Changes

==Hydrologic Changes Due to Urbanization==

Hydrologic Changes Due to Urbanization

<h3>Pre-Development Hydrology</h3>

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 (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.

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 (Sharma et al., 1986).

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%).

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Post-Development Hydrologic Changes:

<h3>Post-Development Hydrologic Changes</h3>

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.  

In urban areas which use stormwater ponds to control the peak flow of runoff entering receiving environs the net volume of runoff remains the same, but the rate of release is controlled (left). In older urban areas where stormwater ponds are not commonly in use, the timing and rate of release of stormwater to the receiving environment is uncontrolled, and this is representative of approximately 85% of the pre-existing urban areas throughout Ontario (right).

 

In urban areas which use stormwater ponds to control the peak flow of runoff entering receiving environs the net volume of runoff remains the same, but the rate of release is controlled (left). In older urban areas where stormwater ponds are not commonly in use, the timing and rate of release of stormwater to the receiving environment is uncontrolled, and this is representative of approximately 85% of the pre-existing urban areas throughout Ontario.

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Caption: The left image depicts a typical urban hydrologic condition wherein an end-of-pipe control (stormwater management pond) is used to control the peak discharge of urban runoff to a receiving water body.  The right image depicts a similar upland condition, but without any sort of end-of-pipe stormwater management facility.  

Caption: The left image depicts a typical urban hydrologic condition wherein an end-of-pipe control (stormwater management pond) is used to control the peak discharge of urban runoff to a receiving water body.  The right image depicts a similar upland condition, but without any sort of end-of-pipe stormwater management facility.  

The large volumes of stormwater runoff produced under such circumstances overstress conventional stormwater systems leading to flooding, erosion, habitat destruction, degraded water quality, damage to infrastructure systems and post-flooding health-related concerns including mould growth and contaminated water.  

The large volumes of stormwater runoff produced under such circumstances overstress conventional stormwater systems leading to flooding, erosion, habitat destruction, degraded water quality, damage to infrastructure systems and post-flooding health-related concerns including mould growth and contaminated water.  

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 (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).  

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 (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.

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 (IBC, 2016).

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).   

   

   

Caption: Radar tracking of the August 10, 2012 extreme rainfall event. The Lake Ontario nearshore experienced sustained intensities approaching 200 mm/hr, while the southern portion of Peel Region had no measurable precipitation. (Source: Risk Sciences International)

Caption: Radar tracking of the August 10, 2012 extreme rainfall event. The Lake Ontario nearshore experienced sustained intensities approaching 200 mm/hr, while the southern portion of Peel Region had no measurable precipitation. (Source: Risk Sciences International)

While urban flooding and extreme rainfall garner the most attention in discussions pertaining to stormwater management, it is crucial that consideration also be given to the management of our water cycle during dry periods as well. Collectively, we need to be able to manage extreme rainfall events such as the July 8, 2013 storm, combined rain and snow events such as that which caused the Bow River flood in Calgary in 2013, and extended periods of drought as occurred in southern Ontario in 2007. Drought preparedness is required if we are to sustain riverine baseflows, ensure the security of drinking water resources and optimize both water and waste water infrastructure. As municipalities grapple with these new climate realities and their associated costs, they are rethinking how to manage stormwater using a variety of innovative solutions. The figure below illustrates what has already happened in Ontario under conditions of prolonged drought. The Island Lake Reservoir, located near the Town of Orangeville, saw significant drawdown during the summer of 2007 after a period of prolonged drought. Reliant on groundwater for its municipal supply, continued pumping by the Town led to a significant drawdown within the reservoir. This was problematic not just for the ecosystem of the Lake, but for the downstream wastewater treatment plan as well, which relies on discharges from the reservoir in order to ensure that treated effluent can safely be assimilated by the receiving watercourse.

 

While urban flooding and extreme rainfall garner the most attention in discussions pertaining to stormwater management, it is crucial that consideration also be given to the management of our water cycle during dry periods as well. Collectively, we need to be able to manage extreme rainfall events such as the July 8, 2013 storm, combined rain and snow events such as that which caused the Bow River flood in Calgary in 2013, and extended periods of drought as occurred in southern Ontario in 2007. Drought preparedness is required if we are to sustain riverine baseflows, ensure the security of drinking water resources and optimize both water and waste water infrastructure.  

 

As municipalities grapple with these new climate realities and their associated costs, they are rethinking how to manage stormwater using a variety of innovative solutions. The figure below illustrates what has already happened in Ontario under conditions of prolonged drought. The Island Lake Reservoir, located near the Town of Orangeville, saw significant drawdown during the summer of 2007 after a period of prolonged drought.  

 

Reliant on groundwater for its municipal supply, continued pumping by the Town led to a significant drawdown within the reservoir. This was problematic not just for the ecosystem of the Lake, but for the downstream wastewater treatment plan as well, which relies on discharges from the reservoir in order to ensure that treated effluent can safely be assimilated by the receiving watercourse.

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Caption: Drought conditions at Island Lake in the summer of 2007

Caption: Drought conditions at Island Lake in the summer of 2007

Alleviating Pressures Using Low Impact Approaches to Development  

'''Alleviating Pressures Using Low Impact Approaches to Development'''

There are many reasons that make LID the smart choice when it comes to stormwater management. The creation of well-designed permeable landscapes provides an opportunity to capture, retain and infiltrate stormwater runoff close to its source.  Rather that treat stormwater as a waste product to be discarded, LID recognizes stormwater for what it is – a resource to be safeguarded and harnessed for the benefit of both the built and natural environment.

There are many reasons that make LID the smart choice when it comes to stormwater management. The creation of well-designed permeable landscapes provides an opportunity to capture, retain and infiltrate stormwater runoff close to its source.  Rather than treat stormwater as a waste product to be discarded, LID recognizes stormwater for what it is – a resource to be safeguarded and harnessed for the benefit of both the built and natural environment.

 

A central tenet underpinning low impact development approaches to stormwater management is the treatment train approach, which describes a hierarchical suite of practices which manage rainfall where it falls, followed by the attenuation, filtration and infiltration of stormwater along its path of travel and – eventually – using an end-of-pipe detention and polishing process. While many LID practices – including bioretention, soakaway pits and others – are not necessarily intended to remedy issues related to urban flooding per se, they are effective at easing the pressure on aging, overburdened stormwater infrastructure. That being said, there are new options in the LID toolkit which have the capacity to provide both peak flow and large event runoff volume control.

A central tenet underpinning low impact development approaches to stormwater management is the treatment train approach, which describes a hierarchical suite of practices which manage rainfall where it falls, followed by the attenuation, filtration and infiltration of stormwater along its path of travel and – eventually – using an end-of-pipe detention and polishing process. While many LID practices – including bioretention, soakaway pits and others – are not necessarily intended to remedy issues related to urban flooding per se, they are effective at easing the pressure on aging, overburdened stormwater infrastructure. That being said, there are new options in the LID toolkit which have the capacity to provide both peak flow and large event runoff volume control.