Mornington Harbour Coastal Processes Investigation · Mornington Harbour Coastal Processes Investigation J648/R01-C, October 2008, Rev 08 Page 23 Mills Beach to the north, and fine

Mornington Harbour Coastal Processes Investigation

J648/R01-C, October 2008, Rev 08 Page 23

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.



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



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.



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 <



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.



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.



• 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

Documents

Mornington Harbour Coastal Processes Investigation · Mornington Harbour Coastal Processes Investigation J648/R01-C, October 2008, Rev 08 Page 23 Mills Beach to the north, and fine