PREPARATION, REVIEW AND AUTHORISATIONdnn.shoalhaven.nsw.gov.au/demosite/environment/coastal/... ·...

Shoalhaven Coastal Zone Management Plan: Callala Bay Coastal Management Scheme January 2008 1

Document / Report Control Form (not for submission to client)

Project Name: Shoalhaven Coastal Zone Management Plan

Project number: 3001209

Report for: Shoalhaven City Council

PREPARATION, REVIEW AND AUTHORISATION

Revision # Date Prepared by Reviewed by Approved for Issue by

1 31/01/08 C. Adamantidis A. Deen A.Deen

Issue Register

Distribution List Date Issued Number of Copies

Shoalhaven City Council

SMEC staff:

Associates:

-insert TOWN- Office Library:

Proposal Project File:

Company details

SMEC Australia

Level 6, 76 Berry Street

NORTH SYDNEY NSW 2060

Tel: +61 2 9925 5555

Fax: +61 2 9925 5566

Email: [email protected]

www.smec.com.au

The information within this document is and shall remain the property of SMEC Australia.

Shoalhaven Coastal Zone Management Plan: Callala Bay Coastal Management Scheme January 2008

Callala Bay Coastal Management Scheme

FOREWORD

Shoalhaven City Council is preparing a Coastal Zone Management Plan guided by the NSW Government Coastal Policy 1997. This report outlines a proposed coastal management scheme for the Callala Bay area.

This report has been prepared and issued by SMEC Australia to Shoalhaven City Council (the Client). Information published in this report is available for general release only by permission of SMEC Australia and the Client.

REFERENCE

Adamantidis, C.A. (2008). “Callala Bay Coastal Management Scheme”, Report No. 3001209-019 prepared by SMEC Australia for Shoalhaven City Council, January 2008.


Callala Bay Coastal Management Scheme

Table of Contents

1 Introduction...............................................................................................5

2 Coastal Processes....................................................................................6 2.1 Introduction ................................................................................................................6 2.2 Wave Climate ............................................................................................................6

3 Design of Coastal Management Scheme..............................................12 3.1 Introduction ..............................................................................................................12 3.2 Coastal Management Alternatives...........................................................................12 3.3 Design parameters for proposed management scheme .........................................13

4 Summary, Conclusions and Recommendations..................................19 4.1 Summary and Conclusions......................................................................................19 4.2 Recommendations...................................................................................................19

5 References ..............................................................................................20


List of Figures

1.1 Callala Bay – Location Plan

1.2 Callala Bay Beach – erosion escarpment

2.1 Wave Transformation Model domain

2.2 Second-level nested SWAN model grid domain

2.3 Detailed bathymetry in Callala Bay area

2.4 Storm wave height duration recurrence

2.5 Predominant wave approach angle

2.6 Nearshore wave approach angle at 2.5m depth vs. offshore wave direction – south of Sheaffe Street

2.7 Nearshore wave approach angle at 2.5m depth vs. offshore wave direction – north of Sheaffe Street

2.8 Callala Bay Wave Transformation coefficient, Hs offshore = 1m, T=10s, offshore wave direction = SSE

2.9 Callala Bay Significant wave height, Hs offshore = 7m, T=10s, offshore wave direction = SSE

2.10 Callala Bay Significant wave height, locally generated seas, T=4.5s, wind direction = SSE

3.1 Determination of value of K parameter in CERC sediment transport formula

3.2 Predicted beach planform for various groyne locations

3.3 Concept design for potential rock groyne at Sheaffe Street

3.4 Design detail for timber groyne at Sheaffe Street

3.5 Design profile for timber groyne at Sheaffe Street

3.6 Beach nourishment construction (Jimmys Beach, Port Stephens)

3.7 Beach scraping at Callala Bay

List of Tables

2.1 ACES analysis of locally generated sea waves

3.1 Estimated potential longshore sediment transport using CERC (1984) formula


1 Introduction

Callala Bay is located on the northern shores of the coastal embayment of Jervis Bay, and is well protected from both offshore ocean swells and locally generated seas by an extensive rock reef at Callala Point to the south. A location diagram of Callala Bay is provided in Figure 1.1.

Despite the relatively benign wave climate at the beach at Callala Bay, the foreshore has experienced continuing erosion. The erosion appears to have been triggered or exacerbated by construction of a boat ramp in the 1970’s, which has interrupted the natural mechanism of littoral drift at Callala Bay (Nielsen, 2004). As a result, the area of beach north of the boat ramp and south of Sheaffe Street (refer Figure 1.1) has undergone beach recession at rates of up to 0.4 m/year (Coastal Engineering Solutions, 2003).

This continuing erosion, while not posing an immediate hazard to beachfront property, has reduced the recreational amenity of the beach and also poses a hazard to recreational beach users by making beach access difficult (Figure 1.2).

To reduce the impact of this beach erosion and help stabilise the beach, Shoalhaven City Council has been implementing a policy of beach scraping, which is a technique used for accelerating beach recovery following erosion by changing the slope of a beach, periodically, to allow the energy of the sea to bring additional sand onshore (Nielsen, 2003). Work was also carried out at the boat ramp in 2000, to make the structure more permeable to littoral drift and allow sand to bypass the structure more effectively and slow down the beach erosion. However, erosion of the beachfront has continued and Council have requested SMEC to prepare a concept design for a scheme to stabilise the beach.

This concept beach management scheme would involve the use of beach nourishment, coupled with construction of a groyne to arrest the beach erosion and restore beach amenity.

This report presents a concept design of the scheme based on an analysis of the coastal processes occurring at the site as well as providing detailed design parameters for all aspects of the proposed scheme.


2 Coastal Processes

2.1 Introduction

Callala Bay is located within Hare Bay in the northern portion of Jervis Bay, and is protected from ocean swell waves by an extensive reef system at Callala Point.

Coastal Engineering Solutions (2003) undertook a detailed analysis of the coastal processes at Callala Bay, investigating the wave climate and the resulting movement of sand between 1997 and 2001. They concluded that it is primarily wave action that moves the sediment on the foreshores of Callala Bay, and that sand moves primarily alongshore. They assessed the net alongshore movement as being approximately 1000 m3/year toward Boorawine and Wowly creeks. They also concluded that sand could move offshore as a result of storm waves, with most of the sand returning to the beach under long, low ambient swell wave conditions. However, they noted the presence of a natural deep “hole” offshore of the beach which may be acting as a sink for sediment, resulting in some loss of sand available for sediment transport.

Coastal Engineering Solutions (2003) also undertook an analysis of historical aerial photography, dating back to 1949. They concluded that the erosion was initiated by the construction of boat launching facilities in the late 1970’s, with the beach north of the boat ramp receding by up to 10 metres since that time.

Evidence from site inspections (including the presence of building materials in the eroded embankment) shows that the embankment north of the boat ramp has been artificially filled at some occasion in the past, perhaps in an attempt to arrest the erosion.

Shoalhaven City Council have provided updated bathymetric and subaerial survey data for Callala Bay to assist in the design of possible remediation works for the erosion and the area north of the boat ramp. The investigation uses that data to attempt to improve the understanding of the coastal processes at Callala Bay, to enable the design of a successful beach stabilisation scheme. This report details wave transformation analyses and wave climate estimation, as well as an estimate of littoral wave transport, to confirm the understanding of the coastal processes at the site.

2.2 Wave Climate

An important step in understanding the coastal processes at the site is to develop an understanding of the wave climate.

The site is sheltered from ocean swell waves by Callala Point and the presence of extensive reefs to the south of the site.

Wave height and direction are the principal drivers of longshore sediment transport at the site. Long period swell waves, which have the potential to cause sediment transport, would undergo severe refraction around Callala Point, and would be expected to arrive at the area north of the boat ramp from a very limited set of directions. In addition, these waves would be limited in height by wave breaking on the reefs surrounding Callala Point.

Locally generated waves within Jervis Bay would have some impact on sediment transport also, and they would undergo less severe refraction as a result of their shorter wavelengths. However, their direction of approach is also limited due to the narrow southerly fetch width at the beach north of the boat ramp.


To examine this understanding of the wave climate in sufficient detail for design of a successful beach stabilisation scheme, a SWAN wave transformation model was set up, with detailed bathymetry provided by a combination of survey data at the site and bathymetric soundings from Admiralty Charts.

2.2.1 SWAN model

SWAN (acronym for Simulating WAves Nearshore − Cycle III version 40.11) is a numerical wave transformation program developed at the Delft University of Technology (Holthuijsen et al., 2000). SWAN can be used to describe wave transformation in shallow water and to obtain realistic estimates of wave parameters in coastal areas, lakes and estuaries from given wind, bathymetric and current conditions. SWAN is based on the wave action balance equation (or energy balance in the absence of currents) with sources and sinks. The background to SWAN is provided in Young (1999) and Booij et al., (1999). The following wave propagation processes are represented in SWAN:

• rectilinear propagation through geographic space;

• refraction due to spatial variations in bottom topography and current;

• shoaling due to spatial variations in bottom topography and current;

• blocking and reflections by opposing currents;

• transmission through, blockage by or reflection against obstacles. The following wave generation and dissipation processes are represented in SWAN:

• generation by wind;

• dissipation by white-capping;

• dissipation by depth-induced wave breaking;

• dissipation by bottom friction;

• wave-wave interactions (quadruplets and triads);

• obstacles. Wave-induced set-up of the mean sea surface is computed in SWAN. In (geographic) 1D cases the computations are based on exact equations. In 2D cases, the computations are based on approximate equations as the effects of wave-induced currents are ignored (in 1D cases they do not exist). Diffraction is not modelled in SWAN, so SWAN can not be used in areas where variations in wave height are large within a horizontal scale of a few wavelengths. Because of this, the wave field computed by SWAN will, generally, not be accurate in the immediate vicinity of obstacles and certainly not within harbours. SWAN does not calculate wave-induced currents. If relevant, such currents can be provided as input to SWAN (e.g. from a hydro-dynamical model, which can be driven by waves from SWAN in an iterative procedure).


SWAN has been validated using field data by Nielsen & Adamantidis (2003).

Bathymetric data for the model comprised:

• digitised soundings on a 1 km grid as provided by Geoscience Australia (Petkovic & Buchanan, 2002);

• digitised soundings and contours from the Admiralty Chart Aus 193, Jervis Bay and Approaches, scale 1:37 500;

• Surveyed soundings to RL -15m AHD directly adjacent to Callala Beach; and

• Surveyed soundings to RL -6m AHD directly adjacent to Callala Bay boat ramp.

Long term wave statistics were derived from a Waverider buoy operated by the Manly Hydraulics Laboratory, DPWS offshore of Port Kembla. The domain of the wave transformation model extended from Cronulla in the north to Eden in the south, extending some 50 km offshore into water depths in excess of 100 m (Figure 2.1). This region was schematised onto a 3 km square grid from data derived from the soundings on the 1 km grid. A 250 m nested grid, covering all of Jervis Bay and the surrounding coast out to 100 m depth, provided a more detailed schematisation of the study region (Figure 2.2). Data for this grid was derived from the 1 km grid as provided by Geoscience Australia supplemented with detail from the Aus. 193 Admiralty Chart Jervis Bay and Approaches, as well as the surveyed soundings in the vicinity of Callala Bay. Details in the nearshore area of interest were schematised on a 50 m grid based on soundings and contours from the Aus. 193 Admiralty Chart Jervis Bay and Approaches, and the surveyed soundings adjacent to Callala Bay and are depicted in Figure 2.3.

2.2.2 Offshore Swell Waves

Summary wave statistics are available from the Manly Hydraulics Laboratory (e.g., as published in Lord and Kulmar, 2000). The wave data show that the predominant swell wave direction is south-southeast (SSE) with over 70% of swell wave occurrences directed from the SSE quadrant. The average deep water significant wave height, as measured at Port Kembla, is around 1.5 m and the average wave period is around 9 s. Analysis of storms recorded at Port Kembla has provided wave height/duration data for various annual recurrence intervals, which are presented in Figure 2.4. The transformation of offshore swell waves to the area of Callala Bay north of the boat ramp was undertaken using the SWAN model, to examine the range of wave direction that is possible at the site. A vector diagram of offshore waves approaching from the SSE is given in Figure 2.5. It can be seen that the wave vectors approach the beach between the boat ramp and Boorawine Creek at an angle to the shoreline, which would tend to induce northward longshore sediment transport toward Boorawine Creek. North of Wowly Creek, the wave vectors approach at an angle normal to the shoreline, which would indicate that here, the shore orientation is in equilibrium with the prevailing wave approach direction. The model was used to examine the range of wave directions that are possible at the site, due to average swell waves with a significant wave height of Hs = 1m, generated offshore from Jervis Bay. Two locations were examined in detail – the area of beach south of Sheaffe Street (between the boat ramp and Sheaffe Street), and the area north of Sheaffe


Street (between Sheaffe Street and Boorawine Creek). It can be seen in Figure 2.6 that, due to the effect of wave refraction for all offshore wave directions between NE and SSW, the range of wave approach directions possible at Callala Bay beach south of Sheaffe Street is very narrow, between 110° and 113° TN. This compares with a shoreline orientation angle of 100°, indicating that swell waves typically approach the shore at an angle of 10° - 13°, which would induce northward longshore transport. North of Sheaffe Street (Figure 2.7), the wave approach angle is 128° to 130°TN, with a shoreline orientation angle of 120°, indicating an approach angle to the shoreline for swell waves of 8° to 10°. This indicates potential for northward longshore sediment transport north of Sheaffe Street, but at a reduced rate to the area south of Sheaffe Street. The impact of storm waves was also examined using the SWAN model. Kulmar et al. (2005) indicate that the offshore significant wave height for storm waves of 24 hour duration is Hs = 7m. While the wave approach angle was reduced relative to the shoreline orientation angle during such storms, the wave height is increased, increasing the potential for longshore sediment transport. In addition, offshore sediment transport would also occur during such a storm event. The wave approach angle relative to the shoreline was assessed to be 2° - 3° for storm events with offshore directions of SSE (Figures 2.6 and 2.7). Wave height coefficients due to wave refraction of average swell waves (Hs offshore = 1m) at Callala Bay are provided in Figure 2.8. It was found that offshore swell waves approaching from the SSE led to the highest waves at Callala Bay, due to maximum penetration of the waves from this direction through the heads at Jervis Bay. It can be seen that, on average, less than 25% of the offshore swell wave energy can reach the area of Callala Bay adjacent to the boat ramp. For offshore storm swell waves of Hs = 7m, the significant wave height reached in the vicinity of the boat ramp is about Hs = 1m (Figure 2.9). It should be noted that maximum wave heights could reach up to twice the significant wave height – this means that, for design purposes, the maximum wave height due to offshore swell waves at the site could reach Hmax = 2.0 m.

2.2.3 Locally Generated Seas

An assessment of the wave climate due to waves generated locally within Jervis Bay was also made. Wind data at Ulladulla and Point Perpendicular provided by the Bureau of Meteorology for 10 minute averaged wind speeds at 9am and 3pm, for 2007, were searched for occurrences of maximum wind speeds. It was found that, at Ulladulla during the storms associated with the East Coast Low of June 8, 2007, the 10 minute averaged wind speed was measured to be 57 km/h from the SSW at 3pm on 8 June, 54 km/h from the S at 9am on 9 June, and 57 km/h from the SSW at 3pm on 9 June (Bureau of Meteorology, 2008). This was the highest 10 minute average windspeed for the year, and it appears to have persisted from the same direction for greater than 24 hours. The beach at Callala Bay is open to a fetch across Jervis Bay from a narrow, southerly band, and the fetch length is a maximum of 16 km. From the Bureau of Meteorology data it was found that averaged hourly wind speeds of up to 60 km/h (close to 35 knots) were entirely possible, and could occur for durations greater than 24 hours. This wind speed was adopted as the design condition. As a result, locally generated waves at Callala Bay are fetch limited rather than duration limited (i.e. the wave height at Callala Bay is limited by the length of open water across Jervis Bay rather than by the duration of the wind). The wind wave climate was derived using the ACES wave forecasting algorithms (Leenknecht et al., 1991). The results of the ACES analysis found that locally generated


waves could reach a significant wave height of 1.6 m, and a wave period of up to 4.6 s. (Table 2.1).

Table 2.1 – ACES analysis of locally generated sea waves �� ACES � Mode: Single Case � Functional Area: Wave Prediction � �� Application: Wind Adjustment and Wave Growth � ��

�� Item � Value � Units � �� Wind Obs Type � �� El of Observed Wind � Zobs: 10.00 � m � � � Overwater (ship) � � Observed Wind Speed � Uobs: 35.00 � knots � �x� Overwater � � Air Sea Temp. Diff. � �T: 0.00 � deg C � � � Shore (windward) � � Dur of Observed Wind � DurO: 10.00 � hr � � � Shore (leeward) � � Dur of Final Wind � DurF: 10.00 � hr � � � Inland � � Lat. of Observation � LAT: 35.00 � deg � � � Geostrophic � � Wind Fetch Length � F: 16.00 � km � �� Wind Fetch Options � � Eq Neutral Wind Spd � Ue: 34.96 � knots � �� Adjusted Wind Speed � Ua: 48.89 � knots � �x�Open Water� �Restricted� � � � � �� Wave Height � Hmo: 1.62 � m � Options: � Wave Period � Tp: 4.60 � sec � F1: New Case �� F3: Print Wave Growth: Deep-water Fetch-limited F10: Exit Applic

As these locally generated waves have a much shorter wavelength than the offshore swell waves, they would undergo less severe refraction around Callala Point. The locally generated wave height and direction at Callala Bay near the boat ramp was transformed using the SWAN model. Figures 2.6 and 2.7 show that for all locally generated seas from the southerly sector, the angle of wave approach is always oblique to the shoreline, generating a northward sediment transport potential. Figure 2.10 shows the local significant wave height that could be expected at the vicinity of the boat ramp, for locally generated seas. It can be seen that, significant wave heights could reach around 0.7 m at the site, due to wave breaking at the reefs surrounding Callala Point. Locally generated seas from the east could lead to a sediment transport potential toward the south. For locally generated seas from the east across Hare Bay, the fetch length is limited to around 4 km. A 35 knot sustained easterly wind would produce a significant wave height of around 0.5 m and wave period around 2.6 seconds in the vicinity of the boat ramp – under these conditions, waves would undergo little refraction and sediment can be transported south. The 3pm annual wind rose statistics from the Bureau of Meteorology (2008) for Point Perpendicular show that easterly winds occur for less than 5% of the time, and easterly wind speeds of greater than 20 km/h occur for less than 1% of the time. This compares with winds from the south occurring 25% of the time, with southerly wind speeds exceeding 20 km/h for more than 10% of the time. Consequently, the potential for sediment transport from north to south due to locally generated seas is much smaller than the potential for sediment transport from south to north. It should be noted that maximum wave heights could reach up to twice the significant wave height – this means that, for design purposes, the maximum wave height due to locally generated seas at the site could reach Hmax = 1.4 m.


2.2.4 Summary of Wave Climate

From the above analysis of the wave climate for the site, it was found that:

• Swell waves can only approach the site from a narrow range of directions, due to severe wave refraction;

• Locally generated seas mainly approach the site from the south due to the narrow range of fetch directions possible at the site;

• The significant wave height under extreme conditions can reach Hs = 1m at the site due to swells and around 0.7m due to locally generated seas;

• The direction of approach of wave energy at the site would mostly favour northward longshore sediment transport in the vicinity of the boat ramp; and

• The potential for sediment transport to the north diminishes north of Sheaffe Street, and the beach plan form is in equilibrium with the mean wave direction north-east of Wowly Creek.

This analysis confirms the understanding of coastal processes at the site as obtained from observations of the beach erosion, with the highest rate of erosion observed immediately north of the boat ramp and south of Sheaffe Street, and the build-up of sediment south of the boat ramp being consistent with the above analysis of the local wave climate.


3 Design of Coastal Management Scheme

3.1 Introduction

The understanding of coastal processes at Callala Bay developed in this report allows the design of a beach stabilisation scheme for the area experiencing the greatest degree of erosion (between the boat ramp and Sheaffe Street, Figure 1.1). In summary, the coastal processes at the site relevant to the design of a successful beach management scheme are:

• Longshore sediment transport from south to north, with sediment being trapped by the boat ramp and the beach realigning in response to the prevailing wave climate;

• Longshore movement of sediment being transported into the mouth of Boorawine and Wowly Creeks by the prevailing wave climate;

• Enhanced longshore sediment transport to the north during storm events;

• Offshore sediment transport by storm waves during severe storm events;

• Onshore transport of sediment as a result of the prevailing long, low swell wave climate that is experienced at Callala Bay.

The proposed beach management scheme presented here has been designed based on this understanding of the prevailing coastal processes at Callala Bay.

3.2 Coastal Management Alternatives

An appropriate coastal management option, based on the coastal processes at the site, comprises a combination of the following elements:

• A groyne located along the beach to allow natural build-up of sediment seaward of the area that is undergoing beach recession;

• Beach nourishment to minimise the impact on the beach downdrift of the groyne;

• Beach scraping to make use of the long, low swell wave climate that naturally induces onshore sediment transport.

Other alternatives for coastal protection are summarised below, with reasoning provided for these alternatives not being adopted:

• Terminal Revetment/seawall – This would protect the escarpment and landward area from further erosion, but does not address the cause of the erosion. Erosion would continue seaward of the wall, resulting in loss of the beach.

• Beach nourishment only – This option considered in isolation would provide some buffer for storms in the short term. However, longshore sediment transport would continue and the beach would need to be renourished on a periodic basis.

• Removal of the boat ramp – This option would result in the loss of a community facility, and the erosion of the escarpment would continue as the beach continues


to realign its planform to the prevailing wave direction. In addition, erosion of the beach south of the boat ramp would occur.

• A groyne without beach nourishment – this option would cause erosion of the beach downdrift of the groyne. In addition, the natural accumulation of sediment updrift of the groyne would be a very slow process, and storms would continue to cause erosion of the embankment until sufficient sand has built up to provide a buffer against such storms.

3.3 Design parameters for proposed management scheme

The design of the proposed beach management scheme involves the following steps:

• quantifying the coastal processes (including quantifying the sediment transport rates);

• consideration of the most appropriate location of the groyne;

• consideration of the design parameters for the groyne (incident wave height, water levels, length of groyne required, height of groyne);

• consideration of potential construction methods/materials, life of structure;

• consideration of potential sources and quantities required for beach nourishment material;

• consideration of the impact of the proposed scheme.

3.3.1 Derivation of Sediment Transport Rate

Crucial to the design of a successful groyne scheme is an understanding of the rate of longshore drift at the site. Coastal Engineering Solutions (2003) estimated the net rate of longshore sediment transport to be 1000 m3/year toward the north. Using the understanding of the coastal processes and the SWAN numerical model from Section 2, a potential longshore sediment transport rate has been estimated for the site. CERC (1984) suggests various methods of deriving longshore sediment transport rates for a site, including using the known transport rate at a nearby site, measured sediment volume changes between two bathymetric surveys of the site, or use of the CERC formula for potential sediment transport (which assumes that the longshore sediment transport rate depends on the longshore component of energy flux in the surf zone. The CERC formula is given by:

( ) lssP

gaK

Q'ρρ −

= (3.1)

where Q = Longshore sediment transport rate K = dimensionless empirical coefficient, related to sediment grain size ρs = sediment density ρ = water density g = acceleration due to gravity a = solids fraction of the in-situ sediment deposit (1 – porosity).


and the longshore component of energy flux in the surf zone is given by:

( )bgbsbls CHg

P θρ 2sin16

2= (3.2)

where Hsb = nearshore breaking height of the significant wave Cgb = wave group speed at breaking, and θb = angle breaking wave crest makes with the shoreline. In shallow water,

bgb gdC = (3.3) where

db = depth of wave breaking, which is assumed to be related to the wave breaking height as bb dH 78.0= .

The values for the parameters in the CERC formula are given below:

K = dimensionless empirical coefficient, related to sediment grain size. The median grain size of sediment (D50) in the surf zone at Callala Bay was found to be 0.35 mm. From Coastal Engineering Manual (2003), an empirically based value for K is around 0.6, based on the median grain size (refer Figure 3.1).

ρs = sediment density = 2650 kg/m3

ρ = water density = 1025 kg/m3 for seawater g = acceleration due to gravity = 9.81 m2/s

a = solids fraction of the in-situ sediment deposit (1 – porosity). Porosity of a typical beach berm is around 40%, so a = 0.4;

Hsb = nearshore breaking height of the significant wave – from the analysis in Section 2, this is around 0.6 - 1 m for storm waves and 0.16 – 0.25 m for typical swell waves;

Cgb = wave group speed at breaking, which varies with the wave height in accordance with Equation 3.3;

θb = angle breaking wave crest makes with the shoreline, which is 8° to 13° for typical swell waves and 2° to 3° for storm waves.

The above parameters were used to derive longshore sediment transport rates for Callala Bay, which are provided in Table 3.1. It can be seen from Table 3.1 that the annual longshore sediment transport potential for the area south of Sheaffe Street is as high as 5,000 to 50,000 m3/year over the active beach profile. However, the net transport rate would be a lot smaller than this, as this estimate does not take into account the availability of sediment for transport, the input of sediment to the system from other sources such as sand bypassing Callala Point, or occasional movements to the south caused by north-easterly winds.


Table 3.1 – Estimated potential longshore sediment transport using CERC (1984) formula

Location Hs offshore Hs nearshore Longshore sediment transport

potential m3/day

Longshore sediment transport

potential m3/yr

South of Sheaffe Street

1.0 m 0.10 – 0.25m 13 – 160 5 000 – 50 000

7.0 m (storm conditions)

1.0 m 800 - 1200 n/a

North of Sheaffe Street

1.0 m 0.06 – 0.14m 3 – 30 1000 – 10 000

7.0 m (storm conditions)

0.65 m 200 - 400 n/a

It is also noted that the potential for longshore sediment transport is increased as a result of storm events, with the potential for around 1000 m3 of sediment transport for a 24 hour duration storm event south of Sheaffe Street, and 300 m3 over 24 hours north of Sheaffe Street. This sediment is likely to accumulate in the entrance to Boorawine and Wowly Creeks or in the dunes to the east. While the net sediment transport rate at the site is not known precisely, it is evident that the main potential is for sediment transport from south to north, and that this potential is highest in the area south of Sheaffe Street. Longshore sediment transport is also likely to occur during storm events, with the direction of this transport also being from south to north.

3.3.2 Length of groyne

The length of groyne required is related to the width of the surf zone over which longshore sediment transport can take place. The depth of longshore sediment transport is estimated as approximately 1.6 times the significant breaking wave height (Hallermeier, 1981). Groynes which extend to a depth beyond the limit of wave breaking will trap 100% of the longshore sediment transport. For a breaking wave of H = 1m, the groyne would need to extend from the landward limit of wave runup to a depth of approximately 1.6 m, to trap the majority of the sediment transport that can occur at the site, under both ambient and storm wave conditions. From the bathymetric survey of Callala Bay, this would require a groyne length of around 60 - 70m. It is advantageous, however, to allow the groyne to have some permeability to sand transport – if the groyne is too long, sediment can be transported to depths beyond which it can be brought back to shore under ambient swell waves, resulting in reduced sand bypassing and beach erosion downdrift of the groyne. Conversely, if the groyne is too short, the distance updrift of the groyne that would accumulate sand would be limited, limiting the benefit obtained from the groyne.


An estimated length of groyne that would provide an optimal combination of sand bypassing and stabilised length of shoreline would be around 50 m, or to a depth of around 1 m below Australian Height Datum (AHD).

3.3.3 Potential groyne locations

Three potential locations for a groyne of approximately 50 m length were considered:

• At Sheaffe Street;

• Between Sheaffe Street and Boorawine Creek; and

• At the southern end of the entrance to Boorawine Creek.

The estimated equilibrium shoreline orientation as a result of these three potential groyne locations, as well as the predicted equilibrium orientation with no groynes, is given in Figure 3.2. This estimated shoreline is based on the assumption that the beach will align itself normal to the angle of the predominant incoming wave energy, which is represented by the wave vectors shown in Figure 3.2, for average offshore swell conditions. From the Coastal Engineering Manual (2003), the recommended spacing of groynes in a groyne field is approximately two to three times the groyne length. A 50 m long groyne approximately three groyne lengths north of the boat ramp would be located around Sheaffe Street (approximately 150 – 200m north of the boat ramp). However, the shoreline north of the groyne would still experience longshore sand transport, and some erosion (albeit at a reduced rate) would still occur here, even when the Sheaffe Street groyne is fully bypassing. If the groyne were to be located between Sheaffe Street and the entrance to Boorawine Creek, the equilibrium planform of the beach is predicted to adjust as shown in Figure 3.2. This location would provide benefit to a greater length of beach, though the benefit would diminish with distance from the groyne, with only minimal benefit to the area of the beach closest to the boat ramp. If the groyne were located at the entrance to Boorawine Creek, the beach north of Sheaffe Street would receive the most benefit, with the beach south of Sheaffe Street receiving minimal benefit from the groyne. In addition, a groyne at this location would act to partially train the entrance to Boorawine Creek, which may have implications for the supply of sediment to the beach system and alter the tidal characteristics of the creek and the estuarine ecology. Two groynes, one at Sheaffe Street and one at Boorawine Creek may be required to stabilise the entire length of beach between the boat ramp and Boorawine Creek. The potential impact of such a system on the shoreline is shown in Figure 3.2. However, a system of two groynes would increase the cost of the project substantially, while having potential negative impacts at the entrance to Boorawine Creek. From the discussion above and with reference to Figure 3.2, a 50 m long groyne located at Sheaffe Street would provide the highest benefit-cost ratio for the project. The shoreline north of the groyne will need to be monitored for any potential impacts, which should be negligible if beach compartment is filled with beach nourishment sand.


3.3.4 Potential Construction Materials for structure

Groynes can be constructed of various materials, including rock/rubble mound, concrete or steel sheet piles, or timber. The construction material is determined in part by the design conditions that the structure would be subject to – including wave loadings, potential for scour, durability requirements, and ease of construction. A typical design for a rock groyne structure that would withstand the wave conditions expected at the site (a breaking wave of 1.6 m) is given in Figure 3.3. While a rock structure would probably be the most durable and require less maintenance than other potential structures, it would also be the most costly to build and would also reduce recreational beach amenity. However, due to uncertainty related to sediment transport rates and impacts on erosion of the beach downdrift of the groyne, it may be preferable to design a structure that can be more easily removed if warranted. A timber groyne structure would serve this purpose as they typically have a design life of around 20 years. A timber groyne structure can be monitored over time and if downdrift impacts become unacceptable, the structure can be removed if warranted. Conversely, if the structure performs well, it can be replaced at the end of its lifetime with a more permanent rubble-mound type structure. A typical construction detail for a timber sheet pile groyne is given in Figure 3.4. Equipment required would include an excavator, pile driver, and timber posts and rails. Typical costs for timber posts are of the order of $20 - $50 per post, whereas quarry rock typically costs around $25 - $35/m3, depending on proximity of the source (Victorian Department of Sustainability and Environment, 2007).

3.3.5 Groyne Profile

A groyne can have a longitudinal profile that approximately acts as a template for the desired profile of the updrift beach. Typically, the landward end of the groyne is set at the elevation of the natural, existing beach berm, with a sloping section through the swash zone at approximately the slope of the beach berm (Coastal Engineering Manual, 2003). The seaward section would be set at a level around mean low water, or lower. This would allow sand to bypass and overpass the structure, minimising the impact downdrift of the groyne. A potential groyne profile for a timber groyne at Sheaffe Street is given in Figure 3.5. The groyne sheet piles should be driven in to sufficient depth for stability and to allow for scour.

3.3.6 Beach nourishment design

The success of the management scheme relies on beach nourishment – without this, a groyne at Sheaffe Street would cause erosion downdrift of the structure, and the area south of Sheaffe Street would take a long time to fill naturally, with erosion continuing in the meantime.


Potential sources for beach nourishment must be located within the same active littoral system. The most suitable source of sand for nourishment appears to be from the entrance to Wowly Creek, located approximately 600 m northeast of the site. Other potential sources, such as south of the boat ramp, and the “hole” offshore from the beach identified by Coastal Engineering Solutions (2003) may be unsuitable. Sand from south of the boat ramp is likely to be bypassing the ramp, thus providing a natural supply of sand to the beach north of the boat ramp. Sand from the “hole” offshore from the beach may be colonised by seagrasses which would be damaged if extracted for sand nourishment. It is likely that sand south of the boatramp is naturally bypassing the ramp – removal of this sand would reduce the bypassing rate, accelerating loss of sand from the nourished compartment. The volume of sand required to create a small dune and sandy beach in the area south of Sheaffe Street is estimated to be approximately 5000 m3. Some beach nourishment would also be required north of the groyne, to mitigate any downdrift impacts. The beach nourishment should be applied at a plan profile that approximates the predicted post groyne plan of the beach – i.e. to create a “pocket beach” that is aligned approximately according to the direction of the incoming wave crests, or as shown in Figure 3.2. Coastal Engineering Solutions (2003) discuss the costs associated with beach nourishment, with deployment of a dredge and equipment for pumping sand costing a minimum of $20,000. Based on a pumping rate of $5/m3 (Coastal Engineering Solutions, 2003), the estimated cost of pumping 5000 m3 of sand would be approximately $25,000. Additional costs for labour (i.e. spreading the sand) and project management would be required. Typically, a sand bund would be built at the water’s edge, with sand pumped in behind the bund to build the beach up to the desired level, as shown in Figure 3.6.

3.3.7 Beach Scraping

The proposed beach management scheme should be implemented in conjunction with beach scraping, as described by Nielsen (2003). The beach scraping strategy enhances the natural beach building process of long, low ambient swell wave conditions bringing sand back onto the beach. The presence of a groyne as part of the management scheme would assist in keeping the scraped material on the beach, rather than losing it to longshore transport. Beach scraping would be especially important north of the Sheaffe Street groyne, as erosion of this area would be expected to continue following groyne construction. The process of beach scraping is illustrated in Figure 3.7.


4 Summary, Conclusions and Recommendations

4.1 Summary and Conclusions

This report has examined the design of a beach management scheme for the area of Callala Bay north of the boat launching ramp and south of Sheaffe Street. This area has been undergoing erosion since construction of the boat ramp in the late 1970s. An examination of the coastal processes in this area was undertaken, based on the results of wave refraction modelling and detailed survey information provided by Shoalhaven City Council. It was found that the area is subject to a high potential for longshore drift, with sand moving from south to north, due to the oblique angle of the ambient wave climate. This understanding is supported by anecdotal evidence at the site. A potential management scheme which would be appropriate for this situation involves the construction of a groyne, coupled with beach nourishment and beach scraping. The design parameters for the groyne were developed based on an understanding of the coastal processes, including the length of the groyne, groyne profile, and optimum location of the groyne. Sand available for beach nourishment would be best sourced from the entrance to Wowly Creek, approximately 600 metres northeast of the site.

4.2 Recommendations

It is recommended that a 50 m long timber sheet pile groyne be constructed in the vicinity of Sheaffe Street, and that the area south of the groyne be nourished with sand to build a dune and beach berm, thus providing some buffer for future storms. It is recommended that some beach nourishment sand also be placed downdrift of the groyne, to minimise downdrift erosion. Further, it is recommended that the completed beach management scheme be monitored for its effectiveness, with the beach response monitored over time. The timber groyne would have a design life of around 20 years, during which the scheme would be operating on a pilot basis, to confirm the longshore sediment transport rates and confirm the understanding of the coastal processes as presented in this report. Following this period, if the groyne is successful, the timber groyne could be replaced by a more durable rock structure. The nourished dune should be vegetated and fenced, in accordance with the NSW Department of Land and Water Conservation (2001) Coastal Dune Management Manual.


5 References

Booij, N., R.C. Ris & L.H. Holthuijsen (1999). “A third-generation wave model for coastal regions, Part I, Model description and validation”, J.Geoph. Research, 104, C4, 7649-7666.

Bureau of Meteorology (2008). “Ulladulla and Jervis Bay, New South Wales Daily Weather Observations, January 2007 – January 2008”, http://www.bom.gov.au/climate/dwo/IDCJDW0200.shtml

Coastal Engineering Manual (2003). “Part V Chapter 3 – Shore Protection Projects”, U.S. Army Corps of Engineers, EM 1110-2-1100, 31 July 2003.

CERC (1984). “Shore Protection Manual”, U.S. Army Corps of Engineers, Coastal Engineering Research Centre, Waterways Experiment Station, Vicksburg, Miss.

Coastal Engineering Solutions (2003). “Callala Bay Shoreline Erosion Study”, Report of Coastal Engineering Solutions prepared for Shoalhaven City Council, February, 2003.

Department of Sustainability and Environment (2007). “Technical Guidelines for Waterway Management”, Department of Sustainability and Environment, Victoria.

Hallermeier, R. J. (1981). “A Profile Zonation for Seasonal Sand Beaches from Wave Climate,” Coastal Engineering, Vol 4, pp 253-277.

Haradasa, D., S. Wylie & E. Couriel (1991). “Design Guidelines for Water Level and Wave Climate at Pittwater”, Australian Water and Coastal Studies Pty Ltd Report 89/23, March, 1991.

Holthuijsen, L.H., Booij, N., Ris, R.C., Haagsma, IJ.G., Kieftenburg, A.T.M.M, Kriezi, E.E. (2000) “SWAN Cycle III version 40.11 User Manual”, Delft University of Technology, October, 2000.

Leenknecht, D.A., A. Szuwalski & A.R. Sherlock (1991). “Automated Coastal Engineering System, Version 1.05”, Coastal Engineering Research Centre, Dept. of the Army, Waterways Experiment Station, Corps of Engineers, Vicksburg, Mississippi, April, 1991.

Lord, D.B. & M. Kulmar (2000). “The 1974 storms revisited: 25 years experience in ocean wave measurement along the south-east Australian coast”, Proc. 27th ICCE, ASCE, Sydney, July, 2000, 559-572.

Nielsen, A.F. (2003). “Callala Bay Beach Scraping”, Report No. 31434-002 prepared by SMEC Australia for Shoalhaven city Council, November 2003.


Nielsen, A. F. and Adamantidis, C. A. (2003) “A Field Validation of the SWAN Wave Transformation Program” Proc. Coasts and Ports Australasian Conference 2003.

NSW Department of Land and Water Conservation (2001), “Coastal Dune Management: A Manual of Coastal Dune Management and Rehabilitation Techniques”, Coastal Unit, DLWC, Newcastle.

Young, I.R. (1999). “Wind Generated Ocean Waves”, Eds. R. Bhattacharyya & M.E. McCormick, Ocean Engineering Series, Elsevier, Amsterdam, 288 p.


Figures

Figure 1.1 – Callala Bay – Location Plan

Figure 1.2 – Callala Bay Beach – erosion escarpment


250000 300000 350000 400000 450000

5950000

6000000

6050000

6100000

6150000

6200000

Figure 2.1 – Wave Transformation Model domain

Area of Nested Grid

Wollongong

Batemans Bay


Figure 2.2 – Second-level nested SWAN model grid domain

Callala Bay

Nested model grid


Figure 2.3 – Detailed bathymetry in Callala Bay area


Port Kembla Wave Data Feb 1974 to Dec 2004

(Manly Hydraulics Laboratory)

0123456789

101112

0.01 0.1 1 10 100 1000

Return Period (years)

Sto

rm S

igni

fican

t Wav

e H

eigh

t Thr

esho

ld (m

)

1hr duration

6hr duration

12hr duration

24hr duration

48hr duration

72hr duration

Figure 2.4 - Storm wave height duration recurrence (from Manly Hydraulics Laboratory)


Figure 2.5 – Predominant wave approach angle

Measurement location (north)

Measurement location (south)

Sheaffe Street


Nearshore wave approach angle vs offshore wave direction, Hs=1m

90

95

100

105

110

115

120

125

45 90 135 180

Offshore Wave Direction (°TN)

Wav

e D

irec

tion

at R

efer

ence

Loc

atio

n

90

95

100

105

110

115

120

125

Wav

e di

rect

ion

loca

l sea

s

T=10sT=4.5sShore angle S Sheaffe StWave angle Hs offshore 7m

Figure 2.6 – Nearshore wave approach angle at 2.0m depth vs. offshore wave direction – south of Sheaffe Street


Nearshore wave approach angle vs offshore wave direction, Hs=1m

110

115

120

125

130

135

140

145

45 90 135 180

Offshore Wave Direction (°TN)

Wav

e D

irec

tion

at R

efer

ence

Loc

atio

n

110

115

120

125

130

135

140

145

Wav

e di

rect

ion

loca

l sea

s

T=10s (2)T=4.5sShore angle N Sheaffe StWave angle offshore Hs = 7

Figure 2.7 – Nearshore wave approach angle at 2.5m depth vs. offshore wave direction – north of Sheaffe Street


Figure 2.8 – Callala Bay Wave Transformation coefficient, Hs offshore = 1m, T=10s, offshore wave direction = SSE


Figure 2.9 – Callala Bay Significant wave height, Hs offshore = 7m, T=10s, offshore wave direction = SSE


Figure 2.10 – Callala Bay Significant wave height, locally generated seas, T=4.5s, wind direction = SSE


Figure 3.1 – Determination of value of K parameter in CERC sediment transport formula (from Coastal Engineering Manual, 2003)


Figure 3.2 – Predicted beach planform for various groyne locations


Figure 3.3 – Concept design for potential rock groyne at Sheaffe Street

NOT TO SCALE

Core (quarry run)

2 layers armour W50 = 1 t D50 = 850 mm

Underlayer W50 = 100 kg D50 = 400 mm

Minimum crest width 2.2 m Crest level = 3m AHD

1V

2H

Geotextile SECTION VIEW

PLAN VIEW

-1 m AHD


Figure 3.4 – Design detail for timber groyne at Sheaffe Street

(after Coastal Engineering Manual, 2003)


-4

-3

-2

-1

0

1

2

3

4

0 10 20 30 40 50 60 70 80 90

Chainage (m)

Ele

vatio

n (m

AH

D)

Sheaffe Street profile (survey)Groyne profileMWL

Figure 3.5 – Design profile for timber groyne at Sheaffe Street


Figure 3.6 – Beach nourishment construction (Jimmys Beach, Port Stephens)


Figure 3.7 – Beach scraping at Callala Bay

TOILETS

EXISTING PLAYEQUIPMENT

T

BBQCA

RPA

RKIN

G

BOORAWINE STREETW

ATT

STR

EET

Water line at 10:30am 18.12.07

TOP OF BANK

BOTTOM OF BANK

TOP OF BANKBOTTOM OF BANK

TOP O

F BANKBO

TTOM

OF BANK BOORAWINE

CREEK

-1.0

-2.0

0.0

EXISTINGBOLLARDS

2.0

3.0

2.0

RAMP ACCESS TO BEACHWITH VIEWING PLATFORM

DINGHYAREA

GEO BAG BANK PROTECTION ANDSTUB GROYNE TO ENGINEERS DESIGN& DINGHY SKID ACCESS

TT

T

T

T

PROPOSEDCAR

PARKING

SLIP RAIL

SLIP RAIL

PROPOSED PATH NETWORK

PROPOSED PATH NETWORK

PROPOSED PATH LINKING JETTY TO PROPOSED PATH NETWORK

EXISTINGBOAT RAMP

0.0

-1.0

-2.0

1.0

3.0

BEACH SCRAPING

SAND BORROW S

ITE

BEACH SCRAPING SAND BORROW SITE

SAND BORROW SITES AROUND BOAT RAMPGOVERNED BY SEA GRASS EXTENTS THAT MAY

IMPROVE BOAT RAMP ACCESS AND SANDBYPASSING UNDER THE RAMP STRUCTURE

SAND NOURISHMENT TORECOMMENDED PROFILES

CA

RPA

RKIN

G

CARPARKING

SIMPLIFY EXISTING PATH SYSTEM

4.0

5.0

6.0

SEWER LINE

1.0

SEWER LINE

FENCE

FENCE

DINGHYAREA

J E R V I S B A Y

SHEA

FFE

STRE

ET

: PROPOSED SAND NOURISHMENT AREA

: PROPOSED SAND BORROWING AREA

: WATER

: PROPOSED CONCRETE PATH

T : EXISTING TABLE

LEGEND

TOP OF BANKBOTTOM OF BANK

Beach

Reserve

EXISTING PEDESTRIAN ACCESS

GEO BAG BANK PROTECTION ANDSTUB GROYNE TO ENGINEERS DESIGN& DINGHY SKID ACCESS

RAMP ACCESS TO BEACHWITH VIEWING PLATFORM

EXISTINGJETTY

: EXISTING TREES

DINGHYAREA

REMOVE PATH

Bu

i l d i ng

e s i g

nD

DWG. DATELOT / DP

NORTH

NO. IN SET

JOB NO.Landscape Concept

Sheaffe St., Callala Bay

Shoalhaven City Council

REVISIONS

Art House 818

P.O. Box268 Vincentia N.S.W. 2540Phone: 02 4443 7999, 0411 162 851

Fax: 02 4443 7992Email: [email protected]

Web: arthousedesign.com.au

SHEET NO.

SCALES

ADDRESS

PROJECT

CLIENT

N

EW

SLandscape Concept PlanScale 1:750 @ A2

As Shown Nov. 2008

1

1A

REVI

SIO

NA

: 4.

11.2

008

- GEO

BA

G B

AN

K P

ROTE

CTIO

N E

XTEN

DED

ALO

NG

BEA

CH

PREPARATION, REVIEW AND AUTHORISATIONdnn.shoalhaven.nsw.gov.au/demosite/environment/coastal/... ·...

Documents

Transcript of PREPARATION, REVIEW AND AUTHORISATIONdnn.shoalhaven.nsw.gov.au/demosite/environment/coastal/... ·...