 000 6 290 000 69 110 000

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Granger Bay Rock Revetment Concept Study ______________________________________________________________________________________________________________

TABLE 6.2: COST ESTIMATES FOR DOLOS REVETMENT WITH RECLAMATION (OPTION 2)

ITEM DESCRIPTION

UNIT

QTY

RATE

AMOUNT

t

53 366

337

17 990 000

86 503 82 276

302 57

26 130 000 4 690 000

44 294 26 055

351 181

15 550 000 4 720 000

m

6 695 3 938

351 181

2 350 000 720 000

Concre te Units Manufacture and place in storage 20 tonne dolos units Transport and place 20 tonne dolos units

No No

1 245 1 245

17 835 3 087

22 210 000 3 850 000

Geotextile Supply and place Bidim A10 geotextile or equivalent

m

2

8 000

53

430 000

Concre te Cap Class 35/19 plain concrete to revetment edge wall + formwork & steel

m

3

270

3 843

1 040 000

RECLAMATION Select, load and transport fill material from stockpile, and place in reclamation: Zone 1: below +1.5m MSL

m

Zone 2: above +1.5m MSL (level and compact to 95% Mod AASHTO)

m

Gravel surfacing to reclamation: place and compact to 98% Mod AASHTO

m

REVETMENT Rock Supply to stockpile core material by selection and sorting from suitable rock and concrete material from demoliiton stockpile Supply core rock (5 - 3000kg) from approved quarry Construct bund with core of grading 5 - 3000kg Supply rock of grading 2 - 4t from approved quarry Construct underlayer with rock of grading 2 - 4t

t 3

m t

3

m t

Supply rock of grading 3 - 6t from approved quarry Construct underlayer with rock of grading 3 - 6t

3

SUB-TOTAL Preliminaries and Generals Design Risk Allowance Engineering TOTAL

3

38 824

0

0

3

106 042

0

0

3

10 850

143

1 560 000

30 25 10

101 190 000 30 360 000 32 890 000 16 450 000 180 870 000

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Granger Bay Rock Revetment Concept Study ______________________________________________________________________________________________________________

TABLE 6.3: COST ESTIMATES FOR DOLOS REVETMENT WITHOUT RECLAMATION (OPTION 3)

ITEM DESCRIPTION

UNIT

QTY

RATE

AMOUNT

t

53 366

337

17 990 000

44 365 57 489

302 57

13 400 000 3 280 000

25 276 14 868

351 181

8 880 000 2 700 000

13 770 8 100

351 181

4 840 000 1 470 000

17 835 3 087

11 030 000 1 910 000

53

390 000

REVETMENT Rock Supply to stockpile core material by selection and sorting from suitable rock and concrete material from demoliiton stockpile Supply core rock (5 - 3000kg) from approved quarry Construct bund with core of grading 5 - 3000kg Supply rock of grading 2 - 4t from approved quarry Construct underlayer with rock of grading 2 - 4t

t 3

m t

3

m t

Supply rock of grading 3 - 6t from approved quarry Construct underlayer with rock of grading 3 - 6t

3

m

Concre te Units Manufacture and place in storage 20 tonne dolos units Transport and place 20 tonne dolos units

No No

618 618

Geotextile Supply and place Bidim A10 geotextile or equivalent

m

2

7 290

SUB-TOTAL Preliminaries and Generals Design Risk Allowance Engineering TOTAL

30 25 10

65 840 000 19 760 000 21 400 000 10 700 000 117 690 000

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Granger Bay Rock Revetment Concept Study ______________________________________________________________________________________________________________

7.

ENGINEERING AND PLANNING CONSIDERATIONS

Three different options have been presented to provide a range of engineering solutions for a final revetment edge along the Granger Bay coastline. Cost estimates indicate that a rock revetment may be the most economical. However, from an engineering and planning perspective extending the existing dolos revetment along a straight line is preferred (Option 2). The following points provide the basis for this preference:



Option 2 requires 145 000 m3 fill material which could be obtained from various V&A based sources. Without this reclamation the surplus material would have to be hauled to an approved disposal site.



Improved geometry. The discontinuity between the dolos and rock revetment is avoided and potential damage to dolos units due to rock displacements under storm conditions avoided. Sharp bends or strong curvature along revetments leads to lower stability which is avoided with a straight alignment.



The straight line edge provides for a much larger set-back distance. This would ensure less impact of overtopping and spray during storm conditions.



Previous model studies have confirmed toe stability for the dolos revetment. The shallower rock revetment may experience toe stability problems (to be confirmed in a model study as part of the final design) that may require founding in deeper water at an increased cost.



The corner between the dolosse and rock has the potential to focus reflections towards the west (Oceana), which is avoided with a straight line. Potential larger reflections off a steeper dolos slope has been confirmed not to be a problem in previous model studies.



By following the proposed straight line the stormwater outfall in the corner of the rock/dolos interface can be properly engineered and accumulation of debris avoided. Landside planning. An abrupt end to the dolos revetment will integrate poorly with the existing road and storm water infrastructure and inhibit continuous public access to the shoreline.

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Granger Bay Rock Revetment Concept Study ______________________________________________________________________________________________________________

8.

CONCLUSIONS

The temporary revetment which was originally designed as a dynamic rubble beach has been severely abraded resulting in much reduced stone sizes with an average rock mass estimated to be in the order of 1kg. The revetment has therefore reached the end of its design life and repair work is required to minimize the risk of damage to property and infrastructure on the landward side.

Furthermore, the unprotected embankment along the beach road towards Granger Bay small craft harbour is also vulnerable to wave attack due to informal protection by uncontrolled random fill, implying a high risk of failure. Collapse of the embankment will also result in failure of the beach road behind it. Storm damage occurred in August 2012 in which more than 4 m of this bank was eroded over significant distances, illustrating the need for an engineered edge.

The damage required

emergency repair work of approximately R1 million.

Three revetment options for the gravel beach were considered. The first option consists of a 3t to 6t rock revetment with a flat slope. The second option is a straight line extension of the 20t dolos revetment which requires additional reclamation with fill material from available V&A sources (surplus excavation material). The third option is a more stable revetment protected with 20t dolos armour units but no reclamation of additional land. In addition to these three revetment options for the gravel beach, a rock revetment is proposed for the unprotected embankment.

Cost estimates for the three revetment design options are as follows (costs for each option are inclusive of revetment costs for the unprotected embankment):



Option 1. Rock revetment

R69 million



Option 2. Dolos revetment with reclamation

R181 million



Option 3. Dolos revetment without reclamation

R118 million

These costs include 30 percent for preliminary and general, 25 percent for contingencies and 10 percent for professional fees.

From an engineering and planning perspective Option 2 is preferred. It allows for the efficient use of surplus fill, has a more stable geometry, provides for a larger set-back distance and has a confirmed toe stability (deeper water) and acceptable wave reflection as determined in a previous model study.

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Granger Bay Rock Revetment Concept Study ______________________________________________________________________________________________________________

REFERENCES

PRDW (2009a)

Durban Point Small Craft Harbour. Project Quality Plan Report No. 463/5/001, September 2009

CEM (2003)

Coastal Engineering Manual. Shore Protection Projects. Part V Chapter 3. US Army Corps of Engineers, EM 1110-2-1100, 31 July 2003.

CIRIA/CUR/CETMEF (2007)

Manual on the use of Rock in Coastal and Shoreline Engineering. CIRIASpecial Publication 83.

CSIR (2006)

DRAFT REPORT: Development Setback Lines for the Northern Beaches, Richards Bay, and Evaluation of the uMhlathuze Beaches

DEA/SSI (2009)

A user - friendly guide to the Integrated Coastal Management Act of South Africa

Pullen et al (2007)

Wave Overtopping of Sea Defences and Related Structures: Assessment Manual, August 2007. EA Environment Agency, UK, ENW Expertise Netwerk Waterkeren, NL, KFKI

Goda, Y (1975)

Irregular wave deformation in the surf zone. Coastal Engineering in Japan, Vol. 18

PRDW (2007)

Reinstatement of the Bluff Access Road. Technical Note 385/C/009 REV 01, November 2007.

PRDW (2010)

Global Climate Change: Consequences for Coastal Engineering Design. Position Paper, Report No. 939/1/001, September 2009.

WPR (1993)

Granger Bay Shoreline Protection: Model Tests on Revetments and Stormwater Outlet. Part 1 – Two Dimensional Tests, June 1993

WPR (1994)

Granger Bay Shoreline Protection Works: Report on Contract Completion and Future Maintenance, September 1994

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Granger Bay Rock Revetment Concept Study ______________________________________________________________________________________________________________

WSP (2010)

Development of a Methodology for Defining and Adopting Coastal Development Setback Lines, April 2010

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Granger Bay Rock Revetment Concept Study ______________________________________________________________________________________________________________

APPENDIX A: CROSS-SECTIONS FOR REVETMENT OPTIONS

______________________________________________________________________________________________________________ Prestedge Retief Dresner Wijnberg (Pty) Ltd

Title:

GRANGER BAY REVETMENT – CONCEPT STUDY

ROCK REVETMENT – MINIMUM CAPITAL COST

Appendix

A1

Title:

GRANGER BAY REVETMENT – CONCEPT STUDY

DOLOS REVETMENT WITH RECLAMATION

Appendix

A2

Title:

GRANGER BAY REVETMENT – CONCEPT STUDY

DOLOS REVETMENT WITHOUT RECLAMATION

Appendix

A3

5kg - 3000kg CORE

1

GEO TEXTILE UNDERLAY

1.5 1

5kg - 3000kg CORE

TYPICAL SECTION

Title:

GEO TEXTILE UNDERLAY

1.5

3t - 6t ROCK

+0.0m MSL

3t - 6t ROCK

2.5 m

2.5

m

+0.0m MSL

A

----

GRANGER BAY REVETMENT - CONCEPT STUDY

ROCK REVETMENT AT UNPROTECTED EMBANKMENT

TYPICAL SECTION

B

----

Appendix

A4

Granger Bay Rock Revetment Concept Study ______________________________________________________________________________________________________________

APPENDIX B: PLAN LAYOUTS FOR REVETMENT OPTIONS

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C

A

100

m

15m

B

LEG

AL

SET

BAC

KL INE

PLAN - GENERAL ARRANGEMENT SCALE: 1: 1000

NOTE: WHERE GROUND LEVEL EXCEEDS 7.5m, CREST EXTENDS TO EXISTING SLOPE NOTE: WHERE GROUND LEVEL IS LESS THAN 7.5m, CREST WIDTH TO BE 7.0m WITH DOWN SLOPE OF 1 : 1.5

43m 7.0m +7.5m MSL

3t - 6t ROCK

1.5

3.6

1

1 5m

5kg - 3000kg CORE

1.5

+0.0m MSL

-3.5m MSL

5kg - 3000kg CORE

1 5kg - 3000kg CORE

TYPICAL SECTION SCALE: 1:250

Title:

A

----

2.5m

1

+0.0m MSL GEO TEXTILE UNDERLAY

m

GEO TEXTILE UNDERLAY

1.5

3t - 6t ROCK

+0.0m MSL

3t - 6t ROCK

2.5

2.5

m

+0.0m MSL

-6.0m MSL (BED ROCK LEVEL)

TYPICAL SECTION SCALE: 1:250

GRANGER BAY REVETMENT - CONCEPT STUDY

PLAN LAYOUT ROCK REVETMENT - MINIMUM CAPITAL COST

TYPICAL SECTION

B ----

SCALE: 1:250

Appendix

B1

C

----

1 : 1 000 0m

50m

100m

200m

A 15m

C

( OF FSE

T FR 100 OM m HIG HW ATE

RM

ARK

)

B

K LINE

BAC L SET

LEGA

PLAN - GENERAL ARRANGEMENT SCALE: 1: 1000

15m 3.5m GRAVEL SURFACING TO RECLAMATION

2t - 4t ROCK UNDERLAYER

+7m MSL 1.5 1

RECLAIMED LAND

1.5

5kg - 3000kg CORE

1.5 1

1.5

m m

GEO TEXTILE UNDERLAY

1

5kg - 3000kg CORE

-2.5m MSL

1.5

1.5 1

1

-6m MSL

5kg - 3000kg CORE

TYPICAL SECTION SCALE: 1:250

Title:

A

----

TYPICAL SECTION

GRANGER BAY REVETMENT - CONCEPT STUDY

PLAN LAYOUT DOLOS REVETMENT WITH RECLAMATION

SCALE: 1:250

B

----

Appendix

B2

+0.0m MSL

2.1 GEO TEXTILE UNDERLAY

m

GEO TEXTILE UNDERLAY

+0.0m MSL

3t - 6t ROCK

+0.0m MSL

3t - 6t ROCK

2.5

2.5 m

+0.0m MSL

20t DOLOS

1

4.2

+2.2m MSL

TYPICAL SECTION SCALE: 1:250

C

----

1 : 1 000 0m

50m

100m

200m

A

15m

B

100

m

--

C --

LEG

AL S

ET B A

CK

LINE

PLAN - GENERAL ARRANGEMENT SCALE: 1: 1000

3t - 6t ROCK +7.5m MSL 1.5 4.2 m

1

5m

+0.0m MSL

3t - 6t ROCK

+0.0m MSL

3t - 6t ROCK

2.1

m

+2.0m MSL 0.0m MSL

5kg - 3000kg CORE

1

GEO TEXTILE UNDERLAY

1.5

1 5kg - 3000kg CORE

1m

1.5

2.5 m

2.5

m

1.5 GEO TEXTILE UNDERLAY

1 5kg - 3000kg CORE

TYPICAL SECTION SCALE: 1:250

Title:

A

----

TYPICAL SECTION SCALE: 1:250

GRANGER BAY REVETMENT - CONCEPT STUDY

PLAN LAYOUT DOLOS REVETMENT WITHOUT RECLAMATION

TYPICAL SECTION

B

----

SCALE: 1:250

Appendix

B3

C

----

-3.5m MSL