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Urban Water Management and Bioswale Design

Combining hardscape and softscape to make cities flood-resistant

June 13, 2019

A median in a road slopes to contain native grasses and other plants as part of the bioswale design
Attractive bioswale design. Greendale Grange Ave Bioswale, 2010; Aaron Volkening, CC BY 2.0.

Floods are increasing in the United States. It is the most common natural hazard, and creates the greatest economic and social impact. Changes in climate and in ecosystems are creating the conditions that lead to the disaster and are forcing cities to become more flood resilient. A report from the National Academies of Sciences, Engineering, and Medicine (NASEM) predicts that many parts of the United States will see a 30-40% increase in extraordinary precipitation events by the late 21st century. Concrete and asphalt create runoff, as does the tilled surface of many agricultural fields. With increased precipitation, there is more water to manage, and less space for it to infiltrate. Forests and wetlands around human settlements once managed floodwater quite well. As cities expand and these natural areas are less prevalent, municipalities are learning to recreate them. Urban forest, raingarden, and bioswale design carefully place porous hardscape and softscape together to bring nature’s successful strategies into the heart of the city.

A car splashes through water accumulating on a road surface during a flash flood
A lot of hardscape is impervious to water and will cause water to pool on the surface.

Trees and flood management

Trees can help manage water by returning it to groundwater or transpiring it into the air.

Parks and other grassy areas in cities often have compacted soils due to the pressure of many feet walking on it. This compaction makes the soil less permeable to water—which reduces the amount of moisture it can absorb. Compacted soils are often surrounded by concrete, stone, or other impermeable hardscape, with drainage going into the sewer system. Sewers often do not replenish groundwater and can be overwhelmed in flood conditions. They can also become clogged with debris, especially in the autumn, in cities with many deciduous trees.

Trees help with the permeability of compacted soils. The roots of a tree twist through the dirt, opening small spaces into which water can seep. The roots of a tree can improve water penetration by 150%. Municipalities in flood zones are using urban forests as a means of managing floodwater.

What is an urban forest?

An urban forest is any group of trees within a city, whether they be on public land or private. Urban foresters care for trees to the benefit of the city—whether they’re alone or in groups—encouraging healthy tree populations. They must manage the tight confines of densely-packed infrastructure, utilities, and buildings.

Urban forests can be found in parks or other natural places, but the term also can refer to trees along streets and in medians. Cities often prioritize tree-lining streets to experience a variety of benefits. Trees offer shade, filter pollutants, cool the air, and produce oxygen. They’re also attractive. People prefer streets with trees: mature trees even raise property values.

Soil compaction can be the enemy of urban trees, especially on busy streets where many people step over and around the tree’s roots. This compaction challenges the tree’s ability to increase water penetration. It can also leave the tree without enough water. Supporting urban trees means protecting the ground around the tree from ongoing soil compaction.

Municipalities protect urban trees with planting, fencing, or tree grates.

A close shot of four adjacent tree grates branded Nashville featuring a white feather stuck in one corner
Preventing soil compaction. Nashville tree grate; Alex Ford, CC BY 2.0.

Tree grates and tree grate arrays

A large amount of uncompacted soil is needed to support a fully mature tree and best manage stormwater. An EPA study suggests that a tree with a canopy of 30 feet needs 1000 cubic feet of uncompacted soil in order to be healthy. This is not as much of a problem in park spaces or green areas, but it can be on a sidewalk. Trees will fight to find air and water, and so if they are given insufficient space or compacted soils, they will often fight the way to the surface, causing the sidewalk to heave and crack.

Tree grates help create more space and air for a tree than do berms in the concrete. They can be sized appropriately given annual rainfall and the species of tree.

Some municipalities have experienced sidewalk heave or tree crowding with the grates and tree species they’re using. To help prevent heave, tree grating can be used like trench grates, placed in a long strip or array. Creating long exposed areas protect soil from compaction while leaving a greater area to offer air and water to the tree system. Tree roots move toward water, which means this type of array can encourage roots to stay within an established zone. The tree’s easiest access to moisture and air is under the array, and a long enough space allows the tree to grow healthily. To prevent heave at the surface and the roots from interrupting utilities, root shielding and redirection should can also be used. With a managed root zone and well-built soils, a tree grate array can help with water management and be a strategy to prevent heave in a busy urban setting.

When encouraging fully mature trees, it’s important to leave enough space so that the tree grate does not choke the growing tree. This can be done in stages as the tree grows, or a large hole can be protected against being a tripping hazard with plantings with a tree guard. All trees should be checked yearly to ensure they’ve room for growth.

In large expanses of hardscape, trees are only one piece of the water management puzzle. Raingardens can help process average rainwater levels, and bioswales are built to handle storm surges.

What is a raingarden?

A raingarden is a small garden planted into a depression the landscape designed to capture storm water. It improves water quality and captures some of the rainwater that would otherwise end up in the sewer system.

What is a bioswale?

Larger than a raingarden, the bioswale is a gently sloped low-lying drainage area, covered with vegetation, built to process water run-off. Carefully designed strata of gravel, soil, and plants create a biological and soil water filter. Like a raingarden, the bioswale design is meant to recharge the groundwater, but it can handle the larger amounts in a storm. It can also clean the water from a much larger area through the action of the vegetation.

Decorative grasses are part of a series of small bioswales along a street curb
Curbside bioswales require litter maintenance. Image by Brad Davis, CC BY 2.0.

Bioswale design

A “swale” is a low or hollow landform, suggesting the sloped characteristics of bioswale design. In general, a slope over a large area is preferred, but slightly sloped bioswales can be built into curbs. The slope directs water away from hardscape surfaces where it might pool. The vegetation within a bioswale is important for maintaining soil permeability, preventing erosion, managing particulate, and breaking down chemicals within the runoff.

There are different types of bioswale appropriate to the goal of the swale. “Wet” bioswales are meant to simulate wetland areas for filtration, and they maintain water at the bottom of the swale. Impermeable structures built at the bottom of the trench help prevent this water from dissipating. Permeability increases when this structure is overtopped. These wet bioswales can also be designed such that the water table is exposed. Wetland-simulating swales work well as biological filtration but are less beneficial at processing storm surges.

“Dry” bioswales are more common. They also provide some of the particulate removal and chemical management of wet swales, but they are primarily designed to help with water permeability. They filter dirty run-off in regular rain cycles and help absorb run off during storm surges.

Long, gentle slopes work best in bioswale design, as sheets of water flow more slowly over these gradients, allowing more time for absorption. These slopes are built with permeable soil, often a sandy loam, with only a small amount of clay to prevent absorption. Pea gravel along inlet areas helps prevent erosion. An underdrain system carries excess out of the area.

A trench grate lies next to the rocks at the edge of a bioswale
Trench grate can also be used on pathways across swale. Image by Chris Hamby, CC BY-AS 2.0

Bioswale edges can be built level with the surface of the roadway. Often, they are surrounded with curbs, which need gaps and slope to feed water into the swale. Trench drains can span a surface and create the slope conditions to bring water down to the bioswale. These trench grates, and beehive grates over the overflow pipes, can help prevent large pieces of debris from settling, although the bioswale is intended to manage particulate. It is useful to have rocks, stepped concrete, or other friction-producing hardscape along dips. This rough terrain slows water down if there is a surge that overflows the swale’s capacity. It helps prevent erosion caused by quickly flowing water.

The vegetation in the bioswale is also important. Native species tend to be more adaptable to the local environment. If they are specialized to a local niche, they are often hardier plants that do not need to be supported by pesticides and herbicides. Clean swales such as these provide habitat and sustenance for bees, butterflies, and other insects. These insects in turn support birds, helping bring these animals back to the city.

Recently, studies have shown that trees and bushes can be very effective when integrated into bioswale design. In the case of an arboretum parking lot, trees provided at least 46% of the water management. Tree roots within a bioswale still must be managed in order not to interfere with the soil filtration of the system. Often, a bioswale provides a large enough area to support a very healthy tree.

Large skyscrapers rise behind a beautiful park filled with trees
Urban landscapes of the future will need to be greener than our current futuristic vision suggests.

The green future of cities

In TV and movies, many of the imagined future cities are ones of concrete, glass, and steel. Often, the images feature very little that is green or wild.

Although we have learned these cityscapes as the image of the future, the imagined cities would be hot, gritty, and vulnerable to storm surges. They also are antithetical to what draws people to a neighborhood. People prefer places with lots of growing green spaces.

A more realistic and sustainable view of future cities would have trees on every block, a weather-respecting ratio of bioswales to hardscape, green roofs, urban farms, and greenbelts that support local bee and bird populations. It is by integrating the city to the environment that we create rugged, sustainable spaces.

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