How to Calculate the Environmental and Economic Benefits of your Bioswales and Rain Gardens

Cities are increasingly turning to green stormwater infrastructure (GSI) to reduce runoff.

Cities are increasingly turning to green stormwater infrastructure (GSI) to reduce runoff, which helps maintain water quality and reduce flooding and erosion.

Stormwater runoff is a major cause of water pollution in cities. When rain falls on roofs, streets and parking lots, the water is not all absorbed into the ground. Instead, stormwater drains through gutters, storm sewers and other collection systems, and is then released into nearby bodies of water. This water often contains pollutants like trash, bacteria, chemicals and heavy metals. And, the quantity of water and speed of flow can cause erosion and flooding, and damage habitat, property and infrastructure.

Cities worldwide are looking to improve water quality and invest in strategies to mitigate stormwater runoff. Historically, cities have managed stormwater through the use of expansive and capital-intensive underground storm sewer systems known as gray infrastructure. However, use of green infrastructure, an approach to water management that protects, restores or mimics the natural water cycle, has become increasingly popular. Unlike gray stormwater infrastructure which is designed to move stormwater away from the built environment as quickly as possible, green infrastructure reduces and treats stormwater at its source. Strategies for incorporating green infrastructure at the site-level include the use of permeable pavements, bioswales, rain gardens, vegetated or “green” roofs, rain barrels, and cisterns.

Bioswale in a parking lot.

The control of stormwater runoff is a major issue in urbanized areas. Bioswales and other green stormwater infrastructure helps reduce runoff. (Source: Rhyne Landscape)

Green infrastructure provides communities with economic, environmental and community benefits yet not all benefits are easily quantified. Understanding the benefits of green infrastructure, and comparing them to their gray infrastructure counterparts requires a common unit of analysis. While it is fairly straightforward to place a monetary value on gray infrastructure, it can be more difficult to do so for green infrastructure. However, looking only at construction and maintenance costs – planning, installation, operation and replacement – ignores the environmental, economic and social benefits green infrastructure provides. In fact, research has found that when you take into account the water quality, air quality, energy and community benefits provided by green infrastructure green approaches are shown to outperform single-purpose gray approaches.

A report from the Center for Neighborhood Technology (CNT) outlines the benefits of five different types of green infrastructure features. The environmental and social benefits accrue at varying scales based on local factors like climate and population.

This diagram outlines the benefits of five different types of green infrastructure features. The environmental and social benefits accrue at varying scales based on local factors like climate and population. (Source: Center for Neighborhood Technology)

With OpenTreeMap you can calculate ecosystem services for two different types of green infrastructure features: rain gardens and bioswales. Once you denote the area of your rain garden or bioswale on OpenTreeMap, the cloud-based platform automatically calculates the total amount of stormwater diverted. The equation used to calculate stormwater diverted was developed by San Francisco, California-based non-profit organization, Friends of the Urban Forest (FUF). OpenTreeMap then converts the amount of stormwater diverted into dollars saved based on the conversion rates used in the U.S. Forest Services’ iTree application.

Annual stormwater diverted = Annual rainfall x (area of polygon + (square footage of area that drains to the rain garden x runoff coefficient))

The runoff coefficient refers to the percentage of rainfall that hits the drainage area that makes it to the rain garden or bioswale. Some portion of rainwater may runoff too quickly to be absorbed by green infrastructure or may be diverted in some other way. In OpenTreeMap, the default coefficient is 0.85, meaning 85% of the rain that hits the area that drains to the rain garden or bioswale will actually make it to the rain garden or bioswale. This coefficient is an average estimate based on data from Ken Edwards, Ph.D., P.E., owner of LMNO Engineering, however, the exact percentage of runoff varies based largely on the type of material in the drainage area (asphalt, brick, cement etc).

To improve the accuracy of the calculation, map owners can customize the units and number of decimal places for annual rainfall, the area of the green infrastructure feature and the area of the adjacent drainage area.

OpenTreeMap software.

With OpenTreeMap, you can map and manage your green stormwater infrastructure along your trees.

Calculating the ecosystem benefits associated with green infrastructure helps planners calculate the savings green infrastructure provides in terms of stormwater filtration, floodwater storage, clean drinking water and carbon sequestration among other benefits. Shared metrics for quantifying ecosystem benefits are important for tracking the effectiveness of investment in green infrastructure and helps reposition the role of urban green space from an optional amenity to key ingredient for building more sustainable communities.

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