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Canadian Water Resources Journal / Revue canadienne
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Flood processes in Canada: Regional and special
aspects
James M. Buttle, Diana M. Allen, Daniel Caissie, Bruce Davison, Masaki
Hayashi, Daniel L. Peters, John W. Pomeroy, Slobodan Simonovic, André StHilaire & Paul H. Whitfield
To cite this article: James M. Buttle, Diana M. Allen, Daniel Caissie, Bruce Davison,
Masaki Hayashi, Daniel L. Peters, John W. Pomeroy, Slobodan Simonovic, André St-Hilaire
& Paul H. Whitfield (2016): Flood processes in Canada: Regional and special aspects,
Canadian Water Resources Journal / Revue canadienne des ressources hydriques, DOI:
10.1080/07011784.2015.1131629
To link to this article: http://dx.doi.org/10.1080/07011784.2015.1131629
Published online: 29 Jan 2016.
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Date: 29 January 2016, At: 06:56
Canadian Water Resources Journal / Revue canadienne des ressources hydriques, 2016
http://dx.doi.org/10.1080/07011784.2015.1131629
Flood processes in Canada: Regional and special aspects
James M. Buttlea*, Diana M. Allenb, Daniel Caissiec, Bruce Davisond, Masaki Hayashie, Daniel L. Petersf,
John W. Pomeroyg, Slobodan Simonovich, André St-Hilairei and Paul H. Whitfieldb,j
Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016
a
Trent University, Peterborough, Canada; bDepartment of Earth Sciences, Simon Fraser University, Burnaby, Canada; cDepartment
of Fisheries and Ocean, Moncton, Canada; dEnvironment Canada, Saskatoon, Canada; eUniversity of Calgary, Calgary, Canada;
f
Environment Canada, Water & Climate Impacts Research Centre, University of Victoria, Victoria, Canada; gCentre for Hydrology,
University of Saskatchewan, Saskatoon, Canada; hDepartment of Civil and Environmental Engineering, Institute for Catastrophic
Loss Reduction, Western University, London, Canada; iINRS-ete, Québec, Canada; jEnvironment Canada, Vancouver, Canada
(Received 13 April 2015; accepted 8 December 2015)
This paper provides an overview of the key processes that generate floods in Canada, and a context for the other papers
in this special issue – papers that provide detailed examinations of specific floods and flood-generating processes. The
historical context of flooding in Canada is outlined, followed by a summary of regional aspects of floods in Canada and
descriptions of the processes that generate floods in these regions, including floods generated by snowmelt, rain-on-snow
and rainfall. Some flood processes that are particularly relevant, or which have been less well studied in Canada, are
described: groundwater, storm surges, ice-jams and urban flooding. The issue of climate change-related trends in floods
in Canada is examined, and suggested research needs regarding flood-generating processes are identified.
Cet article dresse un portrait des principaux processus essentiels à la génération des crues au Canada et conséquemment,
donne le ton pour les autres articles inclus dans ce numéro spécial, dans lesquels on traite d’événements spécifiques et
des processus qui en font la genèse. Le contexte historique des crues au Canada est résumé sous forme régionale, avec
une description des processus spécifiques à chaque région, qui incluent entre autre les crues nivales, celles causées par
des précipitations liquides sur couvert de neige et les crues pluviales. Certains processus jugés particulièrement pertinents
ou qui ont été moins étudiés au Canada sont décrits : eau souterraine, surcotes associées aux tempêtes, embâcles de
glace et les crues en milieu urbain. La problématique des changements climatiques au Canada est aussi examinée et des
pistes de recherche liée aux processus causant les crues sont identifiées.
Introduction
Flooding, the inundation of normally dry areas with
water, is the most common and costliest natural disaster
for Canadians (Sandink et al. 2010), and can be generated by a range of processes. These include snowmelt
runoff, “flash flooding” due to intense rainfall, ice jams
that develop during ice formation or breakup, failure of
natural dams, and coastal flooding from storm surges,
hurricanes and tsunamis (Figure 1). Flooding may also
be induced by human activity, including flooding caused
by urban development and by failure or abnormal operation of engineered flood-management structures such as
dams and levees.
Flooding can occur at almost any time of the year
somewhere in Canada; however, the relative significance
of a specific flood-generating process may vary markedly
throughout the year. Thus, snowmelt-driven floods are
more frequent in spring and early summer and ice jams
are associated with spring breakup of river ice cover,
while flash floods generated by intense rainfalls happen
in summer when atmospheric convection is more
common.
*Corresponding author. Email: [email protected]
© 2016 Canadian Water Resources Association
The spatial ubiquity of flood-generating processes
also varies for the particular process and size of drainage
basin being considered. Much of Canada is seasonally
snow covered, and these regions experience snowmeltgenerated floods often supplemented by rain-on-snow
events. Such floods are often the maxima in large drainage basins, when the entire basin may contribute water
to the outlet. Similarly, floods can be generated across
most of the country by rainstorms with large depths and/
or intensities (Figure 1). Thus, convective and frontal
systems can generate large short-duration rainfall intensities (Alila 2000) which can occur in all regions (Table 1).
Nevertheless, the significance of such storms to flood
generation varies across the country, with the greatest
depths and intensities for short-duration events in southern parts of Canada and the smallest in the Arctic. These
short-duration events are often responsible for flood generation in relatively small drainage basins, given the
greater chance of high-intensity rainfall occurring over
the entire basin (Watt et al. 1989). Rainfall-driven floods
in larger basins are usually associated with long-duration
2
J.M. Buttle et al.
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which have received relatively little attention to date in
Canada are discussed. Climate-related trends in floods
are summarized, and the paper concludes with suggested
research needs regarding flood-generating processes in
Canada.
Figure 1.
and 2013.
Flood disasters in Canada by type between 1990
storms which tend to have greater areal coverage
(Dingman 2002). Such events occur across southern
Canada (Table 1), although the generating mechanisms
may differ. In eastern Canada (east of 83° longitude;
Watt et al. 1989) these floods may be linked to hurricane
remnants (Milrad et al. 2009; Watt and Marsalek 2013).
Long-duration rainfalls in western Canada may be associated with bands of concentrated near-surface water
vapour over the Pacific Ocean (atmospheric rivers
referred to popularly as the “Pineapple Express”, PE)
which can generate intense storms of orographically
enhanced precipitation (P) in coastal mountain regions
(Roberge et al. 2009; Dettinger 2011; Spry et al. 2014),
or mesoscale systems in inland regions such as the
Mackenzie River basin (Smirnov and Moore 2001) or
over Alberta (Milrad et al. 2015).
While snowmelt- and rainfall-driven floods can occur
across Canada (Figure 1), other particular forms of flooding are more geographically restricted. These include
geomorphically generated floods in high-relief areas of
western Canada, and storm surges on the Atlantic, Pacific and Arctic coasts as well as on major inland water
bodies such as the Great Lakes. Similarly, flooding associated with the overwhelming of storm sewer networks
is confined to urban areas, while flooding induced by rising groundwater tables may manifest itself in permeable
alluvial floodplains along large streams and rivers.
This paper provides an overview of the key processes
that generate floods in Canada. An exhaustive review of
flood-generation processes and their relative significance
across the country is beyond the scope of this paper;
instead, the intent is to provide a broader context for the
detailed examinations of specific floods and flood-generating processes that are given in the individual papers in
this special issue. The paper begins with an outline of
the historical context of flooding in Canada, followed by
a summary of regional aspects of floods in Canada and a
description of the processes that drive floods in these
regions. Flood processes that are particularly relevant or
Historical context of flooding in Canada
Flooding is a costly natural disaster for Canadians
(Sandink et al. 2010), claiming the lives of more than
200 people and causing over CAD $2 billion in damage
during the twentieth century (Jakob and Church 2011).
This value is conservative, given the ~CAD $1 billion in
damage from the 1996 Saguenay flood alone (Leclerc
and Secretan 2016). Figure 2 combines flooding information from the Canadian Disaster Database (CDD; Public
Safety Canada 2014) and Brooks et al. (2001) to summarize the number of events, deaths and evacuations, and
damages resulting from floods that caused reportable
damage and/or loss of life from 1900 to 2010 with estimates for the years following. Damage estimates from
the CDD (based on federal, provincial, insurance and
non-governmental organization payments, and municipal
and other government department costs) were used; damages are expressed in 2010 Canadian dollars and were
adjusted by the Consumer Price Index (CPI; as reported
in Public Safety Canada 2014).
There are several deficiencies in using these sources
to construct a flood record for Canada. Not all events in
Brooks et al. (2001) appear in the CDD and vice versa,
suggesting that neither provides a comprehensive listing
of major floods in Canada. Both sources sometimes list
multiple floods as single events, and some major floods
have been categorized as another hydroclimatic event
type (e.g. the CDD categorized the Hurricane Hazel
flood as a hurricane). The CDD provides little or no
description of some floods, and no indication of information sources used to populate the database. This complicates attribution of the flood mechanism to particular
flood events, such as is attempted in Figure 1. Events in
the CDD were only considered to 2010 due to the
absence of a formal updating process (Lara Deacon, Public Safety Canada, pers. comm. 2014) that would have
included floods such as the 2011 Assiniboine River flood
(Blais, Clark et al. 2016, this issue; Blais, Greshuk et al.
2016, this issue) and the 2013 Bow River Flood
(Pomeroy et al. 2016, this issue) in the CDD.
Despite these caveats, several observations can be
made. The first is that the number of significant floods,
where damages were high enough to be reported in the
CDD, was low before 1950 and has been relatively
stable in recent decades, averaging five or six events per
year since the 1970s. Deaths associated with floods were
greatest in 1951–1960, reflecting the impact of Hurricane
Hazel on southern Ontario (ON); fatalities have been
24-hour
160; 6.7 – sw Vancouver
Island
60; 2.5 – w of L Ontario
80; 3.3 – s NB, s NL
Secondary 70; 2.9 – e NS
Maximum
Secondary
Secondary 55; 2.3 – sw ON; n
central ON; nw ON
Maximum 60; 2.5 – e Gaspé, North
Shore; Eastern Townships
Maximum
65; 5.4 – s NB, s NL, e NS
50; 4.2 – e Gaspé, North Shore;
Eastern Townships; Montreal; Quebec
City
50; 4.2 – sw ON; central ON; nw ON
55; 4.6 – w of Lake Ontario
40; 3.3 – central AB
50; 4.2 – se MB, sw AB
70; 2.9 – sw AB
Maximum
Secondary 55; 2.3 – se MB; s
central MB
30; 1.3 – sw NT; se YT
30; 1.3 – sw NT; se YK
80; 6.7 – Lower Mainland
12-hour
100; 8.3 – sw Vancouver Island
Maximum
Secondary 150; 6.3 – Lower
Mainland
Maximum
50; 8.3 – s NB
22; 22.0 – s NL
24; 24.0 – sw NS,
Sable Island
22; 22.0 – w QC
40; 6.7 – w QC
55; 9.2 – s NL, e
NS
30; 30.0 – sw ON;
nw ON
28; 28.0 – e of Lake
Huron, Georgian Bay
30; 30.0 – Eastern
Townships
20; 20.0 – central
AB; central SA
12; 12.0 – sw NT; se
YT
30; 30.0 – se MB
16; 16.0 – Lower
Mainland
1-hour
18; 18.0 – w
Vancouver Island
30; 5.0 – central
and sw AB; central
SA
45; 7.5 – e of Lake
Huron; sw ON
40; 6.7 – n central
ON; nw ON
45; 7.5 – Eastern
Townships
20; 3.3 – sw NT; se
YT
40; 6.7 – se MB
50; 8.3 – Lower
Mainland
6-hour
60; 10.0 – w
Vancouver Island
Mean annual extreme isohyet (mm; mm/hour)
8; 96.0 – central
AB
10; 120.0 – s MB
4; 48.0 – w
Vancouver Island;
se BC
4; 48.0 – sw NT
5-minute
6; 72.0 – w
central BC
22; 44.0 – sw
11; 132.0 – sw
ON; nw ON
ON; nw ON
20; 40.0 – n ON 10; 120.0 – nw
ON
24; 48.0 –
11; 132.0 – sw
Eastern
QC
Townships
18; 36.0 – s
10; 120.0 –
Gaspé
Eastern Townships
18; 36.0 – nw
8; 96.0 – sw NB,
NB, sw NS, s
nw NB
NL
6; 72.0 – sw and e
NS, se NL
10; 20.0 – sw
NT
24; 48.0 – se
MB
18; 36.0 –
central AB
30-minute
12; 24.0 – w
Vancouver
Island
10; 20.0 –
Haida Gwaii
BC: British Columbia, NT: Northwest Territories, NU: Nunavut, YT: Yukon Territory, AB: Alberta, SK: Saskatchewan, MB: Manitoba, ON: Ontario; QC: Quebec; NS: Nova Scotia, PEI: Prince Edward
Island; NB: New Brunswick; NL: Newfoundland and Labrador.
n: north, e: east; se: southeast; s: south; sw: southwest; w: west; nw: northwest.
NS, PEI,
NB,
NL
QC
ON
NT, NU,
YT
AB, SK,
MB
BC
Region
Table 1. Mean annual extreme rainfall depth and hourly intensity for various durations for locations in regions of Canada, as denoted by the maximum isohyet shown in Hogg
and Carr (1985). Depths and intensities are given for locations with secondary maxima within a given region if present.
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J.M. Buttle et al.
Figure 2.
Floods in Canada identified by Brooks et al.
(2001) and the Canadian Disaster Database, CDD (Public
Safety Canada 2014). The CDD does not contain all floods but
only those considered significant because of financial transfers.
Adjusted costs are based on 2010 Canadian dollars and were
adjusted for the Canadian Consumer Price Index.
generally fewer since the 1950s. The number of people
requiring evacuation is quite varied, the maximum being
in the 1941–1950 period associated with floods in Manitoba (MB) and British Columbia (BC). Costs and
adjusted costs of flood events exceeded CAD $1 billion
in the 1990s (all dollar values correspond to the year in
which they were reported) and continue to rise, consistent with an upward trend since the 1960s. Floods in
2011–2013 continue the pattern of increasing costs. One
interpretation would be that Canadian society has taken
steps to reduce the number of lives lost during flood
events but is accepting growing financial cost in
response to the rising frequency of events.
Many authors have provided classifications of flood
processes (Watt et al. 1989; Caissie and El-Jabi 1993;
Pietroniro et al. 2004; Peters et al. 2006; Whitfield
2012). Table 2 provides a typology of flood-generating
processes, including meteorological, hydrological, geomorphic and human induced. Specific generating mechanisms are summarized, and reference made to case
studies in this special issue. The different flood-inundation mechanisms are summarized in Table 3, which lists
conditions that modify the magnitude of floods as well
as risk modifiers. Risk modifiers are aspects of the physical or social setting that either increase or decrease exposure to flood damage; flood preparations such as dykes
or levees reduce exposure while development within the
floodplain increases risk.
Floods within the Maritime Provinces, Newfoundland
and Labrador
The Maritime Provinces comprise New Brunswick (NB),
Nova Scotia (NS) and Prince Edward Island (PEI).
Highland areas (e.g. Central Highlands of NB and Cape
Breton NS) are largely granite and metamorphic rock,
while sedimentary rocks underlie lowland areas (e.g.
NB’s eastern lowlands, all of PEI and the NS lowland
from Annapolis to Sydney). Most drainage basins have
extensive forest cover: boreal forest occurs mainly in
Cape Breton and northern NB whereas the Acadian Forest occupies much of the southern part of the Maritime
Provinces.
Newfoundland and Labrador (NL) can be subdivided
into the Island and Labrador. Labrador’s bedrock geology is dominated by metamorphic rocks of the Precambrian Shield, while Newfoundland consists of
sedimentary, metamorphic and volcanic rocks associated
with the Appalachian orogen plus intrusive rocks (largely granite). Vegetation ranges from tundra in northern
Labrador to mixed deciduous/coniferous forests in southwestern Newfoundland, with extensive peatlands mantling interior plateaus and coastal lowlands of the Island.
The region’s climate is relatively wet and cool, especially along the coast. Nevertheless, summer temperature
can exceed 30°C, particularly in central NB, while mean
annual air temperature varies between 3°C in northern
NB to 7°C in southwestern NS. Annual P is relatively
homogenous throughout NB and PEI at ~1200 mm, with
slightly higher amounts (~1400 mm) in southern NB on
the Bay of Fundy. NS typically receives slightly higher
annual P than NB and PEI (~1400–1500 mm), with the
greatest amount in Cape Breton. The regional air temperature gradient means southern areas (e.g. southwest NS,
16% of annual P as snow) typically have less snow than
northern NB does (35% of annual P as snow). Snowfall
amount and accumulation have significant impacts on
flood magnitude and timing, and on flood-generation
processes in the Maritime Provinces. Northern Labrador
has an arctic climate with a mean annual temperature of
−1°C and mean annual P of ~750 mm (about 50% as
snow). Mean annual temperature decreases to −3°C in
western Labrador, accompanied by an increase in mean
annual P to ~950 mm. Mean annual temperature in Newfoundland increases from 1°C on the Northern Peninsula
to 5°C on the Avalon and Burin Peninsulas, while mean
annual P on the Island ranges from 1000 to almost 2000
mm, with the greatest amounts along the south coast and
the Avalon Peninsula, and over the Long Range Mountains on the west coast. The greatest snowfall in Newfoundland occurs on the western mountains and the east
coast.
Flood magnitude and timing can differ considerably
across the region (Figure 3). Most NS rivers experience
peak flows throughout autumn and winter; however,
flood timing is earlier in mainland NS (Roseway River;
end of March) than for Cape Breton rivers (Northeast
Margaree River; mid-May). NB rivers generally experience peak flows between March and May (typically early
May); however, floods occur later for northern NB rivers
Canadian Water Resources Journal / Revue canadienne des ressources hydriques
5
Table 2. A typology of floods, based upon two levels of flood generating processes, after Whitfield (2012). The final column indicates event case studies detailing events that have occurred since 1995.
Class/type
of flood
Flood generating process
Meteorological
Brief
Convective cells, thunderstorms; may be
torrential
isolated or coupled to monsoon or other large
rain
system; may produce flash floods
Heavy rain
Synoptic or mesoscale systems
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Torrential
rain
The amount of rain is particularly abundant,
had a fast onset and/or lasts for a long period
of time
Extensive low-pressure system which may
move large quantities of water (e.g. Pineapple
Express)
Extensive low-pressure system
Extratropical
storm
Tropical
storm
Tidal/storm
surge
Hydrological
Rain-onRate of snowmelt is enhanced by rain and
snow
warmer temperature, leading to more rapid
melt
Snowmelt
Ice jam/
breakup
Large accumulations of water in snowpack,
and high rate of melt
Rising water levels break surface ice on rivers
and lakes which forms a jam impounding
water, increasing water levels upstream during
and downstream following failure
Groundwater Groundwater levels rise above the soil surface
Geomorphic
Avalanche
Snow pack stability failure creates ponding of
related
surface water, increasing water levels
upstream during and downstream following
failure
Landslide
Landform stability failure creates ponding of
surface water, increasing water levels
upstream during and downstream following
failure
Outburst
Glacial dam fails releasing impounded water,
flood
increasing water levels downstream following
failure
Tsunami
Seismic activity generates low waves that
move rapidly and become high in shallow
and coastal areas
Human induced
Dam/levee
Structural failure
break
Designed
Management decision
releases
Region or zone
Case study in this issue
Prairies, Great Lakes
1996 Saguenay (Leclerc and Secretan 2016,
this issue)
Coastal areas,
mountain fronts
2005 Alberta (Shook 2016, this issue) 2013
Alberta (Pomeroy et al. 2016, this issue)
2014 Assiniboine River (Ahmair et al. 2016,
this issue; Blais, Clark et al. 2016, this
issue)
Mid-latitudes, coastal
areas
Tropics, coastal areas
Coastal areas
Mountainous areas,
cold regions, northern
latitudes/high
elevations
Northern latitudes/high
elevations,
mountainous cold
regions
Cold regions,
mountains, northern
latitudes
1997 Red River (Rannie 2016, this issue)
2009 Red River (Wazney and Clark 2016,
this issue) 2011 Richelieu River (Saad et al.
2016, this issue) 2008 St. John River
(Newton and Burrell 2016, this issue)
2011 Red River (Stadnyk et al. 2016, this
issue)
Valley bottoms
Mountainous areas
Mountainous areas
Glacierized basins
(e.g. Upsalquitch River). PEI rivers experience flood timing similar to mainland NS rivers (e.g. Roseway and
Wilmot Rivers), while flood timing in Cape Breton is
similar to northern NB rivers (e.g. Upsalquitch and
Northeast Margaree Rivers). The lowest flood flows in
NL per unit area are in Labrador (e.g. Ugjoktok River),
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J.M. Buttle et al.
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Table 3. Flood-generating processes, inundation types and modifying conditions and risks. The flood-generating processes are
described in more detail in Table 1. Many of these factors exhibit seasonal variation.
Flood-generating processes
Inundation types
Modifying conditions
Risk modifiers
Meteorological
Brief torrential rain
Extra-tropical storm
Heavy rain
Torrential rain
Tropical cyclone
Tropical storm
Tidal surge
Hydrological
Rain-on-snow
Snowmelt
Ice jam/breakup
Groundwater
Geomorphic
Avalanche related
Landslide
Outburst flood
Human induced
Dam/levee break
Managed release
River floods
Flash floods
Ice-jam floods
Glacial lake outburst floods
Urban floods
Sewer floods
Precipitation intensity
Precipitation volume
Precipitation timing
Precipitation phase (rain or snow)
Antecedent river conditions
Antecedent watershed conditions
Antecedent urban conditions
Status (frozen or not frozen)
Flood plain elevation
Human encroachment
Flood preparation
usually between April and July in response to snowmelt.
Floods on the Island of Newfoundland are the greatest in
eastern and southwestern regions (e.g. Isle aux Morts
River) relative to the central and Northern Peninsula
regions, largely reflecting greater P in the former areas.
Floods in these regions are possible throughout the year.
Despite these differences in runoff and flood timing
in the Maritime Provinces and NL (Figure 3), flood magnitude is consistent across the region (Figure 4), with
similar relationships between the 50-year flood and basin
area. PEI rivers showed slightly lower flood magnitudes
for given basin areas, which partly reflects their limited
basin size (less than 150 km2) and low relief. NS basins
showed more variability (Caissie and Robichaud 2009),
especially for mid-range basins (100–400 km2). Above-
Figure 3. Daily runoff for selected rivers within the Maritime
Provinces and Newfoundland and Labrador.
average floods for a given basin area in NB tended to be
in the Bay of Fundy area, with correspondingly higher
annual P. Major floods in the St. John River basin in
southwestern NB, such as that in 2008, can be generated
by snowmelt and rain-on-snow, augmented by ice jamming (Beltaos and Burrell 2015, Newton and Burrell
2016). Similarly, Cape Breton rivers had above-average
floods for a given basin area in NS, and experience the
highest P in NS. The largest floods in NL occur along
Newfoundland’s south coast in response to the large P
the region receives.
Floods in Québec
Canada’s largest province is dominated by the rock outcrops, thin soils and abundant lake coverage of the Precambrian Shield. This region is bounded by the
extensive wetlands of the James Bay lowlands in the
northwest, and the St. Lawrence lowlands to the south.
The latter comprise a narrow plain with relatively thick
soil cover lining the St. Lawrence River. The Eastern
Townships and Gaspé Peninsula consist of the low
Appalachian Mountains (mean elevation of 300 m) with
thin soils separated by thicker soiled valleys. Vegetation
on the Precambrian Shield (Gouvernement du Québec
2003) grades from herbaceous and shrub arctic tundra in
the extreme north to forest-tundra (spruce) to the boreal
forest zone with spruce in the northern portion and balsam fir–white birch to the south. The balsam fir–white
birch domain also covers the highest portions of the
Gaspé. Balsam fir–yellow birch dominate the lower
elevations of the Gaspé, the Saguenay–Lac St. Jean
region and the southern edge of the boreal forest. The
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Canadian Water Resources Journal / Revue canadienne des ressources hydriques
Figure 4.
Flood flows (50-year flood) for rivers within the
Maritime Provinces and Newfoundland and Labrador (data
from Department of Environment and Lands 1992; Caissie and
Robichaud 2006).
southernmost portion of the Precambrian Shield and the
St. Lawrence lowlands are covered by hardwoods such
as sugar maple, yellow birch, basswood and butternut
hickory.
Southeastern QC’s maritime climate is influenced by
the Gulf of St. Lawrence, while most of the St. Lawrence valley has a humid continental climate which
changes to a cold polar climate in the extreme north.
The result is strong north–south gradients in both air
temperature and P. Average air temperature ranges from
20°C in the south to 3°C in the north (summer), and
−8°C in the south to −25°C in the north (winter)
(Ouranos 2010). Summer mean P increases to the northwest from nearly 450 mm in the south to 120 mm in the
north. Some southern mountainous regions receive 350
mm of winter P with only 50 mm of snow water equivalent (SWE) in the far north (Ouranos 2010). Snow accumulation typically begins in October in northern QC and
November in southern QC. Precipitation generally
remains in the form of snow until March–April in the
south, while northern QC typically does not receive significant rainfall (> 0.2 mm) before May.
Most important floods result from spring snowmelt
(Javelle et al. 2003), and St-Laurent et al. (2009) noted
that 59% of major floods from 1865–2005 in the StFrançois River basin occurred between March and May.
Such floods may be augmented by rain-on-snow events
(e.g. 1913 St-François River flood [Castonguay 2007];
2011 Richelieu River flood [Saad 2014]). In the case of
the latter, heavy snowfalls (> 30% above normal) preceded spring snowmelt, coinciding with above-average
rainfall amount and intensity, producing peak flows with
an estimated return period of 90 years (Saad et al. 2016).
Other major causes of floods include ice jams following
7
rapid mechanical breakup of the ice cover. Rivers often
impacted by ice jam flooding include the Chaudière
River near Québec City (Roy et al. 2003), the steep Matapedia River in the Gaspé region (Beltaos and Burrell
2010), and the Ouelle River in the southwest St. Lawrence region (MacNider-Taylor et al. 2009). Heavy summer rainfall can also generate floods, and short-duration
(5-minute to 1-hour) maximum rainfall intensities equal
or exceed those in other parts of Canada (Table 1).
Intense rainfall (> 250 mm in 72 hours on the Chicoutimi and Ha! Ha! River basins) was the main cause of
the infamous 1996 Saguenay flood. Flood control structures were insufficient and inefficient in controlling
flows, as some dikes toppled and some sluice gates
underperformed (Leclerc and Secretan 2016, this issue).
Dams on many rivers in QC shift flood seasonality
from peak events in the spring to a phase difference
(spring floods upstream of dams and winter floods downstream), and may increase flood duration below the dam
(Fortier et al. 2011). In spite of regulation of the St.
Lawrence River for hydroelectric production, flood control and navigation safety, some relatively large and shallow reaches (e.g. Lake St-Pierre between Sorel and
Trois-Rivière) are subject to major water-level fluctuations because of the flow variability of tributaries (e.g.
Ottawa, Richelieu, Yamaska and St.François Rivers)
entering downstream of the Beauharnois dam. High
water levels in 1976 (1 m above the summer average)
were observed in Lake St-Pierre, and define the upper
limit of sensitive wetland ecosystems (Hudon 1997).
There is a relatively consistent relationship between
maximum discharge (Qmax) and basin area for various
sub-regions (Figure 5) compared to other regions in
Canada (e.g. the Prairies and the Cordillera discussed
below) despite QC’s great areal extent. The exception
appears to be basins in the far north, where Qmax is
much more variable for a given basin area. This may
reflect the role of lakes as key storage elements in the
landscape, reducing flood peaks and detaining flood
waves in basins where their size and number exert a key
control on the flow regime. The Qmax from many rivers
in the Gaspé often plots above those for other sub-regions at a given basin size, consistent with the relatively
steep gradients of streams in the Appalachian Mountains
of this sub-region.
Floods in southern Ontario
Southern ON (Figure 6a) is bounded by the Great Lakes
to the south, west and northwest and the Ottawa and St.
Lawrence Rivers, and has modest relief (Sangal and
Kallio 1977). All river basins drain to the Great Lakes or
the Ottawa River.
The region’s climate is strongly modified by the
Great Lakes. Mean annual P ranges from 660 to 1010
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8
J.M. Buttle et al.
Figure 5. Maximum daily mean discharge vs. basin area for
unregulated Water Survey of Canada (WSC) gauging stations
in sub-regions in Québec. Maximum daily mean discharge was
obtained for all naturally flowing stations in the region with 10
years or more of discharge record with a known drainage area,
and includes all active and discontinued sites (data from WSC
2014).
mm (Brown et al. 1968), with the highest values on
slopes east of Lake Huron and Georgian Bay (Moin and
Shaw 1985). Mean annual snowfall ranges from < 100
cm along the northwest shore of Lake Erie to > 300 cm
around Georgian Bay (Ontario Ministry of Natural
Resources 1984). The portion of annual P falling as
snow increases from south to north (Moin and Shaw
1985), ranging from less than 20% to more than 30%
(Ontario Ministry of Natural Resources 1984). Distribution of P throughout the year is relatively uniform.
Historically, snowmelt and rain-on-snow have been
the most frequent flood-generating processes in southern
ON (Irvine and Drake 1987; Watt et al. 1989; Gingras
et al. 1994), and heavy spring rainfalls following snowmelt are also common flood generators, such as in the
Cambridge area of the Grand River basin in May 1974
(Moin and Shaw 1985). The significance of summer
storms and high-intensity rainfalls in flood generation
(e.g. Buttle and Lafleur 2007) has increased in recent
years. Snowmelt generally starts in March, although
mid-winter melt events are common. The proportion of
snowmelt-generated peaks increases northward and eastward across the region, with the mean duration of snowmelt peaks also increasing from south (< 2 days) to
north (> 8 days) (Irvine and Drake 1987). Another cause
of winter floods is rain on frozen ground (Watt et al.
1989; Buttle 2011), particularly in agricultural areas.
Figure 6. (a) Physiographic regions of southern Ontario identified by Moin and Shaw (1985). (b) Maximum recorded flood
(daily mean discharge) vs. basin area recorded at unregulated
or minimally regulated Water Survey of Canada (WSC) gauging stations in each region with at least 10 years of record to
2012.
The presence of a temporary river ice cover means
that some flooding in southern ON results from ice-jams
that augment snowmelt-generated flooding, usually following an early spring thaw when the ice cover is still
strong. The Moira River in Belleville is subject to this
type of flooding, as are lower sections of the Thames
and Sydenham Rivers in southwestern ON (Gerard and
Davar 1995). Finally, coastal flooding occurs in some
areas along shorelines of the Great Lakes (Watt et al.
1989). High lake levels can lead to flooding of property,
and wave erosion. In some areas (e.g. the downwind end
of Lake Erie) this is exacerbated by wind set-up raising
water levels as much as 2 m above the static level.
Changes in wind velocity and air pressure can also set
up oscillations in lake levels (seiches), which have led to
flooding at the eastern end of Lake Erie (Trebitz 2006).
Coastal flooding may occur along all the Great Lakes’
shorelines bounding southern ON, but is mitigated
slightly along the Lake Ontario shoreline due to partial
control of that lake’s water levels (McRae and Watt
2006).
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Canadian Water Resources Journal / Revue canadienne des ressources hydriques
The general increase in Qmax with basin area in
southern ON (Figure 6b) displays considerable scatter
and overlap between some regions (e.g. Regions 4 and
5). Nevertheless, Qmax values from Regions 1 and 4
diverge, with greater floods for Region 4 beyond a basin
size of ~100 km2. This may reflect water storage in the
numerous lakes and wetlands in Region 1.
Southern ON is one of the most urbanized regions in
Canada, containing such major urban centres as Toronto,
Ottawa, Hamilton, London and Kitchener/Waterloo, and
a later section on floods in urban areas addresses flood
generation in these and other Canadian cities and towns.
Agricultural land in much of ON needs drainage
improvement via subsurface tile drainage systems to
allow farmers earlier access to the land following spring
snowmelt (Irwin and Whiteley 1983). While there is no
consensus as to the impact of land drainage on floods,
enhanced soil storage capacity towards the point of soil
saturation should decrease peak flows at the field scale,
after which tile drainage likely increases peak flow
slightly by adding subsurface outflows to overland runoff
(Irwin and Whiteley 1983). The influence of tile drainage
on flooding at the basin scale is similarly equivocal.
Drainage of surface depressions previously unconnected
to the flow network will likely increase flood peaks;
however, the effect on flooding at a specific location on
the stream network depends in part on the degree of synchronization of flow contributions to the channel (Irwin
and Whiteley 1983).
Floods in the boreal forest
The boreal forest region extends from the Yukon (YT) to
NL and combines Canada’s Boreal Plains and Boreal
Shield ecozones; floods in the Boreal Cordillera zone are
addressed later in the section on western Cordilleran
floods. The Boreal Shield comprises almost 20% of
Canada’s landmass and contains 22% of Canada’s freshwater area (Urquizo et al. 2000). It is a rolling landscape
of igneous and metamorphic bedrock outcrops, generally
thin soils, numerous lakes and wetlands. It contains the
headwaters of several major river systems (e.g. Nelson
and Churchill Rivers in MB, St. Lawrence in ON, Eastmain and Rupert in QC), and is largely forested. Northern areas are dominated by white and black spruce,
balsam fir and tamarack, while white birch, trembling
aspen, balsam poplar, and white, red and jack pine are
common in southern regions. The Boreal Plains extend
from the Peace River district in BC to southeastern MB.
They have fewer lakes and bedrock outcrops relative to
the Boreal Shield, and forest cover consists of jack and
lodgepole pines, white and black spruces, tamarack and
aspen.
Mean annual P in the boreal forest ranges from <
400 mm in extreme northeastern BC to > 1200 mm in
9
eastern QC (Fisheries and Environment Canada 1978).
This general west-to-east increase is interrupted by
increased P in the lee of Lake Superior in ON. There is
also a slight increase along a north–south gradient in ON
and QC. Annual evapotranspiration (ET) usually exceeds
annual P in the northwestern boreal, while P – ET
increases moving eastward. Rainfall intensities are generally lower in the boreal forest than in the Prairies and
southern ON and QC for a given duration and return
period (Table 1). Mean annual snowfall increases from
~160 cm in northeastern BC to > 400 cm in eastern QC,
interrupted by an intermediate peak snowfall in the lee
of Lake Superior. Mean January daily air temperatures
are < −22.5°C in northeastern BC and −12°C in eastern
QC, declining to < −27.5°C in northern Saskatchewan
(SK) and MB. Mean July daily air temperatures are
17.5°C along the southern edge of the boreal forest in
MB, ON and QC, dropping to < 12.5°C in northern QC.
On average, rivers freeze over on 1 December in the
southern boreal in ON and QC, and as early as 1
November in northern SK. The mean date when rivers
become ice-free is 15 April along the boreal forest’s
southern edge, and 1 June along its northern edge (Fisheries and Environment Canada 1978). Permafrost > 10 m
in depth may be present in upland areas of northern portions of the boreal forest, but is generally absent under
major lakes and rivers due to their thermal influence
(Newbury et al. 1984).
The west-to-east shift in annual P – ET from negative to positive values across the boreal forest has significant implications for flood potential in the region.
Negative P – ET in the western Boreal Plains often leads
to disconnection between uplands and stream networks.
This reduces the frequency of significant runoff at the
basin scale, and means that flood potential in response to
spring and summer rainfalls in a given year is maximized when large P inputs in the preceding summer and
fall are combined with large winter snow accumulation
(Devito et al. 2005). Interannual variability in flood
potential in the boreal forest decreases moving eastward
in response to increases in annual P – ET. Granger and
Pomeroy (1997) showed that mature boreal forest stands
had greater ET losses relative to regenerating stands or
recent clearcuts, leading to greater summer and fall soil
moisture in forest clearings and young regenerating
stands (Elliott et al. 1998). Higher soil moisture in
clearcuts combined with compaction of clearcut soils
promoted reduced infiltration capacity. As a result, all
sub-canopy rainfall infiltrated mature forest soils during
a severe summer storm in the central SK boreal forest (>
150 mm), whilst over half of the rainfall formed runoff
in recent clearcuts (Elliott et al. 1998).
Most annual maximum floods in the boreal forest are
driven by snowmelt (Woo and Waylen 1984; Alberta
Transportation 2004), and boreal forest cover affects
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10
J.M. Buttle et al.
snowmelt energetics and rates, meltwater infiltration into
frozen soils, and runoff generation (Prevost et al. 1991).
Wet soils in clearcuts in the fall from reduced summer
ET result in saturated frozen soils in the spring with
restricted infiltration capacity (Pomeroy, Granger et al.
1997). Sublimation losses from intercepted snow
(Pomeroy et al. 1998) lead to maximum spring snowpacks under evergreen forest canopies that are one third
to one half of those in clearings, burned or deciduous
stands (Pomeroy et al. 2002). Melt rates are three times
higher in clearings than under forest stands (Pomeroy
and Granger 1997), generating vastly greater snowmelt
runoff formation in recent clearcuts than in mature stands
(Pomeroy, Granger et al. 1997). Spring and snowmelt
floods are often linked to river ice breakup, which creates water levels far higher than those for equivalent
open-water discharges (Pietroniro et al. 1996). Nevertheless, short-duration intense thunderstorms can produce
severe flooding in small- to medium-sized basins
(Alberta Transportation 2004). Flood hydrology in parts
of the boreal forest is complicated by river impoundment
for hydroelectric generation and the associated flow
diversion, such as the diversion from the Churchill River
Figure 7. Maximum daily mean discharge vs. basin area for
unregulated Water Survey of Canada (WSC) gauging stations
in major drainages across the boreal forest in the Yukon Territory, British Columbia, Northwest Territory, Alberta, Saskatchewan, Manitoba, Ontario, Québec and Labrador. Drainage basin
numbers correspond to the major drainage basins of Canada
used by WSC (see Table 4).Maximum daily mean discharge
was obtained for all naturally flowing stations in the region
with 10 years or more of discharge record with a known drainage area, and includes all active and discontinued sites (data
from WSC 2014).
into the Burntwood–Nelson system at Southern Indian
Lake in northern MB (Newbury et al. 1984).
Our knowledge of flood characteristics of boreal forest rivers is less than for other regions (e.g. southern ON
and QC), partly due to its sparse population. Nevertheless, variability in the Qmax vs. basin area relationship
for the boreal forest (Figure 7) is greatest for rivers
draining to Great Slave Lake, which also have many of
the smallest Qmax values for a given basin size. The latter likely reflect the frequent disconnection between
uplands and stream networks noted earlier for this subregion, often resulting in minor flood peaks relative to
wetter parts of the boreal forest.
Prairie flooding
The plains of the southern portions of Alberta (AB), SK
and MB consist of gently rolling hills separated by deep
river valleys, with a general west–east slope. The sedimentary bedrock is covered by glacial deposits of variable thickness. The region’s natural vegetation is
grasslands, and trees are largely confined to river valleys;
however, it has been heavily altered by agricultural
activity. Southwestern AB and southeastern SK have a
semi-arid cold climate, becoming more humid and colder
to the north and east. Maximum January and July mean
daily air temperatures are generally warmer in this semiarid sub-region (> −10°C and > 20°C, respectively).
Annual P ranges from < 300 mm per year in semi-arid
grassland to > 700 mm in central MB. Mid-winter melts
(frequent in the southwest and infrequent in the northeast) punctuate the region’s protracted winter (usually 4–
5 months). High surface runoff occurs during spring
snowmelt due to relatively rapid water release from
snowpacks to frozen soils (Gray et al. 1985). Most rainfall in spring and early summer is from large frontal systems, and intense rainfall in summer is from convective
storms over small areas (Gray 1970; Shook and Pomeroy
2012; Table 1). High ET with low rainfall and soil moisture from mid-summer to fall result in little runoff (Granger and Gray 1989). This is exacerbated by poorly
drained stream networks such that large areas are internally drained and do not contribute to major river systems except during flooding (Martin 2001; Shook et al.
2013).
Generally low P makes the accumulation, redistribution and ablation of snowcover of critical importance to
the hydrology of the Prairie region. Snowpacks typically
form in November and begin ablation in March and
April. Over-winter wind redistribution and sublimation
reduces peak SWE and increases variability in spatial distribution with large snowdrifts near water courses, densely vegetated sites and topographic depressions, and
wind-scoured zones in summer fallow fields and on hilltops (Fang and Pomeroy 2009). Mid-winter melt and
Canadian Water Resources Journal / Revue canadienne des ressources hydriques
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Table 4.
11
Major drainage basins in Canada.
Code
Major drainage
Selected examples
01
02
03
04
05
06
07
08
09
10
11
Maritime Provinces drainage
St. Lawrence River drainage
Northern Quebec drainage
Southwest Hudson Bay drainage
Nelson River drainage
Western Hudson Bay drainage
Great Slave Lake
Pacific drainage
Yukon River drainage
Arctic drainage
Mississippi River drainage
St. John, St. Croix
Ottawa, Richelieu, Saguenay
Churchill, Grande
Attawapiskat, Hayes, Severn
Assiniboine, Bow, Oldman, Red, Red Deer
Churchill, Kazan, Thelon
Athabasca, Peace, Slave, Smoky
Columbia, Fraser, Lillooet, Skeena, Stikine
Porcupine, White
Liard, South Nahanni, Mackenzie
Frenchman, Milk, Poplar
sublimation in the southwestern Prairies from adiabatically warmed winds in the lee of the Rocky Mountains
(chinooks) reduce SWE substantially. Rain-on-snow contributions are infrequent, although flooding in 2011
showed a rain-on-snow contribution in SK and MB
(Stadnyk et al. 2016). Snowmelt runoff is also strongly
controlled by infiltration to frozen mineral soils, which
itself is sensitive to the influence of cultivation on
macropores and fall soil moisture status. Restricted infiltration through frozen soils may develop with any of
mid-winter melting, early spring rainfall and high fall
moisture content, and can generate flooding (Granger
et al. 1984). The more common limited infiltration condition can reduce flood potential even when SWE is
above normal, but the combination of high snowpacks
and restricted infiltration is most strongly associated with
flood development (Gray et al. 1985, 1986).
In the poorly drained central, northern and eastern
Prairies, snowmelt runoff flooding alone does not generate stream flooding as the landscape is characterized by
numerous small post-glacial depressions known locally
as “sloughs,” “wetlands” or “potholes” and here termed
“ponds.” Most ponds are internally drained, forming
closed basins (LaBaugh et al. 1998; Hayashi et al.
2003), which are non-contributing areas in dry to normal
conditions (Godwin and Martin 1975). However, ponds
connect to one another during floods through the “fill
and spill” mechanism (van der Kamp and Hayashi 2009;
Spence 2010). Peak streamflows are influenced by snowmelt rates and volumes near the ponds, incident P, and
antecedent soil and surface storage status (Fang and
Pomeroy 2008; van der Kamp and Hayashi 2009; Fang
et al. 2010). The storage potential of ponds makes them
important hydrological elements (Hayashi et al. 2003)
which can regulate flood peaks by retaining slope runoff
that might otherwise reach the stream. Land-use alteration in surrounding upland areas can produce noticeable
impacts on snowpack trapped by pond vegetation, surface runoff to ponds, pond levels and subsequent flood
Figure 8. Maximum daily mean discharge vs. basin area for
unregulated Water Survey of Canada (WSC) gauging stations
in sub-regions of the Prairies. Drainage basin numbers correspond to the major drainage basins of Canada used by WSC
(see Table 4). Maximum daily mean discharge was obtained for
all naturally flowing stations in the region with 10 years or
more of discharge record with a known drainage area, and
includes all active and discontinued sites (data from WSC
2014).
generation potential (van der Kamp et al. 2003; Fang
and Pomeroy 2008). Modelling the impact of wetland
drainage on peak flow generation suggests that peak
flows can be increased by approximately one third when
pond drainage approaches 50% in a prairie pothole-dominated drainage (Pomeroy et al. 2014), as a result of the
enhanced efficiency of the drainage network in conveying slope runoff to the basin outlet. The relationship
between Qmax and basin area for Prairie rivers (Figure 8)
shows greater scatter than that in other regions that have
been described. This may partly reflect the internal
12
J.M. Buttle et al.
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drainage of many prairie basins, combined with spatial
variations in the intensity of the aforementioned drainage
of prairie wetlands.
Floods in the western Cordillera and Intermontane
region
The western Cordillera extends from southern BC northwards to the Beaufort Sea, and includes areas of YT and
the western portion of the Northwest Territories (NT).
The region is dominated by three mountain systems:
eastern fold mountains (e.g. Rocky Mountains); the sedimentary, metamorphic and igneous rocks of the interior
plateaus and mountains; and the western system of the
metamorphic and intrusive igneous rocks of the Coast
Range and western-most mountains of Haida Gwaii and
Vancouver Island. Much of the region has considerable
forest cover, and slope aspect plays a key role in local
variations in vegetation cover. Windward slopes on
southern mountain ranges have evergreens such as Douglas fir, western hemlock and red cedar. Trees decrease
in size with increasing elevation, and transition to a tundra landscape above the tree line. Leeward slopes may
be covered by vegetation typical of semi-arid landscapes,
particularly in southern BC.
The Cordillera’s location between the Pacific Ocean
and Canada’s interior, its great latitudinal extent and its
rugged terrain result in a great variety of climates.
Annual P ranges from > 4000 mm on the west coast of
Vancouver Island and Haida Gwaii to < 300 mm in the
southern BC interior and parts of the YT. January mean
daily temperature ranges from ~2.5°C in the BC lower
mainland to < −30°C in the YT, while July mean daily
temperature ranges from > 20°C in the southern BC interior to 5°C along the arctic coast. Relief and slope aspect
play important roles in sub-regional climates. Valleys are
warmer than mountain slopes, and the rain-shadow effect
makes windward mountain slopes generally wetter than
leeward slopes.
Mountainous regions of Canada are characterized by
multiple flood-generating mechanisms (Watt et al. 1989;
Woo and Liu 1994). Floods can be generated by rainfall,
snowmelt and rain-on-snow, whether in coastal basins
(Melone 1985; Loukas et al. 2000) or in the BC Interior
(Eaton et al. 2002). The larger the basin, the more likely
that floods are generated by different processes (Woo
and Liu 1994). Except for the largest rivers, rainfall
floods may be of the highest magnitude but are small in
number. Summer floods produced by heavy rainfalls,
although rare, are more significant in smaller basins. Dettinger (2011) described PE which deliver large amounts
of warm moist air onto the west coast of North America,
often resulting in high snowlines, large rainfalls and
extreme floods (Neiman et al. 2013). Spry et al. (2014)
showed that both P and streamflow rates in the BC
Lower Mainland were greater for PE storms than for
non-PE storms. Frontal boundary floods along the east
slope of the Rockies in AB have occurred on many
occasions (e.g. Ford 1924; Hoover 1929; Pomeroy et al.
2016). The largest are generated when mesoscale convective systems stall over the foothills when moving
upslope towards the mountains. Streams receiving substantial snowmelt inputs generally have annual peak
flows in May and June, and event magnitude is determined by SWE and weather conditions during the melt.
Rain-on-snow floods are common in autumn and winter
in coastal areas, and also occur into late spring in interior
areas (McCabe et al. 2007). They are often the largest
floods in larger basins (e.g. Pomeroy et al. 2016), and
the severity of rain-on-snow events depends on P
amount, elevation of the freezing level and the amount
and spatial distribution of snow (McCabe et al. 2007).
Warm wet Pacific storms (often PE events) cause rapid
melting of the existing snowpack (where sensible- and
latent-heat exchanges supply 60–90% of the energy for
snowmelt), augmenting the event’s discharge.
Floods in the Cordillera can also be generated by ice
jams as in other regions of Canada, particularly on more
northerly or high-elevation streams. In addition, the
region experiences flood mechanisms that are relatively
unique to the Western Cordillera. These include debris
flows (rapid flow of saturated debris in a steep channel),
debris floods (rapid flow of debris-laden water in a steep
Figure 9. Maximum daily mean discharge vs. basin area for
unregulated Water Survey of Canada (WSC) gauging stations
in sub-regions of the Cordillera in British Columbia. Maximum
daily mean discharge was obtained for all naturally flowing stations in the region with 10 years or more of discharge record
with a known drainage area, and includes all active and discontinued sites (data from WSC 2014).
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Canadian Water Resources Journal / Revue canadienne des ressources hydriques
channel) and snow avalanches (Desloges and Gardner
1984; Jakob and Jordan 2001; Jakob et al. 2016), and
glacial outburst floods caused by the rapid drainage of
ice-dammed lakes (Desloges and Church 1992; Geertsema and Clague 2005). Wildfire-induced changes to soil
infiltrability may contribute to debris flow and debris
flood initiation (Jordan and Covert 2009).
Discharge maxima for a given basin area (Figure 9)
are generally greater for the wetter coastal area, while
the lowest maxima are found in the drier south–central
interior of BC. There is considerably greater scatter in
the Qmax vs. area relationship for relatively small (< 100
km2) basins in the Cordillera compared to eastern
Canada (Figures 4–6), which likely reflects the combined
influence of the highly variable climate across the Cordillera and variations in basin characteristics. Maxima
from basins draining the east and west slopes of the
Rocky Mountains do not show consistent differences;
however, there is much less variability in the Qmax vs.
area relationship for west slope relative to east slope
basins. The relatively low maxima for some of the latter
may partly reflect basins with hydrometric stations
located far enough downstream that a greater portion of
the basin is in the rain shadow of the Rocky Mountains.
Regional aspects of floods in the Arctic
Canada’s Arctic comprises over 40% of Canada’s landmass, covering all of Nunavut (NU) and the majority of
the NT (Arctic Islands), as well as the northern edges of
YT, MB, ON, QC and NL. It is north of the boreal forest
of the subarctic region and is predominantly comprised
of tundra and barren landscape underlain by permafrost
(see Prowse et al. 2009 for a more detailed discussion of
the region’s physical geography and climate).
The region has long, cold winters and short, cool
summers, leading to snow and ice cover for most of the
year with short periods of runoff. Spring runoff supplies
the bulk of the annual flow, especially for small- to medium-size basins wholly within the Arctic. Many headwater systems are ephemeral as a result of freezing to the
bed and/or flows confined to early spring. Open-water
flow lasts several weeks in the northern Arctic to several
months in more southerly regions. Several large, northward-flowing perennial river systems originate in the
boreal, plains and mountain ecozones, such as the 1.8million-km2 Mackenzie River basin that is Canada’s largest contributor of water to the Arctic Ocean.
Whilst snowmelt drives Arctic flood discharge, river
ice breakup and subsequent jamming have the potential
to generate extreme flood levels (Watt et al. 1989). Other
important flood mechanisms are snow/ice blockage of
channels, icings and glacier bursting. Snowmelt produces
runoff over large areas but often at a slow rate due to
the variable contributing area of meltwater runoff. This
13
area is usually less than the basin area due to redistribution of snow that results in a highly variable spring
snowpack (Essery et al. 1999), cold content in deep
snow drifts that must be overcome before meltwater can
be generated (Marsh and Pomeroy 1996), and spatially
variable melt energy to the patchy snowcover that persists for many weeks during the snowmelt period (Pohl
and Marsh 2006). These spatially variable effects desynchronize meltwater production and are exacerbated in
mountain tundra environments such as in the YT (Carey
and Woo 1999; Pomeroy et al. 2003). In tundra plains
and uplands, absence of trees promotes substantial snow
redistribution to valleys (Pomeroy, Marsh et al. 1997),
where spring meltwater can accumulate behind snowand ice-choked channels and cause downstream flooding
when temporary dams rupture and release potentially
large volumes of impounded water (Woo and Sauriol
1980; Woo 1983; Church 1988). Drainage of icedammed lakes has the potential to produce extreme
floods (Cogley and McCann 1976; Church 1988).
Rainfall-generated floods in the Arctic are rare and
seldom documented (Kane et al. 2003; Dugan et al.
2009); nevertheless, extreme summer flows may occur
(e.g. Cogley and McCann 1976). Continuous permafrost
underlying Arctic basins precludes typical groundwater
systems and severely limits subsurface storage during the
summer, leading to inflated storm runoff response relative to more southern basins (Kane et al. 2003). Church
(1988) suggested that the most extreme discharge could
ultimately arise from rainfall even in the High Arctic.
Although summer rainfalls are generally of low intensity
and minimal hydrological significance in the polar desert
environment compared to nival floods (Table 1; Dugan
et al. 2009), low-pressure systems over intermediatesized basins allow the entire basin to contribute to storm
flow and produce summer floods three to four times
greater than the maximum snowmelt flood (Woo et al.
2008).
The High Arctic is characterized by a short-lived
nival hydrological regime with the maximum discharge
usually generated by spring snowmelt (Woo 1983;
Church 1988). High-gradient headwater basins are dominated by summer floods, while snowmelt drives flooding
in basins on low-gradient coastal plains (Kane et al.
2008). Snowmelt floods also occur in large basins (e.g.
Mackenzie River) that drain extensive southern areas.
Nevertheless, river ice breakup and associated ice jams
dominate the generation of annual high-water levels
throughout the Arctic (von de Wall et al. 2009). This
flood-generating process is most pronounced in sub-regions possessing the combined effects of low relief and
a cold, dry arctic climate.
Restricted infiltration due to the presence of permafrost in the Arctic promotes flooding of wetland areas
(Woo et al. 2008). Unless winter snowfall is limited, the
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14
J.M. Buttle et al.
wetland streamflow regime is typified by the spring freshet (Woo 1983). Rapid snowmelt releases large water
quantities while the active layer is frozen, producing surface runoff that floods wet meadows and infills ponds
and lakes. For instance, the maximum extent of wetland
flooding usually occurs immediately following spring
snowmelt in the High Arctic polar desert (Woo and
Young 2006).
Wetland ecosystems are also found in deltaic environments. The aquatic, semi-aquatic and terrestrial zones
in deltas form highly diverse and variable ecosystems,
and the Mackenzie River system contains three major
deltas: the Peace–Athabasca Delta (PAD), Slave River
Delta and Mackenzie River Delta (MRD). The ecological
integrity of deltaic floodplains depends on their natural
dynamic character, which is controlled by periodic floodwater inputs, deposition of material, and flushing during
high river-stage events (Peters et al. 2016).
A prominent feature of these deltas is an abundance of shallow, productive wetland and lake basins
(> 25,000 in the MRD) with varying degrees of surface water connectivity to the main flow system
(Lesack and Marsh 2010). Low-elevation wetland and
lake basins are potentially flooded annually and for
relatively long durations, while the frequency and
duration of flooding of basins at higher elevations are
much less (Marsh and Lesack 1996). Wetland and lake
basins in the MRD and PAD perched above the main
flow system depend on high spring snowmelt runoff
and ice jamming to raise backwaters enough to
recharge water levels back to the spill elevation (Peters
et al. 2006; Lesack and Marsh 2010). Although openwater floods can inundate low-lying areas of these deltas, ice-jam flooding is often the only process capable
of recharging wetland and lake ecosystems. These
would otherwise desiccate over decadal timescales in
semi-arid environments where ET generally exceeds P
(Marsh and Lesack 1996; Peters et al. 2006).
Some special aspects of floods in Canada
Many rivers in Canada flow northward, and flooding in
these rivers (e.g. Richelieu, Red, Peace–Athabasca, Hudson Bay rivers) is often connected to delayed melting in
northern portions of the basin relative to earlier snowmelt in southern portions (Bruce 1939). Spring breakup
can be the largest physical disruption in rivers that flow
northward to cooler regions where spring thaw is later
(Smith 1980; Rood et al. 2007). Such floods put cities
such as Winnipeg at risk (Blais, Clark et al. 2016) while
providing an essential ecological service to deltas such
as the PAD (Peters et al. 2006). Several flood case studies in this special issue report on such flooding events
(Rannie 2016; Riboust and Brissette 2016; Saad et al.
2016; Wazney and Clark 2016).
This section describes in more detail several distinct
aspects of flooding in Canada. The first is the little-studied issue of groundwater flooding. The second is ice-jam
flooding, which although not unique to Canada is a common process in cold regions. The third type is stormsurge flooding, which is generally restricted to coastal
areas. Fourth, the concentration of Canada’s population
in cities makes the issue of urban flooding of particular
interest.
Groundwater flooding
Groundwater flood events are generated by four mechanisms (Hughes et al. 2011): (1) natural water table rises
and surface saturation caused by extreme high-intensity
and/or long-duration rainfall (e.g. Robins and Finch
2012); (2) groundwater flow through alluvial deposits
bypassing river channel flood defences; (3) groundwater
level rise due to cessation of groundwater abstraction;
and (4) underground structures creating barriers to
groundwater flow that result in water tables rising to
cause flooding (e.g. Edwards 1997). An additional mechanism is over-irrigation in semi-arid and arid areas,
which causes groundwater levels to rise along with
related salinization (Xiong et al. 1996).
Reports of groundwater flood events in Canada are
uncommon, although numerous studies describe groundwater–surface water (GW–SW) interactions under variable
stream flow conditions (e.g. Cloutier et al. 2014), and
unpublished accounts indicate basement flooding related
to GW–SW interactions in communities built on alluvial
aquifers. One such example comes from the relationship
during the major flood of 20–21 June 2013 between the
groundwater level in Canmore, AB and the hydrograph
of the Bow River (from the Banff Water Survey of
Canada [WSC] gauging station) which flows through the
town (Figure 10). The Canmore groundwater observation
Figure 10. Discharge for the Bow River at Banff, Alberta for
the 20–21 June 2013 flood, and groundwater level in the alluvial aquifer in Canmore, Alberta.
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Canadian Water Resources Journal / Revue canadienne des ressources hydriques
well is 24 km downstream of the Banff WSC station in a
thick (up to 100 m) sand and gravel aquifer (Toop and de
la Cruz 2002). Aquifer thickness at the well is 73 m,
which is 730 m from the current river channel. Bow
River discharge in Banff rose sharply near midnight of 20
June and peaked at 401 m3/s in the late afternoon of 21
June (Figure 10), exceeding the highest instantaneous
flow recorded at this station (399 m3/s on 14 June 1923).
Flood peak travel time between Banff and Canmore is
likely on the order of a few hours. A kinematic wave
induced by a rapid increase in river stage (Jung et al.
2004) likely led to the quick response of the groundwater
level, which reached the maximum rise of ~1.6 m by
midnight on 22 June (Figure 10). Much of Canmore is
built on the alluvial aquifer (Toop and de la Cruz 2002)
and is vulnerable to flooding associated with the rising
water table, which can back up sewer systems and cause
basement as well as surface flooding due to the reduced
sewer drainage capacity. The 20–21 June 2013 flood
event was particularly severe due to the combination of
extremely high river stage and intense local infiltration
caused by heavy rain (Blair Birch, Town of Canmore,
pers. comm. 2014). Approximately 130 km downstream
of Canmore, the Bow River flows through Calgary, where
densely populated urban areas are also located on the
alluvial aquifer (Cantafio and Ryan 2014). Similar cases
of groundwater-induced flooding were reported in Calgary during the same flood (Osborn and Ryan 2014).
15
strong control on flood generation in many areas of
Canada (Ashton 1986; Gray and Prowse 1993). The
establishment of river ice cover decreases flow conveyance by reducing channel cross-sectional area and
flow velocity, and increasing flow resistance through an
enlarged wetted perimeter. The addition of an ice cover
with a similar roughness to the river bed results in ~30%
increase in water depth over open-water channel flow
conditions (Gray and Prowse 1993). On a large river system, water abstraction to feed ice growth and hydraulic
storage behind the accumulating ice can substantially
decrease downstream flow (Moore et al. 2002; Prowse
and Carter 2002). Backwater storage is released during
spring melt, contributing to the largest hydrologic event
of the year for most ice-covered rivers at northern latitudes; e.g. > 20% of the spring freshet volume in the
Mackenzie River was estimated to be over-winter water
released from ice-induced hydraulic storage at the time
of river ice breakup (Prowse and Carter 2002).
Interaction of a large flood wave with an intact and
mechanically strong ice cover can result in dynamic (mechanical) river ice breakup and ice jamming, with potential generation of extreme flood levels (Watt et al. 1989).
Ice jams may present significant resistance and/or
obstruction to streamflow, leading to river stage several
metres greater than for the same discharge under openwater conditions (von de Wall et al. 2010; Figure 11)
and occasional over-banking of channel water (Gray and
Prowse 1993).
Storm surges
Storm surges are water-level oscillations in a coastal or
inland water body with a period of a few minutes to a few
days, resulting from atmospheric weather systems. These
include extra-tropical cyclones, hurricane remnants and
squall lines embedded in larger scale synoptic systems.
Resulting water-level changes along the shoreline can be
on the order of several metres. Storm surges are frequent
in coastal areas of Canada, the deadliest of which was the
1869 Saxby Gale in the Bay of Fundy which killed more
than 100 people in the Maritimes. They have resulted in
near-shore inundation on the Atlantic (Danard et al. 2003)
and Pacific (Murty et al. 1995; Crawford et al. 2000)
coasts, in the Gulf of St. Lawrence, St. Lawrence Estuary,
Bay of Fundy, Hudson Bay, James Bay, Northwest Passage, Beaufort Sea (Harper et al. 1988; Marsh and Schmidt
1993), the Great Lakes (Watt et al. 1989; Trebitz 2006),
and other large lakes such as Lake Winnipeg. With rising
sea levels, storm-surge damage may increase in future
(e.g. Lyle and Mills 2016).
Ice jam-related floods
The formation, growth and ablation of ice cover and the
associated development of ice jams exert a particularly
Figure 11. Open-water and ice-influenced peak water levels
vs. discharge at the Mackenzie River at Arctic Red River Water
Survey of Canada (WSC) station for the years 1972 to 2006
(modified from von de Wall et al. 2010).
16
J.M. Buttle et al.
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Floods in urban areas
A number of processes can produce flooding in cities;
the focus here is on the role of urbanization itself. Flooding in urban areas is a special concern since Canada’s
urban population (people living in urban areas as defined
by national statistical offices) was just above 27 million
(81% of Canada’s total population) in the 2011 census
(Statistics Canada 2014). Urbanization involves the construction of buildings, roads and related infrastructure,
with consequent reduction of the infiltration capacity of
previously permeable surfaces and a general improvement in the urban landscape’s hydraulic efficiency
through the creation of roadside gutters, sewer networks
and lined stream channels. The result is increased surface
runoff, more rapid water transmission through the drainage network, and increased flood risk in terms of both
peak flow magnitude and frequency of occurrence of that
flood magnitude (Leopold 1968; Hall 1984; Leith and
Whitfield 2000).
The most common way of dealing with enhanced
runoff in urban areas has been to carry it away as
quickly as possible via underground pipes and sewers
(Hall 1984). Older (pre-1950s) communities frequently
have combined sewer systems, where rainfall drains into
sewers carrying waste water and both are transferred to
sewage treatment works. Since 1960, many new developments have separate sewer systems where water from
gutters and roads may be carried through pipes to the
nearest watercourse, but often simply joins a combined
sewer (Waller 1976). Most modern urban flood drainage
systems were designed to cope with rainfall events that
occur with a one-in-30-year probability (the design
storm); however, older parts of the system may be operating to a lower standard. It is inevitable that the capacities of sewers, covered urban water courses and other
piped systems will sometimes be exceeded. When the
piped system is overwhelmed or cannot drain effectively
into an outfall, the excess travels down roads and other
paths of least resistance, and floods low-lying areas.
These areas can contain property and infrastructure
where flooding can cause costly damage, distress and
sometimes loss of life.
As an example, Toronto, Canada’s largest city, experienced major flooding due to extreme rainfalls in 2000,
2005 and 2013 (Kovacs et al. 2014). The 8 July 2013
flood in Toronto resulted from 126 mm of rain generated
by two sequential thunderstorm cells. Rainfall intensity
far exceeded storm sewer capacity, and caused runoff to
travel along city streets to creeks and rivers. The storm
caused major transit disruptions and delays, road closures, power blackouts, flight cancellations and flooding
across Toronto and Mississauga, including flooding of
3000 basements. The Insurance Bureau of Canada estimated the 8 July storm costs at close to CAD $1 billion
in damages – the most expensive natural disaster ever in
Toronto and ON.
The wide range of urban stormwater best management practices (BMPs) include such end-of-pipe options
as infiltration basins and stormwater management ponds
(Zimmer et al. 2007); the latter store runoff temporarily
and release it over a protracted period to downstream
drainage systems (Marsalek and Schreier 2009). Increasing emphasis has been placed on low-impact development (LID) based on design features to reduce overland
flow and enhance groundwater recharge, such as the disconnection of downspouts from storm sewers, use of pervious pavement, and inclusion of lot-level storage and
infiltration features such as rain gardens and street swales
(Zimmer et al. 2007). Such BMPs include green roofs,
which may be particularly appropriate for reducing runoff in downtown areas where extensive impervious surface coverage and high land prices in downtown areas
make creation of vegetated space for water infiltration
very expensive (Roehr and Kong 2010).
Observed and anticipated climate-related trends in flood
drivers and flooding in Canada
Research into historical trends of flooding in Canada and
its drivers at the national and regional scales, as well as
anticipated changes in flooding associated with climate
change, has been spurred in part by the expectation that
climate change may intensify the water cycle via
increases in ET and P (Huntington 2006). A major theme
of this work is the attempt to link any changes in flooding to historical and forecast trends in climate variables
such as air temperature and P (Mortsch et al. 2015).
There was no significant trend in mean annual temperatures over the latter half of the twentieth century for the
entire country; however, Zhang et al. (2000) reported
that annual maximum temperatures increased by 1.5–
2.0°C during this period in northern BC and western NT,
and cooled by ~1.5°C in northeastern Canada, with a
similar pattern for annual minimum temperatures. Annual
P increased by 5–30% during this period across Canada,
with statistically significant increases mostly in the Arctic. Such climatic changes (particularly the increase in
annual and in some cases seasonal P) may result in
increased flood risk in various parts of Canada, especially if there is a corresponding increase in P intensity
(Morgan et al. 2004). Zweirs and Kharin (1998) predicted that doubling of atmospheric carbon dioxide
(CO2) would increase rainfalls with a 20-year return period in Canada by ~14% using a global climate model
(GCM). Mailhot et al. (2012) used regional climate simulations to forecast large relative increases in annual
maximum P for a range of durations and return periods
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Canadian Water Resources Journal / Revue canadienne des ressources hydriques
for 2014–2070 for southern ON, southern QC and the
Prairies, with the smallest increases in coastal areas.
Nevertheless, evidence of increases in extreme rainfalls in Canada from the historical record that might support such projections is equivocal. Thus, Kunkel et al.
(1999) found no long-term trend in an extreme P index
for Canada for 1951–1993. Mekis and Hogg (1999)
observed an increased fraction of annual P falling in the
largest 10% of daily events with measurable P for 1940–
1995 over a large portion of Canada; however, this trend
may have been unduly influenced by stations in northern
Canada with relatively short records (Zhang et al. 2001).
Zhang et al. (2001) noted no identifiable trends in either
the frequency or the intensity of extreme rainfalls at the
national scale. They found a significant linear trend in
spring heavy rainfall events for 1900–1998 in eastern
Canada, but no trends in heavy rainfall events in other
seasons or regions. They suggested that the failure of
their analyses to support GCM projections of increased
heavy rainfalls made by Zwiers and Kharin (1998) and
others may possibly reflect the relatively early stages of
global warming induced by greenhouse gases. Shook
and Pomeroy (2012) found no increase in rainfall frequency or depth in single-day events in the Prairies over
records extending to 100 years, but an increase in the
number and intensity of multiple-day rainfall events was
evident over much of the region, and in some cases the
number of multiple-day events has doubled. A substantial increase in early spring and late fall rainfall can be
hydrologically significant since these rains may cap frozen soils with ice layers that restrict infiltration or induce
rain-on-snow flooding.
Research into trends in flooding drivers has examined
the role of teleconnections between two dominant modes
of atmospheric variability – the North Atlantic Oscillation (NAO) and the Pacific/North America teleconnection pattern (PNA) – and P amount and intensity. Stone
et al. (2000) found seasonally increasing trends in total
P resulting from increases in all levels of event intensity
in southern areas of Canada during the twentieth century.
They observed a rainfall response to the NAO in northeastern Canada in summer, while the PNA strongly influenced P variations in southern BC and the Prairies. The
PNA only influenced the frequency of heavier rainfalls
in the autumn in ON and southern QC, with a negative
PNA generally leading to more extreme P events. The
potential link between the NAO and extreme P in QC
may extend to flood characteristics (magnitude, frequency and duration), and Fortier et al. (2011) concluded
that low-frequency oscillations in this teleconnection pattern may exacerbate flood conditions. Gingras and Adamowksi (1995) compared flood magnitudes during and
outside of El Niño–Southern Oscillation (ENSO) periods,
and found a significantly lower probability of floods during warm El Niño periods for five basins in southern
17
ON. They concluded that the frequency of rainfall-generated floods is different during ENSO periods, with
important consequences for water resources managers
particularly in regard to flood forecasting and reservoir
management.
Analyses of temporal trends for floods in the northwestern boreal forest indicate that spring runoff is starting earlier in the year apparently due to increasing
spring air temperatures (Burn et al. 2004). Warm Pacific
Decadal Oscillation (PDO) phases (positive values) are
associated with warmer and drier winters, while cold
PDO phases (negative values) are linked to cooler and
wetter winters. During warm PDO phases, the timing of
annual maximum and snowmelt-induced floods in the
northwestern part of the boreal forest shifts towards the
spring, and moves towards the summer for the cold
PDO. Such decadal-scale climate fluctuations also affect
flood magnitudes in the Boreal Plains, where periods of
above-average P can increase the effective contributing
area for flood generation by increasing hydrologic connectivity in a basin through rising water levels in ephemeral lakes in closed depressions and areas of internal
drainage (Alberta Transportation 2004).
Changes in P associated with such variations in teleconnections may be superimposed on long-term trends.
Thus, St-Laurent et al. (2009) ascribed an increase in the
frequency of occurrence of major floods since the beginning of the twentieth century in the St-François basin to
increased P, although periods of below-average P (1950–
1960 and 1980–1990) were also observed. Cunderlik and
Ouarda (2009) observed no significant trends in the timing of rainfall-driven floods, but significant negative
trends in snowmelt flood magnitude over the last three
decades in southern ON. This was attributed to earlier
snowmelt as result of a warming climate. Cunderlik and
Ouarda (2009) also noted that basins with mainly snowmelt-driven floods have a unimodal flood seasonality,
while those experiencing snowmelt-driven plus rainfalldriven floods in late summer and fall have a bi-modal
seasonality. They suggest the potential for flood seasonality at a station to change from unimodal to bi-modal
with time, which may imply an increasing intensity of
rainfall events in recent years.
In the Prairies, Shook and Pomeroy (2012) noted an
increased rainfall fraction of P in March and clustering
of summer rainfall events with greater numbers of multiple-day rainfall events. Dumanski et al. (2015) found
that these trends are causing an increase in flooding in a
small basin in SK and greater contributions from rainon-snow. The appearance of rainfall-runoff high flows in
2012 and the flood of record in 2014 suggest a rapidly
changing prairie hydrology where snowmelt runoff over
frozen soils is no longer the exclusive mechanism of
flood generation, and flow volume from snowmelt is
exceeded by that due to rainfall-runoff.
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18
J.M. Buttle et al.
Future and present trends in floods in mountainous
regions must consider multiple flood-generating mechanisms and resist the temptation to generalize, since reliable detection of trends in floods in mountainous regions
is complex. This is exacerbated by issues associated with
pooling data with different start times, durations and
basin characteristics (Viviroli et al. 2012). Using a common period of observations, Cunderlik and Ouarda
(2009) showed that flood magnitudes are decreasing in
AB and BC, although there appear to be no significant
trends in the magnitude of rainfall floods in streamflow
records across Canada (Burn and Whitfield 2016). This
is echoed by Harder et al. (2015), who found no trend in
peak flows over 50 years in Marmot Creek, AB which
drains part of the front ranges of the Rocky Mountains.
Byrne et al. (1999) argued that shifts in synoptic types
in future climates could either increase or decrease floods
based on basin location, while Whitfield et al. (2003)
showed that the type of future changes in BC’s mountainous Georgia Basin depends upon the hydrological
regime. In rainfall-driven streams, modelled flood events
increase in number but not magnitude, whereas in hybrid
streams the frequency of winter events increases while
that of summer snowmelt floods decreases. In snowmeltdriven streams, the magnitude and duration of summer
floods decrease. All of these changes reflect the increasing domination of rainfall-driven events in this region
(Whitfield et al. 2003).
The few data available for time series analysis of
river ice breakup events in the Arctic suggest that a general (although not statistically significant) increase in
magnitude and decrease in timing of maximum breakup
water level has occurred since the 1970s (von de Wall
2011). Restricted by similar station limitations and time
period, a time series analysis by Monk et al. (2011)
revealed mostly non-significant trends in peak annual
runoff magnitude.
Conclusions and areas of future research
Flooding in Canada can occur from a broad range of
processes, described here and in accompanying papers in
this special issue. While many processes responsible for
flood generation occur in most if not all regions of
Canada, there are large regional contrasts in the predominance of specific generating mechanisms. These are
expected to differ with both climate variations and
change in the future, and these similarities and differences have been summarized above.
A major research need highlighted in this paper is
the importance of constructing a reliable historical flood
record for Canada. This would assist such efforts as
improving our understanding of the processes driving
flooding at particular locations, and the assessment of
temporal changes in the magnitude and frequency of
floods generated by specific processes at the regional and
national scale. The deficiencies in the Canadian Disaster
Database (CDD) that were highlighted earlier need to be
addressed. Since the public costs of floods are increasing, more should be done to ensure that the CDD is
complete and consistent.
Changes to the family of snowmelt-, rain-on-snow-,
ice jam- and rainfall-generated floods related to climate
variation and change continue to warrant attention, as
these processes affect most areas of Canada. Progressive
warming has meant a shift from snowmelt-dominated
flooding to rain-on-snow or rainfall-runoff flooding in
some areas, and these changes have challenged local
authorities who manage infrastructure or predict flooding.
Rising sea levels and changes in storm frequency suggest
that storm-surge flooding may increase in coastal areas.
This prospect needs to be addressed.
Approaches to improve our understanding of floodgeneration processes will by necessity vary between
regions across the country, given differences in such factors as hydroclimate, geology, physiography and land
cover. For example, in the Prairie landscape there is a
need to consider the role of various hydrological processes in altering the states and locations of water and
the consequent effects on streamflow generation (Shook
et al. 2015). These transformations include wind redistribution and ablation of winter snowfall, snowmelt and
runoff over frozen soils, and filling and spilling of
depressional storage as runoff contributes to streamflow.
Improvements in understanding the flood hydrology of
the Arctic have to be made in the face of a scarcity of
gauged streams (Woo et al. 2008; Dugan et al. 2009)
which limits knowledge of spatial variations in flood
generation across the Arctic, and the lack of complementary data (e.g. P, air temperature and soil moisture)
which complicates building a consensus on runoff
response for the region (Kane et al. 2008). Expansion of
the Arctic hydrometric monitoring network occurred during the International Polar Year Program (2007–2008);
however, wide-scale coverage is constrained by the
expense of installing and maintaining such remote systems. Efforts must continue to integrate data from the
existing network, climate reanalysis products, remote
sensing approaches for estimating discharge and areal
extent of inundation on large rivers, and hydraulic and
hydrologic models to improve our understanding of flood
characteristics across the Arctic.
Similarly, the research needs associated with flood
processes that have received little attention to date or are
particularly relevant to Canada are varied. Thus, the process of groundwater flooding needs to be considered
more explicitly by flood monitoring and risk-assessment
processes. Although groundwater flooding can cause significant social and financial impact, particularly to basements of structures located close to rivers (Jacobs GIBB
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Canadian Water Resources Journal / Revue canadienne des ressources hydriques
Ltd. 2006), impacts from this form of flooding could be
overlooked in the planning process. Engineering solutions to overcome the risk of fluvial flooding, such as
levees, would not protect such developments from flooding caused by groundwater underflow through permeable
materials driven by high river stages.
Flood risks associated with storm surges in Canada
are strongly linked to the potential for climate change to
intensify weather systems and associated wind fields,
thus resulting in bigger surges and an east–west and
north–south shift in the tracks of the weather systems
(Danard et al. 2003). Such changes in storm tracks could
subject new areas to storm surge activity. Future work
needs to build on previous attempts to model stormsurge events (e.g. Bobanović et al. 2006), given the
influence of rising sea levels and increasing development
in low-lying coastal areas on storm surges in future climates. Concurrent needs include an improved capacity to
map potential inundation zones for storm surges that
might occur in populated coastal areas (Danard et al.
2003; Webster et al. 2004, 2006) to reduce or avoid the
flood risk associated with these events.
Climatic shifts in most of Canada to wetter winters
and heavier summer rainfall events (Kunkel et al. 1999;
Institute for Catastrophic Loss Reduction 2012) also may
affect flooding in urban areas. Design storms based on
specified return periods, durations and intensities have
long been used to plan urban drainage systems, despite
the imprecise definition of the design storm concept in
Canada (Marsalek and Watt 1984; Watt and Marsalek
2013). Concerns about climate change and the need to
adapt to it have prompted many municipalities in Canada
to revisit the design storm event issue, particularly in
connection with drainage design since non-stationarity
likely means that design storms simply do not exist as
static measures. This reanalysis has mostly focused on a
single property of design storms: intensity–duration–frequency (IDF) relations and projected increases in rainfall
intensity (Peck et al. 2012; Srivastav et al. 2014). Failure
to revise design criteria in light of climate change effects
on IDF relations may result in undersized infrastructure
and increased flood risk (e.g. Mailhot et al. 2012). Urban
drainage design practice would significantly benefit from
the adoption of a comprehensive approach that considers
all design storm event characteristics and their sensitivity
to climate change and inherent uncertainties in existing
IDF relations, as well as hydraulic design of sewer networks (Buttle and Lafleur 2007; Guo and Zhuge 2008).
Such an approach should incorporate LID practices, and
research should address (amongst other issues) the rainfall magnitudes and frequencies at which LID control
needs to be supplemented by end-of-pipe practices such
as stormwater ponds, as well as the ability of LID
designs to function effectively in Canadian cities during
19
the winter when soil freezing may limit water infiltration
(Zimmer et al. 2007).
Human actions or inactions play an important role in
both the changes of flood magnitude and frequency and
the risk to society. Changes in land use also play an
important and understudied role in changes in floods.
Whether or not future floods increase in frequency or
magnitude, Canadians need to recognize that continued
development in floodplains and areas of urban development increase the risk and potential costs of flooding.
The decisions that our society makes will determine if
flooding is going to remain the most common and costliest natural disaster for Canadians.
Acknowledgements
The authors acknowledge the significant contributions of the
many colleagues who have over the past decades provided the
foundation for the study of floods in Canada. Carole HoltOduro (Alberta Environment and Parks) provided unpublished
groundwater data, and Blair Birch (Town of Canmore) provided
information regarding groundwater levels. The Hydrological
Expertise Center of the Québec Department of Environment,
Sustainable Development and Climate Change (https://www.
cehq.gouv.qc.ca/, accessed 18 September 2014) was an important source of information regarding floods in Québec. We also
express our thanks to the reviewers and editors for their suggestions.
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