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Urban Flood Risk Mapping - Tiered Vulnerability Assessment in Risk Mitigation Frameworks for Existing Communities

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Review of urban flood risk mapping methods to guide risk reduction strategies. Tiered vulnerability assessment for urban storm (overland pluvial and sewer), sanitary / wastewater and riverine systems is illustrated from "Flood Plain to Floor Drain", consistent with the author's design standards guideline for best practices and projects in existing communities. Example risk mapping / tiered vulnerability evaluations in Markham, Ontario and Stratford, Ontario are shown emphasizing where simple and intermediate risk assessment can guide no-regret, practically deployed policies and programs to reduce urban flood risk, and there advanced risk assessment can guide minor and major capital projects as part of more comprehensive studies in high risk areas that warrant infrastructure investments to lower flood damages in specific local systems. Presentation was made as part of the Ontario Urban Flooding Collaborative organized by Green Communities Canada.

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Urban Flood Risk Mapping - Tiered Vulnerability Assessment in Risk Mitigation Frameworks for Existing Communities

  1. 1. Ontario Urban Flooding Collaborative Webinar / Urban Flood Risk Mapping Reducing Flood Risk from Flood Plain to Floor Drain: Tiered Vulnerability Assessment for Riverine, Wastewater and Storm Drainage Systems to Guide Best Practices For Flood Risk Reduction Robert J. Muir, M.A.Sc., P.Eng. Manager, Stormwater City of Markham September 13, 2018 1
  2. 2. OUTLINE 1) Developing a Standard for Design Standard Adaptation in Existing Communities 2) Tiered Vulnerability Assessment (Risk Mapping) to Guide Best Practices & Projects 3) Example Mapping • Storm • Sanitary (Wastewater) Sewer • Riverine 4) Application of Tiered Risk Assessment (Relation to Best Practices and Projects & Limitations) 5) Combining Risk Factors 2
  3. 3. Design Standard Adaptation in Existing Communities 3 • Markham has been a leader in stormwater management since the 1970’s and was a key contributor to the Intact Centre on Climate Adaptation’s new communities Best Practices document. • Rob wrote a standards adaptation framework as input to ICCA’s next existing communities best practices doc (see Rob’s blog cityfloodmap.com) Preventing Disaster Before it Strikes Reducing Flood Risk from Flood Plain to Floor Drain Developing Best Practices for Residential Flood Risk Reduction: Existing Communities, Canada
  4. 4. Design Standard Adaptation in Existing Communities 4 • Framework focuses on old design standard upgrades includes: • History of: – design std. evolution to “characterize common risk challenges” (ICCA) – operation & maintenance – benefit / cost analysis • Best Practices for: – Tiered Risk Assessment – Planning Policies – Risk Reduction Programs – Capital Projects – Operational practices (maint. & inspections). Blog: Reducing Flood Risk from Flood Plain to Floor Drain
  5. 5. Step 1 Study Area Identifi- cation Step 2 Program Implemen- tation Step 3 Class EA Study or Master Plan Step 4 Long-term Capital Projects Process for Defining Policies, Programs, and Projects 5 Tiered Vulner- ability Mapping No- Regrets Programs Capital Projects Policy Updates • Downspout disconnection (combined) • Infill by-laws & SWM over- controls U/S • Plumbing Protection (b- water valves, sump pumps) SIMPLE • Servicing Era • Flood records • Insur. rates INTERMEDIATE • Downsp. survey • Flow mon.(san.) & rain monitor’g • DEM for sanitary MH inflow risk • ArcHydro or DMTI overland risk products ADVANCED • CCTV, fdn. drain, sewer/inlet invert, & basem’t elev. survey • DEM for overland • Resident flood history survey (EA consultation) • Flow mon. (stm.) • Hydrodynamic modelling sewers and/or riverine• Downspout disconnection (selective from sanitary) • Sanitary MH sealing (low lying areas) • Minor & major capital works (little to no generic “best practices”), deferred works • May require funding prgm. / staffing, etc. • Combine w/ CSO / PCP goals / projects • Combine w/ water & road works (asset mgmt.) & any growth-related upgradesAll projects require study “Viable Projects” • High Cost (ROI screening) • Complex • Trade-offs (Regrets)
  6. 6. Step 1 Study Area Identifi- cation Step 2 Program Implemen- tation Step 3 Class EA Study or Master Plan Step 4 Long-term Capital Projects Process for Defining Policies, Programs (Best Practices), and Projects 6 Risk / Vulner- ability Mapping No- Regrets Programs Capital Projects Policy Updates • Downspout disconnection (combined) • Infill by-laws & SWM over- controls U/S • Plumbing Protection (b- water valves, sump pumps) SIMPLE • Servicing Era • Flood records • Insur. rates INTERMEDIATE • Downsp. survey • Flow mon.(san.) & rain monitor’g • DEM for sanitary MH inflow risk • ArcHydro or DMTI overland risk products ADVANCED • CCTV, fdn. drain, sewer/inlet invert, & basem’t elev. survey • DEM for overland • Resident flood history survey (EA consultation) • Flow mon. (stm.) • Hydrodynamic modelling sewers and/or riverine• Downspout disconnection (selective from sanitary) • Sanitary MH sealing (low lying areas) • Minor & major capital works (little to no generic “best practices”), deferred works • May require funding prgm. / staffing, etc. • Combine w/ CSO / PCP goals / projects • Combine w/ water & road works (asset mgmt.) & any growth-related upgradesAll projects require study “Best Practices” “Best Practices” • No Regrets • Low Cost • Practically Deployed • Not Projects
  7. 7. Tiered Vulnerability Assessment 7 Tiered Vulner- ability Mapping (Flood Risk) • Flood records or claims • Insurance Rates / Rank • Servicing Era • In-field high water level observations • ArcHydro or DMTI overland risk products • Flow monitoring • Hydrodynamic modelling sewers and overland • 2D overland or riverine STORM (VA3) • Flood records or claims • Insurance Rates / Rank • Servicing Era • In-field surcharge level observations • Flow monitoring • Hydrodynamic modelling local sewers and trunks (PS’s, WTP, etc.) SANITARY (VA2) • Map top of bank • Historical High Water Level • HEC-GeoRAS and estimated design flows (floodline estimation) • FDRP mapping (regulatory mapping) • Add multiple return periods, assess individual building inundation, consider operational factors, 2D, add sewers, etc. RIVERINE (VA1) SIMPLE INTERMEDIATE ADVANCED
  8. 8. Overland Flow Paths – INTERMEDIATE Study Link Class EA Dual-Drainage - ADVANCED Study Link * * * Storm Sanitary N/A Flooding Reports - SIMPLE Aug. 19, 2005 Storm System Flood Tiered Risk Mapping Surcharge Monitoring - INTERMEDIATE 8 City LotStreet
  9. 9. Downspout Survey - INTERMEDIATE “ArcHydro” Inflow Risk – INTERMED. * * * Storm Sanitary N/A Sanitary Sewer Tiered Risk Mapping I&I Monitoring - INTERMEDIATE 9 Flooding Report - SIMPLE Aug. 19, 2005 City StreetLot
  10. 10. HEC-RAS Flood Response Plan - ADVANCED PCSWMM 2D & Sewer Model - ADVANCED Study Link Geo-RAS Overland - INTERMEDIATE River System Tiered Risk Mapping 10 Dambreak Safety - ADVANCED Catchment River Reach Building
  11. 11. JBA Risk (DMTI Spatial) • 2D ‘GRID/Raster’ identifies riverine & overland risks for range of return periods (20-year to 1500-year) • DEM not conditioned (can overestimate depths behind embankments), good high level info. 11
  12. 12. 12 City Street Lot Steeles Ave. East Overland Catchment Building Risk Score • Overland flow model shows buildings within overland flow path & sags. • Building-scale flood risks aggregated to rank overland flow priority catchments. Storm System - Aggregating Flood Risks
  13. 13. Fraction Surcharged In Sewershed Steeles Ave. East • Calibrated Hydraulic Model (InfoWorks all-pipe model analyzes flows & sanitary sewer surcharge for 100 year storm (existing and future). • Pipe-by-pipe risk aggregated to sewer catchment. Sanitary System – Aggregating Flood Risk Thornhill Unionville Markham Village 2016 Insurance Business Magazine Master Class presentation
  14. 14. WastewaterSanitary Sewer Risk Characterization (Markham) SIMPLE to ADVANCED Old pre-1980 subdivisions Surcharged Sewers Under Today’s 100- Year Storm More Surcharge With Future Storm (+ 30%) 14 Study Link (2018 WEAO paper) Study Link (2018 WEAO presentation)
  15. 15. WastewaterStorm Sewer Risk Characterization (Stratford Master Plan) SIMPLE to ADVANCED - COMBINED 15 GIS Applications in Urban Drainage Master Planning - R. Muir. AWRA GIS Specialty Conference, 2004 (xp software case study) City of Stratford City-wide Storm System Master Plan. Dillon Consulting Limited, 2004. Flooding Report & Density - SIMPLE Service Era & Connection Types - SIMPLE ArcHydro - INTERMEDIATE XPSWMM - ADVANCED Combined Factor Risk Screening Policies, Programs, Project Benefit/Cost Prioritization
  16. 16. Application of Tiered Approach - SANITARY Policies, Programs, Projects 16 Tiered Vulner- ability Mapping SIMPLE • Servicing Era • Flood records • Insurance rates INTERMEDIATE • Downsp. survey • Flow mon.(san.) & rain monitor’g ADVANCED • CCTV, fdn. drain, sewer/inlet invert, & basem’t elev. survey • Resident flood history survey (EA consultation) • Hydrodynamic modelling of sewers (Pumping Stns, WTPs) Policies (Best Practices) Programs (Best Practices) Projects (Minor & Major Capital) No Yes Partially Separated or Combined Monitor Sanitary Sewer Flow and Rain Flood Density > 0.5 Reports/ hectare Insurance Back-up Code Rank > 65% Yes Yes No Yes Base Infilt. > 40%, or Flood Dens. > 1.5 Continue to Monitor Sanitary Refine Areas Yes 25- Year Q > 0.56 L/s/ha No Initiate Downspout Connection Survey Enhanced O&M (CCTV etc.) Initiate Capacity Study to Identify Projects 25- Year Q < 0.56 L/s/ha* Enhanced O&M (CCTV etc.) Yes100-Year Freeboard < 1.8 m No Enhanced O&M (e.g., siphons, critical infr. assets) Assess Project Cost Per Property (B/C) Un- insurable Zones No Yes By-law To Prevent Gravity Sewer Connection Analyze WWF I&I Statistics (Peaks, Volumes) No Benefit /Cost > 2 or ($32k / Home) Implement Capital Project Yes No En- hanced O&M *or Flood Density > 3
  17. 17. Application of Tiered Approach - STORM Policies, Programs, Projects 17 Tiered Vulner- ability Mapping SIMPLE • Servicing Era • Flood records • Insurance rates INTERMEDIATE • In-field WL observations • ArcHydro or DMTI overland risk products ADVANCED • CCTV, fdn. drain, sewer/inlet invert, & basem’t elev. survey • Resident flood history survey (EA consultation) • Hydrodynamic modelling of sewers, overland and riverine Policies (Best Practices) Programs (Best Practices) Projects (Minor & Major Capital) High Overland Risk No Yes No Major Drainage Design Infill Dev’t SWM Flow Controls to Minor System Capacity Flood Density > 0.5 Reports/ hectare Insurance Back-up Code Rank > 65% Yes Yes No Yes Geo-RAS Overland Screening or Regulatory Floodplain Extension (>125 ha) Yes Prioritized System (Sanitary I&I, Culvert Constraints, etc.) Enhanced O&M (Critical Grates, Inlets, Structures, & Channel Debris Removal) Initiate Capacity Study to Identify Projects Yes100-Year Freeboard < 1.8 m No Enhanced O&M Assess Project Cost Per Property (B/C) By-law To Prevent Reverse Slope Driveways Screen Infill Dev’t Sites for Overland Risk Mgmt. High Risk Building Flood Proofing (Non-Reg.’d) No Benefit /Cost > 2 or ($32k / Home) Implement Capital Project Yes No En- hanced O&M ArcHydro Overland Risk Map Limit Dev’t in New Regulated Zones
  18. 18. 94% of Reported Flooding Beyond Regulated Riverine Hazard Zones in Toronto During Past Events (May 12, 2000, August 19, 2005, July 8, 2013) Prioritizing Areas – Low Riverine vs. High Urban Risks 18 Toronto Overland Flow & Basement Flooding – ArcScene 3D Visualization of Fluvial, Pluvial and Sewer Flood Risk Regulated Area Overland Area Urban Flood Risk from Flood Plains to Floor Drains Correlation of basement flooding with overland drainage & topographic risk
  19. 19. Connected Factors – Overland Risk Raises Sanitary Risk 19 • …” Toronto Overland Flow & Basement Flooding – Correlating Reported Basement Flooding During 2000, 2005 and 2013 Up to 3x More Basement Flood Claims in Overland Zone (insurance company analysis for July 8, 2013 storm) Properties In Overland Zones Claim More
  20. 20. • Sewer back-up risk codes consider past flooding risks and correlate to reported flooding records. • Regression to predict risk has been explored in the past (e.g., MRAT ‘heat maps’ using regression on a multitude of municipal system parameters). • Focusing on key risk drivers like peak sanitary wet weather flow and aggregated sewer catchment surcharge shows strong correlation to insurance company sewer back-up risk code rankings in the City of Markham. Explaining Insurance Risk with Municipal Sewer Risk Factors on Flow Stress and System Capacity 20 Multiple Linear Regression Shows Risk = f(inflow stresses + capacity constraints) > 60% of Risk Explained by 2 Variables 3 2 1 3 2 1
  21. 21. • “Local-level detailed hazard mapping” is required for uses of: – Local government planning – Risk mitigation decision making and design • “High-level hazard mapping” supports: – Regional planning and prioritization • High-level trends have ‘scatter’ at the local level. – See R. Muir Review of Toronto Risk Factors Scale of Mapping for Planning, Decision Making & Design 21 Way Forward for Risk Assessment Tools in Canada - Final Report 2017
  22. 22. Conclusions 22 • Risk Mapping Guides Risk Reduction Activities: – Simple methods identify no-regrets best practice Policies – Intermediate methods guide best practice Programs & identify areas (catchments, reaches, sewersheds) for advanced tier vulnerability assessment & study commensurate with risk (prioritization) – Advanced methods are used in studies to identify minor and major capital Projects subject to financial viability • Risk factors may be related from “Flood Plain to Floor Drain” with interaction of river, storm, and sanitary systems. • Multiple factors can quantitatively explain insurance risks (peak sanitary I&I flow and percentage of surcharged pipes explain > 60% of sewer back-up risk codes) and guide priorities for advanced risk mapping and further studies.
  23. 23. Thank You Questions ? More Rob : Blog: www.CityFloodMap.com Podcast: Open During Construction on iTunes Twitter: @RobertMuir_PEng More City of Markham : Web: www.markham.ca Twitter: @CityofMarkham 23

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