AUSTRALIAN MARITIME COLLEGE - City of Prior Lake · 2016-04-29 · AUSTRALIAN MARITIME COLLEGE...

AUSTRALIAN MARITIME COLLEGE

National Centre for Maritime Engineering & Hydrodynamics

COMMERCIAL IN CONFIDENCE

INVESTIGATION INTO THE EFFECT

OF WASH OF BOATS AND WIND WAVES

ON THE SWAN RIVER

for the

SWAN RIVER TRUST

FINAL REPORT NO. 09/G/17

13 August, 2009

EXCELLENCE IN MARITIME TRAINING CONSULTANCY & RESEARCH

AMC Search Report 09G17 Version 2.0 Page 1

Commercial-in Confidence

Project Report

Title:

INVESTIGATION INTO THE EFFECT OF WASH OF BOATS AND WIND

WAVES ON THE SWAN RIVER

Client: Swan River Trust

Client Representatives: Rod Hughes,

Chris Mather,

Andrea Agocs

Project Manager: Gregor Macfarlane

Consultant: Tim Gourlay, CMST

Author(s): Gregor Macfarlane

Date: 13 August, 2009 Project No: 09/G/17 No. of pages: 74

Classification: Commercial-in-Confidence

Distribution List: Project Manager (sign)

Client (two copies)

Towing Tank File

Gregor

Macfarlane

AMC Search CEO (sign)

John Foster

You should be aware of the fact that no matter how accurate and precise the Australian Maritime College is

in the preparation of its modelling, there can be no guarantee, given the multitude of variables which may

affect the outcome, that the result of modelling will be replicated in actual conditions and vessels.



CONTENTS

Page No.

Executive Summary

3

Nomenclature

4

1. Introduction 5

1.1 Study Background 5

1.2 Study Objectives 5

2. Vessel-Generated Waves

7

2.1 Wave Concepts 7

2.2 Vessel Wave Patterns 11

2.3 Propagating Wave Phenomena 16

2.4 Vessel Speed Regimes 19

2.5 Restricted Channel Effects 22

3. Wave Wake and Bank Erosion

23

3.1 Background 23

3.2 Relevant Wave Wake Studies 24

3.3 Wave Wake and Bank Erosion 24

3.4 Relevant Wave Wake Characteristics 25

3.5 Wave Measurement 29

3.6 Useful Wave Wake Measures in Sheltered Waterways 29

3.7 Wave Energy or Power? 30

3.8 Bank Erosion Studies 31

3.9 Operating Criteria 33

3.10 Waterway Types 36

4. Wind and Vessel Wave Predictions for the Swan River

38

4.1 Introduction 38

4.2 Wind Wave Predictions 38

4.3 Vessel Test Program 49

4.4 Vessel Wave Predictions 49

4.5 Comparison of Wind and Vessel Generated Waves 53

5. Conclusions

70

6. References

71



EXECUTIVE SUMMARY

This study of vessel wash on the Swan River has been undertaken on behalf of the Swan River

Trust (SRT). The study has been instigated in response to specific concerns regarding the influence

of vessel wash on identified bank erosion.

The primary aim of this study is to conduct an indicative investigation into the comparative impact

of wash from boat and wind waves. The analysis will contribute to the development of actions to

implement the boat wash recommendations from the Boating Management Strategy for the Swan

Canning Riverpark (Swan River Trust, 2009).

This report provides a comprehensive outline of relevant background information on boat wash

issues, with a particular focus on the operation of commercial and recreational vessels on sheltered

waterways (refer Sections 2 and 3).

Predictions for both vessel and wind wave characteristics are presented from which it is possible to

identify those vessel speeds where bank erosion is likely to occur as a result of vessel operations for

the following two example regions on the Swan River (as nominated by the SRT):

Lower Reach “Open Water”: alongside Mounts Bay Road between Quarry Point and the

Narrows (31˚58.3´S, 115˚50.0´E)

Upper Reach “Sheltered Water”: alongside Ashfield Parade, Ashfield, near Ron Courtney

Island (31˚55.3´S, 115˚56.4´E)

The characteristics of the wind waves for these two example sites are predicted using standard

hindcasting techniques (refer Section 4.2). A test program of typical vessels that are known to

frequent these two example sites on the river was developed in collaboration with the SRT (refer

Section 4.3). This program deliberately included both commercial and recreational vessels that are

known to be presently operating in these regions.

Predictions of the characteristics of the vessel generated waves were then undertaken for each

vessel, vessel speed and lateral distance using a proven prediction technique developed by AMC

(refer Section 4.4). The predictions of the vessel generated waves are then compared against the

predictions of the wind generated waves in Section 4.5.

For the upper river site (Ashfield Parade) it is clear that shoreline erosion is very likely as a result of

vessel generated waves where a blanket speed limit of either 8 or 9 knots (or greater) is imposed as

the energy and power of the maximum waves generated by all vessels far exceed that of the

maximum wind waves over the entire range of lateral distances investigated. It is also clear that a

reduction in vessel speed, down to 6 or 5 knots, should dramatically reduce the potential for

erosion. For example, at a speed of 5 knots the energy and power of the maximum waves for all

vessels are likely to be below that of the maximum wind waves, provided a minimum lateral

distance of 20 metres is maintained between the vessel sailing line and the shore.

The situation is less well defined for the lower reach site (Mounts Bay Road), primarily because

two of the commercial vessels are likely to generate waves that possess far greater energy and

power than the other four vessels investigated and also the predicted maximum wind waves. For

any blanket speed limit to be effective it must be specified for the „worst offender(s)‟, and thus if

this were undertaken in this case it is likely to be overly restrictive for other vessel classes.



NOMENCLATURE

B Vessel beam [m]

gc Wave group velocity [ms-1

]

E Wave energy per metre wave crest length [Jm-1

]

E Wave energy density [Jm-2

]

F Fetch length [m]

Frl Length Froude number [v(gL)-1/2

]

Frh Depth Froude number [v(gh)-1/2

]

Frv Volumetric Froude number [v(g1/3

)-1/2

]

g Acceleration due to gravity [9.81ms-2

]

h Water depth [m]

H Wave height [m]

L Vessel waterline length [m]

n Wave decay exponent

P Wave power per metre wave crest length [Wm-1

]

T Wave period [s]

AU Wind stress factor 23.171.0 U

U Wind speed at 10m elevation

7/110

)(z

zU [m/s]

)(zU Wind speed at elevation z [m/s]

v Vessel speed [ms-1

]

y Lateral distance between vessel sailing line and measurement point [m]

z Elevation [m]

Angle between wave crest and shoreline [radians]

Constant

Pi

Volume [m3]



1. INTRODUCTION

1.1 STUDY BACKGROUND

This study of vessel wash on the Swan River has been undertaken on behalf of the Swan River

Trust. The study has been instigated in response to specific concerns regarding the influence of

vessel wash on identified bank erosion.

The Swan River in general has experienced a gradual increase in vessel traffic over the last century.

Apart from the increase in local population, improvements in transport connections between Perth

and Fremantle and the popularity of the region as a tourist destination have increased the incidental

population, placing pressures on the environment.

The introduction of regular, high-speed commercial ferry services has introduced a wave regime

with erosive components considerably in excess of those previously present. High-speed vessels

create wake waves which may be moderate in height, but long in period. They also create their

highest wave energy during the transition between slow and high speed.

The growing popularity of high-powered, high-speed recreational craft has led to two significant

problems common to most waterways – vessel wash and noise. High-speed craft can generate wash

that has the potential to create considerable damage to the banks of sheltered waterways, which

traditionally have been low energy environments. Noise is a separate issue, though one that is often

cloaked in wash complaints voiced by waterfront dwellers.

Conversely, most of the early river traffic prior to the introduction of high-speed recreational and

commercial vessels would have operated at slower relative speeds where wash damage was less

evident.

Land-use issues have also arisen where land clearing has removed most of the riparian vegetation,

leaving the bank vegetation depauperate. As a consequence, the bank has little resistance to incident

waves of any form.

The demonstrated inability of some sections of the Swan River to achieve a new dynamic

equilibrium condition over the past ten years or so has also led to this vessel wash study.

1.2 STUDY OBJECTIVES

The primary aim of this study is to conduct an indicative investigation into the comparative impact

of wash from boat and wind waves. The analysis will contribute to the development of actions to


Canning Riverpark (Swan River Trust, 2009).

The objectives are to:

• Clarify the relative contribution of wind and boat generated waves;

• Develop a rational basis for the creation of low wash zones; and

• Provide preliminary advice on the efficacy of establishing blanket low speed zones in the

Swan and Canning Rivers.



An additional objective of this study is to gather and present relevant background information and

data that provides a clear description of boat wash issues, with a particular focus on the operation of

commercial and recreational vessels on sheltered waterways. Basic concepts and background is

covered in detail in Section 2 and more in-depth coverage of applied wave wake issues related to

bank erosion is covered in Section 3.

Investigations specific to this study are covered in Section 4 where predictions of both wind and

vessel generated waves for a range of marine craft known to operate on the Swan River are

presented. A summary of, and discussion on, the direct comparison between boat and wind

generated waves for two specific sites are also provided in this Section.



2. VESSEL-GENERATED WAVES

As noted in the Introduction (Section 1), this section has been included to provide background

information on relevant boat wash issues with a particular focus on the operation of commercial and

recreational vessels on sheltered waterways.

2.1 WAVE CONCEPTS

2.1.1 Introduction

As a vessel moves through the water, it creates a pressure field around it that gives rise to surface

waves. The vessel can be of a conventional form, or an exotic form such as a hydrofoil or

hovercraft - the effect is essentially the same.

Water waves are a gravity-dependent phenomenon characterised by several simple parameters

(Figure 2.1), many of which have a mathematical relationship to each other.

Wave height is defined as the height difference between a successive crest and trough, or trough

and crest. Amplitude is defined as the height of the crest above, or depth of the trough below, the

still water level, but is seldom used in wave wake studies.

Wavelength is defined as the horizontal distance between corresponding points on two successive

waves - usually either crest-to-crest or trough-to-trough.

Wave period is the time taken for corresponding points on two successive waves to pass a fixed

point - usually the time difference between successive crests or troughs or zero-crossing points.

Wavelength and period are related quantities - the longer the wavelength, the greater the wave

period.

Wave celerity or wave speed is the speed of the wave relative to the surrounding water. This is also

related to wave period.

elevation

time

Wave Parameters

height

period

zeroup-crossingpoint

crest

trough

still water level

zerodown-crossingpoint

Figure 2.1 – Wave Parameters

ele

vati

on



Beneath a propagating deep-water wave there is a circular motion of water. The further below the

surface, the weaker this circular motion becomes (Figures 2.2 and 2.3), Newman (1977). This

circular motion, where the water particles beneath the crest move forward and the particles beneath

the trough move rearward, is most commonly experienced when swimming in the surf - a swimmer

will be pushed towards the beach by the unbroken wave crest but drawn back out to sea by the wave

trough. The depth of this circular motion is dependent on wavelength. When the water depth is

greater than half the deep-water wavelength, the circular motion at the sea bed is negligible.

Since the strength of this circular motion is dependent on depth, the forward motion at the top of the

wave crest will be greater than the rearward motion in the trough. The result is a nett forward

movement of a small amount of water. Waves are therefore largely a form of energy transport, with

a small amount of mass transport.

sea bed

sea surface

negligible rotation when the water

depth is greater than half the

wavelength

Figure 2.2 – Orbital wave motion

Deep Water Wave Velocity Field

reproduced from J.N. Newman

Marine Hydrodynamics

(not to scale)

direction of propagation

at depths greater than half the deep water

wavelength, the orbital motion is negligible

Figure 2.3 – Deep water wave velocity field

A group of waves, often referred to as a wave packet, may be made up of waves with different wave

periods as well as heights. Ocean waves are a good example of this, with waves of different heights

and periods forming a random sea surface (Figure 2.4), Young (1999). Vessel waves are similar in

nature to ocean waves and vessel wash is a combination of many waves of different dimensions.

This random composition makes wash analysis complicated.



short-period waves

time

elevation

medium-period waves

time

elevation

long-period waves

time

elevation

combined waves

time

elevation

Figure 2.4 – Superposition of waves

2.1.2 The Effect of Depth

Waves travelling in deep water are well defined and understood. For any wave, deep water is

defined as being a depth greater than half the wavelength - this being the point where the circular

motion within the wave begins to feel the bottom. Figure 2.5 shows the water depth required for

waves with different wavelengths, hence wave periods, to be considered deep-water waves.



As a deep-water wave moves into shoaling water where the water depth is less than half the deep-

water wavelength, the wave characteristics change. The circular motion beneath the wave is

compressed into an elliptical shape, concentrating the wave energy into a decreasing depth of water,

slowly at first but increasing as the depth decreases further. Although a wave becomes depth-

influenced when the depth is less than half the deep-water wavelength, the water is considered to be

truly shallow when the water depth is about one-twentieth the deep-water wavelength. It is at this

very shallow depth that shoaling can occur.

The shoaling wave maintains its wave period independent of water depth, but its wavelength and

wave speed decrease. Also, the concentration of wave energy into shallower water causes the wave

to change in height, particularly so for long-period waves. These phenomena can be clearly seen at

a surf beach, where the waves in a set move into shore, slow down, bunch together and grow in

height before breaking.

The speed at which a wave can travel is depth dependent - the shallower the water, the slower the

wave speed. This affects the type of waves that a vessel can create if travelling in shallow water.

Once the local water depth approximately equals the wave height, the wave becomes unstable and

breaks.

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10

period (s)

depth (m)

deep water

shallow water

shoaling depth

Figure 2.5 – Deep / shallow water. The term “shoaling depth” refers to the depth at which a shallow

water wave begins to grow in height.

2.1.3 Environmental Effects

Waves with different characteristics have different effects on the environment.

Wind waves are characterised by low to moderate wave heights, but short periods. As an example,

the wind wave height and period for varying wind speeds and fetch lengths are shown in Table 2.1.

These figures are generated by hindcasting - applying a standard set of equations to generate wave

data given the wind speed and fetch (the distance the wind has been blowing over water). This is

covered in more detail in Section 4.2 and the Shore Protection Manual (1984).



Fetch (m) Wind 5 m/s Wind 10 m/s Wind 20 m/s

100 m 26 mm / 0.5 s 62 mm / 0.7 s 144 mm / 0.9 s

500 m 59 mm / 0.8 s 137 mm / 1.1 s 321 mm / 1.5 s

1,000 m 83 mm / 1.0 s 194 mm / 1.4 s 452 mm / 1.9 s

10,000 m 250 mm / 2.0 s 586 mm / 2.8 s 1,304 mm / 3.8 s

Table 2.1 - Hindcast wind waves (wave height in mm / wave period in seconds)

It is clear that the height of wind waves grows by several hundred percent but the period increases

at a much slower rate with increasing wind speed. This has three particular consequences.

Firstly, wind waves, or chop, cause discomfort to small craft. The waves are close together and

relatively high, so they are considered to be steep. The wave period is often similar to the roll

period of small vessels, causing them to roll synchronously when stationary.

Secondly, the energy in wind waves tends to be more height-dependent than period dependent.

Wave energy is equally a function of both wave height and period. A shoreline naturally subjected

to wind waves may occasionally experience waves with a large height, but the corresponding period

will remain relatively low. Similarly, such a shoreline may be able to withstand vessel wash,

provided the wash is characterised by moderate wave height but low corresponding wave period.

Thirdly, shoaling waves tend to increase in height before breaking. However, for waves with

periods less than 3 seconds, the shoaling is considered to be negligible (less than 10% height

increase), and for waves with periods less than 2 seconds the waves will be close to breaking before

any shoaling occurs. Wind waves, particularly in sheltered and semi-enclosed waters, tend to

maintain their deep-water height before breaking. This can also be seen at a surf beach, where the

long-period swells stand up before breaking but the wind-driven chop simply breaks on the shore.

The last point is of particular interest, as it is commonly believed by most people that vessel wash

height is the primary determinant of erosion potential – wave height is a more visual indicator of

wake waves. However, wave period possibly has a greater effect in sheltered and semi-enclosed

waters, particularly on shorelines with sloping beaches. Such shorelines have natural, dynamic

mechanisms to withstand wind waves, but the sudden introduction of vessel wash containing much

longer-period waves may upset the balance. In the case of the upper reaches of the Swan River

with its steep, compacted soil banks, it is most likely that both wave height and period are equally to

blame for erosion. The wind wave climate experienced by the upper Swan River is extremely

modest in comparison to more open areas.

2.2 VESSEL WAVE PATTERNS

2.2.1 Introduction

The general wave pattern generated by a vessel is largely independent of vessel form, but it is

affected by water depth and vessel speed.

Characteristic vessel wash is divided into three categories, depending on vessel speed and water

depth. The defining parameter is depth Froude number, a non-dimensional relationship between

vessel speed and water depth. Vessels of different configurations travelling at the same depth

Froude number will produce equivalent wave patterns.



The definition of depth Froude number, Frh, is:

gh

vhFr (2.1)

Depth Froude number has its greatest effect when the water depth is less than about one-quarter the

vessel‟s waterline length; it has moderate influence at depths up to one-half the waterline length and

has little influence at depths greater than the waterline length.

The water depth limits the speed at which a wave can travel in shallow water, such that the

maximum speed will be reached when the depth Froude number equals one.

At a vessel speed below a depth Froude number of one, the speed is said to be sub-critical. A depth

Froude number of one is termed the critical speed and speeds around the critical speed are

sometimes referred to as trans-critical speeds (approximately 0.8 ≤ Frh ≤ 1.2). The position of the

upper and lower bounds of the trans-critical range can vary according to vessel and waterway

conditions. Speeds above a depth Froude number of one are said to be super-critical. These zones

are shown graphically in Figure 2.6.

Depth / Speed Zones

0

5

10

15

20

25

30

35

0 5 10 15 20

Depth (m)

Vessel S

peed

(kn

ots

)

Fd=0.8

Fd=1.2

Fd=1.0

sub-critical

super-critical

Figure 2.6 – Critical speed zones

2.2.2 Sub-Critical Speeds

In deep water, defined as being a depth such that the depth Froude number is less than 1 (more

importantly, when the depth Froude number is less than about 0.7), all vessels produce a wave

pattern termed the Kelvin wave pattern, named after Lord Kelvin, an early pioneer of vessel wave

theory, Kelvin (1887). A typical Kelvin wave pattern is presented in Figure 2.7. It is characterised

by two wave types - transverse and divergent waves.

Transverse Waves

These waves are commonly referred to as stern waves and propagate parallel to the vessel's sailing

line. The height of these waves is largely a function of vessel displacement-length ratio, with a

heavy, short vessel producing higher waves. The period of the transverse waves is a function of

vessel speed, as they effectively travel along with the vessel.

Frh=1.2

Frh=0.8

Frh=1.0



Divergent Waves

Commonly referred to as bow waves, the divergent waves propagate obliquely to the vessel's sailing

line at an angle of approximately 55 degrees. This wave formation, referred to as the Kelvin wedge,

subtends an angle of slightly less than 20 degrees to the sailing line, which is constant for all vessel

forms. Many vessels also create stern divergent waves, though this additional wave train usually

melds into the bow divergent system at some point aft of the vessel. Divergent waves are generally

steep and close together near the vessel - carrying as much energy as possible for their wavelength.

35 o 16’

Propagation direction

of diverging waves

54 o 44’

Diverging waves

Transverse

waves

Cusp locus line

28’ 19 o

Figure 2.7 – Kelvin wave pattern

The point of intersection of the transverse and divergent wave trains is termed the cusp and

represents a localised wave height peak. At successive cusps, the divergent waves decay in height

slower than the transverse waves, such that a vessel wake measured far from the sailing line will

feature divergent waves more prominently. The oblique propagation angle of the divergent system

compared with the transverse system means that the divergent system is usually of greater interest

when assessing erosion, as these waves propagate towards the shore.

There are exceptions to this. If a vessel producing a significant transverse wave system, such as a

heavy vessel, changes course, the transverse waves created prior to the course change will continue

to propagate along the original course and may eventually reach the shore. This is commonly noted

when slow speed displacement vessels traverse a narrow river at cruising speed. The river traps the

transverse waves and does not allow them to diffract (spread their energy by growing sideways in

crest length), greatly reducing their height decay. These waves may be evident for several minutes

after the vessel has passed.

2.2.3 Critical Speed

When the depth Froude number approaches unity, which can occur when either the water depth

shoals or the vessel's speed changes relative to the water depth, the wave pattern changes, refer

Figure 2.8, Sorensen (1973a).

As the waves reach their depth-limited speed, the divergent waves increase their angle to the sailing

line, propagating more in line with the stern transverse waves.



At the critical speed, when the depth Froude number equals one, a vessel will experience a peak in

resistance. The relative magnitude of the resistance peak is dependent on the ratio of the water

depth to vessel waterline length, with very shallow water for a given waterline length producing the

most pronounced increase in resistance.

The wave pattern generated may consist of only one long-period wave, termed a wave of

translation, propagating parallel to the sailing line. This single wave travels with the vessel and so

does not radiate from it. It does, however, grow in crest length - the vessel pumps energy into this

wave that is initially accommodated as a height increase, but once height stabilises the wave grows

in crest length. The speed of this crest length growth equals the vessel speed. If banks bound the

water at the sides, limiting energy growth in the single wave, a train of several waves, termed

solitons, may then form ahead of the vessel if the conditions are conducive to their formation

These waves of translation are particularly damaging and are to be avoided. Not only are they

difficult to see, having a long period but low height, they are hard to maintain under real-life

conditions. It is common for vessels operating in shallow water to operate at speeds that may be

depth-critical at times and the Master needs to be aware of this and avoid the critical speed or pass

through it quickly. The damaging effects are a non-linear function of vessel displacement, so larger

vessels are of more concern than smaller vessels.

2.2.4 Super-Critical Speeds

At speeds above the depth-critical speed, a vessel's wave pattern changes again (Figure 2.8).

The transverse waves, which travel at the speed of the vessel, are no longer able to travel at the

vessel speed due to the limiting relationship between maximum wave speed and water depth. As

the vessel accelerates from a sub-critical to a super-critical speed, the transverse waves fall behind

the vessel and disappear altogether. The lack of a transverse wave train reduces vessel wavemaking

resistance, which explains why many vessels go faster in very shallow water.



Figure 2.8 – Wave wake patterns

Frh < 1

Frh = 1

Frh > 1



The divergent waves also re-appear in their more usual form, but propagate at an angle to the sailing

line that is dependent on the vessel's speed, such that the velocity vector parallel to the vessel's

sailing line is not more than the critical speed. The higher the super-critical vessel speed, the less

acute the propagation angle becomes. For very high-speed craft operating in relatively shallow

water, it often appears that the divergent waves propagate almost perpendicular to the sailing line.

When viewed from above (Figure 2.8), the super-critical divergent wave pattern looks different to

the sub-critical wave pattern. The super-critical pattern consists of long-crested waves, whereas the

sub-critical pattern consists of a series of shorter-crested waves.

2.3 PROPAGATING WAVE PHENOMENA

2.3.1 Dispersion

As the propagation of deep-water divergent waves is unaffected by water depth, the waves will

propagate at speeds dependent on their individual wavelengths, Newman (1977). The longer period

waves will travel faster and the shorter period waves will travel slower. If a wave trace is taken at

different distances away from the sailing line, the trace will show the wave packet to be lengthening

further from the sailing line as the individual waves spread out. This phenomenon is termed

dispersion and deep-water waves are considered to be dispersive. Figure 2.9 illustrates dispersion,

where the width of the first group of waves on the left hand side of each trace increases with

increasing lateral distance.

There is also a weak relationship between wave speed and wave height – for a given wavelength the

higher waves travel slightly faster and therefore disperse. Amplitude dispersion is ignored in wave

wake studies.

As the period of the transverse waves is dependent on vessel speed, the transverse waves will all

have the same period when the vessel is travelling at a constant speed. There will be, therefore, no

dispersion evident (in practice there is weak dispersion, probably due to amplitude dispersion cause

by height attenuation away from the vessel).

If the divergent waves propagate from deep to shallow water, or are created in shallow water to

begin with, the waves will begin to feel the bottom and become depth-influenced. When the speed

of each wave is depth-critical, that is, the depth Froude number for each wave equals unity, the

maximum speed of propagation becomes limited to the depth-critical speed. A wave packet will

then stop dispersing and the waves will travel at the same depth-limited speed. Shallow water

waves are therefore termed non-dispersive.

This is not exactly the case for vessel waves in shallow water. With the divergent wave packet

being comprised of many waves with different wavelengths, the waves with long wavelengths will

become speed limited and therefore non-dispersive before those slower waves with shorter

wavelengths. Also, in reality there is some leakage of wave energy in a non-dispersive packet, but

this is only evident over several boatlengths of shallow water wave propagation.

Dispersion can create difficulties when assessing wave traces obtained through the conduct of

physical experiments. Where a trace taken close to a vessel (within, say, half a boat length), the

trace may appear to consist of only a few waves, when in fact these waves represent many more

waves of differing wavelength superimposed. It takes approximately 2-3 boatlengths for waves to

disperse sufficiently such that the period of individual waves can be measured with certainty. Wave

height is affected to a lesser degree.



2.3.2 Attenuation

As the distance abreast of the sailing line increases, the wave height decreases. This height

attenuation is due to diffraction – spreading the wave energy along the wave crest.

Havelock (1908) extended the work of Kelvin (1887) to show that the wave heights at the so-called

cusp points decrease at a rate inversely proportional to the cube root of the distance from the vessel.

Sorensen (1969) used model tests to show that the bow wave data generally conformed to this rate

of decay. Havelock also showed that the transverse waves generally decrease at a rate inversely

proportional to the square root of the distance from the vessel, a fact that applies to all waves that

appear behind the cusp locus line. Therefore, transverse waves tend to decay at a faster rate than the

higher waves that occur along the cusp locus line. Thus, it is reasonable to conclude that the waves

along the cusp locus line (typically the highest divergent waves in a propagating wave packet) will

become even more prominent to the observer as the distance from the vessel increases. Several

studies have shown this to be true, Sorensen (1969), Sorensen (1973a), Renilson and Lenz (1989).

According to deep-water vessel wave theory, the attenuation measured at the cusp (the point of

interaction of the transverse and divergent wave systems) is:

For divergent waves:

H = γy -⅓

(2.2)

For transverse waves:

H = γy -½

(2.3)

The transverse waves attenuate faster with distance from the sailing line and the distant waves will

be dominated by the divergent system.

The wave trace from a single probe is not guaranteed of cutting exactly at the cusp (where the

transverse and divergent waves intersect), so the attenuation exponents (-1/3 and -

1/2) may vary

slightly. However, analysis of the considerable amount of model scale test data available does

justify the use of these exponents.

In shallow water, divergent wave attenuation rates appear to be slightly higher than in deep water

(Sorensen 1973a), and shallow water model scale tests performed at the AMC appear to confirm

this trend. The number of shallow water variables complicates the assessment of shallow water

wave attenuation and the most conservative approach is to apply the deep-water attenuation rates.



0.3L Probe

-200

-150

-100

-50

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80 90 100 110

0.5L Probe

-200

-150

-100

-50

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80 90 100 110

0.7L Probe

-200

-150

-100

-50

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80 90 100 110

1.0L Probe

-200

-150

-100

-50

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80 90 100 110

2.0L Probe

-200

-150

-100

-50

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80 90 100 110

2.5L Probe

-200

-150

-100

-50

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80 90 100 110

Figure 2.9 - Deep water wave dispersion, for different lateral probe positions. (Cox, 2000)

Incr

easi

ng l

ater

al d

ista

nce

fro

m t

he

ves

sel

sail

ing l

ine



2.4 VESSEL SPEED REGIMES

The wave wake generated is directly related to a vessel‟s wavemaking resistance. There are three

distinct speed ranges where a vessel‟s wash changes in magnitude and interactions between these

speed conditions and the three depth-related wash regimes (sub, trans and super-critical) can create

an unacceptable wash. Figure 2.10 shows graphically these speed regimes in terms of a non-

dimensional speed-length ratio, length Froude number (Frl).

The definition of length Froude number, Frl, is:

gL

vlFr (2.4)

In practical terms, the first regime (displacement speed) is best observed in slow, heavy vessels.

These vessels experience a practical upper limit of their speed, termed hull speed, which can only be

exceeded with a substantial increase in engine power. High-speed craft, which have a power-to-

weight ratio such that they can travel faster than their hull speed, first experience a resistance hump

just above hull speed before settling into the high-speed regime. For a planing hull, this will be the

onset of planing.

When the waterline length changes notably with speed it can be more appropriate to use the

volumetric Froude number:

31

vFr

g

v (2.5)

2.4.1 Displacement Speed

All vessels have a displacement speed range where the length of the transverse waves generated is

less than the waterline length. The upper limit of this speed range is when the length Froude

number, Frl, equals 0.399, which reduces to:

v = 2.43√L (2.6)

This maximum displacement speed, or hull speed, represents the condition where the longest wave

generated equals the waterline length of the vessel. To travel faster than this, the vessel must begin

to climb its own bow wave (the common analogy).

Wavemaking resistance in the displacement region is proportional to v6, so small changes in speed

cause large changes in resistance and wash. A clear example of this can be found in Figure 2.11

where the engine power as a function of vessel speed is presented.

In the displacement speed region, wave periods are modest and wavemaking energy transforms into

wash height, creating steep waves. In general, operating at speeds up to 75% of the maximum

displacement speed (or about 1.82√L knots) will produce modest wash height and period.



Figure 2.10 – Vessel speed regimes

Frl < 0.399

Frl = 0.399

Frl = 0.5

Frl > 0.5



Engine Power - Robin-T

0

20

40

60

80

0 5 10

Vessel Speed (knots)

En

gin

e P

ow

er

(hp

)

Figure 2.11 – Engine power against speed curve

2.4.2 Semi-displacement Speed

As a vessel powers through the displacement speed limit, its running trim increases as the transverse

waves move aft of the transom. Vessel wavemaking resistance is high, peaking at a length Froude

number of approximately 0.5.

Wave wake height increases to its maximum and divergent wave periods increase steadily. A

particular operating condition to be avoided is at a length Froude number of 0.5 and depth Froude

number of 1.0, when maximum specific wavemaking resistance and depth effects coincide. This

condition occurs when:

h = 0.25 L and

v = 3.04√L

Semi-displacement speeds, often referred to as hump speeds in planing craft terms (when the vessel

appears to climb over the hump before planing) create damaging wash.

2.4.3 High Speed

As the length Froude number increases above 0.5, specific wavemaking resistance slowly reduces.

The maximum wave height reduces and maximum wave period levels to a relatively constant value.

For a planing hull form, the vessel will be approaching its fully planing condition. Round bilge

multihull forms are simply referred to as high-speed displacement forms. Wetted surface area, the

basis of frictional hull drag, becomes the principal drag component, hence the drop in total wave

wake energy with increasing speed.

It is often said that high-speed vessel wake is preferable to that of the semi-displacement speed,

where waves are high and steep. This may appear to be the case, but the high-speed condition

produces the longest wave periods, which may have as much or greater effect on shorelines and

shoreline structures as wave height. It is important to remember that wave height attenuates with

distance from the sailing line but period remains constant.



2.5 RESTRICTED CHANNEL EFFECTS

In rivers and channels that are shallow and narrow, a vessel may encounter a blockage condition

where it effectively begins to push water along with it. This results in a large surge preceding the

vessel and a drawdown as the vessel passes. If the channel is narrow enough, this surge and

drawdown impinges on the shoreline and leads to damaging erosion. It is particularly noticeable at

speeds around the critical speed and at high speeds.

The most common restricted channel phenomena are:

surge - defined as the rise in surrounding water level preceding an approaching vessel. When a

vessel travels in a uniform channel at close to the critical speed (depth Froude number of unity)

solitary waves, or solitons, can propagate forward of the vessel almost periodically. Dand et al

(1999) concluded that the existance of solitary waves in open water may explain the "rogue" waves

associated with real-world operation of fast craft;

drawdown - sometimes referred to as suction troughs, defined as a lowering of the water level

abreast of a passing vessel. It often appears as a recession of water from a beach or bank as a vessel

passes close offshore;

backwater flow - defined as the aftwards acceleration of water across a shallow seabed as a vessel

passes above.

squat - defined as the mean increase in sinkage and change in dynamic trim when a vessel moves

through water. Although present in deep water, squat is considerably aggravated by restrictions in

water depth and or width, Tuck (1967).

The nett effect of these restricted channel effects is referred to as blockage in ship design. The

calculation of blockage is important in the design and operations of canals and ports, as well as in

ship scale model testing. There are many references available, though the published methods for

calculating blockage can produce widely varying results, depending on the source of the empirical

data and/or the water flow assumptions made, Gross and Watanabe (1972), Millward (1983). What

is clear is that a single definitive methodology or set of equations for calculating blockage effects

does not exist.

The onset of a blockage condition occurs when the ratio of vessel underwater cross-section against

waterway cross-section reaches a particular value. This value is dependent on factors such as the

vessel speed and channel width-to-depth ratio. As a guide, if the vessel underwater cross-section is

less than 1% of the channel cross-section, blockage will be negligible. Vessels can usually operate

with few environmental effects up to a 3-4% ratio.

However, blockage effects such as surge and drawdown are localised phenomena that travel along

with the vessel. Any vessel passing close to a bank, especially in shallow water, can create a

localised blockage effect. This needs to be considered when proposing vessel operating criteria in

restricted waterways. The drawdown in particular can be erosive and, if severe enough, will affect

people and small craft at the water‟s edge.



3. WAVE WAKE AND BANK EROSION

3.1 BACKGROUND

Vessel wave wake has been a prime topic for study over the past two decades, though it no longer

attracts quite the same attention since industry has gained a general understanding of the primary

issues. Sufficient science has been developed to allow for the regulation of the most damaging

vessels without actually perfecting the science.

It is known that wave wake issues can differ considerably depending upon the size and/or speed of

the vessel(s) and the location(s) in which they operate. As a result, it is useful to categorise

particular scenarios into the following three distinct regions, with reference to examples of rivers

and harbours in Australia:

a) Highly Sensitive Regions - This region includes very sheltered waterways such as rivers with very

limited fetch and/or width. They often have steep, cohesive banks that are highly susceptible to

erosion by vessel wave wake. Vessel speeds are likely to be restricted to a small range of sub-

critical depth Froude numbers. Vessel operation at trans-critical depth Froude numbers should be

avoided and operation at super-critical depth Froude numbers may be limited to only very small

craft (less than about five metres length). Examples include the lower Gordon River, upper

reaches of the Parramatta and Swan Rivers and sections of the Noosa River.

b) Moderately Sensitive Regions - This region includes semi-sheltered estuaries such as the lower

reaches of large rivers and harbours or areas where shorelines have been artificially armoured to

withstand increased wave action. Vessel speeds are likely to be restricted to a range of sub-

critical depth Froude numbers. The possible exceptions may include certain small craft and larger

wave wake-optimised craft that could operate at some super-critical depth Froude numbers. In

such cases, multiple criteria may be required to determine acceptable speeds for each vessel type

(this is discussed in more detail in later sections). Operation at trans-critical depth Froude

numbers should be limited to acceleration and deceleration between the sub and super-critical

conditions. Examples include the lower reaches of the Parramatta, Brisbane and Swan Rivers and

sheltered areas of Sydney Harbour.

c) Coastal Regions - In these more exposed regions, wave wake criteria generally only apply to

large high-speed craft operating at trans or super-critical depth Froude numbers. Minimal

problems eventuate from almost all vessels operating at sub-critical speeds. Some existing criteria

applied to high-speed vessels are based on acceptable levels from „conventional‟ (i.e., not high-

speed) vessels operating at sub-critical speeds. Often the criteria are imposed due to adverse

safety risks for other users of the waterway (and shoreline) as a result of large/long vessel waves

generated at high speeds. Examples include Scandinavian coastal regions and Marlborough

Sounds in New Zealand.

The differences between types of waterways are discussed further in Section 3.10.

The limited number of regions where wave wake is of concern within Australia (such as those of the

Gordon, Parramatta, Brisbane and Swan River ferry services) have been the subject of individual

studies that have sought vessel-specific solutions, as opposed to an over-arching methodology that

would allow for a desktop evaluation of any vessel in any waterway, Macfarlane and Cox (2007).

Australia does have a relatively large recreational boating population that utilises the limited

sheltered waterways available. This is not dissimilar to the USA, where the majority of recreational

boating is enjoyed on fresh water lakes and rivers, as well as sheltered coastal waterways, rather



than the open ocean itself. It therefore makes sense to attempt to develop guidelines for vessel wave

wake that allows for the sustainable use of sheltered waterways.

In Australia, this has generally come in response to a perceived erosion event. It can be said with

some certainty that maritime regulatory authorities have generally been reactive in their approach to

wave wake and erosion. A partial exception are the Gordon River services which, operating within a

National Park and World Heritage Area, are regulated by a land management rather than maritime

agency. There the initial response in the early 1990s was reactive, but became proactive with the

implementation of a long-term monitoring and vessel certification process that is on-going today,

Bradbury et al (1995), Bradbury (2007a).

3.2 RELEVANT WAVE WAKE STUDIES

The Australian Maritime College (AMC) has conducted studies of wave wake and erosion in several

recreational sheltered waterways in South-east Queensland, Macfarlane and Cox (2004). The

Queensland Government had a pressing desire to find the causal links between the wave wake of

certain vessels and the erosion they may have caused in the Noosa, Brisbane, Bremer, Maroochy and

Mary Rivers. One aim was to raise awareness of potential effects of new classes of vessels and

activities such as wakeboarding before erosion occurs, so that regulatory bodies are not reliant solely

on reactionary measures.

The AMC‟s studies collected wave wake data from controlled field experiments on a range of small

craft, but without actually measuring corresponding erosion. Instead, erosion studies undertaken on

the Gordon River prior to 1995 were re-analysed in an attempt to derive relationships between

measured small craft wave wake and erosion thresholds. A set of operating criteria were developed

from the re-analysed Gordon River data.

In a separate study of different bank types, similar controlled field experiments were conducted on

the Gordon River between 1997 and 2005, but in this instance both the wave wake and the

subsequent shoreline disturbance were recorded, Bradbury (2007b), Macfarlane (2006).

3.3 WAVE WAKE AND BANK EROSION

3.3.1 Introduction

At the outset, it must be accepted that there may never be a rigorous theory that links vessel wave

wake and erosion. This is similarly the case in coastal engineering, where beach erosion is predicted

by a number of largely empirical and statistical rules developed over many decades. Those rules

may have a grounding in basic science and engineering, but they are underpinned by empirical

equations and a reality that can only be represented statistically, with accompanying error as a

consequence. One only has to review The Coastal Engineering Manual (formerly the Shore

Protection Manual) by The US Army Corps of Engineers (2002), to see that it is weighted heavily

with model test results and empirical tables.

Similarly, it must also be accepted that most natural waterways are dynamic environments subject to

erosional and/or depositional processes. Not all erosion events can be blamed on vessel wave wake.

In many instances local land use practices such as riparian (river bank) vegetation removal and

farming, as well as waterway issues such as regulation, channelisation, extractive processes and up-

or downstream development (of flood protection or harbours, for instance), can be the root cause of

upstream erosion. Boating often simply becomes the focus of attention for an otherwise existing and

complex problem.



In contrast to naturally-occurring wave climates, a vessel‟s wave wake is characterised by short

event duration and a broad spectral spread of wave parameters that do not lend themselves to the

application of conventional statistical methods. Instead, the principle statistics of concern may well

relate to the extent to which certain wave wake parameters exceed those of the existing wave climate

in a particular area.

3.3.2 Recreational Craft

In developing comprehensive, but simple, bank erosion criteria for recreational craft, a number of

factors must be realised:

1. Recreational boating is not a substantial direct revenue source for marine regulatory authorities,

so receives limited attention, hence limited funding.

2. When funding for maritime scientific investigation is limited and a political solution must be

found, recreational boaters are soft targets. It is often easier and cheaper to apply a blanket speed

limit to boating activities than to police it.

3. Vessel wave wake complaints are often used to mask other community concerns such as the

noise generated by high-speed craft and the loss of amenity. Communities and governments

react strongly to tangible evidence such as bank erosion, regardless of the cause, whereas noise

and loss of amenity are more subjective, somewhat less tangible, and therefore less likely to

attract regulation.

4. Shoreline erosion can very often be the result of land use issues, engineering works, river

regulation or climate change and sea level rise.

5. Regulators, builders and owners of small craft have scant information relating to vessel

parameters such as displacement, dimensions and hull design. Often only very simplified

parameters must be relied upon to determine wave wake potential.

6. Every possible combination of bank type, bank material, riparian vegetation and river

bathymetry cannot be covered, and indeed may not need to be.

7. The influence of environmental variables such as shallow water must be limited as they

introduce an exceptional number of additional parameters.

Fortuitously, the vast majority of recreational vessels using sheltered waterways are small, high-

speed monohulls, typically used for water skiing and recreational fishing. There are often a smaller

number of slow-speed vessels such as professional fishing vessels, houseboats and workboats, and

some commercial charter and ferry operators. If the vessel length is sufficiently large and it is

engaged in a commercial operation, case-by-case testing and approval can be implemented.

3.4 RELEVANT WAVE WAKE CHARACTERISTICS

For the purposes of studying small craft wake waves, four parameters are necessary to adequately

describe a wave – height, period, water depth and direction of propagation. For the purpose of

comparing vessels or a single vessel at a range of speeds the last two can often be held reasonably

constant at site appropriate values, leaving height and period as the key variables.

It is necessary to identify the waves of geomorphological interest and focus upon them. Of the two

vessel-generated wave types, transverse and divergent, it is the divergent systems that dominate in

high-speed vessel wakes. Transverse waves can be significant when generated by displacement hull

forms or heavy, transom-sterned high-speed craft traversing at displacement speeds, and especially

where the waterways are very narrow. Transverse wave height (and therefore energy) decays faster

than divergent wave height with lateral separation from the sailing line, but this decay becomes



bounded by the shoreline. Being more of a concern with slow vessel speeds, transverse waves are

best controlled by changes to operating speeds and vessel design.

For other applications a degree of vessel induced erosion may be acceptable but limits may need to

be placed on how much erosion is to be permitted. Earlier work on the Gordon River (von

Krusenstierna 1989, Nanson et al 1994) drew attention to an increase in the rate of erosion as waves

became larger (as opposed to simply higher) and found that simple measures could explain much of

the erosion. However, allowing some erosion is more complicated because one must then consider

the cumulative effects of all waves exceeding the erosion threshold.

In analysing vessel wave wakes, the two parameters of maximum wave height and the

corresponding wave period for the highest wave (often termed the maximum wave) have therefore

been adopted as the primary measures. The importance of quantifying wave wakes with simple

measures is critical when assessing small craft wave wake impacts. If the measures were

complicated, statistically difficult to represent or costly to collect and collate, regulatory authorities

may be reluctant to pursue a path of boating management through scientific understanding. Blanket

speed limits might be a typical response but these, which to be effective must be specified for the

„worst offender‟, are likely to be overly restrictive for other vessel classes.

Our primary measures, height of the maximum wave and its corresponding period, appear to exhibit

certain predictable relationships at high vessel speeds, which is essential to the development of a

simple but sound method for predicting small craft wave wake. Cox (2000) demonstrated for high-

speed craft travelling at sub-critical depth Froude numbers that divergent wave height is largely a

function of length-displacement ratio and the corresponding period is largely a function of vessel

waterline length. Analysis using the AMC‟s wave wake database, Macfarlane and Renilson (2000),

and by others (Warren, 1991) clearly supports this. Vessel hull form has only a limited bearing on

high-speed, deep water wave wake, as demonstrated in Figures 3.1 and 3.2 (data obtained from the

AMC wave wake database). Figure 3.3, taken from field tests conducted by AMC, shows how high-

speed wave period (divided by √L) collapses to a narrow, constant band at high speeds.

In Figure 3.4, the most common wave wake parameters, such as energy, power and height, show

growing values with increasing vessel speed, peaking at a certain speed (normally about Frl = 0.5)

and then decreasing back to a lower level. Similarly, wave period also grows with increasing vessel

speed, peaks, but tends to level off rather than decrease at higher speeds. Regardless of which wave

wake parameter is used as the erosion indicator, it is clear that there may be two distinct operating

speed ranges – slow speed and high speed, with intermediate transitional speeds to be especially

avoided.

Planing craft in particular are burdened by this “transition hump” where resistance and hence wave

wake is high. In some sports, such as wake boarding, this is viewed by the proponents as beneficial.

Many boaters will explain anecdotally how they believe it is better to travel at high speeds in

sheltered areas and this reasoning has long been used as a justification for transiting at speed. The

current science would not support such a generalisation since the waves from small planing vessels

have been demonstrated to be capable of eroding both muddy and sandy banks (even at the greatest

distance allowed by the sheltered waterbodies examined).



0

100

200

300

400

500

600

700

800

4 5 6 7 8 9 10

Slenderness Ratio ( LWL / Volume1/3

)

Ma

xim

um

Wav

e H

eig

ht

(m

m) Monohulls

Multihulls

AMC Wave Wake Database

Vessel Speed = 13 knots

Lateral Distance = 50 metres

Figure 3.1 – Maximum wave height as a function of slenderness ratio

0

200

400

600

800

1000

1200

1400

1600

1800

0 100 200 300 400 500 600 700 800 900 1000

Vessel Displacement (tonnes)

Ma

xim

um

Wa

ve

He

igh

t

(mm

)

Monohulls

Multihulls


Waterline Length = 45 metres


Lateral Distance = 200 metres

Figure 3.2 – Maximum wave height as a function of vessel displacement



0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Wa

ve

Pe

rio

d / L

1/2

Length Froude Number

Houseboat, LWL = 11.5m, Disp.= 15,500kg

Horizon V-Nose Punt, LWL = 3.9m, Disp.= 800kg

Beam Trawler, LWL = 7.3m, Disp.= 6,100kg

Water Bus, LWL = 8.2m, Disp.= 3,900kg

Sea Jay 5.2m Centre Console, LWL = 4.5m, Disp.= 1000kg

Pacific 7.7m Centre Console, LWL = 6.7m, Disp.= 2,500kg

Ski Boat (Inboard), LWL = 5.3m, Disp.= 1,450kg

Ski Boat (Outboard), LWL = 4.6m, Disp.= 1,030kg

12' Aluminium Dinghy, LWL = 3.3m, Disp.= 280kg

16' Aluminium Dinghy, LWL = 4.4m, Disp.= 510kg

Full Scale Trials DataLateral Distance, y = 23 metres

Figure 3.3 – Wave period / L1/2

as a function of length Froude number

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Length Froude Number

Maximum Wave Height

Period of the Maximum Wave

Energy of the Maximum Wave

Power of the Maximum Wave

Slow Speeds

Transition Speeds

High Speeds

Figure 3.4 – Wave height, period, energy and power as a function of length Froude number

Wave P

erio

d / L

1/2



3.5 WAVE MEASUREMENT

If waves are measured too close to the passing vessel (within one boat length laterally, though this

can be speed-dependent), the measured waves may be subject to localised interactions. If they are

measured too far from the vessel, the dispersing wake waves may be substantially affected by the

existing wind wave environment.

For most field studies conducted by AMC the reference point for wave measurement is standardised

so that the results are directly comparable. The nominal lateral distance from the sailing line to the

wave probe is normally fixed at around one-quarter of the width of the river in question as it is

surmised that most vessels navigate in the centre half of the river, so would not normally stray closer

to the bank than this distance.

To compare the results of trials where different lateral separations between sailing line and

measurement point are used, wave heights must be corrected according to their decay with distance.

The relationship between the maximum diverging wave height and lateral distance of deep water

waves was discussed in Section 2.3.2 and varies according to Equation 3.1.

Hm = yn

(3.1)

The variable is a vessel-dependent function of speed. As previously mentioned, the exponent “n”

has theoretical values of -1/3 and -

1/2 for divergent and transverse waves respectively when measured

at the points of intersection of these two wave trains, Sorensen (1973). During field trials the wave

measurement point is most likely not at the point of intersection, so the exponent values may vary. It

was decided that a -1/3 decay exponent was appropriate for deep water divergent waves, recognising

that it is not necessarily absolute (nor applicable in those relatively rare cases where transverse

waves have greater geomorphic effect). Analysis of the AMC‟s wave wake database shows that the

deep water, divergent wave decay exponent generally varies between a range from -0.22 to -0.4,

where -1/3 is considered a reasonable engineering approximation (Macfarlane, 2002).

3.6 USEFUL WAVE WAKE MEASURES IN SHELTERED WATERWAYS

Historically, wave height has been used as the primary comparative measure for vessel wave wake.

It is possibly the simplest parameter to measure and this fulfils another desirable requirement – it is

within public perception where subjective visual observation must substitute for engineering

measurement. Similar comments were made by Lesleighter (1964) in his analysis of ski boat wave

wake on the Hawkesbury River, where he found that inflated anecdotal claims of excessive wave

wake height could not be substantiated by measurement.

In the authors‟ opinion, the historical use of wave height alone, or indeed any single criterion,

cannot possibly reflect the true erosion potential of a vessel‟s wave wake. Wave period is a strong

indicator of the potential to move sediment in any shoreline environment, either through the period-

dependent orbital velocity below the surface of shallow, but unbroken, waves, or through the gravity

driven jets of plunging breakers. Period, along with height, is required to calculate both wave

energy and wave power. Sheltered waterways generally see only a wind wave environment. Wind

waves of short fetch (and even waves of longer fetch, such as wind-driven ocean seas) exhibit a

disproportionate growth relationship between wave height and period, disproportionate in that wave

height grows more rapidly than period but both have equal weighting in calculating wave energy.

An example of hindcast wind wave values for different wind speeds and fetches was provided in

Table 2.1 (refer Section 2.1.3).



It is clear that increasing either fetch or wind speed leads to much faster growth in wave height than

wave period. Consequently it can be argued that sheltered waterways experience occasional wind

wave height variations of several hundred percent, but with limited accompanying wave period

growth. Sheltered shorelines in a wind wave environment are often dynamically stable. Beach areas,

if they exist, adjust in response to the prevailing wave climate and sediment budget. Other landforms

in low wave energy environments may typically owe their genesis to processes not associated with

waves. When there is a substantial increase in incident wave period beyond what such landforms

would normally experience the shoreline may experience erosion. Not only are the longer period

waves more energetic but orbital currents capable of entraining sediment extend to greater depths.

Where mud flats are present, shoaling long-period wake waves may form higher breakers more

likely to plunge. Small craft traversing at high speeds in sheltered waterways can generate wave

periods far longer than those which occur naturally.

The geomorphic impact of wind waves is not evenly felt throughout river systems and the greatest

impacts occur at the downwind ends of reaches. In contrast, vessel wave wake impacts are more

evenly spread throughout the waterway, with diverging waves especially impacting upon shorelines

that would not otherwise be subjected to a significant incident wave climate. The wave wakes of

high-speed craft, in particular, are dominated by the divergent wave system and, as the depth Froude

number becomes super-critical, all waves propagate obliquely to the sailing line.

3.7 WAVE ENERGY OR POWER?

Wave energy is the sum of a wave‟s potential energy and kinetic energy. Wave energy density is

calculated using (SPM 1977 p2-27):

8

2gHE (3.2)

The wave energy density can be multiplied by the wavelength to obtain the energy E in each

wavelength (per unit width of wave crest):

8

2gHE (3.3)

For waves whose length is less than twice the water depth, the “deep-water” assumption can be used

to relate wave length and period. This is expected to be the case for all the wind wave cases

considered in this study given the relatively sheltered nature of the waterways. Therefore the

wavelength can be found from:

2

2gTλ (3.4)

Thus, the energy E in each wavelength (per unit width of wave crest) can be calculated using:

16

g 222 THρE (3.5)

Wave power is the rate at which wave energy is transmitted in the direction of wave propagation.

The average wave power per unit width of wave crest is (SPM 1977 p2-42):

gcEP (3.6)



gc is the group velocity, which is given (when the wavelength is less than twice the water depth,

SPM 1977 p2-25) by:

Tcg

2 (3.7)

Hence the wave power in open water is:

32

22 THgP (3.8)

When waves are impacting directly onto a shoreline, the power calculated above is that which

impacts onto the shoreline. When the wave crests are at an angle to the shoreline, the wave power

transmitted per metre of shoreline is:

cos32

cos22 THg

cEP g (3.9)

Wave energy and wave power are both commonly used in coastal engineering. Assuming a

simplified, sinusoidal wave form, wave energy (per wavelength and unit crest width) is proportional

to H2T

2 and wave power (derived from energy density) is proportional to H

2T (as can be seen in

Equations 3.5 and 3.9 respectively).

Power is a useful descriptor of wave energy over a period of time, such as may be found in the

statistical analysis of an incident wave field acting over a long timeframe. In the case of the wave

wake of a passing vessel, the waves generated are discrete events and so do not necessarily lend

themselves to description on a statistical time basis. It is felt that wave energy may be a better

measure for such discrete events.

Recent studies by the author introduced another derived parameter – wave energy per unit wave

height, or an HT2 relationship when reduced to its principal variables, Macfarlane and Cox (2004).

This parameter has shown empirically to display the most promising correlation between incident

waves and erosion. Short period waves of less than 2 seconds period, such as the maximum waves

generated by small craft and sheltered waterway wind waves do not shoal to any appreciable degree

before they break. Moreover, a wave breaks when the water depth approximately equals the wave

height, so its energy is concentrated into a depth of water equal to the wave height. Consequently, it

is believed that energy per unit wave height is a measure of the energy content in a short-period

wave just at the time of breaking, and therefore the energy being dissipated onto the shoreline. It is

agreed that this explanation has not been tested and serves only as a possible explanation of the

empirical strength of the HT2 relationship with erosion rates.

For the present study it is suggested that the most appropriate measure is wave energy (per

wavelength and unit crest width), however wave power has also been included (along with the basic

characteristics of wave height and period) to provide alternative checks.

3.8 BANK EROSION STUDIES

Often, the only way to successfully gather required bank erosion data is to conduct controlled

experiments. It is helpful to conduct experiments in the specific waterways being studied, though it

is not always necessary to do so if no attempt is made on-site to correlate between the wash

generated and any erosion that might result. Experimental programs to provide such correlation are

often long-term, high-budget studies.



The benefits of on-site testing are three-fold. Firstly, there is always benefit in gaining local

knowledge of waterway and land use issues, by both interacting with other scientific and regulatory

bodies involved as well as speaking with the local waterway users and other stakeholders. Secondly,

the science is always made more robust when researching real issues under real conditions. Thirdly,

it is important for all concerned, including the researchers and regulatory bodies, to be seen to be

doing something with the intention of generating outcomes that balance the environment and the

recreational amenity.

In contrast to conducting controlled experiments, incidental measurements of passing vessel traffic

is largely useless as a means of gathering data that can be analysed in detail. The testing relies on

recording a number of parameters, such as distance between the vessel sailing line and the probe,

vessel speed and vessel condition, and these parameters cannot be recorded adequately from

incidental vessel traffic. The only real uses for incidental data are to collect statistical information on

waterway usage and cumulative wave energy.

In Australia there have been several significant studies that have attempted to measure bank erosion

from vessel wave wake. The first were academic collaborations on the Gordon River in the early

1990s (von Krusenstierna 1990, Nanson et. al., 1994) and the second is an on-going study on the

Gordon River conducted by Bradbury (refer Bradbury et al, 1995). There was a desktop study of the

Swan River (Pattiarachi and Hegge,1990), but its analysis technique was rudimentary and the results

inconclusive.

Von Krusenstierna attempted to measure erosion using erosion pins set into sandy banks, from

which commercial vessels were subsequently banned. The waves of a passing vessel were measured

and the resultant erosion was measured. It was demonstrated that there was a threshold of wave

wake values below which the rate of erosion was regarded as less significant, and such thresholds

were evident for all of the wave wake measures recorded.

From the mid 1990s further experiments were conducted on cohesive muddy banks lining the

Gordon River reaches remaining open to commercial traffic. Although of cumulative concern, the

amount of erosion per vessel pass was expected to be less than 0.1 mm and therefore undetectable

by measurement of erosion pins in the field. Instrumental measurement of the turbidity (degree of

suspension of solid material) resulting from sediment suspension in normally very clear water was

therefore used as a proxy for erosion. Since the land manager had a pressing need for a criterion to

distinguish appropriately „low wake energy‟ vessels the suggestion from early (and limited) data of

an initial threshold to sediment movement at a wave height of 75 mm was used to define the

maximum acceptable wave. Subsequent work demonstrated this to be overly simplistic and that

wave period was also an effective influence. That point was most graphically demonstrated by the

extreme turbidity caused by the low but long waves generated by small planing craft. However

limiting wave height and period independently was found overly restrictive in that it excluded many

of the wave wake events that did not cause any erosion.

Results from one of several sites used in the on-going study are presented here to illustrate the

relationship between commonly used wave parameters and turbidity (Macfarlane et al, 2008).

These figures show turbidity near the river bank (measured at two different water depths) against the

maximum wave parameters of height (Figure 3.5), period (Figure 3.6), energy (Figure 3.7) and

power (Figure 3.8). A fifth graph, Figure 3.9, shows turbidity against the derived parameter of HT2

(energy per unit wave height). From these graphs, several salient features become apparent:



All graphs define very definite threshold values below which turbidity is essentially zero (ie.

within the range of instrumental and background noise).

Wave height is a relatively poor indicator of erosion potential. One wave height value of 178mm

shows zero turbidity, yet the second-highest recorded turbidity event occurs for another wave at

this same height.

There is close correlation between wave period and turbidity.

There is similarly close correlation between both wave energy and power with turbidity.

The derived parameter HT2 exhibits the tightest grouping of all data points.

3.9 OPERATING CRITERIA

The historical application of a single operating criterion, most notably a limit on the height of the

maximum wave, has been demonstrated to be at best unreliable and at worst incorrect. For example,

a single criterion of wave height was adopted for vessel operations on Sydney Harbour yet there

have been reports of significant foreshore damage (Kogoy, 1998). Moreover, attempts to remedy the

lop-sided nature of a single parameter criterion by expanding into a wave energy or wave power

form may be inefficient in containing all erosive components of vessel wave wake, so that some

vessel types might still be over-restricted when limits are based upon experimentally determined

thresholds to erosion.

0

50

100

150

200

250

10 100 1000

Maximum Wave Height (mm)

Ele

va

ted

Tu

rbid

ity (

NT

U)

Water Depth = 1.0 metres


Full Scale Bank Erosion Data

Figure 3.5 – Elevated turbidity as a function of maximum wave height



0

50

100

150

200

250

0.1 1 10

Period of the Maximum Wave (s)

Ele

va

ted

Tu

rbid

ity (

NT

U)




Figure 3.6 – Elevated turbidity as a function of wave period

0

50

100

150

200

250

1 10 100 1000

Energy of the Maximum Wave (J/m)

Ele

va

ted

Tu

rbid

ity

(N

TU

)




Figure 3.7 - Elevated turbidity as a function of wave energy



0

50

100

150

200

250

1 10 100 1000

Power of the Maximum Wave (W/m)

Ele

va

ted

Tu

rbid

ity (

NT

U)




Figure 3.8 – Elevated turbidity as a function of wave power

0

50

100

150

200

250

0.01 0.1 1 10

HmT2

(ms2)

Ele

va

ted

Tu

rbid

ity

(N

TU

)




Figure 3.9 – Elevated turbidity as a function of HmT2



On the other hand, where limits have been determined by desktop (or otherwise incomplete) studies

they have not always been appropriate for all relevant vessel types. For instance, some high-speed

vessels, particularly those claimed to possess “wave wake reducing characteristics” (which are more

strictly often only wave height reducing characteristics by way of high length-displacement ratio)

have the potential to satisfy an apparently reasonable energy criterion but still cause erosion. Prime

examples of this are the various “low-wave wake” ferries operating in Sydney and Brisbane. Such

vessels have been found capable of generating wave periods considerably in excess of the existing

waterway wave climate (up to 4-5 times longer), but with low accompanying height when travelling

at high speed. The possibility of this is illustrated graphically in Figure 3.4, where the most

commonly applied wave wake parameters of energy, power and wave height reduce gradually at

high speeds.

An energy criterion may be reasonably defined for commercial vessels operating at any speed.

However, if commercial vessels are forced to operate at low speed then it is possible that smaller,

high-speed recreational craft may meet the energy criterion yet still create an erosive wave wake,

since aspects of geomorphic response may be linked more to wave period than height.

The particular sensitivity of sheltered waterways to incident wave period led to the belief that

multiple criteria were the key to any operating limits. High-speed, deep water wave period was

shown to remain essentially constant whereas other parameters decreased steadily as speed increased

well into the high-speed range. The other benefit of including a period-based criterion is the strong

correlation at high speed between the period of the maximum wave and a vessel‟s waterline length.

This is an important part of the criterion simplification process necessary for eventual application in

practical situations.

Macfarlane et al (2008) proposed a multi-criteria approach to ensure that all the erosive components

of high-speed, small craft wave wake are accounted for. Two criteria were derived, to be applied

jointly; an energy criterion (based on Equation 3.5) and a wave period criterion. This latter criterion

uses the relationship between wave period and waterline length, as discussed in Section 3.4, to

determine an upper limit of waterline length.

In coastal engineering terms, energy states tend to jump in orders of magnitude, not in incremental

percentages. In many respects the push by designers to improve the wave wake characteristics of

their vessels by a nominal modest percentage is likely to be somewhat inconsequential in erosion

terms. Generally, a design either will or will not work – small changes to design parameters like hull

spacing, waterline beam, draught, etc are unlikely to turn an erosive design into an acceptable one.

These double criteria highlight the design dilemma – lengthening the hull to reduce the

displacement-length ratio and hence wave height will only increase the wave period.

3.10 WATERWAY TYPES

As noted, unless the overall numbers of parameters are reduced and those chosen are simplified, it

would be impossible to derive any useful erosion criteria. From discussions with coastal engineers, it

is believed that there may only need to be as few as two, but probably three, bank types studied. All

are natural depositional landforms. Artificial shorelines are more diverse and should be engineered

to withstand an appropriate wave climate, although that has not always been the case.



The first is typical of very sheltered waterways that may experience little or no tidal range and do

not have a beach structure since the energy climate is not wave dominated. These low-lying banks

tend to be characterised by cohesive muds and substantial sediment trapping riparian or saltmarsh

vegetation (usually not mangroves). The sediments are fine enough to be transported in suspension

by currents and these deposits may represent the accumulation of sediment over extensive

(geomorphological) time-scales. Once such natural features are disturbed by erosion the damage is

effectively permanent.

The second is characterised by some resemblance to a beach, usually consists of fine sand and

muddy sediment (so-called muddy sands) but may not have formed entirely (or at all) in response to

wave driven processes. Cohesive soil banks may lie at the head of the beach, such that the beach

represents an adjustment of the bank which has been exposed to wind wave, tidal and flood

influences. These banks can withstand some wave action, but they often do not have the support of

riparian vegetation. The upper bank structure can be severely weakened if the riparian vegetation is

removed due to anthropogenic intervention such as land development (such as the Brisbane and

Parramatta Rivers), cattle grazing (such as along the Patterson River in NSW, and many others) and

tidal influx (Brisbane River).

The third possible bank type is what we would regard as a sandy beach, with fine to coarse grained

sand (sandy muds or clean sand) that extend well above and below the mean waterline. These

beaches are normally found in open areas in bays and at estuary mouths where there is a substantial

wind wave climate and/or strong tidal flows. It is this third bank type which may be surplus to the

study requirements, as they are somewhat dynamic by nature and are already reasonably understood

by current coastal engineering science. Energetics are typically such that true beaches are not

susceptible to wave wake from small craft although some may be affected by larger, high-speed

ferries.

For small craft operating in sheltered waterways, only the first two bank types are considered to be

of prime importance.



4. WIND AND VESSEL WAVE PREDICTIONS FOR THE SWAN RIVER

4.1 INTRODUCTION

As outlined in Section 1, the primary aim of this study is to conduct an indicative investigation into

the comparative impact of wash from vessels and wind waves.

As discussed in Sections 2.1.3 and 3.6, the energy in wind waves tends to be more height-dependent

than period dependent and wave energy is equally a function of both wave height and period.

However, wave period possibly has a greater effect in sheltered and semi-enclosed waters,

particularly on shorelines with sloping beaches. Intact shorelines have natural, dynamic

mechanisms to withstand wind waves, but the sudden introduction of vessel wash containing much

longer-period waves may upset the balance. In the case of the upper reaches of the Swan River (for

example, alongside Ashfield Parade near Ron Courtney Island) with its steep, compacted soil banks,

it is most likely that both wave height and period are equally to blame for erosion. The wind wave

climate experienced by the upper reaches of the Swan River is extremely modest in comparison to

the more open areas of the lower reaches (such as the shoreline alongside Mounts Bay Road

between Quarry Point and the Narrows).

Therefore, the logical first step of this investigation is to quantify the characteristics of the wind

waves for the sites of most interest. The following two example sites were as nominated by the SRT

(refer Section 4.2 for more details):

Lower Reach “Open Water”: alongside Mounts Bay Road between Quarry Point and the

Narrows (31˚58.3´S, 115˚50.0´E)

Upper Reach “Sheltered Water”: alongside Ashfield Parade, Ashfield, near Ron Courtney

Island (31˚55.3´S, 115˚56.4´E)

The next step in the investigation is to design a suitable test program of typical vessels that are

known to frequent these two example sites on the river (refer Section 4.3) and identify their range of

operating speeds. This was conducted in collaboration with the SRT and deliberately included both

commercial and recreational vessels that are known to be presently operating in these regions.

Predictions of the characteristics of the vessel generated waves were then undertaken for each

vessel, vessel speed and lateral distance (refer Section 4.4). The predictions of the vessel generated

waves are then compared against the predictions of the wind generated waves in Section 4.5.

4.2 WIND WAVE PREDICTIONS

All calculations of wind wave parameters on the Swan River presented within this sub-section were

undertaken by the Centre for Marine Science and Technology (CMST) at Curtin University of

Technology. The SRT was keen on involving CMST within this study, firstly so they could

contribute their expertise in this field and secondly to provide local knowledge of the specific sites

of interest.

The calculations performed by CMST, as determined in consultation with the SRT, are as shown in

Table 4.1.



Wind wave

parameters

calculated

Wave height

Wave period

Wave energy

Wave power

Swan river sites Lower Reach - Mounts Bay Road near Quarry Point

Upper Reach - Ashfield Parade near Ron Courtney Island

Wind conditions Three different wind conditions toward the upper end of measured

wind speeds

Table 4.1 – Wind wave predictions undertaken

4.2.1 Wind Analysis

For the wind wave analysis, it was desirable to use measured wind data that most closely

approximated the wind conditions at the two sites chosen. For both the upper reach (Mounts Bay

Road) and lower reach (Ashfield Parade) sites, the Bureau of Meteorology measured data from

Melville Water was the most appropriate data to use in terms of closeness to the site and

measurement height.

Measured wind data was provided by the Bureau of Meteorology from the Inner Dolphin pylon

anemometer on Melville Water, with details as shown in Table 4.2.

Anemometer

location

Melville Water BOM station 009091

Inner Dolphin Pylon (31˚59.4´S, 115˚49.9´E, 7.5 metres above

Mean Low Water)

Time period Annual data from 2002 (when anemometer was installed) to 2008

inclusive

Data supplied Half-hourly data, giving:

mean wind speed over previous 10 minutes

maximum wind gust speed over previous 10 minutes

mean wind direction over previous 10 minutes

Table 4.2 – Bureau of Meteorology wind wave data

Mean wind speed and gust wind speed data over the 2002 – 2008 period are plotted in Figures 4.1

and 4.2.



0

5

10

15

20

25

30

35Mean wind speed - Melville Water

Time period 2002 - 2008 inclusive

knots

Figure 4.1 – Mean wind speed 2002 to 2008

0

10

20

30

40

50

60Gust wind speed - Melville Water


knots

Figure 4.2 – Gust wind speed 2002 to 2008



The highest mean wind speed is 32.9 knots, while the highest gust speed is 50.2 knots. Bearing in

mind that waves take some time to generate, we shall use the mean wind speed as the basis for wind

wave calculations.

Wind direction is also important, as each site will have a wind direction to which it is most exposed,

which is normally the direction with the longest fetch. In addition, the highest wind speeds are

concentrated at certain directions (for Perth: south-westerly and easterly in summer and south-

westerly through to north-westerly in winter).

The wave power imparted to the shoreline also depends on the relative angle between the shoreline

and the wave direction. Therefore, the wind conditions that produce the largest wave height are

generally not the same as those that produce the largest wave power against the shoreline.

4.2.2 Site Analysis

The two sites are shown in red in Figures 4.3 and 4.4 (screenshots from chart AUS-898, reproduced

with permission from the WA DPI). The angle between the wave crests and shoreline (discussed

in Section 4.2.3) is also shown.

Figure 4.3 – Location of lower reach site (Mounts Bay Road) on Swan River

Wind

75°

165°

α



Figure 4.4 – Location of upper reach site (Ashfield Parade) on Swan River

The relevant factors governing wind-generated waves for each site are as given in Tables 4.3 and

4.4, taken from chart AUS-898. Only wind directions that will produce appreciable waves at the

sites are included.

Lower Reach - Mounts Bay Road

Wind direction Fetch Average width Effective fetch

(SPM 1977 p3-

31)

Average

depth over

fetch

180˚ 1120m 1500m 1070m 5m

195˚ 1770m 1000m 1270m 5m

210˚ 1690m 1000m 1240m 5m

225˚ 1640m 750m 1050m 4m

240˚ 1370m 500m 780m 4m

Table 4.3 – Relevant factors governing wind generated waves for lower reach site

103°

Wind

13°

α



Upper Reach - Ashfield Parade

Wind direction Fetch Average width Effective fetch

(SPM 1977 p3-

31)

Average

depth over

fetch

90˚ 50m 250m 50m 3m

105˚ 50m 250m 50m 3m

120˚ 50m 250m 50m 3m

135˚ 50m 150m 50m 3m

150˚ 150m 120m 130m 3m

165˚ 200m 120m 150m 3m

180˚ 600m 100m 210m 3m

190˚ 900m 100m 230m 3m

Table 4.4 – Relevant factors governing wind generated waves for upper reach site

4.2.3 Wind Wave Hindcasting

Characteristics of the predicted wind waves are calculated according to the Shore Protection Manual

(1984), p3-55. The formulae used are:

4/3

2

2/1

24/3

22

530.0tanh

00565.0

tanh530.0tanh283.0

A

A

AA

U

gd

U

gF

U

gd

U

gH (4.1)

8/3

2

3/1

28/3

2

833.0tanh

0379.0

tanh833.0tanh54.7

A

A

AA

U

gd

U

gF

U

gd

U

gT (4.2)

The above formulae, along with formulae covered in Section 3.7, were utilised to calculate the wind

wave height, period, energy and power over the entire period for which measured wind data were

available.

For the calculation of wave power the shoreline angle was taken to be 75° for the lower reach site

(Mounts Bay Road) and 13° for the upper reach site (Ashfield Parade) (refer Figures 4.3 and 4.4).

The water density was assumed to be 1025kg/m3 at both sites of interest.

4.2.4 Calculated Wind Waves for the Lower Reach

Calculated wind wave parameters at the lower reach (Mounts Bay Road) over the period of

measured wind data are shown in Figures 4.5, 4.6 and 4.7.



0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45Wind wave height - Mounts Bay Rd


metr

es

Figure 4.5 – Wind wave height 2002 to 2008 at lower reach site

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2Wind wave period - Mounts Bay Rd


seconds

Figure 4.6 – Wind wave period 2002 to 2008 at lower reach site



0

50

100

150

200

250

300Wind wave power per metre of shoreline - Mounts Bay Rd


Watt

s p

er

metr

e

Figure 4.7 – Wind wave power 2002 to 2008 at lower reach site

Statistics on the wave conditions produced at the lower reach site (Mounts Bay Road) over the 7

years of measured data are as given in Table 4.5:

Maximum wave height 0.41 metres

Maximum wave period 1.8 seconds

Maximum wave energy per wave 1070 Joules/m

Corresponding wind speed at time of maximum wave height 32.9 knots

Corresponding wind direction at time of maximum wave height 220º

Maximum wave power to shoreline 267 Watts/m

Corresponding wind speed at time of maximum wave power 31.9 knots

Corresponding wind direction at time of maximum wave power 190º

Table 4.5 – Predicted wind wave characteristics at lower reach site

The 1% and 10% exceedence values are given in Table 4.6. As an example, a wind wave height of

0.24m was only exceeded 1% of the time over the 7 years of hindcast data.



1% exceedence 10% exceedence

Wind wave height 0.24m 0.14m

Wind wave period 1.5s 1.3s

Wind wave energy per wave 250 Joules/m 63 Joules/m

Wind wave power

transmitted to shoreline

68 W/m 16 W/m

Table 4.6 – Predicted 1% and 10% exceedence values for wind wave characteristics at lower reach

site

4.2.5 Calculated Wind Waves for the Upper Reach

Calculated wind wave parameters at the upper reach (Ashfield Parade) over the period of measured

wind data are shown in Figures 4.8, 4.9 and 4.10.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

Wind wave height - Ashfield Parade


metr

es

Figure 4.8 – Wind wave height 2002 to 2008 at upper reach site



0

0.2

0.4

0.6

0.8

1

1.2

1.4Wind wave period - Ashfield Parade


seconds

Figure 4.9 – Wind wave period 2002 to 2008 at upper reach site

0

1

2

3

4

5

6

7Wind wave power per metre of shoreline - Ashfield Parade


Watt

s p

er

metr

e

Figure 4.10 – Wind wave power 2002 to 2008 at upper reach site



Statistics on the wave conditions produced at the upper reach site (Ashfield Parade) over the 7 years

of measured data are given in Tables 4.7:

Maximum wave height 0.18 metres

Maximum wave period 1.1 seconds

Maximum wave energy per wave 77 Joules/m

Corresponding wind speed at time of maximum wave height 31.9 knots

Corresponding wind direction at time of maximum wave height 190º

Maximum wave power to shoreline 6.3 Watts/m

Corresponding wind speed at time of maximum wave power 27.0 knots

Corresponding wind direction at time of maximum wave power 160º

Table 4.7 – Predicted wind wave characteristics at upper reach site

The 1% and 10% exceedence values are given in Table 4.8:

1% exceedence 10% exceedence

Wind wave height 0.09m 0.04m

Wind wave period 0.8s 0.6s

Wind wave energy per wave 9.2 J/m 1.0 J/m

Wind wave power

transmitted to shoreline

1.3 W/m 0.3 W/m

Table 4.8 – Predicted 1% and 10% exceedence values for wind wave characteristics at upper reach

site



4.3 VESSEL TEST PROGRAM

The list of typical commercial and recreational vessels, along with their respective details and

typical range of operating speeds, were supplied by the SRT. The recreational vessels selected for

this study were not chosen on brand but on basic length and weight that were deemed to be similar

to a majority of vessels in that size range.

Following a series of preliminary predictions of the likely wave characteristics each vessel may

generate, the AMC recommended to the SRT a range of suitable speeds that should be investigated

for each vessel. These specific speeds took into account a number of factors, including:

the maximum speed each vessel was capable of achieving,

the existing speed limit(s) for the two example sites,

the speeds that were likely to generate the most damaging waves for each of the two

nominated example sites.

The resultant test program is shown in Table 4.11.

Another variable that plays a significant role in the characteristics of vessel generated waves is the

lateral distance between the sailing line of the vessel and the shoreline of interest (as discussed in

Section 2.3). The effect of varying this distance is specifically dealt with in the study (see Sections

4.4 and 4.5), but a nominal typical distance was selected for each of the two sites of interest, as

shown in the test program (Table 4.1).

Normal academic practice is to relate this wave propagation distance to vessel waterline length,

however, it can sometimes be more appropriate to nominate a fixed distance from the sailing line.

This more practical approach will benefit the smaller vessels, as the distance of the measurement

point from the sailing line is a greater percentage of the waterline length. For instance, if we were to

compare the predictions for the 24.9 metre long MV River Lady and 5.0 metre Haines Hunter

Prowler 520, the measurement point relative to the sailing line in the upper reach (where y = 23m) is

0.9L for the MV River Lady and 4.6L for the Prowler 520. The wash of the Prowler 520 would have

attenuated more than that of the MV River Lady by the time it reached the measurement point.

This is a suitable method of comparison, as the width of the river at any point is the same for all

vessels and a wash height measure is needed relative to that fixed width, not relative to individual

vessel waterline lengths. Increasing the sailing line distance from the shore is a method often

employed by vessel operators to decrease wash impact, but where the distance from the shoreline is

limited to the centre of a river, benefit must be given to the wash of smaller vessels and its greater

relative attenuation.

4.4 VESSEL WAVE PREDICTIONS

The predictions of the vessel generated waves are undertaken using AMC‟s wave wake database

which has been specifically developed over the past 15 years to compare the characteristics of the

waves generated by one hull form in a direct and fair manner against another, or many other, hull

forms. In all cases, the characteristics of the vessel generated waves have been obtained through

scale model experiments conducted within AMC‟s controlled environment maritime hydrodynamic

facilities. This prediction tool currently contains wave wake data for approximately 100 different

hull forms.

AMC Search Report 09G17 Version 2.0

Page 50


v

Condition Vessel Description Vessel Name Monohull or Length Beam L / B Displacement L / Volume1/3 Draught Maximum Site Description Lateral Vessel

Number Catamaran Waterline Waterline Loaded Ratio Speed Distance Speed

L B v y v

(m) (m) (t) (m) (knots) (m) (knots)

1 Commercial Vessel Hull Form A MV Captain Cook Monohull 31.0 6.5 4.77 130 6.07 1.8 11 Lower Reach - Open Water 100 9

100 10

100 11

2 Commercial Vessel Hull Form B Sea Cat Catamaran 23.9 8.4 2.85 61 6.02 1.4 21 Lower Reach - Open Water 100 8

100 10

100 14

100 18

100 21

3 Commercial Vessel Hull Form C MV River Lady Monohull 24.9 5.0 4.98 30 7.95 0.8 10 Lower Reach - Open Water 100 6

100 8

100 10

4 Commercial Vessel Hull Form D Star Flyte Express Monohull 38.92 8.5 4.58 151 7.25 1.8 26 Lower Reach - Open Water 100 10

100 14

100 18

100 22

100 26

5 Recreational Vessel Hull Form A Haines Hunter Prowler 680 Monohull 6.6 2.1 3.14 2.4 4.89 0.5 18* Lower Reach - Open Water 100 8

Intermediate Length 100 10

100 14

100 18

6 Recreational Vessel B Mustang Sports Cruiser 2800 Monohull 7.6 2.4 3.17 3.5 4.96 0.55 18* Lower Reach - Open Water 100 8

100 10

100 14

100 18

7 Commercial Vessel Hull Form C MV River Lady Monohull 24.9 5.0 4.98 30 7.95 0.8 10 Upper Reach - Sheltered Water 23 5

23 6

23 7

23 8

23 9

8 Recreational Vessel Hull Form A Haines Hunter Prowler 520 Monohull 5.0 1.8 2.78 1.0 4.96 0.32 18* Upper Reach - Sheltered Water 23 5

Small Length 23 6

23 7

23 8

23 9


Intermediate Length 23 6

23 7

23 8

23 9


Long Length 23 6

23 7

23 8

23 9

11 Recreational Vessel Hull Form B Mustang Sports Cruiser 2800 Monohull 7.6 2.4 3.17 3.5 4.96 0.55 18* Upper Reach - Sheltered Water 23 5

23 6

23 7

23 8

23 9

VESSEL DETAILS SITE DETAILS

Vessel Test Program

* The maximum speed will depend on a number of factors. The value shown is the maximum speed investigated in the present study. Table 4.11 – Vessel test program



The primary aim of the database is to scale all vessels, and corresponding wave wake measurements,

to a series of prescribed input parameters. The database has two alternative manners in which to

base a comparison:

Constant Waterline Length: where the waterline length (L), vessel speed (v) and distance

from the vessel sailing line (y) are the input variables, or;

Constant Vessel Displacement: where vessel displacement ( ), vessel speed (v) and distance

from the vessel sailing line (y) are the input variables.

Once the required input parameters are specified the scale factor for each individual data set is then

calculated by dividing the requested full scale waterline length by the model scale waterline length

(for constant L option), or full scale displacement by model scale displacement (for constant

option). This scale factor can then be used to obtain the corresponding model scale speed for the

requested full scale speed for comparison. Each vessel‟s data set contains and wave period values

for a wide range of corresponding model speeds. If required, an interpolation between the two

closest model scale speeds is undertaken to obtain the corresponding values for and wave period.

Using this , knowledge about the wave decay rate of the divergent waves is used to obtain the

corrected maximum wave height at any requested lateral distance from the sailing line. The scale

ratio for each individual data set can then be used to obtain the full scale corrected maximum wave

height and wave period for the corresponding vessel waterline length or displacement.

The resultant wave energy is then calculated using Equation 3.5. As discussed in Section 3.7, when

waves are impacting directly onto a shoreline, the power calculated is that which impacts onto the

shoreline. When the wave crests are at an angle to the shoreline, the wave power transmitted per

metre of shoreline is as given by Equation 3.9. For the lower reach site, the wave crest angle to

the shoreline was taken to be 0 degrees (parallel to the shoreline) as it is assumed that this is quite

feasible given the relatively large width of the waterway. This will result in a conservative approach.

For the upper reach site, the wave crest angle to the shoreline was taken to be approximately 55

degrees as it is assumed that most vessels will aim to travel roughly parallel to the shoreline within

the relatively narrow width of the waterway (the angle of 54.7 degrees is the propagation angle of

divergent waves, refer Figure 2.7).

It is then a simple process to plot the results in a useful manner. An example of the output from the

database is shown in Figure 4.11 where maximum wave height is plotted as a function of vessel

displacement.

The accuracy of the database predictions have been validated against experimental data obtained

from carefully executed full scale trials by AMC. These full scale experiments have been conducted

on a wide variety of commercial and recreational vessels (approximately 20 in total), of which some

of the results have been published; Macfarlane and Cox 2004, Macfarlane 2006, Macfarlane 2009,

and Macfarlane, Cox and Bradbury 2008.



0

50

100

150

200

250

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Ma

xim

um

Wa

ve

Hei

gh

t (

mm

)

Vessel Displacement (tonnes)


Monohulls

Multihulls

L = 6.6 metres

v = 7.0 knotsy = 23 metres

Figure 4.11 – Example of predictions from the AMC wave wake database

It should be noted that these vessel wave predictions are based on the assumption that all vessel

speeds are considered sub-critical depth Froude numbers, or „deep‟ water (refer Section 2.2.1). This

is entirely valid for vessel operations in the upper reaches of the Swan River as all vessel speeds up

to and including the current maximum speed limit of 8 knots are sub-critical depth Froude numbers

(assuming a nominal water depth of 3 metres).

This is not the case for all vessel speeds investigated for the lower reaches of the Swan River where

only those vessel speeds below approximately 11 knots are considered to be sub-critical. Thus it is

likely that some of the selected vessels will operate at trans-critical and super-critical speeds in the

lower reaches, thus are likely to generate different wave patterns that in some cases may be

potentially damaging to the shoreline (refer Section 2.2).

In order to evaluate just how much of an issue this may present this current study, a series of

preliminary predictions of the likely wave characteristics were conducted for each of the six vessels

of interest (refer Table 4.1). In summary, it was found that two of these vessels are likely to generate

waves whose characteristics may not be closely matched to those predicted using the AMC‟s wave

wake database (sub-critical wave pattern). These two vessels are the larger commercial vessels that

are capable of operating at speeds well in excess of 11 knots (Sea Cat and Star Flyte Express). More

precise predictions could be obtained using alternative methods, particularly for these vessels

operating at trans-critical speeds (which is generally undesirable as this is where the most potentially

damaging waves are generated). From experience, wave periods at trans-critical vessel speeds will

be longer than sub-critical speeds, thus the waves generated are likely to contain more energy.

The other two commercial vessels considered in the lower reaches (MV Captain Cook and MV

River Lady) are only capable of sub-critical speeds (for the given water depths) and thus the wave

prediction technique adopted should be ideal.



The two recreational vessels selected for this part of the study are capable of operating at trans-

critical and super-critical speeds in the lower reaches, however, the preliminary predictions indicated

that the maximum energy of their generated waves are likely to be lower than that of the maximum

wind generated waves expected in the lower reaches. This preliminary study included several

alternative prediction techniques, including full scale experimental data conducted by AMC on

similar sized vessels travelling at trans-critical and super-critical speeds (ie similar water depths and

vessel speeds). Thus, it is considered acceptable to predict the wave characteristics generated by

these vessels at all speeds of interest using the AMC‟s wave wake database.

4.5 COMPARISON OF WIND AND VESSEL GENERATED WAVES

4.5.1 Comparison of Wind and Vessel Waves for the Lower Reach

In this section, the predictions of the wind and vessel waves for the lower reach are compared in

order to determine the effect from the two primary variables of (a) vessel speed and (b) lateral

distance between the sailing line of the vessel and the shoreline.

The effect of varying vessel speed on the height, period, energy and power of the maximum vessel

generated waves for the lower reach are presented graphically in Figures 4.12 to 4.16. In all of these

figures the lateral distance between the sailing line of the vessel and the shoreline has remained

constant at 100 metres.

In Figure 4.12, the height of the maximum waves (y-axis) generated by a range of vessels is plotted

as a function of vessel speed (x-axis). For the lower reach site the range of vessel speeds

investigated is relatively large, ranging from 6 knots up to 26 knots (where applicable). Also shown

in this figure are the predicted heights of the expected wind waves (maximum, 1% exceedence, 10%

exceedence), indicated by the various dashed horizontal lines. As can be seen, only two of the six

vessels investigated are predicted to generate wave heights that exceed the predicted maximum wind

wave height. Not surprisingly, these two vessels are the two largest vessels that can reach speeds in

excess of 11 knots and the maximum vessel wave heights occur at the higher speeds.

In Figure 4.13, the period of the maximum waves generated by the same range of vessels is plotted

as a function of vessel speed. As was the case with the previous figure, the corresponding wind

wave limits (maximum, 1% exceedence, 10% exceedence) are indicated by the various dashed

horizontal lines. As can be seen, almost all vessels at all vessel speeds investigated are likely to

generate waves that exceed the period of the predicted maximum wind waves.

As discussed in Sections 3.6 and 3.7, the use of either wave height or wave period alone cannot

possibly reflect the true erosion potential of a vessel‟s wave wake, thus it is recommended that either

wave energy or power (or multiple criteria) be adopted. Both the wave height and period are

required to calculate these quantities.

In Figure 4.14, the energy of the maximum waves generated by the same range of vessels is plotted

as a function of vessel speed. Here it can clearly be seen that the energy of the waves generated by

the two largest vessels travelling at higher speeds (in excess of 11 knots) are much greater than the

energy of the maximum wind waves (up to approximately 25 times greater at the worst case). In

order to see what is happening closer to the energy of the maximum wind waves the range for the y-

axis has been reduced in Figure 4.15, where it can be seen that all six vessels generate maximum

waves that have less energy than the maximum wind waves for vessel speeds up to and including 10

knots. Both of the recreational craft and the MV River Lady remain below this limit at all speeds

investigated.



In Figure 4.16, the power of the maximum waves generated by the same range of vessels is plotted

as a function of vessel speed. As expected, a similar trend to that found for wave energy is

displayed.

The next series of figures examine the effect on the height, energy and power of the maximum

waves generated by the same range of vessels when the lateral distance between the sailing line of

the vessel and the shoreline is systematically varied. In each set of these figures, the vessel speed

remains constant and the lateral distance varies from 25 metres up to 200 metres. It should be noted

that the period of the vessel generated waves should not change with lateral distance, thus this has

not been shown graphically (wave period will remain approximately as shown in Figure 4.13).

In Figure 4.17 the height of the maximum waves generated by this range of vessels is plotted as a

function of lateral distance for the constant vessel speed of 10 knots. As expected, the wave heights

reduce as lateral distance increases. Similar plots for the wave energy and power are provided in

Figures 4.18 and 4.19 respectively.

A similar set of plots are provided for the vessel speed of 14 knots in Figures 4.20 to 4.22 and for 18

knots in Figures 4.23 to 4.25.

There are a number of key findings that can be concluded from the figures presented in this sub-

section for vessel operations in the lower reach site:

The two „typical‟ recreational craft included in this part of this investigation (Haines Hunter

Prowler 680 and the Mustang Sports Cruiser 2800) are unlikely to generate waves that

exceed the energy or power of the maximum wind waves, provided that these vessels do not

operate at speeds close to 10 knots within a lateral distance of approximately 50 metres of

the shoreline.

The two large commercial vessels capable of speeds in excess of 11 knots (Sea Cat and Star

Flyte Express) generate large waves whose energy and power far exceeds that of the

expected maximum wind waves when these vessels operate at speeds of 14 knots and

higher. Increasing the lateral distance reduces their energy and power, however they remain

in excess of the maximum wind waves even at the largest lateral distance investigated (200

metres).

Of the other two commercial vessels investigated, the MV River Lady appears to generate

waves that do not exceed the energy or power of the maximum wind waves at any

achievable vessel speeds or lateral distances. The MV Captain Cook meets these limits

when operating at 10 knots, provided the lateral distance is 75 metres or greater, but exceeds

these limits when operating at 11 knots.

As discussed in Section 4.4, the two largest and fastest commercial vessels are capable of operating

at trans-critical and super-critical vessel speeds, thus the prediction technique adopted by this study

may not be ideal in these cases. The predictions presented in this chapter are based on the

assumption that the vessels are operating at sub-critical speeds. When a vessel operates at trans-

critical and super-critical speeds different wave patterns are generated, as discussed earlier (Section

2.2 and page 52). From experience, the prediction technique adopted may estimate slightly higher

wave heights but lower wave periods than actually generated at trans-critical and super-critical

speeds. Thus, the wave energy (and power) generated by these two commercial vessels could be less

than that presented here, but not much less. It is also possible that the wave energy could be even

higher.



4.5.2 Comparison of Wind and Vessel Waves for the Upper Reach

In this section, the predictions of the wind and vessel waves for the upper reach are compared in

order to determine the effect from the two primary variables of (a) vessel speed and (b) lateral

distance between the sailing line of the vessel and the shoreline.

The effect of varying vessel speed on the height, period, energy and power of the maximum wave is

presented graphically in Figures 4.26 to 4.29. In all of these figures the lateral distance between the

sailing line of the vessel and the shoreline has been fixed at 23 metres.

In Figure 4.26, the height of the maximum waves generated by a range of vessels is plotted as a

function of vessel speed, which varies from 5 to 9 knots for the upper reach site. This figure also

shows the predicted values for the expected wind waves for this site (maximum, 1% exceedence,

10% exceedence), indicated by the various dashed horizontal lines. As can be seen, all five vessels

investigated are predicted to generate wave heights that exceed the predicted maximum wind wave

height when travelling at a speed of 8 knots or greater (at the lateral distance of 23 metres).

In Figure 4.27, the period of the maximum waves generated by the same range of vessels is plotted

as a function of vessel speed. As might be expected for sheltered waterways like the site near Ron

Courtney Island, the maximum period of the wind waves is very low and as a result it is not

surprising to find that almost all vessels at all vessel speeds investigated are likely to generate waves

that exceed the maximum period of the predicted wind waves, as discussed in Sections 2.1.3 and

3.6.

In Figure 4.28, the energy of the maximum waves generated by the same range of vessels is plotted

as a function of vessel speed. Here it can clearly be seen that the energy of the waves generated by

all six vessels exceed the energy of the maximum wind waves at 7, 8 and 9 knots.

In Figure 4.29, the power of the maximum waves generated by the same range of vessels is plotted

as a function of vessel speed. In this case almost all vessels exceed the power of the maximum wind

waves at 6, 7, 8 and 9 knots. The only exception is the MV River Lady which is predicted to

generate a maximum wave having very similar power to that of the maximum wind waves at a speed

of 6 knots.

The next series of figures (Figures 4.30 to 4.38) examine the effect on the height, energy and power

of the maximum waves generated by the same range of vessels when the lateral distance between the

sailing line of the vessel and the shoreline is systematically varied. In each set of these figures, the

vessel speed remains constant and the lateral distance varies from 10 metres up to 100 metres. As

previously mentioned, the period of the vessel generated waves should not change with lateral

distance, thus this has not been shown graphically (wave period will remain approximately as shown

in Figure 4.27). It should be recalled that the period of the waves generated by all of these vessels

will exceed the period of the predicted maximum wind waves at speeds of 6 knots and greater.

In Figure 4.30 the height of the maximum waves generated by this range of vessels is plotted as a

function of lateral distance for the constant vessel speed of 5 knots. As expected, the wave heights

reduce as lateral distance increases and all are below the maximum wind waves. Similar plots for

the wave energy and power are provided in Figures 4.31 and 4.32 respectively where it can be seen

that the values for almost all cases are below the maximum wind waves. The only exception is the

case where some of these vessels operate at the very small lateral distance of approximately 10

metres from the shoreline where the resultant wave power exceeds that of the maximum wind



waves. It is understood that some vessels may operate at such low lateral distances from the shore

in some locations due to the location of the channel, for example at the Windan Bridge.

A similar set of plots are provided for the vessel speed of 6 knots in Figures 4.33 to 4.35 and for 8

knots in Figures 4.36 to 4.38. At the vessel speed of 6 knots the energy of the maximum wave from

all vessels is below that of the maximum wind waves except at the smallest lateral distance

investigated (y = 10 metres). However, the power of the maximum wave for many of the vessels

exceeds that of the maximum wind waves over a significant lateral distance.

It is clear from Figures 4.37 and 4.38 that both the energy and power of the maximum waves for all

vessels far exceeds that of the maximum wind waves over the entire range of lateral distances

investigated for the vessel speed of 8 knots.

There are a number of key findings that can be concluded from the figures presented in this sub-

section for vessel operations in the upper reach site:

For a vessel speed of 8 knots, the energy and power of the maximum waves for all vessels

far exceeds that of the maximum wind waves over the entire range of lateral distances

investigated.

For a vessel speed of 5 knots, the energy and power of the maximum waves for all vessels

fall below that of the maximum wind waves provided a minimum lateral distance of 20

metres is maintained.

For a vessel speed of 6 knots, the energy of the maximum wave from all vessels is below that

of the maximum wind waves except at the smallest lateral distance investigated (y = 10

metres). However, the power of the maximum wave for many of the vessels exceeds that of

the maximum wind waves over a significant lateral distance.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 5 10 15 20 25 30

Max

imu

m W

ave

He

igh

t (

m)


MV Captain Cook

Sea Cat

MV River Lady

Star Flyte Express

Haines Hunter Prowler 680

Mustang Sports Cruiser 2800

Wind Waves - Maximum

Wind Waves - 1% Exceedence


Effect of Varying Vessel Speed (v)Lower Reach - Open Water

Lateral Distance = 100m

Figure 4.12 – Wave height as a function of vessel speed (lower reach)



0

1

2

3

4

5

6

0 5 10 15 20 25 30

Pe

rio

d o

f th

e M

axim

um

Wav

e

(s)


MV Captain Cook

Sea Cat

MV River Lady

Star Flyte Express








Figure 4.13 – Wave period as a function of vessel speed (lower reach)

0

5000

10000

15000

20000

25000

30000

0 5 10 15 20 25 30

Ene

rgy

of

the

Max

imu

m W

ave

(

J/m

)


MV Captain Cook

Sea Cat

MV River Lady

Star Flyte Express








Figure 4.14 – Wave energy as a function of vessel speed (lower reach)



0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30

Ene

rgy

of

the

Max

imu

m W

ave

(

J/m

)


MV Captain Cook

Sea Cat

MV River Lady

Star Flyte Express








Figure 4.15 – Wave energy as a function of vessel speed (lower reach)

0

500

1000

1500

2000

2500

3000

0 5 10 15 20 25 30

Po

we

r o

f th

e M

axim

um

Wav

e

(W

/m)


MV Captain Cook

Sea Cat

MV River Lady

Star Flyte Express








Figure 4.16 – Wave power as a function of vessel speed (lower reach)



0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0 20 40 60 80 100 120 140 160 180 200

Max

imu

m W

ave

He

igh

t (

m)

Lateral Distance (metres)

MV Captain Cook

Sea Cat

MV River Lady

Star Flyte Express



Wind Waves - Maximum (Lower Reach)

Wind Waves - 1% Exceedence (Lower Reach)


Effect of Varying Lateral Distance (y)Lower Reach - Open Water


Figure 4.17 – Wave height as a function of lateral distance, v = 10 knots (lower reach)

0

500

1000

1500

2000

2500

3000

0 20 40 60 80 100 120 140 160 180 200

Ene

rgy

of

the

Max

imu

m W

ave

(

J/m

)


MV Captain Cook

Sea Cat

MV River Lady

Star Flyte Express








Figure 4.18 – Wave energy as a function of lateral distance, v = 10 knots (lower reach)



0

50

100

150

200

250

300

350

400

450

500

0 20 40 60 80 100 120 140 160 180 200

Po

we

r o

f th

e M

axim

um

Wav

e

(W

/m)


MV Captain Cook

Sea Cat

MV River Lady

Star Flyte Express








Figure 4.19 – Wave power as a function of lateral distance, v = 10 knots (lower reach)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0 20 40 60 80 100 120 140 160 180 200

Max

imu

m W

ave

He

igh

t (

m)


Sea Cat

Star Flyte Express











0

2000

4000

6000

8000

10000

12000

0 20 40 60 80 100 120 140 160 180 200

Ene

rgy

of

the

Max

imu

m W

ave

(

J/m

)


Sea Cat

Star Flyte Express









0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 20 40 60 80 100 120 140 160 180 200

Po

we

r o

f th

e M

axim

um

Wav

e

(W

/m)


Sea Cat

Star Flyte Express











0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 20 40 60 80 100 120 140 160 180 200

Max

imu

m W

ave

He

igh

t (

m)


Sea Cat

Star Flyte Express









0

10000

20000

30000

40000

50000

60000

0 20 40 60 80 100 120 140 160 180 200

Ene

rgy

of

the

Max

imu

m W

ave

(

J/m

)


Sea Cat

Star Flyte Express











0

1000

2000

3000

4000

5000

6000

0 20 40 60 80 100 120 140 160 180 200

Po

we

r o

f th

e M

axim

um

Wav

e

(W

/m)


Sea Cat

Star Flyte Express









0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 1 2 3 4 5 6 7 8 9 10

Max

imu

m W

ave

He

igh

t (

m)


MV River Lady








Effect of Varying Vessel Speed (v)Upper Reach - Sheltered Water


Figure 4.26 – Wave height as a function of vessel speed (upper reach)



0.0

0.5

1.0

1.5

2.0

2.5

0 1 2 3 4 5 6 7 8 9 10

Pe

rio

d o

f th

e M

axim

um

Wav

e

(s)


MV River Lady










Figure 4.27 – Wave period as a function of vessel speed (upper reach)

0

200

400

600

800

1000

1200

1400

0 1 2 3 4 5 6 7 8 9 10

Ene

rgy

of

the

Max

imu

m W

ave

(

J/m

)


MV River Lady










Figure 4.28 – Wave energy as a function of vessel speed (upper reach)



0

20

40

60

80

100

120

140

160

180

0 1 2 3 4 5 6 7 8 9 10

Po

we

r o

f th

e M

axim

um

Wav

e

(W

/m)


MV River Lady










Figure 4.29 – Wave power as a function of vessel speed (upper reach)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0 10 20 30 40 50 60 70 80 90 100

Max

imu

m W

ave

He

igh

t (

m)


MV River Lady





Wind Waves - Maximum (Upper Reach)

Wind Waves - 1% Exceedence (Upper Reach)


Effect of Varying Lateral Distance (y)Upper Reach - Sheltered Water


Figure 4.30 – Wave height as a function of lateral distance, v = 5 knots (upper reach)



0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80 90 100

Ene

rgy

of

the

Max

imu

m W

ave

(

J/m

)


MV River Lady










Figure 4.31 – Wave energy as a function of lateral distance, v = 5 knots (upper reach)

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70 80 90 100

Po

we

r o

f th

e M

axim

um

Wav

e

(W

/m)


MV River Lady










Figure 4.32 – Wave power as a function of lateral distance, v = 5 knots (upper reach)



0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 10 20 30 40 50 60 70 80 90 100

Max

imu

m W

ave

He

igh

t (

m)


MV River Lady











0

20

40

60

80

100

120

140

0 10 20 30 40 50 60 70 80 90 100

Ene

rgy

of

the

Max

imu

m W

ave

(

J/m

)


MV River Lady













0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80 90 100

Po

we

r o

f th

e M

axim

um

Wav

e

(W

/m)


MV River Lady











0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 10 20 30 40 50 60 70 80 90 100

Max

imu

m W

ave

He

igh

t (

m)


MV River Lady













0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50 60 70 80 90 100

Ene

rgy

of

the

Max

imu

m W

ave

(

J/m

)


MV River Lady











0

20

40

60

80

100

120

140

0 10 20 30 40 50 60 70 80 90 100

Po

we

r o

f th

e M

axim

um

Wav

e

(W

/m)


MV River Lady













5. CONCLUSIONS

A study has been conducted to provide an indication on the comparative impact of wash from boat

and wind waves. The results from this study are available for the development of actions to


Canning Riverpark. It is understood that this Strategy has recommended that “low wash zones” be

created in order to control shoreline erosion from vessel generated waves. As a result, the SRT

requested that one of the primary aims of this study was to assist this process by providing a rational

basis for the creation of such zones and provide preliminary advice on the efficacy of establishing

blanket low speed zones in the Swan and Canning Rivers.

This study has presented predictions for both vessel and wind wave characteristics from which it is

possible to identify those vessel speeds where bank erosion is likely to occur as a result of vessel

operations for two distinct regions (lower and upper reaches). For example, for the upper river site

(Ashfield Parade) it is clear that shoreline erosion is very likely as a result of vessel generated waves

where a blanket speed limit of either 8 or 9 knots (or greater) is imposed as the energy and power of

the maximum waves for all vessels far exceed that of the maximum wind waves over the entire

range of lateral distances investigated.

It is also clear that a reduction in vessel speed, down to 6 or 5 knots, should dramatically reduce the

potential for erosion. For example, at a speed of 5 knots the energy and power of the maximum

waves for all vessels are likely to be below that of the maximum wind waves, provided a minimum

lateral distance of 20 metres is maintained between the vessel sailing line and the shore.

The situation is less well defined for the lower reach site (Mounts Bay Road). This is primarily

because there are two relatively large commercial vessels that are capable of operating at higher

speeds which are likely to generate waves that possess far greater energy and power than the

predicted maximum wind waves. The waves generated by the other four vessels investigated at this

site are not as great a concern as they are either much smaller craft and/or they are unable to operate

at speeds that will generate such large waves. For any blanket speed limit to be effective it must be

specified for the „worst offender(s)‟, and thus if this were undertaken in this case it is likely to be

overly restrictive for other vessel classes.

Regardless of what type of regulatory criteria is selected, the aim should be to deliver a result that is

close to a final solution, but not necessarily the final solution. Absolute answers come with a

disproportionate cost and the recommended option is to derive operating criteria that make the

problem manageable and then adjust these criteria over time with the benefit of problem monitoring.

In addition, it is acknowledged that the practical policing of any regulatory criteria is as important as

the criteria themselves, as any wash-limiting operating criteria that is too complicated to be applied

and policed are of little benefit to the community.



6. REFERENCES

Bradbury, J., Lower Gordon River erosion – history and lessons learned,

http://www.dpiw.tas.gov.au/inter.nsf/WebPages/LJEM-6J289T?open. Accessed 18 July 2009.

Bradbury, J., Lower Gordon River turbidity monitoring, April 2003 - December 2004,

http://www.dpiw.tas.gov.au/inter.nsf/WebPages/LJEM-6J289T?open. 18 July 2009.

Bradbury, J., Cullen, P., Dixon, G. and Pemberton, M., Monitoring and management of streambank

erosion and natural revegetation of the lower Gordon River, Tasmanian Wilderness World Heritage

Area, Australia, Environmental Management, Vol. 19, No. 2, pp. 259-272, 2005

Cox, G., Sex, lies and wave wake, Proceedings of the RINA International Conference on the

Hydrodynamics of High Speed Craft – Wake Wash and Motions Control, London, United Kingdom,

2000

Dand, I. W., Dinham-Peren, T. A. and King, L., Hydrodynamic aspects of a fast catamaran

operating in shallow water, Proceedings of the RINA International Conference on the

Hydrodynamics of High Speed Craft, London, United Kingdom, 1999

Gross, A. and Watanabe, K., On blockage correction, Report of Performance Committee, Appendix

3, Proceedings of 13th International Towing Tank Conference, 1972

Havelock, T. H., The propagation of groups of waves in dispersive media, with application to waves

produced by a travelling disturbance, Proceedings, Royal Society of London, England, Series A, pp.

398-430, 1908

Kelvin (Sir William Thomson), On ship waves, Proceedings of the Institute of Mechanical

Engineers, pp. 409 – 433, 1887

Kogoy, P., Tidal wave of protest, Sun-Herald (Sydney), 20th

of September, 1998

Lesleighter, E. J., Hawksbury River - The effect of speedboat activities on bank erosion, NSW

Department of Public Works, Report No. 106, 1964

Macfarlane, G.J., The measurement and assessment of sub-critical vessel generated waves, Master

of Philosophy Thesis, Australian Maritime College, 2002.

Macfarlane, G. J., Correlation of prototype and model wave wake characteristics at low Froude

numbers, RINA Transactions, Part A2, International Journal of Maritime Engineering, 2006

Macfarlane, G. J., Correlation of prototype and model scale wave wake characteristics of a

catamaran, SNAME Marine Technology, Volume 46, Number 1, January 2009.

Macfarlane, G.J. and Cox, G., The development of vessel wave wake criteria for the Noosa and

Brisbane Rivers in Southeast Queensland, Fifth International Conference on Coastal Environment

2004 – Environmental Problems in Coastal Regions, Alicante, Spain, 2004.

http://www.dpiw.tas.gov.au/inter.nsf/WebPages/LJEM-6J289T?open

http://www.dpiw.tas.gov.au/inter.nsf/WebPages/LJEM-6J289T?open



Macfarlane, G. J. and Cox, G., An introduction to the development of rational criteria for assessing

vessel wash within sheltered waterways, IMarEST Journal of Marine Design and Operations, Part

BII, 2007.

Macfarlane, G.J., Cox, G. and Bradbury, J., Bank erosion from small craft wave wake in sheltered

waterways, RINA Transactions, Part B, International Journal of Small Craft Technology, 2008.

Macfarlane, G.J. and Renilson, M.R., Wave wake – A rational method for assessment. Proceedings

of the International Conference on Coastal Ships and Inland Waterways, Royal Institution of Naval

Architects, London, United Kingdom, 1999.

Macfarlane, G.J., and Renilson, M.R., When is low wash low wash – An investigation using a wave

wake database, Proceedings of the International Conference on Hydrodynamics of High-Speed Craft

– Wake Wave wake and Motions Control, Royal Institution of Naval Architects, London, United

Kingdom, 2000.

Millward, A., Shallow water and channel effects on ship wave resistance at high sub-critical and

super-critical speeds, Trans RINA, London, UK, Vol. 125, 1983, pp. 163-169

Nanson, G.C., von Krusenstierna, A., Bryant, E.A. and Renilson, M.R., Experimental measurements

of river bank erosion caused by boat-generated waves on the Gordon River, Tasmania”, Regulated

Rivers, Research and Management Vol. 9, 1-14, 1994.

Newman, J. N., Marine hydrodynamics, MIT Press, 1977

Pattiaratchi, C. and Hegge, B., Impact of ferry and large vessel traffic on Swan River foreshore

erosion, Centre for Water Research Report WP 452 CP, University of Western Australia, 1990.

U.S. Army Corps of Engineers Coastal Engineering Research Centre, Shore Protection Manual,

1984.

U.S. Army Corps of Engineers Coastal Engineering Research Centre, Shore Protection Manual,

1987.

U.S. Army Corps of Engineers Coastal Engineering Research Centre, Coastal Engineering Manual,

Volumes I, II, III, IV, http://140.194.76.129/publications/eng-manuals/ Accessed 18 July 2009.

Renilson, M. R. and Lenz, S., An investigation into the effect of hull form on the wake wave

generated by low speed vessels, 22nd American Towing Tank Conference, pp. 424 – 429, August

1989

Sorensen, R. M., Waves generated by model ship hull, Journal of the Waterways, Harbours Division,

Proceedings of the American Society of Civil Engineers, Volume 95, No. WW4, pp. 513 – 538,

1969

Sorensen, R.M., Water waves produced by ships, The Journal of the Waterways Harbours and

Coastal Engineering Division, Proceedings of the American Society of Civil Engineers, Vo. 99. No.

WW2, pp.245-256, May 1973(a)

http://140.194.76.129/publications/eng-manuals/



Sorensen, R. M., Ship-generated waves, Advances in Hydroscience, Vol. 9, pp. 49 – 84, Academic

Press, New York, 1973(b)

Swan River Trust, Boating management strategy for the Swan Canning Riverpark, January 2009

Tuck, E. O., Sinkage and trim in shallow water of finite width, Schiffstechnik, Bd. 14, 1967

von Krusenstierna, A., River bank erosion by boat-generated waves on the lower Gordon River,

Tasmania, Master of Science Thesis, Department of Geography, University of Wollongong, 1990.

Warren, N., Wash from fast ferries, Ship and Boat International, The Royal Institution of Naval

Architects, pp. 11-13, April 1991.

WA Department of Planning and Infrastructure, Maritime Cartographic Services, Chart 898- Swan

and Canning Rivers, Edition 5, December 2005.

Young, I. R., Wind generated ocean waves, Elsevier Ocean Engineering Book Series, Vol. 2, 1999

AUSTRALIAN MARITIME COLLEGE - City of Prior Lake · 2016-04-29 · AUSTRALIAN MARITIME COLLEGE...

Documents

Transcript of AUSTRALIAN MARITIME COLLEGE - City of Prior Lake · 2016-04-29 · AUSTRALIAN MARITIME COLLEGE...