5.0 TIDAL CURRENTS - Vancouver Fraser Port Authority · PDF fileAfter a period of initial...

Vancouver Port Authority Northwest Hydraulic Consultants Ltd. /Triton Consultants Ltd. Roberts Bank Container Expansion File: 33863 Coastal Geomorphology Study - 58 - November 2004

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.


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


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

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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


Figure 5-3: December Ebb Tide - Existing Conditions

Figure 5-4: December Flood Tide - Existing Conditions


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


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.

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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


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.

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May 8, 2004


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

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May 8, 2004

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Figu

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, 200

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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.


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.


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.


Figu

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