Changes

==Pre-development hydrology==

===Pre-development hydrology===

[[File:Natural Ground Cover.png|thumb|Natural ground cover pre-development conditions]]

[[File:Natural Ground Cover.png|thumb|Natural ground cover pre-development conditions]]

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.

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%).

===Post-development hydrologic changes===

==Post-development hydrologic changes==

====Water quantity changes====

===Water quantity changes===

[[File:Urban_Hydrology_1.png|thumb|This 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.]]

[[File:Urban_Hydrology_1.png|thumb|This 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.]]

[[File:Urban_Hydrology_2.png|thumb|The right image depicts a similar upland condition, but without any sort of end-of-pipe stormwater management facility.]]

[[File:Urban_Hydrology_2.png|thumb|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 [[metal]]s (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>.

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 [[metal]]s (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===

[[File:Lake_Ontario_2012.png|thumb|Six notable extreme rainfall events have occurred within the past thirteen years in the GTHA, resulting in damages due to flooding. This figure shows a notable extreme rainfall “near-miss” event, labelled “Lake Ontario 2012”.]]

[[File:Lake_Ontario_2012.png|thumb|Six notable extreme rainfall events have occurred within the past thirteen years in the GTHA, resulting in damages due to flooding. This figure shows a notable extreme rainfall “near-miss” event, labelled “Lake Ontario 2012”.]]

[[File:Radar_tracking_August_2012.png|thumb|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)]]

[[File:Radar_tracking_August_2012.png|thumb|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)]]

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.

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.

===Alleviating pressures using LID===

==Alleviating pressures using LID==

[[File:Urban_Hydrology_3.png|thumb|Urban hydrology with Low Impact Development]]

[[File:Urban_Hydrology_3.png|thumb|Urban hydrology with Low Impact Development]]

[[File:Pearson_Graph.png|thumb|Typically designed to handle the smaller, most frequent storm events, LID practices in Southern Ontario are usually sized according to the 90th percentile event]]

[[File:Pearson_Graph.png|thumb|Typically designed to handle the smaller, most frequent storm events, LID practices in Southern Ontario are usually sized according to the 90th percentile event]]

Typically designed to handle the smaller, most frequent storm events, LID practices in Southern Ontario are usually sized according to the [[Runoff_volume_control_target| 90th percentile event]]. In many areas this translates into events that are <30 mm in size. Note that 25 mm is considered to be a suitable representation of the ‘first flush’ volume, and that controlling this amount of runoff provides stormwater engineers with control over 90% of the mean annual pollutant load <ref>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</ref>.

Typically designed to handle the smaller, most frequent storm events, LID practices in Southern Ontario are usually sized according to the [[Runoff_volume_control_target| 90th percentile event]]. In many areas this translates into events that are <30 mm in size. Note that 25 mm is considered to be a suitable representation of the ‘first flush’ volume, and that controlling this amount of runoff provides stormwater engineers with control over 90% of the mean annual pollutant load <ref>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</ref>.

[[category:Planning]]

[[category:Planning]]