Changes

==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 <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.

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.

==Post-development hydrologic changes==

==Post-development hydrologic changes==

===Water quantity changes===

===Water quantity changes===

While rainfall intensity, soil and vegetation characteristics, slope length and steepness all play a role in the timing and rate of runoff generation, the creation of impervious surfaces – including rooftops, driveways, roads and parking lots – disrupts rainfall’s ability to penetrate the soil surface and infiltrate.  In heavily urbanized, well-drained areas, the time of concentration is significantly reduced due to the relative smoothness of impervious surfaces, and the dense network of stormwater [[conveyance]] infrastructure including gutters, catch basins and subsurface pipes.  

While rainfall intensity, soil and vegetation characteristics, slope length and steepness all play a role in the timing and rate of runoff generation, the creation of impervious surfaces – including rooftops, driveways, roads and parking lots – disrupts rainfall’s ability to penetrate the soil surface and infiltrate.  In heavily urbanized, well-drained areas, the time of concentration is significantly reduced due to the relative smoothness of impervious surfaces, and the dense network of stormwater [[conveyance]] infrastructure including gutters, catch basins and subsurface pipes.  

===Climate-related impacts===

===Climate-related impacts===

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