Changes

==Hydrologic Changes Due to Urbanization==

==Hydrologic Changes Due to Urbanization==

===Pre-Development Hydrology===

===Pre-Development Hydrology===

In Ontario prior to development, it is typical for rain falling to the surface to be intercepted by the leaves and stems of vegetation, and this is referred to as interception storage. The amount of rain lost to interception storage depends on the kind of vegetation and its growth stage, but abstraction values of 1 – 4 mm are typical (UNFAO, 1991). The presence of vegetation also helps to reduce the incidence of soil crusting which can otherwise occur when raindrops impact bare soil surfaces. The root systems of vegetation help to loosen the soil and increase its connected porosity, and this in turn promotes more rapid infiltration. A landscape’s infiltration capacity is also dependent on soil texture; the highest infiltration capacities are typically found in loose, sandy soils, while heavy clay or clay-loam soils usually have smaller infiltration capacities.

In Ontario prior to development, it is typical for rain falling to the surface to be intercepted by the leaves and stems of vegetation, and this is referred to as interception storage. The amount of rain lost to interception storage depends on the kind of vegetation and its growth stage, but abstraction values of 1 – 4 mm are typical <ref>United Nations Food and Agricultural Organization (UNFAO). 1991. A Manual for the Design and Construction of Water Harvesting Schemes for Plant Production. Available at URL: http://www.fao.org/docrep/u3160e/u3160e00.htm#Contents</ref>. The presence of vegetation also helps to reduce the incidence of soil crusting which can otherwise occur when raindrops impact bare soil surfaces. The root systems of vegetation help to loosen the soil and increase its connected porosity, and this in turn promotes more rapid infiltration. A landscape’s infiltration capacity is also dependent on soil texture; the highest infiltration capacities are typically found in loose, sandy soils, while heavy clay or clay-loam soils usually have smaller infiltration capacities.

If rain falls at rate which is greater than the underlying soils infiltration rate, it will begin to fill depressions, at which point runoff will begin to be generated. The production of runoff is accelerated as surface slope increases and slope lengths decrease, as both considerations increase surface runoff velocities and decrease the time of concentration (Sharma et al., 1986).

If rain falls at rate which is greater than the underlying soils infiltration rate, it will begin to fill depressions, at which point runoff will begin to be generated. The production of runoff is accelerated as surface slope increases and slope lengths decrease, as both considerations increase surface runoff velocities and decrease the time of concentration <ref>Sharma, K.D. 1986. Runoff behaviour of water harvesting microcatchments. Agricultural Water Management 11 (2): 137-144</ref>.

Under natural conditions, the presence of surface vegetation and leaf litter provides ample opportunity for rainfall to be intercepted, detained and infiltrated – even in area with moderate to steep slopes. Generally speaking, only about 10% of the annual rainfall amount in such areas is lost as surface runoff.  The rest of the water supports the growth of vegetation (40%), feeds nearby watercourses (20%) and recharges aquifers (20%).

Under natural conditions, the presence of surface vegetation and leaf litter provides ample opportunity for rainfall to be intercepted, detained and infiltrated – even in area with moderate to steep slopes. Generally speaking, only about 10% of the annual rainfall amount in such areas is lost as surface runoff.  The rest of the water supports the growth of vegetation (40%), feeds nearby watercourses (20%) and recharges aquifers (20%).

'''Water Quality Impacts'''

'''Water Quality Impacts'''

The surface runoff generated in urban areas frequently carries with it a cocktail of pollutants.  Although it is variable in nature, runoff pollutants are typically derived from a combination of fine sediments from atmospheric deposition, oil, grease and heavy metals (including Cd, Cu, Fe, Ni, Pb, Zn, etc.) from vehicular traffic and industrial activities, nutrients derived from lawn fertilizers and pet waste, and – in seasonally cold climates – road salts from winter maintenance activities (Aryal et al., 2010; Bäckstrom et al., 2004; Trenouth et al., 2015). These pollutants accumulate on the road surface during the antecedent dry period between consecutive rainfall events, and are washed off at the onset of rainfall. The majority of particles are washed off with the first flush of stormwater runoff, typically considered to be accounted for with the first 25 mm, or one inch or runoff (Strenstrom and Kayhanian, 2005).  

The surface runoff generated in urban areas frequently carries with it a cocktail of pollutants.  Although it is variable in nature, runoff pollutants are typically derived from a combination of fine sediments from atmospheric deposition, oil, grease and heavy metals (including Cd, Cu, Fe, Ni, Pb, Zn, etc.) from vehicular traffic and industrial activities, nutrients derived from lawn fertilizers and pet waste, and – in seasonally cold climates – road salts from winter maintenance activities <ref>Aryal, R. Vigneswaran, S. Kandasamy, J.; Naidu, R. 2010. Urban Stormwater Quality and Treatment. Korean Journal of Chemical Engineering, 27(5):1343-1359</ref> <ref> Trenouth, W.R. Gharabaghi, B., Perera, N. 2015. Road salt application planning tool for winter de-icing operations. Journal of Hydrology. 524:401-410</ref>. These pollutants accumulate on the road surface during the antecedent dry period between consecutive rainfall events, and are washed off at the onset of rainfall. The majority of particles are washed off with the first flush of stormwater runoff, typically considered to be accounted for with the first 25 mm, or one inch or runoff <ref>Stenstrom, M.K. Kayhanian, M. 2005. First flush phenomenon characterization. Prepared for California Department of Transportation, Division of Environmental Analysis. Available at URL: http://www.dot.ca.gov/hq/env/stormwater/pdf/CTSW-RT-05-073-02-6_First_Flush_Final_9-30-05.pdf</ref>.  

'''Climate-Related Impacts'''

'''Climate-Related Impacts'''

Since 1995, Ontario has had a weather-related state of emergency almost every single year (Swiss Re, 2010). The City of Windsor saw extreme events that caused severe flooding in 2007, 2010, 2016 and 2017 (City of Windsor, 2012). The Ottawa region experienced one extreme event every year for five years, and in the Greater Toronto Area (GTA), there have been four extreme rainfall events in the past ten years (Environment Canada, 2014). Such high intensity events produce heavy rainfall in relatively short periods of time. While it is reasonable to expect runoff to be produced under such conditions – particularly when rain falls which exceeds a soil’s hydraulic conductivity - the production of stormwater is exacerbated in urban areas where the overwhelming majority of surfaces are impervious. The problems associated with managing stormwater volumes are exacerbated when dense stormsewer networks efficiently convey stormwater runoff volumes from a large contributing upland area to a single outlet location, such as a stormsewer outfall in a river or stream.

Since 1995, Ontario has had a weather-related state of emergency almost every single year <ref>Swiss Re (in collaboration with Institute for Catastrophic Loss Reduction) (2010). Making Flood Insurable for Canadian Homeowners. Available at URL: http://www.iclr.org/images/Making_Flood_Insurable_for_Canada.pdf</ref>. The City of Windsor saw extreme events that caused severe flooding in 2007, 2010, 2016 and 2017 <ref>City of Windsor. 2012. Climate Change Adaptation Plan. Available at URL: http://www.citywindsor.ca/residents/environment/environmental-master-plan/documents/windsor%20climate%20change%20adaptation%20plan.pdf</ref>. The Ottawa region experienced one extreme event every year for five years, and in the Greater Toronto Area (GTA), there have been four extreme rainfall events in the past ten years (Environment Canada, 2014). Such high intensity events produce heavy rainfall in relatively short periods of time. While it is reasonable to expect runoff to be produced under such conditions – particularly when rain falls which exceeds a soil’s hydraulic conductivity - the production of stormwater is exacerbated in urban areas where the overwhelming majority of surfaces are impervious. The problems associated with managing stormwater volumes are exacerbated when dense stormsewer networks efficiently convey stormwater runoff volumes from a large contributing upland area to a single outlet location, such as a stormsewer outfall in a river or stream.

In July 2013, the GTA experienced its most severe storm event in 60 years. Nearly five inches (126 mm) of rain fell in a two-hour period. In comparison, during Hurricane Hazel (a devastating event in 1954 where 81 lives were lost), the two-hour maximum precipitation was 91 mm and the total amount of rainfall was 285 mm over nearly two days (Toronto Star, 2013). Conventional municipal drainage systems could not carry stormwater away fast enough. Roads and highways were overcome with floodwater closing major transportation corridors including Highway 427. GO Train passengers were stranded, and power outages and basement flooding were widespread with property damage of more than $1 billion (IBC, 2016).

In July 2013, the GTA experienced its most severe storm event in 60 years. Nearly five inches (126 mm) of rain fell in a two-hour period. In comparison, during Hurricane Hazel (a devastating event in 1954 where 81 lives were lost), the two-hour maximum precipitation was 91 mm and the total amount of rainfall was 285 mm over nearly two days (Toronto Star, 2013). Conventional municipal drainage systems could not carry stormwater away fast enough. Roads and highways were overcome with floodwater closing major transportation corridors including Highway 427. GO Train passengers were stranded, and power outages and basement flooding were widespread with property damage of more than $1 billion <ref>Insurance Bureau of Canada (IBC). 2016. Facts of the property & casualty insurance industry in Canada. 36th edition, ISSN 1197 3404. Available at URL: http://assets.ibc.ca/Documents/Facts%20Book/Facts_Book/2016/Facts-Book-2016.pdf</ref>.

While it is nearly impossible to ascribe the cause of a single event to the broader issue of climate change, the trend is clear: an increasing number of high-intensity, short-duration (HISD) events are impacting our urban areas, exacerbating the stresses on overtaxed stormwater infrastructure. The figure below highlights a series of seven recent extreme rainfall events which have struck the Greater Toronto and Hamilton Area (GTHA).   

While it is nearly impossible to ascribe the cause of a single event to the broader issue of climate change, the trend is clear: an increasing number of high-intensity, short-duration (HISD) events are impacting our urban areas, exacerbating the stresses on overtaxed stormwater infrastructure. The figure below highlights a series of seven recent extreme rainfall events which have struck the Greater Toronto and Hamilton Area (GTHA).   

References:

References:

 

Bäckstrom, M.; Karlsson, S.; Bäckman, L.; Folkeson, L.; Lind, B.  2004.  Mobilisation of Heavy Metals by De-icing Salts in a Roadside Environment.  Water Research, 38:720-732.

Bäckstrom, M.; Karlsson, S.; Bäckman, L.; Folkeson, L.; Lind, B.  2004.  Mobilisation of Heavy Metals by De-icing Salts in a Roadside Environment.  Water Research, 38:720-732.

 

Environment Canada. 2014. Climate. Available at URL: http://climate.weather.gc.ca/  

Environment Canada. 2014. Climate. Available at URL: http://climate.weather.gc.ca/  

 

Pitt, R. 1999.  Small Storm Hydrology and Why it is Important for the Design of Stormwater Control Practices. In: Advances in Modeling the Management of Stormwater Impacts, Volume 7. Computational Hydraulics International, Guelph, Ontario and Lewis Publishers/CRC Press. 1999.  

Pitt, R. 1999.  Small Storm Hydrology and Why it is Important for the Design of Stormwater Control Practices. In: Advances in Modeling the Management of Stormwater Impacts, Volume 7. Computational Hydraulics International, Guelph, Ontario and Lewis Publishers/CRC Press. 1999.  

 

 

 

Toronto Star. 2013. Monday’s storm vs. Hurricane Hazel. Available at URL: http://www.thestar.com/opinion/letters_ to_the_editors/2013/07/14/mondays_storm_vs_hurricane_hazel.html  

Toronto Star. 2013. Monday’s storm vs. Hurricane Hazel. Available at URL: http://www.thestar.com/opinion/letters_ to_the_editors/2013/07/14/mondays_storm_vs_hurricane_hazel.html