3 concept design of preferred optionsdnn.shoalhaven.nsw.gov.au/demosite/environment/coastal/... ·...

Currarong Beach Erosion Design Study 3001859 | Revision No. 4| Page | 55

3 CONCEPT DESIGN OF PREFERRED OPTIONS

Other than dune management options (which are part of Council’s existing beach management practices), preferred options were selected from the Triple Bottom Line assessment and after a workshop with DECCW, community representatives and Council. These options include:

Beach nourishment

Construction of a groyne

The concept design of the different options is based on an analysis of the coastal processes occurring at the site as well as the provision of detailed design parameters for all the aspects of the proposed options. These options would be implemented in conjunction with reconstruction of key beach access points.

3.1 Coastal Processes

3.1.1 Introduction

Currarong Beach is facing north and is sheltered from the southerly wave climate by Beecroft Head. Two main reefs are located along the beach with one located in front of Plutus Creek (western end of the beach) and another one located around 200m west of Currarong Creek entrance.

Coastal Engineering Solutions (2003) undertook a detailed analysis of the coastal processes at Currarong Beach and investigated the wave climate and sediment movement between 1997 and 2001. They concluded that it was primarily wave action that moves sand at Currarong. Estimated gross volume of sand movement along the section of the beach east of the central reef was ±10,000m3/yr and along the section of the beach west of the central reef sand movement was estimated at ±5,000m3/yr. An offshore movement of around 20,000m3 could occur during a 50 year ARI event (wave height of less than 3m) but most of the sand pushed offshore is returned to the beach under average wave conditions. Some sediment movement occurs at the entrance of Currarong Creek where sand is washed into the creek by longshore transport but the twice daily ebb tide washes the sand out of the creek and the ambient waves redistribute it along the shoreline.

Shoalhaven City Council has provided updated bathymetric and subaerial survey data for Currarong Beach to assist in the design of possible remediation works for the erosion. The investigation uses that data to attempt to improve the understanding of the coastal processes at Currarong Beach, to enable the design of successful beach stabilisation options. This report details wave transformation analyses and wave climate estimation, as well as an estimate of littoral wave transport direction, to confirm the understanding of the coastal processes at the site.

3.1.2 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 southerly ocean swell waves by Beecroft Head and the presence of extensive reefs along the beach.

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 and diffraction around Beecroft Head and would be


expected to arrive at the beach from a limited set of directions. In addition, these waves would be limited in height by wave breaking on the reefs located along the beach.

To examine this understanding of the wave climate in sufficient detail for design of a successful beach stabilisation option, 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.

3.1.2.1 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 cannot 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-dynamic 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; and

Surveyed soundings to RL -6m AHD along Currarong Beach.

Long term wave statistics were derived from a Waverider buoy operated by the Manly Hydraulics Laboratory, offshore of Port Kembla. The domain of the wave transformation model extended over 250km from Bulli in the north to Bunga in the south, extending some 200 km offshore into water depths in excess of 100 m (Figure 3.1). This region was schematised onto a 2.5 km square grid from data derived from the soundings on the 1 km grid. A 400 m nested grid, covering the surrounding coast of Jervis Bay from Gerroa in the north encompassing the whole of Jervis Bay, and out to 600 m depth, provided a more detailed schematisation of the study region (Figure 3.1). 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. A 200 m nested grid, covering the coast from Shoalhaven Heads to Beecroft Head provided a more detailed schematisation of the study region (Figure 3.1). 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 and the surveyed

soundings in front of Currarong Beach. A 50 m nested grid, covers the surrounding coast of the Beecroft Peninsula (Figure 3.2). Data for this grid was derived from the Aus. 193 Admiralty Chart Jervis Bay and Approaches and the surveyed soundings in front of Currarong Beach. Details in the nearshore area of interest were schematised on a 15 m grid based on soundings and contours from the Aus. 193 Admiralty Chart Jervis Bay and Approaches,

and the surveyed soundings adjacent to Currarong Beach and are depicted in Figure 3.2.

3.1.2.2 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 SE 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 3.3. The transformation of offshore swell waves to the area of Currarong Beach 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 SE with an 8s wave period is given in Figure 3.4. It can be seen that the swell wave vectors mostly approach the beach at an angle to the shoreline, which would tend to induce westward longshore sediment transport. At the western end of the beach near Plutus 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 swell wave approach direction.


Figure 3.3 - Storm wave height duration recurrence (from Manly Hydraulics Laboratory)

Port Kembla Wave Data Feb 1974 to Dec 2004

(Manly Hydraulics Laboratory)

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101112

0.01 0.1 1 10 100 1000

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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 Currarong. Four locations were examined in detail – the western end of the beach near Plutus Creek, the main body of the beach along Warrain Crescent, the area directly west of the reef located at the centre of the beach and the eastern section of the beach located between the central reef and Currarong Creek entrance. These four locations are shown in Figure 3.4. It can be seen in Figures 3.5 to 3.8 that, due to the effect of swell wave refraction for all offshore wave directions between NE and SSW, the range of wave approach directions possible at Currarong Beach is very narrow, between 30° and 37°TN. This compares with a shoreline orientation angle of 0° to 36.5°TN, indicating that swell waves typically approach the shore at an angle of -3° to +33°, which would mainly induce some westward longshore transport all along the beach with some eastward movement at the eastern end. The impact of storm waves was also examined using the SWAN model. Kulmar et al. (2005) indicate offshore significant wave heights for different storm wave directions. Selected wave heights for qualitative analysis of the sediment transport are shown in Table 3.1. 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. Given the large angle around the reef at the centre of the beach a large storm event can have a significant impact on the longshore sediment transport depending on the storm direction.

Table 3.1 – Significant wave height versus climate condition (and storm duration)

Climate condition Offshore Significant Wave Height (m)

Everyday conditions 1.5

12-hour SE storm (50 yr ARI) 7.5

12-hour ENE storm (50 yr ARI) 5.5

12-hour SE storm (10 yr ARI) 6.2

12-hour ENE storm (10 yr ARI) 4.2

Wave height coefficients due to wave refraction of average swell waves (Hs offshore = 1m) at Currarong Beach are provided in Appendix B. It was found that east and directly west of the reef located at the centre of the beach (respectively RP4 and RP3), the wave transformation coefficient does not exceed 0.08. For the main body of the beach (RP2), offshore swell waves approaching from the SSW to ESE have a wave refraction coefficient of less than 0.06 while the waves approaching from the east to NE range between 0.12 and 0.17. At the western end of the beach, offshore swell waves approaching from the SSW to SE have a wave refraction coefficient of less than 0.09 while the waves approaching from the ESE to NE range between 0.22 and 0.53. These coefficients can be applied to the wave heights presented in Table 3.1 to determine the design significant wave height used to estimate sediment transport rates.

50 year ARI storm waves reach a nearshore wave height of around 0.90m for a SE direction and 1.9m for an ENE direction. These nearshore wave heights have been calculated by using the results of SWAN as input into the nearshore wave transformation model SBEACH. It should be noted that the design wave height H1/10 (i.e. average of the highest 1/10

th of the wave) is 1.27 times larger than the significant wave height Hs – this means that, for

design purposes, the wave height due to swell at the site could reach H1/10 = 2.4m for an ENE swell and 1.1m for a SE swell.


Figure 3.5 – Nearshore swell wave approach angle at 2.0m depth vs. offshore wave direction – Reference Point RP1

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Offshore Wave Direction(°TN)

Nearshore wave approach angle vs offshore wave direction, Hs=1m, RP1

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Shore angle 36.5°



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Shore angle 13°directly east of central reef


3.1.2.3 Diffraction Analysis

Diffraction is a significant phenomenon occurring in the vicinity of Currarong Beach around Beecroft Head and the several reefs located along the shoreline. The main diffraction that occurs around Beecroft Head and the nearshore reefs was determined using the diffraction diagram provided in the Coastal Engineering Manual. Results of the diffraction analysis at Currarong Beach are presented in Figure 3.9. This diagram shows wave crests approaching at an angle to the shoreline.

As the diffraction is associated with a strong refraction around the same features, it cannot be considered entirely independent of refraction. This makes the problem of combining refraction and diffraction processes strongly dependent on nearshore bathymetry variations at small spatial scales, making the problem very complex and intractable without the use of numerical modelling. For this reason, the combined refraction/diffraction model REF/DIF1 was used to determine this combined effect on the wave climate at the beach.

REF/DIF (acronym for REFraction/DIFfraction) is a phase-resolving parabolic refraction-diffraction model for ocean surface wave propagation. It was originally developed by Jim Kirby and Tony Dalrymple starting in 1982, based on Kirby's dissertation work. This work led to the development of REF/DIF 1, a monochromatic wave model. The following wave propagation processes are represented in REF/DIF 1:

Diffraction;

Refraction;

Shoaling;

Energy dissipation (including dissipation due to interaction with the bottom, porous sand and wave breaking); and

Wave-current interaction.

The REF/DIF 1 model, in parabolic form, has a number of assumptions inherent in it. These assumptions are:

Mild bottom slope: The mathematical derivation of the model equations assumes that the variations in the bottom occur over distances which are long in comparison to a wave length. For bottom slopes up to 1:3 the mild slope model was accurate and for steeper slopes it still predicted the trends of wave height changes and reflection coefficients correctly;

Weak nonlinearity based on a Stokes expansion of the water wave problem;

The wave direction is limited to a sector ±70° to the principal assumed wave direction, due to the use of the minimax wide angle parabolic approximation of Kirby (1986b);

Waves which are reflected directly back the way they came are not modelled and are neglected

Combined refraction/diffraction models include both effects explicitly, thus permitting the modelling of waves in regions where the bathymetry is irregular and where diffraction is important. Combined refraction/diffraction models are uniquely suited for the calculation of wave heights and wave direction in areas where one or both of these effects are present such as Currarong Beach. The weakly nonlinear combined refraction and diffraction model described here includes the third order correction to the wave phase speed. The wave height is known to second order (Liu and Tsay (1984)). It should be noted that it is not a complete third order theory, as all the third order terms are not retained. Known ambient currents, which affect the height and direction of wave propagation, are input for the model and enable it to predict waves where currents may be strong.


The domain of the combined refraction/diffraction model covers the surroundings of Beecroft Head and Currarong Beach. It extends around 1.5km offshore to include the various offshore reefs located offshore of Currarong Beach and the western part of Beecroft Head that would have the largest influence on the processes impacting Currarong. It has been schematised onto a 10m square grid. Data for this grid was derived from the Aus. 193 Admiralty Chart Jervis Bay and Approaches and the surveyed

soundings in front of Currarong Beach. The size of the grid is directly linked to the surveyed area as the lack of accurate bathymetry offshore would generate error in the model. Most of the diffraction occurs around the reef and rock head located within the bay in front of Currarong Beach while refraction remains the main factor offshore of Beecroft Head. An example of the result from REF/DIF is illustrated in Figure 3.10.

From the result of REF/DIF, it is observed that, in typical conditions (i.e. Hs = 1.5m and Tp = 10s), the resulting wave heights rarely exceed 0.6m along the extremities of the beach but some focusing occurs at the centre of the beach due to the presence of an offshore reef. Values close to 1m are observed in this area. Much energy is dissipated along Beecroft Head due to the strong refraction and diffraction occurring around it. Currarong Beach is fairly protected from the SSW to ESE swell waves and is influenced by wave from an eastern to northern direction. A moderate eastward sediment movement is observed at the western end of the beach – where the curve of the beach is influenced by the reef in front of Plutus Creek – while a strong westward movement is noted at the centre of the beach. The section of the beach east of the central reef appears to have a moderate eastward movement.

3.1.2.4 Locally Generated Seas

An assessment of the wave climate due to waves generated locally along Currarong Beach was also made. Wind data at Jervis Bay Wind Station and Point Perpendicular provided by the Bureau of Meteorology for 10 minute averaged wind speeds at 9am and 3pm, between 1957 and 2009, were used to estimate a typical condition wind speed. The annual wind rose statistics from the Bureau of Meteorology (2010) for Point Perpendicular Lighthouse show that north-easterly, northerly and north-westerly winds occur for around 15% of the time each and that the median wind speed is around 15-20 km/h (i.e. 5-6m/s). Wind roses are provided for 9am and 3pm wind speed at Point Perpendicular, Jervis Bay in Figures 3.11 and 3.12 respectively. A value of around 15 km/h (i.e. around 5m/s) was selected for the north-westerly winds and of around 20 km/h (i.e. around 6m/s) was selected for the northerly and north-easterly winds. A fetch of 30km was estimated for the northerly wind and of 20km for north-easterly wind while a 5.7km fetch was used to determine the north-westerly wind-generated waves. The 20 km fetch for north-easterly wind was estimated assuming that north-easterly winds are often locally-generated sea to land breezes (due to temperature differences between sea surface and land surface). The fetches are illustrated in Figure 3.13. The wind wave climate was derived using the ACES wave forecasting algorithms (Leenknecht et al., 1991). The wind wave results are gathered in Table 3.2.


Table 3 2 – ACES analysis of locally generated sea waves

Wind Direction Wind Speed (m/s) Fetch length

(km) Significant Wave

Height Hs (m) Peak Period

Tp (s)

NW wind 5

(typical conditions) 5.7 0.2 2

N and NE wind 6

(typical conditions) 20 0.5 3

NW wind 18.2*

(10yr ARI 10-hourly wind speed) 5.7 0.86 3.13

NW wind 20.9*


NW wind 23.1*


N wind 18.2*

(10yr ARI 10-hourly wind speed) 30 2.26 5.70

N wind 20.9*


N wind 23.1*


NE wind 18.2*


NE wind 20.9*


NE wind 23.1*


*Value adapted from the Australian Standard Wind Code AS/NZS1170.2:2002 value.


Figure 3.11 – Annual wind rose for 9am wind speed at Point Perpendicular, Jervis Bay


Figure 3.12 – Annual wind rose for 3pm wind speed at Point Perpendicular, Jervis Bay


3.13


As these locally generated waves have a much shorter wavelength than the offshore swell waves, they would undergo less severe refraction on the different reefs and headlands around Currarong Beach. The locally generated wave height and direction at Currarong Beach was transformed using the SWAN model. A vector diagram of wind-generated waves approaching from the NW with a 2s wave period is given in Figure 3.14. Results of the SWAN model are illustrated in Appendix B. They show that locally generated seas are always oblique to the shoreline, generating an eastward or westward sediment transport potential depending on the wind direction. Locally generated seas from the north-west and north could lead to a sediment transport towards east along most of the beach while north-easterly winds would generate a westward transport. It can be seen that, significant

wave heights could reach around 0.45 m at the site for typical conditions, due to wave breaking at the reefs surrounding Currarong Beach. In storm conditions, wind-waves can reach a significant wave height of up to 1.6-1.9m along the beach.

It should be noted that the design wave height H1/10 (i.e. average of the highest 1/10

th of the wave) is 1.27 times larger than the significant wave height Hs – this means that, for

design purposes, the wave height due to locally generated seas at the site could reach H1/10 = 2.0-2.4m.

3.1.2.5 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 and diffraction around Beecroft Head;

The significant swell wave height under extreme conditions (50 year ARI storm

event) can reach Hs = 1.9m at the western end of the beach due to ENE swells and Hs = 0.90m due to SE swells;

The locally generated waves can reach up to Hs = 0.45m in typical conditions and up to Hs = 1.6-1.9m during a 50 year ARI storm event;

The direction of approach of wave energy at the site would mostly favour westward longshore sediment transport for the swell waves while the wind waves generate an eastward sediment transport.

The conclusion from the results of the detailed diffraction analysis confirms understanding of the coastal processes developed using the SWAN model, with:

Longshore sediment transport from east to west along the beach located west of the central reef, with the beach realigning in response to the prevailing swell climate;

Lower longshore sediment transport from west to east along the section of the beach located between the central reef and Currarong Creek entrance, generated by the prevailing wind wave climate and local diffraction effects;

Enhanced longshore sediment transport during storm events;

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

Offshore sediment transport by tides at Currarong Creek entrance.


3.14


3.2 Design Of Coastal Management Options

3.2.1 Introduction

The understanding of coastal processes at Currarong Beach developed in this report allows the design of beach erosion remediation options for the area experiencing the greatest degree of erosion. In summary, the coastal processes at the site relevant to the design of a successful beach management option are:

Low rates of longshore sediment transport from east to west along the beach located west of the central reef, with the beach realigning in response to the prevailing swell climate;

Longshore sediment transport from west to east along the section of the beach located between the central reef and Currarong Creek entrance, generated by the prevailing wind wave climate and local diffraction effects;

Enhanced longshore sediment transport during storm events;

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

Offshore sediment transport by tides at Currarong Creek entrance.

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

3.2.2 Coastal Management Alternatives

An appropriate coastal management option, based on the coastal processes at the site and the preferred options selected after the workshop with Council and the Coasts and Estuaries Committee, 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 in addition to beach nourishment to minimise the impact on the beach downdrift of the groyne;

Beach nourishment at the central reef where the dwellings behind the dune are the closest to the beach.

The above options would be supplemented with geotextile works along the beach accessways to help protect them against accelerated erosion.

3.2.3 Design Parameters For Proposed Management Options

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

quantifying the coastal processes (including a qualitative estimate of the sediment transport direction at various locations along the beach);

consideration of the most appropriate location of the groyne (at the reef at the centre of the beach, west of the reef or at the eastern end of the beach);

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; and

consideration of the impact of the proposed options.


3.2.3.1 Derivation Of Sediment Transport Pathways

Crucial to the design of a successful groyne scheme is a qualitative understanding of the rate and direction of longshore drift at the site. Estimated potential longshore sediment transport rates are not provided within this report, as they are subject to considerable uncertainty. However, the techniques described below were used to provide a qualitative understanding of sediment transport pathways for design of a successful groyne scheme. 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. The CERC formula assumes that the longshore sediment transport rate depends on the longshore component of energy flux in the surf zone. The CERC formula provides an estimate of the instantaneous (gross) sediment transport, ignoring the effects of currents and onshore-offshore processes. The above parameters were used in conjunction with long-term statistics on swell wave direction to estimate longshore sediment transport rates and directions for Currarong Beach. It should be noted that longshore sediment transport rates derived using the CERC formulation provide at best an order-of-magnitude estimate of the sediment transport, as there is considerable scatter in reported estimates of the dimensionless K value (refer Figure 3.15), and as the formulation does not take the effect of wave period into account in the calculations. The CERC formula is given by:

lssP

ga

KQ

' (3.1)

where Q = Longshore sediment transport rate K = dimensionless empirical coefficient, related to sediment grain size

s = sediment density

= 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

2sin16

2 (3.2)

where Hsb = nearshore breaking height of the significant wave Cgb = wave group speed at breaking, and

b = 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 Currarong Beach was found to be 0.25 mm. From Coastal Engineering Manual


(2003), an empirically based value for K is around 0.75, based on the

median grain size (refer Figure 3.15).

s = sediment density = 2650 kg/m3

= 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 = 0.6; Hsb = nearshore breaking height of the significant wave – from the analysis

in Section 3.1.2.2; Cgb = wave group speed at breaking, which varies with the wave height in

accordance with Equation 3.3;

b = angle breaking wave crest makes with the shoreline, which is -3° to +33°.

The above parameters were used to derive an understanding of longshore sediment transport directions and relative magnitudes for Currarong Beach under the range of wave conditions possible at the site. Potential net longshore sediment transport refers to the amount of sediment that would be transported along the shoreline by wave action over a given period of time, assuming that there is an infinite supply of sand available on the updrift side, and ignoring the effects of currents, coastal structures, bedrock in the surf zone and onshore-offshore transport processes. The net transport rate refers to the sum of the sediment transport for each possible wave condition and duration, summed over a chosen time period.

The longshore sediment transport potential was calculated using the wave occurrence statistics:

For the swell waves, the longshore sediment transport has been weighted for each direction using the occurrence statistics shown in Table 3.3. For an offshore wave height used for the modelling under typical conditions being 1.5m, the wave direction occurrence of the “1.00 1.99” significant wave height were used in the calculation. For example, the sediment transport generated by a SSE swell wave

direction was weighted using a coefficient of

;

For the wind-generated waves, the longshore sediment transport has been weighted for each direction using the percentage of occurrence of the different wind direction as shown by the wind roses (Figures 3.11 and 3.12). It was observed that each of the three directions used for the calculation of the sediment transport (i.e. NW, N and NE) occur 15% of the time.

Two types of wave dynamic generate reversed directions of sediment transport at RP1, RP2 and RP4. The potential volume of sediment transport by wind waves is in the range of 5,000m3/yr – 40,000 m3/yr mostly eastward along Currarong Beach, more than twice the transport induced by swell waves along the beach toward the west. However, the net transport rate would be a lot smaller than potential transport, 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 from Currarong Creek or occasional wind-blown sand transport. Referring to the CERC results, the area east of the central reef is relatively stable with a small longshore sediment transport potential toward the east.


Table 3.3 – Sydney wave height occurrence by direction to December 2004 (Kulmar et al., 2005)

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 east to west along the beach for the swell-generated sediment movement while it is from west to east for the wind-generated sediment movement.


Figure 3.15 – Determination of value of K parameter in CERC sediment transport formula (from Coastal Engineering Manual, 2003)


For comparison purposes, the sediment transport direction and relative magnitude was also evaluated using the Kamphuis (1991) expression. This expression is based on an extensive series of hydraulic model tests, and depends on breaking wave height, wave period, grain size, nearshore beach slope and nearshore wave approach angle. The expression is given by:

where

Qk = sediment transport rate, m3/year

Hsb = breaking wave height

Top = wave period (8s, 10s and 12s for swell waves)

mb = nearshore beach gradient (i.e. 1:10 as measured in the beach survey)

D = sediment grain size (i.e. 0.20mm according to the sand samples taken

along the beach)

b = angle breaking wave crest makes with the shoreline, which is ranging from -3° to +33° for the different swell wave directions.

Kamphuis (1991) method also shows that the main potential is for sediment transport from east to west along the beach west of the central reef and west to east in the section east of the reef. It is noted that the Kamphuis equation takes into account wave period, which is not a parameter used by the CERC equation.

A conceptual sediment transport model for Currarong Beach based on the results of all the previous calculations is illustrated in Figure 3.16. An eastward sediment movement is generated at the reef located in front of Plutus Creek due to the beach angle generated by the reef itself. Along the section of the beach located west of the central reef, the wind generates an eastward sediment transport while the swell generates a westward sediment transport. This westward sand movement potential is exacerbated westward as the swell energy increases. Cross-shore sediment movement occurs mostly during storm events. Along the section of the beach east of the central reef, the impact of swell on the sand movement becomes negligible and the wind impact is predominant. This section of the beach is impacted by the Currarong Creek entrance. The flood tide would push the sediments within the creek and therefore generate siltation of the creek while the ebb tide would slightly redistribute some sand on the beach. Some lower cross-shore sediment transport would occur along this section of the beach as it is much more protected than the western half.

3.2.3.2 Potential Groyne Locations

Four potential locations for a groyne of approximately 30-80 m length were considered:

On the central reef of the beach;

At the eastern end of the beach along Currarong Creek entrance;

East of the central reef; and

Between the central reef and Plutus Creek, about 250m west of the central reef.

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 3.17. 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 Appendix B for average offshore swell and wind conditions.

𝑄𝑘 = 6.4 × 104 𝐻𝑠𝑏

2 𝑇𝑜𝑝1.5𝑚𝑏

0.75𝐷−0.25sin 2αb 0.6


If the groyne were to be located west of the central reef, benefits would be uncertain as wind wave-generated sediment transport is toward the east, and swell-wave generated sediment transport is toward the west. A “null point” where there is a change in net sediment transport direction is located along the western side of the beach, and this null point would move eastward or westward depending on day-to-day weather conditions. If the groyne is located east of the null point at any particular time, it could temporarily cause erosion in front of Warrain Crescent. If the groyne were to be located on the central reef, it would cause sand accumulation along the eastern side of the groyne and shelter a short section of shoreline. A reduction in the amount of sand moving westward past the groyne would cause erosion along the western side of the groyne. Beach erosion along the western side of groyne would therefore not be controlled and the groyne would not impact on sand loss to Currarong Creek. A groyne located just east of the central reef, while easier to construct due to improved access, may increase the potential for the creek entrance to break out through the sand spit. Such a groyne may provide some benefit to the eroded area at the eastern end of Warrain Crescent. If the groyne were to be located at the eastern end of the beach near Currarong Creek entrance, the beach between the central reef and the groyne would receive the most benefit, with the beach west of the central reef receiving minimal benefit from the groyne. The groyne may retain sand within eastern end of the beach and reduce sand loss into Currarong Creek. If such a groyne were to be placed too close to the creek entrance, it would act in conjunction with the existing eastern training wall to train the entrance to Currarong 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.


DECCW advice suggests that a groyne east of the main reef would be required in conjunction with beach nourishment to prevent loss of sand into the creek. The shoreline on the downdrift side of the groyne would need to be monitored for any potential impacts, which should be negligible if the beach compartment is filled with beach nourishment sand.

3.2.3.3 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. The relative groyne length varies with the location since the surf zone width changes with wave height and tidal stage. For the area west of the central reef where a swell breaking wave of around Hsb = 0.45m occurs, the groyne would need to extend from the landward limit of wave runup to a depth of approximately 0.7m, to trap the majority of the sediment transport that can occur at the site under ambient wave conditions. The local wave climate at the central reef and the area near the entrance to the Currarong Creek is dominated by wind to generate a breaking wave in the range of 0.45m. From the bathymetric survey of Currarong Beach, this would require a groyne length of around 50m for the western groyne, 70m for the groyne on the central reef, 30m for the groyne directly east of the central reef and 80m for the eastern groyne. 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. A high groyne has crest elevations above the normal high-tide level and above the limit of wave runup on the beach which would stop the sediment transport over the groyne and transmit little wave energy. A low groyne would allow sediment transport over the groyne to minimise erosion along the beach downdrift. The groyne height depends on the available sand supply for the beach nourishment and would be determined to allow the groyne compartment to be filled with sand allowing an immediate permeability to sediment transport that would avoid erosion on the downdrift side of the groyne.

3.2.3.4 Potential Construction Materials For Structure

Groynes can be constructed of various materials, including rock/rubble mound, concrete or steel sheet piles, timber or geotextile. The materials selected in this study are rock for a permanent groyne and geotextile to use for the construction of a trial groyne as a preliminary structure. 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. Rock Groyne A typical design for a rock groyne structure that would withstand extreme wave conditions expected at the site (a significant wave height of 1.6-1.9m) is given in Figures 3.18-3.21.


NOT TO SCALE

Core (quarry run)

2 layers armour W50 = 506 kg D50 = 670 mm

Underlayer W50 = 51 kg D50 = 310 mm

Minimum crest width 1.7m Crest level = 2m AHD

1V

2H Geotextile

SECTION VIEW

-1 m AHD

Layer thickness: 1.15m

La layer thickness: 0.70m


NOT TO SCALE

Core (quarry run)




1V

2H Geotextile

SECTION VIEW

Reef level (around 0.5m AHD)




NOT TO SCALE

Core (quarry run)




1V

2H Geotextile

SECTION VIEW

-1 m AHD




Sizing of the rock has been undertaken using the Hudson Equation. The rock armour sizes required to maintain stability for the given wave and water level conditions at each groyne were determined using the algorithms in ACES (CERC, 1984). These algorithms use the Hudson Formula (after CERC 1984) for revetment stability, given by:

cot1 3

3

rD

r

SK

HwW

where: W = Weight of an individual armour unit in the primary cover layer, kg; wr = unit saturated surface dry density, kg/m

3 H = design wave height at the structure site, m (corresponding to Hmax)

Sr = specific gravity of armour unit, relative to the water density at the

structure (usually can be approximated as 1.65)

angle of the structure slope, measured in degrees KD = stability coefficient which depends primarily on the shape of the

armour units, roughness of the armour unit surface, sharpness of edges and the degree of interlocking achieved during placement

The above formula is based on comprehensive physical model investigations at the U.S. Army Corps of Engineers. The variable wr depends on the properties of the available rock. A flatter slope or higher stability coefficient (KD) value leads to a decrease in required armour stone weight, W.

Armour units that consist of rough quarried stone will have a higher KD value than smooth, rounded armour stones. A higher KD value can be achieved by special placement of the

armour stones to achieve a high degree of interlocking. Random placement of the stones leads to a lower value of KD, which could lead to the required armour stone size W

exceeding that which is available. Conservative parameters were chosen in order to select the required KD value. Incorporated within the KD value are variables such as the angle of incidence of wave attack, size and porosity of the underlayer material, revetment crest width and the extent of the revetment slope below the still water level. Table 3.4 gives recommended values of KD to use for different situations (after CERC, 1984). Table 3.4 – KD values for Determining Quarrystone Weight*

Suggested "KD" Values for use in Determining Quarrystone Weight

Armour Units (Quarrystone)

Number of layers ‘n’

Placement

Slope

Cotangent

Structure Trunk

Structure Head

Breaking Wave

Non-breaking

Wave

Breaking Wave

Non-breaking

Wave

Smooth rounded 2 Random 1.5 – 3.0 1.2 2.4 1.1 1.9 Smooth rounded >3 Random 1.6 3.2 1.4 2.3 Rough Angular 1 Random 2.9 2.3

1.5 1.9 3.2 Rough Angular 2 Random 2.0 2.0 4.0 1.6 2.8

3.0 1.3 2.3

Rough Angular >3 Random 2.2 4.5 2.1 4.2 Rough Angular 2 Special 5.8 7.0 5.3 6.4

Parallelpiped 2 Special 7.0-20.0 8.5-24.0

Graded Angular Random 2.2 2.5

*After CERC, 1984


Waves will not break directly onto the structure slope. The results from the Hudson analysis assume that no damage to the profile is allowed (static design). From Table 3.5, a revetment consisting of two layers of rough angular armour stones specially placed and subject to breaking waves corresponds to a KD value of 5.8 for the head of the groyne and 5.3 for the trunk of the groyne. This value has been adopted for the Final Design. Results of the static design approach are given in Figure 3.18 - 3.21. In calculating the armour sizes, it has been assumed that the density of individual armour stones is 2,650 kg/m3. Use of a geotextile layer between the underlayer and core layer would reduce the interface shear strength significantly. In addition, a geotextile layer would reduce the permeability of the armour which would decrease its stability under wave action. For this reason, a graded filter layer would be recommended rather than a geotextile to prevent fines from the core washing through the armour layer of the groyne. The following guidance is from the Coastal Engineering Manual Part VI (2003). To prevent loss of core material due to leaching through the filter layer, the grain size diameter exceeded by 85% of the filter material should be less than 4-5 times the grain size diameter exceeded by the coarsest 15% of the foundation or underlying material (CEM, 2003), i.e.

D15(filter)/D85(foundation) < 4 – 5

The coarser particles of the foundation or base material are trapped in the voids of the filter layer, thus forming a barrier for the smaller sized fraction of the foundation material. The same criterion can be used to size successive layers in multilayer filters that might be needed when there is a large disparity between void sizes in the overlayer and particle sizes in the material under the filter. Filter layers overlying coarse material like quarry spall and subject to intense dynamic forces should be designed similar to a rubble-mound structure underlayer with:

W50(filter)/W50(foundation) < 15 – 20 Adequate permeability of the filter layer is needed to reduce hydraulic gradient across the filter. The acceptable permeability criterion is:

D15(filter)/D15(foundation) > 4 – 5

To prevent loss of fine particles through the filter, the filter gradation should not be too wide and should conform to:

D60(filter)/D10(filter) < 10

Filter layers constructed of coarse gravel or larger material should have a minimum thickness at least two to three times the diameter of the larger stones in the filter distribution. In underwater placement, bedding layer thickness should be at least two to three times the size of the larger quarrystones used in the layer, but never less than 30 cm thick to ensure that bottom irregularities are completely covered. Considerations such as shallow depths, exposure during construction, construction method, and strong hydrodynamic forces may dictate thicker filters, but no general rules can be stated. For deeper water the uncertainty related to construction often demands a minimum thickness of 50 cm.


If the underlayer of the groyne matches the above criteria, the filter layer would not be necessary. Moreover, the core material can be selected in order to match the above criteria using the underlayer parameter as filter parameter. Using the Hudson Equation and the above criteria, the filter layer can be removed if the underlayer and core material comply with the following criteria:

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, such as a geotextile groyne. Geotextile Groyne A geotextile groyne structure would typically have a design life of around 5 years. A geotextile 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. The stability of the geotube groynes was analysed by applying the Pilarczyk Formula (Pilarczyk, 2000):

𝐻 𝐷

Where: Hs = Significant wave height at the structure D = Thickness of the geotube

∆ = Relative density of the sand = −

With n = porosity of the geotube, i.e. 40% ρs = density of the sand, i.e. 2650kg/m

3 ρw = density of the water, i.e. 1025kg/m

3

ξop = Irribarren number =

With tan α = slope of the structure, i.e. 1V:2H

λ0 = offshore deepwater wavelength =

g = gravity constant, i.e. 9.81m/s2

T = peak period, i.e. 5.7s for the wind wave and 12s for the swell

D15(underlayer)/D85(core) < 4 – 5

D15(underlayer)/D15(core) > 4 – 5

D60(underlayer)/D10(underlayer) < 10

For Western Groyne: o W50(core) > 2.3 kg o Underlayer Thickness > 700mm

For Central Groyne: o W50(core) > 4.3 kg o Underlayer Thickness > 800mm

For Groynes East of Central Reef: o W50(core) > 2.7 kg o Underlayer Thickness > 700mm


The geotube size was designed for a 10 year ARI storm event and the results of the sizing are given in Table 3.5. Table 3.5 – Design parameter of the geobag groynes using Pilarczyk (2000)

Groyne Location Significant Wave

Height at the Structure Hs (m)

Peak Period at the Structure Tp (s)

Geobag Thickness

D (m)

Nominal Geobag Diameter

(m)

West Groyne 1.52 12 1.58 2.63

Central Reef Groyne

1.79 5.7 1.23 2.05

Groynes East of Central Reef

1.57 5.7 1.11 1.86

A geobag groyne was built along an open-coast beach at Maroochydore Main Beach using 2.5m3 vandal deterrent geotextile containers in 2001 and is still operating in the present day. Limited number of physical model tests in order to get an idea of the stability of the geotextile containers in exposed groyne structures were carried out by Geofabrics Australasia Pty Ltd and from the results of the test, it would appear that the 2.5m3 would be sufficient to withstand the existing wave climate at Currarong. Cross-section of the groyne is illustrated in Figure 3.22. “Armour” geobags layer should be vandal and UV resistant while standard geobags can be used as “core material” as illustrated in Figure 3.22. Once the groyne is built, the outer surface of the containers would become impregnated with sand. The outer sand layer provides additional UV protection. Based on the UV testing data and the successful ongoing 14-year life of the standard containers in the Stockton Beach SLSC revetment, Geofabrics suggested a 25-year minimum life for a 2.5m3 ELCOROCK vandal deterrent container. It was concluded that 2.5m3 geobags can be used for the groyne construction at Currarong.

3.2.3.5 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.

3.2.3.6 Beach Nourishment Design

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

Beach nourishment involves placement of sand onto the beach to create a dune, which provides a buffer against erosion due to storms. Such nourishment depends on locating a suitable source of sand, such as a nearby creek entrance. It works best when the sand placed on the beach closely matches the grain size and characteristics of the native beach sand, or when the sand is sourced from within the same coastal sediment compartment as the beach.


Figure 3.22 – Example of a cross-section using geobags

N.B.: due to the presence of the central reef increasing the ground level, only three layers of geobags would be necessary for the central groyne

UV + Vandal Deterrent

2.5m3 Geobag

Standard

2.5m3 Geobag

~ 2.6m

Natural Ground Level


As potential sources for beach nourishment should ideally be located within the same active littoral system, the most suitable source of sand for nourishment appears to be from the entrance to Plutus Creek and Currarong Creek. An estimate of the available sand at these locations is around 10,000 cubic metres in front of Plutus Creek and around 7,000 cubic metres within Currarong Creek. Some further sediment supply may be extracted from Abraham Bosom Beach and the beach west of Plutus Creek entrance. A check of the entrance stability following dredging of sand from the entrance area for beach nourishment was carried out by means of an Escoffier analysis. Dredging sand from the entrance in the intertidal zone could increase the tidal prism of Currarong Creek. However, it was considered that the change would not be enough to send the entrance into an unstable scouring mode (Nielsen 2004). This means that the entrance will tend to shoal over time, and that this would still be the case following dredging for beach nourishment. The beach profile has been studied to determine if the beach has conceivably reached its equilibrium profile. Bruun (1954) proposed a simple power law to describe the relationship between water depth, h, and offshore distance, x, measured at the mean sea level to determine the equilibrium profile:

h = Ax2/3

where A is a dimensional shape factor, mainly dependent on the grain size. Figure 3.23 (from Dean, 1987) gives an empirical relationship between A and grain size, D.

The median grain size was obtained from sediment samples undertaken at various locations along Currarong Beach. The results of the analysis are provided in Figure 3.24. This gives a value of A for Currarong Beach, based on the median grain size (D50) of

around 0.20 mm, of approximately 0.1. A typical beach profile for Currarong Beach was compared to the equilibrium profile to verify if offshore sediment could be used as beach nourishment source for the beach. The result of the comparison is illustrated in Figure 3.25. It is noted that the existing beach profile is located below the theoretical equilibrium profile. Therefore no offshore sand is available for use as beach nourishment supply. The nearshore profile may indeed be the cause of the existing erosion along Currarong Beach with offshore sand movement unable to be brought back onto the beach, resulting in the beach profile readjusting to reach the equilibrium profile shape. The volume of sand required to provide protection of the area between where the houses are the closest to the beach to where the dune arm is the narrowest along Currarong Creek (i.e. around 230m length of beach), for a design storm event generating a 60m3/m storm bite would be around 13,800m3. An additional sand nourishment of around 1,000m3 on the downdrift side of the groyne would prevent the beach from eroding due to the presence of the groyne. The minimum total volume of sand required for efficient beach nourishment and groyne would therefore be around 15,000m3. This is within the amount of sand available at Plutus Creek and Currarong Creek which is estimated to be 17,000m3. 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 3.26. Cross-sections of the beach illustrating the beach nourishment principle along the beach are provided in Figure 3.27 and the location of the cross-section is shown in Figure 3.26.


Figure 3.23 - Suggested relationship for shape factor A vs. grain size D


Figure 3.24 – Sediment grain size analysis for Currarong Beach

0

10

20

30

40

50

60

70

80

90

100

110

0.01 0.1 1 10

% p

assi

ng

Sand Diameter (mm)

Sieve analysis results

Currarong Beach

Currarong Creek

Plutus Creek Entrance

Abraham Bosom Beach


Figure 3.25 – Comparison of equilibrium profile and existing profile at Currarong Beach

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

0 50 100 150 200 250 300 350

Ele

vatio

n (m

)

Chainage (m)

Currarong Beach Profile Comparison

Equilibrium Profile

Existing Beach Profile


(A) (B)

(C) (D)

Figure 3.27 – Currarong Beach sand nourishment cross-sections at cross-sections CS1 (A), CS2 (B), CS3 (C) and CS4 (D) from Figure 3.25

-1

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100

Beach Profile Before Sand Nourishment

Beach Profile After Beach Nourishment

-1

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70



-1

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70



-1

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70



Volume of 60m3/m

Volume of 60m3/m Volume of 60m3/m


When the borrow sand distribution does not match the native sand distribution, it results in an overfill ratio RA. This overfill ratio is a coefficient depending on the distribution of both the borrow sand and native sand and which is applied to the required volume of sand. For example, if the overfill ratio between the existing sand and the selected source of sand is 1.1, a volume of 1,100m3 of borrow sand would be required to act as efficiently as a volume of 1,000m3 of native sand.

After Coastal Engineering Manual (CEM, 2003), a nourishment project should use fill material with a composite median grain diameter equal to that of the native beach material, and with an overfill factor within the range of 1.00 to 1.05. This is the optimal level of sediment compatibility. Both the overfill factor and equilibrium beach profile concepts indicate that sediment compatibility is sensitive to the native composite median grain diameter. As such, the compatibility range varies depending on the characteristics of the native beach material, with coarse material being less sensitive to small variations between the native and borrow sediments than fine material. As a rule of thumb, for native beach material with a composite median grain diameter exceeding 0.2 mm, borrow material with a composite median diameter within plus or minus 0.02 mm of the native median grain diameter is considered compatible. For native beach material with composite median diameter between 0.15 and 0.2 mm, borrow material can be considered compatible if its composite median diameter is within plus or minus 0.01 mm of the native diameter. For native beach material with a composite median diameter less than 0.15 mm, use of material at least as coarse as the native beach is recommended. Even though material is deemed compatible based on these rules, grainsize differences should be factored into estimates of required fill volume through use of equilibrium beach profile methods, or the overfill factor, or both.

The overfill ratio can be calculated using the following criteria (CEM, 2003):

Where:

x = -log2D(100-x) = Xth percentile of the sediment diameter (phi unit) D(100-x) = sediment diameter exceeded X percent of the time (mm) σϕb = estimated standard deviation for borrow material (phi unit) σϕn = estimated standard deviation for native material (phi unit) Mϕb = estimated mean grain size for borrow material (phi unit) Mϕn = estimated mean grain size for native material (phi unit)

Once these two criteria calculated, the overfill ratio can be read from the diagram shown in Figure 3.28.


At Currarong, some sand has been transported from the beach to Currarong Creek entrance and some sand is available at different locations including:

Currarong Creek Plutus Creek Abraham Bosom Beach

The grain size distribution at these locations as well as at the native beach is illustrated in Figure 3.23 and the values required in the above criteria for each location are provided in Table 3.6. From Table 3.6 and Figure 3.28, an overfill ratio of 1.10 would be required to stabilise the beach nourishment using sand from Currarong Creek, while no overfill would be needed should the sand be sourced from the area in front of Plutus Creek or from Abraham Bosom Beach.

Table 3.6 – Distribution parameters required to determine overfill ratio at the four sampling locations

Location

Diameter Currarong Beach Currarong Creek Plutus Creek

Abraham Bosom Beach

D5 (mm) 0.09 0.08 0.10 0.11

D16 (mm) 0.12 0.11 0.16 0.17

D50 (mm) 0.20 0.20 0.21 0.22

D84 (mm) 0.28 0.28 0.28 0.33

D95 (mm) 0.33 0.37 0.38 0.40

Nourishment work would also involve dune management techniques to revegetate the dune with native species, holding the dune in place and improving the ecology and recreational amenity of the beach.

Typically, a sand bund is constructed at the seaward end of the proposed beach nourishment profile, and a slurry of sand mixed with water is pumped onto the beach. An example of the process of placing beach nourishment at Jimmys Beach in Port Stephens is shown in Figure 2.13. It would require revegetation and fencing works to be carried out at the dune, which would provide a measure of protection to the dune against erosion.

A detailed environmental approvals process (often an REF or EIS) would need to be carried out, as extraction of sand from the estuary could be seen as an “extractive industry” under the Environmental Planning and Assessment Act.

Given the limited volume of sand available from the neighbouring creeks of Currarong and the existing sediment transport scheme presented in Figure 3.16, the “beach nourishment only” option does not appear to be adequate at Currarong. Indeed, this option would need a large amount of imported sand and without any structures retaining the sand, sand would be transported either back into the creek (east of the central reef) or westward (west of the central reef).


Figure 3.28 – Isolines of the adjusted overfill ratio (RA) for values of mean difference and sorting ratio (Shore Protection Manual, 1984)


3.2.3.7 Geotextile Protection of Accessways

Accessways can be rehabilitated using geotextile revetment protections as an emergency measure. Such protections have been designed for a 2 year ARI storm event. Design parameters for these protections were calculated using the Pilarczyk formula as described in Section 3.2.3.4 and results depending on the location of the accessway are provided in Table 3.7.

Table 3.7 – Design parameter of the geobags groynes using Pilarczyk (2000)

Location Significant Wave

Height at the Structure Hs (m)

Peak Period at the Structure Tp (s)

Geobag Thickness

D (m)

Nominal Geobag Diameter

(m)

Section of the Beach West of Central Reef

1.44 12 1.51 2.52

Central Reef 1.64 5.4 1.12 1.87

Section of the Beach East of Central Reef

1.47 5.4 1.03 1.72

The Peel Street accessway would benefit from protection, as shown in Figure 3.29. Geotextile protection provided here would prevent breakout of the creek across the dune on the western side of the beach, and act as a “trip wall”, resulting in a tendency for the creek to break out west of the accessway, thus reducing damage to the accessways and reducing erosion on the western side of the beach caused by the creek flow.

3.2.3.8 Accessway Over Groyne

Should the groyne be built along the beach an accessway allowing pedestrians to conveniently cross from one side of the groyne to the other would be required. If the groyne comprised geobags, settlement would generate some issues if an accessway is placed at the top of the groyne and geotextile may be slippery and cause user safety issues. Therefore, an accessway that would go around the groyne directly landward of it is recommended as illustrated in Figure 3.30. This accessway would consist of a board and chain accessway or timber steps on each side of the groyne leading to a walkways along the dune.


3.29


Figure 3.30 – Suggested access from one side of the groyne to the other (typical accessways pictures from the Coastal Dune Management)

Rock or Geobag

Groyne

Timber Steps Or

Board and Chain Accessway

Elevated Walkways Or

Board and Chain


3.2.3.9 Transition from Geotextile to Rock Groyne

If a geotextile groyne option is selected and proves to be an efficient protection along the beach, geotextile can be replaced by rock. Another possibility would be to use the existing geobags as core material for the rock groyne and capping the existing groyne with rock armour. The advantages of this option are:

Saving on the construction cost as it would require less additional rock core material;

Use of local sand;

No need to remove the existing geotextile groyne.

However, several factors would have to be taken into account:

Settlement of the geobags might generate stability issues of the rock armour if the geobags are not already totally settled;

Additional layers of geotextile would be required to avoid damage to the geobags during the placement of the rock armour;

Design life of the rock groyne would be reduced due to the presence of the geotextile bags that would most certainly cede before 50 year and loss of integrity of the geobags would make the rock armour collapse;

Geobags would create some steps (each bag being a step) along the slope of the groyne and the required armour thickness would have to be taken where the thickness is the thinnest (hence additional armour rock would be required).


4 COST ESTIMATE OF OPTIONS

4.1 Groynes

From the dimension of the groynes and cost estimates for the hourly rate of labour, equipment hire, material used and administration from the Rawlinson book edition 2010, the concept cost were estimated as follows:

If built using rocks:

- the western groyne would cost around A$300,000;

- the central groyne would cost around A$322,000;

- the groyne east of the central reef would cost around A$224,000; and

- the eastern groyne would cost around A$437,000.

If built using geobags:

- the western groyne would cost around A$162,000;

- the central groyne would cost around A$143,000;

- the groyne east of the central reef would cost around A$108,000; and

- the eastern groyne would cost around A$246,000.

4.2 Beach Nourishment

Coastal Engineering Solutions (2003) discuss the costs associated with beach nourishment. A cost of a minimum of $26,000 was updated for the current situation to include including deployment of a dredge and equipment for pumping sand. Based on a pumping rate of $6.50/m3 (updated from Coastal Engineering Solutions, 2003), the estimated cost of pumping 25,000 m3 of sand would be approximately $100,000. Additional costs for labour (i.e. spreading the sand) and project management would be required. Given the limited sand supply (estimated to be 17,000m3), some sand may need to be imported and such sand would cost around $10-$20/m3.


5 SUMMARY, CONCLUSION AND RECOMMENDATIONS

5.1 Summary and Conclusions

This report has examined the design of a beach management scheme for the area of Currarong Beach east of Plutus Creek and west of Currarong Creek. This area has been undergoing severe erosion where the front of the dune escarpment is very steep as a result of storm bite and a sediment budget deficit. An examination of the coastal processes in this area was undertaken, based on the results of wave refraction modeling (SWAN), combined wave refraction and diffraction model (REF/DIF), nearshore wave transformation model (SBEACH), wave forecasting algorithms (ACES) and updated bathymetric/subaerial survey data provided by Shoalhaven City Council. It was found that this area is subject to potential for longshore drift, with sand moving from east to west on the western side of the beach and from west to east on the eastern side of the beach, due to the oblique angle of the ambient wave climate, windborne waves and severe storm events. A potential management scheme which would be appropriate for this situation involves the construction of a groyne, coupled with beach nourishment. 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 Plutus Creek and Currarong Creek which are within the same active littoral system. Some sand may also be available at nearby Abrahams Bosom Beach. A central groyne would not have much impact on the beach alignment and therefore on the protection of the properties along Warrain Crescent. A groyne at the eastern end of the beach may act as a training wall for Currarong Creek and would avoid sand to be moved from the beach back to the creek entrance. However, sand would accumulate between the central reef and the groyne that would not provide any protection to the dwellings located along Warrain Crescent, unless it is constructed to a length of 180m and associated with extensive beach nourishment work as presented by CES (2003). This would have a consequent impact on the environment and cost. A groyne just east of the central reef coupled with beach nourishment would prevent loss of nourishment material back into the creek but may result in an increased risk of breakout of Currarong Creek entrance through the narrow sand spit. Positioning the groyne at the narrowest part of the spit would potentially minimise this risk, while preventing loss of nourishment sand into Currarong Creek and still providing some storm erosion buffer for the eastern half of Warrain Crescent. Detailed design of the tie-in between the groyne and the shoreline would be required to prevent undermining of the landward end of the groyne should Currarong Creek break through the spit in the vicinity of the groyne. Geotextile protection to the beach accessways, especially for the Peel Street accessway, would reduce the impact of the creek breakout on erosion at the western end of the beach. Should the groyne option be selected, a possible accessway was suggested for pedestrians to cross from one side of the groyne to the other. A transition from geotextile to rock groyne by capping the existing geotextile groyne with rock was assessed and a list of advantages/disadvantages were provided.


5.2 Recommendations

It is recommended that beach nourishment be carried out to provide some protection to the eroded beach embankment against storm bite. A geotextile groyne could be constructed east of the central reef to prevent beach nourishment sand from being washed back into the creek. A groyne constructed at the narrowest part of the spit east of the central reef was seen as the most appropriate location, as this would reduce the risk of break-out of Currarong Creek through the spit, prevent loss of sand into Currarong Creek and provide some storm erosion buffer to the eastern end of Warrain Crescent. 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. A geotextile groyne would have a design life of around 5 years, during which the scheme would be operating on a pilot basis, to confirm the local longshore sediment transport directions and confirm the understanding of the coastal processes as presented in this report. Following this period, if the groyne is successful, the geotextile 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.


6 REFERENCES

Antunes do Carmo J., C.S. Reis and H. Freitas (2009). Rehabilitation of a geotextile-reinforced sand dune

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

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

Coastal Engineering Solutions (2003). Currarong Beach Foreshore Erosion and Management Options Study

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

Dean, Robert G. 1987. Coastal Armoring: effects, principles and mitigation. Proceedings

of 20th Coastal Engineering Conference, American Society of Civil Engineers, 1843-57.

Engineering, A.A. Balkema, Rotterdam.

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.

Kamphuis, J. W. (1991). “Alongshore sediment transport rate,” Journal of Waterways, Port, Coastal and Ocean Engineering ASCE, 117(6), 624-641.

Kulmar, M., Lord, D., Sanderson, B. (2005), Future Directions for Wave Data Collection in New South Wales, 2005 Coasts and Ports Australasian Conference.

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

Nielsen, A.F. (2004) “Currarong Creek Entrance Stability”, Letter report for Shoalhaven City Council dated 15 December 2004.

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

http://www.bom.gov.au/climate/dwo/IDCJDW0200.shtml


Petkovic, P., Fitzgerald, D., Brett, J., Morse, M., Buchanan, C. (2001), Potential field and bathymetry grids of Australia's margins . Australian Society of Exploration Geophysicists 2001 Conference, Extended Abstracts.

Pilarczyk, K.W. (2000). Geosynthetics and Geosystems in Hydraulic and Coastal SMEC (2007) Shoalhaven City Council Coastal Zone Management Study and Plan –Currarong Beach Coastal Hazard Study

Townserd, M. (2005) Semaphore Park offshore breakwater – a trial. Proceedings of the Australian coasts Conference, Adelaide, September 2005.

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

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