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5.0 TIDAL CURRENTS
5.1 METHOD OF APPROACH
A suite of numerical models was used to predict tidal currents and resulting sediment transport
patterns under various scenarios and conditions. Details of the numerical model development,
calibration and testing are described in Appendix B. Model development was carried out in three
phases. The first phase involves tidal modelling studies in the Strait of Georgia, Juan de Fuca
Strait and around Vancouver Island using Triton Consultants program “Tide2D”. This “Wide
Area Model” was used to provide tidal height and tidal current boundary conditions along a line
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 are used primarily to drive other detailed tidal models of the Roberts Bank
inter-causeway area.
The second phase involves developing a “Base Model” to simulate hydrodynamic conditions in
the Fraser estuary, Roberts Bank tidal flats and adjacent portions of the Strait of Georgia. The
Surfacewater Modelling System (SMS 8.0) program was used to develop the Base Model. This
package utilizes the US Army Corps of Engineers’ two-dimensional finite element
hydrodynamic and advection-dispersion models, including RMA2, SED-2D and ADCIRC. The
Base Model was used to assess general flow patterns in the area of interest and to identify the
potential extent of impacts from various alternative project developments.
Finally, a “Detailed Model” 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 Detailed Model is 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 wharf extension at 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.
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5.2 BASE MODEL
5.2.1 Base Model Description
After a period of initial model testing it was decided to utilize the ADCIRC program in SMS as
the Base Model. ADCIRC is the Advanced Circulation model for ocean, coastal areas and
estuaries developed by Dr. Rick Luettich at the University of North Carolina at Chapel Hill and
Dr. Joannes Westerink at the University of Notre Dame. The model has been developed for the
U.S. Army and Navy over the last 15 years to compute circulation in coastal oceans associated
with tides, winds and density-driven flows (Luettich et al., 1991). ADCIRC is based on a
reformulation of the shallow water equations and applies a finite element discretization strategy,
which makes it free from spurious oscillations. The equations have been formulated using the
traditional hydrostatic pressure and Boussinesq approximations and have been discretized in
space using the finite element (FE) method and in time using the finite difference (FD) method.
5.2.2 Base Model Development
The bathymetry required for numerical modelling was derived from information provided by
Triton and VPA. Although the Base Model extended well into the Pacific Ocean and up the
Fraser River, the region of interest included a 23 km long section of Roberts Bank, beginning at
Steveston Bend and extending to Point Roberts. Once the model extents were defined a
computational mesh was generated by defining the spatial and hydraulic characteristics of the
bank as a series of nodes and elements. Node spacing was typically 100 m on the tidal flats and
Causeway areas. The computational mesh in the area of interest is shown in Figure 5-1.
5.2.3 Model Verification
The results from the Base Model were compared to tide levels predicted by Tide2D. In addition,
comparisons were made with published tide elevation data supplied by the Department of
Fisheries and Oceans, Canadian Hydrographic Service tidal stations. Appendix B summarizes
these comparisons and shows that the model predictions agreed closely with the published
information. The predicted velocities were also compared to field data recorded by two current
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meters located immediately west of the Deltaport Terminal. The results are summarized in
Figure 5-2.
Figure 5-1: Base Model Computational Mesh in Area of Interest
Figure 5-2: Comparison of Predicted and Observed Velocities near Roberts Bank for Period February 23 to 24, 1984
0
0.2
0.4
0.6
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1
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4753800 4840200 4926600 5013000 5099400Time (s)
Velo
city
(m/s
)
-120
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60
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300
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Dire
ctio
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eg fr
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Predicted Vel Measured Vel Predicted Dir Measured Dir
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5.2.4 Base Model Results
Figures 5-3 and 5-4 illustrate typical tidal patterns on Roberts Bank. Throughout the tidal cycle
velocities remain low within the study area (near the terminal and between the causeways), with
maximum velocities below 0.7 m/s. Between the causeways flow remains aligned with the
causeways except during the low velocity tidal peaks where flow switches from ebb to flood or
flood to ebb. The highest velocities in the inter-causeway area located along the main inter-tidal
channel east of the south turn basin, and along the edges of the causeways. Eelgrass (modelled
with increased roughness) and the presence of defined inter-tidal channels tend to concentrate
flow in these regions.
5.3 DETAILED MODEL
5.3.1 Detailed Model Description
The Detailed Model of the inter-causeway tidal flat area used the program River2D, a model that
is particularly well suited for representing shallow flows on eelgrass covered tidal flats. One of
its unique features is the ability to simulate wetting and drying flow on tidal flats by changing the
surface flow equations to groundwater flow equations in these areas. This procedure allows
calculations to carry on without changing or updating the boundary conditions. The model, like
other 2-D finite element depth averaged models, solves the conservation of mass (continuity
equation) and the conservation of momentum equations in horizontal plane (x and y directions
with z defining the vertical). The resulting steady-state solution is provided in terms of a unit
discharge in both the x and y direction and flow depth. A complete description of the model
along with the solution of the 2-D depth averaged equations and underlying assumptions can be
found in Ghanem et al. (1996).
Figure 5-5 shows the extent and computational mesh of the Detailed Model of the inter-
causeway area of Roberts Bank. Four time frames were identified in order to subdivide the
modelling effort into representative tidal cycles. Three time periods defined typical 2003 neap,
mean and high tide cycles as predicted by both the ADCIRC Base Model and Tide2D. The
fourth time period modelled the May 2004 field investigation for verification of the detailed
model results using predicted tide
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Figure 5-3: December Ebb Tide - Existing Conditions
Figure 5-4: December Flood Tide - Existing Conditions
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Figure 5-5: Detailed Model Extent and Mesh
levels posted by the DFO. The four time periods and tidal cycles are presented in Figure 5-6 and
included:
• Neap Tide – February 6 to 8, 2003
• Mean Tide – May 1 to 3, 2003
• High Tide – December 25 to 27, 2003
• Field Verification – May 6 to 8, 2004
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Figure 5-6: Representative Tidal Cycles
5.3.2 Verification of Detailed Model
A series of field investigations were conducted in April and May 2004 to measure velocity
magnitude and direction on Roberts Bank. The measurements were made with an RDI Rio
Grande ADCP. These data were used to verify the predicted velocities from the Detailed Model
in the ship turning basin and on the tidal flats. Figure 5-7 shows the ADCP current measurements
collected over Roberts Bank and in the tidal channel between the Deltaport and BC Ferry
causeways. Figure 5-8 shows the locations where modelled and measured velocities are
compared.
Figure 5-9 shows predicted velocities in the main trunk of Channel 1 at comparison location 1,
located mid-way along the largest dendritic channel between the two causeways. Both the field
measurements and modelled velocities indicate that the highest velocity in this channel on May
8, 2004, is approximately 0.8 m/s and occurred during ebb tide conditions. The timing of the
maximum velocity lags the highest rate of tidal drop (Tsawwassen Gauge), most likely due to
water retained in the eel grass beds. However, the peak velocity during flood 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.
0
1
2
3
4
5
0 12 24 36 48 60 72
Time (hr)
Wat
er S
urfa
ce E
leva
tion
(m c
hart
dat
um)
Mean Neap High
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Figure 5-7: Location of ADCP Current Measurements on Tidal Flats and in Tidal Drainage Channels
Figure 5-8: Comparison Locations for Modelled and Measured Velocities
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Figure 5-9: Comparison of Predicted and Measured Velocity in Trunk Drainage Channel for Comparison Location 1
Figure 5-10 shows velocities at two points in the ship turning basin, near the vicinity of the
proposed Deltaport expansion. The velocities were much lower due to the larger depths, and the
predicted values were very close to the measured values.
-1.0
-0.8
-0.6
-0.4
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Time (hours)
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city
(m/s
)
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3.0
3.5
4.0
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Tide
Lev
el (m
)
Trunk Channel Vel (1)ADCP Survey DataTide Level
May 8, 2004
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Figure 5-10: Comparison of Predicted and Measured Velocity in Ship Turning Basin for a) Comparison Location 2, and b) Comparison Location 3
5.3.3 Flow Patterns Near Deltaport
Figure 5-11 shows the present flow patterns in the inter-causeway area during a typical Mean
Tide. Results for other representative tide conditions are provided in Appendix B. The predicted
flow patterns are relatively simple and agree well with basic hydrodynamic principles. The flow
is primarily an onshore/offshore exchange of water and velocities vary proportionately with the
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
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0 10 20 30 40 50 60 70 80
Time (hours)
Velo
city
(m/s
)
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1.0
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2.0
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3.0
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4.0
4.5
5.0
Tide
Lev
el (m
)
Turning Basing Velocity (2)ADCP Survey DataTide Level
May 8, 2004
a)
-1.0
-0.8
-0.6
-0.4
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Time (hours)
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city
(m/s
)
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el (m
)
Turning Basing Velocity (3)ADCP Survey DataTide Level
May 8, 2004
b)
Vancouver Port Authority Northwest Hydraulic Consultants Ltd. /Triton Consultants Ltd. Roberts Bank Container Expansion File: 33863 Coastal Geomorphology Study - 68 - November 2004
Figu
re 5
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Flo
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rns d
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ean
Tid
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ycle
– M
ay 3
, 200
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Vancouver Port Authority Northwest Hydraulic Consultants Ltd. /Triton Consultants Ltd. Roberts Bank Container Expansion File: 33863 Coastal Geomorphology Study - 69 - November 2004
rate of change of the tide level. The model accounts for topography and eelgrass; however, some
locations introduce a degree of complexity to the detailed flow pattern that is difficult to
represent in the model. For instance, the model is not able to account for flow through the voids
in the riprap crest protection, which was observed to be a substantial portion of the flow during
certain portions of the tidal cycle.
The eelgrass has a major effect on velocities on the tidal flats. In areas of eelgrass, the maximum
velocity was observed to seldom exceed 0.2 m/s, occasionally approaching 0.4 m/s during high
tidal cycles. A relatively short distance away, 20 m to 50 m, in the adjacent sand-covered
drainage channels the observed velocity frequently reached up to 0.4 m/s and often approached
0.9 m/s. Since the 2-D numerical model representation of eelgrass is an approximation, it is
expected that the eelgrass may have a more profound impact on flow patterns over the tidal flats.
At lower tide stages eelgrass most likely retains more water than the model predicted which may
further magnify the velocities in the drainage channels, reduce velocities over the eelgrass beds,
and more significantly impact the distribution of flow over the flats.
5.3.4 Flow Patterns in Drainage Channels
The scale of many features in the key tidal drainage channels is too small to be resolved in the
topographic surveys and mesh used in the detailed model. Therefore, we have used a
combination of direct observations and numerical modelling to describe the hydraulic conditions
in these channels.
Observed Flow Conditions
Three series of velocity measurements were made on the tidal flats during this investigation. On
March 26, 2004 a Swoffer portable propeller meter was used to measure velocities in the tidal
channels. On April 6-7th, 2004 we used an acoustic Doppler Current Profiler (ADCP) 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 May 8th, with most data collected on
the west side.
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At high tide, the tidal flats are inundated under several metres of water and the flow velocity over
the flats is very low. Velocity increases gradually over the flats as the tide drops and reaches a
peak velocity at about the mid-point of the tidal range. A lag effect, induced by friction and
temporary water storage on the flats, 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. The velocities
in the trunk 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. At tide levels over two metres above the crest protection structure, the
water surface is uninterrupted by the presence of the crest. As the tide drops however, the water
surface shows signs of the flow accelerating over the crest. At approximately one metre above
the crest protection structure, water flows over the entire length of the crest with an average
velocity of about 0.8 m/s. As the tide drops further, flow across the eelgrass beds decreases and
there are obvious jets of higher velocity flow issuing from the channel outlets and flowing across
the crest protection structure, as well as a significant amount of flow being diverted along the
structure and discharging at the southern end of the crest. These jets decrease as the tide
continues to drop until at the lowest level of the tide the water surface in the turning basin is
below the crest protection structure and the majority of flow from the tidal flat channels is
diverted along the structure.
On the rising tide, water begins to flow upwards into the tidal flats in the lowest part of the
channel paralleling the crest protection structure, while higher up on the tidal flats water
continues to drain into the channels and flow down to meet the incoming tide. The most
spectacular change in flow occurs as the rising tide overtops the large sandbar at the head of the
Channel 1 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 ebbing tide were
quickly reversed to reflect the shoreward direction of sediment transport, and anti-dunes forming
on the bar surface indicated super-critical flow.
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Results From 2D Model
The detailed 2-D numerical model provides some additional insight into the hydraulic conditions
forming the tidal channels and more specifically the sand lobe located at the terminus of the main
trunk channel. Flow patterns in the vicinity of the largest sand lobe were analyzed over a large
tidal cycle (December 27, 2003). Figures 5-12 and 5-13 illustrate velocity magnitude and
direction over the sand lobe under ebb and flood tide conditions, respectively. It is important to
note the location and timing of the highest velocities encountered during the tidal cycle. The
highest velocities in the tidal channel occur during ebb tide conditions. This is also the dominant
direction for sediment movement near the seaward end of the channel. However, the model
results also illustrate that velocities near the upslope terminus of the dendritic channel, adjacent
to the sand lobe, are of similar magnitude for the ebb and flood tides.
During flood tides water passes through the dendritic channel unimpeded, then diverges and
spills over the shallow tidal flats at 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.
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Figu
re 5
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Flo
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urin
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bb T
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Nea
r th
e H
ead
of th
e L
arge
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Cha
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Vancouver Port Authority Northwest Hydraulic Consultants Ltd. /Triton Consultants Ltd. Roberts Bank Container Expansion File: 33863 Coastal Geomorphology Study - 73 - November 2004
Figu
re 5
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Flo
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Tid
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ear
the
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the
Lar
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hann
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