Green Technologies for Managing Stormwater

Green Houses Green stormwater management technologies, designed specifically for managing excess stormwater, have proven to be effective in reducing infrastructure costs and protecting hydrological systems. In urban landscapes, these technologies can be integrated into site design and assist with stormwater management at the site-level.

Learn more about the challenges of managing stormwater, and how green technologies are helping.

Stormwater Management

Pipes Stormwater management generally involves the collection, transportation, and treatment of rainfall and snow runoff in urban areas. Physical infrastructure (pipes and pumps) is the most common technique for managing stormwater. Throughout most of the developed world, ‘piping, pumping and treating' stormwater has proven to be successful in keeping communities flood-free and sanitary.

However, as urban residential, commercial and industrial areas continue to expand into surrounding landscapes, the amount of impervious surfaces (paved roads, driveways, parking lots, roofs, etc.) in our watersheds increases. This increase can result in higher peak flow rates and greater volumes of stormwater runoff because impervious surfaces don't allow water to be absorbed by the ground. Without substantial, expensive infrastructure upgrades, our existing pipes and pumps won't have the capacity to effectively manage our future stormwater needs.

Stormwater Pathways

Conventional Stormwater Infrastructure

Canadian municipalities spend $12 - $15 billion annually on infrastructure and Canada's total municipal infrastructure debt has been estimated at $57 billion (van der Roeg, 2003).Conventional infrastructure for stormwater is expensive and it can negatively affect the environment.

Conventional stormwater systems focus on quickly moving water away from urban areas during extreme storms. The fast moving water can lead to erosion and downstream flooding in receiving streams, high nutrient and sediment loadings, lower dissolved oxygen availability, higher temperatures, and reduced biodiversity. This type of event can permanently change the natural hydrologic functions of a landscape, which can affect water-dependent species and sometimes result in property damage.

Pollutants in Stormwater

Stormwater picks up pollutants that are harmful to aquatic life. Major sources of urban pollutants that contribute to these impacts include:

Atmospheric deposition: Fine particles, metals, pesticides;
Landscape maintenance: Pesticides and herbicides;
Litter and leaf fall: Debris, humicorganics, metals;
Waste from urban wildlife and pets: Bacteria;
Vehicles: Fuels, brake/tire wear, fine particles, metals;
Pavement maintenance: Heat, petroleum derivatives;
Pavement de-icing: Salts, chlorides, sulfates, coarse sediments;
Industrial and commercial business: Vehicle fluids, organic and inorganic waste, metals, bacteria;
Construction: Sediments, high pH from concrete, petroleum products from fresh asphalt, organics.

These pollutants can have both acute and cumulative effects. Acute effects are observed immediately and tend to taper off over time. In contrast, cumulative effects multiple over time and incrementally affect the environment as pollutants build up in a watershed. Both acute and cumulative effects should be considered in stormwater management.

Cumulative Effects

Green Stormwater Management Technologies

Green stormwater management technologies, designed specifically for managing excess stormwater, have proven to be effective in reducing infrastructure costs and protecting hydrological systems. In urban landscapes, these technologies can be integrated into site design and assist with stormwater management at the site-level, which is a preferred option for managing peak flows and runoff volume. Because green technologies mimic natural ecological functions, they have the capacity to dramatically increase water productivity in a watershed. When functioning properly these technologies can increase the efficiency of existing water infrastructure networks.

Stormwater Management Pyramid

Collective Benefits of Green Technologies

Improved urban stormwater quality by utilizing the natural processes of vegetation to filter stormwater and take up harmful pollutants;
Opportunities to use and re-use water on-site, improving groundwater recharge and eliminating expensive 'pipes and pumps';
Water quality and quantity issues are managed on-site, reducing stress on existing, aging wastewater and stromwater infrastructure.

Some of the more common green technologies being developed and applied in urban stormwater management include green roofs, constructed wetlands, specific vegetation, and trenches for infiltration.

Green Technology Examples

These are some of the available "Green Technologies" for improving the management of stormwater.

Green Roofs

Green House A green roof uses vegetation as part of the roofing system. It can be installed on flat and some sloped roofs, replacing the traditional shingles. It consists of a waterproof, root-repellent membrane, drainage system, filter cloth, lightweight growing medium, and vegetation ranging from grasses and plants to trees.

There are basically two types of green roof systems: extensive and intensive. Extensive green roofs have low weight, low capital cost, and minimal maintenance. Intensive green roofs have a greater soil depth and more plantings with higher maintenance requirements.

Studies in Berlin have shown that green roofs can absorb 75% of the precipitation that falls on them (CMHC, 2007).Benefits: A significant benefit of green roofs is their ability to absorb precipitation, slowing down runoff rates. This function reduces the volume of runoff that conventional municipal stormwater systems must manage, which reduces the risk of cumulative impacts of stormwater downstream.

Challenges: In Europe green roofs are well established, which is the result of government legislative and financial support. In North America, however, the benefits of green roof technologies are poorly understood and the market remains immature, despite the efforts of several industry leaders.

Major challenges in North America include:

Lack of knowledge and awareness of the systems and associated cost benefits;
Lack of incentives to facilitate implementation;
Lack of specialized products, few Canadian installations, and no technical standards.

For more information on Green Roof Technology see:

Constructed Wetlands

Constructed wetlands are shallow pools developed specifically for stormwater or wastewater treatment with growing conditions suitable for wetland plants. Constructed wetlands improve water quality by minimizing point source and non-point source pollution before it enters streams, natural wetlands, and other receiving waters. They can also play a water quantity management role.

There are two basic types of constructed wetlands:

Subsurface systems have no visible standing water and are designed so that the wastewater flows through a gravel substrate, or layer, beneath the surface vegetation;
Surface flow systems have standing water at the surface and are more suited to larger constructed wetland systems such as those designed for municipal wastewater treatment.

Benefits: Constructed wetlands are large vegetated pervious areas, which collect and absorb stormwater providing more time for infiltration into the water table. Depending on the size of the constructed wetland, it may provide modest reductions in overall runoff quantity.

The vegetation, soils, microbial species, and physical properties (slope, aspect, climate) of a constructed wetland all work together to remove larger sediment and pollutants, thus improving runoff quality.

Challenges: It is difficult to establish wetlands at sites with sandy soils or other soils with high infiltration rates. A careful review of local climate and water table conditions should be conducted before choosing a constructed wetland as a supplemental stormwater management tool. Although constructed wetland design can be adapted for most regions of the country, they are less effective in areas that are excessively arid.

Seasonal climatic fluctuations may cause contaminants to remobilize in anaerobic conditions under ice. Finer sediments may cause ice to be thin or unstable, which can be a safety hazard.

The standing water in constructed wetlands may contribute to thermal pollution and contribute to downstream warming. This may preclude the use of constructed wetlands in areas where sensitive aquatic species live.

Successful constructed wetlands take time to develop. Regular inspection to monitor hydrologic conditions and ensure aquatic, shoreline, and upland plants are surviving is critical. Wetland operators may need to control nuisance insects, odours, and algae, which can be expensive.

Constructed wetlands also require large areas of land.

For more information on Constructed Wetlands in the Bow River Basin see:

http://www.riparia.ca/projects/elbow_valley.htm

Vegetative Practices

Vegetation is often used to slow runoff and help stormwater infiltrate the soil and settle particulates before entering another stage of treatment. Two frequently used vegetative measures, filter strips and grassed swales, are examples of this approach.

Filter strips help control stormwater quality. They are bands of close-growing vegetation, usually grass, planted between upland runoff areas and receiving water or channel. Filter strips can include shrubs or woody plants, which help stabilize the grass strip. They are often used as pre-treatment devices for other stormwater control practices such as infiltration basins and trenches. Such strips are used primarily in residential areas around streams or ponds, where runoff does not tend to be heavily polluted and an additional level of quality control is desired.

Grassed swales are shallow earthen channels covered with a dense growth of a hardy grass. In a residential setting, swales look like an extension of a front lawn, and can be used as alternatives to curb and gutter stormwater systems. This method, again, usually provides pre-treatment before runoff discharges to treatment systems and improves initial water quality.

Benefits: Filter strips and swales reduce stormwater pollution by filtering pollutants such as sediment, organic matter, and trace metals by the filtering action of vegetation, sediment deposition, and infiltration into the soil. However, they are less effective at removing soluble pollutants.

Filter strips and swales also reduce the quantity of stormwater runoff by slowing flows and providing an area for increased infiltration. In addition, these vegetative practices increase groundwater re-charge, an important consideration in urban areas facing groundwater supply concerns.

Grassed swales also help prevent erosion through the presence of stabilizing root systems and vegetation that slows runoff rates.

Vegetative practices are relatively inexpensive to establish and maintain. Grassed swales might be even more economical than curb and gutter drainage systems. Filter strips can be utilized in urban settings for treating rooftop runoff and runoff from lawns and other pervious areas from small properties.

Challenges: Vegetative practices are inappropriate for controlling runoff from larger facilities such as commercial or industrial establishments and are usually insufficient to manage runoff from substantial rainfall events.

Filter strips and swales are less effective in regions with sandy soils because sandy soils contribute to collapse of swale walls.

Filter strips and swales require repair and maintenance including periodic inspection, mowing at least twice each year, fertilizer application, and repair and reseeding of washed out areas and bare spots. Inspections may also show the importance of removing dead vegetation. Inadequate maintenance will result in functional failures.

For more information on filter stripes and bioswales see:

http://www.crd.bc.ca/watersheds/lid/swales.htm

Infiltration Trenches

Infiltration trenches are primarily designed for stormwater quality management and generally provide little or no stormwater quantity management.

A conventional infiltration trench is a shallow, excavated trench that has been backfilled with stone to create a narrow underground reservoir. Stormwater runoff diverted into the trench drains from the bottom of the trench into the subsoil and eventually to the water table.

Benefits: Infiltration trenches can improve the quality of stormwater runoff. A properly maintained trench can remove both particulate and soluble pollutants. Vegetation and soils perform biological and chemical conversion, which can effectively remove sediment, phosphorus, nitrogen, trace metals, coliforms, and organic matter.

Infiltration trenches take up little land and can be located on or close to residential sites or clusters of sites. Smaller infiltration devices such as infiltration basins and dry wells are aptly suited to manage stormwater quantity from roofs or other surfaces.

Infiltration trenches help provide groundwater recharge, control peak stormwater flows, and protect against erosion. A significant advantage of these trenches is that, in areas with a high percentage of impervious surface, infiltration is one of the few means to provide significant groundwater recharge.

Challenges: It is critical that infiltration devices only be used where the soil is porous and can absorb the required quality of stormwater. In areas where runoff is polluted, about 1 metre of clearance above the water table is recommended to help prevent groundwater pollution.

Maintenance requirements include regular inspections, cleaning of inlets to prevent clogging, mowing, and inspection of observation wells to maintain proper operation. Insects, odours, and soggy ground can arise as nuisances.

Rates of pollutant removal depend on the type of soil (sandy soils are less effective at removing nitrates and trace metals than less porous soils). Particulates may also be trapped when an infiltration trench is used in conjunction with other treatment systems (i.e., filter strips, urban forestry, etc.).

For more information on infiltration trenches see:

Sources:

Canadian Mortage and Housing Corporation (CMHC). (2007). Alternative Stormwater Management Practices for Residential Projects.

van der Roeg, C. (2003). A Capital Question - Infrastructure in Western Canada's Big Six. Western Cities Project Report #27, Canada West Foundation, Calgary, AB.

van Duin, B. (2006). Thoughts on Stormwater Quality. WCWWA Stormwater Management Seminar - Stormwater Quality. Powerpoint Presentation. Edmonton, AB. February 22, 2006.