Urban runoff is best addressed with a coordinated set of design and policy choices rather than a single fix. Combining infiltration-focused infrastructure with transportation and development planning reduces flooding, pollution, and stress on urban waterways.

Core idea: combine strategies to maximise infiltration and minimise runoff

Urban runoff problems intensify when impervious surfaces dominate and stormwater is rapidly routed into drains and streams.

Photograph of an urban stormwater outfall where accumulated trash and debris illustrate how runoff can rapidly convey pollutants into waterways. This image supports the link between impervious surfaces, fast drainage routing, and downstream water-quality stress that green infrastructure aims to prevent. Source

“Putting strategies together” means selecting complementary actions that work across sites (lots, streets, neighbourhoods) and scales (parcel to watershed).

Key mechanism: infiltration and slowed flow

Stormwater controls are most effective when they:

• Increase infiltration into soils where water can be stored and filtered

• Reduce runoff volume by capturing rainfall close to where it lands

• Slow runoff velocity to lower peak discharge and erosion risk

• Improve water quality by filtering sediments, nutrients, metals, and hydrocarbons

Strategy bundle 1: redesign hard surfaces to let water in

Permeability upgrades target the largest driver of runoff: paved cover.

Permeable pavement in the “treatment train”

Use permeable pavement where traffic loads and soils allow, especially when paired with:

Diagram of permeable pavement layers (surface course over a stone reservoir) showing stormwater infiltrating downward rather than running off. The labeled sub-base illustrates temporary storage and the potential role of an optional underdrain, linking pavement design to reduced runoff volume and slower discharge. Source

• Sub-base storage (gravel layers) that temporarily holds stormwater

• Overflow routing to nearby planted areas for large storms

• Source control (street sweeping, litter control) to prevent clogging and preserve infiltration

Where it fits best

Integration is strongest when permeable surfaces are prioritised in:

• Parking lanes, low-speed streets, sidewalks, driveways, and overflow parking

• Redevelopment projects (replacing surfaces at end-of-life reduces marginal cost)

• Locations upstream of flood-prone intersections or undersized storm drains

Strategy bundle 2: add vegetation to intercept, store, and infiltrate

Vegetation-based actions are most effective when distributed widely and connected to runoff pathways.

Trees as stormwater infrastructure

Planting trees supports infiltration by:

• Intercepting rainfall on leaves and branches (reducing immediate runoff)

• Increasing soil organic matter and pore space around roots

• Enhancing evapotranspiration, lowering water volume reaching drains

Trees work best when combined with:

• Soil restoration (decompaction, amended soils) to prevent “green on top, sealed below”

• Curb cuts or inlets that direct street runoff into vegetated zones

Schematic of a stormwater bumpout (a vegetated curb extension) showing street runoff being directed into planted soil media for filtration and infiltration. Labels emphasize pollutant filtering by vegetation/soil and added subsurface stone storage, illustrating how green infrastructure slows flow and improves water quality. Source

Integrated placement

To “put strategies together,” place vegetation where it can receive and manage runoff:

• Along streets (continuous canopy plus curbside infiltration zones)

• In medians and traffic-calming islands designed to accept stormwater

• Near outfalls to provide final filtration and energy dissipation

Strategy bundle 3: reduce impervious area through transport and development choices

Infrastructure retrofits are amplified when cities build and travel differently, reducing how much paved area is needed in the first place.

Transportation choices

Increasing public transportation and safe walking/cycling networks can:

• Reduce demand for wide roads and large parking lots

• Support redevelopment of parking areas into permeable or planted spaces

• Lower the frequency of expansion into new impervious surfaces

Compact development (“build up, not out”)

Compact development reduces runoff risk by:

• Limiting the spread of low-density pavement networks

• Preserving open space that can infiltrate stormwater

• Making stormwater systems easier to manage at higher efficiency per unit land area

Putting it together: coordinated planning and co-benefits

Effective runoff reduction comes from aligning engineering, land-use planning, and maintenance.

Integrated design principles

A combined plan typically:

• Treats stormwater at multiple points (roofs → lots → streets → neighbourhood outfalls)

• Prioritises infiltration-first features, with safe overflow for extreme storms

• Targets “hot spots” (frequently flooded blocks, polluted runoff corridors) for bundled upgrades

• Uses zoning, redevelopment standards, and capital projects to steadily shrink effective imperviousness

Benefits when strategies reinforce each other

When permeable surfaces, trees, transit, and compact development are implemented together, communities can see:

• Lower flood frequency and reduced strain on storm drains

• Improved stream health from reduced erosion and pollutant loads

• Increased groundwater recharge where soils and geology permit

• Added urban benefits (shade, cooler streets, improved streetscapes) that support continued adoption and compliance