Why renewal sites are a good fit
Renewal projects, whether a rebuilt street, a redeveloped block or a reclaimed lot, open the ground and rearrange surfaces. That is the moment when planted drainage features are cheapest to add. This article walks through the practical benefits of three common features, rain gardens, swales and tree pits, and the upkeep each one needs. The wider system view is covered separately.
Rain gardens
A rain garden is a shallow, planted basin that collects runoff from nearby roofs or pavement, lets it soak into the ground over hours or a day or two, and filters some pollutants through soil and roots.
Benefits.
- Reduces the volume and peak rate of runoff leaving a site in smaller storms.
- Adds plants and pollinator habitat to places that were bare.
- Can be designed as an attractive amenity in plazas, courtyards and residential blocks.
Practical limits. They need soil that drains reasonably well, or an underdrain if it does not. They should be located away from building foundations. On former industrial land, soil testing is needed before encouraging infiltration.
Swales
A swale is a broad, gently sloping planted channel that carries runoff slowly along a street, parking area or path. Water slows, some soaks in, and sediment settles out.
Benefits.
- Replaces or reduces the length of piped drainage in some layouts.
- Gives the street a green edge, sometimes combined with a trail or sidewalk.
- Filters part of the pollution carried off roads.
Practical limits. Swales require width, gentle slopes and clear driveways crossings. In cold climates, road salt can harm plants. Steep sites may need check dams to slow water.
Tree pits and trenches
Tree pits are planting areas set into sidewalks or plazas. When designed to receive runoff through curb openings, they combine shade with drainage.
Benefits.
- Provide shade that makes walking more comfortable in warm weather.
- Intercept some rainfall on leaves and take up water through roots.
- Raise the visual quality of a street, which can support nearby businesses.
Practical limits. Trees die in cramped, compacted pits. Adequate soil volume and good drainage are essential. Road salt, pet waste, damage from vehicles and compaction by foot traffic reduce survival. Underground utilities frequently constrain locations.
Shared benefits
Across these features, benefits typically include:
- Less runoff reaching sewers, which may ease overflows in places with combined sewers, though the effect depends on how much of the catchment is treated.
- Cleaner water, since soil and plants filter some pollutants.
- Cooler, more pleasant surroundings.
- Habitat for insects and birds.
- Opportunities for education and community pride.
These benefits are real but vary by location. Claims that green features alone solve flooding in extreme storms are not realistic; they work with, not instead of, drainage systems.
Maintenance is the main caveat
The most common reason green infrastructure disappoints is a lack of upkeep. Typical tasks include:
- Removing litter and sediment from inlets.
- Weeding, especially during the first few years while plants establish.
- Watering new plantings during dry spells.
- Replacing dead plants and mulching.
- Checking that water drains within the intended time and that inlets are not blocked.
- Pruning trees and replacing those that fail.
Without a clear owner and budget, features clog and look neglected, which can lead to public frustration and removal. Planning the maintenance before construction, including who does it, how often and how it is paid for, matters as much as the design. Public maintenance funding is often more difficult to secure than construction funding.
Costs and comparison
Construction cost per unit of runoff managed varies widely. Green features can sometimes cost less than extending pipes or tanks, particularly when integrated into planned street work, but they carry ongoing maintenance costs that pipes do not. A fair comparison counts both and also considers the additional benefits that are not easily priced.
Fairness considerations
New green amenities in renewal areas can raise nearby property values. Without protections, longtime residents may bear the cost of success. Involving residents in choosing and caring for features, and pairing investment with housing measures, help the benefits stay local. The result still depends on local conditions and follow-through.