Green infrastructure

Before we began using pipes, drains, pumps, and other "grey infrastructure" to manage stormwater, nature provided the “green infrastructure” to slow, filter, and move water to where it belonged. In forests and wetlands, water is still managed naturally. The foundation of this network is the soil. It is the drain, the pipe, the pump, and the water treatment plant all in one.

As we’ve continued to pave over our soils, however, demands on both natural and manmade stormwater management systems have increased. In cities the use of green infrastructure to alleviate strain on these systems has become a popular alternative to costly grey infrastructure expansion. These green infrastructure techniques rely heavily on the inherent and unique qualities of soil.

Certain soil properties determine how quickly and how much water can be infiltrate, or permeate, the ground—preventing flooding, and the overloading of streams and water treatment plants, as well as recharging groundwater supplies. The soil is also responsible for much of the filtering of contaminants in urban stormwater, which can otherwise lead to serious water quality issues.

Moreover, soil provides the nutrients and water holding capacity needed to support plants, which help prevent erosion, reduce runoff by using and storing water, clean and cool the air, and provide an aesthetic quality to urban spaces. Hence the soil is responsible both for reducing water quantity and improving water quality - the primary goals of stormwater management.

How nature manages water

Why is green infrastructure becoming so important now? Our urbanized areas have greatly expanded to accommodate growing populations. But this growth has come at the cost of forests, wetlands, and undisturbed soils capable of managing stormwater naturally. As a result, water movement in the city is much different from in a field or forest. This is primarily due to the permeability of the surface the rain is falling on.

How water moves in the forest

When it rains in a forest or on fields, the rainwater soaks into the soil. The soil then stores a portion of the water, while the remainder moves slowly to streams or into groundwater reserves.

Once water is in the soil, the speed of its flow slows down. After most storms, the water that soaks into the soil stays there and is taken up by plants. Even after heavier rains, most of the rain will stay in fine-textured and well-developed soils, or move very slowly through them. When heavier rains fall on coarsely textured or sandier soils, the water will move through the soil more quickly.

In either case, water is able to enter the soil because of the soil’s permeability. This movement of water through the soil is called “subsurface flow.” Subsurface flow moves rainwater to streams, lakes, and other surface waters. It can also move water to groundwater where it is stored for centuries. The storage of water in soil is a critical type of water storage, providing water for streams, plants, and people long after the rain has stopped.

If the water travels instead over surfaces such as city streets or bare hillsides, it can carry sediment, nutrients, and other contaminants with it. When this water enters healthy soil, the soil acts as a filter, removing sediment and many of the nutrients and contaminants the water has picked up.

Many of these particulates are absorbed onto soil surfaces where they can either be taken up by plants and soil organisms or incorporated into the soil, making them unavailable to water or living things. That filtered water that results then slowly and steadily recharges groundwater and river systems, helping to keep streams cool, clean, and consistently full for people, fish, and other organisms.

How water moves in cities

When it rains in a city, the story is very different. Much of the land in urban areas is covered by pavement or asphalt. Because rain can’t soak into the soil underneath, these covered areas are referred to as impermeable surfaces. As the amount of impermeable surface increases with urbanization, so too does the amount of runoff.

Even when urban soil is not covered by houses, stores, parking lots, or roads, it’s often compacted. Compaction reduces the pore space in the soil, which drastically slows the rate at which water can infiltrate, or percolate, into the soil. Because compacted soils only let minimal amounts of water percolate through, they act more like asphalt than functional soils.

When only limited places exist where water can infiltrate into the ground, stormwater moves over the ground instead. This process is called “overland flow.” During overland flow, water picks up speed and objects that get in its way, including trash, sediment, and other contaminants such as motor oil, nutrients, and metals.

Many cities have also increased the speed of overland flow and the amount of runoff because gray infrastructure has been designed to move water off streets as quickly as possible through gutters, storm drains, and sewer pipes.

So where does all that runoff go?

The fate of urban stormwater runoff

Urban stormwater that does not percolate into urban soils often flows directly into streams, lakes, or the ocean either by overland flow or through storm drains that discharge directly into natural waters.

Most municipalities also have combined sewer systems, in which stormwater may be carried via pipes to water treatment facilities. This ends in one of two ways: 1) municipalities expend money and resources to clean the stormwater, or 2) large volumes of stormwater, combined with normal sewage levels, overload treatment facilities. 

In the first case, dilute stormwater is energy-intensive to treat as wastewater plants are designed to treat more concentrated influent. The dilute stormwater reduces the operating efficiency of the plants and so wastes energy.

In the second case, large volumes of stormwater can overwhelm the capacity of a wastewater treatment plant, causing it to release a portion of the stormwater, combined with untreated sewage, into natural waters. This type of event is called a “combined sewer overflow”. Municipalities are allowed a certain number of these events each year. But regulations are also tightening due to concerns over water quality.

So, to summarize, if a soil has been compacted or paved over it will have low to zero permeability, preventing water from infiltrating into the soil. This results in larger volumes of water moving across the surface, which in turn causes flooding, water pollution, increased erosion, and decreased storage of water in the ground for later use. In other words, there are two major problems with the methods most municipalities use to manage stormwater: issues of water quantity and water quality. 

Impacts on water quantity

Urbanization can result both in too much water and too little water reaching streams.

In many cities, subsurface flow has nearly been eliminated as a result of so many paved or other impervious surfaces. So, instead of water gradually entering streams through subsurface flow, much more water from a storm enters streams quickly through overland flow.

Water flowing off streets directly into streams causes water levels to rise quickly, making streams much more likely to flood during heavy rains. In periods without rain, in contrast, streams have a tendency to fall below normal levels because of reduced subsurface, or soil, flow. This type of behavior results in streams being described as “flashy”—meaning their water levels rise and fall quickly.

Below are some of the negative impacts flooding can have on both people and wildlife.

Economic costs to communities and individuals from flooding can be high. Floods may risk human life, damage property, and wreak havoc on daily routines in urban areas—causing additional economic costs to people and businesses. Flooding in the United States alone costs about $2 billion per year. In a 2002 report, the U.S. Environmental Protection Agency estimated that stormwater controls would save $14 million dollars annually.

Increased sedimentation carried by floodwaters leads to shallower streams that are more prone to flooding. 

Stress to wildlife caused by changing stream dynamics reduces biodiversity in streams. In the figure at right, an increasing percentage of impervious area due to urbanization correlates with reduced biodiversity in streams. Sedimentation, for example, results in shallower, warmer water with less available oxygen, which in turn creates a stressful environment for aquatic life. Sediments may also carry contaminants, including metals and organic chemicals, that are harmful to fish and other organisms.

Stream shape changes caused by flooding make streams more prone to flooding in the future.

Impacts on water quality

Water washing off city streets, rooftops, eroded hillsides, and other urban surfaces will carry all sorts of things with it, including metals, dirt, and debris. Collectively termed contaminants, these materials are either dissolved in water or more commonly are attached to particles in water. Water-borne contaminants can be divided into a few basic categories.

• Plant nutrients, particularly nitrogen (N) and phosphorus (P)
• Metals, most commonly copper (Cu), zinc (Zn) and lead (Pb)
• Organic chemicals primarily related to gasoline products, but also pesticides and industrial products used in manufacturing and construction
• Pathogens from animal feces
• Trash and debris, especially plastics

A quick way to assess how contaminated water may be is by measuring the amount of total suspended solids. Total suspended solids is a measurement of how many particles are floating around in water. A turbidity meter is used to quantify the amount of particles in a water sample, based on how much light passes through the sample. Whether these contaminants are “bioavailable” (can be taken up by plants and animals) depends on the chemistry of the water and how the contaminants are attached to other particles.

Contaminants can have a negative impact on both humans and wildlife including:

Beach closures caused by contamination, particularly from pathogens, can result in loss of recreational opportunities and economic costs to businesses that depend on tourism.

Behavioral changes in fish and other harmful impacts on wildlife have been connected to metal exposure. For example, researchers in Washington have observed that metals are particularly harmful to spawning Coho salmon, and copper has been shown to inhibit salmons’ ability to detect predators.

Built up trash and debris can clog drains, threaten wildlife, and be aesthetically displeasing.

Eutrophication of water caused by excess nutrients may lead to suffocation of fish and other aquatic biota, as well as water unfit for recreation.

How soil can help

Soil can play an important role in removing suspended solids and dissolved contaminants before they reach natural waters. By slowing the movement of water down, soil gives sediments time to settle out and allows chemicals to adsorb to soil surfaces.

The amount of contamination in stormwater will depend on surrounding land uses as well as the frequency and duration of storm events. Runoff from an industrial site, for example, may carry more metals, whereas runoff from a golf course or residential area may contain more pathogens and pesticides.

So the question is... How do we get our cityscapes to manage water more like a forest?

Mimicking the soil with green infrastructure

Green stormwater infrastructure refers to the network of green spaces in our cities that collectively provide stormwater management, recreation, and wildlife habitat. Central to this concept is the intentional practice of using natural or nature-inspired systems to manage stormwater on site.

This method is quickly being recognized as a low-cost alternative to traditional engineered systems, often termed grey infrastructure. Unlike these traditional solutions, green infrastructure increases the permeability of our cityscapes so that they behave more like natural soils—slowing, holding, and purifying stormwater. By managing stormwater where it falls, runoff to natural waters and the burden on wastewater treatment facilities is also reduced. 

Types of green infrastructure

Green stormwater infrastructure includes permeable pavement, green roofs, urban forests, wetlands, and rain gardens. Green infrastructure can be engineered, as with a bioswale, or nature-made, such as a preserved wetland. Preserved, natural systems have the advantage of not requiring upfront construction and design costs. Depending on their size, they may also provide more recreational opportunities and be capable of processing larger quantities of water.

Manmade systems, on the other hand, can be customized to meet a particular need and may require less land area to be effective.

Central to the success of any individual system is that it’s interconnected with others to create a more powerful whole. This requires strategic planning involving many community stakeholders. To learn how communities around the country are expanding their green infrastructure through highway projects, land preservation, and urban renovations, visit the Conservation Fund’s Green Infrastructure Project Profiles.

The importance of soil

All these tools rely directly or indirectly on soil to achieve stormwater management strategies that are more affordable and effective than traditional grey infrastructure systems. While projects like urban forests rely on native soils, the soils used in green infrastructure systems, including rain gardens and bioswales, are specifically designed to infiltrate water quickly to reduce flooding. At the same time, these special soil mixtures must retain water long enough to remove contaminants and clean the water.

As the ratio of soil surface to rainwater is a lot smaller in cities than in natural systems, more is expected of these soils. For example, a bioswale will be designed to filter stormwater from a surrounding area much larger than the bioswale itself. Therefore these soils must be carefully designed both to manage stormwater quantity and quality.

When designing soil mixtures for bioswales, there are a few system requirements to keep in mind. Bioswale soils must:

• Infiltrate water quickly, but not too quickly. Water needs to travel slowly enough through the soil for filtering and adsorption of contaminants but must also infiltrate fast enough to prevent flooding.
• Resist compaction. Compaction of soils can greatly reduce infiltration rates, making a rain garden or bioswale ineffective.
• Hold enough water and nutrients to maintain healthy plant life. While soil alone can significantly reduce water pollution, no one will want a “rain garden” of bare soil or half dead plants in their front yard.

So what are these special soil mixtures composed of? Two specific soil ingredients are necessary to achieve the objectives above: sand and compost. Other ingredients might include topsoil and other ingredients such as water treatment residuals, wood chips, or perlite. 

What can you do?

Now that you understand the role of soil in green infrastructure, what can you do to reduce stormwater runoff in your community?

1. Conserve the soil

Like any sustainable strategy, the most important starting point is conservation. Conserving our green spaces and caring for the soil that’s already in place is central to creating healthy ground for improved stormwater management. This includes reducing soil compaction and erosion, and promoting soil health. Strategies for improving soil include:

• Amending soils with compost
• Letting leaves and grass clippings decompose in place to restore soil organic matter
• Using compost socks and berms to prevent erosion in areas under construction
• Planting trees and native plants in areas where soil is bare

2. Install a rain garden or other green infrastructure feature

The next step in improving stormwater management in your community is to incorporate green infrastructure, such as rain gardens, in your landscape. Residential rain gardens not only reduce flooding in your neighborhood but can increase property values and are a charming landscape feature. Other green infrastructure tools for reducing runoff include use of rain barrels or cisterns, disconnecting downspouts, and installing green roofs.

In recognition of the value of these tools, many municipalities have developed incentive programs to share the cost of construction. For example, RiverSmart Homes, an incentive program in Washington, DC, provides homeowners with $1,200 towards installing a rain garden or other green infrastructure system on their property.

3. Create a community

The impact of these strategies will be enhanced if they are implemented on a community level. Many of the municipally supported incentive programs are targeted toward neighborhoods where the impacts of green stormwater management will be greatest. Clustering rain gardens in a designated area such as a neighborhood block allows pooling of resources, the potential for shared maintenance, and a greater collective impact on runoff.

Tips for inspiring community stormwater projects:

Communicate! Talk to your neighbors or hold a community event to educate the neighborhood about the issues surrounding stormwater runoff. Such events might include a presentation by a master gardener trained in rain-wise landscaping or just a simple potluck. It sounds obvious but this is a great way to share ideas and find partners.

Collaborate! Once you have identified neighborhood partners, reach out to landscapers, nurseries, and other vendors who might be willing to give you bulk discounts for group projects. Local non-profits or government agencies may also be helpful in getting your project off the ground.

Educate! Post signage in yards or parking strips where projects have been built to bring attention to the economic and environmental values of green infrastructure.

Share! Organize a tour of your rain garden project to inspire other communities to create their own projects.