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Extreme rainfall events are increasing in frequency and intensity worldwide due to climate change. With intense storms comes a growing concern surrounding urban flooding and its destructive impacts.

Major cities across the globe are searching for solutions to curb flooding damage. Urban trees have emerged as a natural and effective approach to enhancing stormwater management. By providing interception, infiltration, and runoff reduction, trees are powerful tools to mitigate flooding. When combining green and blue infrastructure together such as trees, and stormwater management we can combine these efforts and create a system that is more impactful at reducing runoff volumes and minimizing flood risk.

Understanding Urban Flooding and Its Impacts

Urban flooding occurs when intense rainfall overwhelms drainage systems and flows rapidly across impervious surfaces into buildings, homes, and infrastructure. Impermeable roads, parking lots, and sidewalks combined with inadequate sewer systems lead rainwater to pond and flood key areas, rather than draining naturally.

The effects of urban flooding include destruction of property, closure of businesses and government facilities, disruptions to public transit systems, damage to infrastructure like roads and bridges, and increased risk of injury or waterborne diseases from sewer overflows.

As cities continue to densify and replace green space with concrete buildings and asphalt pavement, and as extreme weather events intensify due to climate change, urban flooding will only become more severe if proactive solutions are not prioritized.

Our team of urban greening consultants have over 30 years of experience planting and designing green blue stormwater management plans & can help you design spaces that will grow trees faster and increase the effectiveness of your stormwater management.

Book a free consultation with our team.

How do Urban Trees Prevent Flooding?

Their root networks, often concentrated within the upper layers of soil, help in reducing surface runoff by enhancing soil permeability and capture. These shallow roots aid in soil structure improvement, preventing soil compaction and promoting infiltration, allowing water to percolate more effectively into the ground rather than running off the surface. By absorbing and storing water during rainfall events, trees can significantly reduce the volume and velocity of stormwater runoff, thereby lessening the risk of flash floods and minimizing the burden on stormwater drainage systems.

Additionally, trees contribute to flood reduction by intercepting rainfall with their canopy. The foliage of urban trees acts as a natural barrier, breaking the impact of raindrops and slowing down the descent of water onto the ground.

This helps to reduce soil erosion and surface water runoff by allowing rainwater to be gradually released, giving it more time to infiltrate the soil or evaporate, consequently lowering the amount of water reaching streets, sidewalks, and drainage systems. In combination with their ability to enhance soil permeability and reduce runoff, urban trees serve as valuable components of green infrastructure, effectively mitigating flooding and contributing to the overall resilience of urban areas against storm events.

The trees will then use the water to nourish itself by evapotranspiration and release oxygen & water vapor back into the atmosphere through its leaves to cool the nearby environment.

The Role of Urban Trees in Stormwater Management

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Urban trees offer a nature-based solution to enhance cities’ resilience against flooding. Their canopies intercept rainfall that would otherwise end up as runoff. Their extensive root systems allow more water to infiltrate into subsurface soils rather than flowing across pavement surfaces. And the added roughness or drag created by trees slows down stormwater that does end up traveling as runoff, providing more time for infiltration and evaporation.

Studies show that just 100 mature tree canopies intercept over 100,000 gallons of rainfall per year depending on the climate. Over an acre of tree cover can reduce runoff by up to 62,000 gallons from a one-inch rainstorm event. When quantified, urban trees provide significant volume reduction benefits to curb stormwater runoff and the resulting floods.

How to Think about Urban Tree Flood Mitigation

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There are four ways urban trees reduce stormwater runoff volumes:

Interception: During rainstorms, tree leaves, branches, and trunks intercept precipitation and absorb or block it from reaching the ground. Interception rates vary based on tree size and structure, climate, and precipitation patterns. But large healthy trees and soil prevent thousands of gallons of water annually from ever forming runoff.

Infiltration: Tree roots and the organic matter from decomposing leaves create small channels and pathways for stormwater to infiltrate into the soil at much higher rates than compacted urban soils.

This facilitates groundwater recharge while preventing rainfall from becoming high velocity runoff. Soil quality and compaction is also an important factor as hard compacted surfaces such as those mainly found in urban areas push the water into the cities municipalities at a greater rate increasing the chances of flooding.

Slowing Runoff: Trees and their structural root networks add surface roughness on the landscape which slows down runoff as it flows towards drains and sewer systems. This slowing effect allows more time for the runoff to naturally infiltrate into soils and prevents high velocity flows that contribute to urban flooding.

Capturing Runoff: Integrating Soil Cells into urban trees is rising in popularity due to the flexibility of the system. You can use permeable surfaces to infiltrate rain water into the pit itself which will hydrate and oxygenise the soil and tree roots.

Once the tree roots and soil have cleaned the water and been satiated, the excess water can filter down into a lower holding chamber to reduce the flow during high peak weather events.

To learn more download our free Strataflow PDF

a soil cell being filled with soil

The excess water can be released at a later time into the municipality systems or with proper planning be used for on-site irrigation of surrounding landscapes.

Benefits Beyond Flood Mitigation

In addition to stormwater management, increasing urban trees provides municipalities a number of co-benefits:

Air Quality Improvement: Trees remove air pollutants like ozone, nitrogen oxides, and particulate matter through their natural respiration processes. This leads to improved air quality and public health outcomes.
Related: Best Trees for Reducing Air Pollution.

Energy Conservation: The shade provided by trees reduces summer cooling demands in buildings, leading to energy savings from reduced air conditioning needs.

Property Value Enhancement: Multiple studies confirm that homes and properties landscaped with trees have 3-15% higher property values compared to non-landscaped equivalents, providing a financial incentive.

Mental Health Benefits: Spending time around trees and green spaces improves mental health by reducing stress, anxiety, and depression through both mental relaxation and lowering unhealthy cortisol levels.

Aesthetic Enhancement: Thoughtfully designed green infrastructure and urban tree planting enhances the beauty, visual appeal and livability of neighborhoods by reducing the amount of grey services needed to service the community.

Planting and Caring for Urban Trees

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To properly leverage the stormwater management potential of urban trees, care must be taken to plant appropriate tree species and proactively maintain tree health through pruning, watering, and disease management.

Native tree species that can withstand urban conditions like air pollution, compacted soils, drought, and flood tolerance are ideal. Maples, Elms, Oaks, Magnolias and Birches are great options in many cities. Consult with your local government or nurseries to get a list of appropriate trees.

When siting trees, utilities, infrastructure, soil compaction and drainage, and sunlight exposure must be considered to allow sufficient soil volumes for the tree as it grows to maturity.

Consistent watering of young trees is critical, along with pruning once mature to encourage optimal canopy structure. Organic approaches should be used to prevent and manage pest damage.

Permeable Pavements and Soil Cells Enhance Infiltration

While trees alone provide measurable stormwater management, combining them with permeable paving systems and subsurface water-storing soil cells can capture, infiltrate and transpire even more rainfall.

As you can see from the above video, the trees planted in soil cells with the supported above ground infrastructure to direct and filter stormwater into the treepit grow to a better quality when compared to the same tree planted in the nearby grass verge.

Permeable pavements also allow surface runoff to pass through the paving material and filter down into aggregate storage layers underneath where water can infiltrate soil naturally.

Any excess runoff is conveyed into connected tree soil cells – structural stormwater chambers that feeds stored stormwater and runoff directly to trees at the ideal pace for root absorption.

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This enables trees to grow stronger, healthier, and larger – which further enhances their rainfall interception capacity. Where possible, cities should install connected permeable pavements and subsurface soil cells in parallel with urban tree planting efforts to maximize stormwater volume reduction benefits.

Case Study: Glenora School Stormwater Management

Final Takeaways

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With record rainfall and intense storms threatening communities globally, cities must take urgent action. Strategically planting and maintaining networks of urban trees represent a sustainable, cost effective nature-based solution to enhance stormwater management and curb urban flooding. Their rainfall interception, enhanced infiltration, and runoff reduction benefits are well quantified.

Combined with high performing green infrastructure like permeable pavements and subsurface soil cells that collect, store and infiltrate rainfall, trees can transform cities into more flood resilient, climate adaptive landscapes.

Through proactive planning and design, municipal leaders can leverage trees as vital green infrastructure protecting vulnerable communities against intensifying flood risks.