7. Assessment of Potential Impacts

Mornington Safe Harbour Environment Effects Statement Volume 1 – Main Report

SINCLAIR KNIGHT MERZ PAGE 109

7. Assessment of Potential Impacts 7.1. Wave Climate

7.1.1. Objective

This section details the investigations carried out to assess the implications of the proposed development on the wave conditions within Mornington Harbour.

Numerical modelling carried out in this investigation was used to assess the degree of protection provided by the proposed harbour and pier wavescreens and whether the resulting conditions are likely to meet safe operating standards.

Outcomes of the wave investigation also provided input to the coastal processes assessment.

The Wave Investigation specialist report produced by Water Technology (2008c) is included in Volume 2 of this EES (Appendix A).

7.1.2. Assessment Criteria

Wave investigations were carried out to assess whether the protection created by the proposed wavescreen would comply with guidelines for maximum wave heights at boat berths, as set by AS 3962-2001 Guidelines for Design of Marinas, Standards Australia 2001.



Table 7-1 Good Wave Climate Criteria for Small Craft in Harbours (Guidelines for Design of Marinas, Standards Australia 2001).

Direction and peak period of design harbour wave

Significant wave height (Hs)

Wave event exceeded once in 50 years

Wave event exceeded once a year

Head seas less than 2s Conditions not likely to occur during this event

Less than 0.3 m wave height

Head seas greater than 2s

Less than 0.6 m wave height Less than 0.3 m wave height

Oblique seas greater than 2s

Less than 0.4 m Less than 0.3 m wave height

Beam seas less than 2s Conditions not likely to occur during this event

Less than 0.3 m wave height

Beam seas greater than 2s

Less than 0.25 m wave height Less than 0.15 m wave height

Note: For criteria for an ‘excellent’ wave climate multiply wave height by 0.75 and for a ‘moderate’ wave climate multiply wave height by 1.25. For vessels of less than 20 m in length, the most severe wave climate should satisfy moderate conditions. For vessels larger than 20 m in length, the wave climate may be more severe. Source: Adapted from Mercer, A.G. Isaacson, M. and Mulcahy, M.W. Design wave climate in small craft harbours. 18thConference on Coastal Engineering. Capetown.1982

Table 7-1 shows the amount of wave attenuation that must be achieved for 1 in 1 year and 1 in 50 year wave conditions for “Good Wave Climate” conditions to be met. By reference to the design wave conditions determined in Section 7.1.4, it can be seen that to create a ‘good wave climate’ within the harbour, the 1 in 1 year wave conditions must be reduced to typically 15% of the incident wave height, whereas the 1 in 50 year wave conditions must be reduced to typically 25% of the incident wave height. Given that there is no significant increase in the wave period or wave height from the 1 in 1 year to 1 in 50 year return period events, the 1 year event will be the limiting factor in determining the wave action within the harbour. This is typical of the fetch limited conditions within Port Phillip.

It is noted that, for vessels of less than 20 m in length (the situation generally anticipated at Mornington), the guidelines specify that the most severe wave climate should satisfy ‘moderate’ conditions. The criteria for moderate conditions are determined by multiplying the wave heights in Table 7-1 by 1.25.

7.1.3. Method

The first part of the assessment involved defining the existing wave conditions at the site of the proposed development. This involved:



Review of existing wind data;

Assessment of design wind climates for the site;

Preparation of a regional wave model of Port Phillip;

Determination of design wave conditions offshore of Mornington;

Detailed Boussinesq wave modelling of the harbour area; and

Identification of existing design wave conditions within the harbour.

Potential changes in wave conditions due to the proposed development were then determined through numerical modelling and changes between the existing and developed conditions compared.

Two separate models were applied in this process, as described below.

7.1.3.1. Measurement of Wave Climate

The MIKE 21 Spectral Waves (SW) model package was used to develop an offshore wave climate for the site. MIKE 21 SW is a wind-wave model that describes the propagation, growth and decay of short period and short crested waves. The model takes into account the effects of refraction and shoaling due to varying depth, local wind generation and energy dissipation due to bottom friction and wave breaking. The model is ideally suited to the assessment of wave generation and propagation processes in coastal areas.

7.1.3.2. Measurement of Wave Penetration

To investigate the effect of wave phenomena in and adjacent to Mornington Harbour, Boussinesq wave modelling was undertaken. The numerical wave model was developed using DHI Software’s MIKE 21 Boussinesq Wave (BW) module. MIKE 21 BW is based on the numerical solution of extended Boussinesq equations and is capable of reproducing the combined effects of most wave phenomena of interest in coastal and harbour engineering.

These include wave propagation over complex terrain, shoaling, refraction, diffraction, reflection and partial reflections from porous structures. Both regular (monochromatic) and irregular (random) waves can be used as input to the model.

MIKE 21 BW creates a computer representation of a physical model of the area of interest. The model terrain is represented as depth values over a regular grid. Porosity layers are introduced to represent porous structures, such as rubble mound breakwaters and groynes, and sponge layers are introduced to eliminate reflections along open boundaries and in other areas where wave energy is to be absorbed.



The MIKE21 Boussinesq Wave Model has been extensively validated against a wide range of analytical solutions, as well as against measurements and physical model test results. Within Port Phillip, Water Technology completed a study of the design of the Martha Cove breakwaters, where combined physical and Boussinesq modelling showed excellent correlation of wave heights within the harbour entrance.

The investigation assessed the likely effectiveness of the harbour wavescreen, combined with both a partial length and full length pier wavescreen.

7.1.4. Existing Conditions

7.1.4.1. Water Level

Design water levels for Mornington Harbour, shown in Table 7-2, were determined for both the 1 and 100 year conditions by consideration of tides, storm surge, climate change and wind conditions.

Since spatial variation in tides within Port Phillip is limited, water levels at Mornington were adopted from available long term information at Williamstown, approximately 40km to the North/North West. The tidal range varied typically from 0.2 m to 0.8 m AHD. Storm surge effects from Bass Strait are not significant at Mornington. The largest surges in Bass Strait are correlated with strong west, southwest and southeast winds. However, North West and northerly winds are most critical in terms of wave action at Mornington. Under these conditions, little or no surge is expected from Bass Strait.

Provision has been made for sea level rise due to climate change. This is based on recent advice of the Intergovernmental Panel on Climate Change (IPCC) and is consistent with the policy requirements of the 2008 Victorian Coastal Strategy. In the IPCCs latest report (IPCC, 2007), 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 0.4 m. The Victorian Coastal Strategy (Victorian Coastal Council, 2008) proposes that for planning purposes, a sea level rise of not less than 0.8 m should be assumed by the end of the century.

Given that the increase in sea level rise is likely to be non-linear, a sea level rise of 0.8m by 2100 is in line with a sea level rise of about 0.35m by 2060, encompassing the 50 year design life of the proposed pier and harbour wavescreens. The 2060 hundred year design water level adopted for this study of 1.7m incorporates this sea level rise, as well as an allowance for the effects of tide and storm surge.



It is noted that the projected sea level rise if the current global rate of around 3.2 mm/year was to continue is close to, but slightly below, the maximum predicted by IPCC3 in the 1990 to 2020 range ie 0.59m. It should also be noted that the National Tidal Centre has measured a sea level rise in the Bass Strait region in the order of 1.7 to 1.8mm/year for the last 15 years, which is much less than the global rate of 3.2mm/year.

Table 7-2 Resulting Design Water Levels Water Level Height (m AHD)

100 Year Design Water Level 1.7

1 Year Design Water Level 1.3

(Source Water Technology, 2008c)

In summary, the design water levels adopted for the proposed wavescreens and associated infrastructure comply with the requirements of the Victorian Coastal Strategy policy of planning for a sea level rise including consideration of storm surge and wave activity.

7.1.4.2. Wind Conditions

Of the locations in Port Phillip where long periods of recorded wind data over open water are available, South Channel Island (23 km west southwest of Mornington) was selected to identify typical wind characteristics for use in the modelling exercises. The data from this site had median and maximum wind speeds for each season of the same order of magnitude and showed summer winds coming predominantly from the south, with winter winds predominantly from the north.

However, analysis indicated that the winds measured at South Channel Island were lower than those given in the Australian Standards AS1170.2-2002 ‘SAA Loading Code, Part 2: Wind Loads’. Due to the shorter recording period of winds at South Channel Island compared to the long term dataset used in the Wind Load Standards, it was decided that the 1 year return winds from Standards Australia would provide a conservative estimate of the wind climate at Mornington.

The magnitudes of these 1 year return period winds for the Melbourne region are shown in Table 7-3.



Table 7-3 AS 1170.2-2002 1 Year ARI Winds Speeds

Wind Direction Standards Australia (m/s)

NNE 20.7

N 22.4

NNW 21.9

NW 21.3

WNW 21.9

W 22.4


As with the 1 year ARI wind speeds, the 50 year and 200 year ARI design wind speeds were derived from the “Basic Wind Speed” for the Melbourne region, provided in AS 1170.2 – 1989 “SAA Loading Code, Part 2: Wind Loads”. The magnitudes of these design winds for the Melbourne region are shown in Table 7-4. The wind speeds are given as 50 and 200 year ARI hourly mean values.

Table 7-4 50 and 200 Year ARI Hourly Design Winds

Wind Direction V50 V200

NNE 23.4 24.4

N 25.3 26.4

NNW 24.7 25.7

NW 24.1 25.1

WNW 24.7 25.7

W 25.3 26.4


7.1.4.3. Regional Wave Conditions

The wave climate offshore of Mornington was calculated using the MIKE 21 Spectral Wave (SW) model.



Operational conditions for the creation of a Safe Harbour at Mornington have been adopted as the 1 year average return interval (ARI) wave. This represents the average maximum significant wave height (HS) to occur in any one year, where the significant wave height is defined as the average of the highest one-third of waves in a record.

The 1 year return wave climate offshore of Mornington was determined, using the 1 year return winds specified in Table 7-3. Winds with the greatest fetch length from the west northwest resulted in the highest waves of over 2.0 m. The predominant wave directions propagating into the harbour are from the west through to north northeast, as detailed in Table 7-5.

Table 7-5 1 Year ARI Wind Wave Heights

Wind Direction Wave Direction (deg)

Water Level

(m AHD)

1 Year Wave

Height HS (m)

Period Ts (s)

NNE 1 1.3 1.59 5.0

N 346 1.3 1.98 5.5

NNW 335 1.3 2.05 5.6

NW 321 1.3 2.05 5.6

WNW 305 1.3 2.08 5.7

W 289 1.3 2.06 5.6 (Source Water Technology, 2008c)

Combining the design wind climates with the MIKE21 SW model, resulted in the 50 year and 200 year ARI design waves presented in Table 7-6.

Table 7-6 50 Year and 200 Year ARI Design Waves

Wind Direction

Wave Direction

(deg)

Water Level

(m AHD)

50 Year Wave 200 Year Wave

Height

Hs (m)

Period

Ts (s)

Height

Hs (m)

Period

Ts (s)

NNE 356 1.3 1.86 5.29 1.96 5.38

1.7 1.87 5.3 1.97 5.39

N 346 1.3 2.30 5.8 2.42 5.89

1.7 2.31 5.81 2.44 5.90

NNW 334 1.3 2.37 5.84 2.49 5.95

1.7 2.39 5.85 2.51 5.97



NW 321 1.3 2.38 5.83 2.50 5.91

1.7 2.40 5.84 2.52 5.92

WNW 305 1.3 2.41 5.85 2.53 5.96

1.7 2.43 5.86 2.55 5.99

W 290 1.3 2.38 5.83 2.51 5.94

1.7 2.41 5.84 2.54 5.97 (Source Water Technology, 2008c)

Table 7-7 shows the amount of wave attenuation that must be achieved for the 1 in 1 and 1 in 50 year wave conditions for ‘good wave climate’ conditions to be met.

Table 7-7 Wave Reduction Required for a Good Climate

Wave Direction

Wave Height Required = 0.30m Wave Height Required=0.60m

1 Year Hs Transmitted Wave Proportion

50 Year Hs Transmitted Wave Proportion

NNE 1.59 19% 1.86 32%

N 1.98 15% 2.30 26%

NNW 2.05 15% 2.37 25%

NW 2.05 15% 2.38 25%

WNW 2.08 14% 2.41 25%

W 2.06 15% 2.38 25% Hs refers to significant wave height, (Source Water Technology, 2008c)

7.1.4.4. Wave Conditions at Mornington Harbour

There is currently minimal protection for vessels moored within Mornington Harbour, especially during north and north-westerly events.

Limited protection is provided by the rubble mound structure underneath the landward end of the existing pier and closely spaced piles along the seaward end of the pier (see photographs in Figure 7-1).



Figure 7-1 Current Pier Protection Works Landward and Seaward End

The rubble mound structure also causes significant damage to the pier decking during periods of strong waves as waves reflect off the structure and force the pier decking up and the close piling is considerably weathered and in need of maintenance.

An analysis of the existing wave climate at Mornington, derived from the Boussinesq wave modelling, and based on the operational wave conditions presented in Table 7-8 show the significant wave heights that are required to be moderated by the proposed development at the locations shown in Figure 7-2.


Figure 7-2 Wave Height Locations



Table 7-8 Existing 1 Year ARI Conditions Wave Heights

Location Wave Direction

N NNW NW WNW W

1 1.83 1.74 1.64 1.58 1.17

2 0.97 0.29 0.18 0.13 0.20

3 1.63 1.40 1.15 0.83 0.67

4 1.13 0.61 0.46 0.24 0.22

5 1.81 1.54 1.68 1.66 0.97

6 1.76 1.61 1.70 1.57 0.74

7 1.62 1.51 1.44 1.06 0.62

8 1.82 1.76 1.52 1.63 1.00

9 1.66 1.65 1.56 1.51 0.92

10 0.89 0.88 0.70 0.52 0.27

11 1.86 1.71 1.65 1.62 0.97

12 1.76 1.68 1.55 1.53 1.05

13 1.76 1.66 1.52 1.52 1.07 (Source Water Technology, 2008c)

The analysis has indicated that wave heights throughout the harbour area are all well in excess of the moderate wave climate conditions (i.e., the most severe wave conditions recommended for small crafts in harbours) for all incident wave conditions north of west. The main factors contributing to this adverse wave climate are summarised below.

North northeast waves Waves caused by strong north north-easterly winds refract in the shallow water to create northerly waves. The comparatively small waves have a high potential for damage within the harbour as the current pier structure offers no protection for moored vessels from waves from the north. Wave heights of up to 1.0 m may occur in the region of the current public pier and small finger jetty, and at the boat ramp. Boats moored on swing moorings in the outer harbour will be subjected to waves up to 1.5 m in height.

North waves As with north north-easterly waves, strong winds from the north refract in the shallow waters near Mornington and approach the harbour from a north north-westerly direction. Although the existing pier does provide some protection from these waves, the larger wave heights result in inshore waves of up to 1.6 m at the outer swing moorings and again over 1.0 m at the public pier and jetty moorings. Wave heights at the boat ramp are up to 0.9 m.



North northwest waves Although slightly higher than the north waves, the greater angle of approach of the north northwest waves ensures boats moored at Mornington do receive slightly greater levels of protection than from waves approaching from the north. Wave heights near to the public jetty are close to 0.5 m, and around 0.9 m offshore from the boat ramp. Wave heights within the harbour area can still be expected to be greater than 2.0 m in parts.

Northwest waves The inner harbour areas are again reasonably well protected from a northwest wave, with heights less than 0.3 m at the landward end and less than 0.6 m at the seaward extent. Wave heights offshore from the public boat ramp are around 0.7 m. Swing moorings in the harbour are at risk with wave heights between 1.5 m and 2.0 m expected to occur in the area.

West northwest waves The highest waves to approach the harbour are from the west northwest, and can be expected to reach upwards of 2.08 m. The design of the base of the pier at the tip of Schnapper Point does however protect a large proportion of the harbour during west north westerly waves. Wave heights off the boat ramp are around 0.5 m, and less than 0.2 m on the public jetty. Outer swing moorings are subjected to waves in excess of 1.6 m.

West waves The harbour berths close to the land are reasonably well protected from westerly wind waves by the existing pier structure. Waves along the current jetty moorings and offshore from the boat ramp are less than 0.3 m. Waves at the swing moorings are between 0.6 m and 1.5 m.

Summary The model results indicate that the current protection provided by the existing Mornington Pier is insufficient to qualify the conditions at Mornington as a “Good Climate”. Marina Safety guidelines, as specified by Standards Australia, recommend wave heights of less than 0.3 m when a wave is head on to a vessel, or less than 0.15 m when the wave is beam on within a marina.

The “Good” wave climate with wave heights less than 0.3 m only occurs in very shallow waters where boats are not be able to moor. Wave heights less than 0.15 m are not present in water of any significant depth.

7.1.5. Potential Impacts

Modelling of effects of the full length pier wavescreen (without a pier extension) and the proposed harbour wavescreen are presented in Table 7-9. The results show the significant decrease in wave heights around the harbour, with most of the mooring area within the harbour



meeting the 0.3 m wave height criterion for “good” wave climate. However, the area behind the outer sections of the pier, and the eastern end of the yacht pens, still retain some exposure to northerly waves.

Table 7-9 1 year ARI Wave Heights with Proposed Development

Location Wave Direction

N NNW NW WNW W

1 0.60 0.48 0.39 0.37 0.21

2 0.40 0.19 0.18 0.14 0.12

3 0.30 0.17 0.15 0.15 0.09

4 0.23 0.17 0.15 0.13 0.08

5 0.18 0.15 0.13 0.13 0.07

6 0.20 0.15 0.13 0.13 0.07

7 0.21 0.16 0.13 0.14 0.07

8 0.34 0.23 0.20 0.25 0.15

9 0.34 0.23 0.18 0.23 0.12

10 0.28 0.21 0.16 0.17 0.08

11 1.79 1.60 1.62 1.56 0.84

12 1.73 1.40 1.08 1.02 0.40

13 1.49 1.00 0.67 0.62 0.25


7.1.5.1. North waves

The full length pier wavescreen works reduce the amount of wave energy penetrating the western entrance to the harbour and, combined with the harbour wavescreen, result in a wave height of less than 0.3 m throughout most of the harbour, and at the boat ramp (Water Technology, 2008c). Berths along the outer part of the pier remain exposed to waves from this direction. Reflected wave heights above 2.0 m in the area offshore of the wavescreen have the potential to affect navigation along the fairway to the public berths. Navigation around the eastern side of the harbour may still be an issue.



The reductions in wave heights are shown in Figure 7-3. This indicates a wave height reduction of greater than 1.0 m for much of the floating berth area. Wave heights around the western end of the wavescreen have also been reduced by the full length pier wavescreen works and decrease the standing wave effect in this area. Wave heights at the swing moorings are still greater than 1.4m, but have been reduced by up to 1.0m at the shoreward end.

Figure 7-3 has been included in this volume of the EES as waves from the north represents the worst case condition. The complete Water Technology specialist report is included in Volume 2 of this EES (Appendix A) and contains figures and an analysis of all potentially important wave conditions.


Figure 7-3 Operational Wave Conditions- North

7.1.5.2. North northwest waves

The full length pier wavescreen, combined with the harbour wavescreen (but without the pier extension), provides a much greater reduction in wave heights within the harbour area from north northwest waves. A wave height of less than 0.3 m is present for almost all public and yacht club berths, tending higher along the outer yacht club berths and along the pier. The



seaward end of the pier would remain unsuitable for mooring in rough weather as wave heights can be in excess of 1.0 m. Reflections of the harbour wavescreen into the fairway may affect navigation to the public berths.


Figure 7-4 Change in Wave Height Versus Existing –North

7.1.5.3. Northwest waves

The harbour is well protected from the north westerly waves with a large proportion of the proposed berths experiencing wave heights of less than 0.3 m. Wave heights along the pier are too high to be considered a “good climate” under AS 3962-2001 guidelines, although they are much reduced from the existing conditions.

Figure 7-4 shows a reduction in wave height of over 0.5 m for much of the length of the pier. Wave heights around the proposed berths have been reduced by between 1.0 and 1.5 m.



7.1.5.4. West Northwest waves

The proposed development provides good protection from waves from the west northwest. Wave heights within the proposed berth areas are well below 0.3 m in the yacht club harbour area and along the new public berths, whilst berths along the existing pier are below 0.3 m for approximately 70 m of the pier. Wave reflection off the wavescreens is again predicted to result in a standing wave pattern to form, and navigation into the western section of the harbour may be difficult with waves of over 2.0 m forming near the entrance. The reduction in wave height from the existing conditions illustrates the sheltering effect of wavescreens during the high WNW wave events.

7.1.5.5. West waves

Waves generated from westerly winds refract into the coast at a more north westerly direction and are successfully blocked by the proposed pier protection works. The harbour wavescreen prevents wave diffraction around the seaward end of the pier from affecting the proposed berths. Wave heights within the harbour are in the range prescribed by the Australian Standards for both beam- and head-on waves. Navigation into the harbour is less likely to be difficult under westerly waves either as wave heights are around 1.4 – 1.6 m offshore from the wavescreen. There is, however, a large reflected wave pattern predicted to form offshore of the pier protection works. Wave heights at the swing moorings are between 0.2m inshore to 0.85m at the outer swing moorings.

The area adjacent to the protected pier experiences a drop in wave height of over 0.75 m for almost the entire length, whilst wave height reductions greater than 1.0 m are apparent in the area sheltered by the harbour wavescreen.

These results are summarised in Table 7-10.

Table 7-10 1 Year ARI Wave Heights – Good Climate

Location 1 2 3 4 5 6 7 8 9 10

Wave direction

N Wave height

Existing 1.83 0.97 1.63 1.13 1.81 1.76 1.62 1.82 1.66 0.89

Base Case 1.36 0.61 0.75 0.54 0.33 0.37 0.37 0.44 0.40 0.59

Option 2 0.60 0.40 0.30 0.23 0.18 0.20 0.21 0.34 0.34 0.28

NNW Wave Existing 1.74 0.29 1.40 0.61 1.54 1.61 1.51 1.76 1.65 0.88



height Base Case 1.34 0.31 0.52 0.40 0.25 0.25 0.30 0.35 0.37 0.49

Option 2 0.58 0.29 0.22 0.20 0.17 0.17 0.19 0.29 0.27 0.25

NW Wave height

Existing 1.64 0.96 1.15 0.46 1.68 1.70 1.44 1.52 1.56 0.70

Base Case 1.20 0.39 0.45 0.34 0.25 0.23 0.28 0.37 0.37 0.41

Option 2 0.60 0.23 0.25 0.21 0.19 0.19 0.22 0.27 0.23 0.25

WNW Wave height

Existing 1.58 0.60 0.83 0.24 1.66 1.57 1.06 1.63 1.51 0.52

Base Case 0.92 0.27 0.36 0.25 0.19 0.20 0.22 0.31 0.24 0.35

Option 2 0.37 0.14 0.15 0.13 0.13 0.13 0.14 0.23 0.19 0.17

W Wave height

Existing 1.17 0.20 0.67 0.22 0.97 0.74 0.62 1.00 0.92 0.27

Base Case 0.57 0.12 0.21 0.12 0.11 0.12 0.13 0.18 0.14 0.19

Option 2 0.21 0.12 0.09 0.08 0.07 0.07 0.07 0.15 0.12 0.08

Key

Excellent Wave Climate Good Wave Climate Moderate Wave Climate


The Base Case presented in Table 7-10 includes the partial length pier wavescreen and harbour wavescreen. Option 2 refers to the proposed development being the harbour wavescreen and the wavescreen along the full length of the pier.

As shown in Table 7-10, under existing conditions the wave heights throughout the harbour area are all well in excess of the moderate wave climate conditions (i.e., the most severe wave conditions allowable) for all incident wave conditions north of west.

Table 7-10 also shows that Option 2, being the proposed harbour wavescreen and wavescreen along the full length of the pier, provides greater protection than partial length pier wavescreen and the harbour wavescreen to the western part (Points 1 - 4 and 10) of the harbour. Excellent to good wave conditions are achieved throughout most of the harbour area under all incident wave conditions, except at the seaward end of the existing pier. Wave heights meet moderate wave conditions at the eastern end of the pen for waves from the north.



When considering these results, it should be noted that the modelling did not include the effects of:

Additional wave attenuation that will be provided by floating berths within the harbour area; and

Timber waling that has been proposed for mooring boats along the inside of the new pier connecting the harbour wavescreen to the land.

The former would be expected to provide additional reductions in wave height within the moorings. As a result, it is expected that the proposed development would result in good wave climate throughout the moorings under virtually all incident wave conditions.

The latter would be expected to reduce wave action at the western end of the moorings, but wave reflections may increase wave action in the area between the harbour pier and the yacht club pier.

7.1.5.6. Wavescreen Reflection

In addition to the above, it is noted that reflections from the outer face of the wavescreens can set up a partial standing wave pattern in the area immediately offshore from them. This has the potential to have an adverse effect on the navigation fairways, and under storm conditions, to cause overtopping of the wavescreens. The effects would be most noticeable under northwest waves for the pier wavescreen, and under north waves for the main harbour wavescreen.

In this respect, it is noted that north and northwest waves will occur mostly in winter, when there will be reduced usage of Port Phillip for recreational boating. The effects of reflections from the wavescreens would be expected to be reduced significantly during summer, when the waves come predominantly from the southwest.

North Waves

Northerly waves generate the highest reflections from the harbour wavescreen. For northerly wave conditions that could be expected to occur, on average, once per year (Volume 2, Appendix A, Water Technology 2008c, Figure 5-36). The partial standing wave pattern in front of the harbour wavescreen can be seen as alternating lines of high and low wave conditions parallel to the wavescreen. Under these conditions, the reflections would be expected to have little effect on the navigation fairway leading to the main boat harbour.

Reflections from the pier wavescreen would be deflected to the southwest and would lose most of their energy breaking at the coast. Under these conditions the reflections would be expected to have little effect on the navigation fairway leading to the area behind the pier wavescreen.



Northwest Waves

Northwest waves generate the highest reflections from the pier wavescreen. For northwest wave conditions that could be expected to occur, on average, once per year (Volume 2, Appendix A, Water Technology 2008c, Figure 5-37). The reflections have their greatest effect to the northwest of the wavescreen. The navigation fairway leading to the area behind the pier wavescreen is relatively unaffected.

Reflections from the main harbour wavescreen would be deflected to the northeast and could be expected to have some effect on wave conditions in the navigation fairway leading to the main harbour area. The reflection would however, be less severe than those from the pier wavescreen and if necessary, could readily avoided by approaching the harbour further from the east, or by seeking temporary refuge in the area behind the pier wavescreen.

Under all wave conditions, the navigation fairway leading to the area behind the pier wavescreen remains relatively unaffected by wave reflections.

Summary

In summary, from an operational perspective, the majority of emergency refuge requirements in the area are likely to be caused by strong southwest winds accompanying the passage of a “cold front”. These conditions occur throughout the year, and are associated with the normal west to east movement of high and low pressure systems across southern Australia. Cold fronts can result in ideal boating conditions deteriorating to potentially quite dangerous conditions in a matter of minutes. The harbour is well protected from the southwest waves and little reflection will occur from the harbour wavescreen under these conditions.

By comparison the strong north and northwest winds that will cause most of the reflections from the wavescreens occur mostly in winter, and are associated with larger pressure systems that are likely to build up more gradually. Under these conditions, boats are less likely to leave the harbour, and, if they do, are more likely to be prepared for adverse conditions, and are less likely to need to find emergency refuge at Mornington. Nevertheless, should a boat seek refuge under adverse north or northwest wave conditions, it can be seen that:

The navigation fairway leading to the public berthing area behind the pier wavescreen remains relatively unaffected by wave reflections;

The navigation fairway leading to the main boat harbour would be relatively unaffected by wave reflections from northerly waves; and

Although the navigation fairway leading to the main boat harbour can be affected by wave reflections from northwest waves, these reflections can be avoided by approaching the



harbour further from the east, or by seeking temporary refuge in the area behind the pier wavescreen.

7.1.6. Management Measures

To improve the wave protection in the public berthing areas adjacent to Mornington Pier the proposed development was modified to include an extension to the outer end of the pier. The aim of the pier extension is to reduce the penetration of waves diffracting around the end of the pier and into the area adjacent to the pier.

A number of orientations were analysed for the pier extension. This included east west extensions to the end of the pier, and a perpendicular extension aligned roughly northwest to southeast. It was found that significantly improved protection to the pier moorings could be achieved by a perpendicular extension of approximately 20m in length on the end of the pier wavescreen. As shown in Table 7-11, the 20m perpendicular extension provides improved wave protection, in particular from the strongest waves of the northwest through west.

Table 7-11 1 Year Wave Climate Along Pier, Perpendicular Wavescreen Extension

Point NNE N NNW NW WNW W

Incident wave 1.54 1.93 1.92 2.00 2.03 1.99

1 0.18 0.21 0.21 0.11 0.11 0.11

2 0.19 0.22 0.22 0.12 0.12 0.12

3 0.21 0.24 0.25 0.13 0.13 0.13

4 0.23 0.26 0.27 0.14 0.14 0.14

5 0.25 0.29 0.29 0.15 0.15 0.15

6 0.27 0.31 0.32 0.17 0.17 0.17

7 0.29 0.35 0.37 0.20 0.20 0.20

8 0.34 0.40 0.43 0.22 0.23 0.22

9 0.44 0.53 0.54 0.28 0.28 0.28




The proportion of the month the maximum daily wave exceeded the good and excellent conditions are shown below in Table 7-12. This shows that mooring conditions along the inside of the pier are greatly improved with the perpendicular extension. Mooring conditions along the pier are considered excellent throughout the summer, and only less than “good” one day per month in winter.

Table 7-12 Number of Days Conditions Exceeded (per month on average) – Perpendicular Wavescreen Extension

Summer Winter

Moderate (<0.38m) <1 <1

Good (<0.3m) <1 1

Excellent (<0.23m) <1 2 (Source Water Technology, 2008c)

In summary, a perpendicular extension of approximately 20m in length on the end of the pier wavescreen, as shown in Figure 7-5 would provide greater protection to the outer berths from waves from the north and north-northeast which cause the highest waves along the inside of the pier. The precise orientation and length of the extension can be optimised at the detailed design stage following consideration of the impacts of wavescreen and the benefits for permanent or day moorings along the pier. As discussed in Sections 7.2 and 7.3, the pier extension will not cause any additional effects to hydrodynamics or coastal processes.




Figure 7-5 Perpendicular Pier Extension

7.1.7. Conclusion

Under current conditions, vessels moored within Mornington Harbour are relatively unprotected, and especially at risk to waves generated by winds from the north through NNW. One year return period waves offshore of the harbour are in excess of 2.0 m and can result in waves within the harbour area of up to 2.2 m. Wave heights throughout the harbour area are well in excess of the moderate wave climate conditions (i.e. the most severe wave conditions recommended by the guidelines) for all incident conditions north of west.

The initially proposed development, comprising a full length pier wavescreen and harbour wavescreen (but without a pier extension), would result in a good wave climate throughout the yacht club moorings under virtually all incident wave conditions. The EES investigations have indicated that the public berthing areas adjacent to Mornington Pier would also receive significantly greater protection. However, the area behind the outer section of the pier would retain some exposure to northerly waves. The proposal was consequently modified to incorporate a pier extension at the end of Mornington Pier to further enhance conditions in this



area, increasing usability and safety of the pier berths without causing any additional effects to hydrodynamics or coastal processes.

Although the areas offshore from the wavescreens can be affected by wave reflections from north to northwest waves, safe emergency refuge will still be available within the harbour.

7.2. Hydrodynamic Assessment

7.2.1. Objective

The objective of this assessment was to address coastal hydrodynamic and mixing issues related to the proposed Safe Harbour development. The assessment did this by investigating existing conditions and developed conditions created as a result of the proposed development. The complete Hydrodynamics Investigation specialist report produced by Water Technology (2008b) is included in Volume 2 of this EES (Appendix B).

7.2.2. Assessment Criteria

This study assessed the potential for the proposed development to result in reduced flushing of the harbour, leading to increased residence times and possible consequential effects on water quality in Mornington Harbour.

The State Environment Protection Policy (SEPP), Water of Victoria Schedule F6: Waters of Port Phillip specifies a range of beneficial uses to be protected, including:

Maintenance of aquatic ecosystems and associated wildlife;

Water- based recreation;

Commercial and recreational use of edible fish and crustaceans; and

Navigation and shipping.

Water quality conditions in the harbour depend on the turnover time and the quality of adjacent source waters (i.e. those of adjacent sections of Port Phillip). An average ‘residence time’ of 30 days has been set as the target for a number of enclosed and semi-enclosed water bodies adjacent to Port Phillip. These include Patterson Lakes, Martha Cove and the proposed Wyndham Harbour.

7.2.3. Method

The following sets out the methodology of the works undertaken as part of the hydrodynamic part of the investigations relevant to the assessment of the impact of the proposals on coastal processes.



Existing Conditions were established through:

Review and analysis of relevant weather and tide data;

Assessment of seasonal and year round wind climates for the area, which were used in order to establish the most relevant design conditions for an impact assessment of the project;

Preparation of a detailed hydrodynamic model of the local area, nested into the wider area Port Phillip model; and

Review of the existing tidal regime, hydraulics and water circulation patterns using the nested hydraulic model.

7.2.3.1. Assessment of the Impacts of the Proposal on Hydrodynamics Processes

The assessment of potential impacts of the proposed development on hydrodynamic processes consisted of:

Preparation of a detailed hydraulic model of the developed conditions including addition of the wavescreen structures and changes in the roughness parameters to represent the floating moorings;

Using the ‘developed conditions’ model within the larger nested model setup, to simulate the tidal regime, hydraulics and water circulation patterns;

Reviewing the tidal regime, hydraulics and water circulation patterns with the proposed development;

Assessment of the impact on tidal regime and current conditions in the area caused by the development by comparing the results of the developed simulations with those found during the existing conditions investigation; and

Analysis of water quality, circulation and flushing within Mornington Harbour and changes from the existing conditions by comparing the results of the developed simulations with those found during the existing conditions investigation.

Numerical modelling was undertaken to assist in the assessment of tide and circulation effects using the MIKE21 Flow modelling system. MIKE21 Flow is a comprehensive modelling package for simulating two-dimensional free-surface flows. It is applicable for modelling hydrodynamic and related phenomena in lakes, bays, estuaries and coastal areas where the effects of stratification can be neglected.

The model of the eastern part of the Bay was established to assist in identifying potential impacts of the proposed development. It was developed with a grid size of 135 m, with nested finer grids reducing successively from 135 m to 45 m, 15 m and 5 m. The fine scale 5 m grid



area made it possible to obtain high resolution of the flows in the immediate vicinity of the proposed harbour development.

The nested model was also used with the MIKE 21 Advection Dispersion (AD) module, an add-on to the MIKE 21 Flow Model. The AD module simulates the spreading of dissolved substances such as salinity, temperature, and contaminated compounds when subjected to advection – the movement of fluid particles due to the flow processes – and dispersion – scattering of fluid particles due to non-resolved flow processes.

7.2.4. Existing Conditions

Water level, tides, surge and climate change factors have been previously described in Section 7.14.

7.2.4.1. Water Circulation

Currents in the eastern part of Port Phillip are generally weak and driven by the effects of tide and wind. Therefore, the impact of the proposed development on water circulation was assessed using representative summer and winter wind conditions, with corresponding tidal variations in Bass Strait.

7.2.4.2. Wind Conditions

As explained in Section 7.1.4, one year return period winds from Standards Australia were adopted as providing a conservative estimate of the wind climate at Mornington.

7.2.4.3. Hydrodynamic Conditions

Existing tide and circulation conditions during the summer

Typical southerly wind conditions encountered during summer result in a generally northeast flow offshore of the proposed development site. As the circulation in the northern part of the Bay is primarily wind driven, the observed current speeds depend on the wind conditions affecting the Bay. The circulation patterns illustrated in Figure 7-6 show peak current speeds offshore of Mornington of approximately 0.20-0.30 m/s during typical summer conditions, peaking to over 0.4 m/s as the water passes Schnapper Point. Within the harbour area currents are kept low, with a maximum speed of less than 0.05 m/s and 0.10 m/s occurring closer to the shoreline at Mothers Beach and Shire Hall Beach respectively.



(Water Technology, 2008b)

Figure 7-6 Summer Circulation under Existing Conditions

Existing tide and circulation conditions during the winter

Typical winter northerly wind conditions result in a generally southwest flow along the coastline around the proposed development site. The circulation patterns illustrated in Figure 7-7 show peak current speeds offshore of Mornington of approximately 0.20-0.30 m/s during typical winter conditions, peaking over 0.4 m/s as the water flows around the headland at Schnapper Point. Again current speeds within the harbour area are relatively low as the water moves offshore at Red Bluff to pass Schnapper Point and continue heading south. Current speeds within the harbour are generally less than 0.15 m/s. An eddy (the swirling of fluid and the reverse current created when the fluid flows past an obstacle) can form to the southwest of Schnapper Point in the sheltered waters of Fisherman Beach.




Figure 7-7 Winter Circulation Under Existing Conditions

Existing water quality conditions

The harbour provides excellent tidal flushing and residence times well below 30 days. Accordingly, appropriate water quality conditions in the harbour will depend largely on the quality of the source water (i.e. the adjacent Bay waters). The nearby Port Phillip water typically exhibits good water quality characteristics. However, following significant rainfall, poor water quality conditions can exist resulting from the discharge of pollutants in stormwater runoff. These may come from stormwater outlets and in particular Tanti Creek. Tanti Creek discharges to the Bay approximately 600 m northeast of the Mornington Harbour, on the north side of Red Bluff cliffs.

Typical circulation patterns in this region of the Bay, particularly in winter, are from northeast to southwest, resulting in the transport of waters and pollutants discharging from Tanti Creek past the harbour. The concentrations of the pollutants in stormwater discharges can be high, but are typically transient, reducing to background levels within a few days of the event. Additionally, there will be significant dilution and mixing of creek water by the time it reaches the harbour area.



Residence times Existing residence times during summer conditions

Typical summer conditions are dominated by southerly winds and a corresponding northeast flow regime in the vicinity of the development. An initial concentration of 100 units was distributed evenly throughout the area of the proposed Safe Harbour and the modelling carried out to simulate tidal and wind driven exchange over the 1 month period.

The results from the summer simulations indicate that the existing conditions have a residence time typically less than 1 day. This currently results in excellent exchange of harbour waters with the Bay and the maintenance of good water quality conditions inside the harbour. The results indicate that pollutant concentrations in the harbour area drops to less than 50% of the original concentration after 3 - 9 hours and less than 25% of the original concentration after approximately 6 - 9 hours.

Winter conditions

Typical winter conditions are dominated by northerly winds and a corresponding southerly flow regime in the vicinity of the development. At the beginning of the simulation undertaken for this study, a period of southerly winds drove the currents and tracer to the north before the northerly winds began to dominate and the tracer began to flow southward past Schnapper Point. The eddying systems offshore and within the harbour discussed are evident along the shoreline in the harbour after 12 hours, and also occur in the half-moon shaped Fishermans Beach to the south.

Tracer flushing in the harbour was again very fast, with less than 50% remaining in the harbour after 12 hours, dropping to below 25% after 1 day before rising to around 35% after 2 days as the northerly winds bring the tracer back into the harbour area on its way southward around Schnapper Point.

7.2.5. Potential Impacts

7.2.5.1. Tides and Circulation

Potential Impacts during Summer Conditions

Figure 7-8 illustrates the impact of the proposed development on current speeds for typical summer conditions. Current speeds of less than 0.05 m/s are predicted within the protected harbour. This would result in a speed reduction of between 0.05 and 0.1 m/s in the area around the proposed harbour wavescreen. Offshore of Schnapper Point, speeds exceed 0.4 m/s and are somewhat greater than those occurring under existing conditions due to the reduced water flow underneath the existing pier following construction of the protection works. Currents in the shallow areas are similar to those observed in the existing case.




Figure 7-8 Summer Circulation Under Developed Conditions


Figure 7-9 Change in Summer Circulation Post Development



As indicated in Figure 7-9 the proposed works result in very little change in current speeds within the harbour during typical summer wind conditions. Changes offshore from the harbour are due to the Parks Victoria pier wavescreen deflecting flows further offshore, and deflecting the eddies that shed from Schnapper Point further offshore.

Potential Impacts during Winter Conditions

The current speeds within the harbour are again very low, mostly less than 0.1 m/s, especially in shallower waters and around the new wavescreens. Currents offshore of Schnapper Point exceed 0.4 m/s, and a large eddy forms on the south side of Schnapper Point as strong currents flow south the offshore zone

There is some decrease in current speed within the harbour; particularly around the wavescreen compared to existing winter conditions (see Figure 7-10). A slight increase in current speed can be seen to the west of Schnapper Point where currents travelling south are less than 0.15 m/s quicker, most likely caused by a change to the eddy formation southwest of Schnapper Point. However with such small changes to the already weak currents, it is not expected to cause a large impact on the local coastal processes.


Figure 7-10 Winter Circulation Under Developed Conditions




Figure 7-11 Change in Winter Circulation Post Development

Overall impacts

Numerical modelling results indicate that the impact of the proposed development on circulation patterns is localised to an area within about 200m of the harbour wavescreen, for both typical summer and winter conditions. During summer currents speeds can be expected to rise slightly on the northwest side of Schnapper Point as the northerly currents must pass around the pier rather than going east through the pier piles. In winter there is a slight increase in current speed offshore from Schnapper Point and a general reduction in current speed in the mooring areas within the harbour. The proposal would result in negligible changes to the regional current regime and would have no impact on tidal variations in Port Phillip. The low currents speeds which are present at Mothers, Scout and Shire Hall Beaches would remain relatively unchanged following development.

7.2.5.2. Water Quality

Residence times

Potential Impacts during summer conditions

The typical summer conditions are dominated by southerly winds and a corresponding northeast flow regime in the vicinity of the development. The results from the summer simulations indicate that the existing conditions have a residence time typically less than one day. This



results in excellent exchange of harbour waters with the Bay and maintenance of good water quality conditions inside the harbour.

Modelling results shown in Figure 7-12 illustrate the changed flushing capacity of the harbour following addition of the full length pier wavescreen and the harbour wavescreen. As shown, the harbour and surrounding waters are predicted to be flushed free within 72 hours. These are considered to be very good flushing conditions. The reduced residence time for the proposed development is due to a combination of the more permeable wavescreen layout and a more accurate and advanced modelling applied to this study compared with earlier investigations.


SINCLAIR KNIGHT MERZ

PAGE 140

(Water Technology 2008b)

Figure 7-12 Summer Flushing Developed Conditions

Potential Impacts during winter conditions

Typical winter conditions are dominated by northerly winds and a corresponding southerly flow regime. The model indicates that flushing in the harbour is again very fast, with total flushing of the harbour occurring within three days.

Figure 7-13 illustrates the changed flushing capacity of the harbour following addition of the full length pier wavescreen and the harbour wavescreen. The model results indicate that total residence time in the harbour is still less than four days, resulting in the maintenance of very good water quality.

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7. Assessment of Potential Impacts