Review Panel Secretariat, Roberts Bank Terminal 2 Project | … · Review Panel Secretariat,...

From: Belland,Brenna [CEAA]To:Cc: Myles,Debra [CEAA]; Ronzio,Joseph [CEAA]; Lim,Julian [CEAA]; Sabo,Samantha [CEAA]; Panel RBT2 /

Commission RBT2 (CEAA/ACEE)Subject: Potential Effects of Opening the Causeway documentDate: September 9, 2016 4:38:55 PMAttachments: RBT2-Potential-Effects-of-Opening-the-Causeway-June-2005-NHC-Memo-January-2014.pdf

Hello Panel Members, As requested by the Panel, here is the document titled: Potential Effects of Opening the Causewayby Northwest Hydraulic Consultants dated June 14, 2005, referenced in Appendix 7.2-B of the EIS. The document will be posted to the CEAA Registry shortly. Thank you,Brenna Belland Review Panel Secretariat, Roberts Bank Terminal 2 Project | Secrtariat de la commissiond'examen, Projet du Terminal 2 Roberts Bankc/o Canadian Environmental Assessment Agency | a/s de lAgence canadienne d'valuationenvironnementale22nd Floor, 160 Elgin St. Ottawa ON K1A 0H3 | 160, rue Elgin, 22ime tage, Ottawa ON K1A [email protected] / [email protected]: 613-957-0626 / toll free 1-866-582-1884 | Tl. : 613-957-0626 / sans frais: 1-866-582-1884

"Jocelyne Beaudet"; "Diana Valiela"; "David Levy"
mailto:/O=EC/OU=NCR/CN=RECIPIENTS/CN=BRENNA.BELLANDmailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]

Roberts Bank Terminal 2 Project Potential Effects of Opening the Causeway January 2014

Roberts Bank Terminal 2 Project

2005 Memorandum Concerning Potential Effects of Opening the Roberts Bank Causeway In response to requests during Pre-Design Consultation (October/November 2013), Port Metro Vancouver (PMV) is releasing the following Memorandum concerning the previously undertaken analysis of the potential effects of opening the Roberts Bank causeway. The Memorandum is dated June 14, 2005 and authored by Northwest Hydraulic Consultants (NHC). Roberts Bank Terminal 2 Project Coastal Geomorphology Studies The Roberts Bank Terminal 2 Project (Project) is a proposed new three-berth marine terminal at Roberts Bank in Delta, B.C. that could provide 2.4 million TEUs (twenty-foot equivalent unit containers) of additional container capacity. The Project is part of Port Metro Vancouvers Container Capacity Improvement Program, a long-term strategy to deliver projects to meet anticipated growth in demand for container capacity to 2030. As part of ongoing environmental and technical work for the Project, PMV has been and continues to undertake field studies at Roberts Bank and the surrounding areas. The Coastal Geomorphology study program for the Project, a combination of field studies, interpretative geomorphic studies, empirical calculations, and numerical modeling, is currently underway. The local study area includes the Roberts Bank tidal flats extending from the seaward side of the dikes to -60 m chart datum (CD) depth, and extending from the BC Ferries terminal to Canoe Passage. To establish tidal conditions for the study area, numerical modeling incorporates the southern part of the Strait of Georgia. Study outcomes will provide a comprehensive understanding of the natural rates of change on the tidal flats and delta foreslope (e.g., accretion or erosion, tidal channel positions, and sedimentary characteristics) in relation to the Project. Consideration of Opening the Causeway PMV-led consultation undertaken in October/November 2013 suggests continued public and First Nation interest in breaching the Roberts Bank causeway to allow flow exchange between the north side of the causeway and the inter-causeway area. In 2005, as part of the Deltaport Third Berth Project, NHC issued a Memorandum to clarify, amongst other items, the potential geomorphological effects of opening the Roberts Bank causeway. NHC found that the volume of tidal water that could be exchanged across the causeway by adding a culvert would be insufficient to improve flushing conditions and may induce potentially damaging local morphological effects. Because of these findings, the Projects Coastal Geomorphology study program currently underway does not directly address breaching the Roberts Bank causeway. The 2005 NHC Memorandum is provided below. It should be noted that the Memorandum reflects plans current at the time. As the Project proceeds through an environmental assessment process, additional studies may be added or changes to the studies may be made based on recommendations from regulators. Field Studies Information Sheets PMV produces monthly field studies information sheets providing information about work occurring at Roberts Bank and the surrounding areas. These information sheets describe the purpose, area, methods and timing of studies.


Roberts Bank Terminal 2 Project 2

To sign up to receive monthly field studies information sheets, please visit www.portmetrovancouver.com/RBT2. For Further Information For further information, please visit our website at www.portmetrovancouver.com/RBT2 or contact us: Phone: 604-665-9337 Fax: 1-866-284-4271 Email: [email protected]
http://www.portmetrovancouver.com/RBT2http://www.portmetrovancouver.com/RBT2mailto:[email protected]

Ref. No. 33863 This technical note was prepared at the request of the Vancouver Port Authority (VPA) to clarify some issues related to physical processes and hydraulic behaviour in the Inter-Causeway area of Roberts Bank. These issues arose during discussions on May 30, 2005 between VPA, Fisheries & Oceans Canada (DFO) and Canadian Wildlife Service (CWS) concerning habitat compensation and mitigation measures for the Deltaport Third Berth Project. Draft minutes of the meeting were provided to NHC and comments have been prepared on three main issues that were discussed:

1. potential effects of opening a breach in the causeway by installing a culvert near the landward end of the flats;

2. clarification of the effectiveness of the crest protection and factors governing growth of the largest tidal channel in the Inter-Causeway area;

3. potential measures for controlling future evolution of the large tidal channel and sand lobe. The comments are based primarily on the results of our previous numerical modelling and geomorphic studies. Some additional simplified hydraulic calculations were made to assess the effect of a breach in the causeway. 1. POTENTIAL EFFECTS OF OPENING THE CAUSEWAY Proposed Opening The concept of installing a 1 m diameter culvert through the causeway was proposed to allow flow exchange between the north side and the Inter-causeway area. It was proposed that the culvert be set near Higher High Water (HHW) to reduce the duration of flows and minimize the risk of any potential adverse impacts. Analysis Carried Out Results from our two dimensional numerical model simulations were reviewed and a time series of water levels was output at specified locations on the north and south sides of the causeway. These water levels were then compared to determine the head difference across the causeway during various phases of the tide. A culvert program (HYDRO-CULV) was used to compute the discharge through the opening under the specified tailwater and headwater conditions. Assumptions It was assumed the invert of the culvert was set 1.0 m below the HHW (mean tide). The available charts show the upper mud flats are generally 0.5 m to 1.0 m higher on the north side of the causeway than on the south side. The culvert was set approximately 1,000 m from the shoreward end. The ground elevations were estimated as follows: north side: 3.5 m south side: 3.0 m

nhc memorandum

northwest hydraulic consultants

30 gostick placenorth vancouver, bc

V7M 3G2, canadatel: (604) 980-6011fax: (604) 980-9264

www.nhcweb.com

To:

Mr. Trevor Peach, Project Manager, Vancouver Port Authority

Date: June 14, 2005

From: David McLean Page: 1 of 8 Cc: Mr. Darrell Desjardins Re:

Questions on Coastal Geomorphology

nhc memo Page 2 of 8

Results of Analysis Water levels on north side and south side of the Causeway were predicted from the two dimensional hydrodynamic base model of the Fraser estuary and Georgia Strait. Levels for a mean tide (December 2004) were used in this analysis. The differential head Hdif was computed as: Hdif= WLnorth side WLsouth side During the flooding tide, water levels are higher on the south side. During the ebbing tide, water levels are higher on the north side. However, the differential head across the causeway is very small at the times when the culvert would be flowing. Typical water level differences ranged between +0.1 m to -0.1 m. Actual water level differences could be different from these predictions due to wind set up and wave effects. The amount of flow passing through a 1 m circular culvert was computed for a tailwater level of 4.0 m (Chart Datum) and a differential head of 0.1 m. The flow through the culvert was substantially less than 0.5 m3/s. The velocity in the inlet and outlet was computed to 0.4 m/s. A repeat calculation was then made for a 4.0 m long x 1.5 m high box culvert. The maximum discharge through the culvert was computed to be 1.0 m3/s. Entrance velocities remained near 0.4 m/s. The amount of flow draining off the tidal flats in the Inter-Causeway area was measured directly during our 2004 field program using an Acoustic Doppler Current Profiler (ADCP). This involved surveying transects from the Roberts Bank Causeway to the Tsawwassen Ferry Causeway and measuring the total discharge of water passing the transect line (the approximate location of this transect line is shown on Figure 1). The measured discharges reached up to1,600 m3/s during these investigations. The hydrodynamic model was also used to compute the total discharge across the line. During the December 2004 mean tide, the computed maximum discharge near the peak ebb reached 2,000 m3/s. This discharge is comparable to the magnitude of the Lower Fraser River at Hope. The existing flow off the tidal flats in the Inter-Causeway area is three orders of magnitude greater than the proposed culvert discharge. Figure 1: Location of Ebb Discharge Measurements From Tidal Flats

Velocity

0.00

0.60

0.54

0.48

0.42

0.36

0.30

0.24

0.18

0.12

0.06

0.00Velocity1.0 m/s

Distance500.0 m

Discharge line


Assessment of Results The hydrodynamic model results and ADCP discharge measurements demonstrate that the flow through a small culvert opening will be insignificant compared to the large tidal exchanges that take place on the tidal flats under existing conditions. Therefore, in our opinion, adding a culvert could not significantly improve flushing conditions in Inter-Causeway area. The large tidal flows that take place on and off the flats each tidal cycle do not seem to have been fully understood when concerns were expressed about possible stagnant conditions and low circulation in the Inter-Causeway region. In our opinion, the existing hydrodynamic model is the most appropriate tool for quantitatively assessing changes in flushing characteristics in this area. Providing an opening in the causeway could induce local morphological effects on both sides of the opening. The magnitude of the impacts would depend on the size of the opening, the invert elevation of the culvert and its location along the causeway. Local increases in velocities at the outlet would induce scour which could lead to dendritic channel formation on the tidal flats. Typical channels that have formed in this manner are visible near the upper end of the tidal flats adjacent to the dyke and drainage outlets along the Tsawwassen Band. It is expected that these new dendritic channels would develop on both sides of the causeway due to the reversing nature of the flows. The risk of channel formation is probably higher on the south side since the ground elevation is noticeably lower on the south side relative to the north side. This will cause groundwater seepage flow to continue after the tide level has fallen below the culvert invert levels. In our opinion, there is a substantial risk that a new opening in the causeway could initiate a new sequence of morphological changes on the tidal flats, which could impact the existing habitat conditions. Conclusion In summary we do not anticipate that opening the causeway would provide a benefit with regard to introducing any significant flows into the intercauseway area. Additionally, we feel that such openings would likely be detrimental in that they would result in the generation of new channels and concurrent erosion of the existing eel grass. 2. ROLE OF CREST PROTECTION IN CONTROLLING TIDE CHANNELS Historical charts, air photos and results from previous investigations (Triton, 1996) were used to document the historical evolution of the drainage channels on the tidal flats. The main findings are described in Appendix C (Section C3.5) of the Coastal Geomorphology report. Some of the key points from the investigation are summarized below. The crest protection structure was constructed in 1982, after headcutting and tidal channel formation had already been initiated by earlier dredging operations (1979 photo in Figure C5-6). Some of the topographic changes which promoted subsequent channel formation were already underway before the structure was installed, making the conditions for mitigating impacts more difficult. The crest protection was successful in controlling channel formation on the north side of the dredged basin. The channels that had started in 1982 have completely disappeared or been buried by subsequent eel-grass formation in this area (1995 photo in Figure C5-7). Therefore, we consider the crest protection has been effective in preventing the expansion of dendritic channels over extensive portions of the tidal flats. Subsequent expansion of eel-grass changed the hydraulic characteristics of the tidal flats and concentrated drainage flow into the central portion of the Inter-Causeway area. The expansion of the eel-grass is directly linked with the growth of the large tidal channel in this area. Without this concentration


of flow, the drainage would not have been sufficient to incise a channel into the tidal flats. Flow near the outlet of the largest tidal channel was sufficiently strong to incise a new channel along the back side of the crest protection (2002 Photo in Figure C5-7). As a result, instead of being ponded by the crest protection, some of the flow by-passed the structure and developed a new channel behind the riprap. A wider crest protection or provision of a broad cobble/boulder apron behind the crest protection might have been sufficient to armour the bed and prevent this channel formation from occurring. The total volume of material eroded from the channel seaward of the crest protection was estimated to be 23,000 m3. The total volume of material excavated from the channel between the crest protection to the start of the sand lobe is 57,000 m3. The dimensions of the tidal channels on Roberts Bank and in Boundary Bay appear to scale with the magnitude of the tidal flows being conveyed. The width and depth of the channels diminish with higher elevation on the flats because the tidal prism (volume of water discharged during a tide cycle) decreases. The channels eventually dissipate into a series of braided, shallow sheet flows and can not incise into the sandy tidal flats. The large active tide channel in the middle of the Inter-Causeway area appears to be approaching an equilibrium situation and will not expand indefinitely up the slope. The 1991, 1995 and 2002 air photos (Figure C5-6 and C5-7) show a sand lobe has developed near the junction of two tributary channels that join the largest active channel in the Inter-Causeway area. The sand lobe has evolved gradually over the last 20 years. The sand lobe appears to have developed as a result of flood-dominated tidal flow processes and was not formed by a single storm event (as apparently reported in the May 30th meeting). Field observations show much of the sand lobe is subjected to shallow, high velocity flows and rapid sediment transport during flood tides as a result of differences in water levels between water ponded on the bar and water flowing in the adjacent channels.


3. MITIGATION OF TIDAL CHANNEL EXPANSION It was indicated in the minutes of the May 30th meeting that there was an interest by environmental agencies to construct weirs or other rock structures in the largest tidal channel to control future changes. We do not recommend constructing new hard protection in this channel since there is some risk that new works could initiate other unexpected channel shifting on the tidal flats, which could impact the existing eel-grass habitat. There are two examples of placing hard rock structures such as short weirs or spurs in tidal channels:

1. a short spur off the end of the crest protection near the outlet of the large tidal channel (marked as A on Figure 4);

2. a short submerged weir near the artificial marsh off the Tsawwassen Ferry Causeway. In both cases, the tidal flows scoured deep channels (up to 5 m deep) and flowed around the ends of the structures. The structures can be easily outflanked since there is no inerodible ground to the protection tie into. Based on the recent site observations and past experience, we recommend using an adaptive approach (including some pilot scale mitigation) to develop measures for improving the stability of the tidal flats in the vicinity of the main channel and the sand lobe. An initial prescription for stabilizing the extent of sand deposition in the vicinity of the sand lobe is illustrated on Figure 5. The work involves:

constructing small cobble berm or sill in the tributary channels to reduce their growth; excavating a small channel through the sand bar to stabilize its extent.

Systematic monitoring should be carried out to evaluate the effectiveness of the measures. The work could be extended or optimized using the experience gained from the first interventions. It should be understood that this work would not be carried out to mitigate against any impacts caused as a result of the Deltaport Third Berth Project. We expect the Third Berth Project will not affect the existing tide channel or sand bar. The landforms and habitat in this area has developed over the last 35 years in response to the expansion of eel-grass and the inter-related morphological changes that developed after dredging the ship turning basin and approach channel.


Figure 4: Crest Protection and Downstream End of Tide Channel Figure 5: Initial Prescription for Stabilizing Portion of the Mid-Tidal Flat

A

Berm

Excavate
2014-01-09-01-HEM-Causeway Breach 2005 Cover Letter-Rev B (2)NHC Causeway Breach Memo June 2005


Roberts Bank Terminal 2 Project

2005 Memorandum Concerning Potential Effects of Opening the Roberts Bank Causeway In response to requests during Pre-Design Consultation (October/November 2013), Port Metro Vancouver (PMV) is releasing the following Memorandum concerning the previously undertaken analysis of the potential effects of opening the Roberts Bank causeway. The Memorandum is dated June 14, 2005 and authored by Northwest Hydraulic Consultants (NHC). Roberts Bank Terminal 2 Project Coastal Geomorphology Studies The Roberts Bank Terminal 2 Project (Project) is a proposed new three-berth marine terminal at Roberts Bank in Delta, B.C. that could provide 2.4 million TEUs (twenty-foot equivalent unit containers) of additional container capacity. The Project is part of Port Metro Vancouvers Container Capacity Improvement Program, a long-term strategy to deliver projects to meet anticipated growth in demand for container capacity to 2030. As part of ongoing environmental and technical work for the Project, PMV has been and continues to undertake field studies at Roberts Bank and the surrounding areas. The Coastal Geomorphology study program for the Project, a combination of field studies, interpretative geomorphic studies, empirical calculations, and numerical modeling, is currently underway. The local study area includes the Roberts Bank tidal flats extending from the seaward side of the dikes to -60 m chart datum (CD) depth, and extending from the BC Ferries terminal to Canoe Passage. To establish tidal conditions for the study area, numerical modeling incorporates the southern part of the Strait of Georgia. Study outcomes will provide a comprehensive understanding of the natural rates of change on the tidal flats and delta foreslope (e.g., accretion or erosion, tidal channel positions, and sedimentary characteristics) in relation to the Project. Consideration of Opening the Causeway PMV-led consultation undertaken in October/November 2013 suggests continued public and First Nation interest in breaching the Roberts Bank causeway to allow flow exchange between the north side of the causeway and the inter-causeway area. In 2005, as part of the Deltaport Third Berth Project, NHC issued a Memorandum to clarify, amongst other items, the potential geomorphological effects of opening the Roberts Bank causeway. NHC found that the volume of tidal water that could be exchanged across the causeway by adding a culvert would be insufficient to improve flushing conditions and may induce potentially damaging local morphological effects. Because of these findings, the Projects Coastal Geomorphology study program currently underway does not directly address breaching the Roberts Bank causeway. The 2005 NHC Memorandum is provided below. It should be noted that the Memorandum reflects plans current at the time. As the Project proceeds through an environmental assessment process, additional studies may be added or changes to the studies may be made based on recommendations from regulators. Field Studies Information Sheets PMV produces monthly field studies information sheets providing information about work occurring at Roberts Bank and the surrounding areas. These information sheets describe the purpose, area, methods and timing of studies.


Roberts Bank Terminal 2 Project 2

To sign up to receive monthly field studies information sheets, please visit www.portmetrovancouver.com/RBT2. For Further Information For further information, please visit our website at www.portmetrovancouver.com/RBT2 or contact us: Phone: 604-665-9337 Fax: 1-866-284-4271 Email: [email protected]
http://www.portmetrovancouver.com/RBT2http://www.portmetrovancouver.com/RBT2mailto:[email protected]

Ref. No. 33863 This technical note was prepared at the request of the Vancouver Port Authority (VPA) to clarify some issues related to physical processes and hydraulic behaviour in the Inter-Causeway area of Roberts Bank. These issues arose during discussions on May 30, 2005 between VPA, Fisheries & Oceans Canada (DFO) and Canadian Wildlife Service (CWS) concerning habitat compensation and mitigation measures for the Deltaport Third Berth Project. Draft minutes of the meeting were provided to NHC and comments have been prepared on three main issues that were discussed:

1. potential effects of opening a breach in the causeway by installing a culvert near the landward end of the flats;

2. clarification of the effectiveness of the crest protection and factors governing growth of the largest tidal channel in the Inter-Causeway area;

3. potential measures for controlling future evolution of the large tidal channel and sand lobe. The comments are based primarily on the results of our previous numerical modelling and geomorphic studies. Some additional simplified hydraulic calculations were made to assess the effect of a breach in the causeway. 1. POTENTIAL EFFECTS OF OPENING THE CAUSEWAY Proposed Opening The concept of installing a 1 m diameter culvert through the causeway was proposed to allow flow exchange between the north side and the Inter-causeway area. It was proposed that the culvert be set near Higher High Water (HHW) to reduce the duration of flows and minimize the risk of any potential adverse impacts. Analysis Carried Out Results from our two dimensional numerical model simulations were reviewed and a time series of water levels was output at specified locations on the north and south sides of the causeway. These water levels were then compared to determine the head difference across the causeway during various phases of the tide. A culvert program (HYDRO-CULV) was used to compute the discharge through the opening under the specified tailwater and headwater conditions. Assumptions It was assumed the invert of the culvert was set 1.0 m below the HHW (mean tide). The available charts show the upper mud flats are generally 0.5 m to 1.0 m higher on the north side of the causeway than on the south side. The culvert was set approximately 1,000 m from the shoreward end. The ground elevations were estimated as follows: north side: 3.5 m south side: 3.0 m

nhc memorandum

northwest hydraulic consultants

30 gostick placenorth vancouver, bc

V7M 3G2, canadatel: (604) 980-6011fax: (604) 980-9264

www.nhcweb.com

To:

Mr. Trevor Peach, Project Manager, Vancouver Port Authority

Date: June 14, 2005

From: David McLean Page: 1 of 8 Cc: Mr. Darrell Desjardins Re:

Questions on Coastal Geomorphology


Results of Analysis Water levels on north side and south side of the Causeway were predicted from the two dimensional hydrodynamic base model of the Fraser estuary and Georgia Strait. Levels for a mean tide (December 2004) were used in this analysis. The differential head Hdif was computed as: Hdif= WLnorth side WLsouth side During the flooding tide, water levels are higher on the south side. During the ebbing tide, water levels are higher on the north side. However, the differential head across the causeway is very small at the times when the culvert would be flowing. Typical water level differences ranged between +0.1 m to -0.1 m. Actual water level differences could be different from these predictions due to wind set up and wave effects. The amount of flow passing through a 1 m circular culvert was computed for a tailwater level of 4.0 m (Chart Datum) and a differential head of 0.1 m. The flow through the culvert was substantially less than 0.5 m3/s. The velocity in the inlet and outlet was computed to 0.4 m/s. A repeat calculation was then made for a 4.0 m long x 1.5 m high box culvert. The maximum discharge through the culvert was computed to be 1.0 m3/s. Entrance velocities remained near 0.4 m/s. The amount of flow draining off the tidal flats in the Inter-Causeway area was measured directly during our 2004 field program using an Acoustic Doppler Current Profiler (ADCP). This involved surveying transects from the Roberts Bank Causeway to the Tsawwassen Ferry Causeway and measuring the total discharge of water passing the transect line (the approximate location of this transect line is shown on Figure 1). The measured discharges reached up to1,600 m3/s during these investigations. The hydrodynamic model was also used to compute the total discharge across the line. During the December 2004 mean tide, the computed maximum discharge near the peak ebb reached 2,000 m3/s. This discharge is comparable to the magnitude of the Lower Fraser River at Hope. The existing flow off the tidal flats in the Inter-Causeway area is three orders of magnitude greater than the proposed culvert discharge. Figure 1: Location of Ebb Discharge Measurements From Tidal Flats

Velocity

0.00

0.60

0.54

0.48

0.42

0.36

0.30

0.24

0.18

0.12

0.06

0.00Velocity1.0 m/s

Distance500.0 m

Discharge line


Assessment of Results The hydrodynamic model results and ADCP discharge measurements demonstrate that the flow through a small culvert opening will be insignificant compared to the large tidal exchanges that take place on the tidal flats under existing conditions. Therefore, in our opinion, adding a culvert could not significantly improve flushing conditions in Inter-Causeway area. The large tidal flows that take place on and off the flats each tidal cycle do not seem to have been fully understood when concerns were expressed about possible stagnant conditions and low circulation in the Inter-Causeway region. In our opinion, the existing hydrodynamic model is the most appropriate tool for quantitatively assessing changes in flushing characteristics in this area. Providing an opening in the causeway could induce local morphological effects on both sides of the opening. The magnitude of the impacts would depend on the size of the opening, the invert elevation of the culvert and its location along the causeway. Local increases in velocities at the outlet would induce scour which could lead to dendritic channel formation on the tidal flats. Typical channels that have formed in this manner are visible near the upper end of the tidal flats adjacent to the dyke and drainage outlets along the Tsawwassen Band. It is expected that these new dendritic channels would develop on both sides of the causeway due to the reversing nature of the flows. The risk of channel formation is probably higher on the south side since the ground elevation is noticeably lower on the south side relative to the north side. This will cause groundwater seepage flow to continue after the tide level has fallen below the culvert invert levels. In our opinion, there is a substantial risk that a new opening in the causeway could initiate a new sequence of morphological changes on the tidal flats, which could impact the existing habitat conditions. Conclusion In summary we do not anticipate that opening the causeway would provide a benefit with regard to introducing any significant flows into the intercauseway area. Additionally, we feel that such openings would likely be detrimental in that they would result in the generation of new channels and concurrent erosion of the existing eel grass. 2. ROLE OF CREST PROTECTION IN CONTROLLING TIDE CHANNELS Historical charts, air photos and results from previous investigations (Triton, 1996) were used to document the historical evolution of the drainage channels on the tidal flats. The main findings are described in Appendix C (Section C3.5) of the Coastal Geomorphology report. Some of the key points from the investigation are summarized below. The crest protection structure was constructed in 1982, after headcutting and tidal channel formation had already been initiated by earlier dredging operations (1979 photo in Figure C5-6). Some of the topographic changes which promoted subsequent channel formation were already underway before the structure was installed, making the conditions for mitigating impacts more difficult. The crest protection was successful in controlling channel formation on the north side of the dredged basin. The channels that had started in 1982 have completely disappeared or been buried by subsequent eel-grass formation in this area (1995 photo in Figure C5-7). Therefore, we consider the crest protection has been effective in preventing the expansion of dendritic channels over extensive portions of the tidal flats. Subsequent expansion of eel-grass changed the hydraulic characteristics of the tidal flats and concentrated drainage flow into the central portion of the Inter-Causeway area. The expansion of the eel-grass is directly linked with the growth of the large tidal channel in this area. Without this concentration


of flow, the drainage would not have been sufficient to incise a channel into the tidal flats. Flow near the outlet of the largest tidal channel was sufficiently strong to incise a new channel along the back side of the crest protection (2002 Photo in Figure C5-7). As a result, instead of being ponded by the crest protection, some of the flow by-passed the structure and developed a new channel behind the riprap. A wider crest protection or provision of a broad cobble/boulder apron behind the crest protection might have been sufficient to armour the bed and prevent this channel formation from occurring. The total volume of material eroded from the channel seaward of the crest protection was estimated to be 23,000 m3. The total volume of material excavated from the channel between the crest protection to the start of the sand lobe is 57,000 m3. The dimensions of the tidal channels on Roberts Bank and in Boundary Bay appear to scale with the magnitude of the tidal flows being conveyed. The width and depth of the channels diminish with higher elevation on the flats because the tidal prism (volume of water discharged during a tide cycle) decreases. The channels eventually dissipate into a series of braided, shallow sheet flows and can not incise into the sandy tidal flats. The large active tide channel in the middle of the Inter-Causeway area appears to be approaching an equilibrium situation and will not expand indefinitely up the slope. The 1991, 1995 and 2002 air photos (Figure C5-6 and C5-7) show a sand lobe has developed near the junction of two tributary channels that join the largest active channel in the Inter-Causeway area. The sand lobe has evolved gradually over the last 20 years. The sand lobe appears to have developed as a result of flood-dominated tidal flow processes and was not formed by a single storm event (as apparently reported in the May 30th meeting). Field observations show much of the sand lobe is subjected to shallow, high velocity flows and rapid sediment transport during flood tides as a result of differences in water levels between water ponded on the bar and water flowing in the adjacent channels.


3. MITIGATION OF TIDAL CHANNEL EXPANSION It was indicated in the minutes of the May 30th meeting that there was an interest by environmental agencies to construct weirs or other rock structures in the largest tidal channel to control future changes. We do not recommend constructing new hard protection in this channel since there is some risk that new works could initiate other unexpected channel shifting on the tidal flats, which could impact the existing eel-grass habitat. There are two examples of placing hard rock structures such as short weirs or spurs in tidal channels:

1. a short spur off the end of the crest protection near the outlet of the large tidal channel (marked as A on Figure 4);

2. a short submerged weir near the artificial marsh off the Tsawwassen Ferry Causeway. In both cases, the tidal flows scoured deep channels (up to 5 m deep) and flowed around the ends of the structures. The structures can be easily outflanked since there is no inerodible ground to the protection tie into. Based on the recent site observations and past experience, we recommend using an adaptive approach (including some pilot scale mitigation) to develop measures for improving the stability of the tidal flats in the vicinity of the main channel and the sand lobe. An initial prescription for stabilizing the extent of sand deposition in the vicinity of the sand lobe is illustrated on Figure 5. The work involves:

constructing small cobble berm or sill in the tributary channels to reduce their growth; excavating a small channel through the sand bar to stabilize its extent.

Systematic monitoring should be carried out to evaluate the effectiveness of the measures. The work could be extended or optimized using the experience gained from the first interventions. It should be understood that this work would not be carried out to mitigate against any impacts caused as a result of the Deltaport Third Berth Project. We expect the Third Berth Project will not affect the existing tide channel or sand bar. The landforms and habitat in this area has developed over the last 35 years in response to the expansion of eel-grass and the inter-related morphological changes that developed after dredging the ship turning basin and approach channel.


Figure 4: Crest Protection and Downstream End of Tide Channel Figure 5: Initial Prescription for Stabilizing Portion of the Mid-Tidal Flat

A

Berm

Excavate
RBT2-Potential-Effects-of-Opening-the-Causeway-June-2005-NHC-Memo-January-2014 (2).pdf2014-01-09-01-HEM-Causeway Breach 2005 Cover Letter-Rev B (2)NHC Causeway Breach Memo June 2005

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