mornington harbour coastal processes investigation

Mornington Harbour Coastal Processes Investigation

J648/R01-C, October 2008, Rev 08 Page 8

Climate Change

Design water levels for the assessment of impacts of the proposed pier protection and harbour

wavescreen have been adopted from the 100 and 1 year return levels with an additional

provision for a predicted sea level rise due to climate change of 0.4m by 2100.

The Intergovernmental Panel on Climate Change (IPCC) was formed in 1988 to assess the

potential impacts of climate change (greenhouse effect) around the world. It is operated by

the World Meteorological Organization (WMO) and the United Nations Environment

Programme (UNEP) and is recognised as an independent panel. The role of the IPCC is to

review the available literature and provide summary reports and information to governments

and other organisations. Their latest report (Climate Change 2007 – The Physical Science

Basis) provides a range of predicted sea level rises from numerous scenarios and different

predictive models. In this latest IPCC report, the range of predicted sea level increases for a

100 year horizon (to the year 2100) varies from 0.18 to 0.59 m with a mean value of about

0.40 m.

It is noted that the draft third Victorian Coastal Strategy (Victorian Coastal Council, 2007)

reviews the main findings of the IPCC Fourth Assessment and other recent Climate Change

material and concludes that “for planning purposes, we will assume a sea level rise of

approximately 0.4 to 0.8 m by the end of the century.”

Given that coastal structures, such as piers and breakwaters have a nominal life span of 50

years, it was therefore considered reasonable to include an allowance of 0.4m in the design

water levels to allow for the effects of sea level rise associated with climate change and global

warming. The resulting design water levels are presented in Table 3-2, below.

Table 3-2 Design Water Levels – Mornington Harbour

Water Level

Height

(m AHD)

100 Year Design Water Level 1.7

1 Year Design Water Level 1.3

Along with increased water levels, the main threats associated with Climate Change will be

an increase in the frequency and intensity of storms, and the corresponding increases in wind

speeds, wave heights and storm surge levels. There is, however, little information available at

present to quantify these risks.

3.3 Wind Conditions

Long periods of recorded wind data over open water are available in Port Phillip Bay at 3

locations – Fawkner Beacon, South Channel Island and Point Wilson. Periodic readings have

also been taken at Hovell Pile during previous studies (Hydrodynamics Investigation, Water

Technology 2008); however these recording periods have been limited to a short amount of

time and are not considered reliable for purposes of this investigation. Similarly, the location

of the Point Wilson anemometer means winds measured here cannot be considered indicative

of conditions near Mornington.



Review of the available wind data showed that the data from Fawkner Beacon and South

Channel Island was likely to be the most representative of the conditions at Mornington. The

data from both these sites had median and maximum wind speeds for each season of the same

order of magnitude, with a median wind speed approximately 13kts. They also showed

summer winds coming predominantly from the south, while winter winds came

predominantly from the north.

After further review of the two sites, it was decided that South Channel Island (23km west

southwest of Mornington) were a more realistic representation of winds at Mornington as it

presented considerably more wind action from the northwest, west and south west than

Fawkner Beacon (located 30km north northwest of Mornington), as would be the case on the

eastern coast of Port Phillip Bay. The South Channel Island wind record was subsequently

chosen as the most suitable for use in modelling exercises.

A wind rose showing data recorded at South Channel Island between 1999 and 2007 is shown

below in Figure 3-2.

Figure 3-3 illustrates the seasonal variation in wind speed and direction where summer winds

are dominated by southerly winds and northerly winds dominate in the winter months. Spring

and autumn have a comparably even spread of wind speeds and directions.

Figure 3-2 South Channel Island Wind Climate, 1999 – 2007

Wind Speed (m/s)

Above 1512.5 - 15

10 - 12.57.5 - 10

5 - 7.52.5 - 5

1 - 2.5Below 1

N

Calm2.24 %

10 %


Figure 3-3 South Channel Island Seasonal Wind Variation, 1999 – 2007

N

Calm1.10 %

10 %

N

Calm3.14 %

10 %

N

Calm4.13 %

10 %

N

Calm0.95 %

10 %

Summer

Winter

Autumn

Spring



4 REGIONAL WAVE CONDITIONS

The wave climate offshore of Mornington has been calculated using the MIKE 21 Spectral

Wave (SW) model (Section 2.1.1).

4.1 Bathymetry

The bathymetry for the MIKE 21 SW model was developed using a digital version of

Admiralty Charts of Port Phillip (CMAP). The model uses a flexible mesh grid allowing finer

detail along the coast and areas of steep gradients, while reducing computations in areas

where topography is constant in slope and shape. The topography used to develop the models

is shown in Figure 4-1. The model does not extend outside of Port Phillip Bay as wave energy

from Bass Strait does not penetrate into the main body of the Bay.

Figure 4-1 MIKE 21 SW Model Bathymetry

4.2 Model simulations

Winds from 15 degree bands were simulated for 12 hours each through the model for a range

of winds speeds from zero to 30 m/s.

Wave height, period and direction at a point offshore of Mornington were extracted from the

results. These wave properties were used to create a matrix for which known wind conditions

could be combined to provide an accurate estimate of the wave climate for any given wind

input.

It should be noted that the incident wave direction is not necessarily the same as the direction

of the wind that generates the wave. This is due to topographic and bathymetric effects that

modify the wave’s direction. When considering the position of Mornington Harbour within

Port Phillip Bay, it can easily be established that the main wave directions will be from west



through to north. Refraction occurs as the waves propagate into shallow water, with refraction

greatest for waves whose angle of incidence is highly oblique to the shore normal direction.

Thus, shore normal NW winds tend to generate NW waves, whereas obliquely incident NNE

winds (shore parallel) result in almost northerly waves at the site (Figure 4-2).

This report refers to the northerly wave event as being the wave climate generated by the

northerly wind, and the northwesterly wave referring to the wave climate generated by a

northwesterly wind, and so on.

Figure 4-2 Mornington Harbour wind-wave interaction

4.3 Offshore Wave Climate

Using the method described above in Section 4.2, the 9 year record of wind data at South

Channel Island was transferred to an offshore wave climate at Mornington. This is shown in

the form of wave roses in Figure 4-3. These roses are presented in terms of the significant

wave height Hs, and are given for overall conditions, summer conditions and winter

conditions.

It is noted that the significant wave height is defined as the average height of the highest one-

third of waves within a record. The significant wave height is used to describe the general

character of the sea. The actual highest wave height within a record may be much higher (up

to two times higher) than the significant wave height and is used in design to ensure structures

will withstand the maximum height expected in a given return period.


Figure 4-3 Offshore Wave Climate at Mornington

The wave roses in Figure 4-3 show waves coming from the north through west to southwest.

Highest waves come from the north-northwest through to west-northwest, reflecting the

longer fetches (i.e., the distance over which the wind can generate wave action) in these

directions.

The seasonal variation noted in the winds in Section 3.3 is evident in the wave climate such

that summer conditions are dominated by waves from the southwest whilst winter conditions

have predominantly north to west-northwest waves. Further, with the longer fetches to the

northwest, the winter waves tend to be higher than those occurring in summer.

In addition to the above, it is noted that the beaches between Schnapper Point and Red Bluff

are directly exposed to incoming wave action from the north to northwest, but become

progressively more sheltered from more west and southwest waves. Thus, the summer

southwest waves are not only lower initially than the winter northwest waves, but they also

undergo greater attenuation due to diffraction around Schnapper Point. As a result, it is

expected that most of the sediment transport occurring along the beaches will take place

during the winter months.

Boussinesq wave modelling has been carried out to provide an understanding of the offshore

to nearshore wave transformations due to shoaling, refraction and diffraction of waves into

the harbour. To keep the number of simulations within reasonable limits, the 1 month

average return period wave conditions offshore from Mornington for each direction was

calculated for input to the Boussinesq model. These conditions represent the average

maximum significant wave heights Hs to occur in any one month. The results are shown in

Table 4-1.

Wave height (m)

Above 1.21 - 1.2

0.8 - 10.6 - 0.80.4 - 0.6

0.2 - 0.40.1 - 0.2

Below 0.1

N

Calm33.55 %

10 %

N

Calm35.55 %

10 %

N

Calm27.75 %

10 %

1999 – 2007 Wind-waves

Summer

Winter



Table 4-1 1 Month ARI Wind Wave Heights

Wind

Direction

Wave

Direction

(deg)

Water

Level

(m AHD)

1 Month Wave

Height

Hs (m)

Period

Ts (s)

NNE 23 1.3 0.43 2.67

N 352 1.3 1.01 3.79

NNW 344 1.3 1.25 4.14

NW 310 1.3 1.24 4.15

WNW 296 1.3 1.31 4.26

W 267 1.3 1.08 3.87



5 BOUSSINESQ WAVE MODELLING

Wave energy is the dominant factor for determining sediment transport along the Mornington

Harbour beaches. Numerical Boussinesq wave modelling has been carried out to investigate

the transformation of the offshore wave conditions, determined in the previous section, to the

nearshore areas where most of the sediment transport takes place. The wave model was

developed using DHI Software’s MIKE 21 Boussinesq Wave (BW) module (see Section

2.1.2). The wave conditions derived from the Boussinesq modelling are then used in the

sediment transport calculations described below in Section 6.5.

Some limited wave protection in the harbour is currently provided by the existing pier works.

These consist of a rubble mound structure underneath the landward end of the pier, and

closely spaced piles along the seaward end. However, there is little protection for the

majority of the harbour area and beaches, especially during north and north north-westerly

events.

The proposed harbour works investigated in this report consist of a wavescreen along the

seaward side of the existing public pier and a second wavescreen oriented roughly east-west

along the 7m depth contour to the east of the pier. The wavescreen along the pier was

considered as extending along the full length of the pier, approximately 130m.

The wavescreens were modelled as full depth vertical walls reflecting 100% of the wave

energy approaching the site. It is noted that there is a perception that partial wavescreens will

provide better flushing and have less impact on coastal processes than full wavescreens.

However, the use of full depth wavescreens was considered to be more appropriate for

Mornington. This was because partial depth wavescreens can result in the following issues:

• Reduced wave blocking ability of the wavescreen (due to a proportion of the wave

energy passing under the wavescreen) can lead to an undesirable wave climate within

the harbour

• Acceleration of wave-induced flows under the screen can lead to localised erosion,

turbidity generation, reduced effectiveness of the screen, and possible stability issues

for the structure.

Further, the wavescreens will change the wave climate within the harbour, irrespective of

whether there is or isn’t a gap underneath. For example, to reduce a 2.0m incident wave to

0.3m or less (required to obtain a good wave climate within the harbour), the energy of the

transmitted wave will need to be reduced to just over 2% of that of the incident wave. That is,

the existence of the gap under the wavescreen will not significantly reduce the impact of the

wavescreen on coastal processes and wave induced sediment transport. Further, as shown in

the Hydrodynamics Report (Water Technology, 2008) excellent water circulation and

flushing will be maintained within the harbour, even with a full depth wavescreen.

Given the potential problems, and lack of significant benefits associated with the use of an

above bed wavescreen, it was strongly recommended that full depth wavescreens be used at

Mornington.

5.1.1 Bathymetry

The bathymetry for the model was created by combining the digitalised Admiralty Charts of

Port Phillip Bay (CMAP) with the surveyed data of Mornington recorded in 1995. There is



not expected to have been a significant change in bed levels since 1995 and the survey data is

adequate for modelling purposes.

The configuration of the development provided by SKM is combined with the Admiralty

Charts and 1995 survey data as shown below in Figure 5-1. This information was used to

create bathymetries for the Boussinesq modelling.

Figure 5-1 Bathymetric Data Inputs

Separate model grids were developed to simulate incident waves from north-northeast, north,

north-northwest, northwest, west-northwest and west. To improve their computational

effectiveness, the bathymetries used for waves from the west northwest to north northwest

were rotated by 45 degrees. This enables the wave generation lines, which create the waves in

the BW model, to be more closely aligned with the model grid.

The model bathymetries had a grid spacing of 0.5m or 0.6m, for the north and north northeast,

and west to north northwest wave models respectively. This is due to the smaller period of the

north and north northeast waves demanding a finer grid for accurate computation. A time step

of 0.039 seconds was used, over a total simulation time of 25 minutes to allow the wave to

fully propagate into the harbour and beach area.

The six grids covered an area of approximately 1km2 each, depending on the direction of

incoming wave. The bathymetry for the north wave is shown in Figure 5-2 below. Areas not

relevant to the harbour area are filled with land values to reduce the computational size and

simulation time by decreasing the number of grid cells calculations are performed in. This is

true for all bathymetries.



Figure 5-2 Boussinesq Wave Modelling – North wave model bathymetry

5.1.2 Input parameters

Once the model bathymetry has been determined, the remaining input parameters required to

define the model are described below. These have been developed in line with current wave

modelling practice.

Boundaries

The model contained only land boundaries. Land surface level was set at 5m AHD. The

water surface elevation was set at 0m AHD.

Porosity Layer

A porosity layer was established for each simulation to account for the effects of wave

dissipation upon structures with sloping banks. The layer contained porosity values of 1 for

the open water, 5 for land values, and a 6m thick buffer of 0.7 around Schnapper Point and

the rock protection on the landward end of the harbour area.

Sponge Layer

A sponge layer was used in the model to represent the dissipative effect of waves breaking on

the beach. Sponge layers were also added along the northern, eastern and southern boundaries

to allow wave energy to radiate out of the model domain without reflection.

The sponge layer along the boundaries was 80 grid cells thick, whilst along the beach the

sponge layer is only 5 grid cells wide as the model reduces wave energy due to wave breaking

in shallower waters and little wave energy is required to be dissipated at the land-water

interface. The sponge layer for the north wave simulation can be seen in Figure 5-3 below.



Figure 5-3 Boussinesq Wave Model Sponge Layer (North wave simulation)

Wave Generation

The models were tested for waves coming from the north-northeast, north, north-northwest,

northwest, west-northwest and west. The one in one month return period wave conditions for

each of these directions are specified in Table 4-3 in Section 4.3 above.

The MIKE 21 Toolbox Random Wave Generator was used to create the incident waves using

the above parameters. The waves were generated as irregular waves using a standard

JONSWAP spectrum with a peak enhancement factor of γ = 3.3. Long crested “uni-

directional” waves were used as input at the wave generation lines. This made it easier to

determine the relationships between the inshore and offshore wave conditions.

The wave climate produced by the MIKE 21 Toolbox has a significant wave height equal to

that stated in Table 4-1.

Each simulation covered a 25 minute period of which the final 5 minutes were used to

calculate the wave response coefficients within the harbour.

Model Validation

The MIKE21 Boussinesq Wave model has been extensively validated against a wide range of

analytical solutions, as well as against measurements and physical model tests results for

numerous applications around the world. Within Port Phillip Bay, Water Technology

completed a study for the design of the Martha Cove breakwaters where combined physical

and Boussinesq modelling showed excellent correlation of wave heights within the harbour

entrance.



5.2 Existing Wave Climate

Resulting wave properties from the Boussinesq modelling were used to create “effective”

wave climates at the seaward end of 5 profiles shown in Figure 5-4. Results from the 1

month ARI model simulations were extracted along each profile at a depth of 2m. This area

represents the beginning of the wave breaker zone.

The wave conditions for each modelled direction at the 2m depth mark were transformed to a

deep water equivalent offshore wave using linear refraction and shoaling coefficients. This

was done assuming the beach had long straight parallel depth contours. The difference

between the modelled wave transformations and the linear refraction and shoaling

transformations was then used to provide an estimate of the effects of diffraction at the start of

the breaker zone.

The wave time series established in Section 4.3 was broken down into a matrix which

described the proportion of time a wave height with a particular period occurred. Wave

heights were broken down into 0.1m bands and wave periods were separated into 0.5 second

bands.

Equivalent offshore waves were then derived for the different wave direction and period

bands using the results of the one month return period Boussinesq model results as a guide to

quantifying the effects of diffraction. Additional changes in wave height and direction due to

variations in wave shoaling and refraction due to changes in the incident wave period and

direction were then calculated using linear wave theory and Snell’s Law.

An equivalent wave height, direction and period wave climate could be then calculated for use

at the offshore offends of the beach profiles as shown below in Figure 5-5.

Figure 5-4 Wave profile locations


Figure 5-5 Existing Wave Climate

5.3 Developed Wave Conditions

The developed condition was considered to consist of the harbour wavescreen and a full

length wavescreen along the existing pier. The full length pier wavescreen was considered to

be the worst case scenario (compared to the existing or partial length pier wavescreen) for the

coastal process investigations as it will have the greatest impact on wave climate within the

harbour. As discussed above, both wavescreens were assumed to be full depth structures.

5.3.1 Developed Wave Climate

Using the same approach as for the existing conditions, described above, the equivalent

offshore wave climates for each of the beach profiles under consideration were determined.

These are presented in Figure 5-6 and show a significant reduction in the wave climate at the

western end of the beach.

At Profile 1, the wave action has become almost non-existent and wave conditions would be

expected to be calm for the vast majority of the time. At Profile 2, the wave action is

significantly attenuated, there are a greater percentage of calms, and there is now no wave

action from the north, or west of north. At Profile 3 the degree of attenuation is somewhat

N

Calm70.62 %

10 %

N

Calm64.94 %

10 %

N

Calm62.38 %

10 %

N

Calm63.25 %

10 %

N

Calm61.36 %

10 %

Wave height (m)

Above 1.21 - 1.2

0.8 - 1

0.6 - 0.80.4 - 0.60.2 - 0.4

0.1 - 0.2Below 0.1

Profile 1

Profile 2 Profile 3

Profile 4 Profile 5


less, but there is a greater incidence of calms relative to existing conditions, and there is no

wave action from west of north. At Profile 4, the wave climate is less affected, however,

there is a slightly higher incidence of calms, and the main direction of in-coming waves has

been rotated slightly from northwest to north-northwest. At Profile 5 the wave climate is only

slightly affected by the presence of the wavescreens.

Figure 5-6 Developed Wave Climate

N

Calm99.93 %

2 %

N

Calm92.12 %

10 %

N

Calm74.81 %

10 %

N

Calm67.34 %

10 %

N

Calm62.93 %

10 %

Wave height (m)

Above 1.2

1 - 1.2

0.8 - 1

0.6 - 0.8

0.4 - 0.6

0.2 - 0.4

0.1 - 0.2

Below 0.1

Profile 1

Profile 2

Profile 3

Profile 4

Profile 5



6 COASTAL PROCESSES

6.1 Regional Geomorphology

The coastal alignment of the Mornington Peninsula between Frankston and Mt Martha is

parallel to the Selwyn Fault which bounds the western side of the Mornington Plateau.

The coastal slopes in the region are related to lithology and rock structures, with resistant

units of granite and ferruginous sandstone forming headlands interspaced with weathered and

poorly consolidated beds which have been eroded to form steep and unstable cliffs along the

shore.

Schnapper Point, on the western extent of Mornington Harbour, is a prominent headland of

red-brown ferruginous sandstone with a boulder and rough platform shoreline. The point is

comprised of sediments from the Baxter Formation which are variable in composition -

dominated by iron-stained coarse to medium-grained cross-bedded, there are mottled sands

with thin finely-laminated clay and fine sand and occasional gravel beds which dip gently to

the south. The gravel beds are occasionally iron-cemented and form angular blocks in the

intertidal shore platforms in front of the cliffs and bluffs. In this respect, the geology is

distinctly different in origin and structure from the volcanic and granite rocks elsewhere on

the Mornington Peninsula.

To the south of Mornington Harbour, Linley Point provides protection from southerly weather

at Fishermans beach, whilst Royal Beach on the south side of Schnapper Point is a small

sandy beach backed by 15 to 20m high sandstone cliffs, protected from the northerly weather

by Schnapper Point.

To the east of Mornington Harbour, Tanti Point (also known as Red Bluff) is surrounded by a

broad rocky reef which separates the mouth of the Tanti Creek and Mills Beach from the

harbour area.

Schnapper Point marks a local change in coastal orientation from north-south to east-west for

a short distance east of the point and the northerly trend continues north of Tanti Point.

6.2 Regional Coastal Processes

The regional coastal processes are dominated by wave conditions, sediment size and local

geography. The Mornington area from Mt Eliza in the north to Mt Martha in the south is

geographically different to the surrounding areas, as shown in Figure 6-1. To the north,

Frankston is the southern boundary of a long, uninterrupted lowland area with a long sandy

beach extending to Mentone. To the south, there is a similar low sandy beach extending

westward from Safety Beach to Blairgowrie.

The Mornington area itself is part of a raised rocky area, consisting of sandstone and clay

cliffs, opposed to the siliceous and calcareous sands found to the north and south.

The long sandy beaches to the north and south of Mornington have a different sediment

transport system, with sediment able to travel uninterrupted for kilometres under wave and

current movements. The beaches in the Mornington area however, consist more of a series of

“closed” cell systems where rock and clay headlands and outcrops prevent significant

amounts of sediment from moving more than a few 100m in any direction.

Sediments within the Mothers, Scout and Shire Hall Beach system are more than likely to

have been supplied by the erosion of the coastal cliffs backing the beaches rather than

transport from another part of the coast, with the exception of a small amount of supply from

mornington harbour coastal processes investigation

Documents