ICLR conducted a Friday Forum webinar on June 18, 2021 titled 'Floodplain mapping over Canada: Investigating performance at inundation level and understanding dynamics of population flood exposure', led by Dr. Slobodan Simonovic, Director of Engineering Studies, ICLR/Professor Emeritus, Department of Civil and Environmental Engineering, Western University. Surface runoff estimates from atmospheric re-analysis datasets are increasingly preferred by hydrologists for modelling floods in regions where traditional observations are not sufficiently available. This presentation explores the fidelity of four widely used re-analyses runoff products as hydraulic forcings to a flood inundation model in describing inundation dynamics over Canada. The re-analysis obtained runoff is used with the Catchment-based Macroscale Floodplain (CaMa-Flood) global hydrodynamic model, to derive high-resolution floodplain maps for 100 and 200-yr return periods. The floodplain maps derived from each reanalysis dataset are compared with the regional developed or ‘benchmark floodplain maps’ over six selected flood-prone basins (test basins) in Canada through a set of performance statistics. Using the superior reanalysis runoff dataset, a few historic flood events over the test basins are simulated and subsequently compared with MODIS satellite-derived floodplain information. We notice that more than 75% of the inundation is precisely captured for these events. The second part of the presentation will focus on the use of four global population datasets (together with census data from Statistics Canada as the reference), their performances and skill in flood exposure assessment across Canada. The flood exposure is quantified based on a set of floodplain maps for Canada derived from the CaMa-Flood global flood model. To obtain further insights at the regional level, the methodology is implemented over six flood-prone River Basins in Canada. We find that about 9% (3.31 million) and 11% (3.90 million) of the Canadian population resides within 1 in 100-yr and 1 in 200-yr floodplains. This work (i) strongly supports the need for careful selection of a re-analysis dataset while performing inundation modelling for large regions: and (ii) also highlights the need for careful selection of population datasets for preventing further amplification of uncertainties in flood risk. The results derived from this study may be useful for flood risk management and contribute to understanding other disaster impacts on human-environment interrelationships.

1. FLOODPLAIN MAPPING OVER CANADA
(performance at inundation level; population exposure; climate change impacts)
Slobodan P. Simonović
Fellow Royal Society of Canada
Fellow Canadian Academy of Engineering
FCSCE, FASCE, FIWRA
Director of Engineering Studies, ICLR
Professor Emeritus, Western University

2. PRESENTATION
Outline
2|
▪ Introduction
▪ Floodplain mapping
▪ Methodology
▪ Use of reanalysis data
▪ Use of CaMa-Flood hydrodynamic model
▪ Population flood exposure assessment
▪ Use of global population datasets
▪ Investigation of the changes in floodplain regimes for future climates

3. INTRODUCTION
Project details
3|
Funding
• NSERC CRD with Chaucer Synd.: 2015-2022 $1,375,600
Research team
• Prof. Slobodan P. SIMONOVIC
• Dr. Mohit Mohanty
• Ms. Bogdana Sredojevic

4. INTRODUCTION
4|Project justification

5. INTRODUCTION
5|
▪ Development of the methodology for high resolution flood inundation
analyses over large regions
▪ Investigation of the fidelity of four reanalysis runoff products
▪ Investigation of the utility of four global population datasets
for population flood exposure assessment
▪ Investigation of the changes in floodplain regimes for the future periods
using the 17 latest GCMs from CMIP6 and new shared socioeconic
pathways
▪ (i) changes in flood inundation extents,
▪ (ii) changes in flood hazards, and
▪ (iii) changes in flood frequency.
Project objectives

6. 6|
▪ Flood inundation analyses over large regions

7. METHODOLOGY
7.|Floodplain mapping
The runoff data:
CFSR every 6 h, 1979-2010 at a
surface grid resolution of 0.3◦.
ERA 3 h, 1979- present, 0.75◦.
MERRA 1 h, 1979- present, 2/3◦ ×
1/2◦.
NARR 3 h, 1979 –present, 0.3◦.
MODIS – near real-time global
flood mapping project: a few
historic flood events.

8. METHODOLOGY
8|Data

9. METHODOLOGY
9|
▪ Download runoff data for each reanalysis product
▪ At station locations – comparison between observed and
reanalysis runoff values (correlation coefficient)
▪ Fitting GEV distribution to the data – extracting 100 year and 200
year
▪ Use gridded 100 and 200-yr runoffs with the CaMa-Flood model
to derive maximum flood depth (m) and inundation extents (km2)
for entire Canada
▪ Downscaling maps to 1 km x 1 km spatial resolution
▪ Discretization of water depts into five classes based on the
degree of severity to humans and economic losses
▪ Clip the maps for six selected basins
▪ Compare the maps using four common performance statistics
Implementation steps

10. METHODOLOGY
10|Implementation steps

11. RESULTS
11|Runoff comparison
(a) NARR, (b) ERA Interim, (c) CFSR and (d) MERRA reanalyses

12. RESULTS
12.|Inundation comparison
100 yr (a) NARR, (b) ERA Interim, (c) CFSR and (d) MERRA reanalyses

13. RESULTS
13|Inundation comparison
200 yr (a) NARR, (b) ERA Interim, (c) CFSR and (d) MERRA reanalyses

14. RESULTS
14|Comparison with benchmark data
100 and 200 yr (a) (e) NARR, (b) (f) ERA Interim, (c) (g) CFSR and (d) (h) MERRA reanalyses
Red River Basin

15. RESULTS
15.|Comparison with benchmark data
100 and 200 yr (a) (e) NARR, (b) (f) ERA Interim, (c) (g) CFSR and (d) (h) MERRA reanalyses
Lower Fraser River Basin

16. RESULTS
16.|Comparison of NARR flood plain data with historic floods
(a) 1997 flood in Red River Basin, (b) 2006 flood in Red River Basin, (c) 2013 flood in Bow and Elbow
River Basin (d) 2008 flood in St. John River Basin, and (e) 2011 flood in Assiniboine River Basin.

17. CONCLUSIONS
17|
▪ Methodology for high resolution flood inundation analyses over large
regions successfully developed and tested
▪ The runoff observations from four reanalyses products were evaluated
as primary inputs into the CaMa-Flood, a global hydrodynamic model, to
derive 100 and 200-yr design floodplain maps.
▪ The gridded runoff values were at first compared with the observed
hydrometric data over the country.
▪ The suitability of reanalyses for floodplain mapping is evaluated over six test
basins by comparing the simulated maps with benchmark floodplain maps.
▪ The NARR product did show superior performance.
Floodplain mapping

18. 18|
▪ Population flood exposure assessment

19. METHODOLOGY
19|Population exposure assessment
▪ Two sets of floodplain maps: all
categories of flood depth (>0 m)
and only high and very high flood
depths (>1.5 m)
▪ Census population dataset 2006-
2019
▪ Global population datasets 2015

20. DATA
20|Population exposure assessment
Aggregated population at various census divisions in 2015 as observed with
(a) Statistics Canada, (b) GHS-POP, (c) GPW, (d) LandScan and (e)WorldPop

21. DATA
21.|Population exposure assessment
Difference in population at various census divisions in 2015 between Statistics
Canada and (a) GHS-POP, (b) GPW, (c) LandScan and (d)WorldPop

22. EXPERIMENTS
22|Population exposure assessment
▪ Experiment 1: assessment of population flood exposure over Canada
and six flood-prone basins during 2015
▪ Experiment 2: assessment of flood exposure over Canada and six
flood-prone basins for 1 in 100-yr and 1 in 200-yr flood events
▪ Experiment 3: understanding the dynamics of flood exposure and
vulnerability

23. RESULTS
23|Population exposure assessment

24. RESULTS
24|Population exposure assessment

25. RESULTS
25.|Population exposure assessment
Divisions with various degrees of flood exposure from 2006 to 2019.

26. RESULTS
26.|Population exposure assessment

27. CONCLUSIONS
27|Population exposure
▪ Global population datasets can be used for population flood exposure
assessment
▪ WorldPop and LandScan provide the closest estimates to the census
data across Canada
▪ Most of the high and very-high exposed divisions are located in the
southern and western provinces of Ontario, Alberta, and British
Columbia.
▪ Up to 4 million Canadians are exposed to high flood vulnerability

28. 28|
▪ Changes in floodplain regimes under climate change

29. METHODOLOGY
29.|Climate change impacts
▪ Coupled Model Intercomparison Project
6 (CMIP6)
▪ 17 GCMs considered (runoff)
▪ SSP2 4.5 (medium range of future
forcing pathway) and SSP5 8.5 (high
range of future forcing pathway)
scenarios used
▪ Three timeframes (historical, near
future and far future)

30. METHODOLOGY
30|Implementation
▪ Comparison of runoff observations in GCMs and RHBN
▪ Flood inundation modelling (including extreme value analysis)
▪ Comparison of inundation over six flood-prone basins and
performance metrics
▪ Changes in floodplain regimes in the near, and far-future
▪ Inundation extent
▪ Flood hazard (five classes of depth)
▪ Quantification of changes in the frequency of flood events

31. RESULTS
31.|Runoff comparison

32. RESULTS
32|Comparison of floodplain maps

33. RESULTS
33|Flood inundation extent
200-yr flood
100-yr flood

34. RESULTS
34|Flood hazard
100-yr flood hazard maps Canada: (a) Historical, (b) Near-future –SSP2 4.5, (c)
Near-future –SSP5 8.5, (d) Far-future –SSP5 4.5, and (e) Far-future –SSP5 8.5.

35. RESULTS
35|Flood hazard
200-yr flood hazard maps Canada: (a) Historical, (b) Near-future –SSP2 4.5, (c)
Near-future –SSP5 8.5, (d) Far-future –SSP5 4.5, and (e) Far-future –SSP5 8.5.

36. RESULTS
36|Flood hazard change
100-yr flood hazard change between historical and (a) Near-future –SSP2 4.5, (b)
Near-future –SSP5 8.5, 545 (c) Far-future –SSP5 4.5, and (d) Far-future –SSP5 8.5
periods.

37. RESULTS
37|Flood hazard change
200-yr flood hazard change between historical and (a) Near-future –SSP2 4.5, (b)
Near-future –SSP5 8.5, 545 (c) Far-future –SSP5 4.5, and (d) Far-future –SSP5 8.5
periods.

38. RESULTS
38|Frequency change
Changes in the frequencies of historical
1 in 100-yr flooding in the far-future for (a) SSP2 4.5, and (b) SSP5 8.5; and
1 in 200-yr flooding for (c) SSP2 4.5, and (d) SSP5 8.5.

39. CONCLUSIONS
39|Climate change
▪ First time use of the latest CMIP6 project for understanding the
changes in floodplain regimes in the future over a large country
▪ The near-and far-future 1 in 100-yr and 1 in 200-yr flood events will
add to a rise in the high-, and very-high flood hazards
▪ Flood frequencies in the far future will increase in several regions
(in the western, and northern parts and a few more in the eastern
parts)

40. RECOMMENDATIONS
40|
▪ Using information from this study create a national flood
hazard atlas for Canada
▪ Perform detailed exposure analyses for regions identified as highly
affected
▪ Include coastal floods in the analyses

41. REFERENCES
41|
Mohanty, M. and S.P. Simonovic (2021) “Fidelity of Reanalysis Datasets in
Floodplain Mapping: Investigating Performance at Inundation Level over Large
Regions”, Journal of Hydrology, Vol.597, 125757, available online at
https://doi.org/10.1016/j.jhydrol.2020.125757
Mohanty, M. and S.P. Simonovic (2021) “Understanding dynamics of population
flood exposure in Canada with multiple high-resolution population datasets”,
The Science of Total Environment, Vol.759, 143559, available online at
https://www.sciencedirect.com/science/article/abs/pii/S004896972037090X
Mohanty, M. and S.P. Simonovic (2021) “Changes in floodplain regimes over
Canada due to climate change impacts: observations from CMIP6 models”, The
Science of Total Environment, accepted for publication (June).
Additional resources
www.slobodansimonovic.com