Transportation networks and land-development patterns strongly shape stormwater runoff.

This cross-sectional illustration shows stormwater directed into a planted basin where it is treated and then infiltrates downward toward the water table. It helps connect land-use choices to groundwater recharge by depicting infiltration as a physical process that can be restored with green infrastructure. Source

Choices that reduce paved area and concentrate growth can lower flooding risk, improve water quality, and support more natural infiltration into soils.

Core idea: runoff is a land-use outcome

When communities prioritise wide roads, abundant parking, and spread-out development, they create more hard surfaces that shed water quickly into drains and streams.

More impervious cover generally means:

This comparative diagram shows how the same rainfall is partitioned into runoff versus infiltration across land-use types with increasing impervious cover (from woods/meadow to residential and urban business districts). It reinforces that higher imperviousness shifts water away from soil storage and groundwater recharge and toward rapid surface runoff. Source

• Faster, higher peak flows during storms (greater flood risk)

This hydrograph schematic plots discharge versus time and highlights how a “flashy” runoff response can create a high peak flow that may exceed sewer carrying capacity. It also illustrates the idea of attenuating peak flow by spreading discharge over a longer period (lower peak, longer duration), linking land-cover-driven runoff dynamics to flood risk. Source

• Less infiltration (reduced soil moisture and baseflow to streams)

• Greater transport of pollutants from streets (oil, metals, sediment) into waterways

Transportation choices: reduce paved area per person

The syllabus emphasises that increased use of public transportation can reduce impervious surfaces. The key mechanism is lowering the need for extensive road and parking infrastructure.

Public transportation and shared mobility

• Buses, light rail, and subways move more people per unit pavement than single-occupancy vehicles.

• Higher ridership can justify fewer lane expansions and smaller parking footprints at destinations.

• Park-and-ride can still add pavement; design choices matter (e.g., structured parking vs. large surface lots).

Active transport and “right-sized” streets

Policies that support walking and cycling can reduce short car trips and parking demand, which can translate into:

• Fewer or narrower travel lanes over time (reduced total impervious area)

• Less curbside and off-street parking

• More opportunity to keep land vegetated rather than paved

Parking management (often overlooked)

Because parking lots are major impervious features, communities can reduce runoff pressures by changing how parking is supplied:

• Removing or reducing minimum parking requirements

• Using shared parking between businesses with different peak hours

• Prioritising pricing and time limits over building new surface lots

Development choices: build up, not out

The syllabus also emphasises building up, not out—compact development that accommodates growth with less land conversion and less pavement.

Compact, infill, and mixed-use development

• Infill development places new housing or businesses within existing urban areas, using existing streets and utilities rather than extending them outward.

• Mixed-use neighbourhoods shorten trip distances (home–work–shopping), supporting transit and walking while reducing the need for new roads and parking.

Limiting sprawl to limit impervious expansion

Low-density sprawl typically requires:

• More lane-miles of roadway per household

• More driveways and wider local streets

• More dispersed parking lots These features increase impervious cover and can reduce infiltration across an entire watershed.

Aligning growth with transit (“transit-supportive” patterns)

Concentrating housing and jobs near frequent transit can:

• Increase transit ridership (reducing per-capita pavement needs)

• Encourage smaller parking supplies

• Support shorter, more walkable street networks

Planning tools that connect transportation and runoff outcomes

Common approaches for implementing the syllabus idea include:

• Zoning updates that allow greater height/density in suitable areas

• Urban growth boundaries or service-area limits to discourage outward expansion of roads and utilities

• Transportation funding that prioritises maintaining existing roads over building new ones

• Stormwater requirements that account for impervious area created by new developments (linking land-use decisions to runoff impacts)