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Stormwater Blog #1
Permeability, Stormwater, and the Urban Environment


Stormwater is made up of rainwater and snowmelt that lands on rooftops, roadways, parking lots, and other surfaces and then runs off into storm drains and waterways.


If stormwater can be absorbed into the ground it’s no problem; but in urban areas with limited exposed ground and vast expanses of impermeable surfaces, stormwater can be problematic.


Permeability and the Water Cycle


Permeability refers to the ability of a material to allow liquid to move through it. In the context of stormwater, permeability refers to the capacity of surfaces to absorb rain and snowmelt. In an undisturbed state, many natural surfaces such as soils, sand, gravel, and other organic materials allow rainfall or surface water runoff 

to percolate down into the soil. When precipitation is absorbed into the ground, the water is filtered before eventually reaching waterways or recharging groundwater sources.


When humans alter the natural environment to create spaces for living, travel, and recreation, the resulting developments often interrupt the natural water cycle through the installation of impermeable surfaces like asphalt and concrete. Imagine a raindrop landing on a newly-sealed paved driveway. What happens to that raindrop? Since the paved driveway does not allow water to pass through – it is an impermeable surface – the raindrop will sit on top of the pavement until enough rain falls that the water begins to flow.

Why does permeability matter?


When the ground’s ability to absorb water is compromised by development, stormwater can pose several issues in the urban environment. When stormwater lands on an impermeable surface, it begins to flow instead of soaking in. Stormwater flow can pick up speed quickly, causing flash-flooding and erosion of urban infrastructure and nearby stream banks and shorelines.


Stormwater has become one of the top sources of water pollution on the planet. When stormwater flows over land, it can pick up contaminants like motor oil, windshield fluid, detergents, road salt, fertilizers, bacteria, garbage, and pet waste before flowing into nearby aquatic ecosystems and water sources. This contaminated runoff can compromise water quality and negatively impact wildlife.

Additionally, the increased volume of stormwater surface flow associated with urban development can put strain on wastewater infrastructure. Since precipitation can no longer sink into the ground, high volumes of stormwater enter wastewater pipes and wastewater treatment plants, leading to increased costs for wastewater treatment. When the wastewater collection system reaches capacity, excess wastewater overflows directly into local waterways without being treated. Local flooding and sewer back-ups can also occur.


The good news…


Increasing the permeability of urban environments can have a big impact on stormwater management! 


Creating permeable surfaces in urban environments helps manage stormwater by 1) slowing it down, 2) soaking it in, and 3) filtering out debris and pollutants.


Some tips to help manage stormwater by increasing permeability in your own yard include:


  • Installing a rain garden, or bioswale or infiltration trench in areas where water flows during storm events.

  • Laying down permeable pavers instead of single-layer asphalt for walkways and driveways.

  • Plant native trees and plants to help slow down and direct stormwater into the soil.

  • Redirect downspouts and incorporate rain barrels into your home management system to conserve water -- no need to turn on the hose to water your garden!

  • Expand the green space on your property and reduce hard surfaces where possible.

  • Consider replacing large expanses of lawn with deep-rooting perennial plants which absorb more water than a conventional grass lawn.

Stormwater Blog #2

Green Infrastructure: Working with Nature to Manage Stormwater

Mitigating stormwater in urban environments usually involves directing flows of water, creating catchment areas where water can pool, and increasing permeability in areas where water can soak into the ground. While approaches to stormwater management have typically focused on the engineering of structures and waterways to control flows, in recent years much attention and action has focused on the development of green infrastructure systems which help slow down and soak in stormwater. The United Nations 2018 World Water Day campaign asks, “How can we reduce floods, droughts and water pollution?” Their answer is “By using the solutions we already find in nature” ( This is precisely the aim of green infrastructure installments which create opportunities for stormwater to soak into the ground as it would in a natural, undeveloped environment.

What is green infrastructure? 


The preservation and development of green infrastructure has been described as a preferred and emerging standard for stormwater mitigation in urban environments. On a watershed or regional scale, green infrastructure refers to the naturally occurring remnants of forest and vegetation that surround and grow throughout urban environments and provide valuable ecosystem services. On a community or neighbourhood scale, green infrastructure describes development practices which reduce stormwater runoff, such as permeable parking lots and urban forests. At the site scale – a residential, business, or industrial property – green infrastructure refers to low impact developments (LIDs) which mimic natural conditions and processes using vegetation, soils, and engineered landscapes to slow down stormwater, allow infiltration, retention, and evaporation of water on site. Some examples of green infrastructure include rain gardens, bioswales, infiltration strips, green rooftops, planters, and retention ponds which incorporate vegetation into their design.


The interception of stormwater by vegetation serves many functions in an urban environment. Green infrastructure helps protect waterways and waterbodies near urban environments by acting as a filter. Stormwater that is directed from impermeable surfaces like rooftops and paved areas into varying combinations of vegetation, soil, sand, gravel, and organic matter is cleansed of many pollutants and debris that was picked up in the urban environment. This directly reduces the amount of polluted run-off that makes its way into nearby waterways and waterbodies,


Green infrastructure also reduces strain on wastewater pipes and water treatment plants by slowing down surface runoff and by reducing the volume and polluted content of runoff that does enter the wastewater system. Thus, green infrastructure directly protects water quality and wildlife, and can reduce strain on grey infrastructure.





Green infrastructure makes good ‘cents’


When compared with grey infrastructure, green infrastructure is less expensive to install and maintain, and can be just as effective at managing stormwater as grey infrastructure, with the added bonus of inherent ecological benefits. The installation of green infrastructure elements has been shown to increase the environmental benefits associated with stormwater management and reduce costs over the lifetime of the installation. Unlike grey infrastructure which requires regular maintenance to remain operative and structurally sound, green infrastructure tends to increase in effectiveness over time as plants become established and as root networks expand. For example, the absorptive capacity of a 30 year old oak tree is greater than a young sapling, and a mature rain garden can filter and absorb more water than a newly planted garden. Additionally, when systematically installed green infrastructure can reduce the volume of water which flows into storm drains and water treatment plants, thus reducing water treatment costs as a whole.


The proof is in the… reduced stormwater runoff!


Green infrastructure has taken root in urban centres around the world that have recognized its potential to manage stormwater. Many cities are leveraging annual precipitation as a resource to sustain vibrant pockets of green habitat that naturally reduce the volume of stormwater runoff.

The city of Vancouver, for example, aims to intercept and treat 90% of the area’s annual precipitation through their green infrastructure plan, thus greatly reducing strain on wastewater systems and saving tax payers money (


New York City implemented a hybrid green and grey infrastructure plan in 2010 in an effort to reduce combined sewer overflows and protect local waterbodies. Since implementation, the project has led to the installation of thousands of bioswales and a reduction of over 200 million gallons per year of stormwater flow into wastewater pipes! (




In many areas of China, ‘sponge cities’ which hinge on the deployment of green infrastructure are being developed to combat local flooding and projected sea level rise ( In areas of Shanghai, the installation of permeable pavement, rain and rooftop gardens, and wetland areas have kept things on track to realize the goal of soaking in 70% of stormwater runoff by the year 2020.













These are just a few examples which demonstrate the effectiveness and value of green infrastructure in action!


That’s not all…


In addition to providing effective stormwater mitigation services, green infrastructure provides an array of valuable ecosystem services. The development of natural systems which retain and conserve fresh water play an important role in managing the global water crisis. Plants, and in particular trees, have a tremendous capacity to influence local hydrological cycles. The interception, direction, filtering, absorption, and transpiration of water through plants helps retain water locally, and promotes improved water quality as many pollutants have been filtered out by the time stormwater finally reaches local waterbodies and underground aquifers.


Global biodiversity losses are expected to be exacerbated by climate change. Green infrastructure provides habitat for many species that have been displaced by urban development, and the installation of green infrastructure directly promotes biodiversity. As well, increasing the number of native trees, shrubs, and plants in an urban setting can increase habitat connectivity for organisms by creating links with nearby natural areas.


Not only does green infrastructure help regulate local water cycles and promote biodiversity, it also helps improve air quality, sequesters and stores carbon, and reduces urban heat impacts. As well, green infrastructure promotes human health by creating a sense of well-being and place through the beautification of concrete landscapes.  


Nature-based solutions in your own backyard


The benefits of working with nature to mitigate stormwater are many and expand beyond the simple redirection and absorption of rainfall and snowmelt. While the deployment of green infrastructure on a large, regional scale is desirable, residents can make a difference by creating absorbent pockets of vegetation on their own properties. Investing in green infrastructure is a direct investment in climate change mitigation, and the strategic installation of plant matter and landscape design on residential lots can help protect homes and out-buildings from flooding and decrease strain on wastewater systems.

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Rain Garden
Infiltration Strip 
Sponge City
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