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 ±10,000m 3/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
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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:
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 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.
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Storm Significant Wave
Height Threshold (m)
Port Kembla Wave Data Feb 1974 to Dec 2004
(Manly Hydraulics Laboratory)
12
11
10
9
8
7
6
5
4
3
2
1
0
0.01
1hr duration
6hr duration
12hr duration
24hr duration
48hr duration
72hr duration
0.1
1
10
100
1000
Return Period (years)
Figure 3.3 - Storm wave height duration recurrence (from Manly Hydraulics Laboratory)
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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 H 1/10 = 2.4m for an
ENE swell and 1.1m for a SE swell.
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Nearshore wave approach angle vs offshore wave direction, Hs=1m, RP1
Wave Direction at Reference Location RP1
44
42
40
38
8s
10s
36
12s
Shore angle 36.5°
34
32
30
45
90
135
180
Offshore Wave Direction(°TN)
Figure 3.5 – Nearshore swell wave approach angle at 2.0m depth vs. offshore wave direction – Reference Point RP1
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Nearshore wave approach angle vs offshore wave direction, Hs=1m, RP2
33
Wave Direction at Reference Location RP2
31
29
27
25
8s
10s
23
12s
21
Shore angle 17°
19
17
15
45
90
135
180
Offshore Wave Direction(°TN)
Figure 3.6 – Nearshore swell wave approach angle at 2.0m depth vs. offshore wave direction – Reference Point RP2
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Nearshore wave approach angle vs offshore wave direction, Hs=1m, RP3
Wave Direction at Reference Location RP3
35
30
25
20
8s
10s
15
12s
Shore angle 0°
10
5
0
45
90
135
180
Offshore Wave Direction(°TN)
Figure 3.7 – Nearshore swell wave approach angle at 2.0m depth vs. offshore wave direction – Reference Point RP3
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Nearshore wave approach angle vs offshore wave direction, Hs=1m, RP4
Wave Direction at Reference Location RP4
40
35
30
8s
25
10s
20
12s
15
Shore angle 0°
along dune arm
10
Shore angle 13°
directly east of
central reef
5
0
45
90
135
180
Offshore Wave Direction(°TN)
Figure 3.8 – Nearshore swell wave approach angle at 2.0m depth vs. offshore wave direction – Reference Point RP4
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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 refractiondiffraction 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.
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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.
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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*
(50yr ARI 10-hourly wind speed)
5.7
1.03
3.33
NW wind
23.1*
(100yr ARI 10-hourly wind speed)
5.7
1.17
3.48
N wind
18.2*
(10yr ARI 10-hourly wind speed)
30
2.26
5.70
N wind
20.9*
(50yr ARI 10-hourly wind speed)
30
2.71
6.06
N wind
23.1*
(100yr ARI 10-hourly wind speed)
30
3.1
6.34
NE wind
18.2*
(10yr ARI 10-hourly wind speed)
20
1.84
4.98
NE wind
20.9*
(50yr ARI 10-hourly wind speed)
20
2.21
5.30
NE wind
23.1*
(100yr ARI 10-hourly wind speed)
20
2.53
5.54
*Value adapted from the Australian Standard Wind Code AS/NZS1170.2:2002 value.
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Figure 3.11 – Annual wind rose for 9am wind speed at Point Perpendicular, Jervis Bay
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Figure 3.12 – Annual wind rose for 3pm wind speed at Point Perpendicular, Jervis Bay
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3.13
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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.
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3.14
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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.
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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:
Q
K
P
 s   ga' ls
(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:
Pls 
where
Hsb
Cgb
b
g 2
H sbCgb sin 2b 
16
(3.2)
= nearshore breaking height of the significant wave
= wave group speed at breaking, and
= angle breaking wave crest makes with the shoreline.
In shallow water,
Cgb  gdb
where
db
(3.3)
= depth of wave breaking, which is assumed to be related to the wave
breaking height as H b  0.78db .
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
Currarong Beach Erosion Design Study 3001859 | Revision No. 4|
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s

g
a
Hsb
Cgb
b
(2003), an empirically based value for K is around 0.75, based on the
median grain size (refer Figure 3.15).
= sediment density = 2650 kg/m3
= water density = 1025 kg/m3 for seawater
= acceleration due to gravity = 9.81 m2/s
= solids fraction of the in-situ sediment deposit (1 – porosity). Porosity
of a typical beach berm is around 40%, so a = 0.6;
= nearshore breaking height of the significant wave – from the analysis
in Section 3.1.2.2;
= wave group speed at breaking, which varies with the wave height in
accordance with Equation 3.3;
= 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.
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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.
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Figure 3.15 – Determination of value of K parameter in CERC sediment transport formula (from Coastal Engineering Manual, 2003)
Currarong Beach Erosion Design Study 3001859 | Revision No. 4|
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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:
2 1.5 0.75 −0.25
𝑄𝑘 = 6.4 × 104 𝐻𝑠𝑏
𝑇𝑜𝑝 𝑚𝑏 𝐷
sin 2αb
0.6
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.
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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.
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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.
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Minimum crest width 1.7m
Crest level = 2m AHD
2 layers armour
W 50 = 506 kg
D50 = 670 mm
Layer
thickness:
1.15m
Underlayer
W 50 = 51 kg
D50 = 310 mm
La layer thickness: 0.70m
-1 m AHD
1V
Geotextile
SECTION VIEW
Core (quarry run)
2H
NOT TO SCALE
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Minimum crest width 2.1m
Crest level = 3m AHD
2 layers armour
W 50 = 945 kg
D50 = 820 mm
Layer
thickness:
1.4m
Underlayer
W 50 = 94 kg
D50 = 380 mm
La layer thickness: 0.80m
Reef level
(around 0.5m AHD)
1V
Geotextile
SECTION VIEW
Core (quarry run)
2H
NOT TO SCALE
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Minimum crest width 1.8m
Crest level = 2m AHD
2 layers armour
W 50 = 587 kg
D50 = 700 mm
Layer
thickness:
1.2m
Underlayer
W 50 = 59 kg
D50 = 330 mm
La layer thickness: 0.70m
-1 m AHD
1V
Geotextile
SECTION VIEW
Core (quarry run)
2H
NOT TO SCALE
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Minimum crest width 1.8m
Crest level = 2m AHD
2 layers armour
W 50 = 587 kg
D50 = 700 mm
Layer
thickness:
1.2m
Underlayer
W 50 = 59 kg
D50 = 330 mm
La layer thickness: 0.70m
-1 m AHD
1V
Geotextile
SECTION VIEW
Core (quarry run)
2H
NOT TO SCALE
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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:
W
wr H 3
K D S r  13 cot 
where:
W
wr
H
Sr
=
=
=
=


=
KD
Weight of an individual armour unit in the primary cover layer, kg;
unit saturated surface dry density, kg/m3
design wave height at the structure site, m (corresponding to Hmax)
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
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
Breaking
Wave
Smooth rounded
Smooth rounded
Rough Angular
2
>3
1
Random
Random
Random
Rough Angular
2
Random
>3
2
2
Random
Special
Special
Random
Rough Angular
Rough Angular
Parallelpiped
Graded Angular
*After CERC, 1984
1.5 – 3.0
1.5
2.0
3.0
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1.2
1.6
Nonbreaking
Wave
2.4
3.2
2.9
2.0
4.0
2.2
5.8
7.0-20.0
2.2
4.5
7.0
8.5-24.0
2.5
Structure Head
Breaking
Wave
1.1
1.4
1.9
1.6
1.3
2.1
5.3
Nonbreaking
Wave
1.9
2.3
2.3
3.2
2.8
2.3
4.2
6.4
Page | 91
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:
W 50(filter)/W 50(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.
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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:






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
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 rubblemound type structure.
The stability of the geotube groynes was analysed by applying the Pilarczyk Formula
(Pilarczyk, 2000):
𝐻
𝐷
Where:
Hs
=
D
=
Significant wave height at the structure
Thickness of the geotube
∆
=
Relative density of the sand =
With
n
ρs
ρw
=
Irribarren number =
With
tan α
=
slope of the structure, i.e. 1V:2H
λ0
=
offshore deepwater wavelength =
g
T
=
=
gravity constant, i.e. 9.81m/s2
peak period, i.e. 5.7s for the wind wave and 12s for the swell
ξop
=
=
=
−
porosity of the geotube, i.e. 40%
density of the sand, i.e. 2650kg/m3
density of the water, i.e. 1025kg/m3
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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 2.5m 3 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.
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UV + Vandal Deterrent
2.5m3 Geobag
Standard
2.5m3 Geobag
~ 2.6m
Natural Ground Level
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
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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 (D 50) 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 60m 3/m
storm bite would be around 13,800m3. An additional sand nourishment of around 1,000m 3
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.
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Figure 3.23 - Suggested relationship for shape factor A vs. grain size D
Currarong Beach Erosion Design Study 3001859 | Revision No. 4|
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Sieve analysis results
110
100
90
80
% passing
70
60
Currarong Beach
Currarong Creek
50
Plutus Creek Entrance
Abraham Bosom Beach
40
30
20
10
0
0.01
0.1
1
10
Sand Diameter (mm)
Figure 3.24 – Sediment grain size analysis for Currarong Beach
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Currarong Beach Profile Comparison
5
4
3
2
Elevation (m)
1
0
-1
0
50
100
150
200
250
300
350
Equilibrium Profile
Existing Beach Profile
-2
-3
-4
-5
-6
-7
Chainage (m)
Figure 3.25 – Comparison of equilibrium profile and existing profile at Currarong Beach
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Currarong Beach Erosion Design Study 3001859 | Revision No. 4|
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(A)
(B)
10
10
9
9
8
8
7
7
6
6
5
Beach Profile Before Sand
Nourishment
5
4
Beach Profile After Beach
Nourishment
4
3
3
2
2
1
1
0
0
0
20
40
60
80
Beach Profile After Beach
Nourishment
0
100
Beach Profile Before Sand
Nourishment
Volume of 60m3/m
10
20
30
40
50
60
70
-1
-1
(C)
(D)
10
10
9
9
8
8
7
7
6
6
Volume of 60m3/m
5
4
Beach Profile Before Sand
Nourishment
5
Beach Profile After Beach
Nourishment
4
3
3
2
2
1
1
0
0
0
10
20
30
40
50
-1
60
70
Beach Profile Before Sand
Nourishment
Volume of 60m3/m
Beach Profile After Beach Nourishment
0
10
20
30
40
50
60
70
-1
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
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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:
=
D(100-x) =
σϕb =
σϕn =
Mϕb =
Mϕn =
x
-log2D(100-x)
= Xth percentile of the sediment diameter (phi unit)
sediment diameter exceeded X percent of the time (mm)
estimated standard deviation for borrow material (phi unit)
estimated standard deviation for native material (phi unit)
estimated mean grain size for borrow material (phi unit)
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.
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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
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
Diameter
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).
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Figure 3.28 – Isolines of the adjusted overfill ratio (RA) for values of  mean difference and  sorting ratio (Shore Protection Manual, 1984)
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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.
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3.29
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Rock or Geobag
Groyne
Elevated Walkways
Or
Board and Chain
Timber Steps
Or
Board and Chain Accessway
Figure 3.30 – Suggested access from one side of the groyne to the other (typical accessways pictures from the Coastal Dune Management)
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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).
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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 17,000m 3), some sand may need
to be imported and such sand would cost around $10-$20/m3.
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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.
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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.
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6 REFERENCES
Antunes do Carmo J., C.S. Reis and H. Freitas (2009). Rehabilitation of a geotextilereinforced 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, 76497666.
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.
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Page | 112
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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APPENDIX A – TRIPLE BOTTOM LINE ASSESSMENT
Social
Total
Rank
1
Disruption to natural coastal
processes
Environmental
1
Impact on ecology, flora and
fauna
1
Preservation of natural
character of the beach
0.5
Approval required
1
Beach/Recreational Amenity
4
Effectiveness of option
0.5
Visual Impact
Economic
2.5
Total Cost
(construction+maintenance)
Weighting
Planned Retreat (Road Relocation) + Dune Management
8.3
8
5
4
7
10
5
10
77.3
1
Beach Nourishment + Vertical Wooden dune fence
7.2
8
5
8
7
8
7
8
76.5
2
Beach Nourishment + Reinforcement with Geotextile
6.7
8
6
8
7
7
5
8
76.3
3
Beach Nourishment
7.8
9
4
8
7
10
5
8
74.5
4
0
8
8
6
7
10
8
10
73.5
5
Dune Management (in isolation) ie Accessway
8.9
9
1
6
10
9
8
10
68.8
6
Planned Retreat (Planning Control) + Dune Management
9.4
8
2
5
9
8
5
10
68.0
7
Groyne Centre (rock)+Beach Nourishment
5.6
4
6.5
6
5
4
4
4
62.5
8
Do Nothing
10
8
0
5
10
8
5
10
62.0
9
Revetment (200m)+Beach nourishment
3.9
5
8
5
4
4
2
3
60.3
10
Groyne Centre (geotubes)+Beach Nourishment
6.1
2
6
5
6
2
3
5
58.3
11
Revetment (200m) + Groyne + Beach Nourishment
2.2
3
9
4
3
3
2
3
56.5
12
Artificial Reef
2.8
8
6
5
1
8
7
1
56.5
12
Groyne Eastern End (rock)+Beach Nourishment
4.4
4
6
6
4
4
4
3
56.0
14
Revetment (entire beach, rock) + Beach nourishment
1.1
4
10
4
2
3
1
2
55.8
15
Groyne Eastern End (geotubes)+Beach Nourishment
5
2
5.5
5
5
2
3
4
52.0
16
Vertical Seawall (200m)
3.3
1
8
3
4
1
1
2
49.8
17
Vertical Seawall (entire beach)
0.6
0
10
2
2
0
0
1
45.5
18
Currarong Creek Entrance Relocation with training walls
1.7
2
6
4
2
1
1
2
38.3
19
Planned Retreat (Purchase) + Dune Management
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