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Mills Beach to the north, and fine sediments held in suspension around Schnapper Point from
the south.
Figure 6-1 Regional Geography
6.3 Sediment Characteristics
The sand on the beaches within the harbour area ranges in sediment grain size. Mothers Beach
has a low angled beach face that is typical of fine sand beaches, while Shire Hall Beach has a
steeper beach face representative of a more medium grain sandy beach.
Sediment sampling was undertaken to confirm the characteristics of the sediment along the
beach and in the harbour. Figure 6-2 shows the sediment sampling locations. The results of
particle size sieve analysis are summarised in Table 6-1.
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Figure 6-2 Sediment Sampling Locations
Table 6-1 Sediment analysis results
Sediment size
(Proportion %)
Location
1 2 3 4 5 6
Coarse (<4.75 mm) 29 38 0 3 0 94
Medium (<0.60 mm) 46 44 3 97 2 6
Fine (<0.30 mm) 25 18 97 14 98 2
The sieve analysis indicates that there is variability within the enclosed system as sediment in
the Mothers Beach area contains a high proportion of fine grained material whilst the sample
taken from Shire Hall Beach to the east has a high proportion of coarse and medium grained
material on- and off-shore respectively.
The samples taken within the Harbour in the approximate area of the harbour wavescreen
show a high proportion of organic material. The sediment analysis shows a well mixed
sample, although physical examination of the sample showed a large proportion of shells and
shell grit within the sample which was black in colour and cohesive.
Further sediment samples along the beach and nearshore area (approximately 0.5m in depth)
again showed grain size variation along the shoreline as stated above. The five samples taken
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along the beaches, shown as a – e in Figure 6-2 are roughly characterised by visual inspection
as predominantly fine, medium or coarse grained in Table 6-2 below.
Table 6-2 Sediment characteristics
1.1 Sample location
Beach position a b c d e
High water level fine medium-
fine
medium-
coarse medium medium
Nearshore zone medium-
coarse1,2
medium-
fine1 coarse
1,2 coarse
1,2 fine
1
1 Some shell particles present; 2 Some rock particles present
6.4 Historical Analysis
A search of available aerial photography was undertaken at VicImage. Copies of suitable
photographs were obtained for 1949, 1957, 1962, 1975, 1985 and 1991. SKM provided an
aerial photograph taken in 2004. A 2008 image was sourced from Google Earth.
The aerial photographs were scanned and geo-referenced using the ArcMap GIS system.
Figure 6-3 presents a history of shorelines between Schnapper Point and Red Bluff.
A review of historical shoreline changes in the vicinity of the proposed development provides
the following key observations relating to coastal processes in the area:
• The beaches clear of the headlands are generally around 20m wide.
• Offshore from the coast there is a rocky layer under the sand.
• There has been little change to the coastline over the 55 years from 1949 to 2004.
Beach widths have changed by less than 10m in the majority of the area. Given the
relatively poor quality of some of the images, this is within the uncertainty in the geo-
referencing process.
• The rate of sand transport along the coast appears to be relatively small.
• Pre 1970 photos show Mothers Beach extending into what is now the boat parking
area. The 1949 photo was not clear in this area
• The 1985 photo shows a much wider beach at Mothers Beach. It is unknown if this is
due to a low tide at the time or a genuine build up in sediment
• There is a natural variability of between 10 and 20m at the eastern end of Shire Hall
Beach
• Following large storm events (1985 and 2008) there is increased erosion at the eastern
end of Shire Hall Beach – up to 40m shorter beach than the average alignment
• Where erosion has occurred at the eastern end of the beaches, accretion has occurred
at either Scout or Mothers Beaches
• The current (as of 2008) alignment of the beach to the eastern end of Shire Hall Beach
is the shortest analysed here.
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Overall, the beach alignment is relatively stable at Mornington with small variations from
year to year and occasionally larger changes following significant storm events. There has
been no trend toward increasing erosion or accretion of the beaches over the last 60 years and
it is therefore concluded that the beaches in the area have been quite stable over that period.
Figure 6-3 Changes in shoreline alignment
6.5 Alongshore Transport Potential
Wave Energy
Waves are the primary factor governing sediment transport along the Mornington beaches.
Waves are generated predominantly by the action of wind over the water, but also from ships
and boats passing nearby. The potential for waves to generate sand transport is dependent
upon the amount of energy associated with the waves. In this respect, it is noted that the wave
energy density (i.e., the wave energy per unit surface area) is defined as:
E = 1/8 ρ . g . H
2
Where: ρ is the density of sea water (1.025 kg/m3), and
g is acceleration due to gravity (9.81 m/s2).
Thus wave energy is proportional to the wave height squared. As wave height doubles, wave
energy quadruples. Similarly, as the wave height halves, the wave energy is reduced to 25%
of the original energy. This is an important factor when considering the effects of a
wavescreen on coastal processes. To provide the necessary protection to boats within the
harbour, the incoming wave climate must be reduced significantly (from Hs > 2.0m to Hs <
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0.3m). This in turn will have an even greater reduction in the incoming wave energy (to <
3% of the incoming wave energy). Even relatively small changes in wave height could be
expected to have some impact on the sand transport at the coast. It is not possible to reduce
the wave climate without having some effect on coastal processes.
Sediment Transport
The main coastal processes generated by wave action include onshore/offshore transport and
alongshore transport. These are discussed briefly below.
Onshore Transport
Onshore/Offshore transport of sand is part of the natural cycle on most beaches. In relatively
calm to moderate wave conditions, there is a net onshore transport of sand along the sea bed.
This results in a gradual build up of sand at the beach and gradual depletion of the offshore
bar. During storm conditions, larger, steeper waves tend to erode sand from the beach and
deposit it in the offshore.
Alongshore Transport
Waves breaking at an angle to a beach generate an alongshore current in the direction of the
alongshore component of the in-coming wave action. These currents combine with the
turbulence generated by the breaking waves to cause a net alongshore transport of sand, and
much of the transport occurs in the breaker zone along the offshore bar.
In closed or semi-closed beach systems, such as at Mornington, the beaches tend to become
aligned perpendicular to the dominant incident wave direction.
At Mornington, Mothers Beach is protected from west and southwest waves and is aligned
perpendicular to a dominant wave direction that is east of north. By comparison, Shire Hall
Beach is more exposed to westerly waves and is aligned perpendicular to waves coming fro
the northwest. Any changes in the incoming wave height and direction distribution would
therefore be expected to result in a change in the stable beach alignments in the area.
Transport Rate
The alongshore transport rate is usually measured in cubic meters of sand per year (m3/year).
This can be a gross estimate of the whole transport occurring at a beach in both directions
(i.e., the sum of the total transport to the east plus the total transport to the west) occurring
over the year), or a net transport which considers the net transport to either the west or east
(i.e., the total transport to the east minus the total transport to the west) occurring over the
year. The longshore transport rate can also be described in terms of cubic meters per meter
length (m3/year/m) of the beach profile. This can provide a better indication of the locations
in which the transport is occurring.
6.5.1 Alongshore Transport Modelling
Numerical modelling using the Danish Hydrological Institutes LITPACK program (Section
2.2) has been undertaken to estimate the alongshore transport potential along the Mornington
Harbour beaches. The alongshore transport potential is the volume of material that, if
available, could be mobilised. Simulations were carried out at a number of cross-shore
profiles within the Mornington area as shown in Figure 5-4.
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6.5.2 Model Inputs
Potential sediment transport in LITPACK is calculated using physical information on the
shore profile, sediment characteristics and bed properties, as well as the environmental
conditions such as the wind and wave climate and tidal regime.
Shore profiles
The bathymetry of the shore profiles were extracted from survey data provided by SKM.
Where survey data was unavailable in the shallows, depths were estimated from GIS mapping
and visual inspection.
Wave Climate
The local wave climate at each profile was calculated using results from Boussinesq
modelling and wave transformation calculations as discussed in Section 5. The wave climates
at the seawards end of each profile for the existing and developed conditions are shown in
Figure 5-5 and Figure 5-6.
6.5.3 Model Results – Existing Conditions
Table 6-3 below presents results from the MIKE 21 LITPACK modelling. The LITPACK
results are used to give an indication of the potential sand transport along the harbour beaches.
With the long-term stability of the existing coastline, it is expected that the net alongshore
transport potential at each profile will be zero. That is, the gross eastwards sand transport
potential will be balanced by the gross westward transport potential at each profile, when
averaged over the year. The overall gross transport potential is the sum of the gross eastward
transport potential plus the gross westward transport potential.
Table 6-3 Existing Alongshore Transport Potential
Location Gross Transport
East
(m3/yr)
Gross Transport
West
(m3/yr)
Net
Transport
(m3/yr)
Gross Transport
Potential
(m3/yr)
Profile 1 400 400 0 800
Profile 2 1,100 1,100 0 2,200
Profile 3 2,200 2,200 0 4,400
Profile 4 1,500 1,500 0 3,000
Profile 5 2,000 2,000 0 4,000
The results in Table 6-3 show that the gross transport potential varies from just under 1,000
m3/year at the more sheltered western end of the beach, and increases to become of the order
of 4,000 m3/year at the more exposed eastern end.
It is noted that LITPACK computes the potential rate of alongshore sand transport, and that
the actual transport rate will only equal the potential rate when there is a sufficient supply of
sand. In this respect it is noted that:
• At Profile 5, there is virtually no beach, and the seabed consists mostly of rocky reef.
There is almost no sand available for transport, and the actual gross transport rate would
be expected to be close to 0 m3/year.
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• At Profile 4, there is a sandy beach and an uninterrupted sandy sea bed in the offshore
area. Here the actual gross transport rate would be expected to be close to the potential
rate of 3,000 m3/year.
• At Profiles 3, 2 and 1 there are varying degrees of rocky reef exposed along the sea bed.
This will limit the amount of sand available for transport, and the actual sand transport
rates would be expected to be somewhat less than the values given in Table 6-3.
The results indicate that whilst the harbour beaches are currently in a stable alignment, there
is the potential to generate a gross eastward and westward transport of up to about 1,500
m3/year along Shire Hall Beach.
Figure 6-4 to Figure 6-8 below show the predicted potential rate of sediment transport over
the beach profile.
Figure 6-4 LITPACK Profile 1 Results – Existing
Figure 6-5 LITPACK Profile 2 Results - Existing
Profile 1
-6
-5
-4
-3
-2
-1
0
1
0 25 50 75 100 125 150 175 200
Distance from shore (m)
Bed L
evel (m
AH
D)
-4
-3
-2
-1
0
1
2
3
Drift (m
3/y
/m)
Bathymetry East drift West drift Net drift
Profile 2
-6
-5
-4
-3
-2
-1
0
1
Distance from shore (m)
Bed L
evel (m
AHD)
-8
-6
-4
-2
0
2
4
6
Drift (m
3/y
/m)
Bathymetry East drift West drift Net drift