This document summarizes a webinar on urban flood risk mapping presented by Robert Muir from the City of Markham. The webinar outlined a tiered vulnerability assessment approach for mapping riverine, wastewater, and storm drainage flood risks to guide best practices and projects. Simple, intermediate, and advanced risk mapping methods were described for each system. The risk maps can be used to identify policies, programs, and capital projects to reduce flood risk from flood plains to floor drains. Combining risk factors across systems was also discussed.

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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
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. 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. 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. 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. 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. 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. 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. 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. • 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. • “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. 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. 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