Inspection and Maintenance: Green Roofs

Inspection & Maintenance Guidance of Green roofs best management practices. These specialized roofs are specially engineered and designed to support the growth of vegetation while protecting the structural integrity of the roof, aiding in helping to cool the building and effectively infiltrate, absorb and retain precipitation (TRCA, 2018).

Overview

Green roofs are engineered rooftop design features that allow the growth of vegetation on rooftops and have numerous benefits. These LID BMPs can also be referred to as vegetated roofs, rooftop gardens or eco-roofs. A green roof acts like a lawn, meadow or garden by intercepting and absorbing a portion of the rainwater or snowmelt that falls on it. The typical layers of a green roof (in ascending order from the roof surface) include a water-proofing membrane, drainage layer, lightweight growing media layer and the vegetation. Excess water that is not absorbed by the growing media or vegetation is collected by the underlying drainage layer, directed to outlet structures and conveyed via the roof drainage system to another BMP or the municipal storm sewer system. A portion of the water absorbed by green roofs is returned to the atmosphere by evaporation and transpiration by plants. Green roofs are typically designed to retain precipitation from small to medium-sized (e.g., 5 to 25 mm rainfall depth) storm events. Overflow outlets are necessary to safely convey flows from major storm events.

Generalized cross-section view of a green roof showing key components and common layers that make up this LID BMP (TRCA, 2018)

Some of the benefits of green roofs include:

• The ability to reduce the quantity of pollutants and runoff being discharged to municipal storm sewers and receiving waters (i.e., rivers, lakes and wetlands);
• Growing media and plants retain pollutants deposited from the atmosphere and reduce metals and other pollutants from conventional roof materials transported by runoff;
• Improve the energy efficiency of the building due to their insulating properties;
• Reduce the urban heat island effect;
• Can provide food and shelter for pollinators;
• Can provide aesthetic value as attractive landscaped features.
  

Key components of Underground Infiltration Systems to pay close attention to are the:

• Filter Bed / Growing Medium
• Vegetation
• Protective layers
• Irrigation System
• Overflow outlets

Associated Practices

• Intensive green roofs contain greater than 15 cm depth of growing media, can be planted with deeply rooted plants (e.g., shrubs and trees) and can be designed to handle pedestrian traffic.
• Extensive green roofs consist of a thinner growing media layer (15 cm depth or less) and are typically planted with.
• Blue roofs are systems that temporarily capture rainwater using the roof as storage and allow it to evaporate and/or to be used for non-potable requirements (i.e. irrigation, toilet flushing, truck washing) and ultimately offset potable water demands.

Inspection and Testing Framework

Maintneance of the Green roofs filter bed and vegetation composition/condition being performed by staff (TRCA, 2018).

Visual Indicators Framework - Green Roofs

Component	Indicators	Construction Inspection	Assumption Inspection	Routine Operation Inspection	Verification Inspection
Perimeter
	BMP dimensions	x	x		x
Growing medium/filter bed
	Green roof structural integrity		x	x	x
	Standing water		x	x	x
	Filter bed erosion		x	x
Vegetation
	Vegetation cover	x	x	x	x
	Vegetation condition		x	x
	Vegetation composition	x	x	x
Overflow outlets
	Overflow outlet obstruction	x	x	x	x

Testing Indicators Framework - Green Roofs

Component	Indicators	Construction Inspection	Assumption Inspection	Routine Operation Inspection	Verification Inspection
Testing Indicators
	Soil characterization testing	x	x		(x)
	Green roof irrigation system testing	x	x	x
	Green roof leak detection testing		x		x
Note: (x) denotes indicators to be used for Performance Verification inspections only (i.e., not for Maintenance Verification inspections)

Construction Inspection Tasks

Construction inspections take place during several points in the construction sequence, specific to the type of LID BMP, but at a minimum should be done weekly and include the following:

1. During site preparation, prior to BMP installation to ensure the roof structure is ready for green roof construction work and confirm that BMP layout area matches approved design drawings and that construction materials meet design specifications
2. After installation of leak detection system (if applicable) to ensure it was done properly
3. At installation of water-proofing membrane, prior to installation of root barrier, drainage layer and overflow outlets to ensure it was done properly and to confirm that slopes are acceptable
4. After installation of root barrier, drainage layer (including filter fabric/layer) and overflow outlets, prior to installation of growing medium and plants to ensure it was done properly and confirm that depth and slopes are acceptable
5. After installation of growing medium layer and plants to ensure it was done properly and to confirm depth, slopes and elevations at overflow outlets are acceptable; After installation of irrigation system to confirm system is functioning
6. Prior to hand-off points in the construction sequence when the contractor responsible for the work changes (i.e., hand-offs between the building and green roof installation contractors)
7. After every large storm event (e.g., 15 mm rainfall depth or greater) to ensure roof drainage or flow diversion devices are functioning and adequately maintained. You can also download and print the table here
   

A technician using a non-invasive locate method to determine the source of a leak in a Green roof membrane (Construction Canada, 2012.)

An example of a conceptual planting design for a green roof project in Mill Valley, California (O’Connell Landscape, 2010).

Green Roofs: Construction Inspections

Construction Sequence Step & Timing	Inspection Item	Observations*
Site Preparation - During site preparation, prior to BMP installation	Ensure the roof structure is ready for green roof construction work
	BMP layout area and dimensions match approved design drawings
	CDA is stabilized or runoff is diverted around BMP layout area
	Construction materials have been confirmed to meet design specifications
Leak detection System - After installation of leak detection system (if applicable), prior to installation of water-proofing membrane	Quality control check leak detection system installation
Water Proofing Membrane – After installation of waterproofing membrane, prior to installation of root barrier, drainage layer and overflow outlets	Quality control check membrane installation
	Confirm that slopes conform with approved design drawings
Root barrier / Drainage layer and Overflow outlets – After installation of root barrier, drainage layer (including filter fabric) and overflow outlets, prior to installation of growing medium layer and plants	Quality control check root barrier and drainage layer installations
	Installation of drainage layer (e.g., depth and slope) is acceptable
	Installations of overflow outlets (e.g., elevation and slope) are acceptable
Filter bed – After installation of filter bed (growing medium layer and plants)	Quality control check installation of any structural components of growing medium layer (if applicable)
	Installation of growing medium (e.g., depth, elevations at overflow outlets) is acceptable
	Growing medium is free of ruts, local depressions
	Planting material meets approved planting plan specifications (plant types and quantities)
	Quality control check installation of erosion matting/protection (if applicable)
Irrigation System - After installation of irrigation system	Confirm installation is acceptable and system is functioning (through testing)

Routine Maintenance - Key Components and I&M Tasks

Regular inspections (twice annually, at a minimum) done as part of routine maintenance tasks over the operating phase of the BMP life cycle to determine if maintenance task frequencies are adequate and determine when rehabilitation or further investigations into BMP function are warranted.

The table below describes routine maintenance tasks for green roofs, organized by BMP component, along with recommended minimum frequencies. It also suggests higher frequencies for certain tasks that may be warranted for BMPs located in highly visible or high pedestrian traffic locations or intensive green roofs featuring shrubs, trees and a wider variety of vegetation types. Tasks involving removal of trash, debris and weeding/trimming or replacement of dead plants may need to be done more frequently in such contexts. For further guidance on maintenance of vegetation cover on green roofs, refer to ASTM D2400/E2400M-06 Standard Guide for Selection, Installation and Maintenance of Plants for Green Roof Systems (ASTM International, 2015)

Individuals conducting vegetation maintenance and in particular, weeding (i.e., removal of undesirable vegetation), should be familiar with the species of plants specified in the planting plan and experienced in plant identification and methods of removing/controlling noxious weeds. Key resources on these topics are provided below at the links provided:

• Agriculture and Agri-food Canada’s Weed Info database
• Ontario Ministry of Agriculture, Food and Rural Affairs’ Ontario Weed Gallery
• Ontario Ministry of Agriculture, Food and Rural Affairs’ Noxious Weeds In Ontario list
• Ontario Invasive Plant Council’s Quick Reference Guide to Invasive Plant Species
• Oregon State University Stormwater Solutions, 2013, Field Guide: Maintaining Rain Gardens, Swales and Stormwater Planters, Corvallis, OR.
• Plants of Southern Ontario (book), 2014, by Richard Dickinson and France Royer, Lone Pine Publishing, 528 pgs.
• Weeds of North America (book), 2014, by Richard Dickinson and France Royer, University of Chicago Press, 656 pgs.

Green Roofs: Key Components, Descriptions and Routine I&M Requirements

Component	Description	Inspection & Maintenance Tasks	(Pass) Photo Example	(Fail) Photo Example
Perimeter	Separates the green roof from the roof edges and other structures; kept clear of vegetation and natural debris as a fire prevention measure. Parapets or other wind break structures may also be present around the perimeter to help prevent wind scour of growing media.	Check for damage or vegetation; Remove any vegetation or natural debris annually to biannually.	The footprint area of the green roof matches what was specified in the final design.	The footprint area of the green roof is significantly smaller than what was specified in the final design.
Growing medium/filter bed	Flat or gently sloping area covered by growing medium and a mixture of plants. Growing medium varies according to the green roof system or product but is generally designed to be porous and lightweight with adequate fertility and moisture retention to support plant growth while maintaining drainage of excess water within a few hours after a storm or snowmelt event. May be covered by matting/other erosion control product to prevent rain/wind scour while plantings are becoming established.	Check for standing water, animal burrows or damage from foot traffic, rain or wind scour; Replace erosion/scour protection where missing or uplifted; Remove trash and natural debris annually to biannually.	There is no standing water on the green roof surface shortly after a storm event.	Standing water is present on the green roof surface and in the underdrain/sub-drain system and bare soil areas are visible (Photo Source: J.V. Heidler)
Vegetation	Plants should be tolerant of the harsh conditions prevalent on rooftops. Use of a wide variety of plants may improve resilience but complicates maintenance and may require staff or contractors trained in horticulture	Maintenance is similar to a conventional perennial planting bed; Water bi-weekly in the absence of rain during first two months and as needed for the remainder of the first growing season; Remove undesirable vegetation biannually to quarterly; Prune shrubs and trees annually, and replace dead plants to maintain a minimum of 80% cover by the third growing season.	The green roof vegetation looks healthy and well maintained.	A portion of the vegetation on the green roof is dying or not thriving.
Overflow Outlet	Flows exceeding the storage capacity of the BMP are conveyed to an adjacent drainage system via an overflow outlet structure and the roof drainage system.	Remove trash, natural debris and clippings biannually to quarterly; Flush with hose or pressure washer annually to biannually	The overflow outlets of this green roof are free of damage and obstruction and function as designed to safely convey excess water from the BMP (Photo Source: Vegetal I.D.).	Sediment is accumulating at the overflow outlet which could impair its drainage function and cause surface ponding and vegetation die-off (Photo Source: Jorg Breuning).
Irrigation System	Most green roofs will require watering over their operating life cycle, especially during the first 2 months. Systems can range from simple hose bibs, garden hoses and sprinklers to intelligent automated systems that schedule watering based on weather forecasts and cistern water levels.	In the spring, reconnect all parts to the water supply, flush lines to clear out any debris or sediment and test to confirm that the system is undamaged and functioning well; In the late fall/early winter, disconnect the system from the water supply, connect it to an air compressor and blow air through it to remove water and ensure the lines and parts are dry, shut off water supply to the roof, and drain all hose bibs; Remove any debris/sediment accumulated on filters biannually.	The green roof is well covered by dense, attractive vegetation which helps maintain its stormwater treatment function and aesthetic value (Photo Source: Earth Rangers Centre)	A major portion of the green roof contains no living vegetation cover (Photo Source: Kevin Songer).
Protective layers	May be one or two layers; designed to protect the roof deck from water damage, including a waterproofing membrane, a root barrier that protects the water-proofing membrane from root penetration and degradation by microbial activity.	Repair isolated leaks in the water-proofing membrane through deconstruction of a small portion of the green roof, patching with new material, and reconstruction.	There are no signs of damage to the concrete parapets along the perimeter and no uplift of green roof layers.	One of the green roof growing media structures has been displaced and requires replacement and repair. (Photo Source: Kevin Songer

Tips to Preserve Basic BMP Function

• To avoid over-compaction of the filter media, any maintenance tasks involving foot traffic on the filter bed should not be performed during wet weather.
• Pavers or walkways should be placed at roof access locations and along primary paths to facilitate access and avoid walking on planted portions during inspection and maintenance work.
• To perform installation, maintenance or repair work, do not use sharp tools, lawn staples and stakes which can damage the drainage layer, root barrier and water-proofing membrane. All sharp pieces of metal and fasteners should be removed from the media area with care.
• For green roofs with succulents (e.g., sedum) as vegetation cover, trim off top stems annually in the spring during the first two years of establishment and leave on the growing media surface to encourage colonization or purchase and spread fresh cuttings.
• Transplant vegetation that is established in the vegetation free perimeter areas to supplement plantings on the filter bed if species are appropriate (See appropriate Green Roof Plants here).
• Pruning of mature trees should be performed under the guidance of a Certified Arborist.
• Establish procedures and timing for irrigation system startup and winterization to avoid damage to system components from freezing.
• Routinely check that the automated irrigation systems is free of damage and delivering water evenly to vegetated areas.
• For green roofs with automated irrigation systems using municipal/drinking water, schedule watering to occur at night or early in the morning to minimize the loss of water to evaporation;
• For green roofs with automated irrigation systems using cistern water, irrigating during the day when evaporation rate is high will make greater use of stored rainwater, thereby freeing up more storage in the system for the next rain storm and helping to reduce site runoff volume.

Rehabilitation & Repair

Table below provides guidance on rehabilitation and repair work specific to green roofs organized according to BMP component.

testing and repair work]] must be scheduled shortly to identify the problem area. (Photo Source: Vegetal I.D., n.d.)

Green Roofs: Key Components, Typical Issues and Rehabilitation Requirements

Component	Problem	Rehabilitation Tasks
Growing Media	Eroded growing media area ≥ 30 cm in length or other damage is present	Restore Growing Media to required depth with material that meets design specifications, replant and irrigate bi-weekly or as needed until plantings are established. If problems persist, consider covering with matting or other erosion control measure until plantings are established or adding parapets or other wind break structures.
	Surface ponding remains for > 3 hours after the end of a storm event because water does not infiltrate through the growing media	Aerate (i.e., rake) or replace growing media in problem areas taking care not to damage the drainage or protective layers.
	Surface ponding remains for > 3 hours after the end of a storm event because the drainage layer is clogged.	Consult with green roof designer or product manufacturer/vendor to determine corrective actions.
Vegetation	Plants are not thriving and [[Green roof media|Growing Media] is low in organic matter (< 3 %) or available phosphorus (< 2.2 mg/kg)	Amendment or fertilizer application should be prescribed by the medium manufacturer or product vendor.
Overflow outlets	Surface ponding remains for > 3 hours after the end of a storm event because overflow outlet is obstructed.	Remove the obstruction which may require the use of a pressure washer or drain-snaking service.
Irrigation system	Distribution line, fitting or sprinkler nozzle is leaking, damaged or misaligned.	Identify the location of the damaged system component through testing (i.e., running the system in each zone while making observations). Turn off the system and schedule the repair work.
Protective Layers	Water-proofing membrane has reached 40 years of age and is due for replacement.	Deconstruct the green roof, replace the water-proofing membrane with new material, and reconstruct with materials that meet design or product specifications.

Inspection Time Commitments and Costs

Estimates are based on an extensive green roof with 15 cm of growing medium, irrigation and water-proofing membrane with leak detection system; estimates for other combinations of these variables are described in the Low Impact Development (LID) Stormwater Management Practice Inspection and Maintenance Guide

Per-task cost estimates for maintenance and rehabilitation of an extensive green roof design with 15 cm growing medium, irrigation and water-proofing membrane with leak detection (in 2016 $ figures).

General time commitments and costs for inspection of an extensive green roof design with 15 cm growing medium, irrigation and water-proofing membrane with leak detection (in 2016 $ figures).

Construction and life cycle cost estimates for extensive green roofs with 15 cm growing medium, irrigation and water-proofing membrane with leak detection (in 2016 $ figures).

Estimates of the life cycle costs of inspection and maintenance have been produced using the latest version of the LID Life Cycle Costing Tool for three design variations (full infiltration, partial infiltration and no infiltration) to assist stormwater infrastructure planners, designers and asset managers with planning and preparing budgets for potential LID features.

Assumptions for the above costs and the following table below are based on the following:

• Design variations for permeable pavements can be broken down into three main categories:
  • Full Infiltration design, where the pavement drains through infiltration into the underlying subsoil alone (i.e., no sub-drain);
  • Partial Infiltration design, where drainage is through the combination of a subdrain and infiltration into the underlying subsoil (i.e., with a sub-drain); or,
  • No Infiltration (i.e., filtration only design) that includes an impermeable liner between the base of the BMP and the underlying native sub-soil, where drainage is through a sub-drain alone (i.e., with a sub-drain and impermeable liner).
• For permeable pavements it is assumed that rehabilitation of the pavement surface will be needed once the BMP reaches 30 years of age in order to maintain surface drainage performance at an acceptable level. Included in the rehabilitation costs are (de)mobilization costs, as equipment would not have been present on site.
• Design costs were not included in the rehabilitation as it was assumed that the original LID practice design would be used to inform this work. The annual average maintenance cost does not include rehabilitation costs and therefore represents an average of routine maintenance tasks, as outlined in Table Permeable Pavements: Key Components, Descriptions and Routine I&M Requirements, above. All cost value estimates represent the NPV as the calculation takes into account average annual interest (2%) and discount (3%) rates over the evaluation time periods.
• The costing presented in this section is specific to permeable interlocking concrete pavers (PICP), as defined in the Tool. This product has been selected for costing due to its popularity and well understood maintenance needs.
• For all permeable pavement design variations, the CDA has been defined as 2,000 m² of which 1,000 m² is impermeable pavement draining to the pavers, and 1,000 m² is permeable pavement. The impervious area to pervious area ratio (I:P ratio) used to size the BMP footprint is 1:1, which is in accordance with recommendations in the LID SWM Planning and Design Guide (CVC & TRCA, 2010).
  • The Full Infiltration design does not include a sub-drain and assumes a native sub-soil infiltration rate of 20 mm/h. The base granular reservoir is 350 mm deep and is capable of storing runoff from a 61 mm rain event over the CDA. A monitoring well is included for inspection purposes.
  • The Partial Infiltration design includes a sub-drain and assumes a native sub-soil infiltration rate of 10 mm/h. The base granular reservoir is 350 mm deep and is capable of storing runoff from a 9 mm rain event before the stored volume reaches the perforated underdrain pipe located 50 mm above the native sub-soil. Although a flow restrictor is recommended to maximize infiltration, the cost of this feature is not included due to its relatively low cost.
  • The No Infiltration design includes a sub-drain pipe installed on the bottom of the sub-surface water storage reservoir and an impermeable liner. All other features are the same as the Partial Infiltration design variation.
• Estimates of the life cycle costs of PICP permeable pavements in Canadian dollars per unit CDA ($/m²) are presented in the table below. LID Life Cycle Costing Tool allows users to select what BMP type and design variation applies, and to use the default assumptions to generate planning level cost estimates.
  • Users can also input their own values relating to a site or area, design, unit costs, and inspection and maintenance task frequencies to generate customized cost estimates, specific to a certain project, context or stormwater infrastructure program.
  • For all BMP design variations and maintenance scenarios, it is assumed that rehabilitation of the pavement surface will be necessary when the BMP reaches 30 years of age to maintain acceptable surface drainage performance. Rehabilitation of PICP pavements is assumed to typically involve the following tasks and associated costs:
    • Remove pavers, bedding and joint fill and top 5 cm (2”) of base aggregate and replace with new material that meets design specifications
    • Construction and Assumption inspection and testing associated with rehabilitation work to confirm that materials meet design specifications and installation is acceptable, including compaction and surface infiltration rate testing.
      

Life cycle cost estimates for all infiltration types of PICP permeable pavements an under minimum and high frequency scenarios (in 2016 $ figures).

Notes:

1. Estimated life cycle costs represent NPV of associated costs in Canadian dollars per square metre of CDA ($/m2).
2. Average annual maintenance cost estimates represent NPV of all costs incurred over the time period and do not include rehabilitation costs.
3. Rehabilitation cost estimates represent NPV of all costs related to rehabilitative maintenance work assumed to be needed after 30 years in service, including those associated with inspection.
4. Full Infiltration design life cycle costs are lower than Partial and No Infiltration designs due to the absence of a sub-drain to construct, inspect and routinely flush.
5. Rehabilitation costs for Full Infiltration designs are estimated to be 54.8% to 55.6% of the original construction costs for High and Minimum Recommended Frequency maintenance program scenarios, respectively.
6. Rehabilitation costs for Partial Infiltration designs are estimated to be 53.5% to 54.3% of the original construction costs for High and Minimum Recommended Frequency maintenance program scenarios, respectively.
7. Rehabilitation costs for No Infiltration designs are estimated to be 47.4% to 48.1% of the original construction costs for High and Minimum Recommended Frequency maintenance program scenarios, respectively.
8. Maintenance and rehabilitation costs over a 25 year time period for the Minimum Recommended maintenance scenario are estimated to be 22.3%, of the original construction costs for Full Infiltration design, 22.6% for Partial Infiltration design, and 20.0% for No Infiltration design.
9. Maintenance and rehabilitation costs over a 25 year time period for the High Frequency maintenance scenario are estimated to be: 39.0% of the original construction costs for Full, 38.9% for Partial Infiltration designs, and 34.5% for No Infiltration designs.
10. Maintenance and rehabilitation costs over a 50 year time period for the Minimum Recommended Frequency maintenance scenario are estimated to be approximately: 1.07 times the original construction cost for Full, 1.06 times the original construction costs for Partial Infiltration designs, and 93.9.% the original construction cost for No Infiltration designs.
11. Maintenance and rehabilitation costs over a 50 year time period for the High Frequency maintenance scenario are estimated to be approximately: 1.43 times the original construction cost for Full, 1.41 times the original construction costs for Partial Infiltration designs, and 1.25 times the original construction cost for No Infiltration designs.

State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include Source, Conveyance and End-Of-Pipe Controls.

Any form of rain or snow.

Toronto and Region Conservation Authority

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.

A system flow of gully inlets, pipes, overland flow paths, open channels, culverts and detention basins used to convey runoff to its receiving waters. City of Toronto 45 Wet Weather Flow Management November 2006

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Abiotic transfer of water vapour to the atmosphere.

Watercourses and Lake Ontario, to which Stormwater and Combined Sewer Overflows discharge.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.

Water ponded on the ground surface.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

Includes the protection of soil from dislocation by water, wind or other agents.

Water ponded on the ground surface.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Tank used to store rainwater (typically roof runoff) for later use.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

The technique of removing pollutants from runoff as it infiltrates through the soil.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

A perforated pipe used to assist the draining of soils.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include Source, Conveyance and End-Of-Pipe Controls.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.