Nooksack River Overflow Flood Mitigation Plan
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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 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. 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: Report No. ENG 050-2020 Page 2 of 6 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. Report No. ENG 050-2020 Page 3 of 6 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 Report No. ENG 050-2020 Page 4 of 6 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. Report No. ENG 050-2020 Page 5 of 6 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.