Green Infrastructure Basics: Nature-Based Stormwater Solutions

Green infrastructure (GI) uses natural processes to manage stormwater, providing environmental, economic, and social benefits that traditional “gray” infrastructure cannot. This guide introduces the key concepts and practices.

What is Green Infrastructure?

Green infrastructure encompasses a range of practices that use vegetation, soils, and natural processes to manage water and create healthier urban environments.

Key Principles:

• Manage stormwater at the source
• Mimic natural hydrology
• Use infiltration, evapotranspiration, and reuse
• Provide multiple co-benefits

Why Green Infrastructure?

Environmental Benefits

• Reduces runoff volume - Infiltration and evapotranspiration
• Improves water quality - Filters pollutants naturally
• Reduces flooding - Slows and stores runoff
• Recharges groundwater - Replenishes aquifers
• Reduces urban heat - Vegetation cools surroundings
• Improves air quality - Plants filter particulates
• Supports biodiversity - Habitat for birds and pollinators

Economic Benefits

• Lower infrastructure costs - Can reduce pipe sizes
• Reduced flood damage - Less downstream flooding
• Property value increases - Attractive landscaping
• Energy savings - Reduced cooling needs
• Extended pavement life - Less stress on surfaces
• Reduced maintenance - When designed properly

Social Benefits

• Improved aesthetics - Green spaces vs. concrete
• Community spaces - Parks and gardens
• Mental health - Access to nature
• Physical health - Encourages outdoor activity
• Environmental education - Learning opportunities

Core Green Infrastructure Practices

1. Rain Gardens (Bioretention)

What it is: Shallow, vegetated depressions that collect and infiltrate runoff.

How it works:

• Runoff flows into depressed garden area
• Plants and soil filter pollutants
• Water infiltrates into ground or drains slowly
• Overflow handles large storms

Typical Components:

• Shallow ponding area (6-12 inches)
• Engineered soil media (18-36 inches)
• Plants adapted to wet/dry cycles
• Underdrain (optional, for poor soils)

Best Applications:

• Residential yards
• Parking lot islands
• Street corners
• Building perimeters

Sizing Rule of Thumb:

Where:

• A = Area
• Rv = Runoff coefficient
• D = Design rainfall depth
• f = Infiltration rate
• t = Drain time
• d = Ponding depth

Calculate bioretention sizing →

2. Bioswales

What it is: Vegetated channels that convey AND treat stormwater.

How it works:

• Runoff flows through linear channel
• Vegetation slows flow, filters sediment
• Check dams create ponding zones
• Water infiltrates or flows slowly downstream

Typical Components:

• Shallow channel (trapezoidal or parabolic)
• Dense vegetation (grass or sedges)
• Check dams (optional)
• Engineered soil (optional for enhanced infiltration)

Best Applications:

• Parking lot edges
• Street medians
• Along buildings
• Linear spaces

Design Criteria:

• Side slopes: 3:1 or flatter
• Bottom width: 2-8 feet
• Maximum velocity: 1-2 fps
• Maximum ponding: 12 inches

3. Permeable Pavement

What it is: Paving materials that allow water to pass through to the soil below.

Types:

• Pervious concrete
• Porous asphalt
• Permeable interlocking pavers
• Grid systems (plastic or concrete)

How it works:

• Rain falls on permeable surface
• Water drains through pavement
• Aggregate base stores water
• Water infiltrates into subsoil

Typical Cross-Section:

1. Permeable surface (2-4 inches)
2. Bedding aggregate (1-2 inches)
3. Open-graded base (12-24 inches)
4. Filter fabric (optional)
5. Subgrade

Best Applications:

• Parking lots (light traffic)
• Driveways
• Patios and walkways
• Overflow parking
• Fire lanes

4. Green Roofs

What it is: Vegetated roof systems that capture and retain rainfall.

Types:

How it works:

• Plants and media absorb rainfall
• Evapotranspiration returns water to atmosphere
• Excess drains to roof drain system
• Reduces runoff volume and rate

Benefits Beyond Stormwater:

• Reduces building energy use (15-30% cooling)
• Extends roof life (2-3x)
• Reduces urban heat island
• Provides habitat
• Aesthetic value

Calculate green roof performance →

5. Rainwater Harvesting

What it is: Capturing roof runoff for later use.

Components:

• Gutters and downspouts
• First-flush diverter
• Storage tank (cistern or rain barrel)
• Overflow connection
• Distribution system (pump or gravity)

Uses for Captured Water:

• Irrigation
• Toilet flushing
• Cooling tower makeup
• Laundry (gray water systems)

Sizing:

Where:

• V = Volume (gallons)
• A = Roof area (sq ft)
• P = Rainfall (inches)
• C = Collection efficiency (0.8-0.9)

6. Urban Trees

What it is: Strategic planting of trees for stormwater interception.

How it works:

• Canopy intercepts rainfall (reduces direct runoff)
• Roots promote infiltration
• Evapotranspiration removes water
• Reduces runoff volume significantly

Stormwater Benefits:

• Large tree can intercept 1,000+ gallons annually
• Root zone promotes 10-15x more infiltration
• Reduces peak flows and volumes

Co-Benefits:

• Shade and cooling
• Air quality improvement
• Carbon sequestration
• Property value increase

7. Infiltration Practices

Infiltration Trenches:

• Narrow, gravel-filled trenches
• Water infiltrates through bottom and sides
• Good for linear applications

Dry Wells:

• Point infiltration from roof drains
• Gravel-filled pit
• Single downspout or small area

Infiltration Basins:

• Larger-scale infiltration
• Surface or underground
• Requires permeable soils

Calculate dry well sizing →

Site Assessment for GI

Before selecting practices, assess site conditions:

Soil Investigation

Infiltration Rate:

• 2 in/hr: Excellent for infiltration

• 0.5-2 in/hr: Good, may need underdrain
• <0.5 in/hr: Underdrain required

Testing Methods:

• Double-ring infiltrometer
• Percolation test
• Soil boring and classification

Groundwater Depth

Minimum Separation:

• 2 feet from bottom of practice to groundwater (typical)
• Some areas require 4+ feet
• Check local requirements

Slope

Maximum Slopes:

• Bioretention: 15% (contributing area)
• Permeable pavement: 5%
• Green roof: Varies by system

Drainage Area

Contributing Area Ratios:

• Rain gardens: 5:1 to 15:1 (drainage:garden)
• Bioswales: 5:1 to 20:1
• Permeable pavement: 1:1 to 3:1

Contamination Concerns

Avoid infiltration where:

• Contaminated soils exist
• Hot spots drain to practice
• Groundwater is drinking source (without pretreatment)

Designing a GI System

Step 1: Establish Goals

What are you trying to achieve?

• Volume reduction
• Peak flow control
• Water quality treatment
• Regulatory compliance
• Multiple benefits

Step 2: Assess Site

• Soils and infiltration
• Slopes and drainage
• Existing features
• Constraints and opportunities

Step 3: Identify Opportunities

• Where does runoff originate?
• What spaces are available?
• Can impervious area be reduced?
• Where can GI fit naturally?

Step 4: Select Practices

Match practices to site conditions and goals:

Step 5: Size and Design

Use appropriate methods:

• Volume-based sizing (capture target depth)
• Hydrologic modeling (SWMM, etc.)
• Local criteria (many areas have specific methods)

Step 6: Plan for Maintenance

Design for long-term success:

• Access for maintenance
• Simple, robust designs
• Clear maintenance responsibilities
• Training for maintenance staff

Maintenance Requirements

Green infrastructure requires ongoing maintenance:

Rain Gardens / Bioretention

Permeable Pavement

Green Roofs

Common Challenges and Solutions

Challenge: Poor Infiltration

Solutions:

• Add underdrain system
• Use larger facility
• Consider lined, treatment-only design
• Change to less infiltration-dependent practice

Challenge: Clogging

Solutions:

• Provide pretreatment (forebay, filter strip)
• Regular maintenance
• Design for sediment access
• Proper construction practices

Challenge: Plant Failure

Solutions:

• Select locally adapted species
• Match plants to water regime
• Provide establishment irrigation
• Replace failed plants promptly

Challenge: Mosquitoes

Solutions:

• Drain within 72 hours
• Avoid standing water
• Maintain flow-through design
• Consider Bti treatment if needed

Challenge: Aesthetics/Acceptance

Solutions:

• Incorporate design features (borders, signs)
• Use attractive plants
• Maintain neat appearance
• Educate stakeholders

Cost Considerations

Initial Costs

GI can cost more or less than gray infrastructure, depending on:

• Site conditions
• Practice selection
• Level of treatment needed
• Regional costs

Typical Cost Ranges:

Lifecycle Costs

Consider full lifecycle:

• Installation
• Maintenance (annual)
• Replacement (less frequent)
• Avoided costs (pipe sizing, treatment)

GI often has lower lifecycle costs when co-benefits are valued.

Getting Started

For Homeowners

1. Start with downspout disconnection
2. Add a rain barrel
3. Plant a rain garden
4. Replace concrete with pervious options
5. Plant trees strategically

For Developers

1. Minimize impervious area
2. Preserve natural areas
3. Integrate GI into landscaping
4. Consider incentive programs
5. Work with experienced designers

For Municipalities

1. Update codes to allow/encourage GI
2. Develop design standards
3. Create incentive programs
4. Train staff in maintenance
5. Lead by example on public property

Summary

Green infrastructure offers a sustainable approach to stormwater management that:

• Manages water at the source rather than collecting and piping
• Provides multiple benefits beyond stormwater control
• Mimics natural hydrology rather than fighting it
• Improves community livability through green spaces

Success requires:

• Understanding site conditions
• Selecting appropriate practices
• Proper design and construction
• Ongoing maintenance commitment

Related Calculators

• Bioretention Calculator →
• Green Roof Calculator →
• Dry Well Calculator →
• Cistern Sizing Calculator →

References

1. Environmental Protection Agency. (2021). Green infrastructure for stormwater management. EPA.

2. Low Impact Development Center. (2020). LID urban design tools. LID Center.

3. Prince George’s County, MD. (1999). Low-impact development design strategies. Department of Environmental Resources.

4. American Society of Landscape Architects. (2019). Professional practice: Green infrastructure. ASLA.

5. Center for Neighborhood Technology. (2010). The value of green infrastructure. CNT.

6. University of New Hampshire Stormwater Center. (2019). Stormwater management fact sheets. UNH.