What Are the Control Measures for Stormwater Runoff

What is stormwater runoff and why does it need to be controlled?

Stormwater runoff occurs when precipitation from rain or snowmelt flows over land surfaces instead of seeping into the ground. This natural process becomes problematic in urban and developed areas where impervious surfaces like roads, parking lots, and buildings prevent water from infiltrating the soil. As a result, large volumes of water accumulate rapidly and flow into storm drains, sewer systems, and water bodies.

The need to control stormwater runoff stems from its significant environmental and infrastructural impacts:

Water Pollution: Stormwater collects pollutants as it flows over surfaces. These contaminants include oil, grease, pesticides, fertilizers, sediment, and bacteria. When this polluted water enters rivers, lakes, and coastal waters, it degrades water quality and harms aquatic ecosystems.

Flooding: Excessive runoff overwhelms drainage systems and natural waterways, leading to flooding in urban areas. This can cause property damage, disrupt transportation, and pose risks to public safety.

Erosion and Sedimentation: High-velocity stormwater flows erode stream banks and transport sediment. This process alters stream morphology, damages aquatic habitats, and can lead to increased flooding downstream.

Groundwater Depletion: When rainwater quickly runs off instead of infiltrating the soil, it reduces groundwater recharge. This can lead to lowered water tables and decreased availability of water for wells and natural springs.

Infrastructure Stress: Large volumes of runoff put pressure on stormwater management systems, potentially leading to overflows, backups, and increased maintenance costs for municipalities.

Heat Island Effect: In urban areas, stormwater that flows over hot surfaces can increase the temperature of receiving water bodies, impacting aquatic life and exacerbating the urban heat island effect.

The environmental and economic consequences of uncontrolled stormwater runoff necessitate effective management strategies. By implementing control measures, we can mitigate these impacts and promote more sustainable urban development.

Regulatory Framework: Many countries have established regulations to address stormwater runoff. In the United States, the Clean Water Act requires municipalities to obtain permits for stormwater discharges and implement control measures to reduce pollutants.

Ecosystem Services: Proper stormwater management can provide additional benefits beyond flood control and pollution prevention. Well-designed control measures can create habitats for wildlife, enhance urban green spaces, and improve the overall quality of life in communities.

Climate Change Adaptation: As climate change leads to more frequent and intense precipitation events in many regions, effective stormwater management becomes increasingly crucial for community resilience.

To illustrate the impact of urbanization on stormwater runoff, consider the following table:

Land Use Type	Impervious Surface Coverage	Runoff Coefficient
Natural Forest	0-10%	0.05-0.20
Suburban	20-40%	0.25-0.40
Urban	60-100%	0.70-0.95

The runoff coefficient represents the fraction of rainfall that becomes runoff. As impervious surface coverage increases, so does the runoff coefficient, indicating a higher volume of stormwater that needs to be managed.

Understanding the nature of stormwater runoff and its impacts is the first step in developing effective control measures. The subsequent sections will delve into various strategies and techniques used to manage stormwater and mitigate its negative effects on our environment and infrastructure.

How do structural control measures mitigate stormwater runoff?

Structural control measures are physical installations designed to manage stormwater runoff by controlling its flow, reducing its volume, or improving its quality. These engineered solutions play a crucial role in comprehensive stormwater management strategies, offering tangible and often quantifiable benefits.

Detention Basins: These large, shallow depressions temporarily store stormwater during peak flow events. Detention basins slow the release of runoff into receiving water bodies, reducing the risk of downstream flooding and erosion. They also allow sediment and associated pollutants to settle out of the water.

Key features of detention basins:
– Designed to empty within 24-72 hours after a storm event
– Can be dry (normally empty) or wet (with a permanent pool of water)
– Often integrated into parks or open spaces for multiple uses

Retention Ponds: Unlike detention basins, retention ponds maintain a permanent pool of water. They provide both stormwater storage and treatment through settling and biological processes. Retention ponds can significantly improve water quality by removing pollutants and nutrients.

Benefits of retention ponds:
– Enhance aesthetic value of landscapes
– Provide habitat for aquatic and terrestrial wildlife
– Can serve as recreational amenities in urban areas

Infiltration Trenches and Basins: These structures are designed to capture runoff and allow it to percolate into the ground. By promoting infiltration, they reduce runoff volume, recharge groundwater, and filter pollutants through soil layers.

Characteristics of infiltration systems:
– Typically filled with permeable materials like gravel or crushed stone
– Require proper site selection to ensure adequate soil permeability
– Can be combined with pretreatment measures to prevent clogging

Constructed Wetlands: Engineered to mimic natural wetlands, these systems use vegetation, soils, and microbial activity to treat stormwater. Constructed wetlands excel at removing pollutants, particularly nutrients and suspended solids.

Advantages of constructed wetlands:
– Provide valuable habitat for diverse plant and animal species
– Offer educational opportunities for environmental awareness
– Can be designed as attractive landscape features

Permeable Pavements: These specialized surfaces allow water to pass through and infiltrate into underlying layers. Permeable pavements reduce runoff volume, filter pollutants, and can contribute to groundwater recharge.

Types of permeable pavements:
– Porous asphalt
– Pervious concrete
– Interlocking pavers with gaps for water infiltration

Green Roofs: Vegetated roof systems that capture and retain rainwater, reducing runoff from buildings. Green roofs also provide insulation, reduce urban heat island effects, and create habitats for birds and insects.

Components of a typical green roof:
– Waterproofing membrane
– Drainage layer
– Growing medium
– Vegetation layer

Rainwater Harvesting Systems: These systems collect and store rainwater from roofs and other impervious surfaces for later use. By capturing runoff at its source, they reduce the volume of stormwater entering drainage systems.

Applications of harvested rainwater:
– Landscape irrigation
– Non-potable indoor uses (e.g., toilet flushing)
– Industrial processes

To illustrate the effectiveness of various structural control measures, consider the following table comparing their pollutant removal efficiencies:

Control Measure	Total Suspended Solids (TSS) Removal	Total Phosphorus (TP) Removal	Total Nitrogen (TN) Removal
Detention Basin	60-80%	20-40%	20-40%
Retention Pond	50-80%	30-50%	30-50%
Infiltration Trench	80-100%	60-80%	40-60%
Constructed Wetland	80-90%	40-60%	30-50%
Permeable Pavement	70-90%	50-70%	30-50%

Note: Removal efficiencies can vary based on design, maintenance, and local conditions.

Bioretention Systems: Also known as rain gardens, these landscaped depressions use engineered soils and vegetation to filter and treat stormwater. Bioretention systems can be integrated into urban landscapes, providing both functional and aesthetic benefits.

Key components of bioretention systems:
– Mulch layer for initial filtration
– Engineered soil mix for pollutant removal
– Native plants adapted to local climate conditions
– Underdrain system (in some designs) for excess water removal

Swales and Filter Strips: These vegetated channels or slopes convey stormwater while promoting infiltration and filtration. Swales and filter strips are often used alongside roads or parking lots to manage runoff from impervious surfaces.

Design considerations for swales and filter strips:
– Gentle slopes to slow water flow and increase contact time
– Dense vegetation to enhance pollutant removal
– Proper length and width to handle expected runoff volumes

Oil-Water Separators: These devices are designed to remove oil, grease, and other petroleum hydrocarbons from stormwater runoff. They are particularly useful in areas with high vehicular traffic or industrial activities.

Types of oil-water separators:
– Gravity separators
– Coalescing plate separators
– Air flotation units

The effectiveness of structural control measures depends on proper design, installation, and maintenance. Regular inspections and upkeep are essential to ensure these systems continue to function as intended over time. Additionally, the selection of appropriate control measures should consider site-specific factors such as soil conditions, land use, climate, and regulatory requirements.

Structural control measures often work best when combined with non-structural approaches, creating a comprehensive stormwater management strategy. The integration of multiple techniques can provide redundancy and enhance overall system performance, leading to more resilient and sustainable urban water management.

What non-structural control measures can be implemented for stormwater management?

Non-structural control measures for stormwater management focus on preventive actions, policies, and practices that reduce runoff and pollutants at their source. These measures often require changes in behavior, land use planning, and institutional practices. While they may not involve physical installations like structural controls, non-structural measures can be highly effective and cost-efficient in managing stormwater runoff.

Land Use Planning and Zoning: Thoughtful urban planning can significantly reduce stormwater runoff by preserving natural drainage patterns and limiting impervious surface coverage.

Key strategies in land use planning:
– Cluster development to minimize disturbed areas
– Preservation of open spaces and natural vegetation
– Establishment of riparian buffers along water bodies

Public Education and Outreach: Raising awareness about stormwater issues and promoting responsible practices among residents, businesses, and institutions is crucial for successful stormwater management.

Effective education initiatives:
– School programs on water conservation and pollution prevention
– Community workshops on rain barrel installation and rain garden creation
– Informational campaigns on proper disposal of household chemicals and pet waste

Street Sweeping: Regular street sweeping removes accumulated debris, sediment, and pollutants from road surfaces before they can be washed into storm drains.

Benefits of street sweeping:
– Reduces pollutant loads in stormwater runoff
– Improves aesthetics of urban areas
– Can extend the life of stormwater infrastructure by reducing sediment buildup

Catch Basin Cleaning: Routine maintenance of catch basins prevents accumulated sediment and debris from entering the stormwater system.

Importance of catch basin cleaning:
– Enhances the effectiveness of the drainage system
– Reduces the risk of localized flooding
– Prevents pollutants from being flushed into receiving waters during storm events

Erosion and Sediment Control: Implementing practices to minimize soil erosion and prevent sediment from entering stormwater systems, particularly at construction sites.

Common erosion control measures:
– Silt fences and sediment traps
– Temporary seeding of disturbed areas
– Mulching and erosion control blankets

Illicit Discharge Detection and Elimination: Programs to identify and remove unauthorized connections to the stormwater system, such as illegal dumping or improper sewage connections.

Components of an effective program:
– Mapping of the stormwater system
– Regular inspections and water quality monitoring
– Enforcement procedures for violations

Good Housekeeping Practices: Implementing proper storage, handling, and disposal procedures for materials that could contaminate stormwater.

Examples of good housekeeping:
– Covered storage areas for chemicals and hazardous materials
– Spill prevention and response plans
– Regular maintenance of vehicles and equipment to prevent leaks

Pesticide and Fertilizer Management: Promoting responsible use of lawn chemicals to reduce nutrient and pesticide runoff.

Best practices for chemical management:
– Soil testing to determine appropriate fertilizer application rates
– Use of slow-release fertilizers
– Integrated pest management techniques to reduce pesticide use

Pet Waste Management: Encouraging proper disposal of pet waste to prevent bacterial contamination of stormwater.

Strategies for pet waste management:
– Installation of pet waste stations in parks and public areas
– Educational campaigns on the environmental impacts of pet waste
– Enforcement of local ordinances requiring waste pickup

To illustrate the potential impact of non-structural measures, consider the following table comparing the effectiveness of selected practices:

Non-Structural Measure	Potential Pollutant Load Reduction	Cost-Effectiveness	Implementation Difficulty
Public Education	10-50%	High	Low to Moderate
Street Sweeping	30-90% (for solids)	Moderate	Low
Catch Basin Cleaning	20-40%	Moderate	Low
Erosion Control	60-90%	High	Moderate
Illicit Discharge Detection	50-100% (for specific sources)	High	Moderate to High

Note: Effectiveness can vary based on local conditions and implementation quality.

Water Conservation Practices: Promoting efficient water use can indirectly reduce stormwater runoff by decreasing the amount of water that becomes runoff from over-irrigation.

Water conservation strategies:
– Installation of water-efficient fixtures and appliances
– Promotion of drought-tolerant landscaping (xeriscaping)
– Rainwater harvesting for non-potable uses

Tree Planting and Urban Forestry: Trees play a crucial role in intercepting rainfall, reducing runoff, and improving water quality through natural filtration.

Benefits of urban trees for stormwater management:
– Canopy interception of rainfall
– Increased soil infiltration rates
– Reduction of urban heat island effect

Pollution Prevention Plans: Developing and implementing plans to minimize the generation of pollutants that could contaminate stormwater, particularly for industrial and commercial sites.

Key elements of pollution prevention plans:
– Identification of potential pollution sources
– Implementation of best management practices
– Regular monitoring and reporting

Impervious Surface Reduction: Encouraging the use of permeable materials and minimizing unnecessary impervious surfaces in new developments and redevelopment projects.

Strategies for reducing impervious surfaces:
– Use of permeable pavement for low-traffic areas
– Implementation of shared parking arrangements
– Design of narrower residential streets where appropriate

Stream and Wetland Restoration: Restoring natural water bodies and their surrounding ecosystems can enhance their capacity to manage stormwater and improve water quality.

Benefits of restoration projects:
– Increased flood storage capacity
– Enhanced pollutant removal through natural processes
– Improved habitat for aquatic and terrestrial species

Non-structural control measures often require collaboration between various stakeholders, including government agencies, property owners, businesses, and community organizations. Their success depends on consistent implementation, ongoing education, and adaptive management to address changing conditions and emerging challenges.

While these measures may not provide immediate, visible results like structural controls, they can lead to long-term, sustainable improvements in stormwater management. When combined with structural measures, non-structural approaches create a comprehensive strategy that addresses stormwater issues at multiple levels, from individual actions to community-wide policies.

How do Low Impact Development (LID) techniques address stormwater runoff?

Low Impact Development (LID) is an innovative approach to stormwater management that aims to mimic natural hydrologic processes in urban and suburban environments. LID techniques focus on managing rainfall at its source, using small-scale, distributed practices that capture, filter, and infiltrate stormwater runoff. This approach contrasts with conventional stormwater management, which often relies on centralized collection and treatment systems.

Core Principles of LID:

Preservation of Natural Features: LID emphasizes the conservation and utilization of existing natural site features that are beneficial to stormwater management.

Key strategies:
– Preserving mature trees and native vegetation
– Maintaining natural drainage patterns
– Protecting sensitive areas such as wetlands and riparian zones

Minimization of Impervious Surfaces: LID techniques seek to reduce the amount of impervious cover in developments, thereby decreasing runoff volume.

Approaches to minimize impervious surfaces:
-## What is stormwater runoff and why does it need to be controlled?

Stormwater runoff occurs when rain or melting snow flows over land or impervious surfaces instead of soaking into the ground. In natural, undeveloped areas, most precipitation is absorbed by soil and vegetation or evaporates back into the atmosphere. However, as urban development increases, more surfaces become impervious – rooftops, roads, parking lots, and other structures prevent water from infiltrating the ground.

This altered hydrology leads to several environmental and public health concerns that necessitate stormwater control:

Water pollution

As stormwater flows over developed areas, it picks up pollutants like:

• Sediment from construction sites and eroded areas
• Oil, grease, and toxic chemicals from vehicles
• Pesticides and nutrients from lawns and gardens
• Bacteria from pet waste and failing septic systems
• Trash and debris

These pollutants are then carried into nearby water bodies, degrading water quality and harming aquatic ecosystems. Excess nutrients can cause algal blooms that deplete oxygen levels. Sediment clouds water and smothers aquatic habitats. Toxic contaminants can make water unsafe for human and wildlife use.

Flooding and property damage

The increased volume and velocity of runoff from impervious surfaces overwhelms natural and man-made drainage systems. This leads to:

• Flash flooding in urban areas
• Erosion of stream banks and damage to infrastructure
• Sewer system overflows in combined sewer systems
• Basement flooding and property damage

Reduced groundwater recharge

When precipitation cannot infiltrate into the ground, it reduces the replenishment of groundwater aquifers. This can lead to:

• Lower water tables and reduced stream baseflows
• Decreased water availability for wells and water supplies
• Land subsidence in some areas

Stream channel erosion

High stormwater flows erode stream banks and alter natural stream channels. This degrades aquatic habitats and can damage nearby property and infrastructure.

Thermal pollution

Runoff heated by warm impervious surfaces raises water temperatures when discharged to streams. This thermal pollution stresses cold-water species like trout.

Economic impacts

The cumulative effects of uncontrolled stormwater runoff result in significant costs:

• Flood damage to property and infrastructure
• Decreased property values near eroded streams
• Increased water treatment costs
• Loss of recreational opportunities and tourism revenue
• Reduced fish and shellfish harvests

To illustrate the magnitude of the stormwater runoff problem, consider these statistics:

Impact	Statistic
Annual stormwater runoff volume in U.S.	~3 trillion gallons
Impervious surface area in average city	45-50% of land area
Increase in runoff from 1 acre parking lot vs. 1 acre meadow	16 times higher
Number of water bodies impaired by urban runoff	~13% of rivers, 18% of lakes
Annual flood damages in U.S.	~$8 billion

Given these wide-ranging and costly impacts, controlling stormwater runoff is critical for protecting water resources, public health and safety, property, and the environment. Effective stormwater management aims to reduce runoff volumes, slow peak flows, remove pollutants, and restore more natural hydrology in developed areas.

How do structural control measures mitigate stormwater runoff?

Structural control measures are engineered systems and facilities designed to manage stormwater runoff quantity and quality. These measures work by capturing, detaining, infiltrating, filtering, or treating runoff before it reaches receiving water bodies. Here’s an overview of common structural controls and how they mitigate stormwater impacts:

Detention basins

Detention basins temporarily store runoff and release it at a controlled rate. This reduces peak flows and flooding downstream. Dry detention basins are designed to completely drain between storm events, while wet detention ponds maintain a permanent pool of water.

Key benefits:
– Peak flow reduction
– Flood control
– Sediment removal (40-60%)

Design considerations:
– Sized to detain runoff from design storm (e.g. 10-year event)
– Outlet structure controls release rate
– Forebay for sediment capture
– Safety bench around permanent pool

Retention ponds

Retention ponds, also called wet ponds, maintain a permanent pool of water and provide additional storage above the normal water level. The permanent pool allows for settling of pollutants between storm events.

Key benefits:
– Peak flow reduction
– Pollutant removal (sediment 50-80%, nutrients 30-50%)
– Habitat creation

Design considerations:
– 3:1 length to width ratio for pollutant settling
– Aquatic bench for safety and vegetation
– Sediment forebay at inlets
– Minimum drainage area of 10-25 acres

Constructed wetlands

Constructed wetlands are shallow marsh systems planted with wetland vegetation. They use natural processes to remove pollutants as runoff flows through.

Key benefits:
– High pollutant removal (sediment 80%+, nutrients 40%+)
– Habitat creation
– Aesthetic value

Design considerations:
– Shallow depths (6-18 inches) for vegetation growth
– Multiple cells for treatment train
– Diverse native plant selection
– Sediment forebay for pretreatment

Bioretention systems

Bioretention systems, including rain gardens and bioswales, are vegetated depressions that filter runoff through engineered soil media. They provide both water quality treatment and runoff reduction through infiltration and evapotranspiration.

Key benefits:
– Runoff reduction (40-80%)
– High pollutant removal
– Groundwater recharge
– Aesthetic landscaping

Design considerations:
– Sized for water quality volume (0.5-1 inch runoff)
– Engineered soil mix (sand, compost, native soil)
– Underdrain in less permeable soils
– Native plants adapted to wet/dry conditions

Infiltration trenches/basins

Infiltration practices are designed to capture runoff and allow it to soak into the ground, mimicking natural hydrology. They work best in permeable soils.

Key benefits:
– Runoff volume reduction
– Groundwater recharge
– High pollutant removal

Design considerations:
– Sized for design storm volume
– Pretreatment to remove sediment
– Observation well for monitoring
– Overflow for large events

Permeable pavement

Permeable pavement systems allow stormwater to infiltrate through the surface into an underlying stone reservoir. This reduces runoff from typically impervious areas.

Key benefits:
– Runoff reduction (45-75%)
– Pollutant removal in underlying layers
– Groundwater recharge

Design considerations:
– Appropriate for low traffic areas
– Underlying soil infiltration rate >0.5 in/hr
– Regular vacuum sweeping maintenance
– Overflow for large storms

Green roofs

Green roofs consist of a waterproof membrane covered with soil and vegetation. They retain and evapotranspire a significant portion of annual rainfall.

Key benefits:
– Runoff reduction (50-80% of annual rainfall)
– Reduced heat island effect
– Extended roof life
– Energy savings

Design considerations:
– Structural capacity of roof
– Slope stability
– Drought-tolerant vegetation
– Irrigation for plant establishment

Manufactured treatment devices

Various proprietary systems use settling, filtration, or other processes to remove pollutants from runoff. These include hydrodynamic separators, media filters, and catch basin inserts.

Key benefits:
– Targeted pollutant removal
– Small footprint for retrofit applications

Design considerations:
– Sized for water quality flow rate
– Regular maintenance and cleanout
– Performance varies by manufacturer

This table summarizes typical pollutant removal efficiencies for selected structural controls:

Control Measure	TSS Removal	TP Removal	TN Removal
Detention basin	60-70%	20-30%	20-30%
Retention pond	70-80%	40-50%	30-40%
Constructed wetland	80-90%	50-60%	40-50%
Bioretention	80-90%	60-80%	40-50%
Infiltration trench	90-95%	60-70%	50-60%
Permeable pavement	80-90%	50-60%	40-50%

TSS = Total Suspended Solids, TP = Total Phosphorus, TN = Total Nitrogen

By implementing these structural control measures, stormwater managers can effectively reduce runoff volumes, peak flows, and pollutant loads to receiving waters. The selection and design of specific practices depends on site conditions, regulatory requirements, and treatment goals.

What non-structural control measures can be implemented for stormwater management?

Non-structural control measures focus on preventing or reducing stormwater runoff and pollution at the source through planning, design, and operational practices. These measures are often more cost-effective than structural controls and can be implemented in both new development and existing urban areas. Here are key non-structural approaches for stormwater management:

Land use planning and zoning

Thoughtful land use planning can minimize impervious surfaces and protect sensitive environmental areas. Strategies include:

• Cluster development to preserve open space
• Transit-oriented development to reduce parking needs
• Mixed-use zoning to reduce vehicle trips
• Conservation subdivisions to protect natural areas
• Impervious cover limits in zoning codes

Site design techniques

Low Impact Development (LID) site design aims to maintain natural hydrology by minimizing disturbance and impervious cover. Key techniques include:

• Preserving natural drainage patterns
• Minimizing soil compaction and grading
• Reducing road widths and driveway lengths
• Using shared parking arrangements
• Disconnecting impervious areas with vegetated areas

Riparian and wetland buffers

Preserving or restoring vegetated buffers along streams, wetlands, and water bodies provides multiple benefits:

• Filtering of runoff pollutants
• Bank stabilization and erosion control
• Flood storage and peak flow reduction
• Wildlife habitat and corridors
• Temperature moderation for streams

Tree preservation and planting

Urban trees play a crucial role in stormwater management by:

• Intercepting rainfall in their canopies
• Increasing infiltration around root zones
• Reducing erosion through root systems
• Providing evapotranspiration
• Mitigating urban heat island effects

Erosion and sediment control

Proper erosion control during construction is critical for preventing sediment pollution. Key practices include:

• Minimizing disturbed area and construction time
• Phasing construction to limit exposed soils
• Installing perimeter controls (silt fences, berms)
• Stabilizing exposed soils quickly with vegetation or mulch
• Protecting storm drain inlets from sediment

Street and parking lot sweeping

Regular street sweeping removes accumulated pollutants before they can be washed into storm drains. Modern regenerative air and vacuum sweepers can remove fine particles most associated with pollutants.

Catch basin cleaning

Routine cleaning of catch basins and storm drain inlets removes accumulated sediment and debris. This prevents resuspension and transport of pollutants during storms.

Pet waste management

Pet waste is a significant source of bacteria in urban runoff. Management strategies include:

• Public education on proper disposal
• Providing bag dispensers and waste receptacles in parks
• Enforcing pet waste ordinances

Fertilizer and pesticide management

Reducing excess fertilizer and pesticide use helps prevent nutrient and chemical runoff. Approaches include:

• Soil testing to determine fertilizer needs
• Using slow-release fertilizers
• Integrated Pest Management to minimize pesticide use
• Proper timing and application methods

Salt and sand management

Winter deicing practices can contribute significant pollutant loads. Best practices include:

• Calibrating spreaders for optimal application rates
• Using alternative deicers (e.g. brine solutions)
• Proper salt storage and handling
• Sweeping sand after winter season

Illicit discharge detection and elimination

Programs to identify and eliminate non-stormwater discharges to storm sewer systems are crucial for preventing pollution. Key elements include:

• Storm sewer system mapping
• Dry weather outfall screening
• Tracing and removing illicit connections
• Public reporting hotlines

Public education and outreach

Educating residents, businesses, and municipal staff on stormwater impacts and pollution prevention is essential. Outreach methods include:

• Stormwater-focused websites and social media
• Brochures and fact sheets on BMPs
• Storm drain marking programs
• Workshops and training events
• School curricula on watershed protection

Pollution prevention plans

Developing and implementing pollution prevention plans for municipal operations and industrial facilities helps minimize contaminants exposed to stormwater. Key elements include:

• Good housekeeping practices
• Spill prevention and response procedures
• Employee training
• Proper material storage and handling
• Preventive maintenance of equipment

This table summarizes the relative effectiveness and cost of selected non-structural measures:

Non-Structural Measure	Effectiveness	Relative Cost
Land use planning	High	Low
Site design techniques	High	Low-Medium
Riparian buffers	High	Medium
Street sweeping	Medium	Medium-High
Catch basin cleaning	Medium	Medium
Public education	Medium	Low-Medium
Pollution prevention plans	Medium-High	Low-Medium

Implementing a combination of these non-structural approaches can significantly reduce stormwater runoff volumes and pollutant loads while often being more cost-effective than structural controls alone. Many of these measures also provide additional community benefits like improved aesthetics, habitat creation, and enhanced quality of life.

How do Low Impact Development (LID) techniques address stormwater runoff?

Low Impact Development (LID) is an innovative approach to stormwater management that aims to mimic natural hydrology by managing rainfall at its source. LID techniques use small-scale, distributed practices to reduce runoff and treat stormwater close to where it falls. This approach contrasts with conventional stormwater management, which often relies on large, centralized facilities to collect and treat runoff.

Key principles of LID include:

• Preserving natural landscape features and minimizing development impacts
• Minimizing and disconnecting impervious surfaces
• Managing stormwater as a resource rather than a waste product
• Using natural systems for infiltration, evapotranspiration, and runoff treatment
• Controlling stormwater at the source through distributed, small-scale practices

Here’s how specific LID techniques address stormwater runoff:

Bioretention systems

Bioretention systems, including rain gardens and bioswales, are shallow depressions filled with engineered soil media and planted with vegetation. They capture, filter, and infiltrate runoff from impervious surfaces.

Runoff reduction: Bioretention can reduce annual runoff volumes by 40-80% through infiltration and evapotranspiration.

Pollutant removal: These systems effectively remove sediment, nutrients, metals, and bacteria from runoff.

Design considerations:
– Sized to capture the water quality volume (typically 0.5-1 inch of runoff)
– Engineered soil mix of sand, compost, and native soil
– Native plants adapted to both wet and dry conditions
– Underdrain in less permeable soils
– Overflow for large storm events

Permeable pavement

Permeable pavement systems allow stormwater to infiltrate through the surface into an underlying stone reservoir. This reduces runoff from typically impervious areas like parking lots and driveways.

Runoff reduction: Permeable pavement can reduce annual runoff volumes by 45-75%, depending on design and underlying soils.

Pollutant removal: These systems filter pollutants as water percolates through the pavement and underlying layers.

Design considerations:
– Appropriate for low traffic areas
– Underlying soil infiltration rate >0.5 in/hr
– Regular vacuum sweeping maintenance
– Overflow for large storms

Green roofs

Green roofs consist of a waterproof membrane covered with soil and vegetation. They retain and evapotranspire a significant portion of annual rainfall, reducing runoff from rooftops.

Runoff reduction: Green roofs can retain 50-80% of annual rainfall, with performance varying by climate and design.

Additional benefits: Green roofs provide insulation, reduce urban heat island effects, and create habitat.

Design considerations:
– Structural capacity of roof
– Slope stability
– Drought-tolerant vegetation
– Irrigation for plant establishment

Rainwater harvesting

Rainwater harvesting systems collect and store roof runoff for later use, typically for irrigation or non-potable indoor uses.