Request Number IR1-12: Flow Passage. Information Request

Transcription

1 Request Number IR1-12: Flow Passage Information Request Provide additional information about the 100 metre flow passage channel scenario between the Westshore Terminals and the proposed Project terminal including: 1. A drawing of the flow channel, including a cross-section. 2. Information about the design of the flow channel, including the approximate discharge capacity of the flow channel. 3. Information about the amount of flow draining into the tidal flats in the inter-causeway area and how it would be influenced by different ebb and flood tides. 4. Information about the maximum potential size for the flow channel and a description of the advantages and/or disadvantages from the use of various sized flow channels or the use of multiple flow channels. 5. Mitigation measures that would be required to protect the flow channel and adjacent areas. VFPA Response Background A 100 m wide flow passage channel was investigated as part of a project optimisation study, in which three conceptual optimisations to reduce potential scour on the preferred terminal orientation were considered, including: Rounding the northwest corner of the terminal; Rotating the terminal; and Adding a flow passage channel A combination of these optimisations was also considered. The channel was included in the study as one possible way to reduce the potential area of scour that was expected to develop at the northwest corner of the terminal in response to the diversion of tidal flow around the terminal during both rising and falling tide conditions (see EIS Section ). Flow passing through a channel between the existing Westshore Terminal and the proposed Project was analysed to determine if this could potentially reduce the volume of water that would otherwise be forced to flow around the proposed terminal The results of the coastal geomorphic studies undertaken indicated that rounding the northwest corner of the terminal would have the greatest positive effect in terms of decreasing Sufficiency Information Request #12 (IR1-12) Page 1

2 16 17 the areal extent of footprint-related effects (i.e., scour induced by flow acceleration around the west side of the terminal). 1. A drawing of the flow channel, including a cross-section As part of the information gathering stage of the investigation to identify possible ways to reduce the potential area of scour at the northwest corner of the terminal, the project team discussed the engineering requirements and limitations of the three conceptual optimisations that were being considered. The 100 m wide channel was adopted for the investigation as it could be accommodated relatively easily within the existing terminal design without significant changes to the terminal configuration. A two-dimensional hydrodynamic model (TELEMAC-2D) was used to evaluate the 100 m flow passage channel Figure IR shows a plan drawing of the flow channel as represented in the TELEMAC-2D model. The dashed lines show the location of data that was extracted to generate a longitudinal profile (Figure IR1-12-2) and a cross-section of the channel (Figure IR1-12-3). In the absence of design information available at the time the analyses were conducted, the channel bottom was assumed to conform to the existing bed elevations, and the sides of the channel were assumed to be free of rock protection, therefore they rise vertically from the bed. This channel configuration was represented in the model. These assumptions would tend to result in an over-prediction of the channel capacity. Sufficiency Information Request #12 (IR1-12) Page 2

3 Figure IR Plan View of the 100 m Channel as Represented in the TELEMAC-2D Model Proposed RBT2 Terminal Westshore Terminal Note: The dashed lines show the locations of longitudinal profile (Figure IR1-12-2) and cross-section (Figure IR1-12-3) information that was extracted from the model. Figure IR Longitudinal Profile of the 100 m Channel, as Represented in the TELEMAC-2D Model Sufficiency Information Request #12 (IR1-12) Page 3

4 Figure IR Typical Cross-section of the 100 m Channel, as Represented in the TELEMAC-2D Model 2. Information about the design of the flow channel, including the approximate discharge capacity of the flow channel As noted above, the channel was not designed at the time of the project optimisation study. A 100 m wide channel with no bank details was chosen for the analyses because this was thought to be readily accommodated within the existing terminal design (referred to as the idealised channel below) Discharge through the idealised channel would be primarily controlled by the difference in tide height at either end of the channel (the hydraulic head) and the depth of flow, which is in itself a function of tide height. Discharge would drop to zero when tide height at the ends of the channel are the same and when tide height drops below the highest bed level at the shoreward end of the channel. The capacity would be primarily dictated by the potential for the rising and falling tide to drive water through the channel. Manipulation of the channel characteristics, for instance by installing scour protection (see Question 5 response below), is a secondary factor that would determine discharge. Figure IR shows a plot of tide height (primary axis) and discharge (secondary axis) in the channel over a tidal cycle. These data were extracted from the TELEMAC-2D model, and show the approximate range of flows that could occur. By convention, the discharge (i.e., volume rate of water flow passing through the channel) is expressed as negative values when the tide is ebbing southward and positive values when the tide is flooding northward. The maximum absolute discharge during this tide cycle is just over 300 m 3 /s through the idealised flow channel. Sufficiency Information Request #12 (IR1-12) Page 4

5 Figure IR Plot of Tide Height Versus the Estimated Discharge through the 100 m Channel for One Tide Cycle 3. Information about the amount of flow draining into the tidal flats in the intercauseway area and how it would be influenced by different ebb and flood tides Flow from the 100 m flow channel would convey water between the intertidal area shoreward of the proposed RBT2 terminal and the Strait of Georgia. There would be no flow via the hypothetical 100 m channel into the inter-causeway area as there is no direct hydraulic connection For information on tidal influences in the inter-causeway area, which is bounded to the south by the B.C. Ferries causeway, to the north by the Roberts Bank causeway, and at the shoreward extent by sea dykes, refer to EIS Section Information about the maximum potential size for the flow channel and a description of the advantages and/or disadvantages from the use of various sized flow channels or the use of multiple flow channels The size of the flow channel is related to constraints such as shifting of the terminal northwestward, and the need to provide access by road and rail across the channel. In general, a wider channel would require a greater shift in terminal location and a multi-span bridge with in-channel bridge piers at approximately 30 m to 40 m spacing. The presence of in-channel piers and the associated scour protection that would be required (see response to Question 5 below) would reduce the effective flow capacity of the channel. Sufficiency Information Request #12 (IR1-12) Page 5

6 For the purposes of reducing scour at the northwest corner of the Project, a wider flow channel would have the advantage of accommodating more flow that would otherwise flow around the structure; however, it is recognised that local scour would also be associated with the flow channel and there may be no net reduction in seabed scour overall (i.e., combined change at the northwest corner and from the flow channel not beneficial). For these reasons, a maximum channel width of 100 m was considered in the project optimisation study There is no advantage to the use of multiple flow channels. Flow capacity would be greater in a single large channel than two or more smaller channels with a sum width similar to the single large channel. Multiple channels would disturb a larger area of the seabed while carrying less flow. 5. Mitigation measures that would be required to protect the flow channel and adjacent areas Mitigation would be required to protect the flow channel and adjacent areas as the 100 m flow passage channel was estimated to generate local velocities of up to 1.1 m/s, which is above that required to transport sand. It is assumed that a requirement for mitigation for the flow channel would be related to ensuring that the adjacent terminal structures are not undermined through scour and lowering of the seabed. Such mitigation could take the form of rock rip-rap along the edges of the flow channel, possibly in combination with general armouring of the bed Scour of the adjacent areas of the tidal flats and subtidal portions of the foreslope may not result in conditions that would affect the stability of the terminal structures. In these adjacent areas, scour could potentially be mitigated by the application of a scour blanket of suitably sized gravel or rock. Because a scour blanket would change the substrate characteristics and modify the physical environment for marine organisms, it may be more desirable to allow the seabed to scour in areas that would not be expected to affect the stability of the terminal structures. Alternatively, adjacent areas could be pre-dredged to the expected scour depth to avoid the deposition of the eroded sediment onto adjacent areas. Sufficiency Information Request #12 (IR1-12) Page 6