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Network Narratives: Tree Trench Guide

Providence, RI is a dense urban area with an industrial history and high levels of impervious surface, resulting in stormwater management and water quality issues that are expected to worsen with the impacts of climate change. To address these challenges, the City experiments with innovative green infrastructure practices in Roger Williams Park, an urban park with severely degraded water quality that is home to the Providence Stormwater Innovation Center (PSIC).  In addition to improving conditions in the park, PSIC provides hands-on training and examples for other municipalities and practitioners on the successes and failures of design, installation, and maintenance for different types of green infrastructure. 

In 2020, the City of Providence Parks Department, PSIC, and PSIC partner Audubon Society of Rhode Island applied for, and was awarded engineering support to develop a guide to tree trench design through the SNEP Network’s Call for Participants in 2020. Network support connected sustainable environmental design consultants from Horsley Witten Group to provide technical assistance and content recommendations for the tree trench design guide.

Providence Stormwater Innovation Center volunteers plant shrubs along the banks of Bellefonte Brook in Roger William Park to improve water quality and reduce erosion.

Tree trenches are a green stormwater infrastructure technique that consists of a series of trees connected by underground infiltration structure. Tree trenches are designed to mimic nature and use the natural filtration properties of soil and plants to manage and remove pollutants from stormwater runoff, while also adding trees to the landscape. Tree trenches can consist of various designs and green infrastructure practices, including infiltration gutters, tree filters, tree filters with storage, bioretention swales, biostrips, and bioretention curb inlet planters.

Subsurface tree trench with space under the pavement for root growth and stormwater storage.

As Bryan Byrnes, Deputy Superintendent of the Providence Parks Department, began to explore potential locations and designs for tree trenches within Roger Williams Park, he noted that proprietary options for tree trench design and materials are often expensive, and there can be challenges to having trees survive and thrive in tree trench projects. There was a need – and an opportunity – to clarify the components of a tree trench and make the design options understandable to a large audience through the creation of a Tree Trench Guide.

“We have found that oftentimes people charged with getting [tree trenches] installed by mandates or ordinances may have little experience in this type of installation,”  Byrnes explained. “Giving them a little guidance hopefully results in them saving money and getting a better product as an informed buyer.”

The project team began with reviewing existing projects in Roger Williams Park, along with a green infrastructure guide from the Boston Parks and Recreation Department, and the Rhode Island DOT’s linear manual. The guide compiles a range of possible tree trench designs, breaking down the potential components to address the considerations for each.

Through a collaborative work process, the guide was developed with feedback from PSIC members and a wide range of stakeholders, including municipal staff, environmental groups, researchers, designers, maintenance personnel, and landowners. Ellen Biegert, Landscape Architect at Horsley Witten Group, noted the benefits. “As far as the process, the biggest lesson would be to gather input from a group and multiple sources. This ensures the information has been looked at from several angles and that it is understandable to a larger audience.” 

The newly-launched Stormwater Tree Trench Design Options Guide, available online, will help municipalities or nonprofits determine the best system for their project based on cost, maintenance, city resources, desired pollutant reduction, aesthetics, and tree health. The guide also provides the background and terminology to help discuss these projects in detail with engineers and designers.

Matrix of tree trench design options from the Guide.

The team hopes the real-world examples and scenarios will share the adaptability of tree trenches to different site constraints, and demonstrate how to create a system that helps trees thrive. “The value of trees as a stormwater management option is often underappreciated and overlooked,” said project partner Brian Kuchar, Landscape Architect with Horsley Witten Group. The Stormwater Tree Trench Design Options Guide is an important resource to advance stormwater management and water quality goals through a practice that promotes the long-term health of trees and the multifunctional benefits they provide to communities.

The SNEP Network’s support is key to the rollout and promotion of the guide through web content, articles, and a webinar that will formally present the guide to the public. The project team will hold a training session using real-world examples, site conditions and scenarios to explain best practices for using the Stormwater Tree Trench Guide when planning a green infrastructure project. To register for the Stormwater Tree Trench Design Options Guide Training, click here.

The SNEP Network is a project of the New England Environmental Finance Center and is funded by the U.S. Environmental Protection Agency’s Southeast New England Program. For more information about upcoming SNEP Network webinars and events, other assistance and training opportunities and resources, or to get in touch with us, visit the SNEP Network’s website at snepnetwork.org.