Nooksack River Overflow Flood Mitigation Plan

COUNCIL REPORT

Executive Committee Report No. ENG 050-2020

Date: December 09, 2020 File No: 5225-03/Nooksack

To: Mayor and Council From: Stella Chiu, Senior Engineer, Drainage and Wastewater Subject: Nooksack River Overflow Flood Mitigation Plan

RECOMMENDATION

THAT staff continue to work with the Province to encourage the United States to complete the cost-benefit analysis and fulfill the purpose and mandate of the International Task Force.

REPORT CONCURRENCE

General Manager City Manager

The General Manager concurs with the The City Manager concurs with the recommendation of this report. recommendation of this report.

SUMMARY OF THE ISSUE

Major flooding of West Sumas Prairie and the Washington State cities of Everson, Sumas and the unincorporated areas of Whatcom, occurred in November 1990 when the overflow from the Nooksack River flooded into the Sumas River basin. The Nooksack River Overflow Flood Mitigation Plan (“Study”) is one of the next steps to address the international and transboundary issue from the United States (“US”) into Canada. The work was recently completed with funding from the National Disaster Mitigation Program; the Province supported the work in a technical advising role. This report provides a high-level summary of the findings and recommends next steps.

BACKGROUND

A Nooksack River International Task Force (NRITF) was established in response to the flooding. The Task Force comprises members from both Canada and the US. Canadian members are from Federal, Provincial and the City of Abbotsford. The focus of the Task Force is on the following four strategies:

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1. Improving emergency response to Trans-Boundary flooding 2. Improving floodplain management 3. Restoring the early 1970’s Nooksack River flow capacity 4. Developing a comprehensive Flood Damage Reduction Plan

The recent focus has been on Strategy No. 4.

In October 2019, the City engaged the services of Kerr Wood Leidal Consulting Engineers (KWL) to undertake the Nooksack River Overflow Flood Mitigation Plan. The work was recently completed with funding from the National Disaster Mitigation Program; the Province supported the work in a technical advising role.

The Study is one of the next steps to address an international and transboundary issue from the US into Canada. The Study estimated flood damages on the Canada side, developed mitigation options and performed cost benefit analyses associated with flood mitigation options.

The US was working to refine a cost-benefit analysis model for use in integrated planning efforts being led by Whatcom County. The flood damage assessment work within this Study will provide information to allow for a meaningful comparison of flood damages on both sides of the border for a 100-year event, which is the standard used by the US. Upon request by the Province, this Study also includes analysis for a 200-year event and climate change scenarios.

The ultimate goal is to provide sufficient data and background information for the Province to have discussions with Washington State officials to consider economic strategies on the Nooksack River.

This report provides a high-level summary of the study findings and recommends next steps. An executive summary of the Nooksack River Overflow Flood Mitigation Plan is included as Attachment A.

DISCUSSION

The following flood scenarios were selected for the Study and simulated using the 2D MIKE FLOOD model:

 Scenario 1: November 1990 flood (35-year)  Scenario 2A: Nooksack River overflow with embankment breaches (100-year)  Scenario 2B: Nooksack River overflow with embankment overtopping (without breaches) (100-year)  Scenario 2C: No Nooksack River overflow (100-year)  Scenario 3: Nooksack River overflow with embankment breaches (200-year)

In addition, climate change analysis was conducted for 100-year and 200-year design flood scenarios (Scenarios 2 and 3). A factor of 1.3 was selected for this analysis, based on a thorough review of available data source for this region from the University of Washington, Western Washington University, Pacific Climate Impacts Consortium (PCIC) and Western University.

Flood maps are included in Attachment B.

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A) Flood Damages Flood damage assessments were completed for the above five flood scenarios. Damage assessments involved determining the following items:

 Quantitative impacts: o Structure and content damage o Agricultural losses o Transportation and business economic losses

 Qualitative impacts, including environmental impacts (septic systems, lagoons, hydrocarbons, asbestos sediments, contamination to wells), lifeline and utility disruption, impacts to First Nations and the potential for a Nooksack River avulsion.

The total quantitative flood damages from the five flood scenarios assessed are summarized in Table 1 below. Details are included in Attachment C.

Table 1 – Total Quantitative Flood Damages Estimated Damages ($M) Structure and Agricultural Economic Scenario Content Damages Damages Losses Total 1 105 41 4 150 2A 316 136 10 462 2B 307 144 10 461 2C 123 84 7 214 3 551 271 14 836

The impacts of climate change and sea level rise were found to exacerbate 200-year flooding damages to $960M.

B) Mitigation Options A number of flood mitigation options for the Sumas Prairie were suggested and modelled since the November 1990 flood event. These options were reviewed as part of this study. Following discussions with the NRITF, three options were selected for costing, modelling and benefit-cost analysis, as shown in Table 2 below. These options are estimated in Class D costs and were designed to the 200-year climate change scenario, which has an existing climate conditions return period of 360 years. Additional information on these options is included in Attachment D.

Table 2 – Flood Mitigation Option Flood Mitigation Option Capital Annual O&M Cost ($M) Cost ($M) No. 1 – Marshall Creek Sump Floodway with Sumas Mountain Tunnel 580 1.7 No. 2 – Dyke Raise and Flood proofing 339 1.0 No. 3A – Eliminate Nooksack Overflows (Everson, US) 29 0.1 No. 3B – Eliminate Nooksack Overflows (US/Canada Border dyke) 310 0.9

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C) Benefit-Cost Analysis Benefit-cost analysis was undertaken for the three options based on their capital costs, annual maintenance costs and annual damages that they prevent for an assumed 100-year lifespan. Table 3 summarizes the Canada-side benefit-cost ratios at 2% and 8% discount rate.

Table 3 – Benefit-cost ratios Flood Mitigation Option Benefit-cost Benefit-cost ratio at 2% ratio at 8% discount rate discount rate No. 1 – Marshall Creek Sump Floodway with Sumas Mountain Tunnel 0.06 0.02 No. 2 – Dyke Raise and Flood proofing 2.0 0.6 No. 3A – Eliminate Nooksack Overflows (Everson) 16.1 5.1 No. 3B – Eliminate Nooksack Overflows (US/Canada Border dyke) 1.6 0.5

Mitigation Option No. 1 was found to provide the least benefit for its cost. It provides minimal flood protection benefits and does not prevent overtopping and failure of the Sumas Dyke during the 200-year climate change flood.

Mitigation Option No. 2 has a medium benefit-cost ratio and prevents all forms of flooding within the Sumas Lake Bottom and the developments in Huntingdon and Arnold area.

Mitigation Option No. 3A provides the highest benefit; however, the analysis only includes impacts and damages on Canadian soil. Additional work is needed on the US side to provide the overall benefit-cost ratio that covers both sides of the border. In addition, it involves mitigation works to be constructed in the US.

The benefit-cost ratios for Mitigation Option No. 3B are similar to Option No. 2; however, these ratios are also from a Canadian perspective only as they do not include flood impacts, mitigation measures and damages within the US. The feasibility of this option also requires further investigation as the dyke would require significant land acquisition, building relocation and road realignment.

Additional analysis is needed on the US side to include the impacts that occur and damages that are prevented within the US, as well as additional mitigation that would be required along the Nooksack River. Until this additional work is completed, none of the three mitigation options evaluated for this study is recommended to be implemented at this time.

D) Risk of Nooksack River Avulsion The risks of an avulsion occurring along the Nooksack River near Everson were previously investigated in a 1993 study, which notes that a flood event larger than the November 1990 flood would be needed to cause an avulsion. At a joint probability of 0.2%, an avulsion would form a channel beginning at Everson and spanning 2 to 3km in length and could double the flows into Sumas Prairie during the 100-year to 200-year flow events, resulting in over $1B in damages.

In addition, the 1993 avulsion study notes that major environmental damages could occur within the Sumas Prairie if the avulsion washes out the Puget Sound Pipeline within the US. Oil spill clean-up costs within the Sumas Prairie, using bioremediation and agriculture losses, would total $80B in 2019 dollars, assuming the same level of agriculture in the 1993 study.

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As this study was completed in 1993, changes since this time to the riverbed, overflow banks, climate and flood frequency estimates have likely impacted the probability of an avulsion occurring, and the avulsion risk analysis should be updated in the future.

E) Next steps During the course of study, the NRITF technical committee met to review the scope of work and draft findings. On May 15, 2020, the NRITF was reconvened by the Province to review its purpose and progress to date and the damage assessment and mitigation options work undertaken in the Study.

As a next step, the Province is working with the US to prepare Status Report No. 4 to introduce the Nooksack River Overflow Flood Mitigation plan and confirm desire for the International Task Force to continue its effort. The report will then be presented to the Environmental Cooperation Committee, to whom the International Task Force reports. The US expressed desire to have further presentations of its progress to date to be included in the Status Report and recommended a future meeting.

It is recommended that staff continue to work with the Province to encourage the US to complete the cost-benefit study they initiated and fulfill the purpose and mandate of the International Task Force.

FINANCIAL PLAN IMPLICATION

The Nooksack River Overflow Flood Mitigation Plan is 100% funded by the Province and Federal governments, through the National Disaster Mitigation Program (NDMP). The City looks to senior governments to provide funding for any related works as it is of international and transboundary nature.

Komal Basatia Acting GM, Finance and Corporate Services Signed 12/6/2020 9:34 AM

IMPACTS ON COUNCIL POLICIES, STRATEGIC PLAN AND/OR COUNCIL DIRECTION

The Nooksack River Overflow Mitigation Plan supports the four Cornerstones of Council’s Strategic Plan by:

 Vibrant Economy: Reducing the risk of flooding to the Sumas Prairie, major local and regional infrastructure in Abbotsford, including the Trans-Canada Highway 1.

 Complete Community: Upholding public safety by reducing the risk of flooding and associated damages.

 Fiscal Discipline: Reducing the risk of flooding and negative impacts to the City’s infrastructure.

 Organizational Alignment: Upholding public trust by providing a high quality of life for residents.

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SUBSTANTIATION OF RECOMMENDATION

The Nooksack River Overflow Flood Mitigation Plan is one of the next steps to address the international and transboundary issue from the US into Canada. The work was recently completed with funding from the National Disaster Mitigation Program; the Province supported the work in a technical advising role. Five flood scenarios (1990 overflow, three 100-year design floods and a 200-year design flood) were simulated using a 2D computer model. In addition, climate change analysis was conducted for 100-year and 200-year design flood scenarios. Flood damage estimates range from $150M to $960M. Three mitigation options were selected with capital cost estimates between $29M to $580M. Benefit-cost analysis was undertaken with ratios between 0.02 to 16, at 2% and 8% discount rate. Additional analysis is needed on the US side to include the damages that are also prevented within the US, as well as additional mitigation that would be required along the Nooksack River. It is recommended that staff continue to work with the Province to encourage the US to complete the cost-benefit analysis and fulfill the purpose and mandate of the International Task Force.

Stella Chiu Senior Engineer, Drainage and Wastewater Signed 12/4/2020 12:22 AM

Rob Isaac GM, Engineering & Regional Utiities Signed 12/7/2020 11:19 AM

ATTACHMENTS: Attachment A Attachment B Attachment C Attachment D PP - Nooksack River Overflow Flood Mitigation

Final Report - Revised Nooksack River Overflow Flood Mitigation Plan

November 30, 2020 KWL File No. 510.184-300

Submitted by:

City of Abbotsford Nooksack River Overflow Flood Mitigation Plan Final Report – Revised November 30, 2020

Executive Summary This report presents the work and results for modelling, flood damage assessment, flood mitigation analysis and benefit-cost analysis associated with flooding in the Sumas Prairie completed by Kerr Wood Leidal Associates Ltd. (KWL) as part of the work for the Nooksack River Overflow Flood Mitigation Plan for the City of Abbotsford (City). This work has been completed with support and funding from the National Disaster Mitigation Program (NDMP), jointly funded from the Province of BC and Federal Government. The work stems from a key strategy of the Nooksack River International Task Force (NRITF) to develop a comprehensive Flood Damage Reduction Plan for flooding in the United States (US) and Canada caused by Nooksack River overflows. As the US has been working to refine a benefit-cost analysis model for use in ongoing integrated planning efforts being led by Whatcom County, the work presented in this report aims to develop a similar model to determine the extent of flood damages in the Sumas Prairie on the Canadian side of the border, which primarily consists of a major agricultural region within the City of Abbotsford. The ultimate goal of this project is to provide sufficient data and background information for Canadian officials to have discussions with Washington State officials to consider economic strategies on the Nooksack River, and develop a mitigation plan to address the flooding issue. Flooding in the Sumas Prairie is caused by a combination of high flows in the Sumas River watershed and flows from the Nooksack River that overtop its right bank near Everson, Washington and are then conveyed along Johnson Creek and the Sumas River to the Canadian border. Floodwaters cross the Canadian border as overland flows toward low-lying areas referred to as the Marshall Creek sump, Saar Creek sump and Arnold Slough sump. During extreme flood events, floodwaters in the Sumas Prairie have the potential to overtop and breach the dike system protecting the Old Sumas Lake Bottom, a low-lying agricultural area that was formerly a lake and provided additional storage during flood events. All flows ultimately drain to the Barrowtown Dam, which consists of a flood box and pump system to convey flows from the Sumas River and the Old Sumas Lake Bottom into the lower reach of the Sumas River that joins the Vedder River and discharges into the Fraser River. Water levels along this lower reach of the Sumas River are therefore impacted by backflows from the Vedder River and the Fraser River. A calibrated 2D MIKE FLOOD model was previously developed in 2014 for the Sumas Prairie in Canada based on the 35-year Nooksack River overflow flood event that occurred in November 1990, and three variations of the 100-year flood scenarios were simulated. The MIKE FLOOD model was used in this study to (1) simulate a 200- year flood event, (2) simulate climate change impacts on the three 100-year flood scenarios and the 200-year flood event, and (3) simulate three flood mitigation options selected as part of this study. Flood damage assessments were completed for the November 1990 flood (based on 2019 building, agricultural, business and traffic conditions), the three 100-year flood scenarios, the 200-year flood event and the flood mitigation scenarios. Damage assessments involved determining the following quantitative and qualitative items: Structure and content damages: estimated using a HEC-FIA model based on flooding of residential and non-residential structures. Agricultural losses: estimated using the Food and Agriculture Organization of the United Nations Damages and Losses method based on the areas flooded within each agricultural parcel. Affected populations: estimated based on flooding of residential structures. Transportation and business economic losses: estimated based on highway and railway closure times and flooding of non-agricultural businesses. Qualitative impacts: evaluated environmental impacts, lifeline and utility disruption, impacts to First Nations and the potential for a Nooksack River avulsion.

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City of Abbotsford Nooksack River Overflow Flood Mitigation Plan Final Report – Revised November 30, 2020

Multiple flood mitigation options for the Sumas Prairie have been suggested and modelled in numerous studies since the November 1990 flood event. These options generally include the following measures: Increasing the capacity at Barrowtown Dam through modifications to the floodboxes or pumps; River modifications downstream of Barrowtown Dam such as channel improvements or channel separation including tunneling through Sumas Mountain; Constructing floodways to relieve Marshall Creek sump or improve conveyance to Saar Creek sump; Raising or constructing relief spillways for the dike system protecting the Old Sumas Lake Bottom; Raising or reinforcing the Southern Railway; Floodproofing individual properties; and, Carrying out measures in Washington State such as blocking the Nooksack overflow at Everson. A thorough review of the above solutions was carried out, and improvements to these solutions were investigated including an alternative floodway configuration along the Sumas River corridor and local area dikes for higher density areas. The three following options were ultimately selected for costing (Class D), modelling, damage assessment and benefit-cost analysis: Mitigation Option #1: construct a new floodway from Marshall Creek sump through Whatcom Road in combination with tunneling Sumas River high flows through Sumas Mountain (capital cost: $580 million). Mitigation Option #2: Raise dikes protecting the Old Sumas Lake Bottom in combination with floodproofing each building (ring dikes) and constructing area dikes for high-density areas (capital cost: $339 million). Mitigation Option #3: construct a structure at Everson to block all overflows from the Nooksack River (capital cost: $29 million) The three mitigation options were modelled for the 100-year flood under existing climate conditions and the 200- year flood under future climate change conditions. Pre-mitigation damages for these two floods were estimated to total $462 million and $960 million, respectively. Benefit-cost analysis was then carried out for the three options based on their capital costs, annual maintenance costs and the annual damages that they prevent for their assumed 100-year lifespans. Benefit-cost ratios were estimated for the three options to be 0.06, 2.0 and 16.1 at a 2% discount rate and 0.02, 0.6 and 5.1 at an 8% discount rate. A benefit-cost analysis was also carried out for a fourth option consisting of constructing a US/Canada border dike and is provided in Appendix J. Mitigation Option #1 was found to provide minimal benefit for its cost and is therefore not recommended. While the capital cost of this option is primarily driven by the high costs of the tunnel, this option also provides minimal flood reduction benefits and does not prevent overtopping and failure of the dike system protecting the Old Sumas Lake Bottom during the 200-year climate change flood. Mitigation Option #2 is recommended from a benefit-cost analysis, although non-monetary factors should be of particular consideration for this option where the cost of mitigation could be similar to the cost of the mitigated damages. This option also completely prevents all forms of flooding within the Old Sumas Lake Bottom and the communities of Huntingdon and Arnold, whereas it increases water levels in the remaining areas of the Sumas Prairie where floodproofing would only be carried out to protect structures. Additional benefit-cost analyses of area dike options for Huntingdon and Arnold as standalone projects are provided in Appendix I. Mitigation Option #3 provides the highest benefit when looking only at Canada-side damages, as the cost to expand the existing levee system at Everson to block the Nooksack River overflows is significantly lower than the flood damages in the Sumas Prairie that are avoided by preventing the overflow flood. However, additional analysis work is needed on the US side to provide the overall benefit-cost ratio that covers the benefits and costs on both sides of the border, including further damages and mitigation needed along the Nooksack River.

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Copyright Notice: These materials are copyright of Kerr Wood Leidal Associates Ltd. (KWL). City of Abbotsford is permitted to reproduce the materials for 2.5 - 3 archiving and for distribution to third parties only as required to conduct business specifically relating to the Nooksack River Overflow Flood Mitigation Plan. Any other use of these materials without the written permission of KWL is prohibited. > 3

Project No. 510.184 Date May 2020 Scenario 1 Hydraulic Model Results: 1990 Flood Event Scale 0250 500 1,000 (m) 1:60,000 Figure A-1 Path: Z:\0000-0999\0500-0599\510-184\430-GIS\MXD-Rp\510184_Fig_AppendixA.mxd Date Saved: 5/14/2020 4:01:15 PM | Author: GOConnell

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Date May 2020 Scenario 2A Hydraulic Model Results: 100-Year Flood Including Nooksack

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Date May 2020 Scenario 2B Hydraulic Model Results: 100-Year Flood including Nooksack

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Project No. 510.184

Date May 2020 Scenario 3 Hydraulic Model Results: 200-Year Flood including Nooksack

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Scale Disclaimer: The map scale of 1:60,000 is only 2 - 2.5 valid on a 11"x17" print.

Project No. 510.184 Scenario CC-2A Hydraulic Model Results: 100-Year Flood under Climate Date May 2020 Change Conditions including Nooksack Overflow, Embankment Breaches Scale 0250 500 1,000 (m) 1:60,000 and Dike Breaches Figure B-1 Path: Z:\0000-0999\0500-0599\510-184\430-GIS\MXD-Rp\510184_Fig_AppendixA.mxd Date Saved: 5/14/2020 4:01:15 PM | Author: GOConnell

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Campb ell Rd

G i l l

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e e k k

Wel ls Line Rd

Vye Rd c

u h Legend ARNOLD g a B r o o k Municipal Boundary

HUNTINGDON CANADACANADA Abbotsford Dike U.S.A. Watercourse

Water Depth (m)

0.0 - 0.5

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1 - 1.5

Service Layer Credits: Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User 1.5 - 2 Community

Coordinate System: NAD 1983 UTM Zone 10N

Scale Disclaimer: The map scale of 1:60,000 is only 2 - 2.5 valid on a 11"x17" print.

Project No. 510.184 Scenario CC-2B Hydraulic Model Results: 100-Year Flood under Climate Date May 2020 Change Conditions including Nooksack Overflow, No Embankment Breaches Scale 0250 500 1,000 (m) 1:60,000 and No Dike Breaches Figure B-2 Path: Z:\0000-0999\0500-0599\510-184\430-GIS\MXD-Rp\510184_Fig_AppendixA.mxd Date Saved: 5/14/2020 4:01:15 PM | Author: GOConnell

City of Abbotsford Nooksack River Overflow Flood Mitigation Plan

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R V E F R A R C h a d s e y C r e e k

r Cr e e

Barrowtown Dam s Cr e

g h K e n r s t r e a m S l o u n e d Wa

t e c s L an

k r e e a y C M c k r

r a T No 3 Rd

an n e Br o e r R i v P o s i g n m a a n S u

h B ro o u k C l a y b k C r e e k N r o i c h o a s r o o k

S t e w a r t C OLD SUMAS LAKE BOTTOM

Campb ell Rd

G i l l

C r e e k

e e k k

Wel ls Line Rd

Vye Rd c

u h Legend ARNOLD g a B r o o k Municipal Boundary

HUNTINGDON CANADACANADA Abbotsford Dike U.S.A. Watercourse

Water Depth (m)

0.0 - 0.5

0.5 - 1

1 - 1.5

Service Layer Credits: Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User 1.5 - 2 Community

Coordinate System: NAD 1983 UTM Zone 10N

Scale Disclaimer: The map scale of 1:60,000 is only 2 - 2.5 valid on a 11"x17" print.

Project No. 510.184 Scenario CC-2C Hydraulic Model Results: 100-Year Flood under Climate Date May 2020 Change Conditions with No Nooksack Overflow, No Embankment Breaches Scale 0250 500 1,000 (m) 1:60,000 and No Dike Breaches Figure B-3 Path: Z:\0000-0999\0500-0599\510-184\430-GIS\MXD-Rp\510184_Fig_AppendixA.mxd Date Saved: 5/14/2020 4:01:15 PM | Author: GOConnell

City of Abbotsford Nooksack River Overflow Flood Mitigation Plan

±H

R V E F R A R C h a d s e y C r e e k

r Cr e e

Barrowtown Dam s Cr e

g h K e n rs t r e a m S l o u n e d W a

e s t L a n c

k r e e a y C M c k r

r a T No 3 Rd

an n e Br o v e r R i P o s i g n m a a n S u

h B r o o u k C l a y b k C r e e k o N s r i c h o a ro o k

S t e w a r t C OLD SUMAS LAKE BOTTOM

Ca mpbell Rd

G i l l

C r e e k

e e k k

Wel ls Lin e Rd

Vye Rd c

u h Legend ARNOLD g a B r o o k Municipal Boundary

HUNTINGDON CANADACANADA Abbotsford Dike U.S.A. Watercourse

Water Depth (m)

0.0 - 0.5

0.5 - 1

1 - 1.5

Service Layer Credits: Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User 1.5 - 2 Community

Coordinate System: NAD 1983 UTM Zone 10N

Scale Disclaimer: The map scale of 1:60,000 is only 2 - 2.5 valid on a 11"x17" print.

Project No. 510.184 Scenario CC-3 Hydraulic Model Results: 200-Year Flood under Climate Date May 2020 Change Conditions including Nooksack Overflow, Embankment Breaches Scale 0250 500 1,000 (m) 1:60,000 and Dike Breaches Figure B-4 Attachment C ‐ Total Quantitative Flood Damages ‐ Breakdown

Damages ($ M) (1) Structure and (2) Agricultural (3) Economic Losses Total Scenario Content Damages Damages 1 105 41 4 150 2A 316 136 10 462 2B 307 144 10 461 2C 123 84 7 214 3 551 271 14 836

1) Flood Damage results for Structures and Contents

2) Agriculture Damages 3a) Economic Impacts ‐ Highway Closures

Note: ‐ Closure of Highway 1 would result in $2,504,000 in traveller costs per day. ‐ Closure of Sumas border and Highway 11 would result in $47,000 in traveller costs per day.

3b) Economic Impacts ‐ Railway

‐ Economic losses resulting from flooding of Southern Railway would be minimal, as traffic on this railway is minimal and local customers could use alternative interchanges with CNR during floods. ‐Repair costs for a breach of Southern Railway similar to the breach that occurred during the 1990 flood were estimated to be approximately $290,000. ‐Repair would be expected for all flood sceniaors except Scenario 2B and 2C, which are 100‐year events when the railway embankment is assumed to not breach.

3c) Economic Impacts ‐ Business Disruption

Note:

Business disruption impacts were estimated based on number of businesses impacted by flooding and the expected closure durations.

3) Business and transportation Economic Impacts Attachment D Flood Mitigation Options

Mitigation Option #1 – Marshall Creek Sump Floodway with Sumas Mountain tunnel

Construct a new floodway from Marshall Creek sump through Whatcom Road in combination with tunneling Sumas River high flows through Sumas Mountain

Potential alignment:

Mitigation Option #2 – Dyke Raise and Floodproofing

Raise Interceptor and Sumas River dykes in combination with floodproofing each building (ring dykes) and constructing area dykes for high-density areas

Potential alignment:

Mitigation Option #3A – Eliminate Nooksack Flows (Everson, US)

Expand the existing levee system at Everson o block all overflows from the Nooksack River

Potential alignment:

Mitigation Option #3B – Eliminate Nooksack Overflows (US/Canada Border Dyke)

Construct an earth-filled dyke/flood wall, complete with flow control structures and flood gate, along the US/Canada border to block all overflows from the Nooksack River

Potential alignment:

Nooksack River Overflow Flood Mitigation Plan

Executive Committee December 21, 2020 Engineering and Regional Utilities Agenda • Purpose • Context • Discussion – Flood scenarios – Flood damages – Mitigation options – Benefit-cost analysis • Next steps • Recommendation

2 www.abbotsford.ca Purpose To provide a high level summary of the findings in the Nooksack River Overflow Flood Mitigation Plan

3 www.abbotsford.ca Context Barrowtown Pump • Major flooding in station Canada Interceptor Dike due to Nooksack Overflow (Nov 1990) • Nooksack River International Task City of Force (NRITF) was Sumas established • Recent focus to develop Flood Overflow Damage Reduction Reach Plan

4 www.abbotsford.ca Context

• In October 2019, Nooksack River Overflow Flood Mitigation Plan was initiated – One of the next steps to address an international and transboundary issue from the US into Canada – Received funding from the National Disaster Mitigation Program – Province supported the work in a technical advising role

• The goal is to provide sufficient data and background information for discussion between Province/Washington State re economic strategies of the Nooksack River

5 Nooksack River Flood Mitigation Plan • Five flood scenarios were established and simulated using a 2D mode: Scenario Description Climate Change scenarios 1 35-year November 1990 flood 2A 100-year Nooksack River overflow with embankment Y breaches 2B Nooksack River overflow with embankment Y overtopping (without breaches) 2C No Nooksack River overflow Y 3 200-year Nooksack River overflow Y

• Climate change factor of 1.3 was used

6 Flood Scenarios (Existing Climate)

Scenario #1 Scenario #2A Scenario #3

November 1990 Overflow Nooksack Overflow with Nooksack Overflow Embankment breaches 35-year 100-year 200-year Flood Damages • Quantitative impacts: Estimated Damages ($M) Structure and Content Agricultural Economic Scenario Damages Damages Losses Total 1 105 41 4 150 2A 316 136 10 462 2B 307 144 10 461 2C 123 84 7 214 3* 551 271 14 836 *Note: Climate Change and sea level rise exercerbate 200-year flooding damage to $960M. • Qualitative impacts, including: Environmental impacts (septic systems, lagoons, hydrocarbons, asbestos sediments, contamination to wells), lifeline and utility disruption, impacts to First Nations and the potential for a Nooksack River avulsion.

8 Mitigation Options

• A number of flood mitigation options for the Sumas Prairie have been modelled since the November 1990 flood event. • Following discussions with NRITF, three options were selected for costing, modelling and benefit-cost analysis: o Option #1: Marshall Creek sump floodway with Sumas Mountain Tunnel o Option #2: Dyke Raise and Floodproofing o Option #3A: Eliminate Nooksack Overflow (Everson, US) o Option #3B: Eliminate Nooksack Overflow (US/Canada Border Dyke)

9 Mitigation Option #1 Marshall Creek Sump Floodway with Sumas Mountain tunnel Potential Alignment: Construct a new floodway Floodway from Marshall Creek sump through Whatcom Road in W h a

t combination with tunneling Highway 1 c o m

R Sumas River high flows o a d through Sumas Mountain

Tunnel

Barrowtown Dam Sumas Mountain Capital Cost: $580M Annual O&M Cost: $1.7M

10 Mitigation Option #2 Dyke Raise and Floodproofing Potential Alignment:

Raise Interceptor and Sumas River dykes in combination with floodproofing each building (ring dykes) and constructing area Capital Cost: $339M dykes for high-density areas Annual O&M Cost: $1.0M

11 Mitigation Option #3A Eliminate Nooksack Flows (Everson, US) Potential Alignment: Expand the existing levee Main Street USGS Gauge system at Everson to block Everson Existing Levee all overflows from the Nooksack River

New Levee

B Massey Road i s s e t

R o a d Capital Cost: $ 29M Annual O&M Cost: $0.1M

12 Mitigation Option #3B Eliminate Nooksack Overflows (US/Canada Border Dyke)

Potential Alignment: Construct an earth-filled dyke/flood wall, complete with flow control structures and flood gate, along the US/Canada border to block all overflows from the Nooksack River

Capital Cost: $310M Annual O&M Cost: $0.9M

13 Base Case Benefit- Note: - Flood maps indicate 200 year Cost climate change scenario - Benefit cost ratio are estimated at 2% and 8% Analysis discount rate

Option #1 Option #2 Option #3

Mitigation Opton #1 Mitigation Opton #2 Mitigation Option #3 (A/B) Benefit-Cost Ratio: Benefit-Cost Ratio: Benefit-Cost Ratio: 0.02 to 0.06 0.6 to 2.0 A- 5.1 to 16.1 B- 0.5 to 1.6 High capital cost; Prevents flooding within Lake Analysis only includes Canadian minimal14 benefit bottom and dense areas wwwdamages.abbotsford.ca Next steps

• Additional analysis is needed on the US side to complete the benefit-cost analysis • NRITF was reconvened by the Province on May 15, 2020 to review its purpose and progress to date and the Nooksack River Overflow Flood Mitigation Plan work • Province is working with US to prepare Status Report #4 to introduce the Flood Mitigation Plan and confirm desire for the Task Force to continue its effort

15 www.abbotsford.ca Recommendations

• THAT staff continue to work with the Province to encourage the US to complete the cost-benefit analysis and fulfill the purpose and mandate of the International Task Force.

16 www.abbotsford.ca