9.4 Stormwater Management

Stormwater management is crucial in urban and agricultural areas. It tackles runoff from rain and snow, which can cause flooding, erosion, and water pollution. Engineers use various techniques to control stormwater and minimize its negative impacts on the environment.

Low Impact Development and Green Infrastructure are key strategies in modern stormwater management. These approaches mimic natural processes, using tools like bioswales and green roofs to reduce runoff and filter pollutants. They aim to create more sustainable urban environments while protecting water resources.

Stormwater Runoff: Sources and Impacts

Urban and Agricultural Sources

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• Stormwater runoff generates from precipitation events and snowmelt flowing over land or impervious surfaces without percolating into the ground
• Common sources include urban areas, construction sites, agricultural lands, and industrial facilities
• Impervious surfaces (roads, parking lots, rooftops) significantly increase the volume and rate of stormwater runoff
  • Example: A typical city block generates 5 times more runoff than a woodland area of the same size
• Urban heat island effect exacerbates by stormwater runoff transferring heat from impervious surfaces to water bodies
  • Example: Runoff from hot asphalt can raise stream temperatures by several degrees, impacting aquatic ecosystems

Environmental Impacts

• Runoff carries pollutants (sediments, nutrients, heavy metals, bacteria) leading to water quality degradation in receiving water bodies
  • Example: Phosphorus from fertilizers in agricultural runoff can cause algal blooms in lakes and rivers
• Increased volume and velocity of runoff causes erosion, flooding, and habitat destruction in streams and rivers
  • Example: Urban streams often experience "flashy" flows, with rapid increases in water levels during storms
• Negatively impacts groundwater recharge altering the natural hydrologic cycle in watersheds
  • Example: In highly urbanized areas, groundwater recharge can be reduced by up to 35% compared to natural conditions

Sustainable Stormwater Management

Low Impact Development and Green Infrastructure

• Aims to mimic natural hydrologic processes by promoting infiltration, evapotranspiration, and water reuse
• Low Impact Development (LID) focuses on managing stormwater at its source through small-scale, distributed practices
  • Example: Bioswales along roadsides to capture and filter runoff
• Green Infrastructure (GI) utilizes natural systems and engineered solutions to manage stormwater while providing multiple environmental benefits
  • Example: Green roofs that reduce runoff and provide insulation for buildings

Best Management Practices and Design Principles

• Best Management Practices (BMPs) implement structural and non-structural measures to reduce stormwater runoff and improve water quality
  • Example: Constructed wetlands that filter pollutants and provide habitat
• Disconnecting impervious areas involves redirecting runoff from impervious surfaces to pervious areas for infiltration and treatment
  • Example: Downspout disconnection to route roof runoff to rain gardens
• Emphasizes integration of stormwater controls into site design and landscaping to create multifunctional spaces
  • Example: Incorporating infiltration basins into public parks for recreation and stormwater management
• Water conservation and reuse strategies (rainwater harvesting) integrate into sustainable stormwater management practices
  • Example: Collecting rainwater from roofs for landscape irrigation or non-potable indoor use

Stormwater Management System Design

Retention and Detention Systems

• Retention basins hold a specific volume of stormwater runoff allowing for infiltration, evaporation, and sediment settling
  • Example: A wet pond that maintains a permanent pool of water and provides additional storage during storm events
• Detention basins temporarily store stormwater runoff and release it at a controlled rate to reduce peak flows downstream
  • Example: A dry basin that empties completely between storm events, often used in combination with outlet structures

Infiltration and Permeable Surfaces

• Infiltration trenches create linear excavations filled with permeable materials allowing stormwater to infiltrate into the surrounding soil
  • Example: A gravel-filled trench along a parking lot edge to capture and infiltrate runoff
• Permeable pavements design with void spaces to allow water to pass through the surface and infiltrate into underlying layers or subsoil
  • Example: Permeable interlocking concrete pavers used in a plaza to reduce runoff and recharge groundwater

Design Considerations and Tools

• Design considerations include site topography, soil characteristics, rainfall intensity, and runoff volume calculations
  • Example: Conducting soil infiltration tests to determine the feasibility of infiltration-based BMPs
• Hydraulic and hydrologic modeling tools size and optimize stormwater management components based on local regulations and design storms
  • Example: Using EPA SWMM software to model the performance of a proposed stormwater management system
• Integration of pre-treatment measures (sediment forebays, vegetated filter strips) enhances the performance and longevity of stormwater management systems
  • Example: Installing a grass filter strip upstream of an infiltration basin to remove sediment and extend the basin's lifespan

Evaluating Stormwater Management Effectiveness

Performance Monitoring and Analysis

• Effectiveness evaluation involves monitoring and analyzing key performance indicators (runoff volume reduction, peak flow attenuation, pollutant removal efficiency)
  • Example: Measuring the reduction in peak flow rates at the outlet of a detention basin during various storm events
• Water quality monitoring techniques (grab samples, automated samplers) assess the impact of stormwater management practices on receiving waters
  • Example: Collecting water samples before and after a constructed wetland to measure pollutant removal efficiency
• Hydrograph analysis compares pre- and post-development runoff patterns and evaluates the effectiveness of flow control measures
  • Example: Analyzing stream gauge data to assess the impact of LID practices on reducing peak flows in an urban watershed

Long-term Assessment and Adaptive Management

• Long-term performance assessment considers factors (maintenance requirements, sediment accumulation, vegetation establishment)
  • Example: Monitoring the infiltration rate of a bioretention cell over several years to determine maintenance needs
• Cost-benefit analysis evaluates the economic feasibility and environmental benefits of different stormwater management strategies
  • Example: Comparing the lifecycle costs of traditional gray infrastructure to green infrastructure alternatives
• Adaptive management approaches allow for continuous improvement of stormwater management practices based on monitoring results and emerging technologies
  • Example: Adjusting the plant species in a rain garden based on observed performance and local climate conditions
• Life cycle assessment of stormwater management systems considers the environmental impacts and resource consumption throughout their design life
  • Example: Evaluating the carbon footprint of various stormwater BMPs, including construction, operation, and maintenance phases