Green Stormwater Infrastructure

• Full document [PDF/33.9MB] (273 pages)
• GSI Design Guidelines for the Capital Region [PDF/1MB]
• Appendix A - Rainfall Capture Targets and GSI Selection [PDF/893KB]
• Appendix B - Absorbent Landscape [PDF/2.3MB]
• Appendix C - Vegetated and Grassy Infiltration Swale Systems [PDF/4.8MB]
• Appendix D - Infiltration Rain Garden [PDF/4.6MB]
• Appendix E - Infiltration Curb Extensions and Traffic Islands [PDF/3.7MB]
• Appendix F - Infiltration Flow Through Planters [PDF/2.7MB]
• Appendix G - Structural Soil Cells [PDF/2.2MB]
• Appendix H - Green Roof [PDF/3.2MB]
• Appendix I - Constructed Wetlands [PDF/3.5MB]
• Appendix J - Cisterns and DetentionTanks [PDF/2.3MB]
• Appendix K - Pervious Paving Systems [PDF/2.8MB]
• Appendix L - Infiltration Trench and Soakaway Manhole [PDF/2.6MB]
• Supplemental 1 - Planting Templates and Plant Lists [PDF/3.9MB]
• Supplemental 2 - Leaky Sewers and Green Stormwater Infrastructure [PDF/1.1MB]

Rain Gardens are landscape features designed to treat stormwater runoff from hard surface areas such as roofs, roads and parking lots. They consist of sunken garden spaces where runoff can pond and infiltrate into deep constructed soils and then into the native soils below.

Where native soils have low infiltration rates, rain gardens often have a drain rock reservoir and perforated drain system to take excess water to the storm drain system. The constructed soils of the rain garden, and an overlying mulch layer, are designed to replicate many of the pollutant removal mechanisms that operate in forested ecosystems.

Stormwater enters the rain garden via an inlet pipe or sheet flow. Small storm events can usually be temporarily stored until they infiltrate into the ground. Most rain gardens are designed to pond no more than 2-3 inches above the soil bed.

While usually designed as stand-alone facilities without conveyance, new designs are evolving that group a series of rain gardens along linear features such as roads, and which include weirs and surface conveyance similar to bioswales.

The soil mix in rain gardens is usually about 1200mm deep and has a high proportion of sand and organics. Most sources now recommend adding a 50-75mm layer of organic mulch above the soil for erosion control, pollutant removal and to maintain infiltration capacity.

1. Full Infiltration

   These rain gardens are used where all inflow is intended to infiltrate into the underlying subsoil. Sources suggest candidate sites have soil permeability greater than 30 mm/hr. An overflow for large events is provided by a pipe or swale to the storm drain system.

2. Full Infiltration with Reservoir

   Infiltration gardens with a reservoir have a drain rock reservoir so that surface water can move quickly through the installed growing medium and infiltrate slowly into subsoils from the reservoir below. This type of garden is a candidate for sites with subsoil permeability greater than 15 mm/hr.

3. Partial Infiltration

   These rain gardens are designed so that most water can infiltrate into the underlying soil, while any surplus overflow is drained by perforated pipes that are placed near the top of the drain rock reservoir. This type of garden is suitable for sites with subsoil permeability of greater than 1.0 mm/hr and less than 15 mm/hr.

4. Partial Infiltration with Flow Restriction

   Flow restrictor rain garden variations are used where subsoil permeability is less than 1 mm/hr. The added feature is a flow restrictor assembly with a small orifice which slowly decants the top portion of the reservoir and rain garden. This type of rain garden provides water quality treatment and some infiltration, while acting like a small detention facility.

Runoff Volumes

Rain gardens can be sized to temporarily store runoff from smaller to medium sized storm events in the depression area itself, the constructed soils, and any constructed reservoir.

Pollutant Removal

Studies have shown that vegetated soils remove more stormwater pollutants than non-vegetated soils through processes of absorption, filtration, sedimentation, infiltration, phytoremediation, volatilization, surface resistance and thermal attenuation. Bioretention systems have demonstrated excellent removal for heavy metals, with some research showing the most uptake occurs in the mulch layer. Estimates from research suggest that metal accumulation would not create any environmental concerns for at least 20 years in these systems.

Hydrocarbons are removed via sorption during the storm event, then biodegradation in the mulch layer by microbial populations. Some US research on older bioretention facilities is showing that they maintain soil functions that actually enhance pollutant processing capability over time. Phosphorous removal rates are also high.

Pollutant Load

Bioretention areas can be used to treat highly contaminated runoff from some land uses where pollutant concentrations exceed those typically found in stormwater. Rain gardens can be used for these types of land uses (stormwater “hot spots”) as long as an impermeable liner is used at the bottom of the filter bed.

Groundwater Recharge

Because stormwater in rain gardens is detained for period of time, it has a chance to infiltrate, replenishing soils and replicating the natural hydrology. Bioretention areas can be applied in almost any soils or topography, since runoff percolates through a man-made soil bed and the design can return excess flows to the stormwater system.

Siting

Bioretention facilities are ideally suited to many ultra-urban areas, such as parking lots. While they consume a fairly large amount of space (approximately 5-20 percent of the area that drains to them), they can be fit into existing parking lot islands or other landscaped areas.

Heat Pollution

Some aquatic wildlife is sensitive to changes in water temperature. Bioretention is a good option for cold water streams because water ponds in them for only a short time, decreasing the potential for warming. Rain gardens have been shown to decrease the temperature of runoff from certain land uses, such as parking lots.

Infrastructure Performance/Sizing

If runoff from a portion of the site can be diverted to a rain garden, the size of other on-site stormwater techniques can either be reduced accordingly, or its effectiveness will be enhanced.

Retrofitting

Bioretention can be used as a stormwater retrofit, by modifying existing landscaped areas, or if a parking lot is being resurfaced. In highly urbanized areas, this is one of the few retrofit options that can be employed.

Longevity/Maintenance

Maintenance is similar to other landscaped garden beds but care must be taken to remove any sediments that accumulate, to check for erosion at the inlet and for any blocking of outlets.

Aesthetics

Vegetated infiltration basins can have an informal or formal design and are easily integrated into the overall landscape or site design. To avoid becoming a breeding ground for mosquitoes, sources recommend that rain gardens be designed to drain no more than a set number of hours (approximately 30-72) after a rainfall event has ended.

Costs

Some sources note that costs of building rain gardens compare favourably with conventional stormwater management facilities. When comparing the cost of rain gardens with other stormwater management techniques, the cost of the landscaping components can be left aside as many municipalities would require still landscaping for commercial and multi-family development.

The use of bioretention areas may reduce the need for other BMPs that require large tracts of contiguous land. As a result, the true cost of the practice is less than the construction cost reported. Similarly, maintenance activities conducted on bioretention areas are not very different from maintenance of a landscaped area; however, bioretention areas may actually lower utility costs by requiring less watering than similarly landscaped areas.

Size

Bioretention systems are generally applied to small sites (less than 2 ha), and in a highly urbanized setting. Literature suggests rain garden areas of about 10-20% of the upstream impervious area. An optimum rain garden size is about 50 square metres, draining 250 square metres of impervious area. When used to treat larger areas, they can clog. In addition, it is difficult to convey flow from a large area to a bioretention area.

Siting

Smaller, distributed rain gardens are better than single large scale facilities. Many jurisdictions specify separation distances for locating rain gardens near wells, property lines, building foundations, footing drains and steep slopes. Rain gardens cannot be placed over utility crossings unless trench dams are installed. Rain gardens should be separate from the ground water table to ensure the table never intersects with the bed of the bioretention facility. This prevents possible ground water contamination.

Slopes

Rain gardens are best used on relatively shallow slopes (approximately 5%). However, sufficient slope is needed to ensure that the bioretention area can be connected with the storm drain system. This stormwater management practice is most often applied to parking lots or residential landscaped areas, which generally have shallow slopes.

Longevity/Maintenance

Because rain gardens can look just like standard landscaped garden beds, there is some concern by local governments and designers that, over time, owners and maintenance companies will not understand their dual function, possibly making alterations that prevent gardens from functioning properly.

CRD Resources

• Gardening With Native Plants
• Native Plants for Moist/Wet Sites 
• Native Plant Ground Covers [PDF/325KB] 
• Building a Rain Garden 

External Resources

• Plant List from Rain Garden Handbook for Western Washington [PDF/666KB] 
• Rain Garden Handbook for Western Washington Homeowners: Washington State University 
• Rain Gardens: A how-to manual for homeowners - Wisconsin Department of Natural Resources

A green roof is a living roof — a vegetated building envelope system consisting of a layer of live plants in a lightweight growing medium, root barrier membrane and drainage layer over a waterproof membrane. The engineered soil used on a green roof is specially formulated to be lighter than regular garden soil, to reduce the weight load on the building. The green roof must be designed so that the weight of the soil and plants is less than or equal to the weight the roof structure can safely support. Green roofs are designed to help manage rainwater by reducing the rate and amount of runoff into the storm drain system.

Did You Know?

The CRD Headquarters have two green roofs. Monitoring results showed rainfall retention of almost 40% and good thermal insulation provided by these green building features. 

Green roofs range from small, simple, owner-constructed designs on sheds and homes, to extensive proprietary systems installed on large commercial and industrial buildings. Of the latter installations, there are two basic types: intensive and extensive.

1. Intensive Green Roofs

   Intensive roofs typically have deeper soils and irrigation systems, allowing for a much wider range of plant types and sizes. They provide significantly better insulation. Intensive green roofs are usually designed to be accessible (available to be walked on or used as amenity space). This type of eco-roof requires more technical expertise to design and install, and more maintenance once built than extensive types.

   Intensive Green Roof Diagram [PDF/6.8MB]

2. Extensive Green Roofs

   These roofs are characterized by their low weight, low capital cost and minimal maintenance. Soils (or growing medium) are very thin, and irrigation is rare, allowing only smaller, hardier plants to survive. Extensive green roofs are more suitable for larger areas and for sloped roofs. They are easier to retrofit onto existing roofs but are not usually designed for public accessibility.

   Extensive Green Roof Diagram [PDF/82B]

While differences exist between the range of plants possible for extensive versus intensive green roofs, those best suited to any rooftop environment are:

• Drought-tolerant, requiring little or no irrigation after establishment
• Self-sustaining, without the need for fertilizers, pesticides, or herbicides
• Able to withstand heat, cold, and high winds
• Very low-maintenance, needing little or no mowing or trimming

• Perennial or self-sowing

  Suitable plants also have growth patterns that allow them to thoroughly cover the soil and include a variety of species to ensure the establishment of a self-maintaining community which is appealing to animal and human users alike. 

Reduced runoff into storm sewers by absorbing more rainfall

Growing medium, and any additional substrate or integrated water storage, can store significant volumes of rainwater, preventing runoff in small storm events, and delaying peak runoff for larger storms. Runoff is extended over several hours, reducing peak flows. The plants can return some of this moisture to the atmosphere via evapotranspiration. Studies in Berlin show that green roofs absorb up to 75% of the precipitation falling on them. A study from Portland confirms similar results, with green roofs mitigating from 65% to 94% of runoff.

As green roofs reduce runoff volumes for most rainfall events, this can improve the performance of other on-site cleaning, infiltration, detention or storage facilities.

Energy conservation by heat flow reduction

Green roofs help reduce a building’s cooling costs because the plants shade the roof surface. This prevents the roof from heating up, thus reducing the need for air conditioning and decreasing energy consumption. In a process called evapotranspiration, plants release or transpire water through pores in their leaves (stomata). Water on the plants’ leaves absorbs heat as it evaporates, cooling the surrounding air in the process.

Additionally, research also shows that green roofs in winter months can reduce the energy used for heating by more than 10 percent during the cold season. (Test houses used include the National Research Council of Canada's field roofing facility in Ottawa).

Roof lifespan can be extended by up to 30 years

Cleaner air and decreased carbon dioxide (CO2) emissions

Design & Construction

Sources agree that a flat surface is suitable for the installation of a green roof, but they differ on the maximum slopes recommended. Recommendations range from 30% (4:12 pitch) up to 40% (or 5:12 pitch).

Green roofs can be used on most types of commercial, multi-family, and industrial structures, as well as single-family homes and garages. Though green roofs are simpler to create in new construction, it is possible to retrofit an existing roof to become a green roof provided the roof has suitable structural strength.

Sound Insulation

Green roofs can be designed for noise attenuation. The growing medium blocks lower frequencies of sound and the plants block the higher frequencies. Tests show that 12cm of growing medium alone can reduce sound by 40 dB.

Longevity/Maintenance

Green roofs protect roofing membranes against ultraviolet radiation, extreme temperature fluctuations and physical or puncture damage from recreation or maintenance. US sources estimate that green roofs last twice as long as conventional roof surfaces.

Building Incentives

Some cities such as Toronto, Ontario award amenity bonuses for proposed buildings with green roofs, increasing the building space that would otherwise be allowed. Many green building rating systems such as LEED^TM assign points in their recognition schemes to green roofs.

Food Production/Amenity

Green roofs hold potential for local food production and have special applicability for outdoor classrooms and horticultural therapy gardens if used on institutional buildings. They can also provide beautiful and useful green spaces for people living in otherwise hard-surfaced urban landscapes.

Root penetration

To avoid damage to the roof and building, plants with known invasive root systems should not be used on green roofs. Many new installations also use an industry-standard chemical or physical root penetration barrier.

Roof Design & Construction

As with all roofs, proper waterproofing, leak detection systems and drainage is required, as is protection from damage during construction. With green roofs, leaks are more difficult to find than on a standard roof. Design and plantings must allow easy access to drainage outlets for inspection and maintenance.

Costs

From a simple financial perspective, eco-roofs cost more to construct than conventional roofs. However, several sources list them as competitive on a life-cycle basis, because of reduced maintenance and replacement costs. In addition, other advantages (energy savings, air cleansing, green space provision, habitat replacement, stormwater detention and cleansing, etc.) should be factored into any cost considerations.

Weight

When calculating structural loads, designers must consider the saturated weight of each material. The structural strength of existing buildings must be assessed to ensure they can accommodate the additional weight of a green roof retrofit.

Fire Safety

Sedum and other succulents are naturally fire resistant, almost eliminating fire concerns. There is evidence from European manufacturers which suggests that green roofs can help slow the spread of fire to and from the building through the roof, particularly where the growing medium is saturated. Other types of vegetation could be of concern and may need to be watered, mowed, and/or maintained to prevent fire hazards before they go dormant during the dry season.

Aesthetics

Plants on roofs are exposed to very harsh conditions and proper maintenance is required to ensure long-term appeal. Some observers find that extensive eco-roofs are not appealing during the dormant season, but this should be tempered by a fair comparison to standard roof finishes, particularly flat roofs, and not to standard ground planted gardens.

• City of Toronto Green Roofs
• Portland Bureau of Environmental Services
• Green Roofs for Healthy Cities
• BCIT Centre for the Advancement of Green Roof Technology
• The Greenroof Projects Database

Living walls are also known as green-, bio-, vegetated-, living- or eco-walls. At their simplest, they are vertical gardens and can include any type of vegetative covering of a standard wall, such as hanging gardens and climbing vines. The term has recently come to include specialized and engineered envelope systems where vegetation is planted, irrigated and grown in modular elements which are secured to or integrated with the wall of a building.

• Living Wall Diagram [PDF/3MB]

In these latter systems, plants typically grow without soil between layers of fibrous material (such as felt or plastic mesh), or in pre-vegetated panels, that are suspended in front of a building wall. They are not planted in the ground or in planter boxes. Based on the principles of hydroponics, water with added nutrients drips slowly to the bottom of the wall where any excess is pumped up and re-circulated. Some living walls incorporate a pool at the base of the structure which can include fish and small animals such as amphibians.

Structural weight, moisture retention, nutrient supply and water distribution are important design considerations in all living walls.

1. Green facades

   These feature vertical structural systems that support climbing plants on the building exterior. Climbers and vines are supported by stainless steel cables, webbing or metal grids and grow up from grade or planters.

2. Active Walls

   Active walls such as the one at Queen’s University in Ontario, are indoor features joined to the building’s air circulation system where fans draw air through the living wall before being circulated through the building for increased oxygen and reduced pollutant levels.

3. Inactive Walls

   These are also indoor features, but rely instead on passive open design for free air circulation rather than on mechanical air systems.

4. Outdoor Living Walls

   Outdoor living walls are the engineered building envelope systems that allow a screen or layer of living plant material to be suspended at some distance from the outside wall of a building. Specialized membranes and drainage layers support the growth of a range of mosses, vines and perennial plants. Most of the information on this fact page relates to this type of living wall, along with the indoor walls, rather than green facade types.

Stormwater Retention

Though there is little evidence that stormwater is a design consideration, living walls can be designed to slowly use up stormwater which lands on the roof or other hard surfaces of a building site. Plants in a soil-less design need a relatively constant supply of water. This could be provided with the aid of a cistern placed higher than the top of the growing medium. Some cleansing would be provided by the plants and soils, and by the bacteria which would eventually inhabit the growing medium and root surfaces. Indoor and outdoor living walls could both take advantage of stormwater for re-use.

Pollutant Removal

Living walls trap many airborne pollutants and particulates on the plant surfaces. In addition, plants take up Carbon Dioxide. A three store high living wall inside the Queen’s University Faculty of Applied Sciences, Live Building Integrated Learning Centre, is designed to remove Volatile Organic Compounds (VOCs) and Carbon Dioxide from the indoor air and its performance is constantly being monitored. A Toronto company, Quality Air Solutions, markets their ‘bio-wall’ as designed for bio-filtration of interior environments, including the removal of volatile organic compounds (VOCs).

Reduced Footprint

Living walls make excellent use of vertical space within cities, providing micro-habitat, aesthetic benefits and air cleansing where none would have typically existed before. The high ratio of wall to roof area in urban spaces means the potential to generate positive environmental changes via green walls versus green roofs is also much higher. 

Energy Savings

Living walls add thermal mass to a building. They also provide shade and an insulating dead air space on the surface of the building wall. Vegetation also lowers adjacent air temperatures by evaporating enormous amounts of water from leaf surfaces. All of these processes help moderate indoor and outdoor building temperatures. One Canadian study found the reduction of summer cooling load by living walls was even more dramatic than for green roofs. The same study showed that significant reductions in the urban heat island effect could be attained if living wall technology was used extensively.

Weight

Despite being constantly wet, the engineered soil-less systems (PVC layer, felt & metal frame for example) can weigh less than 30 kg/m² so are considered fairly light-weight for adding on to existing walls.

Habitat

Living walls could meet some of the habitat requirements of small wildlife species, such as birds and insects, especially if suitable native plants are included.

Aesthetics/Livability

Many of the new living walls, both indoor and outdoor projects, small and large, are designed for artistic effect and to enhance livability by providing calming greenery in very urban spaces. Some of the best known and most dramatic large scale examples in Europe, Asia and the US were designed by botanist Patrick Blanc of France.

Noise Reduction

Green walls can help reduce sound transmission into buildings due to the layer of plants, growing medium and, depending upon the design, the dead air space between the living and conventional walls.

Food Growing

While it is hard to find an example of a living wall designed for food growing, some proponents suggest homemade versions for greenhouse walls and vegetable gardens.

Maintenance

Some sources note that, at least for indoor projects, monthly maintenance programs would be similar to other indoor garden requirements. Other sources suggest that vegetated walls require a much higher level of maintenance than climbers on a vertical frame. Practically speaking, the sheer height of some outdoor living walls will likely pose challenges in terms of maintenance access.

Energy and Water Use

The more complicated engineered systems have been criticized for using energy to supply light (indoor applications only) and pump water and nutrients through the system, and for using embodied energy in the building components. Living walls can be water-use intensive depending on their exposure. Irrigation systems are usually required to supplement rainfall.

Mould/Moisture Problems

Proper air flow and water movement must be established to help ensure harmful moulds do not grow, particularly in indoor applications. In addition, the constant presence of moisture means that the walls must be well separated from the adjacent structure.

Pollens

Designers should consider pollen generation when choosing plants, especially for indoor applications or beside operating windows.

• Patrick Blanc: The Vertical Garden Site
• Greenscreen
• Greenwall Company
• Aquaquest, The Marilyn Blusson Learning Centre: Greenroofs Project Database

Permeable pavement, also known as pervious or porous paving, is a type of hard surfacing that allows rainfall to percolate to an underlying reservoir base where rainfall is either infiltrated to underlying soils or removed by a subsurface drain.

Permeable pavement can be used instead of standard asphalt and concrete for surfacing sidewalks, driveways, parking areas, and many types of road surfaces. Standard asphalt and concrete are considered to be “impermeable.” Precipitation that falls on or drains to them cannot flow through the surface to the soils below, but runs to the lowest points to be drained away.

Permeable pavements include:

• individual unit paving blocks or cobble stones,
• plastic or fibrous grid systems filled with sand, gravel or living plants, or
• specialty mixes of both concrete and asphalt

Permeable asphalt and concrete do not look markedly different from their impervious counterparts, but a close inspection can reveal larger surface “pores.”  Permeable unit pavers and grid systems are usually easier to distinguish from a distance.

Unit Pavers

These consist of interlocking concrete paving blocks separated by narrow gaps (pores) which are filled with sand and/or gravel, as specified by the manufacturer. These gaps allow stormwater to drain into a stone filled reservoir base below the surface, and then into the underlying soils. If the native soil below the paved area has poor permeability, the reservoir can be designed to store rainwater.  Typically, overflow from extremely large storms is conveyed to municipal drainage systems off-site. Permeable pavers are most often seen in use for private driveways, walkways, parking areas at the edge of roadways and parking lots. They are not considered appropriate for heavy volume roads and highways.

Grass Pavers

Grass pavers consist of concrete cells or a strong plastic grid system with large pore spaces filled with a growing medium planted with grass or a low growing herb. This type of product is often used in low-traffic vehicle movement areas such as fire access lanes, long term parking slots and private driveways. Areas often include reservoir bases and underdrain systems similar to unit pavers.

Gravel Pavers

These are similar to grass pavers except that the growing medium is replaced with gravel and no plant materials are used. The look is similar to a simple gravel parking lot but the grid system helps keep gravel pieces in place over time, preventing ruts and worn spots.

Permeable Asphalt

This pavement consists of an open-graded coarse aggregate, bonded together by asphalt cement, with sufficient interconnected open spaces to make it highly permeable to water.

Permeable Concrete

This concrete has a much larger than usual void space, with little or no “fines” material in the mix.  This allows water and air to move quickly through the material to the soils or the base layer below. It typically consists of specially formulated mixtures of Portland cement, uniform, open-graded coarse aggregate, and water.  Porous concrete has been used on highways to reduce hydroplaning.

Runoff Volumes

Research has shown that pavers can significantly reduce runoff volumes, thereby reducing the erosive power of stormwater entering creeks and intertidal areas. This helps to protect backwater refuges, brings less sediment to spawning areas, and prevents erosion of stream banks and loss of bank stability. 

Pollutant Removal

Long-term research on permeable pavers shows their effective removal of pollutants such as total suspended solids, total phosphorous, total nitrogen, chemical oxygen demand, zinc, motor oil, and copper. In the void spaces, naturally occurring micro-organisms break down hydrocarbons and metals adhere.

Groundwater Recharge

In areas with suitable soils, permeable pavements allow stormwater to enter the sub-soils, replicating the natural hydrological cycle by allowing for groundwater recharge and moderating the fluctuations of flows in watercourses.

Heat Pollution

Porous pavement can help lower high runoff water temperatures commonly associated with impervious surfaces. Stormwater pools on the surface of conventional pavement, where it is heated by the sun and the hot pavement surface. By rapidly infiltrating rainfall, porous pavement reduces the water’s exposure to sun and heat. Cool stream water is essential for the health of many aquatic organisms, including trout and salmon.

Infrastructure Performance

Using permeable pavement surfaces reduces the amount of effective impervious area (EIA), in an existing development.  (EIA is the hard surface area directly connected to municipal drainage systems.) Reduction of EIA improves the performance of existing on-site cleansing, infiltration and storage facilities, which thus process less stormwater flow.

Infrastructure Footprint

A reduction in EIA can help reduce the size of the on-site stormwater storage technique required in many municipalities, potentially freeing up land surface for other more valuable uses.

Longevity/Maintenance

While there is little historical evidence, many concrete paver manufacturers claim their product will last 50 years or more. In comparison, asphalt parking lots last a far shorter time, especially in freeze/thaw climates. Frequent crack filling and overlaying, some restriping and at least one reconstruction, would be required within a 50-year span.

Conversely, the maintenance required on a permeable concrete paver system is claimed, depending upon the source, to be from minimal to onerous. Maintenance consists of restriping and occasional cleaning of the aggregate within the pore area by vacuum truck. The latter needs to perform only on a case-by-case basis, depending on how the pavement is performing

Runoff Volumes

Permeable pavements are designed to replace effective impervious areas, not to manage stormwater from other impervious surfaces on site. Use of this technique must be part of an overall onsite management system for stormwater and is not a replacement for other techniques.

Pollutant Load

Highly contaminated runoff can be generated by some land uses where pollutant concentrations exceed those typically found in stormwater. These "hot spots" include commercial nurseries, recycling facilities, fuelling stations, industrial storage, marinas, some outdoor loading facilities, public works yards, hazardous materials generators (if containers are exposed to rainfall), vehicle service and maintenance areas, and vehicle and equipment washing and steam cleaning facilities. Since porous pavement is an infiltration practice, it should not be applied at stormwater hot spots due to the potential for groundwater contamination.  All contaminated runoff should be prevented from entering municipal storm drain systems by using best management practices for the specific industry or activity.

Weight & Traffic Volumes

Reference sources differ on whether low or medium traffic volumes and weights are appropriate for porous pavements. For example, around truck-loading docks and areas of high commercial traffic, porous pavement is sometimes cited as being inappropriate.  However, given the variability of products available, the growing number of existing installations in North America and targeted research by both manufacturers and user agencies, the range of accepted applications seems to be expanding. Some concrete paver companies have developed products specifically for industrial applications. Working examples exist at fire halls, busy retail complex parking lots, and on public and private roads, including intersections in parts of North America with quite severe winter conditions.

Siting

Permeable pavements may not be appropriate when land surrounding or draining into the pavement exceeds a 20% slope, where pavement is downslope from buildings or where foundations have piped drainage at their footers. The key is to ensure that drainage from other parts of a site is intercepted and dealt with separately rather than being directed onto permeable surfaces.

Climate

Cold climates may present special challenges. Road salt contains chlorides that could migrate through the porous pavement into groundwater. Snow plow blades could catch block edges and damage surfaces. Infiltrating runoff may freeze below the pavement, causing frost heave, though design modifications can reduce this risk. These potential problems do not mean that porous pavement cannot be used in cold climates. Porous pavement designed to reduce frost heave has been used successfully in Norway. Furthermore, experience suggests that rapid drainage below porous surfaces increase the rate of snow melt above.

Cost

Some estimates put the cost of permeable paving at two to three times that of conventional asphalt paving. Using permeable paving, however, can reduce the cost of providing larger or more stormwater BMP’s onsite and these savings should be factored into any cost analysis. In addition, the off-site environmental impact costs of not reducing on-site stormwater volumes and pollution have historically been ignored or assigned to other groups (local government parks, public works and environmental restoration budgets, fisheries losses, etc.) The City of Olympia in Washington State is studying the use of porous concrete quite closely and finding that new stormwater regulations are making it a viable alternative to stormwater ponds.

Longevity/Maintenance

Grass pavers require supplemental watering in the first year to establish the vegetation, otherwise they may need to be re-seeded. Regional climate also means that most grass applications will go dormant during the dry season. While brown vegetation is only a matter of aesthetics, it can influence public support for this type of permeable paving.

Olympia, Washington has found that porous concrete mix quality can be difficult to control, as it is sensitive to water and difficult to blend correctly.  The city is still working on how, and how often, to clean, porous concrete. Olympia expects to solve these problems as it gains more experience.

Results of studies have shown that permeable pavement systems dramatically reduce surface runoff volumes and peak discharge. As well, standard water quality indicators are significantly reduced. Research also indicates that, as with any stormwater management technique or device, permeable paving performs well over time if properly installed and maintained.

• Does Not Drain to Bay: The Ecology Center
• Pervious Concrete: Pervious Pavement
• The Portland Bureau of Environmental Services

Also known as stormwater runoff control, erosion prevention and sediment control is not one technology, but rather a suite of methods that can be used to both prevent soils from eroding from a piece of land, and to capture any that do erode. These individual techniques, or erosion best management practices (BMPs) are each applicable to different situations and must be chosen carefully for each project.

Eroded particles of soil, or sediments, can easily be moved off construction and landscaping sites by flowing water and end up in natural water bodies. These sediments can cause damage to receiving water bodies. Sediment in water bodies can reduce the amount of sunlight reaching aquatic plants, clog or abrade fish gills causing suffocation, smother aquatic feeding sites and spawning areas and interfere with fishes’ ability to navigate. Preventing and controlling erosion is essential to protection of natural streams, rivers and salt water ecosystems.

In Canada, the Fisheries Act prohibits the deposit or release of a deleterious (toxic) substance to fish-bearing waters. In high concentrations, sediment is recognized as a deleterious substance. In addition, most municipalities have bylaws making it illegal to allow sediment-laden water to enter municipal storm drains or ditches. Provincial legislation in British Columbia also makes it an offence to pollute a stream. As a result, everyone who undertakes a construction or landscaping project that could cause erosion is advised to consider erosion prevention before they begin their project. For more information about watershed-wise development, see the CRD Developer’s Guide.

Whether simply replacing a driveway, re-landscaping a yard, or constructing a large and complex urban building, planning ahead for erosion prevention and sediment control can save time, money, potential litigation and help protect our environment.

While there are many sediment and erosion Best Management Practices (BMPs) available, the key facts to consider before beginning any construction or land clearing activity are that water moves downhill and, any unprotected soils can be moved off the site by any kind of water moving across the land. Typical sources of water on work sites include rainwater, drainage channels that cross the site and pump-out water from excavation holes.

Examples of BMP’s for erosion and sediment control include:

• Compost: One relatively new technique is the use of high organic content soils (compost) in socks, berms and blankets both at construction sites, and as important components of roadside bioswales and rain gardens.
• Bio-Engineering: Another technique being used more often in North America is bio-engineering, where living plant material is used strategically to help restore eroding stream banks, stabilize slopes of all types, and deactivate roads in logging areas.

In addition, a standard historical practice, the use of straw bales on construction sites, has been largely discredited. At best, hay bales don’t work, at their worst, they can make erosion problems worse. Both the US Environmental Protection Agency and Metro Vancouver have excellent information publications detailing alternatives to the straw bale.

The first and most important BMP is an Erosion Control Plan. Some municipalities require these as part of the building permit approval process, often basing the complexity of the plan required on the size of the building project.

The following points can help guide the development of an erosion control plan that takes a comprehensive approach to addressing construction site runoff. Hiring a qualified professional to design a plan for your site is recommended.

Minimize Clearing & Grading

Avoid clearing/grading any soils that are not absolutely required to be cleared. Map and flag all areas to be protected on site, sharing this information with site crews.

Protect Waterways

Clearing and grading activities near streams should be minimal, and appropriate BMPs installed to prevent any sediment from moving into the stream.

Phase Construction to Limit Soil Exposure

Ideally, construction site soils would not be exposed during the rainiest seasons, but activities can at least be broken into phases. Construction scheduling should allow for the installation of erosion BMPs prior to the start of construction; soil stabilization after grading and BMP maintenance.

Immediately Stabilize Exposed Soils

Exposed soils should be stabilized as soon as possible, and any stockpiles covered when not being worked.

Protect Steep Slopes & Cuts

Cutting and grading of steep slopes (>15 percent) should be avoided. If a steep slope exists, any water flowing onto it should be redirected with appropriate BMPs. Further techniques will be needed to protect the slope from erosion, stormwater, and slippage.

Install Perimeter Controls to Filter Sediments

Specialized BMPs should be properly installed around the perimeter of the construction site. Catch basins receiving stormwater flows from the construction site must be also protected with adequate BMPs.

Employ Advanced Sediment Settling Controls

BMPs, such as sediment basins, should be installed on-site to allow time for sediments to settle out.

Train Contractors on Erosion Control Plan Implementation

Site crews should be trained in erosion control practices or an environmental consultant hired to oversee all aspects of the BMP installation and maintenance.

Control Construction Waste

A plan should describe the type of waste anticipated for the site (such as discarded building materials, concrete truck washout, chemicals, litter, and sanitary waste) and how that waste will be managed to prevent impacts on water quality.

Inspect & Maintain BMPs

Erosion Control BMPs will not work unless properly & regularly maintained, especially before and after rainfalls. Always assign inspection responsibility to a specific crew member(s).

• International Erosion Control Association
• Land and Water Magazine

Also known as infiltration swales, biofilters, grassed swales, or in-line bioretention, bioswales are vegetated open channels specifically designed to attenuate and treat stormwater runoff for a defined water volume. Like open ditches, they convey larger stormwater volumes from a source to a discharge point, but unlike ditches, they intentionally promote slowing, cleansing and infiltration along the way. A sloped base to facilitate this water movement distinguishes bioswales from rain gardens.

There are some design variations of the bioswale, including grassed channels, dry swales and wet swales. These designs may also include an underlying rock reservoir, with or without a perforated underdrain. The specific design features and treatment methods differ in each variation, but all are considered improvements on traditional drainage ditches.

Each type of swale incorporates modified geometry and other design features to allow it to treat and convey stormwater runoff. A typical swale bottom is flat in cross-section, 600 to 2400 mm wide, with a 1-2% longitudinal slope, or dished between weirs on steeper slopes. Bioswale side slopes are usually no more than 3:1, horizontal to vertical.

Bioswale vegetation is typically lawn grasses, but more of the low-volume swales being built in North America are finished with a combination of grasses, perennials, shrubs, groundcover, and trees to meet other community goals in addition to stormwater management.

Grassed Channels

These are similar to a conventional drainage ditch, with the major differences being flatter side slopes and longitudinal slopes, and a slower design velocity for water quality treatment of small storm events. Grass channels are the least expensive option, but also provide the least reliable pollutant removal. The best application of a grassed channel is as pretreatment to other structural stormwater treatment practices.

A major difference between the grassed channel and other stormwater treatment practices is the method used to size the practice. Most stormwater treatment practices are sized by volume of runoff. That is, the process captures and treats a defined water quality volume, or the volume of water. The grassed channel, on the other hand, is based on flow rate (i.e., a peak flow from the water quality storm; this varies from region to region but a typical value is the one inch storm), grass channels should be designed to ensure that runoff takes an average of ten minutes to flow from the top to the bottom of the channel.

Wet Swales

These swales intersect the groundwater, and behave almost like a linear wetland cell. The design variation incorporates a shallow permanent pool and wetland vegetation to provide stormwater treatment. Wet swales are rarely used in residential settings because the shallow standing water is often unpopular with homeowners.

Dry Swales

Dry swales incorporate a deep fabricated soil bed into the bottom of the channel. Existing soils are replaced with a sand/soil mix that meets minimum permeability requirements. An underdrain system is also placed under the soil bed. Typically, the underdrain consists of a layer of gravel encasing a perforated pipe. Stormwater treated by the soil bed flows into the underdrain, which conveys treated stormwater back to the storm drain system.

Runoff Volumes

Even where soils have very poor hydraulic conductivity (around 1 mm/h), a 4 m swale/trench can reduce the volume of runoff from a typical local road to about 25% of total rainfall. In general, infiltration facilities along roads are more effective than on-lot infiltration facilities because there is typically less concentration of runoff (i.e. the ratio of impervious area to infiltration area tends to be lower).

Pollutant Removal

As stormwater runoff flows through bioswales, pollutants are removed through filtering by vegetation and soils. Above-ground plant parts (stems, leaves, and stolon) slow flow and thereby encourage particulates and their associated pollutants to settle. The pollutants are then incorporated into the soil where they may be immobilized and/or decomposed.  Bacteria within healthy soils can help break down carbon-based pollutants like motor oil.

Study data suggest high removal rates for some pollutants, but negative removals for some bacteria, and modest removal capability for phosphorus. Phosphorus removal in bioretention soils increases with the depth of the facility. Sorption onto certain components of the soil is the mechanism. Low pH or oxygen conditions can cause phosphorus to de-sorb however, so the design should allow for dewatering, and pH should be monitored annually if Phosphorus is a concern. Nitrate removal is highly variable. Where it is a concern an elevated under-drain design that creates a fluctuating aerobic/anaerobic zone below the drainpipe can be used to enhance the de-nitrification process.

Some of the oldest bioretention facilities in the US seem to have developed soil structures and functions that enhance pollutant removal ability. 

Groundwater Recharge

Grassed channels and dry swales provide some groundwater recharge if a high degree of infiltration is achieved by the practice. Wet swales typically do not contribute to groundwater recharge, as infiltration is lessened by the accumulation of organic debris on the bottom of the swale.

Siting

Grassed swales can be applied in most development situations, including residential areas, office complexes, rooftop runoff, parking and roadway runoff, parks and green spaces. Swales are well-suited to treat highway or residential road runoff because of their linear nature and because they are designed to receive stormwater runoff via distributed sheet flow, which travels through a grassy filter area at the swale verges. Bioswale design can easily incorporate driveway crossings.

Provision of underground overflow allows the use of the technique in most soils, including clay with infiltration rates as low as 0.6mm/hr.

Retrofitting

One common retrofit opportunity is to use grassed swales to replace existing drainage ditches. Ditches are traditionally designed only to convey stormwater away from roads. In some cases, it may be possible to incorporate features to enhance their pollutant removal or infiltration using check dams (i.e., small dams along the ditch that trap sediment, slow runoff, and reduce the longitudinal slope).

One well-publicized example of a retrofit roadway is the Seattle Street Edge Alternative (SEA) project.  The drainage goals for this project included conveyance, flood control, and minimizing the flow of stormwater off-site. The designers sculpted the project area to move water away from the roadway and homes and into planted swales along both sides of an existing road. They replaced impervious road surface area with bioswales, and helped address traffic goals in the community, at the same time improving local aesthetics and increasing the amount of urban forest to intercept rainfall.

Size

Individual grassed channels are designed for drainage areas of less than two hectares. If grass channels are used to treat larger areas, the flow velocity within the bioswale becomes too great to treat runoff or prevent erosion in the channel. Literature suggests swale areas of about 10-20% of upstream impervious area.

Pollutant Removal

If designed improperly, bioswales will have very little pollutant removal.  They also do not seem to be effective at reducing bacteria levels in stormwater runoff. 

Pollutant Load

Highly contaminated runoff can be generated by some land uses where pollutant concentrations exceed those typically found in stormwater. These hot spots include commercial nurseries, recycling facilities, fuelling stations, industrial storage, marinas, some outdoor loading facilities, public works yards, hazardous materials generators (if containers are exposed to rainfall), vehicle service and maintenance areas, and vehicle and equipment washing and steam cleaning facilities. With the exception of the dry swale design (see Design Variations), hotspot runoff should not be directed toward grassed channels. Swales infiltrate stormwater and can intersect the water table, thereby increasing the risk that hotspot runoff will become a threat to groundwater quality.

All contaminated runoff should be prevented from entering municipal storm drain systems by using best management practices for the specific industry or activity.

Groundwater Recharge

Designers should identify potential high-pollutant sources, particularly industrial/commercial hotspots that would dictate pre-treatment or source control upstream of a bioswale. Restrictions on the depth of groundwater depend on the type of channel used. Some sources suggest a minimum depth from the base of the drain rock reservoir to the water table of 600 mm to prevent a moist swale bottom or groundwater contamination.

Siting

While some sources recommend that bioswales should be used on sites with relatively flat slopes (i.e., less than 4%), others note that the use of properly spaced weirs can allow siting on slopes up to 10%.  When slopes become too steep, runoff velocities become fast enough to cause erosion and prevent adequate infiltration or filtering in the channel.

Maintenance

Maintenance requirements are similar to those for ditches: inspecting for bank slumping & erosion, replanting any bare patches where vegetation has been unsuccessful or removed, maintaining ideal vegetation heights by mowing, and removing garbage. Additionally, sediment build-up within the bottom of the swale should be removed once it has accumulated to 25% of the original design volume.

Design

Sources suggest a thick vegetative cover is needed for proper bioswale function. Water level fluctuation, long-term inundation, erosive flow, excessive shade, poor soils, and improper installation were found to be the most common causes of low vegetation survival in a King County study. The Seattle SEA Street project has had good success with vegetation survival. Neighbours have agreed to care for their boulevards, and careful plant selection was based on non-invasive, low-maintenance plants suited to the moisture regime of their location within the bioswales.

Because of the linear nature of bioswales, stormwater should ideally enter via sheet flow.  Pre-treatment (such as grassed verges) and erosion control must be part of the design in order to avoid sedimentation of the channel. 

• Low Impact Development: Puget Sound Action Team
• Stormwater Management Manual: City of Portland

Intensive Green Roof

The CRD intensive green roof supports a mix of native plants typical of a Garry Oak woodland as well as herbs and shrubs, fruit trees and berries. Food security was a consideration in the design, and there is space designated for staff volunteers to grow vegetables and salad fixings. The space also features espaliered peach and apple trees, kiwi vines and several berry-producing shrubs.

Extensive Green Roof

The roof of the CRD building was retrofitted with an extensive green roof. This system was chosen because of its minimal maintenance requirements and lightweight, consisting of low-growing, drought-tolerant plants (Sedum) rooted in shallow growing medium. Engineers calculated the maximum weight of the green roof that could be supported by the existing roof structure, and the soil depth of the extensive green roof was adjusted accordingly. 

On a west-facing exterior wall of the CRD office building is a 26m2 living wall. Plants in this vertical garden are growing in 10cm of soil in compartments anchored to the wall. The exterior stone of the building envelope was removed and the living wall panel inserted. The wall is irrigated and requires a higher level of maintenance than the green roof systems. Plants selected for the wall are perennial and evergreen native and horticultural varieties that thrive in the sun and wind exposure found at this location. Bees and hummingbirds are frequent visitors to the flowering plants on the living wall.

• Living Wall Diagram [PDF/3MB]

BC Institute of Technology Centre for Architectural Ecology designed and installed systems to monitor the performance of the green roof and living wall and conducted two years of monitoring. This is the first data of this kind on living walls in Canada. A weather station on the roof and sensors on the green roof, living wall and interior locations collect data on rainwater retention, thermal performance and plant growth.

Performance of the green roof and living wall was assessed by:

• Collection and analysis of roof runoff to determine how much water is held by plants on the roof and retained under different weather conditions
• Building heat loss and gain data collected to determine changes in energy consumption, heating and cooling costs and greenhouse gas generation
• Collection of data to document the integrity of the roof

Monitoring data showed that the green roof retains an average of 36% of precipitation, with peak runoff flow reduced by over 90% and delayed nearly 2 hours. Rainwater retention improves as the roof dries out and vegetation comes out of dormancy in spring. The extensive roof on this building is 38mm thick and can absorb 25mm of rain when dry. The green roof showed good thermal performance, greatly reducing the cooling load in the summer. Overall average heat flow through the roof structure is equivalent to a very high insulation value (RSI 6-8). These results indicate that the green roof is indeed a sustainable building feature.

The main contribution of the living wall is to reduce heat loss by providing insulation (RSI 0.6). Monitoring of soil moisture content and plant performance indicated that irrigation is required during dry spells. Results show that living wall performance is very sensitive to the specific micro-conditions of the site.