Traditionally, the goal of stormwater management and drainage engineering has been to convey flow away from a site as quickly as possible. Over time, the increase in impervious surfaces associated with urbanization created severe flooding and water quality issues. Planners, engineers, and scientists needed to figure out a way to design systems that provided adequate site drainage in a way that minimized the impact on local waterways. To address these issues, the United States Federal Government enacted the Clean Water Act (CWA) in 1972. Section 402 of the CWA regulates stormwater discharge from new development, industrial and commercial facilities, and municipalities through National Pollutant Discharge Elimination System (NPDES) permits. These permits require the use of stormwater management practices that reduce and treat stormwater runoff. Stormwater best management practices (BMPs) are often used to meet the requirements of these NPDES permits. Structural stormwater BMPs, which are often referred to as green infrastructure, are typically landscaped or vegetated areas that are designed to capture and treat stormwater runoff. Over time, people began to realize that these structures, if properly designed, can be very effective in minimizing flooding and reducing soil erosion. For this reason, stormwater engineers have begun to focus on finding nature-based solutions to urban drainage issues. According to the Federal Emergency Management Agency (FEMA), nature-based solutions are sustainable planning, design, environmental management, and engineering practices that weave natural features or processes into the built environment to promote adaptation and resilience. Unlike traditional “gray” infrastructure, nature-based solutions/green infrastructure can provide many benefits in addition to mitigating flood risk. The following blog post will explore some of these benefits.
1. Potential to Build Flood Resilience and Drought Resilience At The Same Time
Nature-based stormwater management solutions can potentially build both flood resilience and drought resilience at the same time. Nature-based solutions build flood resilience by allowing water to infiltrate into the soil (making the ground act more like a sponge) rather than running off into a storm drain. Green infrastructure can build drought resilience by incorporating native and/or climate-appropriate plantings into stormwater BMPs. These structures require less irrigation, which minimizes the strain on the municipal water supply. Nature-based stormwater management can also help build drought resilience by promoting other sustainable water management practices such as rainwater harvesting, groundwater recharge, and water reuse/recycling. These practices allow us to store water from the wet periods to use during the dry periods.
2. Improved Soil Health
Soil degradation is a huge problem in the United States of America – in both agricultural settings and urban areas. This is because soil health affects multiple aspects of the environment, including drainage and stormwater management. Healthy soil is probably the most effective flood infrastructure. Hugh Hammond Bennet, the first Chief of the Soil Conservation Service (now the NRCS) and the “Father of Soil Conservation” said, “every additional gallon of water that can be stored in the soil through the use of conservation measures means one gallon less contributed to flood flows.” The soil in urban areas tends to be compacted due to construction and heavy foot traffic. Compaction adversely impacts soil structure because it reduces pore space. As a result, there is less air space for water to move through the soil profile to facilitate drainage. The good news is that we can improve soil health by implementing practices that build soil organic matter (SOM). This is important because SOM has a high water-holding capacity and attracts earthworms and other creatures that move through the soil to improve soil structure. Applying compost and mulch to your landscaped areas is one way to build SOM. Unfortunately, it does take a long time for vegetation and leaves to break down and transform into SOM. Nonetheless, it is worthwhile as building healthy soil makes the ground more like a sponge that absorbs water and less like a parking lot that allows water to run off into the storm drain and/or local waterway.
3. Improved Water Quality
For a long time, we have known that nature-based stormwater management can improve water quality. There are numerous studies that have shown that structural stormwater best management practices (BMPs) remove pollutants that are common in urban settings. Such pollutants include nitrogen, phosphorus, metals, bacteria, and sediment. Stormwater BMPs improve water quality through filtration, reducing erosion, and infiltration. Each of these mechanisms is discussed in further detail below.
Firstly, nature-based stormwater practices filter pollutants from stormwater runoff. The vegetation, soil, and other natural materials act as a filter as water moves through them. In addition, large particles settle out on the soil surface. For example, a study conducted in the Bay Area of California evaluated rain gardens in 12 watersheds and found that the rain gardens successfully removed an average of 96% of anthropogenic debris and 100% of black rubbery fragments. These results suggest that rain gardens and other nature-based stormwater management practices could serve as effective mitigation strategies for removing microplastic pollution from stormwater runoff (Werbowski et al., 2021).
As previously mentioned, nature-based stormwater management reduces the velocity of stormwater runoff. In this way, green infrastructure improves water quality by minimizing erosion. Erosive flows carry a significant amount of sediment with them into nearby streams and waterways. By slowing down the flow of water, nature-based stormwater management reduces the amount of sediment that flows into nearby streams. This is important for stream health because excess sediment can cause waterways to become murky, which inhibits the ability of natural vegetation to grow.
Many nature-based solutions, such as permeable pavement and bioswales, promote the infiltration of stormwater runoff into the ground. This can help recharge groundwater and reduce the volume of water that enters stormwater systems and waterways. In this way, infiltration improves water quality by allowing the soil in a structural stormwater BMP to remove pollutants from water that flows through the soil profile rather than allowing that flow to carry pollutants to nearby waterways.
Finally, nature-based stormwater management practices improve water quality by decreasing water temperature by providing shade. In addition, buried pipes reduce the thermal load of water discharged from a stormwater BMP. Lower water temperature helps water quality because cooler water can hold more dissolved oxygen, which allows for better conditions for aquatic life.
4. Urban heat island reduction
Urban areas with large amounts of impervious surfaces, such as pavement and buildings, can create “heat islands” that raise the temperature of stormwater runoff which contributes to water pollution. This is because, in part, warmer waters create an environment that is conducive to the growth of some harmful algae and other microbes. Nature-based stormwater management practices can reduce the urban heat island effect by promoting evapotranspiration, shading, insulation, reflectivity, and wind movement. Evapotranspiration is sort of like the plant equivalent to sweat. Plants absorb water from the soil through their roots. When the water vapor pressure deficit of the surrounding air is lower than the water potential of the leaves, transpiration occurs. Transpiration requires heat from the surrounding air, which cools the area around transpiring vegetation. In addition, nature-based stormwater management reduces the urban heat island effect by providing shade, which reduces the amount of solar radiation that can reach the ground surface. Green spaces with several trees can also reduce the urban heat island effect by creating air movement that cools the surrounding area. Finally, some nature-based stormwater management practices, such as green roofs, have a high albedo/reflectivity, which means they reflect more sunlight than traditional roofs. This can help reduce the amount of solar radiation that is absorbed by buildings and other surfaces, thus reducing the heat island effect.
5. Wildlife habitat
Nature-based stormwater structures such as rain gardens can also provide habitat for local wildlife, especially if native plants are incorporated into the design. Plants that do well in rain gardens and bioswales can often serve as habitats for a variety of pollinators including bees, wasps, and butterflies. It is important to provide a habitat for pollinators because these insects are vital to food production. Forbes estimated that pollination from honey bees and flies is responsible for between $235 and $577 billion worth of food each year in the United States. Unfortunately, a combination of factors including climate change, drought, and logging has diminished the breeding places for pollinators such as monarch butterflies. The migratory monarch butterfly population has decreased by 85% in the last two decades. Adding native plants such as milkweed into your garden can provide habitat and food for these important insects.
6. Address Environmental Injustice
Historically, urban flooding has disproportionately impacted minority communities. This is because these communities are often located in low-lying areas. Low-income and minority communities are also more likely to be located in areas where a railroad embankment can cause drainage issues. The increasing interest in nature-based stormwater management is an opportunity to address this environmental injustice. For example, practitioners could use green infrastructure to address environmental injustice by prioritizing nature-based stormwater management practices in areas that have been subjected to environmental injustice.
In addition to addressing inequity in stormwater/drainage infrastructure, nature-based stormwater management can also help address inequitable access to green space. A study conducted by an economist at the Los Angeles Regional Water Quality Control Board evaluated how the amount of impervious area differed based on the predominant racial group in various neighborhoods throughout Los Angeles County. This analysis was performed using census tract data and geospatial information from the National Land Cover Database. Then populations of race were then divided into quintiles, with each quintile representing a 20th percentile of a population. This means the first quintile represents the lowest 20th percentile and the fifth quintile represents the highest 20th percentile. When the results of this study were shown in a graphical format, it became apparent that, on average, the percentage of impervious surfaces decreased as the white population increased. For predominantly black neighborhoods, impervious surface levels are about equal for the first three quintiles and then increase for neighborhoods with the highest percentages of black people. For the Hispanic population, as their numbers increase in neighborhoods, so do impervious surface levels. In summary, the results of this analysis demonstrate that access to green space is different for people of color. It is worth noting that the analysis also controlled for owner-occupancy, land use, population density, and income. Even when controlling for these factors, it is clear that the amount of impervious area decreases as the population of white people in a neighborhood increases. The good news is that nature-based stormwater management is a good tool for addressing the racial inequity associated with stormwater pollution. As we all know, green infrastructure not only addresses stormwater pollution problems but makes spaces more beautiful. These spaces can also enhance residents’ life by providing a place for recreation.
Link to the study: https://gispublic.waterboards.ca.gov/portal/apps/storymaps/stories/34dc4888ab3a45bfaa91ab748df4597a
7. Carbon sequestration
Carbon sequestration is the capture, removal, and storage of carbon dioxide (CO2) from the earth’s atmosphere. It should be noted that carbon sequestration differs from carbon storage. This is because carbon sequestration is a term that is focused on capturing and storing carbon in a form that cannot immediately be released. Carbon dioxide is a greenhouse gas, which means it traps heat near the surface of the earth and causes an increase in temperature. The earth naturally releases greenhouse gasses, which keep the planet warm enough to support life. However, human activities have increased the release of greenhouse gasses, which has had a detrimental effect on the health of the planet. This phenomenon is often referred to as climate change. Nature-based stormwater management facilitates carbon sequestration by capturing and storing carbon in the soil profile. A study conducted by Kavehei et al. (2019) found that carbon quickly accumulates in the top 5 cm of the soil profile and accumulates at a slower rate at greater depths. In addition to carbon sequestration, nature-based stormwater management reduces our carbon footprint by reducing the amount of energy required to treat stormwater runoff.
8. Community engagement
Nature-based solutions are a great opportunity to enlist the help of volunteers for planting and even maintaining green infrastructure. A community in San Francisco, California uses volunteer labor to help maintain bioretention cells. Signage contains a QR code that allows people to sign up to volunteer for maintenance duties. After completing a brief training program, volunteers are then responsible for maintaining a particular bioretention cell. This reduces cost for the City and promotes the education of the public. While it may seem unlikely that a program like this could attract enough volunteers to be effective, you would be shocked at how interested people are in helping with these types of projects. I am involved in the Ocean Friendly Gardens (OFG) program for the Surfrider Foundation. We do not have an issue with attracting volunteers to help with work days as long as the work day/event is adequately advertised.
9. More Cost Effective Than Grey Infrastructure
In contrast to nature-based stormwater management, grey infrastructure is a term that refers to traditional, man-made structures such as pipes, dams, levees, and concrete channels. Both nature-based solutions and grey infrastructure are necessary for different situations to address drainage issues. The cost of the materials used to build grey infrastructure (concrete, pipes, etc.) is much higher than the materials required to construct nature-based solutions (e.g., plants, mulch, etc.). The design cost associated with nature-based stormwater management is often lower than the design cost associated with grey infrastructure.
10. Opportunities for Creative Funding
While many people understand the importance of proper stormwater management, funding remains an issue for many municipalities. This is because urban stormwater management requires a significant amount of infrastructure that is largely hidden from the public eye (pipes). The good news is that because stormwater management practices can provide multiple benefits to communities, there is an opportunity to explore multiple funding avenues and develop partnerships for projects that will have a positive impact on a community. This is especially true for projects that incorporate nature-based stormwater management. Creative funding tends to work well for projects that are a hybrid of traditional gray infrastructure and nature-based stormwater management. For example, the LB-MUST project was able to secure funding from a variety of sources due to the multiple benefits it would provide to the community. In addition to enhancing water resiliency in Long Beach, this project included adding green space to an industrial area. A project that I am currently working on has received grant money from two sources because the park will provide stormwater treatment and attract tourists to the area.