Notable Project Submission: Columbus Drive Sustainable Streetscape, Self-Watering Tree Wells

March 2021

Proposed condition: This image depicts bump-out tree wells, which are self-watering via the use of street run-off. Narrow travel lanes result in decreased speeds in the residential neighborhood with bike lanes and on-street parking on both sides.

While attending an ISA Arborist seminar – I heard the words often: “If you can manage the water, you can manage the trees.” That overriding arc of thought became the basis for this urban design, sustainable streetscape concept.

Existing Conditions: This image depicts Columbus Drive with its current state of open asphalt paving, on-street parking, no crosswalks, or bike lanes. It also has no integrated street trees or pedestrian safety zones.

The basis for this concept of Urban Tree Management is utilizing natural rainfall that is collected on urban streets and intercepting the run-off to the trees before discharging into storm inlets. Provide water to trees for long-term sustainability is a difficult element of design in urban streetscape. I often run into two obstacles:

1. Providing irrigation to a hard paved urban environment is not only difficult, but it is often impossible. This is due to digging or trench sidewalks and common areas. Irrigation lines must run to trees; however, finding the water source and power source for controllers, the infrastructure, and the manpower to maintain these systems are also challenging.

2. The second obstacle is providing truck watering for the proposed landscape. In theory, this is a great solution, but once the 12-month contract runs out, no-one is contractually obligated to water the trees. If the trees are not yet established, then the landscape materials stress in dry months, and eventually, they slowly fail due to the annual stressing of the dry season and minimal water resources.

For this project, we ultimately decided to use natural rainfall to water the trees. Because the rainfall is already concentrated by the hard surfaces of streets and paving, it makes sense to redirect the water to the trees as a sustainable water source which can continuously maintain itself.

The tree wells were designed as a part of the stormwater system as a preliminary interception, before draining into the traditional storm inlets. Also incorporated into the tree well was an interception to collect trash and soils from street runoff. This is a critical item which limits the potential pollution of the tree root system from street runoff. The first half-inch of water is diverted into the tree wells directly through the curb’s gutter. Then, after the box fills up, the water overflows directly into the storm drain system. The top of the tree’s root ball is covered in gravel to prevent erosion from heavy remains. The bottom of the box is open to allow for drainage, and the sides are open to allow for tree root growth into the available green spaces. The trees will be planted in structured soils to allow for roadway construction and sidewalk construction over the trees, minimizing settlement. No irrigation is required as the natural rainfall provides the soil moisture.

A bump-out graphic which shows the concept of water along the curb redirected and flowing to the tree well.

Bald Cypress trees were selected for their narrow canopy in the urban environment. Their tolerance for the wet and dry cycle of natural rainfall and the minimal sidewalk uplift of surface roots were additional reasons for selection. On the other side of Columbus Drive, we were limited by TECO requirements, reducing our canopy tree plantings. Instead, we used Chinese Fan Palms in those particular tree wells. We also added shrub beds of Ixora to some of the open areas.

The project is currently under construction and is scheduled for completion in 2022.

A depiction of a tree well graphic and its various components.

Proposed concept sketch of a self-watering tree well which uses stormwater run-off from the street to water the tree. It then overflows into a storm drain.

Construction document detailing a tree well in a bump-out and showing root growth areas, openings in a tree well box, tree grate, and structured soils.

Construction document detailing a tree well in a bump-out and showing root growth areas, openings in a tree well box, tree grate, and structured soils. Tree wells use a standard-sized, 72" square stormwater box with modified openings for the inlets and root growth areas.

The construction document site plan of proposed landscape bump-outs and street tree program.

by Jonathan Toner

Jonathan has worked with several well know Engineering & Architectural Firms in the Tampa Bay area, including Design Advocates, Inc. before starting Terra Tectonics design group, inc., in 1992 and has since produced a wide range and variety of projects. Adding to his knowledge base, and broadening the understanding of trees in the landscape, studies were undertaken to obtain Certification as an ISA Arborist in 2001. Jonathan brings a solid understanding of local climate, environmental, aesthetic and economic issues, as well as a core understanding of project management, based upon experiences from similar projects.