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Asset Type : Enhanced

Water Classification:
Stormwater
Source Water Recharge
Combined Sewer Overflows

Construction Rank : 3

O&M Rank : 4

How it Works : A stormwater tree trench is a system of trees that are connected by an underground infiltration structure. On the surface, a stormwater tree trench looks just like a series of street tree pits. However, under the sidewalk, there is an engineered system to manage the incoming runoff. This system is composed of a trench dug along the sidewalk, lined with a permeable geotextile fabric, filled with stone or gravel, and topped off with soil and trees. Stormwater runoff flows through a special inlet (storm drain) leading to the stormwater tree trench. The runoff is stored in the empty spaces between the stones, watering the trees and slowly infiltrating through the bottom. If the capacity of this system is exceeded, stormwater runoff can bypass it entirely and flow into an existing street inlet.

O&M Required : Provide regular irrigation and weed control until trees are well established. Prune trees once they are established to prevent safety hazards to pedestrians, overhead utility lines, or adjacent buildings. Remove any accumulated trash or debris. Monitor vegetation for damage by insects and other pests. Control invasive species. Remove trees that are in decline. Water, mulch, treat diseased trees, and remove litter as needed. Do an annual inspection for erosion, sediment buildup, and vegetative conditions. Do a biannual inspection of cleanouts, inlets, outlets, etc. Maintenance cost for a prefabricated tree pit is100-$500 per year.

Design Considerations : Tree boxes and tree trenches should be located to avoid interference with underground utilities, particularly water and sewer lines. It is important to consider overhead clearance, the root zone, the water table and soil permeability/limiting zones.

Costs : $850 per tree, $10-$15 per square foot, $8000-$10,000 to purchase one prefabricated tree pit system including filter material, plants, and some maintenance; $1500-$6000 for installation.

Benefits : Tree trenches remove pollutants and nutrients from stormwater runoff by filtering them through the soil and absorption by the tree. They reduce stormwater runoff by collecting and retaining water that would otherwise be discharged into sewer systems or surface waters of streams or rivers. The increased tree canopy also enhances the aesthetic value of the community.

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References:

Tree Trench Fact Sheet
Tree Trench Guidelines

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