7.0 COASTAL GEOMORPHOLOGY - Roberts Bank · Expansion - Coastal Geomorphology Study (2004). 7.2 STUDY AREA The study area for the coastal geomorphology study is delimited by Steveston

Deltaport Third Berth Project January 2005 Environmental Assessment Application Page 165

7.0 COASTAL GEOMORPHOLOGY

7.1 INTRODUCTION

Coastal geomorphology is the study of the landforms and physical processes of the coast. The

study of water movement, predominantly caused by tides and wind is termed “hydrodynamics”.

Consideration of hydrodynamics is important when major development is proposed in coastal

settings, because these developments may change coastal processes, which can impact areas both

adjacent to, and remote from, the development site.

This chapter describes the existing hydrodynamics and coastal processes at Roberts Bank and

assesses any impacts of the proposed Deltaport Third Berth Project. It is based on investigations

undertaken by Northwest Hydraulic Consultants Ltd. and Triton Consultants Ltd. Their complete

study is located in the companion report Technical Volume 2, titled Roberts Bank Container

Expansion - Coastal Geomorphology Study (2004).

7.2 STUDY AREA

The study area for the coastal geomorphology study is delimited by Steveston Jetty to the north,

Point Roberts to the south, the 100 m water depth contour to the west and the top of the bank

along the eastern Roberts Bank shoreline (Figure 7.1). Additional field investigations and

studies were carried out in Boundary Bay to provide complementary information on tidal

channel formation and beach processes. The limits of many numerical models were extended

beyond the proposal boundaries to improve the results.


7.3 METHODOLOGY

Three complementary methods were used to assess the physical response to the proposed

development:

• interpretive geomorphic studies, using historical data, site observations and

measurements;

• analytical computations, using empirical or theoretical relations describing

sediment transport, erosion and deposition processes; and

• numerical computer modelling of waves and tidal currents.

This approach is consistent with experience and recommendations on similar types of projects

which concluded that available numerical modelling techniques are limited, and more than one

method was required to describe the major active processes (Teeter et al. 2001). The major

processes are, however, reflected in the morphology, and can be quantified and assessed using a

range of interpretive methods including historical mapping, field observations and other

analytical computations. These geomorphic investigations can, to some degree, overcome the

limitations of numerical models. The approach adopted with this study integrates

hydrodynamics, sedimentation, and geomorphology and focuses on developing an understanding

of the long-term physical processes that drive morphological change in the project area.

7.3.1 Interpretive Geomorphic Studies

Background information was obtained from reports by Mathews and Shepard (1962), Luternauer

and Murray (1973), Thomson (1981), Kostaschuk et al. (1992) and Clague et al. (1998). Other

reports, peer reviewed journal articles, bathymetric surveys and tide charts were also used. A

comprehensive time series of maps, charts and aerial photographs was compiled and compared to

assess historical changes on Roberts Bank. Interpretative studies were made to assess trends and

to identify the key factors that governed the historical changes. Site observations and field

inspections were made to Roberts Bank and Boundary Bay during a range of tidal conditions to

assess differences and similarities of the physical environments.


7.3.2 Analytical Computations

Topographic changes were assessed by comparing historical survey information using GIS

techniques. Field measurements of tidal channel velocities and discharges were correlated with

channel geometry and the topographic features on the tidal flats. The plan-form geometry of the

tidal channel system was also analyzed using drainage basin properties and stream network

concepts. Empirical and analytical relations governing tidal channel dimensions, channel

networks and sediment movement were assessed and compared with the observed conditions on

Roberts Bank.

7.3.3 Modelling Wave Climate

To develop a statistically meaningful long-term record of wave conditions in the study area two

numerical models were used. A numerical model of offshore wave conditions was developed

using WaveHind software. This model was calibrated and verified against the measured offshore

data for a range of winds. Once calibrated, the model was used to generate a time series of

incident offshore Strait of Georgia wave conditions coinciding with measured wind data. A

second model, SWAN (Simulating Waves Nearshore, Delft University 2003) was used to

compute the propagation of offshore Strait of Georgia waves inshore to the site. To achieve a

reasonable balance of computational efficiency and spatial resolution, the models were

constructed as nested grids with a 100 m resolution in the outer domain, and a 25 m resolution in

the immediate vicinity of the terminal.

A single wave condition is associated with each individual wind speed and wind direction range.

The derived transfer function can then be applied to the available 40 years of offshore wave data,

yielding a continuous hindcast of wave conditions at any inshore location.

A comparison of measured and predicted wave conditions (at the Halibut Bank19 wave sensor)

was made for several representative time periods and verified the model’s accuracy.

19 Halibut Bank is located in Georgia Strait mid-way between Point Atkinson and Nanaimo


7.3.4 Modelling of Tidal Currents

A suite of numerical models was used in three separate phases to predict tidal currents and

resulting sediment transport patterns under various scenarios and conditions. The first phase

involved tidal modelling studies Tide2D; a “Wide Area Model” used to provide tidal height and

tidal current boundary conditions in the deeper waters of the Strait of Georgia parallel to Roberts

Bank, extending from just north of Sandheads at the mouth of the Fraser River to the southern tip

of Point Roberts. These boundary conditions were primarily used to drive other detailed tidal

models of the Roberts Bank inter-causeway area.

The second phase involved developing a “Base Model” using Surfacewater Modelling System

(SMS 8.0) to simulate hydrodynamic conditions in the Fraser Estuary, Roberts Bank tidal flats

and adjacent portions of the Strait of Georgia. The Base Model was used to assess general flow

patterns, and to identify the extent of potential impacts from project developments.

Finally, a “Detailed Model” using River2D was developed specifically for the Deltaport Third

Berth Project to assess local flow conditions in the inter-causeway area between the Tsawwassen

Ferry Terminal and Roberts Bank Causeway. This was particularly useful for assessing shallow

flows on the tidal flats and in eelgrass covered areas as well as for assessing flow effects induced

by structures such as the proposed Deltaport Third Berth and the existing crest protection weir on

the tidal flats. The computational mesh for the Detailed Model has a resolution of between 5-10

m in critical areas near the proposed developments in order to adequately represent project

impacts and complex flows in adjacent tidal drainage channels.

Base Model

The bathymetry for the numerical Base Model included a 23 km long section of Roberts Bank,

from Steveston Bend to Point Roberts. A computational mesh was generated by defining the

spatial and hydraulic characteristics of the bank as a series of nodes (spaced at 100m intervals)

and elements. The results were verified by comparing them to tide levels predicted by Tide2D

(the wide area model), and with tide elevation data from the Department of Fisheries and Oceans

tidal stations. The model predictions agreed closely with this information. The predicted

velocities were also compared to field data recorded by two current meters located immediately

west of the Deltaport Terminal; again the agreement was found to be good.


Detailed Model

Four time frames subdivided the modelling effort into representative tidal cycles; these were

typical 2003 neap, mean and high tides as predicted by the Base and Wide Area models. The

fourth modelled the field investigations, and was used to verify the detailed model results.

A series of field investigations were conducted in April and May 2004 to measure current

velocity and direction on Roberts Bank, and ultimately to verify the predicted velocities from the

Detailed Model in the ship turning basin and on the tidal flats. The measurements were made

with an RDI Rio Grande Acoustic Dopler Current Profiler (ADCP).

Both the field measurements and modelled velocities indicate that the highest velocity (on 8 May

2004) was approximately 0.8 m/s in the trunk of Channel 1 mid-way along the channel between

the two causeways. This occurred during the ebb (outgoing) tide. The timing of the maximum

velocity lags the highest rate of tidal drop probably due to water retained in the eelgrass beds.

The peak velocity during flood (incoming) tide, 0.6 m/s, occurs at the same time as the peak rate

of tidal rise as water is able to pass directly into the drainage channel from the turning basin.

Velocities at two points in the ship turning basin, near the vicinity of the proposed Deltaport

expansion were much lower due to the larger depths; again the predicted values were very close

to the measured values.

Flow Patterns in Drainage Channels

The scale of many features in the key tidal drainage channels is too small to be resolved by the

topographic surveys and mesh used in the Detailed Model. To address this, a combination of

direct observations and numerical modelling was used to describe the hydraulic conditions.

Three series of velocity measurements were made on the tidal flats. On 26 March 2004 a Swoffer

portable propeller meter was used to measure velocities in the tidal channels, and on 6 and 7

April 2004 an ADCP was used to measure velocities and current directions on the east side and

west side of the causeway and in the main tidal channels. These measurements were repeated

again on 8 May 2004.


7.3.5 Modelling of Sediment Transport

Sediment Transport By Waves

Sediment transport was computed using the methods of van Rijn (1989) in order to provide a

means of comparing the relative significance of wave and current induced transport. The

sediment mobility parameter (T) was used to characterize the sediment transport conditions:

c

cTτ

ττ −=

where τ is the local bed shear stress, and τc is the critical shear stress for initiation of

sediment movement

The sediment transport parameters were computed at each grid point in the SWAN numerical

wave model using the program Tecplot. This produced contour plot outputs of sediment mobility

and sediment transport. Additional post-processing, extracting values along selected profiles and

cross section transects, was carried out to make comparisons between runs. The sediment grain

size used for this analysis are described in section 7.4.1.

Sediment Transport By Currents

Sediment transport by currents was estimated using the total load equation (van Rijn 1989) and

equations for sand-bed channels (Engelund and Hansen 1967). The use of sediment transport

equations under tidally-varying flow conditions is somewhat more reliable than for waves, but

the same qualifications should be considered when using the results. Again the output of the

sediment transport by current computations was primarily used as a tool for assessing relative

changes in transport conditions in the study area under various scenarios.

For the total load equation, the volumetric bedload transport rate was estimated from relations

between sediment size, specific gravity and the critical shear stress required to initiate bed

movement The suspended bed material load is then computed using the sediment transport

equation, assuming the sediment concentration near the bed is equal to the bedload

concentration. This provides a means for integrating the suspended load from the bed to the

water surface in order to estimate the total load. The main advantage of this method is that it has

been applied to both current and waves, but a disadvantage is that it is computationally


complicated, which may not be warranted given the uncertainties in the data used to develop and

test the equations. Limited field-testing of the equations on the sand-bed channel of the Fraser

River have shown reasonably good results.

The equations for sand-bed channels use unit weight of sediment, mean velocity, specific gravity

and median sediment size (0.2 mm). The bed shear stress is calculated from the mean velocity

and bed friction factor. This method has been demonstrated to provide good predictions on a

wide range of sand bed channels.

7.4 EXISTING ENVIRONMENT AND GEOMORPHIC PROCESSES

7.4.1 Coastal Features and Processes Affecting Roberts Bank

Strait of Georgia

The Strait of Georgia between the mainland and Vancouver Island influences tidal flows, and

therefore sediment transportation in the study area (Figure 7.2). It is approximately 220 km long,

28 km wide and has an average depth of 155 m (Thomson 1981). The Strait of Georgia is linked

to the Pacific Ocean on its north end by several long, narrow channels and at the southern end by

Juan de Fuca Strait. Although water is transported into and out of the Strait of Georgia from both

ends, the exchange through the south end is about 15 times greater than through the north end.

Tidal flows are relatively strong in the south end, typically reaching 0.5 m/s and setting to the

northwest along the eastern shore. The tidal currents decrease to the north of Point Roberts due

to the increase in cross sectional area of the channel. Tides are predominantly mixed, mainly

semi-diurnal, with a range of up to 5m near Tsawwassen. The mean tidal height is 3.1 m.

Fraser River

The Fraser River drains 250 000 km2 of southern British Columbia (Figure 7.2). At Mission, 85

km from the sea, the Fraser River changes abruptly from a steep, anabranching (multiple

channels of similar size) gravel bed to an irregularly meandering sand bed channel. The river

transports an average of 17.3 million tonnes of sediment, containing 35% sand, 50% silt and 15%

clay, annually (McLean et al. 1999).


Fraser Delta

Lower water speeds associated with the change in gradient below Mission causes the coarser

suspended sediments to deposit in the channel. Over a number of years the deposition and re-

working of sediments has formed the Fraser River delta (Figure 7.2). The modern delta

commences below New Westminster and is a broad plain, which encompasses Richmond,

Ladner and Tsawwassen. It extends approximately 27 km into the Strait of Georgia along its

western margin and includes Sturgeon Bank and Roberts Bank. Boundary Bay is on the inactive

southern side of the delta.

Below New Westminster, the river flows in a number of active distributary channels including the South Arm, North Arm and Canoe Passage. The present flow split in these channels is 85%, 10% and 5% respectively. Flows in the distributary channels dictate the placement and movement of sediments from the Fraser River.

Virtually all of the fine sand, silt and clay is transported through the distributary channels to the river mouth as “wash load” and is distributed into the Strait of Georgia by the action of waves and currents. A portion of the coarse sand fraction (approximately one half of the bed material load) is deposited in the main channel below New Westminster and must be removed by dredging. The amount of sand delivered to the delta through Canoe Passage, the closest channel to Roberts Bank, is very small, estimated to be in the order of 50,000 tonnes/yr.

Marsh

Salt marshes on the delta extend about 1 km seaward from near the high tide mark. They are formed by current, wave and tidal distribution of sediments from the Fraser River, and consist of hummocky, vegetated topography incised by meandering tidal creeks and ponds. Marshes along the western front of the delta (Roberts and Sturgeon banks) are underlain by interbedded organic mud and sand.

Tidal Flats

Gently sloping (0.0004 to 0.0019 average gradient) tidal flats extend for up to 6 km on Sturgeon and Roberts banks. The width of the tidal flats is governed primarily by the mean tidal range (approximately 4 m), the wave climate and sediment characteristics. The tidal flats generally consist of medium to fine sand with silty sand. The surface sediments are finer in the upper flats near the +2 m contour (GeoSea Consulting 1996). Below these, on the lower and middle tidal


flats, are horizontally bedded fine to medium grained sand, which are commonly bioturbated (mixed by organisms) and contain shell fragments. These relatively clean sands grade into silty sands of the mid to upper tidal flats (Mathews and Shepard 1962). Further offshore, the sediments remain relatively coarse medium sand, typically 0.25 mm diameter to a depth of around 100 m. At greater depths the sediments become noticeably finer sandy-silts to silts.

The Roberts Bank tidal flats are generally featureless except for the development of dendritic (branching) tidal channels (Howard 1967) analogous to a typical terrestrial-fluvial drainage network. However, on Sturgeon Banks, particularly adjacent to jetties, sand waves with typical heights of 0.5 m and wavelengths of 50-100 m are present (Luternauer 1980).


Sediments on the tidal flats were probably derived from a combination of sources:

• fluvial and deltaic deposition from former Fraser River distributary channels;

• littoral drift from cliff erosion at Point Roberts.

Most sediment transport on Roberts Bank now involves re-distribution of material contained in

the sediment reservoir of the tidal flats themselves, rather than from external sources. Evidence

for this assertion comes from a number of observations.

Finer Fraser River sediments, (silt and clay fraction) are dispersed over a wide area of the Strait

of Georgia, but the Roberts Bank tidal flats contain relatively little of this material because most

of it is re-suspended by waves and currents. In addition, the Roberts Bank causeway has

deflected the Fraser River plume from the inter-causeway area. The amount of coarser sediment

(sand) discharged onto Roberts Bank via Canoe Passage is very low (50 000 tonnes/yr)

compared to that discharged from the other channels of the Fraser River. In addition the sediment

movement near the mouth of Canoe Passage is dominated by fluvial (river) processes (McLaren

and Buckingham 1983) rather than by the action of currents and waves, which have the ability to

distribute sediment more widely.

Past experience suggests that actual sediment transport rates in most of the inter-causeway area

are very low. For example, no significant infilling has occurred in the dredged approach channel

or ship turning basin since their construction. In addition, the B.C. Hydro power line trench,

dredged south of the B.C. Ferries terminal in 1959, has also not experienced significant infilling.

A northwestward pattern of sediment movement has been suggested by a number of studies

(Luternauer and Murray 1973 and HayCo 1996), and is consistent with the dominant flood tide

direction in this area. Construction of the B.C. Ferries terminal has obstructed nortwestward

moving sediments along the tidal flats (but not in deeper water on the delta fore-slope).

Fore-slope

The delta profile gets steeper from the tidal flats to deep water (Figure 7.3). The sediments here

consist mainly of mud derived from suspended sediment in the Fraser River plume and coarser

sand transported by gravity flows down submarine channels or valleys in the delta slope.


Generally Roberts Bank is covered with medium- to fine-grained sand extending from the tidal

flats down to the fore-slope. Sedimentation rates range from less than 1 to 2 cm/year over much

of the Sturgeon Banks fore-slope and in the Strait of Georgia; much higher sedimentation rates

have been measured off the mouth of the main channel. Little or no sediment is being deposited

today over most of the Roberts Bank fore-slopes. Bottom currents on the fore-slope are strong

enough to generate bedforms in deep water (Luternauer et al. 1977), these findings accord with

tidal current observations, which show north flood tidal currents stronger than southward ebb

tidal currents.

Retrogressive slope failures and sediment-laden gravity flows have eroded the active deep valley

off the mouth of the South Arm near Sandheads, and smaller valleys are evident off of Canoe

Passage and the North Arm (Christian et al. 1998). A large area of disturbed sediments on the

south side of Roberts Bank has been described as the Roberts Bank Failure Complex (Luternauer

et al. 1998). This feature is believed to have formed in the past at a former river mouth, in a

similar manner to the processes that are occurring at Sandheads. The Roberts Bank Failure

Complex is not related to present-day sedimentary processes (Luternauer et al. 1998).


7.4.2 Roberts Bank Tidal Flat Morphology Prior to Terminal Developments

Prior to any substantial marine developments at Roberts Bank a distributary channel drained

from the main arm of the Fraser River near Ladner and flowed south through the Tsawwassen

First Nations Reserve, eventually discharging into the inter-causeway area near the present

marsh (Figure 7.4). This channel was eventually closed off, although local drainage is

discharged onto the inter-causeway area through a pump station.

In 1949 the tidal-flat near the present causeway had northwest trending lineations indicating that

tidal currents swept across the flats between Canoe Passage and the present location of the B.C.

Ferries terminal (Figure 7.5). A navigation marker on the tidal flats, visible as a white triangle,

was causing an accumulation of sediment in the lee of the structure indicating that dominant

drainage and sediment transport was northwest to southeast. Sub-parallel lineations were visible

near the top of the beach, indicating the direction of drainage away from the coarser beach

sediments at the top of the beach. Behind the beachfront, drainage from the salt marsh created a

more complex pattern of small channels and deposits. In the southwest corner of the photo,

deeper grooves indicate that drainage from the tidal flats became slightly channelized near the

lower limit of the tide.

The historical photos indicate that some drainage channel features existed on Roberts Bank

before major developments occurred. However, the network of channels that is present in the

inter-causeway area today, was absent.


7.4.3 Roberts Bank Tidal Flat Morphology After Terminal Developments

Substantial human modification to the tidal flats of Roberts Bank began in 1958 with

construction of the B.C. Ferries terminal and causeway, which was completed in 1960 (Hemmera

2003). Construction of the Roberts Bank port facility began in the early 1960s and was

completed in 1969 (Chapter 3 Project Background and Figure 3.2). The B.C. Ferries terminal

and causeway construction involved dredging a deep trench in the tidal flats south of, and

parallel to, the causeway to provide fill material. Two expansions were completed in 1976 (three

additional berths added to the south west end of the terminal) and 1991 (addition of a new

parking area and eelgrass compensation site to the north). The Roberts Bank Port Facility was

completed in 1969 by dredging southeast of the facility to provide construction material.

Between 1980 and 1982 the dredged basin was significantly enlarged, to provide material for

three additional pods, and for a turning basin for large ships. A crest protection structure was

built around the upper rim of the basin to limit drainage channel formation, and the causeway

was widened. Subsequent expansion occurred between 1994 and 1996 when additional docking

facilities were built on the southeast corner of Pod 4.

West Side of Roberts Bank

Historic changes on the tidal flats between the mouth of Canoe Passage and the Deltaport

causeway include confining Canoe Passage between Westham Island and Ladner by dykes

(Figure 7.6). However, further downstream where the channel spills across the tidal flats the

outlet has experienced considerable channel instability and shifting. The active width of the

channel has varied, narrowing appreciably since 1966 due to bar growth at the south edge of the

channel (Figure 7.6). Changes occurred on the upper tidal flats near the Roberts Bank causeway,

probably in response to construction of the causeway, which modified the drainage paths

(Figures 7.6 and 7.7). The predominant tidal flow was shifted from a northwest to southeast

trending direction, to a generally north-south direction, parallel to the two causeways.

Aggradation (build up) of 0.5 to 1.0 m is evident near the mouth of Canoe Passage and

degradation (lowering) is evident along the edge of the fore-slope (Figure 7.8); these are

consistent with other findings (Stewart and Tassone 1989). Bed level changes on the west tidal

flats appear to have been minimal.


Inter-causeway Area

Notable morphological changes have occurred in the inter-causeway area between 1970 and

2002 (Triton 1996 and Figures 7.9 to 7.11). The most visible development was the initiation and

growth of a network of tidal drainage channels from around the periphery of the dredged basin.

The characteristic “dendritic” pattern, first described in Tarbotton et al (1993), distinguishes the

main difference of these channels from the drainage features that existed previously (see Section

7.4.2 Roberts Bank Tidal Flat Morphology Prior to Terminal Developments). The channels were

initiated following the construction of the Westshore terminal and the dredging of the initial

basin in 1969. The channels are visible one year after completion of the Roberts Bank causeway

in 1970, and in 1979 (Figure 7.9). The channels reached a moderate size by 1979. These initial

drainage channels were largely obliterated or significantly modified when the larger ship-turning

basin and crest protection structure were completed in 1982 (Figures 7.10 and 7.11). The

growth of the two largest trunk channels during this time shows a general increase in size (Triton

1996) and landward migration from 1984 to 2002. By 2002 the channel was 45 m wide and 380

m long; an average growth of about 20 m/yr. A short spur was constructed on the crest protection

structure in 1992 to control the advancing channel (Tarbotton et al. 1993). A 5 m deep scour hole

developed near the end of the spur, indicating a considerable portion of the flow now passes

behind the crest protection structure, instead of flowing over the top.

Topographic changes in the inter-causeway area between 1968 and 2002 (Figure 7.12) show net

lowering of portions of the tidal flats landward of the ship turning basin, on both sides of the

crest protection structure. The drainage channels have incised into the tidal flats by typically 1.5

m. The head of the main trunk channel has experienced net elevation gain of 0.5 m. Net elevation

gains of 1.0 m to 2.0 m near the causeway and Deltaport structures are fill material added during

construction and do not represent sediment accretion.


7.4.4 Existing Coastal Geomorphic Environment

The existing drainage channel system is characterised by a trunk (main) channel that extends

from the crest protection structure up the tidal flats (Figure 7.11). Two main tributaries and a

series of small channels drain water off the tidal flats on the ebb tide, conveying flow into the

trunk channel. The channels have a sinuous planform (shape viewed from above) and are

connected in a dendritic pattern. At low tide, the large channels have defined banks while the

small channels are shallow depressions in the tidal flat. The planform of the trunk channel is

generally straight and the upper end terminates bluntly at a sandy bar deposit. The trunk of

Channel 1 is approximately 90 m wide near the crest protection structure, and extends for 700 m.

It splits at the crest protection structure, and runs parallel to it for a further 370 m before draining

into the ship turning basin. The main trunk of Channel 2 is approximately 40 m wide near the

crest protection structure and extends 350 m up to the tidal flats.

Factors Governing Morphologic Changes

Construction of the two causeways and related dredging altered several processes governing tidal

flat morphology and physical habitat, including: tidal flow direction and magnitude, wave

climate; and influx of the Fraser River sediment plume. One of the major results of these changes

has been the development of drainage channels, which were driven by two main factors;

dredging of the ship turning basin triggering headcutting and expansion of eelgrass over the flats,

promoting flow concentration.

Dredging and Headcutting

Dredging of the turning basin has lowered the erosional base level within the tidal range. Prior to

dredging, the tidal flats extended out to approximately elevation –4.5 m at a nearly constant

slope of 0.0011 before dropping off to the sea floor (Figure 7.4). The slope of the tidal flats was

in equilibrium with the long-term physical conditions. Excavations of the borrow pit in 1969, and

the turning basin in 1982 created a step in the tidal flat profile (Figure 7.12). The excavation

extended into the flats above the lowest low water level (LLW) so flow draining off the tidal

flats at the lower part of the tide intersected the edge of the excavation and channels were

initiated through a process of headcutting up the tidal flats (Figure 7.13).


Effects of Eelgrass

Eelgrass beds on Roberts bank have expanded by approximately 100% since 1950 due to a

number of inter-related reasons (Triton 1996):

• increase in water clarity and light penetration due to the causeways and terminals

preventing turbid, sediment-laden water from the Fraser River, which previously

limited eelgrass growth on the tidal flats (Koch 2001);

• initiation of a positive feedback loop that promotes further expansion of eelgrass

beds through ponding of water and stabilisation of the substrate(Triton 1996);

• introduction of the non-native eelgrass Zostera japonica, which occupies higher

(and drier) habitat than the native Z.marina; and

• modification to tidal drainage pathways has likely accelerated the spread of both

eelgrass species through the spread of seeds and rhizomes.

A comparison of the available historic survey information suggests there have been some

localized topographic changes on portions of the inter-causeway tidal flats. For example,

comparison of surveys in 1968 and 2002 suggested net lowering averaged between 0.25 to 0.5 m

over an area of 18 ha landward of the crest protection structure (Figure 7.14). The total volume

of material eroded during this 34 year period amounts to 57,000 m3. However, the apparent

topographic differences are probably within the accuracy of the available surveys. Bed lowering

could represent an important factor in the expansion of the eelgrass beds because of the

sensitivity of this species to desiccation at low tide. The physical presence of eelgrass beds on

the tidal flats appears to be playing a major role in the expansion and maintenance of the tidal-

flat channels by increasing flow resistance. Increased flow resistance results in reduction in flow

velocity within the eelgrass beds and a drainage lag that causes a head differential between the

tidal flats and the tide level in the open water.

7.4.5 Conceptual Model of Tidal Flat Response

A conceptual model of tidal flat response has been developed on the basis of the historical developments that have occurred on Roberts Bank. This assessment was based on a review of seven different tidal channels on Roberts Bank and Boundary Bay.


Stage 1: Initiation Dredging for the ship turning basin initiates channel formation by a process of headcutting and flow concentration. The channel formation will be initiated when the excavation extends into the tidal flats above LLW. If the limits of the excavation do not extend above LLW the risk of channel formation is minimal.

Headcutting will develop along the edge of the dredged basin during the ebb tide in response to the locally high velocities and higher water surface slopes. This effect is greatest near low tide when the “sump” action of the basin causes more concentrated water flow. The processes of headcutting and flow concentration define distinct “drainage basins” on the tidal flats. Eelgrass affects this stage by acting as a reservoir, delaying the drainage of water off higher elevations during the ebb tide. This creates a phase lag between the tide level in the Strait and the runoff draining down the slope, particularly near low tide. This contributes to high rates of shallow or sheet flow with greater velocities than would normally develop. Variations in eelgrass extent or density also create local variations in resistance, which contribute to local flow concentrations.

Stage 2: Channel Incision Once the drainage paths develop and flow is concentrated, channel incision will begin to develop and migrate landward as a “knickpoint”. It appears that channel incision starts when velocities exceed about 0.5 m/s. Depending on the geometry of the dredged basin, one or more trunk channels will begin to develop at the outlet. The overall dimensions (width and depth) are related to the peak discharges that are produced during the ebb tide and these dimensions appear to be predictable. A network of tributary channels joins the trunk channel.

Stage 3: Shoreward Channel Progression The trunk channel will extend in a landward direction due to headcutting. The driving force for the headcutting is no longer the initial knickpoint from the dredged basin. Instead, it is the invert level of the trunk channel, which may be 2m lower than the adjacent level of the tidal flats. Sediment transport appears to be ebb-dominated during this phase and the channel extension occurs primarily on the ebb tide. The rate of landward extension of the largest trunk channel averaged 40 m/yr; even after mitigation measures (the crest protection structure) were installed. As the channel grows, it may erode into eelgrass, creating further flow concentration and increased tendency for development of the channel.


Stage 4: Bifurcation

The main (trunk) channels on Roberts Bank generally developed landward, then eventually

bifurcated (divided in two), and formed a branched channel system. The point of bifurcation

appears to be controlled by elevation and local drainage features on the tidal flats. This point is at

about 1.5 m on Roberts Bank. During incoming flood tides, the bifurcation causes the flow to

diverge and the resulting reduction in sediment transport capacity triggers localized deposition in

the form of a sand bar.

Complex hydraulic conditions develop during the flood tide near the bifurcation point,

particularly when the water inundates the intervening flats between the two channel branches.

Local differences in water levels in adjacent channels produces short periods of high velocities

and very high sediment transport, which cause development of chutes and scour holes and the

loss of eelgrass or other vegetation. These processes are governed by the micro-topography of

the tidal flats and the local distribution of eelgrass and cannot be modelled deterministically.

Landward growth of the trunk channel may alter the overall drainage patterns on the middle and

upper portions of the flats. This may capture other channels or drainage areas on the flats, which

in turn, could initiate changes in the magnitude of runoff conveyed by the channel. For example,

capturing other drainage surfaces would cause the main trunk channel to respond by widening

and deepening. This would in turn initiate further headcutting.

Stage 5: Establishment of Final Equilibrium Condition

The trunk channel will extend landward reaching a point where a stable slope can be achieved. This will occur when the discharge intensity in the landward end of the channel is reduced to the point that a stable channel is achieved. The stable point will be governed by the elevation of the surrounding tidal flats, since this level controls the volume of water that is drained. The stable level is approximately 1.5 m to 2.0 m above LLW at Roberts Bank.

7.4.6 Results of Analytical Computations and Numerical Modelling

The analytical computations and numerical modelling generally support the findings previously presented for the interpretative geomorphic studies. However, they provide quantitative data and a great deal more specificity on the wind and current components that dictate sediment movement in the study area. A short description of the results of wave and current analyses to


support the sediment movement results are presented here. For more detail on these analyses see the complete report, Technical Volume 2.

Wave Climate

Based on these results the largest offshore waves are generated from the southeast, south and northwest. The significant wave height (Hs) exceeds 2.0 m for a total period of 105 hr/yr on average (approximately 4 days/yr). Of this, waves from the southeast and south directions account for 100 hours/yr, while the remaining 5 hours/yr are from northwest waves. The predominant wave height is less than 1.0 m, with little seasonal variation.

The transformation of waves that occurs as a result of refraction (Figure 7.15), wave breaking

and friction shows that on the east side of the Deltaport causeway, waves from the northwest and

west undergo greatest reduction, while waves from the south experience the least. Opposite

Deltaport, a 3 m incident northwest wave will be reduced to 0.6 m, while a 3 m south wave will

be reduced to 2.4 m. Further landward on the upper tidal flats, a 3 m incident wave will be

reduced to a height of approximately 0.4 m, regardless of the incident wave direction.

Tidal Currents

The Base model and Detailed hydrodynamic models predicted that tide current velocities are

generally low on the tidal flats, with the velocity magnitude varying proportionately with the rate

of change of the tide level. In the inter-causeway area, the direction is primarily on-shore/off-

shore, with the highest velocities occurring in the main tidal channels. Eelgrass, which produces

substantial flow resistance, limits velocities on the tidal flats and tends to concentrate the flow in

the unvegetated channels.


Maximum velocity in areas of eelgrass seldom exceeded 0.2 m/s, occasionally approaching 0.4

m/s during high tide. However, a relatively short distance away, in the adjacent drainage

channels the observed velocity frequently reached 0.4 m/s, and often approached 0.9 m/s. Since

the 2D numerical model representation of eelgrass is an approximation, it is expected that

eelgrass may actually have a more profound impact on flow patterns over the tidal flats. At lower

tide stages eelgrass probably retains more water than the model predicted, which might further

magnify velocities in the drainage channels, reduce velocities over the eelgrass beds, and more

significantly impact the distribution of flow over the flats.

The actual measurements of tide current flows corroborate the model predictions. When the tidal

flats are inundated at high tide the flow velocity is very low. Velocity increases gradually as the

tide drops, reaching a peak at about the mid-point of the tidal range. A lag effect, induced by

friction and temporary water storage, causes water to continue to drain from the tidal flats and

into the drainage channels even as the tide reaches its minimum and begins to rise. Velocities in

the main drainage channels are sufficient to develop dunes on the streambed.

Flow velocity and flow patterns at the crest protection structure undergo constant transition

through the tidal range as the water surface is uninterrupted by this structure.

On the flood tide, water begins to flow up into the tidal flats in the lowest part of the channel

parallel to the crest protection structure, while higher up on the tidal flats water continues to

drain into the channels and flow down to meet the flood tide. A change in flow occurs as the

flood tide overtops the large sandbar at the head of the main trunk channel. Once the bar is

overtopped the velocity increases rapidly and there is considerable transport of sand. Velocities

of up to 0.7 m/s were measured in water depths of only 0.1 m. The direction of bar-surface

ripples that were preserved during the ebb tide were quickly reversed to reflect the shoreward

direction of sediment transport during the flood tide.


The Detailed Model provides some additional insight into the hydraulic conditions forming the

tidal channels and more specifically, the sand lobe at the terminus of the main trunk channel.

Model predictions over a large tide cycle (27 December 2003) indicated that the highest

velocities in the main trunk tidal channel occurred during the ebb tide. This is the dominant

direction for sediment movement near the seaward end of the channel (Figure 7.16). However,

the model results also illustrates that velocities near the upslope terminus of the channel, adjacent

to the sand lobe, are of similar magnitude for the ebb and flood tides. During flood tides water

passes through the channel unimpeded, then diverges and spreads over the shallower flats near

the head of the channel. Local velocities of up to 0.6 m/s towards the sand lobe redistribute

sediment over and around the lobe and into the smaller dendritic channels toward the shore.

During ebb tides a significant amount of water is retained in the eelgrass, which creates a larger

head differential between water levels over the tidal flats and tide levels in Strait of Georgia. In

the vicinity of the sand lobe, local velocities of up to 0.6 m/s are predicted, similar to the peak

flood tide velocities.

Sediment Transport by Waves

Sediment transport was characterized in terms of the van Rijn mobility parameter (T), as defined

in Section 7.3.5 Modelling of Sediment Transport. The threshold of sediment movement occurs

when T = 0, and the transport intensity increases with increasing values of T (as illustrated in the

model outputs in Figures 7.17 and 7.18). The spatial distribution of sediment mobility for

southeast and south waves (Figure 7.17) and west and northwest waves (Figure 7.18) at high,

mean and low tide conditions show that in deep water where the bed velocities were virtually

zero, conditions were below the threshold for sediment movement. Initiation of motion occurred

at depths of approximately 7 to 10 m for incident wave heights greater than 1.5 m. Transport

intensities peaked in depths of 3-4 m, then decreased shoreward due to the reduction in wave

heights by refraction, shoaling and attenuation.

South East Waves

Incident southeast waves experience a substantial reduction in height due to refraction as they

pass over the relatively shallow tidal flats (Figure 7.17). This also causes the waves to change

direction appreciably and become aligned with the tidal flat bathymetry. The highest sediment

mobility (T> 6) occurred in depths between 3 and 4 m west of the Roberts Bank causeway near


the point of initial wave breaking. The B.C. Ferries and Roberts Bank causeways shelter the

inter-causeway area, greatly reducing velocities and sediment transport conditions, and as a

result, conditions are below sediment movement thresholds in the ship turning basin.

South Waves

Incident south waves approach Roberts Bank virtually straight on, and experience less refraction

than other waves (Figure 7.17). Even though the incident deep-water wave height was less than

for southeast waves, the local sediment transport conditions on the shallow tidal flats were

similar or higher in magnitude. In the ship turning basin, conditions were well below the

threshold. The greatest sediment mobility occurred around mean tide on the lower portions of the

tidal flats. At high tide, the bed velocities, and therefore sediment transport, were reduced due to

the deeper water. Transport intensities were high at low tide, but at this time only a relatively

small area of the lower tidal flats was exposed to waves from this direction.

West Waves

Incident west waves were refracted as they passed over the relatively shallow tidal flats and

quickly aligned nearly straight on to the slope of the flats (Figure 7.18). The highest sediment

mobility occurred west of the Roberts Bank causeway and on the west side of the B.C. Ferries

terminal. The east side of the inter-causeway area was sheltered by the Roberts Bank causeway

and was below the threshold for significant sediment movement. As with other scenarios

relatively high sediment transport intensities occurred at mean tide.

North West Waves

In localized areas on the east side of the Roberts Bank causeway incident northwest waves were

turned nearly 180 degrees by refraction (Figure 7.18). This effect, and direct shelter of the

causeway resulted in conditions being below the sediment movement threshold in the inter-

causeway area. Relatively low intensity sediment movement occurred west of the causeway.


Sediment Transport by Currents Fore-slope Sediment transport rates and sediment concentrations were computed from the hydrodynamic model runs as well as from direct measurements of current velocities at two DFO current meter sites offshore from the Roberts Bank causeway. The long-term time series of tidal current measurements was used to estimate frequencies of sediment movement and net transport rates over the year. Results are summarised in Table 7.1 At Location 1 (4 m below LLW) the threshold for sediment transport was exceeded 19 % of the time (68 days/yr), but the peak transport intensities were low (Table 7.1). For example the bed mobility exceeded twice the threshold for movement for a duration of only 3.9 days/yr. Net transport was towards the northwest at current meter sites, reflecting the dominance of flood tides. Annual transport rates during flood and ebb tides, and the resulting net transport quantity were also computed. These results indicate the net sand transport to the northwest is approximately 200,000 tonnes/yr across a 1 km wide zone of the fore-slope off the causeway. Estimates of sediment transport rates by theoretical equations are subject to considerable uncertainties. Good predictive equations may provide estimates that range between one half to two times the actual transport rates.

Table 7.1 Annual transport rates at two locations on the fore-slope.

Frequency (days/year) Transport Parameter (T) Location 1 Location 2

>0 68 108 >2 3.9 13 >3 0.65 >4 14

Inter-causeway Area Computations of sediment transport in the inter-causeway area from the 2D hydrodynamic model showed maximum shear stresses are well below the threshold for sand transport in the navigation channel, ship turning basin and around Deltaport, and they were weak or below the threshold over most of the tidal flats (Figure 7.19). The shear stresses were generally lower than estimates on the fore-slope because the peak current speeds in the Strait of Georgia are higher than on the shallow, vegetated tidal flats. Areas of high transport conditions in the inter-causeway area were restricted to the tidal channels, where conditions were sufficiently high to develop dunes.


The net sediment transport rate was computed at 15-minute time steps at a number of points in

the inter-causeway area for three different tidal ranges (large tide, mean tide and neap tide). In

the main tidal channel net transportation increased in the seaward direction during the ebb tide

due to the increased flow in the channel. By comparison, the transport rate was virtually constant

along the channel during the flood tide. There was a net seaward movement of sediment near the

channel outlet and very little near its landward end.

Annual net sediment transport was estimated by integrating the loads in three representative tidal

ranges over the year, combined with their frequency of occurrence. The computed net seaward

transport in the channel varied down the channel between 3,000 and 15,000 tonnes/yr using the

sand-bed channels equation (Engelund and Hansen 1967) and between 500 tonnes/yr and 3,000

tonnes/year using the van Rijn equation (van Rijn 1989). The computed net transport rates were

very sensitive to small changes in flow parameters. The net seaward transport provides further

evidence that the channel is still in a state of adjustment, and that headcutting is still occurring.

7.4.7 Summary of Existing Coastal Geomorphology Trends

The following qualitative assessment of the general morphological trends at Roberts Bank is

based primarily on historical observations made over the last 75 years, and on an understanding

of the physical processes occurring on Roberts Bank. It predicts likely changes on the tidal flats

if the existing conditions persist; i.e. no further developments on Roberts Bank.

Historical evidence and results from the numerical models indicate that sediment transport rates

on most of Roberts Bank are relatively low. Furthermore, most long-term morphological changes

are associated with tidal drainage channel formation in the inter-causeway area, or channel

shifting near the outlet of Canoe Passage. Future morphologic changes in the vicinity of

Deltaport are expected to be minor. However, it is apparent that the network of drainage

channels that formed after the earlier terminal and causeway developments is still evolving and

has not reached a final equilibrium state. In particular, the largest drainage channel (Figure 7.11)

is still extending seaward and landward. The large area of deposition near the head of the

channel also appears to be extending further landward and accreting to a higher elevation. Field

investigations and analytical studies indicate the trunk channel will continue to migrate landward

until reaching approximately elevation 2 m (chart datum). The zone of sand deposition at the


head of the channel will also shift landward. This should not impact native eelgrass (Zostera

marina) since it is confined to lower elevations. There is a chance that the smaller drainage

channel to the east, may also continue to grow, particularly if the outlet can by-pass the crest

protection structure by joining with the larger channel along the creek. Only minor erosion is

necessary to cause this development.

7.5 ASSESSMENT OF IMPACTS

7.5.1 Analysis Framework

The following geomorphological conditions were analysed to assess the significance of impacts

to the environment:

• the extent of potential impacts on wave conditions;

• the extent of potential impacts on tidal currents;

• the extent of potential impacts on sediment transport conditions; and

• the potential for new drainage channel development in response to the Project.

Based on experience and knowledge gained from earlier developments on Roberts Bank, the aforementioned geomorphological conditions have the potential to cause: local erosion or scour adjacent to new structures; impacts to overall delta stability caused by interruption of sediment transport; changes to deposition patterns in dredged basins and adjacent areas; and eelgrass erosion on the tidal flats due to new drainage channel development. These environmental effects are discussed in more detail below.

Given the relatively small scale of the proposed development in comparison to previous developments, potential impacts were likely to be more localized, and the analysis required a relatively fine resolution to detect any impacts.

The assessment of morphologic response to the Deltaport Third Berth proposals was based on two approaches. First the numerical wave and hydrodynamic models was used to generate representative hydrodynamic simulations of “With Project” and “Without Project” scenarios. The difference between the velocities and bed shear stresses in each scenario was mapped to present the area of potential impact. Additional sediment transport computations were then made


to further assess the effect of these hydraulic changes on sediment movement patterns. Secondly, independent of the numerical modeling, interpretative geomorphic assessments from the experience of earlier developments at the two terminals and the field observations were made.

7.5.2 Impacts on Wave Conditions

Outputs from the two scenarios were compared by computing the changes between them at each point in the model grid. Two parameters were used to represent the project impacts: changes in bottom velocity (U1-U0) and changes in sediment mobility (T1-T0). The impact on waves from the southeast (Figure 7.20) and south at low, medium and high points of a mean tide indicate the impact zone was very localized, being limited virtually to the footprint of the main structures. The wave height was increased directly in front of the wharf extension. The wave height was reduced directly behind the wharf and along the side of the causeway directly behind the third berth wharf extension due to sheltering. No significant changes in velocity or sediment mobility due to waves occurred in the vicinity of the drainage channels or on the upper tidal flats.

7.5.3 Impacts on Tidal Currents

The projected bathymetry after construction of Deltaport Third Berth was digitized and used to generate a new computational mesh for the inter-causeway area, with the project area added as a no-flow boundary. The model was re-run for a range of different tide conditions. The impacts of Deltaport expansion were very localized, and confined to the immediate flow field around the modified structures and the dredged channel. The absolute magnitude of the changes was also very small, and to illustrate these changes detailed comparisons of the local area around the project site were prepared for representative large and mean (Figures 7.21 and 7.22) tidal ranges during flood and ebb tides. During flood tides the end of the present wharf extends into the flow, inducing a zone of flow divergence and a weak re-circulating eddy behind it (Figure 7.21). Extending the wharf further displaces this flow pattern landward, but does not appear to change the overall pattern. During the ebb tide, the wharf causes the flow to accelerate around its end creating a zone of local flow concentration (Figure 7.22). Extension of the wharf creates higher flow concentration at the end of the structure. However, the absolute magnitude of the peak velocities remains very low and is typically less than 0.1 m/s. These results indicate that the primary impact of the proposed Deltaport expansion will be to shift the existing flow patterns shoreward.


The velocity differences between the two scenarios were computed at each node in the mesh, then plotted and contoured to illustrate the spatial extent of the project impacts (Figure 7.23). Velocity differences of less than 0.05 m/s were considered to be insignificant as they are within the tolerance of the numerical model computations. The impacts are highly localized and so small as to be near the tolerance of the numerical computations (apparent velocity differences far from the project are induced by numerical imprecision rather than actual project impacts).

7.5.4 Impacts on Sediment Transport

The magnitude of the wave bottom velocities and tidal current velocities in the vicinity of the

proposed expansion area are well below the threshold for significant sediment transport of fine

sand. Therefore, no significant alterations in sediment transport patterns are expected to occur.

Furthermore, no significant local scour or infilling is anticipated adjacent to the wharf,

navigation channel or in the tug basin.

7.5.5 Impacts on Tidal Drainage Channels

The main excavation for the Deltaport Third Berth Project involves deepening the approach

channel to the wharf extension (Figure 7.24). This excavation occurs in deep water where the

existing ground level is -12 m CD. Based on our understanding of past tidal channel formation

on Roberts Bank, there is no potential for significant headcutting to occur from an excavation at

this depth. Furthermore, due to the large depths, the peak velocities generated by tidal currents in

this area are very low; typically less than 0.2 m/s. Therefore, the risk of any significant new

channel formation occurring from deepening the north end of the existing ship channel appears

to be negligible.

Preliminary plans show the tug basin extending into the existing crest protection, which is at 0 m

CD. Some modification to the structure is anticipated, although details are not finalized.

Excavation of the tug basin may involve cutting into portions of the tidal flats that are above

LLW. Theoretically, there is potential for minor headcutting to be initiated, particularly if the

crest protection was eliminated. However, given the limited extent of the excavation and its

orientation with respect to the existing drainage patterns, the potential for headcutting is expected

to be minor. Furthermore, this effect will be controlled if the crest protection structure is

modified to tie-in with the tug basin.


7.5.6 Conclusions

Based on the combined analysis of the numerical modelling and geomorphic investigations six

conclusions regarding the proposed Deltaport Third Berth Project have been reached. Points 1

through 3 relate specifically to the proposed Project, while 4 through 6 are ongoing effects from

existing developments at Roberts Bank and are not affects from the Project.

1. Impacts of the proposed Deltaport Third Berth Project on tidal currents and waves

will be small and confined to the immediate flow field around the Deltaport Third

Berth wharf extension and the dredged channel. Tidal velocities will increase

locally along the front of the wharf extension, and decrease behind it. The

magnitude of the velocities in the impact zone is well below the threshold for any

significant sediment transport. No significant morphological impacts are expected

to develop in response to changes in tidal flow magnitude or direction.

2. The end of the present wharf extends into the current flow, which causes zone of

flow divergence and a weak eddy to form behind the wharf during flood tides.

Extending the wharf shifts the flow pattern landward but does not appear to

significantly change the overall pattern. During the ebb tide, flow accelerates and

concentrates at the end of the wharf. Extending the wharf will create higher flow

concentration at the end of the structure, however the absolute magnitude of the

peak velocities remains very low and is typically less then 0.1 m/s, which is very

similar to the present situation.

3. The risk of new tidal drainage channels forming in response to planned navigation

dredging appears to be very low. The proposed channel excavation would occur

in relatively deep water (below LLW and the tidal flats where headcutting is

initiated) and will not induce significant headcutting on the tidal flats. There is

some potential for localized disturbance of the tidal flats near the proposed tug

basin, which could induce shallow, small-scale channels, although this can be

controlled if the existing crest protection structure is modified. Further design

information on the tug basin and re-aligning the crest protection structure needs to

be reviewed and assessed for more certainty.


4. Previous developments on Roberts Bank induced long-term morphological

changes on the tidal flats, which continue at the present time. The main changes

involved initial headcutting on the tidal flats, expansion of eelgrass beds and the

formation of a network of tidal drainage channels.

5. The large drainage channel in the middle of the inter-causeway area continues to

expand in both seaward and landward directions. Sand from this channel is being

transported up onto the tidal flats during flood tides to form a large sand lobe. It is

expected that the channel will expand further shoreward before eventually

stabilizing. Shoreward expansion is limited by the higher elevation of the tidal

flats and the reduced amount of runoff that can be generated during the tidal

exchange.

6. The crest protection structure appears to have effectively limited the growth of at

least two of the smaller channels. However, the large channel near the centre of

the inter-causeway area has partially by-passed the structure and continues to

expand. This site, as well as similar experience at the smaller structure near the

B.C. Ferries terminal, illustrates the difficulty of fully preventing channel

formation when the contributing drainage area leading to the channel is large.

This situation does not arise at the Third Berth Project.

7.5.7 Significance of Effects

Based on the significance criteria for this assessment (the extent of changes to coastal processes

and the potential for the development of new drainage channels), there are no significant effects

of the Deltaport Third Berth Project on the coastal environment or processes at Roberts Bank.

Changes continue to occur as a result of earlier developments on Roberts Bank, but this

investigation into the introduction of the Deltaport Third Berth Project has not shown any

increase in the magnitude or extent of these changes.


7.6 MITIGATION

The results of this investigation show that the effects of the proposed Deltaport Third Berth

Project on the hydrodynamics and coastal processes at Roberts Bank are essentially unchanged.

Therefore, no specific mitigation measures are proposed.

The wave and current climates in conjunction with the layout of the coastal developments drive

sediment transport changes at Roberts Bank. Many design features were incorporated into the

Deltaport Third Berth Project to minimise or avoid potential impacts on the hydrodynamics and

coastal processes of Roberts Bank. These include:

• sloping rock revetment design to minimise reflection and propagation of waves;

• crest protection – Subject to detailed engineering design on the wharf structure,

the extension of the crest protection into the new Tug Basin will be assessed from

a geomorphological perspective and planned and constructed in advance of

dredging for the basin to mitigate for potential impacts on hydrodynamics and

coastal processes.

7.7 RESIDUAL EFFECTS

As there are no significant effects on coastal processes arising from the construction of the

proposed Deltaport Third Berth there can be no residual effects after mitigation.

Documents

7.0 COASTAL GEOMORPHOLOGY - Roberts Bank · Expansion - Coastal Geomorphology Study (2004). 7.2 STUDY AREA The study area for the coastal geomorphology study is delimited by Steveston