On January 26, ICLR held a Friday Forum webinar title 'Urban Rainfall: Assessing climate change and extreme rainfall events' led by Andrew Wiens, Senior Water Resources Engineer at Associated Engineering. Extreme rainfall and the anticipated effects of climate change pose a significant urban flood risk. Recently, the City of Surrey engaged Associated Engineering to develop numerical models for evaluating flooding from rainfall events and for estimating economic losses due to flooding. This study also examined the anticipated changes to rainfall patterns as a result of climate change and evaluated several potential methods for mitigating flood damage to private property. This presentation covered the following information: 1. Sources of Urban Flooding; 2. Urban Flood Risk Importance; 3. Risk Assessment Components: a. Estimating rainfall and runoff amounts b. Estimating water levels and flood extents c. Estimating economic losses due to flooding; 4. Case Study: City of Surrey’s Climate Change Rainfall Adaptation Strategy. Andrew Wiens, Senior Water Resources Engineer, provides technical leadership and specialist hydro-technical expertise for Associated Engineering projects across Western Canada. Andrew specializes in hydraulic analysis and economic flood risk assessment of urban drainage infrastructure during extreme rainfall events and climate change. Andrew has been analyzing large-scale urban stormwater systems using dual drainage models for over 10 years. As part of this work, he has developed credible methods for estimating surface flooding extents and flood routing during extreme rainfall events. In addition, Andrew has developed methods for quantification of economic damage to private property as a result of flood damage due to rainfall, river-based flooding and dam breach analysis.

1. Urban Flooding
Assessing Climate Change and Extreme Rainfall Events
January 26, 2018
Andrew Wiens, P.Eng.

2. Outline

3. Outline
• Case Study:
• City of Surrey: Climate Change Rainfall
Adaptation Strategy
• Other Project Examples:
• City of Burnaby
• Insurance & DFA
• Summary
Special Thanks to:

4. Case Study:
Climate Change Rainfall Adaptation Strategy

5. Project Objectives
Review impacts
of City’s climate
change rainfall
projections as
applied to three
case study areas
Develop
screening level
assessment to
identify high
risk areas
Evaluate
performance
of stormwater
system
Develop and
evaluate
adaptation
measures
1 2 3 4

6. City of Surrey
• Area: 317 sq.km.
• Population: over 500,000

8. Future Climate Change Projections
Average increase in projected rainfall statistics:
Planning Horizon Increase Relative to 2013 IDF
2030’s 20%
2050’s 35%
2080’s 68%

9. Why does this matter?

10. Why does this matter?
• Key objective is not pipe sizing –
it is identifying impact to society
(economic losses)
• River & coastal flood risk is well understood;
rainfall flooding risk not so much…

11. Why does this matter?

12. Why does this matter?

14. Weather-related Loss Events Worldwide 1980-2015

15. Top Flood/Storm Events for Insurance Payouts

16. Top Flood/Storm Events for Insurance Payouts
Extreme Rainfall

17. The Hydrologic Model

18. Hydrology: Rainfall

19. Hydrology: Rainfall
• Different Terms for Probability
• Return Period
• Flood Frequency
• Exceedance Probability

20. Hydrology: Rainfall
• Different Terms for Probability
• Return Period
• Flood Frequency
• Exceedance Probability
100 Year Flood
The magnitude of flooding which has a
1% chance of occurrence in any year

21. Hydrology: Rainfall
• Different Terms for Probability
• Return Period
• Flood Frequency
• Exceedance Probability
100 Year Flood
The magnitude of flooding which has a
1% chance of occurrence in any year
Key Point: Annual Probability

22. Hydrology: Runoff
• Definition:
• Varies By Catchment Parameters
• Area, Slope, Soil Type,
Land Cover
• Varies By Rainfall
“The volume of rainfall which
flows off an area.”

23. Hydrology: Runoff

24. Hydrology: Climate Change
Adapted from PIEVC Risk Table, Engineers Canada
1:5001:1 000 1:200 1:100 1:50 1:1
Adaptation

25. The Hydraulic Model

26. The Hydraulic Model
MH
Pipe
The Traditional Approach

27. The Hydraulic Model
CatchmentCatchment
100% collection efficiency
No exchange of flow between minor and major system
Excess flow stored at nodes
No transference of flow to next d/s node
The Traditional Approach

28. The Hydraulic Model
MH
Pipe
The Traditional Approach

29. 5-year (2013)

30. 100-year (2080)

31. The Hydraulic Model
Pipe
CB
MH
CB
Pipe
Major System
can flood
roads and
properties
The New Approach

32. The Hydraulic Model
Pipe
CB
MH
CB
Pipe
Major System
can flood
roads and
properties
The New Approach
Why Model this?
- Don’t we have a Major System?
- Isn’t it supposed to be safe?

33. Imperial Street between
Walker and Canada Way
Sag

34. Sag

35. The Sag

36. Sag

37. 5-year (2013)

38. 100-year (2080)
What does Flooding Mean?

39. The Flood Damage Calculations

40. The Flood Damage Calculations
• Depth-Damage Curves
Overview…

41. Depth Damage Curves
$-
$250
$500
$750
$1,000
$1,250
$1,500
$1,750
0 0.5 1 1.5 2 2.5
Depth-Damage Relationship by Landuse
(average of sub-types - includes structure and contents)
Basement
Residential
Commercial
Ind./Inst.
Park Structure
Water Depth by Floor, m
Damage,$/m2

42. Average Annual Risk

43. Average Annual Risk
• Where:
• n = The total number of return periods
• D = Damage at a given return period ($)
• R = Return Period (i.e. 50, 100, etc.)
• Y = Record of Interest (years)

44. The Model Results

45. The Model Results
Analysis
• Hydraulic Analysis to estimate flood extents
• Flood Damage Calculations
Time Horizon 5-year (20% AEP) 100-year (1% AEP)
2013 P P
2030 P P
2050 P P
2080 P P

46. The Model Results
Area 1 – Older Residential Area
5-year (2013)
Does it Matter?

47. The Model Results
Area 1 – Older Residential Area
5-year (2013)

48. The Model Results
Area 1 – Older Residential Area
5-year (2080)

49. The Model Results
Area 1 – Older Residential Area
100-year (2080)

50. The Model Results
Area 2 – Newer Residential Area
5-year (2013)

51. The Model Results
Area 2 – Newer Residential Area
5-year (2080)

52. The Model Results
Area 2 – Newer Residential Area
100-year (2080)

53. Mitigation Options

54. Available Adaptation Options
• Backflow Preventers
• Lot-Level Storage
• Sump Pumps
• Basement Window Well
Protection
• Bylaw Changes to Reduce
Flood Vulnerability
• Modify Overland Flow
Paths
• Lot Grading Strategy
• Purchase & Abandon
Property
• Minimum Building Elevation
• Upgrade Pipe Conveyance
• Pipe Storage
• Peak Flow Diversion
• New Community Ponds
• Retrofit Existing Ponds

55. Sewer Backflow Preventers

56. Mitigation Options
Sewer Backup Valves
• Benefit–Cost Ratio:
200 to 400
• Easy Implementation

57. Upgrade Pipe Conveyance

58. Mitigation Options
Pipe Upgrades
• Sized for 5-year 2080
• Benefit-Cost Ratio =
15 to 50

59. MBE Strategy

60. Mitigation Options
Minimum Building Elevation
• Sized for 100-year 2080
• Benefit-Cost Ratio
= 15 to 80
• Long
Implementation

61. Modify Overland Flow Paths

62. Mitigation Options
Modify Overland Flow Paths
• Sized for 100-year 2080
• Benefit-Cost Ratio
= 15 to 80
• Long
Implementation

63. Insurance & DFA Assistance

64. Questions?
• Home Insurance….

65. Questions?
• So, I’m moving to Calgary…

66. Questions?
• So, I’m moving to Calgary…

67. Questions?
• So, I’m moving to Calgary…

68. Summary

69. Summary
• Why evaluate overland flow?
Don’t assume “safe” overland flow

70. Summary
• Why evaluate overland flow?
It’s not always a problem

71. Summary
• Why evaluate overland flow?
Find Simple Solutions

72. Questions?

73. Contact Andrew Wiens, P.Eng.
email: wiensa@ae.ca
office: 403-262-4500