6.2 Stormwater management and green infrastructure

Stormwater management is a critical challenge in urban areas due to impervious surfaces and pollutants. Unmanaged runoff can lead to flooding, erosion, and water quality issues. Green infrastructure offers innovative solutions to mitigate these problems.

Green infrastructure mimics natural processes to manage stormwater while providing multiple benefits. Bioretention systems, permeable pavement, green roofs, and constructed wetlands are some key strategies. These approaches improve water quality, reduce flood risk, and enhance urban ecosystems.

Stormwater runoff challenges

• Stormwater runoff is a major issue in urban areas due to the high concentration of impervious surfaces and human activities that generate pollutants
• Unmanaged stormwater runoff can lead to a range of environmental, social, and economic problems that negatively impact the sustainability and resilience of cities

Urban development impacts

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• Urbanization alters the natural hydrologic cycle by increasing the amount of impervious surfaces (rooftops, roads, parking lots) that prevent water from infiltrating into the ground
• Development activities (construction, land clearing) can expose soils and increase erosion, leading to higher sediment loads in stormwater runoff
• Urban land uses (industrial, commercial, residential) introduce a variety of pollutants (heavy metals, nutrients, pathogens) into stormwater through activities such as vehicle use, landscaping, and waste disposal

Impervious surface effects

• Impervious surfaces reduce the amount of rainfall that can infiltrate into the ground, leading to increased surface runoff volumes and peak flow rates
• Runoff from impervious surfaces tends to have higher velocities and erosive power, which can cause channel erosion and streambank instability
• Impervious surfaces also contribute to the urban heat island effect by absorbing and retaining heat, which can raise the temperature of stormwater runoff

Water quality degradation

• Stormwater runoff can transport a wide range of pollutants (sediment, nutrients, bacteria, oils, metals) from urban surfaces into receiving water bodies (streams, rivers, lakes, estuaries)
• Excess nutrients (nitrogen, phosphorus) in stormwater can lead to algal blooms and eutrophication, which deplete dissolved oxygen levels and harm aquatic life
• Bacterial contamination from sources such as pet waste and leaking septic systems can make water unsafe for human contact and recreation

Flooding and erosion

• Increased runoff volumes and peak flows from impervious surfaces can overwhelm the capacity of stormwater infrastructure (pipes, channels) and cause localized flooding
• Flooding can damage property, disrupt transportation and utility services, and pose risks to public safety
• High-velocity runoff can erode soils, undermine structures, and deposit sediment in downstream areas, affecting water quality and aquatic habitats

Green infrastructure solutions

• Green infrastructure refers to a network of natural and engineered systems that mimic or restore natural hydrologic processes to manage stormwater runoff
• Incorporating green infrastructure into urban landscapes can help mitigate the negative impacts of stormwater runoff while providing multiple co-benefits (habitat, recreation, aesthetics)

Bioretention systems

• Bioretention systems (rain gardens, bioswales) are vegetated depressions that collect and infiltrate stormwater runoff
• They typically consist of a ponding area, mulch layer, engineered soil media, and native plants that tolerate both wet and dry conditions
• As runoff flows through the system, pollutants are removed through physical (filtration), chemical (adsorption), and biological (plant uptake, microbial degradation) processes

Permeable pavement

• Permeable pavement (porous asphalt, pervious concrete, interlocking pavers) allows stormwater to pass through the surface and infiltrate into the underlying soil
• It can be used in parking lots, sidewalks, and low-traffic roads to reduce runoff volumes and improve water quality
• Permeable pavement systems often include a gravel storage layer and underdrain to manage excess water during heavy rainfall events

Green roofs

• Green roofs are vegetated systems installed on building rooftops to intercept and retain rainfall
• They typically consist of a waterproofing membrane, drainage layer, growing media, and drought-tolerant plants (sedums)
• Green roofs can reduce the volume and peak flow of runoff, improve building insulation and energy efficiency, and provide habitat for pollinators

Rainwater harvesting

• Rainwater harvesting involves collecting and storing rainfall from rooftops or other surfaces for later use
• Rainwater can be stored in cisterns, barrels, or underground tanks and used for non-potable purposes (irrigation, toilet flushing, cooling systems)
• Harvesting rainwater can reduce the demand on potable water supplies, lower water utility costs, and decrease the volume of stormwater runoff

Constructed wetlands

• Constructed wetlands are engineered systems that mimic the functions of natural wetlands to treat stormwater runoff
• They typically consist of a series of shallow pools or cells with emergent vegetation (cattails, rushes) that slow down flow and promote sedimentation and filtration
• Wetland plants and microorganisms can uptake nutrients, break down pollutants, and provide habitat for wildlife

Stormwater management benefits

• Implementing green infrastructure and other stormwater management practices can provide a range of environmental, social, and economic benefits
• These benefits can help cities become more sustainable, resilient, and livable by improving water quality, reducing flood risks, and enhancing ecosystem services

Improved water quality

• Green infrastructure practices can remove pollutants from stormwater runoff through physical, chemical, and biological processes
• Vegetated systems (bioretention, green roofs, wetlands) can filter out sediment, nutrients, and other contaminants before they reach receiving waters
• Infiltration practices (permeable pavement, infiltration basins) can reduce the volume of runoff and allow for natural treatment in the soil

Reduced flood risk

• By retaining and slowly releasing stormwater, green infrastructure can help reduce peak flows and alleviate pressure on downstream drainage systems
• Distributed practices (rain gardens, green roofs) can capture runoff close to the source and prevent localized flooding
• Preserving and restoring natural floodplains and wetlands can provide additional storage capacity and attenuate flood waves

Groundwater recharge

• Infiltration-based practices (permeable pavement, bioretention) can allow stormwater to percolate into the soil and replenish groundwater aquifers
• Recharging groundwater can help maintain baseflow in streams, support wetland ecosystems, and provide a more stable water supply during dry periods
• In coastal areas, groundwater recharge can help prevent saltwater intrusion and maintain the integrity of freshwater aquifers

Ecosystem services

• Green infrastructure can provide a variety of ecosystem services beyond stormwater management
• Vegetated systems can sequester carbon, mitigate urban heat island effects, and improve air quality by filtering pollutants
• Green spaces can provide habitat for wildlife, increase biodiversity, and enhance recreational opportunities for urban residents

Planning and design considerations

• Effective stormwater management requires careful planning and design to ensure that green infrastructure practices are appropriately sized, sited, and integrated into the urban landscape
• Several factors must be considered when planning and designing green infrastructure systems to optimize their performance and minimize maintenance requirements

Site assessment factors

• Conducting a thorough site assessment is essential to identify opportunities and constraints for green infrastructure implementation
• Factors to consider include: soil type and infiltration capacity, slope and drainage patterns, existing vegetation and land cover, utility locations, and adjacent land uses
• Geotechnical investigations (soil borings, percolation tests) may be necessary to determine the feasibility of infiltration practices

Hydrologic modeling

• Hydrologic modeling tools (TR-55, SWMM, HEC-HMS) can be used to simulate the effects of green infrastructure on stormwater runoff quantity and quality
• Models can help determine the optimal size, location, and configuration of practices based on site-specific conditions and performance goals
• Continuous simulation models can provide a more comprehensive assessment of long-term performance under a range of rainfall scenarios

Vegetation selection

• Selecting appropriate vegetation is critical for the success and long-term performance of green infrastructure practices
• Plants should be native or adapted to local climate conditions, tolerant of both wet and dry periods, and resistant to pests and diseases
• Diverse plant communities (trees, shrubs, herbaceous plants) can provide multiple benefits, such as erosion control, pollutant uptake, and habitat value

Maintenance requirements

• Regular maintenance is essential to ensure the continued function and effectiveness of green infrastructure practices
• Maintenance activities may include: sediment removal, vegetation management, mulching, erosion repair, and drain cleaning
• Developing a clear maintenance plan and budget is important to allocate responsibilities and resources over the life cycle of the project

Policy and regulations

• Implementing green infrastructure at a citywide scale requires supportive policies, regulations, and incentives to encourage adoption and ensure consistent standards
• Several policy and regulatory tools can be used to promote stormwater management and green infrastructure in urban development and redevelopment projects

Stormwater management ordinances

• Local governments can adopt stormwater management ordinances that require new development and redevelopment projects to implement green infrastructure practices
• Ordinances typically specify performance standards (runoff reduction, water quality treatment) and design criteria for various practices
• Some ordinances may also require the use of green infrastructure for certain land uses or in specific watershed areas

Low impact development incentives

• Incentives can be used to encourage developers and property owners to voluntarily implement low impact development (LID) practices that minimize stormwater impacts
• Examples of incentives include: density bonuses, expedited permitting, reduced stormwater fees, and tax credits
• Incentives can help offset the perceived higher costs and risks associated with green infrastructure and promote innovation in stormwater management

Stormwater utility fees

• Stormwater utility fees are charges levied on property owners to fund the construction, operation, and maintenance of stormwater infrastructure
• Fees are typically based on the amount of impervious surface area on a property, which is used as a proxy for stormwater runoff contribution
• Some utilities offer fee discounts or credits for properties that implement green infrastructure practices, providing a financial incentive for stormwater management

NPDES permit compliance

• The National Pollutant Discharge Elimination System (NPDES) permit program regulates stormwater discharges from municipal separate storm sewer systems (MS4s) and construction activities
• MS4 permits often require the development of stormwater management programs that include green infrastructure and LID practices
• Incorporating green infrastructure into NPDES permit compliance can help cities meet water quality standards and avoid costly violations

Implementation strategies

• Successful implementation of green infrastructure requires a collaborative and multifaceted approach that engages various stakeholders and leverages multiple funding sources
• Several strategies can be employed to facilitate the widespread adoption of green infrastructure practices in urban areas

Public-private partnerships

• Public-private partnerships (P3s) can be used to finance, design, construct, and maintain green infrastructure projects
• P3s can leverage private sector expertise and resources while sharing risks and rewards between public and private entities
• Examples of P3s in green infrastructure include: stormwater credit trading, performance-based contracts, and green infrastructure banks

Retrofit opportunities

• Retrofitting existing urban areas with green infrastructure can provide significant opportunities for stormwater management and other co-benefits
• Potential retrofit locations include: streets and rights-of-way, parking lots, building rooftops, and vacant lands
• Retrofits can be implemented through capital improvement projects, redevelopment activities, or voluntary programs

Funding mechanisms

• Securing adequate and sustainable funding is critical for the long-term implementation and maintenance of green infrastructure
• Potential funding sources include: stormwater utility fees, general funds, grants, loans, and public-private partnerships
• Developing a diverse funding portfolio can help ensure the resilience and adaptability of green infrastructure programs over time

Community engagement

• Engaging the community in the planning, design, and stewardship of green infrastructure can help build support, ownership, and long-term sustainability
• Community engagement activities may include: public meetings, workshops, volunteer events, and educational programs
• Collaborating with community-based organizations, schools, and neighborhood groups can help align green infrastructure projects with local priorities and values

Monitoring and evaluation

• Monitoring and evaluating the performance of green infrastructure practices is essential to assess their effectiveness, identify areas for improvement, and communicate benefits to stakeholders
• A comprehensive monitoring and evaluation program should include both quantitative and qualitative metrics, as well as a framework for adaptive management and continuous improvement

Performance metrics

• Establishing clear and measurable performance metrics is important to track the progress and success of green infrastructure projects
• Metrics may include: runoff volume reduction, peak flow attenuation, pollutant load removal, infiltration rates, and plant survival rates
• Other metrics related to co-benefits (e.g., urban heat island mitigation, biodiversity, community engagement) can also be considered

Adaptive management approach

• Adaptive management is a systematic approach to decision-making that incorporates monitoring, evaluation, and adjustment of management practices over time
• It involves setting objectives, implementing actions, monitoring outcomes, and adapting strategies based on new information and changing conditions
• Adaptive management can help green infrastructure programs remain flexible and responsive to uncertainties and challenges

Long-term effectiveness studies

• Long-term studies are needed to evaluate the performance and durability of green infrastructure practices over extended periods (e.g., 10-20 years)
• These studies can provide valuable insights into the maintenance requirements, life cycle costs, and resilience of practices under various environmental conditions
• Collaborating with academic institutions and research organizations can help advance the knowledge base and inform best practices for green infrastructure

Continuous improvement process

• Establishing a continuous improvement process can help green infrastructure programs evolve and optimize their performance over time
• This process involves regular review and analysis of monitoring data, identification of opportunities for enhancement, and implementation of corrective actions
• Sharing lessons learned and best practices among practitioners and stakeholders can help accelerate the adoption and scaling of green infrastructure solutions