Eden Wharf Extension - Preliminary Dredge Plume Modelling

Eden Wharf Extension -

Preliminary Dredge Plume

Modelling

Report

October 2016

Level 17, 141 Walker St

North Sydney NSW 2060

Australia

301020-07698-001

www.advisian.com

Department of Primary Industries

Eden Wharf Extension - Preliminary

Dredge Plume Modelling

Report

Advisian ii

Synopsis

This report provides an update on the preliminary dredge plume modelling carried out in August

2015 for inclusion in the EIS to include updated dredge quantities and assess the impact of two

different dredging methodologies as considered in the Final Dredge Plan (September 2016).

Disclaimer

This report has been prepared on behalf of and for the exclusive use of Department of Primary

Industries, and is subject to and issued in accordance with the agreement between Department of

Primary Industries and Advisian.

Advisian accepts no liability or responsibility whatsoever for it in respect of any use of or reliance

upon this report by any third party.

Copying this report without the permission of Department of Primary Industries and Advisian is not

permitted.

Project No: 301020-07698-001 – Eden Wharf Extension - Preliminary

Dredge Plume Modelling: Report

Rev Description Author Review

Advisian

Approval Date

A Draft for Internal

Review

C Adamantidis

B Williams

C Adamantidis

B Draft

C Adamantidis

B Williams

C Adamantidis

C Final

C Adamantidis

B Williams

C Adamantidis

24/10/16




Report

Advisian iii

Table of Contents

Executive Summary ............................................................................................................................ v

1 Introduction ........................................................................................................................... 1

1.1 Background .............................................................................................................. 1

1.2 Consultant Engagement ...................................................................................... 1

1.3 Scope .......................................................................................................................... 1

1.3.1 Exclusions ................................................................................................................. 2

2 Physiography ......................................................................................................................... 3

2.1 Bathymetry ............................................................................................................... 3

2.2 Tides and Currents ................................................................................................. 3

2.3 Waves ......................................................................................................................... 4

2.4 Wind ............................................................................................................................ 4

2.5 Sediment Properties ............................................................................................ 10

3 Model Setup ........................................................................................................................ 12

3.1 Model Description ............................................................................................... 12

3.2 Model Domain ...................................................................................................... 12

3.3 Simulation Period ................................................................................................. 14

3.4 Dredge Plan ............................................................................................................ 15

3.4.1 Location and Extent of the Dredge Pocket ...............................................15

3.4.2 Preferred Dredge Method ...............................................................................15

3.4.3 Option 1 BHD (Base Case) ...............................................................................17

3.4.4 Option 2 BHD + TSHD (Single handling method) ..................................17

3.4.5 Option 3 BHD + TSHD (Partial Double-handling method) .................18

3.5 Assumptions in dredge plume dispersal ..................................................... 19




Report

Advisian iv

3.5.1 Sediment Classes and Settling Velocity ......................................................19

3.5.2 Deposition..............................................................................................................20

3.5.3 Erosion .....................................................................................................................20

3.5.4 Flocculation ...........................................................................................................21

3.5.5 Parameters Summary.........................................................................................21

3.6 Model Scenarios ................................................................................................... 21

3.6.1 Scenario 1 – Backhoe Dredging ....................................................................21

3.6.2 Scenario 3 - Trailer Suction Hopper Dredging.........................................22

3.7 Plume Dispersion Methodology ..................................................................... 25

3.7.1 Scenario 1 (BHD) .................................................................................................26

3.7.2 Scenario 3 (Combination BHD and TSHD) .................................................26

4 Model Results ...................................................................................................................... 28

4.1 Presentation of Model Results ........................................................................ 28

4.1.1 Visual Impact of TSS...........................................................................................28

4.2 Model Results – Percentile Exceedance Maps ........................................... 30

4.3 Impact of Sediment Disposal at Offshore Spoil Ground ....................... 40

5 Discussion and Recommendations.............................................................................. 42

Appendix List

Model results for E and SE wind directions Appendix A:




Report

Advisian v

Executive Summary

This report provides an update on the preliminary dredge plume assessment carried out in August

2015 for the Eden Port Expansion project (WorleyParsons 2015a, Report no. 301020-07698-MA-

REP-0002). It considers an increase in the volume to be dredged and explores the relative impact

of an additional dredging scenario, where Trailer Suction Hooper Dredging (TSHD) is considered in

conjunction with Backhoe Dredging (BHD), as described in the Final Dredging Plan (Report no.

301020-07698-MA-PLN-0002).

The generation, transport and fate of dredge plume material has been simulated at Port of Eden

using a state-of-the-art 3D hydrodynamic and sediment transport model. Two cases have been

considered in the dredge plume modelling:

Use of a medium-sized backhoe dredger (BHD) to dredge the entire dredge footprint in

conjunction with two barges;

Used of a Trailer Suction Hopper Dredge (TSHD) to deliver dredged materials to the

offshore disposal site. Most of the material would be bulked out direct by the TSHD, with

material in areas inaccessible or containing material too hard for a TSHD dredged by the

BHD and loaded into hopper barge(s). The hopper barge(s) would bottom dump the

material in deep water within the dredging footprint in an area accessible by the TSHD (i.e.

there would be double-handling of material within the dredge footprint).

Multiple sediment sources have been included at various locations in the model, to provide a more

realistic simulation of the likely sources of sediment from the dredging and the sediment disposal

when compared with the earlier preliminary dredge plume modelling. However, the analysis

presented herein remains preliminary, as no field data are yet available for model calibration or

validation.

The preliminary far-field dredge plume simulations suggest that, for the dredge methodology

considered, the footprint of areas that exceed 10mg/l above background will not occur beyond the

boundaries of the dredge pocket, for both the BHD only and combined BHD/TSHD scenarios with

double-handling. As wind-driven and tidal currents within Snug Cove are rather low, the dredge

plume is only slowly dispersed in to the wider marine environment.

Assessment of the potential for sediment dispersal from the OSG to Snug Cove has also been

carried out. Maximum near-surface simulated TSS concentrations did not exceed 1mg/L beyond

approximately 2 km from the offshore spoil disposal site. This implies that, once added to TSS

concentrations generated by the local dispersal of fine sediments from the dredge plant, TSS

concentrations will not exceed ANZECC environmental trigger levels at (10mg/L) at the mussel

farm.

A field data collection program has been commenced as of September 2016, which will provide

data for calibration and validation of a detailed dredge plume assessment.




Report

Advisian 1

1 Introduction

1.1 Background

The Port of Eden is the southernmost “declared port” in NSW. The Port services the south coast of

NSW including the towns of Bega, Merimbula, Bombala and Cooma. The Port of Eden is also home

to a large fishing fleet and services the needs of a variety of importers and exporters, including

woodchip, pine logs, explosives and general cargo.

The Port of Eden has a deep water harbour with three existing wharves, including a multi-purpose

(Navy) wharf, a private berth operated by a woodchip exports company, and a breakwater berth

located in Snug Cove.

Substantial funds have been committed by various Government authorities operating at the

Federal, State and Shire level to expand the capacity of the breakwater wharf to accept cruise ships

up to 300m in length, thus facilitating access to the cruise ship industry at Eden and thereby

increasing Tourism at Eden.

1.2 Consultant Engagement

Advisian have been engaged by NSW Department of Primary Industries – Lands to provide design

services for the development of the new wharf and breakwater extension.

This report addresses, in a preliminary fashion, concerns about the impact dredge operations may

have on a nearby shellfish farm, which is sensitive to the level of fine suspended sediments within

the water column. The impact is assessed through the numerical simulation of the expected

advection and dispersal of the dredge plume during excavation of the new berth pocket, which is

deepened to accommodate the larger class of cruise vessel.

The position and plan alignment of the mussel farm is given in Australian Hydrographic Service

Chart A00192 (Figure 1-1).

1.3 Scope

The purpose of this preliminary dredge plume dispersion study is to assess the dispersal and fate

of dredge spoil material within Eden, and the potential for dredge spoil material to settle on the

shell fish farm adjacent to Snug Cove.

The study is a simplified assessment, assuming a few meteorological conditions that are of

relevance to the dispersal of any dredge plume material towards the shellfish farm. The preliminary

studies are designed to provide a first-estimate on the expected concentration of suspended

material within Snug Cove from dredge plant operations.

It is expected that, following this preliminary investigation, a more thorough numerical modeling

analysis would be conducted to calibrate the model against observed wave and current activity at




Report

Advisian 2

the site, to more properly assess the range of environmental forcing at Eden and the associated

occurrence probabilities, and to undertake an appropriate level of statistical analysis on the

simulation results to define the range of expected TSS concentrations and exceedance probabilities

during the dredge season.

This report updates previous modelling undertaken in August 2015 (WorleyParsons 2015a) to

account for a larger volume of dredging than previously envisaged (so as to accommodate cruise

ships up to 325 m length), as well as updated dredge methodology as documented in the Eden

Breakwater Wharf Extension – Final Dredging Plan (Report no. 301020-07698-MA-PLN-0002).

Figure 1-1 - Location of mussel farm (“Marine Farm”) relative to Snug Cove and Eden Breakwater.

Source: Chart A00192.

1.3.1 Exclusions

The following are excluded from the preliminary modelling:

Detailed statistical analysis of the numerical model results

Collection of field data

Calibration of the model to field data

Consideration of the full range of environmental conditions.




Report

Advisian 3

2 Physiography

2.1 Bathymetry

Bathymetry of Twofold Bay is described by AUS Charts 192 and 806. Detailed hydrographic survey

data have also been made available by Crown Lands, dated May 2015. This information has been

combined within the sediment plume model to provide the most accurate possible description of

the surrounding seafloor topography and adjacent coastline.

Incorporated into the model bathymetry is the proposed dredge footprint as provided in the Final

Dredging Plan (Report no. 301020-07698-MA-PLN-0002).

2.2 Tides and Currents

Tide conditions for the Eden area are derived from a Global Inverse Tide reanalysis model derived

from satellite altimeter data. The TPXO 8.0 global model of ocean tides best-fits, in a least-squares

sense, the Laplace Tidal Equations and along-track averaged data from TOPEX/Poseidon and Jason

satellite constellations (Egber, Bennet & Foreman, 1994; Egbert and Erofeeva, 2002). The tides are

provided as complex amplitudes of earth-relative sea-surface elevation for eight primary (M2, S2,

N2, K2, K1, O1, P1, Q1), two long period (Mf, Mm) and three non-linear (M4, MS4, MN4) harmonic

constituents. These constituents are used to provide boundary conditions for the tidal

hydrodynamic model.

Previous analysis of tidal elevations and tidal currents are given in Cardno (2011) and GHD (2013).

Tidal planes at Port of Eden are given in Table 2-1. Peak spring tidal currents are in the region of

0.03m/s at the south-west tip of the breakwater. This compares with a 5-year ARI wind-driven

current magnitude of 0.4m/s in the vicinity of Snug Cove and the breakwater (GHD, 2013).

Table 2-1 Tidal Planes at Eden, relative to Chart Datum. Reproduced from Australian National Tide

Tables (2009).

Highest Astronomical Tide (HAT) 2.1m

Mean Higher High Water (MHHW) 1.8m

Mean Lower High Water (MLHW) 1.2m

Mean Sea Level (MSL) 1.0m

Mean Higher Low Water (MHLW) 0.8m

Mean Lower Low Water (MLLW) 0.2m




Report

Advisian 4

2.3 Waves

Wave conditions within Twofold Bay consist of (a) swell waves generated within the Pacific Ocean

basin, and (b) wind-sea wave conditions generated by local squall activity generated by weather

fronts passing over Eden. In general, wave energy is low within Twofold Bay. GHD (2013) report 1-

year ARI wave heights at the south-west tip of the breakwater to be in the region of <1.1m (wind-

sea) and ~2.0m (swell). Associated peak wave periods for wind-sea are in the region of 4 seconds,

whilst those for swell are in the region of 9 - 12s (MHL, 2007; GHD, 2013).

Although wind-sea Hm0 during squalls are typically of the same magnitude as longer-crested swell

waves refracting and diffracting in from the Pacific, the potential for sediment resuspension during

dredge operations will be governed by swell wave activity due to much greater orbital velocities at

the sea floor (~10m below Chart Datum at the dredge pocket) . Locally generated wind-sea waves

are therefore excluded from this analysis.

Seasonal mean swell wave parameters obtained from analysis of a global wave hindcast model are

given in Table 2-2 for position 37.0°S, 150.5°E. The Hm0 given is the seasonal mean value. The

imposed wave direction is taken as the energy-weighted peak wave direction averaged across the

season. The peak wave period is derived from a fitting relationship between Hm0 and Tp observed in

the data for waves occurring within ±45° of the seasonal wave direction.

Table 2-2: Seasonally representative integral wav parameters derived from analysis of NOAA

WaveWatch3 global hindcast model at location 37.0°S, 150.5°E.

Season Hm0 (m) Tp (s) MWD (°TN)

Summer 1.73 8.4 62.6

Winter 1.72 9.3 77.0

The swell wave parameters are imposed as uniform boundary wave conditions in the wave model

and propagated to shore.

2.4 Wind

Wind data for the region is taken from Bureau of Meteorology station Merimbula Airport AWS

(36.91°S, 149.90°E) for a period of 10 years encompassing 1998 to 2008. Prior to analysis wind

speeds were adjusted to 10m above mean sea level using a 1/7th

power law relationship.

Merimbula Airport AWS was selected over other weather stations due to the proximity of the

weather station to Eden (18km), the completeness of data records (close to 100%) and the relative

modernity of the observations.

It is noted that Cardno (2011) and GHD (2013) produced a wind climate by blending a combination

of CFS wind fields used in the NOAA WaveWatch3 wave hindcast model and wind observations

made at Green Cape, which is a similar distance to the south of Eden. It is considered unnecessarily




Report

Advisian 5

complex to reproduce this wind climate for the purpose of dredge plume simulations at this initial

stage, noting that the blended Green Cape wind field did not successfully reproduce measured

wind-sea wave heights at the EWS observation point (Cardno, 2011).

Figure 2-1 to Figure 2-4 show seasonal wind roses for spring, summer, autumn and winter. In

general, the most common directions are south-west and north-east. However the most sensitive

sectors in terms of dredge plume impacts will be for winds approaching from the north-east to

south-east, as these will induce currents in the water column that will move dredge plume material

towards the mussel farm.




Report

Advisian 6

Figure 2-1: Wind rose at Merimbula Airport AWS for Autumn, 1998 – 2008.




Report

Advisian 7

Figure 2-2: Wind rose at Merimbula Airport AWS for Summer, 1998 – 2008.




Report

Advisian 8

Figure 2-3: Wind rose at Merimbula Airport AWS for Autumn, 1998 – 2008.




Report

Advisian 9

Figure 2-4: Wind rose at Merimbula Airport AWS for Winter, 1998 – 2008.




Report

Advisian 10

2.5 Sediment Properties

A summary of the sediment characteristics are provided in Table 2-3 based on the results and

findings as outlined in the Eden Breakwater Wharf Extension, Sediment Sampling and Analysis Plan.

(Australasian Marine Associates, 15/8/2016, Rev 3). The sediments are dominated by the sand

fraction with a mean of 83% sand, and minor components of clay and silt (maximum of 12% fines).

A variable percentage of gravels was recorded, up to 32%.

Table 2-3: Summary of Sediment Particle Size Analysis

Cla

y

(<2µ

m)

(%)

Sil

t (2

–

60

µm

)

(%)

San

d

(60

µm

–

2m

m)

(%)

Gra

vel

(>2m

m)

(%)

Co

bb

les

(>6 c

m)

(%)

Number of

samples 44 44 44 44 44

Mean 5 2 83 10 <1

Standard

Deviation 2 1 10 9 -

Maximum 8 4 96 32 <1

Minimum 1 <1 61 1 <1

The sediment properties at the site of the proposed dredging were estimated by examining the

individual vibrocores within the dredge footprint. That analysis found that the Clay and Silt

components are approximately 5% each of the sediment composition throughout the dredge

footprint, as illustrated in Figure 2-5. The updated modelling has therefore assumed a sediment

composition of 5% silt, 5% clay and 90% sand throughout the dredge area.




Report

Advisian 11

Figure 2-5 – Percentage of sand, silt and clay at each location within the dredge footprint as indicated

in the map above.

000000000 250250250250250250250250250 500500500500500500500500500

metresmetresmetresmetresmetresmetresmetresmetresmetres

D1D2

D3D4

D5

D7

D6

05

101520253035404550556065707580859095

100

D1 D2 D3 D4 D5 D6 D7

% o

f se

dim

en

t co

mp

osi

tio

n

Sediment source location

Clay

Silt

Sand




Report

Advisian 12

3 Model Setup

3.1 Model Description

The dredge plume dispersion simulations are undertaken Delft3D, which is a fully integrated

computer software suite for a multi-disciplinary approach and 3D computations for coastal, river

and estuarine areas. It can carry out simulations of flows, sediment transports, waves, water quality,

morphological developments and ecology.

The FLOW module, which is used to simulate tidal and wind-driven currents, is a multi-dimensional

(2D or 3D) hydrodynamic (and transport) simulation program which calculates non-steady flow and

transport phenomena that result from tidal and meteorological forcing on a curvilinear, boundary

fitted grid. For the 3D simulations considered in this study, the vertical grid is defined following the

σ co-ordinate approach. That is, the model is split in to a number of layers that are defined as a

constant percentage of water depth. Four layers have been used in the hydrodynamic model, with

a fifth “bed layer” describing sedimentation.

As a refinement on the previous dredge plume modelling, the water quality model used to

describe the dredge plume behaviour was D-WAQ PART, a 3D random walk particle tracking

model which is part of the Delft 3D suite and is coupled to the hydrodynamic model. Re-

suspension of material is included within the model predictions, with all fines released from the

dredging and suspended during the dredge disposal settling through the water column and being

available for later re-suspension as based on a critical shear-stress formulation.

3.2 Model Domain

A domain-decomposition approach is used to downscale wave and hydrodynamic processes

occurring on the continental shelf (grid resolution order of 1km) to Eden Breakwater (grid

resolution 20m). Four domains are used, with simulated variables dynamically passed between

each domain:

A ‘coarse’ domain, horizontal grid resolution 540m by 540m, driven by tidal elevation

boundaries derived from TPXO 8.0 global tide inversion model;

An ‘outer’ medium resolution domain of grid cell size 180m by 180m;

An ‘inner’ medium resolution domain of grid cell size 60m by 60m, encompassing Twofold

Bay;

A ‘fine’ model of grid cell size 20m by 20m, encompassing Snug Cove, the breakwater

extension, the dredge pocket, and the adjacent mussel farm.

Each model domain was implemented in 3D with four σ-layers in the vertical, each layer

comprising 25% of the local water column thickness.

Figure 3-1 and Figure 3-2 show the model domains and bathymetry used in the dredge plume

model.




Report

Advisian 13

Figure 3-1: Extent of Delft3D model domains comprising Eden hydrodynamic model. Location of OSG

dredge spoil disposal site in model (see Section 4.3) shown by white asterisk.

OSG




Report

Advisian 14

Figure 3-2: Expanded view of model domain and bathymetry around Snug Cove.

3.3 Simulation Period

All simulations are run for a total of

2 days to achieve numerical stability within the model from cold start, and allow wind-

driven currents to achieve quasi-equilibrium;

1 day to allow local quasi-equilibrium of the dredge plume in the immediate vicinity of the

dredger;

29 day lunar cycle during which simulation results are output and analysed for statistics.

A 29 day lunar cycle is considered long enough to allow a reasonable estimate of the dredge

plume dispersal to the wider environment. The ultimate selection of simulation time is a

compromise between accuracy (longer is better) and computational efficiency (shorter is better).

The key criterion is that some form of quasi-equilibrium is achieved with the dispersal of fine

suspended sediment at the site. As a number of simplifying assumptions are made about the




Report

Advisian 15

dredge plant operations (see Section 3.6) to maximise environmental impact to the mussel farm,

there is no need to simulate the entire dredge season at this level of assessment.

3.4 Dredge Plan

3.4.1 Location and Extent of the Dredge Pocket

Figure 3-3 shows the location and footprint of the dredge pocket at -10.5m CD.

Since the Concept Dredging Plan some refinements have been made to the design of the dredge

basin including:

The berth pocket has been defined (refer to Drawing 301020-07698-MA-DWG-3002 Rev A

- Figure 3-3)

Some 231,500 m3 (net) of dredged materials is expected to require dredging. This includes

an average over-dredge allowance of 0.5m over an area of 116,500 m2 but excludes

contingency from sedimentation since the time of the most recent hydrographic survey

(2015).

Approximately 6,000 m3 of rock or rock like materials is included in the above figure.

3.4.2 Preferred Dredge Method

The Concept Dredging Plan compared the use of a Backhoe Dredge (BHD), Cutter Suction Dredge

(CSD) or Trailer Suction Hopper Dredger (TSHD) working independently over the entire dredge

footprint.

The report recommended a medium sized BHD for the following main reasons:

Availability of three vessels of this type operating in south east Australia and New Zealand

The three BHD’s are expected to be able to dredge most, if not all, of the fractured rock

that may be encountered without secondary pre- treatment.; and

The material can be dredged at or near its natural moisture content to minimise the

generation of plumes.

Subsequently to the Concept Dredging Plan, a final design workshop was undertaken to compare

the relative advantages and disadvantages for a dual dredge spread (using a BHD in combination

with a TSHD) compared to a single medium sized BHD operating independently. The principal

objective of the workshop was to establish if the environmental assessment process, related

studies and tender documents are to allow for potential use of a TSHD to undertake part of the

dredge works.

For all options, work would be carried out on a 24 hours, 7 days a week basis. A shorter duration

time is beneficial in reducing any risk of harm to the environment, regardless of the dredging

method.




Report

Advisian 16

Figure 3-3 – Revised dredge pocket extent




Report

Advisian 17

A description of three dredging options considered in the Final Dredge Plan is provided below.

Option 1 and Option 3 have been modelled, with Option 3 likely to represent the worst case

environmental scenario with the highest potential for adverse impacts from sediment dispersal.

3.4.3 Option 1 BHD (Base Case)

The backhoe dredger excavates material and places into the accompanying hopper barge(s). The

material is then transported and disposed by bottom dumping in the marine disposal area. This

method generally requires two barges to facilitate continuous dredging. For a disposal ground

within 6.0 nautical miles, each barge would require a hopper capacity of 900 to 1200 m3. The BHD

removes material at or near its natural moisture content to minimise the generation of plumes.

The BHD is an essential component of the dredge project as it will be the only effective means of

removing rock.

Typical plant likely to be used to fulfil the above dredge method includes:

Medium BHD currently in use in the region (equivalent to approx. 200 tonne excavator and

up to 10m3 bucket capacity);

Two appropriately sized barges (i.e. 900 m3 to 1,200 m

3) in size either towed by tugs or

self-propelled; and

Appropriately sized tugs or work boats to assist with dredger positioning, move barges

and take barges offshore as necessary.

In addition to the base case option of using a BHD for dredging and hopper barge to transport the

material to the offshore spoil ground, there are two additional methods which are considered

viable/feasible for dredging work at Eden. These are described below:

3.4.4 Option 2 BHD + TSHD (Single handling method)

This involves bulk dredging of OTR materials using the TSHD and use of the BHD and hopper

barges to remove rock and sediments near existing structures and in areas where the TSHD cannot

manoeuvre. It is envisaged the TSHD would undertake most of the dredging of Zone 2 and the

BHD would dredge Zone 1 and a portion of Zone 2, including areas alongside the existing

Multipurpose Jetty and Breakwater Wharf. In this scenario both dredges would transport dredged

materials directly to the Offshore Disposal Site (ODS). The BHD would require two barges in the

order of 900 to 1200m3 for efficient operation and one tow vessel. A towed barge would achieve

up to 6 knots in open waters. Accordingly, return transit times of 2 to 3 hours would be expected

for each barge movement.

The single handling method:

Reduces barge movements to ODS, increases the safety of dredge operations and reduces

the risk to other users.

Has the potential to reduce the execution time by up to 30% to approximately 10 weeks;

and




Report

Advisian 18

Reduces the overall cost of the works by up to 20% compared to the base case method.

Typical plant likely to be used to fulfil the above dredge methods include:





self-propelled;


and take barges offshore as necessary; and

TSHD such as the MV Brisbane (2,900m3 hopper capacity).

3.4.5 Option 3 BHD + TSHD (Partial Double-handling method)

In order to maximise overall safety during the works, this method uses the TSHD to deliver most (if

not all dredged materials to the ODS. Most of the material would be bulked out direct by the

TSHD. Material in areas inaccessible or containing material too hard for a TSHD would be dredged

by the BHD and loaded into hopper barge(s). The hopper barge(s) would bottom dump the

material in deep water within the dredging footprint in an area accessible by the TSHD. Any large

rock that cannot be handled by the TSHD would either be disposed of elsewhere within the site

(e.g. along the lee side of the breakwater) or at the ODS.

The partial double-handling method:

Minimises barge movements to ODS, maximises the safety of dredge operations and

minimises the risk to other users.

Has the potential to reduce the execution time by up to 50% to approximately 6 weeks;

Reduces the overall cost of the works by up to 30% compared to the BHD option; and

Requires a temporary stockpile site within the dredge footprint. A suitable site which

could be considered in the environmental impact assessment would be the lee-side of the

existing breakwater in between the fender line of the proposed wharf and the toe line of

the breakwater. This site is in approximately 8m of water, and is sufficiently clear from

moorings and other marine structures to allow continued access to these assets.

Typical plant likely to be used to fulfil the above dredge methods include:





self-propelled;


and take barges offshore as necessary; and

TSHD such as the MV Brisbane (2,900m3 hopper capacity).




Report

Advisian 19

3.5 Assumptions in dredge plume dispersal

The TSHD dredges by pulling a drag head across the seafloor, mobilising the seabed material into

the water column, while sucking the subsequent water and suspended material up through the

drag head. This mixture of water and dredged material is then pumped through external pipes

into a hopper. Heavier particles settle to the bottom of the hopper and lighter particles remain in

suspension within the hopper. Once the hopper is full of water, the hopper is typically allowed to

overflow with the low-density mixture overflowing back into the sea, while continuing to capture

the denser material within the hopper. This reduces the number of trips required from the dredge

area to the offshore disposal area and therefore reduces the overall duration of the dredging

campaign. Sediment is introduced into the water column from the TSHD near the seabed at the

drag head and from propeller wash, and near the surface from the overflow.

In comparison, sediment from the BHD is introduced near the bed only at the dredge area. For this

reason, Option 3 is considered to present the greatest potential for the generation of sediment

plumes in Snug Cove, as sediment is introduced both at the bed and near the surface, and this

option involves double-handling of material.

The dredge plume simulations consider the dredge pocket excavated to a depth of 10.5m below

CD, as this represents the most conservative scenario in terms of environmental impact.

3.5.1 Sediment Classes and Settling Velocity

Sediment plume dispersion studies typically require three to five particle size classes to be defined,

based on their distinct settling patterns and potential for re-suspension as a result of current

and/or wave action. As the plume dispersion model is used to simulate the dispersion of material

suspended into the water column, typically only the fine particles, including silts and clays, are

included in the model as coarser materials, including sands and gravels, settle out almost

immediately.

For this preliminary plume model study, four particle settling velocity classes were used for the

assessment, which was considered sufficient to characterise the material fractions at the various

locations where dredging is to take place. These settling velocity classes are presented in Table 3-1

together with the corresponding particle fraction.

Table 3-1 Particle size classes and settling velocities used for the modelling assessment

Particle Faction [micron] Settling Velocity [cm/sec]

<2 (Clay) 0.00035

2 – 65 (Silt) 0.035

65 – 125 (Fine Sand) 1.2

> 125 (Coarse Sand) 3.0




Report

Advisian 20

The sediment velocity classifications listed above are based on an unflocculated sediment sample,

that is, all the particles were separated in the analysis. In reality at high suspended sediment

concentrations (>300 mg/l) the finer clay and silt particles are likely to floc together in the plume

upon entering salt water, resulting in larger particles (or flocs) that have much higher settling

velocities. Below this level of concentration flocculation appears to be of minor importance (van

Rijn, 1989). The modelling presented in this report has assumed that the particles will not flocculate

and have a constant settling velocity. This assumption is expected to give slightly conservative

estimates of the plume dimensions and concentrations as particles with lower settling velocities

will remain in suspension for longer periods, allowing the plume to travel further.

3.5.2 Deposition

In the model, the deposition rate is formulated as a function of the settling velocity, the near-bed

concentration and a threshold bed shear stress below which sediment is deposited (‘critical shear

stress for deposition’). For the current study, a critical bed shear stress for deposition of 0.05 N/m2

was employed for the finer size classes (clays and fine silts), consistent with recommendations for

dredge dispersion studies in areas of similar seabed characteristics (Doorn-Groen & Foster 2007;

Van Rijn, L.C. 1989).

3.5.3 Erosion

The erosion rate depends on the seabed properties, whether the seabed is dense and consolidated

or soft and only partly consolidated. In the present model, the bed is described as one layer with

the material deposited and resuspended solely that which results from the dredging works at the

project site. This enabled the impact of the proposed dredging works to be isolated in the analysis.

The layer contains the material which is re-suspended and subsequently settled during each tidal

cycle. A threshold shear stress determines whether the deposition material is re-suspended or not.

The criterion for erosion is exceeded corresponding to the driving forces exceeding the sediment

stabilising forces.

Partheniades (1965) and Parchure & Mehta (1985) investigated the critical shear stress for erosion

of cohesive sediments as listed in Table 3-2. For the present modelling study the critical shear

stress parameter in the model was set to the value of 0.2 N/m2 as the particle tracking analysis is

primarily concerned with the transport and fate of unconsolidated silt and soft mud suspended by

the dredging processes, rather than the rapidly settling sand component which does not remain in

the water column and therefore does not contribute to turbidity.

Table 3-2 - Criteria Shear Stress for Sedimentation and Erosion. (Partheniades (1965) and Parchure &

Mehta (1985))

Mud Type Density (kg/m3)

Typical critical shear

stress (N/m2)

Mobile fluid mud 180 0.05 – 0.1

Partly consolidated

mud 450 0.2 – 0.4

Hard mud 600+ 0.6 – 2.0




Report

Advisian 21

3.5.4 Flocculation

In marine environments the settling velocity of fine sediments is largely governed by the extent of

flocculation of the individual particles. The settling velocity of the floccs is strongly related to the

salinity, sediment concentration, water depth, flow velocity and the type of type of instrument used

to make the observation (Van Rijn, 1993).

Experimental laboratory research shows a clear effect of salinity on settling velocity for salinities up

to 10PSU when the sediment concentration is smaller than 1000 mg/l (Krone, 1962), which is within

the expected range of concentrations at Eden for back-hoe dredger operations. Beyond 10PSU,

which is the case for Eden, the settling velocity for sediment suspensions below 1000 mg/l is

essentially that of the unflocculated particles. Therefore only the unflocculated settling velocities

are considered within the model.

The settling velocity of each fraction is considered constant during the simulations.

3.5.5 Parameters Summary

The dispersion coefficient is a critical parameter with respect to the spatial excursion of the

sediment plume. This and other sediment parameters used in the model set-up, based upon

Advisian’s experience in other sediment dispersion studies, have been summarised in Table 3-3.

Table 3-3 - Key parameters and formulations used in the dredge plume models

Model Parameter

Critical shear stress for deposition 0.05 N/m2

Critical shear stress for erosion 0.2 N/m2

Horizontal dispersion coefficient 1.0 m2/s

Vertical dispersion coefficient 0.01 m2/s

Number of vertical sigma layers 4 (equal layers each spanning 25% of the depth)

3.6 Model Scenarios

3.6.1 Scenario 1 – Backhoe Dredging

Under this scenario, a backhoe dredger is proposed to be used for dredging of the entire dredge

pocket.

The backhoe dredger comprises a long-reach excavator mounted on a floating pontoon. The

dredger excavates the seabed and fills two non-propelled split-hopper barges with a hopper




Report

Advisian 22

capacity of 900 m3 to 1,200 m

3. Once filled, the barges are propelled to the spoil disposal ground

by a tug and would typically sail at around 5 knots, for a sailing time from the dredge site to the

spoil ground of around 80 minutes. The two non-propelled barges are assumed to work in series,

with the dredger filling one barge while the other is being towed by the tug to the spoil disposal

area.

Advisian’s experience with similar dredge studies has shown that the following production rates for

backhoe dredging can be expected:

14,000 m3 per week in “hard” material (i.e. “rock”);

24,000 m3 to 28,000 m

3 per week in “soft” material (sand and silt, which comprises the

majority of sediment to be removed from the dredge pocket).

Table 3-4 summarises the production rate characteristics that have been assumed for the purposes

of the modelling.

Dredging cycle times have been estimated from Table 3-4 and assuming both split hopper barges

being used, for an overall dredge cycle time of 280 minutes.

Based on the volume of dredging required and the assumed production rate of the dredger, using

the backhoe dredger would result in an estimated dredging campaign duration of at least 10

weeks, assuming that dredging will occur for 24 hours a day, 7 days per week.

Table 3-4 - Summary of dredging characteristics (adapted from WorleyParsons 2005)

Backhoe TSD 2900 (Brisbane)

Sand Silt Sand Silt

Time to fill (min) 179 195 120 120

Time of overflow (min) - - 10 10

In-situ sediment volume to fill barge incl. bulking (m3) 625 600 2,074 1,160

Mean filling sediment discharge (kg/s) 1.2 9

5.8 5.22

Mean overflow discharge (kg/s) 19.1 19.1

Amount suspended per barge load 12,900 105,300 24,200 11,600

3.6.2 Scenario 3 - Trailer Suction Hopper Dredging

Under Option 3, most of the material would be bulked out direct by the TSHD. Material in areas

inaccessible or containing material too hard for a TSHD would be dredged by the BHD and loaded




Report

Advisian 23

into hopper barge(s). The hopper barge(s) would bottom dump the material in deep water within

the dredging footprint in an area accessible by the TSHD. Any large rock that cannot be handled

by the TSHD would either be disposed of elsewhere within the site (e.g. along the lee side of the

breakwater) or at the ODS.

The assumed TSHD dredge for this project is the BRISBANE or similar (Figure 3-4). The

characteristics of the Brisbane vessel and proposed modes of operation relevant to the modelling

are summarised below:

Average sailing speed = 10 knots (loaded and empty)

Production rate between 750 and 1000 m3/hr (based on filling times between 1.5 and 2

hours and an average load of 1,500 m3)

Draft of 6.2 m when loaded . Propeller wash will apply for dredging at Eden, due to the

relatively shallow dredging depth.

Overflow released through “green valve” at the vessel’s draft level of 6 m

During dredging, fine sediment will be generated from bottom disturbance and from overflow

from the hopper barges as well as potentially from propeller wash. The majority of the visual

turbidity at the dredging sites is likely to be attributed to the overflow water from the barges rather

than effects from bottom disturbance.

The sediment characteristics and rate of overflow is dependent on the type of dredge and varies

with make and model. WorleyParsons (2005) undertook a detailed analysis of various options of

dredge to assess filling times and the length of time the barge will overflow.

Economic loading times and length of overflow discharge were assed in WorleyParsons (2005),

considering the following factors:

Type of dredge

Hopper capacity

Type of material dredged

Behaviour of sediment during dredging

Fully laden sailing speed

Length of the dredging area

Distance to disposal site

Time to turn dredger

Time to dispose (dump) sediment




Report

Advisian 24

Figure 3-4 – Brisbane TSHD (http://www.marinetraffic.com/en/ais/details/ships/503380000)

Concentrations of suspended sediment released during the dredging operation (excluding

overflow) were estimated by the S factor method that provides estimates of sediment that passes

out of the immediate dredging area, typically 50 m from the dredger. The S factor is defined as the

following:

𝑆 =𝐴𝑚𝑜𝑢𝑛𝑡 𝑟𝑒𝑠𝑢𝑠𝑝𝑒𝑛𝑑𝑒𝑑

𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑆𝑜𝑖𝑙 𝐷𝑟𝑒𝑑𝑔𝑒𝑑 (𝑘𝑔/𝑚3)

Typical expected “S factors” assuming that overflow and degassing discharge is prohibited for TSD

are in the order of 5 kg/m3. Calculated S Factor for silt based on the material behaviour during

extraction has previously been estimated at approximately 180 kg/m3, and has been assumed for

this analysis.

Suspended sediment concentration of the overflow discharge was based on the WorleyParsons in-

house database of recorded overflow concentrations for dredging in Western Australia, as reported

in WorleyParsons (2005). Typical mean overflow concentrations are generally in the order of 6,000

to 8,000 mg/l. For this dredge plume study 8,000 mg/l is considered.

The sediment during dredging is likely to behave as non-cohesive sediment. It is expected that

during dredging via TSHD the material will breakdown from the cohesive state and behave similar

to a non-cohesive material.

http://www.marinetraffic.com/en/ais/details/ships/503380000




Report

Advisian 25

Table 3-4 summarises the production rate characteristics that have been assumed for the purposes

of the modelling.

Scenario 3 uses a combination of BHD and TSHD, with the BHD dumping material within the

dredge pocket and the TSHD removing this material and disposing it at the Offshore Spoil Disposal

site. While the quantity of material that would be removed by BHD is not known precisely, for the

purposes of the preliminary modelling it is assumed that 50,000 m3 of material would need to be

removed using the BHD and that this would be likely located near the breakwater. Based on the

volume of sediment and an assumed production rate of 750 m3/h for the TSHD, the dredge

campaign is assumed to take approximately 3 weeks.

3.7 Plume Dispersion Methodology

The release and settlement of sediments during dredging has been simulated using the particle

tracking model. The particle tracking extension to Delft3D incorporates variable settling rates and

allows the particles to be grounded and resuspended.

For the TSHD, sediment releases would occur from the drag head (i.e. distributed in Layer 4 of the

model, i.e. near the bed at 80% of the water depth), and overflow releases nearer to the surface

based on the overflow release valve at the vessel’s draft level of 6 m (Layer 2 of the model at 40%

of the water depth). While there would be little contribution from propeller wash expected, as the

draft of the vessel is 6 m over an overall water depth of 11 m, this was included in the near-bed

sediment contribution.

At the spoil disposal ground and for disposal of the BHD spoil within the dredge footprint,

sediment suspended during disposal was added to model at 20%, 40% and 60% of the model

depth (Layer 1, Layer 2 and Layer 3) over 20 minutes. The discharge depths are based on physical

measurements presented in US Army Engineer Waterways Experiment Station (1992).

For the backhoe dredging, it is assumed that at the dredge site, sediment is released near the bed

– i.e. in the bottom layer of the model domain (Layer 4).

The following assumptions were used in modelling the production rate of sediment at the spoil

ground and at the dredging site:

For the TSHD, the overall cycle time used in the model is 220 minutes, which includes 120

minutes dredge time (to fill the hopper), 10 minutes overflow time at the dredge site, 40

minutes sailing time, 20 minutes dumping at the spoil ground (and within the dredge

footprint) and 40 minutes return sailing time.

For the backhoe dredging for Option 1, the cycle times used in the model includes 195 min

filling, 80 minutes sailing (@ 5 knots), 20 minutes dumping at the spoil ground, 20 minutes

return sailing time and two hopper barges working in series, 24 hours per day, 7 days per

week.

For the backhoe dredging for Option 3, the material is dumped within the dredge pocket

and so there would be no sailing time required.




Report

Advisian 26

Based on previous experience with capital dredging works it is assumed that 80% of the

reworked dredged material will settle immediately to the seabed with the dumped mass.

The remainder will slowly settle and disperse into the environment.

The location of each sediment source in the dredge area has been assumed to be

sequential.

Multiple sediment sources were introduced into the model to indicate the likely sequence of

dredging in the different areas, as well as the main sediment disposal location at the spoil ground,

as indicated in Figure 3-5.

3.7.1 Scenario 1 (BHD)

For the BHD scenario, a full 29 day lunar cycle of BHD was simulated, with sediment being released

near the bed at the 7 locations (D1 – D7) indicated in Figure 3-5. Sediment was released at the

spoil ground over 20 minutes for each dredging cycle.

3.7.2 Scenario 3 (Combination BHD and TSHD)

For Scenario 3, dredging was assumed to occur over the first three weeks of the month, to simulate

the expected duration of dredging under this method, and also the persistence of the plume

following cessation of dredging. Sediment was released near the bed at 3 locations (D1 to D3)

indicated in Figure 3-5, which were assumed to require BHD. It was then assumed that the

sediment from the BHD would be released at location D7 over the top three layers of the model.

The TSHD would then remove sediment from Locations D4 to D7, from which sediment is released

near the bed (from the drag head) and at around 40% of the depth (from the dredge overflow).

Sediment was then released at the spoil ground over 20 minutes for each dredging cycle.




Report

Advisian 27

000000000 250250250250250250250250250 500500500500500500500500500

metresmetresmetresmetresmetresmetresmetresmetresmetres

D1D2

D3D4

D5

D7

D6

Location (D7) where material placed

from BHD for removal by TSHD

0 1 2

kilometres

Mussel Farm

Offshore Spoil

Disposal Ground

Figure 3-5 – Sediment sources in the model

Area where BHD assumed under

Scenario 3

Area where TSHD assumed under

Scenario 3

Eden Wharf Extension - Preliminary Dredge Plume Modelling

Documents

Transcript of Eden Wharf Extension - Preliminary Dredge Plume Modelling