7.0 coastal geomorphology - port of vancouver · expansion - coastal geomorphology study (2004)....

Deltaport Third Berth Project January 2005 Environmental Assessment Application 65

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.

Deltaport Third Berth Project January 2005 Environmental Assessment Application Page 167

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.

Deltaport Third Berth Project January 2005 Environmental Assessment Application 2

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.

7.0 coastal geomorphology - port of vancouver · expansion - coastal geomorphology study (2004)....

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