Urbanization

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Natural ground cover pre-developmentrefers to the characteristics and functions of a system prior to urban development. conditions
This image depicts a typical urban hydrologic condition wherein an end-of-pipe controlA structural best management practice that is located at the end of a flow conveyance route. End-of-Pipe Controls on surface and below ground but are not limited to wet ponds, constructed wetlands and other similar systems. (stormwater management pondA body of water smaller than a lake, often artificially formed.) is used to control the peak dischargeThe greatest volume of stream flow occurring during a storm event. of urban runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. to a receiving water body.
Urban hydrology with Low Impact DevelopmentLow impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Pre-developmentrefers to the characteristics and functions of a system prior to urban 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 interceptionThe interception, storage and eventual evaporation of rainfall from vegetation canopies. storage. The amount of rain lost to interceptionThe interception, storage and eventual evaporation of rainfall from vegetation canopies. storage depends on the kind of vegetation and its growth stage, but abstraction values of 1 – 4 mm are typical [1].

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 clay1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System). or clay1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).-loam soils usually have lower infiltration capacities.

If rain falls at rate which is greater than the underlying soils infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour., it will begin to fill depressions, at which point runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. will begin to be generated. The production of runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. is accelerated as surface slope increases and slope lengths decrease, as both considerations increase surface runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. velocities and decrease the time of concentrationThe time needed for a drop of water to reach the outlet of a catchment from the most remote location within the catchment. [2]. 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.

About 10 % of the annual rainfall amount in such areas is lost as surface runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface.. The rest of the water supports the growth of vegetation (40 %), feeds nearby watercourses (20 %) and recharges aquifersLayer of rock or soil that holds or transmits water. (20 %).

Post-development hydrologic changes

Water quantity changes

While rainfall intensityThe rate of rainfall in millimeters per hour., soil and vegetation characteristics, slope length and steepness all play a role in the timing and rate of runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. generation, the creation of imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. 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 concentrationThe time needed for a drop of water to reach the outlet of a catchment from the most remote location within the catchment. is significantly reduced due to the relative smoothness of imperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. surfaces, and the dense network of stormwater conveyance infrastructure including gutters, catch basinsGround depression acting as a flow control and water treatment structure, that is normally dry. and sewers.

In urban areas which use stormwater ponds to control the peak flow of runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. entering receiving environs the net volume of runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. remains the same, but the rate of release is controlled. In older urban areas where stormwater ponds are not commonly in use, the timing and rate of release of stormwater to the receiving environmentRefers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer. is uncontrolled, and this is representative of approximately 85% of the pre-existing urban areas throughout Ontario.

The large volumes of stormwater runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. produced under such circumstances over stress conventional stormwater systems leading to flooding, erosion(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc)., habitat destruction, degraded water quality, damage to infrastructure systems and post-flooding health-related concerns including mould growth and contaminated drinking water supplies.

Water quality impacts

Surface runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. generated in urban areas frequently carries with it a cocktail of pollutants. Although it is variable in nature, runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. pollutants are typically derived from a combination of fine sedimentsSoil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls. 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 salt from winter maintenance activities [3][4]. These pollutants accumulate on the road surface during the antecedent dry periodThe period of time between consecutive storms or rainfall events. between consecutive rainfall events, and are washed off at the onset of rainfall. The majority of particles are washed off with the first flushThe delivery of a disproportionately large load of pollutants during the early part of storms due to the rapid runoff of accumulated pollutants. The first flush of runoff has been defined several ways (e.g., 10 mm per impervious area).Initial pulse of stormwater runoff which picks up the pollutants that have settled on surfaces during the dry period. The first flush contains the highest pollutant concentrations. of stormwater runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface., typically considered to be accounted for with the first 25 mm of runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. [5].

Alleviating pressures using LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

There are many reasons that make LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. 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 runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. close to its source. Rather than treat stormwater as a waste product to be discarded, LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. recognizes stormwater for what it is – a resource to be safeguarded and harnessed for the benefit of both the built and natural environmentRefers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.. A central tenet underpinning low impact developmentA stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. approaches to stormwater management is the treatment trainStormwater management following the hierarchical approach: Source Control measures, Conveyance Control measure and End of Pipe treatment to achieve the water quality and water balance target for lot level development of the preferred strategy.A combination of lot level, conveyance, and end-of-pipe stormwater management practices. approach, which describes a hierarchical suite of practices which manage rainfall where it falls, followed by the attenuationReduction of peak flow and increase of the duration of the flow event., filtrationThe technique of removing pollutants from runoff as it infiltrates through the soil. and infiltration of stormwater along its path of travel and – eventually – using an end-of-pipe detentionThe temporary storage of stormwater to control discharge rates, and allow for sedimentation. and polishing process. While many LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. practices – including bioretention, soakawayA pit into which liquids may flow and then percolate slowly into the subsoil.An excavated area lined with geotextile filter cloth and filled with clean granular stone or other void forming material, that receives runoff and allow it to infiltrate into the native soil; can also be referred to as infiltration galleries, French drains, dry wells or soakaway pits. 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 LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. toolkit which have the capacity to provide both peak flow and large event runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. volume control.


  1. 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
  2. Sharma, K.D. 1986. Runoff behaviour of water harvesting microcatchments. Agricultural Water Management 11 (2): 137-144
  3. Aryal, R. Vigneswaran, S. Kandasamy, J.; Naidu, R. 2010. Urban Stormwater Quality and Treatment. Korean Journal of Chemical Engineering, 27(5):1343-1359
  4. Trenouth, W.R. Gharabaghi, B., Perera, N. 2015. Road salt application planning tool for winter de-icing operations. Journal of Hydrology. 524:401-410
  5. 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