Minnesota Stormwater Manual test page 5

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Overview of Typical O&M Issues

Street trees and tree boxes provide a range of community benefits, including the precipitation capture and runoff reduction. Trees also contribute to air pollution reduction, carbon sequestration, reduction in heat island effect, habitat creation for wildlife, and can be used as tools for climate adaptation. Green stormwater infrastructure (GSI) trees are often planted in highly visible areas and are subject to public interaction, vandalism, and winter deicing, but trees typically remain resilient with proper maintenance.

O&M of tree trenches and boxes generally requires a low level of effort and expertise, and street trees can have life spans of up to 60 years (Cappiella et al. 2016). Green infrastructure managers do share common issues and concerns related to trees. These most common O&M concerns for tree trenches and tree boxes include:

• General poor health
• Insufficient water for growth
• Accumulation of litter and debris in the inlet
• Spent and lost mulch

The sections below describe best practices to prevent or minimize these common problems.

Design Phase O&M Considerations

Tree trenches and tree boxes have a very small footprint and should be designed to treat a relatively small drainage area. Surface area and surface area to drainage area ratio are key design characteristics. Design specifications will vary based on the selected filter media and filter box manufacturer. Properly sizing and designing the tree box will help prevent O&M issues. Designers should consider:

• Minimizing the surface area to drainage area ratio, based on the manufacturer guidelines.
• Properly sizing the tree box or tree trench to manage an appropriate surface area based on local precipitation patterns.
• Selecting regionally appropriately species that are tolerant to the soil and growth media (see Design Guidelines for Tree Quality and Planting).
• Selecting species that will grow in the limited space provided by the filter box see (Design Guidelines for Tree Quality and Planting).

Designers should also consider the maintenance schedules and tasks when locating tree trenches or tree boxes on their site. The small surface area of tree boxes makes them versatile, however, they should be placed within areas that are easily accessible to ensure proper maintenance. Maintenance of tree trenches and tree boxes does not require large or heavy equipment, but routine maintenance should be expected once or twice a year. Designers can incorporate solutions to facilitate these maintenance activities:

• Incorporating multiple and easy access points
• Place near supportive companion plants to prevent diseases
• Installing observation wells
• Specifying the optimized soil media composition and depth to effectively trap or sequester nutrients (phosphorus in particular), and that can also support the desired species
• Site-specific species selections that take into account sun exposure, shade, proximity to traffic corners (visibility issues), salt-tolerance, etc.
• Providing educational signage to increase public awareness.
• Installing measures like low fencing to prevent damage from pedestrian foot traffic.

Designers should consult and include any local requirements regarding green infrastructure. O&M considerations often depend on whether the practice is located on public land, private land, or in the public right of way. For example, plantings in the public right of way that conflict with any traffic safety considerations could require increased O&M, such as pruning or complete removal.

The designer should also provide a site-specific O&M plan that includes the following:

• Construction inspection schedule and checklists
• Post-construction routine maintenance schedule and checklists
• Operating instructions for the practice (if applicable)

Additional information that should be included in O&M plans is described in the “Post-Construction Phase O&M Considerations” section. Example O&M plans are also provided further down.

Construction Phase O&M Considerations

Proper construction methods and sequencing play a significant role in reducing O&M problems. Some key items during the construction phase include:

1. Before construction begins:
   1. Ensure that the contributing drainage area is fully stabilized with vegetation prior to the beginning of construction. Also make sure that impervious areas in the contributing drainage area are clean. If this is not possible, use barriers or diversions to direct stormwater flows from the contributing drainage area away from the practice.
   2. Install any needed erosion and sediment controls in your construction site and prepare a storm water pollution prevention plan (SWPPP).
   3. Designate a stormwater supervisor to make sure someone is responsible for erosion and sediment control.
   4. Hold a pre-construction meeting to review the construction plans and the sequencing of construction.Other agenda items could include discussing the status of the pre-construction drainage, erosion, and sediment controls; and reviewing or assign points of contacts for the stormwater and/or green infrastructure key personnel.
2. During construction:
   1. Construct any pre-treatment devices before installing any tree boxes or trenches. Depending on the site design and drainage pattern, this may not be necessary, but stormwater should be directed away from the boxes during the construction phase.
   2. Ensure heavy equipment does not enter the footprint of the practice to avoid compaction of the infiltration medium.
   3. Store any soil, mulch, or gravel media away from the practice footprint to avoid clogging the infiltration medium.
   4. Inspect the practice during construction to ensure that the tree trenches or tree boxes are built in accordance with the approved design and standards and specifications. Use a detailed inspection checklists that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided below.
3. After construction:
   1. Verify that the tree trenches or tree boxes were built in accordance with the approved design and standards and specifications.
   2. Verify that tree is staked and supported with guy wires to promote stable growth, if necessary.
   3. Verify that the contributing drainage area is fully stabilized with vegetation prior to removing any barriers, diversions, or erosion and sediment control measures.
   4. Verify that the practice actually captures and infiltrates runoff. Conduct a full inundation test to inspect the underdrain and outflow function.
   5. Use a detailed inspection checklists that include sign-offs by qualified individuals at the completion of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided below.

Post-Construction Phase O&M

Post-construction maintenance is performed on tree trenches and tree boxes to maintain proper infiltration, filtration, and to promote healthy vegetation. Immediately after construction, short-term maintenance goals include efforts to establish and keep the tree healthy. Not all trees are guaranteed to survive the establishment phase, so care should be taken to inspect the health following construction. Important post-construction considerations are provided below.

• A site-specific Operations and Maintenance Plan should be prepared by the designer prior to putting the stormwater practice into operation. This plan should provide any operating procedures related to the practices. The plan should also provide clear maintenance expectations, activities, and schedules. Include photos if possible. Be clear about who is responsible for the maintenance and the type of expertise that will be needed for distinct O&M activities. The O&M plan should include an anticipated budget for O&M activities. The O&M plan should also include an example O&M inspection checklist and an example maintenance report. Example O&M plans are provided here.
• A legally binding and enforceable maintenance agreement should be executed between the practice owner and the local review authority. Example maintenance agreements are provided here.
• Inspection and maintenance activities are distinct and can be done as separate activities or together. Inspection will typically assess the practice for any O&M issues, whereas maintenance will address the O&M issues identified by the inspection. A dedicated inspection effort on a large number of BMPs can help prioritize maintenance activities.
• Maintenance activities should be careful not to cause compaction. No vehicles or stockpiling should be allowed within the footprint of the practice. Foot traffic should be kept to a minimum.
• BMP areas generally should not be used as dedicated snow storage areas.

Overview and schedule of general maintenance activities for tree trenches and tree boxes
Link to this table

First Year of Operation
Activity	Frequency	Time period	Level of effort	O&M benefita
Check that there is no ponding within the box or trench.	At least twice after storm events > 0.5 inches	Within the first 6 months	1-2 hours	1,5
Check for evidence of clogging in the media.	At least twice after storm events > 0.5 inches	Within the first 6 months	1-2 hours	1
Supplemental watering, during drier periods, particularly if keeping stormwater offline trees are established.	1/week initially	During first two months of the growing season	1-2 hours	2,3,4,5,6
	As needed	First growing season	1-2 hours	2,3,4,5,6
Remove any weeds present	As needed	First growing season	1-2 hours	2,3,4,5,6
Remove any trash or debris	As needed	First growing season	1-2 hours	1,2,3,4,5,6
Add mulch if layer is less than 3 inchesb	As needed	First growing season	1-2 hours	4,5
Clean root collar	Annually	During growing season	1-2 hours	1
Remove stakes	As needed	End of growing season	1-2 hours	5
Inspect tree health	Biannually and after large storms	First growing season	1-2 hours	1,2,5,6
After First Year of Operation
Activity	Frequency	Time period	Level of effort	O&M benefita
Check that there is no ponding within the box or trench.	Biannually	Any time when ground is not frozen	1-2 hours	1,5
Check for evidence of clogging	Biannually	In spring and fall	1-2	1
Supplemental watering during drier periods	As needed or when soil is dry 3 inches below the surface	Dry periods until roots are established	1-2 hours	2,3,4,5,6
Remove any weeds present	As needed	During growing season	1-2 hours	2,3,4,5,6
Remove any trash or debris	As needed	Any time	1-2 hours	1,2,3,4,5,6
Add mulch if layer is less than 3 inches	Annually	During growing season	1-2 hours	4,5
Remove stakes (if still present)	Once	End of second growing season	1-2 hours	5
Prune excess growth	Once in year 2 or 3, Every three years between years 4-10, Every five years as needed after 10 years	End of winter or early spring	1-2 hours	2,5,6
Inspect tree health and safety	Biannually and after large storms	During growing season	1-2 hours	1,2,5,6
After 5+ Years of Operation (non-routine maintenance)
Activity	Frequency	Time period	Level of effort	O&M benefita
After long term operation of the practice, some occasional and infrequent maintenance activities might be required, such as tree or media replacement.	As needed	As needed	Could be significant depending on the activity	1,2,3,4,5,6
aKey to Maintenance Benefits: Proper stormwater flow and infiltration Creation and maintenance of wildlife habitat Creation and maintenance of pollinator habitat Nutrient cycling and storage Aesthetics and public enjoyment Carbon sequestration bNote that many practitioners are minimizing the use of mulch or using alternatives to mulch to control weeds. Using mulch can cause clogging of inlet, outlet, and bypass pipes, and can introduce invasive species such as jump worms. Alternatives to mulch include ground vegetation such as clover or sedges, or arranging plantings in more dense configurations so as to minimize use of mulch.

Common problems and how to troubleshoot them for tree trenches and tree boxes

Maintenance Costs

Maintenance costs will vary on a number of factors, including but not limited to:

• Size of the practice and its contributing drainage area
• Type of plantings used
• Site visit frequency
• Level of maintenance needed
• Local weather conditions
• Staffing needs (number of staff, external vs. internal staff, etc)
• Travel time between sites
• Efficiencies of scale (single GI vs. a cluster of GI)
• Equipment needed

Maintenance costs for trees and tree boxes should be relatively low compared to other green infrastructure practices, partially due to its size. Routine maintenance can take as little as 30 minutes per unit and does not require any special training, tools, or machinery. Because routine maintenance does not require any specialized training, the range of annual maintenance cost is around $100-$500 (CRWA). A study published in 2017 by ASCE describes the annual maintenance cost for a tree planter to be $260 per year, based on 2015 data from Fort Collins, CO (Clary, 2017).

Useful Resources

Additional Detailed O&M Information

• More detailed information regarding specific maintenance activities are provided here: https://docs.google.com/document/d/1nIz8g947JdefxTxUPv_JKAP-_tecrUUPeuv5p1XXhZA/edit?usp=sharing. Topics discussed include:
  • Supplemental Watering
  • Pruning
  • Staking and Straightening
  • Protecting the Trunk
  • Mulching
  • Fertilizers
  • Checking Tree Safety
  • Tree Health and Troubleshooting Guidelines
  • References

Case Studies

• Application of Stormwater Tree Trenches in the City of Vancouver, 2018 – https://sustain.ubc.ca/sites/default/files/2018-52%20Application%20of%20Stormwater%20Tree%20Trenches%20in%20the%20City%20of%20Vancouver_Vega.pdf
• https://stormwater.pca.state.mn.us/index.php?title=Case_studies_for_tree_trenches_and_tree_boxes

Maintenance Training Documents and Videos

• International Society of Arboriculture – Tree Risk Assessment Qualification: https://www.isa-arbor.com/Credentials/ISA-Tree-Risk-Assessment-Qualification
• Tree Care Industry Association (TCIA) – ANSI A300 Tree Care Standards: https://www.tcia.org/TCIA/Build_Your_Business/A300_Standards/A300_Standards.aspx
• Shade Tree Maintenance: https://youtu.be/8emNNf3QyWA
• Tree planting aftercare: https://extension.psu.edu/planting-aftercare-webinar-essential-to-tree-establishment-and-survival

Example O&M Plans, Checklists, Reports, and Maintenance Agreements for tree trenches and tree boxes

References

• Cappiella, Karen, Sally Claggett, Keith Cline, Michael Galving, Peter MacDonagh, Jessica Sanders, Thomas Whitlow, and Qingfu Xiao. 2016. Recommendation of the Expert Panel to Define BMP Effectiveness for Urban Tree Canopy Expansion.
• Charles River Watershed Association (CRWA). 2008. Stormwater Tree Pit Low Impact Best Management Practice (BMP) Information Sheet.
• Clary, J., Piza, H. 2017. Cost of Maintaining Green Infrastructure. American Society of Civil Engineers (ASCE), Reston, VA.

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Street sweeping quiz

1. In areas with high tree canopy cover (e.g. 30% or more), which sweeping regime is likely to work best for reducing phosphorus loads?
   1. Monthly
   2. Bi-weekly
   3. Bi-weekly to monthly in summer, weekly in fall
   4. Weekly in fall
2. Which of these is not a concern with mechanical broom sweepers?
   1. They grind up organic material, thus increasing the surface area of the material, which leads to increased phosphorus loss
   2. They cannot remove material from cracks
   3. They are ineffective at capturing fine sediment
   4. They are ineffective at capturing coarse sediment
3. True or false: Street sweeping debris must be landfilled
4. If you were planning a street sweeping program for your city, which of these would be least important?
   1. Mapping areas that do not have curbs
   2. Ensuring my drivers are trained and certified
   3. Identifying watersheds with phosphorus or sediment impairments
   4. Implementing parking restrictions when streets are swept
5. True or false: For a medium to large city with mechanical broom sweepers and multiple impaired waters, it is cost effective to upgrade to regenerative air sweepers
6. True or false: Very few cities sweep in late winter or early spring because there is little water quality benefit.

Answers

1. (c)
2. (d) Read more here
3. False. It can be reused in areas where human contact is limited (e.g. don't use in residential areas, playgrounds, athletic fields, etc.)
4. (a) While the presence or absence of curbs is important, if drivers are properly trained they will know how to sweep correctly in uncurbed areas, thus eliminating the need to map these areas
5. True Read more here
6. False. Late winter runoff contributes significant amounts of sediment, phosphorus, and salt. Winter sweeping is generally avoided for logistical reasons. Newer, regenerative sweepers can be used year-round.

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