Mornington Harbour Wave Investigation
Figure 5-36
Wave Reflections for 1 year ARI Northerly Waves
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, the Boussinesq
model results have been reproduced in Figure 5-37. The partial standing wave pattern in front
of the pier wavescreen can be seen as alternating lines of high and low wave conditions
parallel to the wavescreen. It can be seen that the reflections have their greatest effect to the
northwest of the wavescreen and that 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 reflections would, however, be less severe than those from the pier
wavescreen and, if necessary, could readily be 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 shown in Figures 5-25 to 5-34, the navigation fairway leading to
the area behind the pier wavescreen remains relatively unaffected by wave reflections.
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Mornington Harbour Wave Investigation
Figure 5-37
Wave Reflections for 1 year ARI Northwesterly Waves
The average number of days where a standing wave pattern forms offshore from the harbour
wavescreen, producing a maximum wave height greater than 1.0m, 1.5m, and 2.0m can be
seen below in Table 5-5. The Table shows that the standing wave pattern producing waves of
some height occurs more often in winter than summer, and on average, will produce a wave
crest above 2.0m approximately twice per month during winter, based on the available record
of data.
Table 5-5
Number of Days Standing Wave above 1, 1.5 and 2m at Harbour
Wavescreen
(per month on average)
Height Standing
Wave (m)
Summer
Winter
Year-round
1.0 < H < 1.5m
3
9
6
1.5 < H < 2.0m
2
7
4
H > 2.0m
<1
2
1
The average number of days where a standing wave pattern forms offshore of the pier
protection wavescreen is shown in Table 5-6. The Table indicates that there will be a
significant reflected wave height offshore of the pier wavescreen up to 3 days per month
during the winter months. As expected, this is higher than at the harbour wavescreen, and is
due to the higher west through north waves occurring during winter which the pier
wavescreen is more exposed to. Year round conditions offshore of the pier protection
wavescreen are similar to the harbour wavescreen.
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Mornington Harbour Wave Investigation
Table 5-6
Number of Days Standing Wave above 1, 1.5 and 2m at Pier Protection
Wavescreen
(per month on average)
Height Standing
Wave (m)
Summer
Winter
Year-round
1.0 < H < 1.5m
3
8
5
1.5 < H < 2.0m
2
7
4
H > 2.0m
<1
3
2
Summary:
From an operational perspective, it is considered that 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.
• 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.
5.6 Conclusions
From the above, it is concluded that the full length Parks Victoria wavescreen in combination
with the MBHL wavescreen will provide the most significant decrease in wave heights at
Mornington. A pier extension will be required to prevent the most severe wave conditions
allowable being exceeded in the public berthing areas behind the Parks Victoria wavescreen.
The proposed swing moorings remain exposed to higher wave conditions from the northnortheast through west-northwest and would provide only slightly improved conditions from
the existing swing moorings under these conditions.
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Mornington Harbour Wave Investigation
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.
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Mornington Harbour Wave Investigation
6
ADDITIONAL PIER PROTECTION
As described in Section 5.6, additional works will be required to provide functional and safe
moorings along the inside of the public pier under all incident wave conditions. The
configuration investigated above provides adequate protection behind the harbour wavescreen
and along the existing pier from the west through north-northwest waves, but still allows
relatively high wave conditions to propagate along the inner face of the pier during north
through north-northeast wave conditions.
To assess the effects of wave diffraction around the end of the pier wavescreen, and around an
extension of the pier wavescreen, an analytical investigation was undertaken as described
below.
6.1 Pier protection wavescreen
A wave diffraction investigation was carried out for Parks Victoria to assess the wave
protection afforded by the pier protection wavescreen alone. The investigation used standard
diffraction solutions as provided by the U.S. Army Core of Engineers Shore Protection
Manual (1984) to estimate the diffracted wave height along the shoreward side of the pier. 1
year ARI wave heights along the pier for a range of incident wave directions (at locations
indicated in Figure 6-1) are shown in Table 6-1.
Figure 6-1
Wave height locations along the pier
The moorings along the inside of the pier will be parallel to the pier. As such, the diffracted
waves will be head on to the moored boats. As described in Table 4-3, a “good” wave climate
for “head on” wave conditions is achieved at locations where the significant wave height does
not exceed 0.3m, on average, once per year. “Excellent” wave conditions are achieved when
the 1 year wave height limit is reduced by 25% to 0.23m, and “moderate” conditions are
achieved when it is increased by 25% to 0.375m. Further, moderate wave conditions are the
most severe wave conditions allowable for mooring vessels of less than 20m in length.
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Mornington Harbour Wave Investigation
Table 6-1
Point
Incident
wave
1
2
3
4
5
6
7
8
9
1 Year ARI Wave Climate Along Pier, Full length pier protection
NNE
N
NNW
NW
WNW
W
1.54
0.25
0.27
0.30
0.34
0.37
0.41
0.47
0.57
0.84
1.93
0.22
0.24
0.26
0.28
0.32
0.34
0.40
0.50
0.71
1.92
0.22
0.24
0.26
0.28
0.31
0.35
0.40
0.51
0.74
2.00
0.20
0.22
0.23
0.25
0.28
0.30
0.35
0.46
0.67
2.03
0.19
0.20
0.22
0.23
0.25
0.28
0.34
0.42
0.60
1.99
0.17
0.18
0.20
0.21
0.23
0.25
0.30
0.38
0.55
From Table 6-1, it is clear that a good wave climate for moorings is only achieved at the
innermost part of the pier, and that the maximum allowable conditions are exceeded for most
of the outer section of the pier. The proportions of time the daily maximum wave heights are
expected to exceed the 1 year ARI limits at the outer end of the pier are given in Table 6-2
below. These results show that the 1 year wave limit for good wave conditions would be
expected to be exceeded for almost 50% of the time during winter months and approximately
13% of the time in summer at the outer end of the pier.
Table 6-2
Number of Days Conditions Exceeded (per month on average)
Summer
Winter
Moderate (<0.38m)
2
4
Good (<0.3m)
4
14
Excellent (<0.23m)
7
18
This indicates that the initial configuration does not provide functional and safe unsupervised
mooring conditions along the inside of the public pier. As such, it was considered that an
extension of the wavescreen from the end of the existing pier would be required to provide
better berthing conditions for a greater period of time.
6.2 Pier Protection Wavescreen Extension
6.2.1 Orientation
A number of orientations were analysed for the wavescreen extension. This included eastwest extensions to the end of the pier, which it was initially thought would provide the
greatest protection, and a perpendicular extension aligned roughly northwest to southeast.
Through the investigations, it was found that the latter provided the most effective protection
to the moorings along the inside of the pier.
6.2.2 Length
Table 6-1 above indicates that there is no where along the pier with a wave climate considered
“excellent”. Points 1 to 4 meet the “good” criteria under all but the north northeast waves;
however Points 7, 8 and 9 exceed this good criterion with waves from all directions.
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Mornington Harbour Wave Investigation
Wave heights at Point 8 are roughly 70% of those calculated at Point 9, whilst wave heights at
Point 7 are around 55% of those at Point 9. This indicates that the wave height reduces
quickly as distance from the end of the breakwater increases. Therefore, a small increase in
pier protection wavescreen should be sufficient to provide good to excellent conditions along
the pier.
6.2.3 East-west pier protection (20m)
A pier extension oriented roughly east-west with a length of 20m (approximately two-thirds
of a 1 year ARI wave length) was first analysed for wave protection along the pier. The
diffraction investigation described above was repeated to include the effects of the 20m
wavescreen extension. The results are presented in Table 6-3 below.
Table 6-3
Point
Incident
wave
1
2
3
4
5
6
7
8
9
1 Year ARI Wave Climate Along Pier, Wavescreen Extension = 20m
NNE
N
NNW
NW
WNW
W
1.54
0.22
0.23
0.25
0.27
0.30
0.32
0.35
0.39
0.51
1.93
0.22
0.24
0.26
0.28
0.31
0.34
0.37
0.45
0.56
1.92
0.19
0.20
0.21
0.23
0.25
0.26
0.28
0.37
0.48
2.00
0.17
0.18
0.20
0.22
0.24
0.26
0.29
0.34
0.43
2.03
0.16
0.18
0.20
0.22
0.24
0.25
0.28
0.32
0.41
1.99
0.16
0.18
0.19
0.21
0.22
0.24
0.26
0.32
0.40
The proportions of the month the maximum daily wave exceeded the good and excellent
conditions have been calculated and are shown below in Table 6-4. These show a general
improvement in mooring conditions, but there is still a significant proportion of the time
where the maximum allowable (moderate) mooring conditions are exceeded.
Table 6-4 Number of days conditions exceeded (per month on average) – east-west
wavescreen extension
Summer
Winter
Moderate (<0.38m)
1
4
Good (<0.3m)
1
4
Excellent (<0.23m)
3
12
6.2.4 East-west pier protection (30m)
An east-west extension of 30m (approximately the length of a 1 year ARI wave) was then
analysed to see what improvement could be achieved with a 50% increase in the length of
extension. The results presented in Table 6-5 show that excellent wave conditions are now
present for Points 1 through 4 in all conditions except for the north northeast wave, and good
conditions are met for Points 1 through 7, again with the exception of the north northeast
wave.
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Mornington Harbour Wave Investigation
Table 6-5
Point
Incident
wave
1
2
3
4
5
6
7
8
9
1 Year ARI Wave Climate Along Pier, Wavescreen Extension = 30m
NNE
N
NNW
NW
WNW
W
1.54
0.21
0.21
0.23
0.25
0.28
0.30
0.33
0.36
0.42
1.93
0.22
0.23
0.25
0.27
0.30
0.32
0.34
0.39
0.49
1.92
0.19
0.19
0.20
0.22
0.23
0.25
0.26
0.31
0.41
2.00
0.16
0.17
0.19
0.21
0.23
0.25
0.27
0.31
0.37
2.03
0.16
0.17
0.19
0.21
0.23
0.24
0.26
0.29
0.36
1.99
0.16
0.17
0.18
0.20
0.21
0.23
0.24
0.29
0.36
The proportion of the month the maximum daily wave exceeded the moderate, good and
excellent conditions have been calculated and are shown below in Table 6-6. This table
indicates that the 30m east-west extension provides significantly improved mooring
conditions along the inside of the pier, with good and moderate mooring conditions only
being exceeded approximately one day per month in winter.
Table 6-6
Number of days conditions exceeded (per month on average) – 30m
wavescreen extension
Summer
Winter
Moderate (<0.38m)
<1
1
Good (<0.3m)
<1
1
Excellent (<0.23m)
2
8
6.2.5 Perpendicular pier protection
A perpendicular extension of length 20m (approximately two-thirds of a 1 year ARI wave
length) was then analysed for wave protection along the pier. The results are presented in
Table 6-7 below. These show that the 20m perpendicular extension provides improved wave
protection, in particular from the strongest waves of the northwest through west. Improved
conditions at the outer berths could be achieved by extending the return a short distance,
however the higher waves from the north through north-northeast are less likely to occur, as
shown in Table 6-8.
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Mornington Harbour Wave Investigation
Table 6-7
Point
Incident
wave
1
2
3
4
5
6
7
8
9
1 Year ARI Wave Climate Along Pier, Perpendicular Wavescreen
Extension
NNE
N
NNW
NW
WNW
W
1.54
0.18
0.19
0.21
0.23
0.25
0.27
0.29
0.34
0.44
1.93
0.21
0.22
0.24
0.26
0.29
0.31
0.35
0.40
0.53
1.92
0.21
0.22
0.25
0.27
0.29
0.32
0.37
0.43
0.54
2.00
0.11
0.12
0.13
0.14
0.15
0.17
0.20
0.22
0.28
2.03
0.11
0.12
0.13
0.14
0.15
0.17
0.20
0.23
0.28
1.99
0.11
0.12
0.13
0.14
0.15
0.17
0.20
0.22
0.28
The proportion of the month the maximum daily wave exceeded the good and excellent
conditions are shown below in Table 6-8. 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 6-8
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
6.2.6 Summary
A number of different orientations and lengths of pier return were investigated to achieve
improved mooring conditions. It was found that significantly improved protection to the pier
moorings could be achieved by a perpendicular extension of about 20m in length on the end
of the pier wavescreen as shown in Figure 6-2. This extension 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 the wavescreen and the benefits of permanent
or day moorings along the pier.
Interaction between the harbour wavescreen and pier return have not been considered,
however, due to its relatively small size, there is unlikely to be a significant change in the
reflected wave conditions caused by the perpendicular extension.
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Mornington Harbour Wave Investigation
Figure 6-2
Perpendicular pier extension
6.3 Effect on Hydrodynamics and Coastal Processes
6.3.1 Hydrodynamics
As in the Hydrodynamics Investigation Report (Water Technology R01-H, 2008), the existing
current speeds in Mornington Harbour are low, with a dominant northerly current in summer,
and a dominant southerly current in winter. Efficient flushing properties of the existing
harbour permits residence times for a pollutant released within the harbour are approximately
1-2 days under typical summer or winter conditions. Even under a worst-case no wind
scenario, the residence times were still only in the order of 4 to 5 days.
The effect on current speeds of the pier protection wavescreen and the harbour wavescreen
are predominantly experienced in the offshore area outside of the pier wavescreen and the
inshore area inside the harbour wavescreen. Similarly, the effects of the pier protection and
harbour wavescreens on residence times are minor with an increased residence time to less
than 2.5 days in summer and remaining consistent at less than 3.5 days during typical winter
conditions.
Considering the minor changes to current speeds and residence times following addition of
the pier protection and harbour wavescreen, the additional wave protection, described above,
will not result in any significant additional changes to current speeds or residence times at
Mornington. For the no wind worst-case scenario, scenario, the residence times would remain
in the order of 4 to 5 days, which will ensure that good water quality can be maintained within
the harbour.
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Mornington Harbour Wave Investigation
6.3.2 Coastal Processes
The main feature affecting the coastal processes at Mornington Harbour is the harbour
breakwater. The pier protection extension will have only a small impact on a narrow band of
waves at the entrance to the harbour which do not impact the coastal processes further
inshore. There will be no significant impact on the waves which generate the coastal
processes at Shire Hall Beach or Scout Beach.
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Mornington Harbour Wave Investigation
7
CONCLUSION
The proposed wavescreens provide the harbour area with a significantly increased level of
wave protection. Conditions post development with the full length pier protection in place
show a greater reduction in wave heights across a larger area when combined with the harbour
wavescreen than post development conditions with the partial length pier protection and
harbour wavescreen in place.
As a result, it was concluded that the full length Parks Victoria wavescreen, in combination
with the MBHL wavescreen, had the greatest potential for providing a Safe Harbour at
Mornington. The wavescreens will not provide significant protection for the proposed swing
moorings, and wave heights here will be similar to those under existing conditions.
This proposed configuration allowed the north and north-northeast waves to generate an
elevated wave climate along the public berths along the inside of the existing pier. An
extension to the pier wavescreen was found to provide greater protection to the pier,
increasing usability and safety along the pier berths without causing any additional effects to
the hydrodynamics or coastal processes.
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Mornington Harbour Wave Investigation
8
REFERENCES
Australian Standard AS3962-2001 SAA Guidelines for Design of Marinas
GHD Mc Knight 1987, “Mornington Safe Harbour Feasibility Study” GHD Mc Knight,
November 1987
GHD 2002, “Mornington Pier Storm Protection” GHD Consulting, 2002
IPCC 2007, “Climate Change 2007: The Physical Science Basis – Summary for Policy
Makers” Contribution to the Fourth Assessment Report of the Intergovernmental Panel on
Climate Change, February 2007
Lawson and Treloar 1996, “Mornington Pier – Investigation into the likely effects of sheet
piling” Report J5059/R1697, December 1996
Loder & Bayly 1990, “Mornington Boating Facilities Study and Environmental Effects
Statement” Loder and Bayly Consulting Group, January 1990
Water Technology 2008, “Mornington Harbour Hydrodynamics Assessment” Report
J648.01H-07, prepared by Water Technology Pty Ltd, February 2008
Water Technology 2008, “Mornington Harbour Coastal Processes Assessment” Report
J648.01C-07, prepared by Water Technology Pty Ltd, February 2008
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9
GLOSSARY
AHD: The Australian Height Datum is a theoretical reference surface (datum) for altitude
measurement in Australia.
Bathymetry: The measurement of depths of water in oceans, seas and lakes;
Beach profile: A cross-section taken perpendicular to a given beach contour;
Breaker zone: The zone within which waves approaching the coastline commence breaking
Cross-shore: Perpendicular to the shoreline
Design storm: Coastal protection structures will often be designed to withstand wave attack
by the extreme design storm. The severity of the storm (i.e. return period) is chosen in view of
the acceptable level of risk of damage or failure. A design storm consists of a design wave
condition, a design water level and duration.
Design wave: The wave having the characteristics against which protection is desired, e.g. 1
in 50 year return period wave.
Diffraction: The phenomenon occurring when water waves are propagated into a sheltered
region that interrupts a portion of the otherwise regular train of waves, resulting in the multidirectional spreading and bending of the waves.
Dissipation: The loss of energy due to the action of friction or turbulence.
Eddy: A current fluid, forming on the side of the main current, especially one moving in a
circle; in extreme cases a whirlpool
Fetch: The length of unobstructed open sea surface across which the wind can generate
waves
Flushing time: The time required to replace all the water in an estuary, harbour, etc., by
action of current and tide
Geomorphology: Branch of physical geography which deals with the form of the Earth and
the investigation of the history of geologic changes through the interpretation of topographic
forms.
Groyne: A structure extending from the shore into the water to provide protection from wave
action, or to restrict the transport of sediment in the area.
Groyne bay: The beach compartment between two groynes
Hindcasting: In wave prediction, the retrospective forecasting of waves using measured wind
information
Incident wave: Wave moving landward
Longshore: Parallel and close to the coastline
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Mornington Harbour Wave Investigation
Mean higher high water (MHHW): The arithmetic average of the elevations of the higher
high waters of a mixed tide over a considerable period.
Mean high water springs (MHWS): The average height of the high water occurring at the
time of spring tides.
Mean lower low water (MLLW): The average height of the lower low waters over a
considerable period.
Mean low water (MLW): The average height of the low waters over a considerable period.
Mean low water springs (MLWS): The average height of the low waters occurring at the
time of the spring tides.
Mean sea level: The average height of the surface of the sea for all stages of the tide over a
considerable period, usually determined from hourly height readings
Monochromatic waves: A series of waves generated in a laboratory, each of which has the
same length and period
Porosity: A measure of the void spaces in a material, and is measured as a fraction, between
0–1, or as a percentage between 0–100%
Reflected wave: That part of an incident wave that is returned (reflected) seaward when a
wave impinges on a beach, seawall or other reflecting surface
Refraction: The process by which the direction of a wave moving in shallow water at an
angle to the bottom contours is changed. The part of the wave moving shoreward in shallower
water travels more slowly than that portion in deeper water, causing the wave to turn or bend
to become parallel to the contours
Return period: Average period of time between occurrences of a given event
Sheet Piling: Interlocking member of wood, steel, concrete, etc., subject to lateral pressure,
driven individually to form an obstruction to percolation, to prevent movement of material for
seawalls, stabilization of foundations, etc.
Significant wave height: Average height of the highest one-third of the waves for a stated
interval of time
Snells Law: In optics and physics, Snell's law (also known as Descartes' law or the law of
refraction), is a formula used to describe the relationship between the angles of incidence and
refraction, when referring to light or other waves, passing through a boundary between two
different isotropic media, such as air and glass. The law says that the ratio of the sines of the
angles of incidence and of refraction is a constant that depends on the media.
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Standing wave: A type of wave in which the surface of the water oscillates vertically
between fixed nodes without progressing. A wave of essentially stable form which does not
move with respect to a selected reference point
Wave height: The vertical distance between the crest (the high point of a wave) and the
trough (the low point)
Wave period: The time, in seconds, required for a wave crest to traverse a distance equal to
one wave length
Wave length: The distance, in metres, between equivalent points (crests or troughs) on waves
Glossary extracted from: http://www.csc.noaa.gov/text/glossary.html and
http://en.wikipedia.org
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APPENDIX A MIKE 21 SPECTRAL WAVE MODEL
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Page A-1
Mornington Harbour Wave Investigation
APPENDIX B MIKE 21 BOUSSINESQ WAVE MODEL
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Page B-1