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URBAN FLOOD RISKS
from FLOOD PLAINS to
FLOOR DRAINS
CORRELATION OF BASEMENT FLOODING WITH
OVERLAND DRAINAGE & TOPOGRAPHIC ...
OUTLINE
•Severe Basement Flooding Events – Toronto, ON
•Risk Factor Mapping
•Overland Drainage (on table land, beyond vall...
SEVERE FLOOD EVENTS - TORONTO
MAY 12, 2000
3162 REPORTS
AUGUST 19, 2005
3640 REPORTS
JULY 8, 2013
1934 REPORTS
(sample of ...
MAPPED FLOOD REPORTS (PROPERTIES)
• Locations georeferenced from Toronto basement
flooding Class EA study reports and maps...
FLOOD LOCATION - VALLEY vs TABLE LAND
• Large Toronto valleys protected, regulated as
natural flood plain hazard, so basem...
FLOOD LOCATION - VALLEY vs TABLE LAND
3162 REPORTS
2.0 %
98 %
3640 REPORTS
0.6 %
99 %
1934 REPORTS*
2.8 %
97 %
Flooded Pro...
FLOOD LOCATION - VALLEY vs TABLE LAND
Regulated Valley ± 6%
(includes flood vulnerable and table land)
River Flood Vulnera...
OBSERVATIONS
• WHAT’s WORKING WELL? NATURAL HAZARD POLICY IN ONTARIO
• Flood hazard regulation in Ontario is effective.
• ...
DEFINING OVERLAND DRAINAGE RISKS
• Ontario digital elevation models (DEMs), derived data products and
ArcHydro GIS tools u...
Elevation Model
GIS ANALYSIS
Regulated Valley
GIS ANALYSIS
Overland Path
10 Hectare Area
GIS ANALYSIS
Overland Path
2 Hectare Area
GIS ANALYSIS
Overland Path
1 Hectare Area
GIS ANALYSIS
Multiples of 100
Year Flow Spread
on 10 Hectare Path
Identify Risk Areas
HYDROLOGIC / HYDRAULIC ANALYSIS
100 Year Flow
Cal...
EXAMPLE 1 - OVERLAND DRAINAGE RISKS
Example 1
EXAMPLE 1 - OVERLAND DRAINAGE RISKS
Limit of Valley / River Feature
EXAMPLE 1 - OVERLAND DRAINAGE RISKS
Limit of Regulated Area
EXAMPLE 1 - OVERLAND DRAINAGE RISKS
Basement
Flood Cluster
Basement
Flood Cluster
Basement
Flood Cluster
Flow Spread
EXAMPLE 2 - OVERLAND DRAINAGE RISKS
Example 2
EXAMPLE 2 - OVERLAND DRAINAGE RISKS
Limit of Valley / River Feature
Limit of Regulated Area
EXAMPLE 1 - OVERLAND DRAINAGE RISKS
Overland Path
10 Hectare Area
EXAMPLE 1 - OVERLAND DRAINAGE RISKS
Wide Flow
Spread (Low
Slopes)
Basement
Flood Cluster
Basement
Flood Clusters
Basement
...
CORRELATING FLOODS TO OVERLAND PATH RISK
• 98% of flooded properties are beyond river flood
vulnerable areas (part of regu...
CORRELATING FLOODS TO OVERLAND PATH RISK
• Distance to the overland flow path was measured for
each 2000, 2005 and 2013 fl...
CORRELATING FLOODS TO OVERLAND PATH RISK
• Correlation weakened by factors such as :
• Flooding caused by local sewer capa...
CORRELATING FLOODS TO FLOW SPREAD RISK
• Distance to the overland 100 year flow spread was
measured for each flood locatio...
CORRELATING FLOODS TO CATCHMENT SLOPE RISK
• Flood density was highly correlated to Catchment Slope.
CORRELATING FLOODS TO CATCHMENT SLOPE RISK
• Lowest slope areas have up to 10x higher flood density.
Over 4 floods / ha fo...
CORRELATING FLOODS TO CATCHMENT SLOPE RISK
• Ironically, along valley flood plains, the higher slopes
reduces risk of base...
URBAN FLOOD RISK SUMMARY
• Catchment slope and proximity to overland flow can partially
explain basement flooding risk (at...
EXAMPLE OVERLAND RISK MAPPING
• All Toronto addresses have been classified by proximity
to overland flow, as multiples of ...
BACK-UP AND OVERLAND PERIL CONSIDERATIONS
• WHAT IS RELATED?: Overland flood risks and basement back-up
risks are shown to...
DE-RISK OPPORTUNITIES
• Risk assessment, management and mitigation efforts should focus
on highest risks, recognize correl...
DE-RISK OPPORTUNITIES (CONT’D)
• Riverine Flood Risk:
1. Identify properties in regulated floodplain affected by frequent
...
FUTURE STUDY – NEXT STEPS
• Overland Flood Risk:
1. Confirm risk factors by assessing insured flood reports and
spatial di...
FUTURE STUDY – NEXT STEPS
• Basement (Sewer Back-up) Risk:
1. Review sewer remediation priorities considering overland ris...
END
EXAMPLES
• Managing Overland Flow Paths and
Flood Risks
• Walmsley Brook (former East York)
• Newtonbook Area (former Nort...
WALMSLEY BROOK – EAST YORK
• Table land overland drainage feature filled during
development. Severe upstream basement floo...
WALMSLEY BROOK – EAST YORK
• Approx. 4.5 km of “Walmsley Brook” and branches
(e.g., Northlea Stream) were enclosed.
Filled...
2015
Filled-in /
piped> 250
flooded
basements
May 12,
2000
storm
NEWTONBROOK AREA - NORTH YORK
• Low catchment slopes (less than 1.5%), wide
overland flow spread, limited overland outlet....
NEWTONBROOK AREA - NORTH YORK
• Overland drainage affects sewer back up risk. Over
450 reported basement floods on August ...
Urban Flood Risk from Flood Plains to Floor Drains
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Urban Flood Risk from Flood Plains to Floor Drains

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Correlation of basement flooding with overland drainage & topographic risk factors during severe storms (Toronto, Ontario, May 12, 2000, August 19, 2005, July 8, 2013 severe storms).

Outline:
Severe Basement Flooding Events – Toronto, ON
Risk Factor Mapping
Overland Drainage (on table land, beyond regulated valley / river flood vulnerable area)
Catchment Relief (topographic slope factors)
Correlation of Observed Flooding, Flood Density and Risks
Joint Back-up and Overland Peril Considerations
De-risk Opportunities
Next Steps for Further Study

Published in: Engineering
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Urban Flood Risk from Flood Plains to Floor Drains

  1. 1. URBAN FLOOD RISKS from FLOOD PLAINS to FLOOR DRAINS CORRELATION OF BASEMENT FLOODING WITH OVERLAND DRAINAGE & TOPOGRAPHIC RISK FACTORS DURING SEVERE STORMS Robert Muir, M.A.SC., P.Eng.
  2. 2. OUTLINE •Severe Basement Flooding Events – Toronto, ON •Risk Factor Mapping •Overland Drainage (on table land, beyond valley) •Catchment Relief •Correlation of Flooding and Risks •Joint Back-up and Overland Peril Considerations •De-risk Opportunities •Next Steps
  3. 3. SEVERE FLOOD EVENTS - TORONTO MAY 12, 2000 3162 REPORTS AUGUST 19, 2005 3640 REPORTS JULY 8, 2013 1934 REPORTS (sample of 4700 total)
  4. 4. MAPPED FLOOD REPORTS (PROPERTIES) • Locations georeferenced from Toronto basement flooding Class EA study reports and maps for severe flood events.
  5. 5. FLOOD LOCATION - VALLEY vs TABLE LAND • Large Toronto valleys protected, regulated as natural flood plain hazard, so basement flooding incidents mostly beyond valleys, on table land.
  6. 6. FLOOD LOCATION - VALLEY vs TABLE LAND 3162 REPORTS 2.0 % 98 % 3640 REPORTS 0.6 % 99 % 1934 REPORTS* 2.8 % 97 % Flooded Properties within Regulated Valley Area : 5.1 – 7.3 % (includes River Flood Vulnerable Areas) Properties in River Flood Vulnerable Areas : Properties On Table Land (beyond vulnerable areas) : MAY 12, 2000 AUGUST 19, 2005 JULY 8, 2013
  7. 7. FLOOD LOCATION - VALLEY vs TABLE LAND Regulated Valley ± 6% (includes flood vulnerable and table land) River Flood Vulnerable ± 2% Table Land ± 98%
  8. 8. OBSERVATIONS • WHAT’s WORKING WELL? NATURAL HAZARD POLICY IN ONTARIO • Flood hazard regulation in Ontario is effective. • Policies prevent flood risks, direct development beyond valleys and river flooding areas (flood plains). Municipal OPs must follow suit. • Only 1-2 % of flooded properties during Toronto flooding are in ‘flood vulnerable areas’ defined by the local conservation authority, and only 5- 7 % are in defined valley features regulated under the CA Act. • WHAT CAN BE IMPROVED? MANGEMENT OF URBAN FLOOD RISK • Urban flooding from overland flow beyond river systems represents the majority of damages (Toronto 2000, 2005, and 2013 example). • Resources should be directed to assessing and mitigating greatest risks (urban, overland flooding on table lands).
  9. 9. DEFINING OVERLAND DRAINAGE RISKS • Ontario digital elevation models (DEMs), derived data products and ArcHydro GIS tools used to define hydrologic features (catchments and drainage line network). • Spatial Analyst GIS tools used to extract hydrologic parameters for peak flow analysis (upstream drainage area, upstream flow length, upstream catchment slope). • Spatial Analyst GIS tools used extract hydraulic characteristics of drainage network segments. • Toronto design rainfall intensity data combined with GIS data to derive 100 year peak flow rates for each drainage network segment. • Empirical hydraulic equation used to create function relating peak flow and segment hydraulics with overland flow depth and flow spread, assuming conventional roadway cross section geometry.
  10. 10. Elevation Model GIS ANALYSIS
  11. 11. Regulated Valley GIS ANALYSIS
  12. 12. Overland Path 10 Hectare Area GIS ANALYSIS
  13. 13. Overland Path 2 Hectare Area GIS ANALYSIS
  14. 14. Overland Path 1 Hectare Area GIS ANALYSIS
  15. 15. Multiples of 100 Year Flow Spread on 10 Hectare Path Identify Risk Areas HYDROLOGIC / HYDRAULIC ANALYSIS 100 Year Flow Calculated for Each Cell 100 Year Flow Calculated for Each Overland Network Link
  16. 16. EXAMPLE 1 - OVERLAND DRAINAGE RISKS Example 1
  17. 17. EXAMPLE 1 - OVERLAND DRAINAGE RISKS Limit of Valley / River Feature
  18. 18. EXAMPLE 1 - OVERLAND DRAINAGE RISKS Limit of Regulated Area
  19. 19. EXAMPLE 1 - OVERLAND DRAINAGE RISKS Basement Flood Cluster Basement Flood Cluster Basement Flood Cluster Flow Spread
  20. 20. EXAMPLE 2 - OVERLAND DRAINAGE RISKS Example 2
  21. 21. EXAMPLE 2 - OVERLAND DRAINAGE RISKS Limit of Valley / River Feature Limit of Regulated Area
  22. 22. EXAMPLE 1 - OVERLAND DRAINAGE RISKS Overland Path 10 Hectare Area
  23. 23. EXAMPLE 1 - OVERLAND DRAINAGE RISKS Wide Flow Spread (Low Slopes) Basement Flood Cluster Basement Flood Clusters Basement Flood Clusters
  24. 24. CORRELATING FLOODS TO OVERLAND PATH RISK • 98% of flooded properties are beyond river flood vulnerable areas (part of regulated valley areas) and are on table land, impacted by overland surface flow. • Statistically, properties with reported basement flooding are closer to the overland surface flow path. • On average, for the closest 30% of properties, flooded properties are located at 58% of the distance to the overland path of all other properties (control group of 500k+ addresses). • For the next closest 30% of properties, flooded properties are at 74% of the distance of the control group, and remaining flooded properties are at 93% distance (less correlation farther away).
  25. 25. CORRELATING FLOODS TO OVERLAND PATH RISK • Distance to the overland flow path was measured for each 2000, 2005 and 2013 flood location and compared to over 500,000 Toronto addresses (control). 0 50 100 150 200 250 300 1 2 3 4 5 6 7 8 9 10 DistancetoFlowPath(m) Decile 2000 Flood Report 2005 Flood Report 2013 Flood Report All Properties (Avg) Flood locations can be 30 to 50 % closer to overland flow path than all addresses (control group).
  26. 26. CORRELATING FLOODS TO OVERLAND PATH RISK • Correlation weakened by factors such as : • Flooding caused by local sewer capacity deficiencies • Property traits (basement depth, reverse drive, back-flow valve) • Correlation strengthened by factors such as : • Under-reporting of flood incidents to municipality (e.g., reports only to insurance) • Adjusting for variable intense rainfall location (e.g., July 2013 rain low in east Toronto, low overland risk, dilutes data in large areas)
  27. 27. CORRELATING FLOODS TO FLOW SPREAD RISK • Distance to the overland 100 year flow spread was measured for each flood location, and all Toronto addresses and normalized as multiples of flow spread. 0 5 10 15 20 25 1 2 3 4 5 6 7 8 9 10 DistanceasMultplesof100YearFlowWitdh Decile 2000 Flood Report 2005 Flood Report 2013 Flood Report All Properties (Avg) Flood locations can be multiple times closer to 100 year flow spread than all addresses (control group).
  28. 28. CORRELATING FLOODS TO CATCHMENT SLOPE RISK • Flood density was highly correlated to Catchment Slope.
  29. 29. CORRELATING FLOODS TO CATCHMENT SLOPE RISK • Lowest slope areas have up to 10x higher flood density. Over 4 floods / ha for slope < 1 % Less than 2.5 floods / ha for slope 1-3 % Less than 1.5 floods / ha for slope 3-5 % Less than 0.5 floods / ha for slope > 5 %
  30. 30. CORRELATING FLOODS TO CATCHMENT SLOPE RISK • Ironically, along valley flood plains, the higher slopes reduces risk of basement flooding (river not a factor). High slope / low risk Low slope / high risk
  31. 31. URBAN FLOOD RISK SUMMARY • Catchment slope and proximity to overland flow can partially explain basement flooding risk (at individual properties and by local density of flooding). • Catchment slope explains 5-10 % of flood density for the 3 events. • In lowest slope catchments, basement flooding occurs at up to 15 times the 100 year flow spread away from the 10 ha overland flow path (North York sag ‘Example 2’ slides, Class EA Area 28). • In other catchments, concentrated basement flooding has been experienced up to 8 times away. • Low catchment slope and proximity to overland flow path do not necessarily imply direct overland flood risk, but may indicate a property’s susceptibility to neighbourhood minor system (sewer) risks. Sewers have a wide influence, beyond any direct overland flooded properties (i.e., direct overland flood enters lowest property basement, then sewer and surcharges through sewer system, in inverse proportion to system slopes (lower slopes = higher spread)).
  32. 32. EXAMPLE OVERLAND RISK MAPPING • All Toronto addresses have been classified by proximity to overland flow, as multiples of 100 year flow spread. 31% are within 4 times the 100 year spread.
  33. 33. BACK-UP AND OVERLAND PERIL CONSIDERATIONS • WHAT IS RELATED?: Overland flood risks and basement back-up risks are shown to be correlated. Traditionally, back-up is associated with the ‘minor’ sewer system and not the ‘major’ overland system. • HOW ARE RISKS RELATED?: Sewer back-up can occur in spatially random locations based on local factors / deficiencies, but is shown to be more often clustered for severe storms, reflecting neighbourhood scale factors including overland drainage system. • WHERE ARE THE RISKS?: 98% of basement flooding is on table land. Riverine food risks are low where flood hazard regulations are enforced, such that only 2% of Toronto severe weather flooding is within ‘flood vulnerable’ areas along valleys. • CAUSE?: Overland risk factors are seldom mapped or managed. Pre-1980’s drainage design did not preserve overland system. Infill / intensification can increase overland flows, adversely affect overland drainage path (filled in / blocked), & stress minor system with overland inflows (into homes, ponding/inflows over MHs).
  34. 34. DE-RISK OPPORTUNITIES • Risk assessment, management and mitigation efforts should focus on highest risks, recognize correlation of back-up & overland risks. • Overland Flood Risk: 1. Map and manage overland flow paths, preserve / restore /enhance function during (re)development and remediation. 2. Develop / apply policies to mitigate overland flow risks, e.g., adapt river approach to table land, that restricts some activities. 3. Lot level at-surface flood proofing of lowest / closest properties (reduce inflows & surrounding sewer back-up risk). • Basement (Sewer Back-up) Risk: 1. Education on risk, using overland mapping to promote voluntary lot level damage reduction measures, e.g., backflow valves. 2. Mandate lot level damage reduction measures, e.g., backflow valves in areas with high risk. 3. Set sewer remediation priorities considering overland risks.
  35. 35. DE-RISK OPPORTUNITIES (CONT’D) • Riverine Flood Risk: 1. Identify properties in regulated floodplain affected by frequent return period events, assess benefit / cost of risk reduction activities (e.g., relocation or flood proofing). Numbers expected to be limited. 2. Define and regulate overland risk as natural hazard in largest, highest risk overland flow areas without floodplain regulation mapping (e.g., minimum 150 - 200 ha drainage area). 3. Assess remediation (culvert improvements, etc.) that can reduce frequent damages in flood vulnerable areas (benefit / cost). • Stormwater and Subwatershed Management: 1. Require water quantity overcontrol for (re)development in catchments with downstream overland flow constraints or significant risk (e.g., development through large unregulated overland flow areas, low slope catchments, high historical damages / flood incidents, known overland obstructions)
  36. 36. FUTURE STUDY – NEXT STEPS • Overland Flood Risk: 1. Confirm risk factors by assessing insured flood reports and spatial distribution of claims / damages vis a vis overland path and catchment slope. Assess all Toronto 2013 municipal flood reports (only sample used here). Evaluate other cities to confirm / refine factors (e.g., city with new dev’t / overland protection). 2. Expand risk mapping to include ‘sinks’ with limited overland flow relief as an additional risk factor (e.g., use enhanced elevation model, identify overland back-up areas). 3. Identify operational / maintenance risk factors and de-risk opportunities (channel maintenance, clearing grate obstructions overland grading restoration). Mandate / regulate operational practices (similar to Ontario drinking water systems). Identify pilot / demonstration for operational measures. 4. Mandate / promote overland insurance in highest risk areas. 5. Identify pilot / demonstration for lot level flood proofing.
  37. 37. FUTURE STUDY – NEXT STEPS • Basement (Sewer Back-up) Risk: 1. Review sewer remediation priorities considering overland risks. 2. Identify operational / maintenance risk factors and de-risk opportunities (sewer system inspection / flushing). 3. Identify and mitigate inflow risks in overland flow / ponding areas. • Riverine Flood Risk: 1. Identify pilot / demonstration for benefit / cost assessment and risk reduction activities (e.g., relocation or flood proofing). 2. Extend regulation / mapping in high risk overland areas. • Stormwater and Subwatershed Management: 1. Implement water quantity overcontrol for (re)development in high risk catchments. 2. Prioritize retrofits in highest risk catchments (LIDs, etc.)
  38. 38. END
  39. 39. EXAMPLES • Managing Overland Flow Paths and Flood Risks • Walmsley Brook (former East York) • Newtonbook Area (former North York)
  40. 40. WALMSLEY BROOK – EAST YORK • Table land overland drainage feature filled during development. Severe upstream basement flooding. Filled-in / pipedHigh Risk > 400 ha Drainage Area
  41. 41. WALMSLEY BROOK – EAST YORK • Approx. 4.5 km of “Walmsley Brook” and branches (e.g., Northlea Stream) were enclosed. Filled-in / piped Walmsley Brook Northlea Stream
  42. 42. 2015 Filled-in / piped> 250 flooded basements May 12, 2000 storm
  43. 43. NEWTONBROOK AREA - NORTH YORK • Low catchment slopes (less than 1.5%), wide overland flow spread, limited overland outlet. High Risk Low Slope Drainage Area
  44. 44. NEWTONBROOK AREA - NORTH YORK • Overland drainage affects sewer back up risk. Over 450 reported basement floods on August 19, 2005. Filled-in / piped

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