Green Roofs

Green Roofs are native plant installations on the roof of a building. They can be placed on new or existing construction although they are typically installed in concert with new construction as the design considerations can be taken into account when the building is planned. Roofs are one of the primary sources of runoff as they are large impervious areas. By placing hardy native vegetation on the roof, the building can greatly reduce the amount of stormwater it produces by using the plants to evapotranspire much of the water received during a storm. Additionally, there are energy benefits inherent to green roofs as they act as an insulating layer both in the summer and during the winter.

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Design Considerations

Siting of Facility

Each rooftop should be considered unique. Retrofitting existing buildings or new construction should take the following into consideration: local climate conditions, roof slope, function of the roof (e.g. stormwater management, public access and/ or habitat creation), size of the project, budget, degree of accessibility, structural loading and infrastructure located on top of the building.

Structural Loading

The load bearing capacity of a green roof’s underlying structural support is a major factor influencing the design. The load bearing capacity of a roof must consider: dead load - the total weight of materials including soil, plants, snow, ballast and any other roof materials and live load - people, including maintenance workers and any other activities that the roof will need to support.

Type of Green Roof

There are two main divisions of green roofs - extensive and intensive. Generally, extensive roofs are lighter, have less than 4 inches of planting medium, use drought tolerant vegetation and are able to handle a limited number of people for maintenance concerns. Intensive systems are heavier with a much deeper planting medium, allow for greater live loads and require a much higher structural capacity load.

Vegitation

Successful rooftop vegetation must be able to: rapidly stabilize soil, quickly repair itself from damage, absorb and transpire water despite extreme conditions of heat and cold, wind and drought. In general, as the planting medium’s depth increases, so does the list of viable plant species. Sedums and mosses have been successfully used in shallow depth areas, while native grasses and forbs may be used in deeper soils.

Growing Medium

The growing medium must meet the selected planting’s nutrient, water, oxygen and pH needs. However, the structural load capacity of the building often determines the depth and material of the medium, which ultimately determines the vegetation that can be supported. Most growing mediums are made up of approximately 3/4 mineral material and 1/4 organic material. Mineral materials can be natural - sand or gravel, artificial - perlite or vermiculite, or created from recycled building materials.

Filter Fabric

Filter fabric below the growing medium prevents soil particles and other debris from migrating to and clogging the layers below. The drainage layer is protected while aeration is provided for the growing medium. Non-woven, non-biodegradable fabrics typically come in rolls and must be overlapped and secured to one another.

Water Drainage and Storage

The drainage layer helps prevent leaks by moving excess water away from the waterproof layer. Roof drainage design must consider: stormwater management goals, roof slope and the depth and nature of the drainage material. There are 3 main types of drainage material including granular materials (coarse gravel, stone, expanded clay, etc.) that have a large proportion of open space when packed together, sponge-like porous mats that can absorb and hold water, and several different types of synthetic drainage modules. Most of the synthetic modules create a rigid platform to keep growing medium and vegetation from contacting the waterproofing. Several of the synthetic modules have small depressions to store excess water.

Root Barrier

A physical or chemical barrier is needed to protect organic based waterproofing (i.e. asphalt) that can be penetrated by roots and broken down by microorganisms. The most common root barrier is a high-density polyethylene (HDPE) membrane. Metal and plastic base plates and PVC rolls are also used. PVC can also serve as waterproofing itself and is available in recycled form. Root barriers and waterproofing must be raised above the planting medium at roof perimeters and at any projections that penetrate the roof vertically, (chimneys, vents, walls, etc.) in order to completely enclose the soil and vegetation.

High Quality Waterproofing

One of the most important aspects of an effective green roof is maintaining a waterproof seal. While there are several types of membranes that can make up the waterproof layer, built-up systems that use bituminous materials are the most common. Another system seals overlapping rolls of synthetic materials, such as poly(vinyl chloride) PVC, rubber (EPDM), hypolan (CSPE) or thermoplastic polyolifins, together in a single-ply membrane. Hot or cold liquid systems can be sprayed or painted onto the roofing deck to create a joint free seal. Overlying soils and vegetation can extend the life of these waterproofing systems by protecting them from damaging ultraviolet light and extreme temperature fluctuations.

Irrigation

Proper green roof design and plant selection should alleviate or limit irrigation needs to new vegetation establishment and/ or prolonged periods of drought. Typical irrigation methods include: surface spray - hoses and sprinkler heads, drip and tube - subgrade tubes deliver water directly to the root zone, capillary - mats hold water under the root zone for plants to take up, and standing water - water is captured from large storms and held for future use.

Leak Detection System

Roof leaks are a hazard to any roofing system; however, locating damaged waterproofing under several layers of a green roof can prove to be very difficult and costly. Electric field vector mapping (EFVM) is a relatively inexpensive leak detection system that charges the planting medium with electricity and looks for grounds, where moisture contacts the metal or concrete roof deck. Modular systems that use trays can easily be removed to fix damaged waterproofing, but locating the source of the leak is still difficult without a leak detection system.

Links

• Wikipedia
• Green Roofs for Healthy Cities
• Center for Green Roof Research (Penn State)

Infiltration Trenches

• Rainfall
• Storage
• Infiltration

• Rainfall
• Storage
• Infiltration

• Rainfall
• Storage
• Infiltration

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Design Considerations

Pretreatment Needs

To ensure the long term effectiveness of infiltration trenches, pretreatment should be used to prevent the facility from clogging. Pretreatment is most effective when multiple practices are used in series. These practices include vegetated filter strips, grass swales, grit chambers, sedimentation basins and sediment traps or forebays. The pretreatment system should be designed to remove 25-30% percent of sediment loads. Accessibility to accommodate periodic maintenance is a critical design factor for pretreatment systems.

Contributing Drainage Area

Typically, infiltration trenches are designed for small sites (e.g. five acres or less) but can be applied to larger areas if designed properly. Consideration should be given to the slopes of the contributing drainage area.

Ponding Area

The ponding area provides temporary runoff storage prior to infiltration, filtration, or evaporation. Ponding depths vary with size, treatment area and infiltration capacity of insitu soils but should be able to drain before the next storm event. Drain times between 6 and 72 hours ensure satisfactory pollutant removal and further capacity for the next storm.

Observation Well

A perforated pipe should be installed in the infiltration trench to monitor water levels and drawdown time. The pipe should be flush with the bottom of the trench and should be anchored by a foot plate. The top of the well should be capped and locked.

Storage Area

Use 1-3 inch washed stone aggregate that is open-graded and of a narrow size range so that voids between aggregates are not filled by smaller particles, thus contributing to lower efficiencies or clogging. Filter fabric on the sides and top of the trench help prevent surrounding soil from clogging the facility. The transmissivity of the filter fabric should be considered (e.g. no less than 100 gpm). An optional layer of pea gravel on top of the filter fabric at the top of the trench, can maximize sediment and pollutant removal and easily be replaced if the facility starts to clog. Typical trench depths range from 3’-12’.

Bypass and Outlets

Infiltration trenches should be designed with a bypass to direct excess flow away from the trench to appropriate locations downstream. This can be done overland or in storm pipes, but should minimize concentrated erosive flow.

Insitu Soil

Soils may be the most important factor in infiltration trench site suitability. To verify existing soils information, perform 1-3 soil borings in the location of the proposed infiltration practice to confirm permeable soils, and depth to the seasonally high water table, bedrock or other impeding layer. Soil borings should be performed to a minimum 5’ below the bottom of the proposed infiltration trench. Soils should have low clay and silt content and have infiltration rates greater than 0.5 in/hr.

Depth to Water Table and/or Bedrock

Provide a minimum 3’ distance below the bottom of the practice and the water table or bedrock (Protecting Water Quality in Urban Areas: A Manual, Minnesota Pollution Control Agency. March 1, 2000). This separation is required to maintain groundwater quality and the hydraulic capacity of the practice.

Siting of Facility

Each site should be considered unique. Infiltration trenches should be strategically located to collect sheet flow from impervious surfaces such as roads, sidewalks, down spouts, etc. Generally, water that enters as sheet flow should be perpendicular to the main axis of the trench. Water entering as channel flow should be parallel to the main axis and direction of flow. Trenches should be located at least 100 feet upgradient from private wells (greater for public wells), 100 feet from septic fields, 25 feet from building foundations and 100 feet from surface waters.

Pervious Pavement

• Rainfall
• Storage
• Infiltration

• Rainfall
• Storage
• Infiltration

• Rainfall
• Storage
• Infiltration

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Design Considerations

Contributing Drainage Area

Offsite runoff onto pervious pavement should be limited to impervious surfaces. The ratio of contributing impervious area to the pervious paver surface should not exceed 3:1. If runoff is coming from adjacent pervious areas, it is important that the area be fully stabilized.

Siting of Facility

Each site should be considered unique. Pervious pavement systems are typically used in low-traffic areas such as: parking pads in parking lots, overflow parking areas, residential driveways, residential street parking lanes, recreational trails and emergency vehicle and fire access lanes. These systems are not recommended on sites with a slope greater than 2%.

Load Bearing Surface

An appropriate modular porous paver should be selected for the intended application. If it is a load bearing surface, then the pavers should be able to support the maximum load.

Insitu Soil

The surface of the insitu soils should be lined with filter fabric or an 8” layer of sand. It should be graded so it is completely flat to promote infiltration across the entire surface. Insitu soils should have field-verified, minimum permeability rates of greater than 0.5 inches per hour to a depth of 3 feet below the bottom of the stone reservoir. During construction and preparation of the subgrade, special care must be taken to avoid compaction of the soils.

Pervious Paver Infill

The pervious paver infill selection is based upon the intended application and required infiltration rate. Masonry sand (or equal) has high infiltration rates and should be used where no vegetation is desired. A sandy loam soil has a substantially lower infiltration rate, but will provide a growth medium for vegetative cover.

Filter Layer

The layer of filter fabric is located below the pervious paver infill and the above the storage layer: 1) Pervious pavers 2) Pervious paver infill 3) Filter fabric 4) Base course/ storage layer.

Base Course/Storage Layer

Regulatory requirements will determine the design storage volume. The stone aggregate used should be washed, bank-run gravel, 1.5 to 2.5 inches in diameter with a void space of approximately 40%. The gravel base course must have a minimum depth of 9 inches. The system should be constructed to infiltrate the design storm within a maximum of 48 hours (24 recommended).

Underdrains and Overflows

Underdrains are used to provide overflow drainage for low permeable subgrades and/or for storms exceeding the design storm. If the in- situ soils exhibit low permeability, the underdrain may be located in the gravel bed and the pervious pavement system will operate as a biofiltration practice. Designs need some method to convey larger storms to the storm drain system. One option is to set storm drain inlets slightly above the surface elevation of the pavement. This allows for temporary ponding above the surface if the surface clogs, but bypasses larger flows that are too large to be treated by the system.

Depth to Water Table and/or Bedrock

Pervious pavement systems should have a minimum of 3 feet between the bottom of the practice and the seasonally high water table or bedrock. This separation is required to maintain groundwater quality and the hydraulic capacity of the practice.

Rain Gardens

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• Storage
• Infiltration

• Rainfall
• Storage
• Infiltration

• Rainfall
• Storage
• Infiltration

Rain gardens are a good solution to treat stormwater that runs off of impervious areas. They help to reduce the amount of runoff leaving the site, they increase the water quality of that which remains, and they provide an attractive landscaping element. Pictured above are the rain gardens which were installed at the Hugo city hall in 2003.

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Design Considerations

Contributing Drainage Area

Typically raingardens are designed for small sites (e.g. five acres or less) but they can be applied to larger areas if designed properly (e.g. incorporating multiple raingardens in a series).

Pretreatment Needs

A grass buffer strip should be provided adjacent to the raingarden to reduce runoff velocity and filter out particulates before they reach the raingarden. Slopes should not exceed 3:1 for erosion protection and maintenance purposes.

Organic Mulch Layer

The organic mulch layer serves a number of functions: protects the plant bed from erosion; retains moisture in the plant root zone; provides a medium for biological growth and decomposition of organic matter; and filters pollutants.

Ponding Area

The ponding area provides temporary runoff storage prior to infiltration, filtration, evaporation, or transpiration. Ponding depth is a factor of the desired treatment volume, the area, the infiltration capacity of the insitu soils and the landscaping/planting plan. Typically ponding depths range from 6” to 3’ and should be designed to drain within 72 hours. Outlets and underdrains can be used to direct excess flow away from the raingarden.

Planting Soil Bed

For ideal infiltration, filtration and healthy plant growth the growing medium should consist of a blend of organic matter (20%), sandy soil (50%) and top soil (30%). Some clay is desirable, because clay particles adsorb heavy metals, hydrocarbons, and other pollutants. However, the clay content should not exceed 10% of the medium. To maintain infiltration rates of 0.5-3.0 in/hr native soils with higher clay content should be amended with imported sandy soil. Soils lacking organic matter should be amended with compost. A soil pH of 5.5 to 6.5 is ideal for pollutant removal by microbial activity. Depth of planting soil in Raingardens with an amended sand/gravel bed should be a minimum of 2’.

Optional Sand or Gravel Bed

A sand or gravel bed below the raingarden area should be provided to aerate and drain the planting soil depending on the permeability of the underlying material. If the insitu soils exhibit low permeability, the underdrain may be located in the gravel bed and the raingarden will operate as a biofiltration practice.

Outlets and Underdrains

Raingarden(s) should be designed with an emergency overflow to direct excess flow away from the raingarden and prevent undesirable inundation/flooding. An underdrain is a perforated pipe installed in the gravel bed that collects filtered runoff and directs it toward an approved location, such as a drainage swale or an existing drainage system. The use of an underdrain results in a raingarden that will operate as a biofiltration practice.

Depth to Water Table and/or Bedrock

A minimum of 3 feet should be provided from the bottom of the practice to the seasonally high water table or bedrock. This separation is required to maintain groundwater quality and the hydraulic capacity of the practice.

Insitu Soil

Raingardens can be applied to most situations. In some cases, runoff percolates through an engineered soil bed and is returned to the stormwater system via an under drain (biofiltration). In other cases, runoff percolates through the engineered soil bed down into the naturally permeable underlying material (bioretention). In this case, the raingarden acts as an infiltration practice. For more information on the infiltration capacity of soils, see the information provided for infiltration basins and/or trenches.

Siting of Facility

Each site should be considered unique. Microclimates (light, temperature and wind) and the size of the drainage area will influence the size of the raingarden and plants selection process. Raingardens should be strategically located to receive waters from impervious surfaces (sidewalks, driveways, rain gutters etc.) Raingardens should be located a minimum of 10 to 50 feet from existing structures at a minimum 1% slope in order to keep water away from foundations.

Vegetated Swales

• Rainfall
• Storage
• Infiltration

• Rainfall
• Storage
• Infiltration

• Rainfall
• Storage
• Infiltration

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Design Considerations

Swale Types

Vegetated swales can be applied to most situations. In dry swales, runoff percolates through highly permeable soils or an engineered soil bed. If the underlying soils are permeable, the swale can be designed for volume control (infiltration). If the underlying soils are less permeable, runoff can be returned to the stormwater system via an under drain (biofiltration). Wet swales are used in areas that intersect groundwater levels or in soils that have low percolation rates. Water is stored in shallow pools to settle pollutants and allow vegetation to treat runoff.

Contributing Drainage Area

Typically vegetated swales are designed for small sites (e.g. five acres or less) but they can be applied to larger areas if designed properly (e.g. using vegetated swales in conjunction with other best management practices).

Pretreatment

Depending on the function of the vegetated swale, they can either serve as pretreatment for another BMP or they may require pretreatment. If the swale is designed to retain and infiltrate the runoff generated for smaller rainfall events, consideration should be given to the pretreatment of runoff before it enters the swale (e.g. provide a filter strip upstream of the swale or situate the swale downstream of a water quality pond).

Residence Time

While vegetated swales are typically designed to treat runoff from small storms, they can also be designed to provide volume control if situated in permeable material. By increasing the residence time (the period it takes runoff to travel from one end of the swale to the other) of a dry swale, the practice will be more effective in removing pollutants and infiltrating runoff. Designing the swale with check dams promotes additional storage and infiltration while reducing flow velocities.

Geometry

The main components of a swale’s geometry include: length and longitudinal slope of the swale (design with minimal to no longitudinal slope not to exceed 5 percent), side slopes (minimum 1:3), shape of its cross-section and roughness created by vegetation. The Manning’s equation is a useful tool in determining the swale’s dimensions; however, each swale is unique and all aspects of its geometry should be fitted to site conditions and goals. The geometry of the swale should allow larger storms to safely pass through without scouring or erosion.

Vegitation

Vegetation stabilizes soil, filters pollutants and slows runoff flows increasing residence time. Selected plants should provide dense cover with a root structure that resists erosion and must be able to survive in local soil and microclimate conditions. Dry swale plants must be able to handle periodic inundation while wet swale vegetation may be inundated for longer periods of time. Wetland vegetation is often used in wet swales.

Optional Sand or Gravel Bed

A sand or gravel bed below the vegetated swale should be provided to aerate and drain the planting soil depending on the permeability of the underlying material. If the insitu soils exhibit low permeability, the underdrain may be located in the gravel bed and the vegetated swale will operate as a biofiltration practice.

Outlets/Emergency Overflows

Vegetated swales should be designed to handle larger storms without scouring and erosion. An emergency overflow should be provided to direct excess flow away from the swale and prevent undesirable inundation/ flooding. The overflow should be designed with an energy dissipater to prevent scouring.

Optional Underdrains

An underdrain is a perforated pipe installed in the gravel bed that collects filtered runoff and directs it toward an approved location, such as an existing drainage system.

Depth to Water Table/Bedrock

Dry swales should have a minimum of 2 feet between the bottom of the practice and the seasonally high water table or bedrock. This separation is required to maintain groundwater quality and the hydraulic capacity of the practice. Wet swales may intersect the water table in areas where there is no potential to transport pollutants.

Siting of Facitity

Each site should be considered unique. Vegetated swales should be strategically located to receive runoff from impervious surfaces (sidewalks, roads, parking lots) along the entire length of the swale. If water entering the practice is concentrated, erosion protection (e.g. rip-rap) should be used to dissipate energy and spread the flow across the width of the swale. Vegetated swales are not recommended in areas with steep slopes, sandy soils, concentrated flows, or any other factors that could erode the swale’s banks.

Cold Climates

Swales may be used for snow storage if the vegetation is salt tolerant and the following issues are taken into consideration: swale is located a minimum of three feet above the water table, the swale is located on flat, well-drained soils, and the snow does not contain unusually high concentrations of chlorine.

Other BMPs

There are many more stormwater Best Management Practices than those detailed in these pages. Below are listed a few more BMPs including sample designs if you are thinking of applying them in a project.

Detention Pond

• Design Details
• Outlet Structure Design

Infiltration Basin

• Design Details

Subsurface Infiltration

• Design Details

Pond

• Design Details

Shallow Wetland

• Design Details

Surface Sand Filter

• Design Details

Perimeter Sand Filter

• Design Details

Underground Sand Filter

• Design Details