URBAN FLOOD RISKSfrom FLOOD PLAINS to

FLOOR DRAINS

CORRELATION OF BASEMENT FLOODING WITH

OVERLAND DRAINAGE & TOPOGRAPHIC RISK

FACTORS DURING SEVERE STORMS

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

SEVERE FLOOD EVENTS - TORONTO

MAY 12

3162 REPORTS

AUGUST 19

3640 REPORTS

JULY 8

1934 REPORTS(sample of total)

MAPPED FLOOD REPORTS (PROPERTIES)

• Locations georeferenced from Toronto basement flooding Class EA study reports and maps for severe flood events.

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.

FLOOD LOCATION - VALLEY vs TABLE LAND

3162 REPORTS

5.6 %

2.0 %

98 %

3640 REPORTS

5.1 %

0.6 %

99 %

1934 REPORTS*

7.3 %

2.8 %

97 %

Flooded Properties within Regulated Valley Area :

Flooded Properties within River Flood Vulnerable Areas :

Flooded Properties On Table Land Beyond River Vulnerable Areas

:

FLOOD LOCATION - VALLEY vs TABLE LAND

Regulated Valley ± 6%

River Flood Vulnerable ± 2%

Table Land ± 98%

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 is 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).

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.

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 ANALYSIS100 Year Flow

Calculated for

Each Cell

100 Year Flow

Calculated for

Each Overland

Network Link

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

Flood Clusters

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 basementflooding 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).

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

Dis

tance

to F

low

Pa

th (

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).

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 :

• Considering under-reporting of flood incidents to municipality (e.g., reports only to insurance)

• Adjusting for variable intense rainfall location (e.g., July 2013rain low in east Toronto, low overland risk, dilutes data in large areas)

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

Dis

tance

as

Multple

s of

100 Y

ear

Flo

w W

itdh

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).

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 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 %

CORRELATING FLOODS TO CATCHMENT SLOPE RISK

• Ironically, along valley flood plains, the higher slopesreduces risk of basement flooding (river not a factor).

High slope /

low risk

Low slope /

high risk

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 15times 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)).

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.

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).

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.

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)

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.

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.)