Thursday, February 9, 2023

Using trees to manage stormwater: City council hears Wilmington case study

11:00 p.m. Water Street in downtown Wilmington near the Cape Fear River. (Port City Daily photo/Mark Darrough)
Storm flooding in downtown Wilmington. (Port City Daily photo/Mark Darrough)

WILMINGTON — Most people probably don’t think about trees when they think of an urban setting, but so-called urban forests play a significant role in these environments.

The City of Wilmington was just one of two cities in the state selected to participate in an urban forest study conducted by the Green Infrastructure Center (GIC). In early 2019 City Council was given the results of the study which suggested the city spend more money on tree preservation in urban areas.

On Monday, City Council was given another update to the ongoing efforts to protect the urban canopy and was offered suggestions on further actions. Aaron Reese, city arborist, gave the presentation (you can watch the presentation online).

One of the more significant issues the city has faced over the past few years is the increase in flooding, especially from storms like Hurricane Florence but also from comparatively minor thunderstorms. Urban forests can play a large role in absorbing stormwater and offsetting the damages it causes.

“This project, called Trees to Offset Water, is a study of Wilmington’s forest canopy and the role that trees play in up taking, storing and releasing water. This study was undertaken to assist Wilmington in evaluating how to better integrate trees into their stormwater management programs. More specifically, the study covers the role that trees play in stormwater management and shows ways in which the city can benefit from tree conservation and replanting. It also evaluated ways for the city to improve forest management as the city develops,” according to the case study report.

The project has been in the works since 2016 and the final report has been made public. While Wilmington does have room for improvement, it is worth noting that the city does have 48% canopy coverage — when compared to other cities this is at the upper end of the list.

Mayor Bill Saffo acknowledged that one of the things he heard most from voters this past election season was the need to plant more trees and protect trees in Wilmington.

“It’s good news that we have 48% canopy coverage in the city, which is great … I would like to make a challenge to this council and this community, we are at 48% and I always want to beat Charleston is whatever they are doing and for them to be at 50% we should at least be at 51% or 52% and I think we can do that within the next 365 days,” Saffo said.

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A graph from the GIC study shows how much tree canopy could be added (Port City Daily/Courtesy City of Wilmington)

The GIC study did offer 18 total recommendations to the city to .help reach those goals and preserve the urban forest.

They are listed, according to the GIC report, in priority order (note, the following list is verbatim from the report, it has not been edited).

  1. Use the GIC’s stormwater uptake calculator to determine the benefits of maintaining or increasing tree canopy goals by watershed. The calculator provided to Wilmington allows the city to determine the stormwater benefits or detriments (changes in runoff) from adding or losing trees and calculates the pollution loading reductions for nitrogen and phosphorus, and sediment.
  2. Develop an Urban Forestry Management Plan (UFMP) which includes statistics on the community values of trees, measurable and achievable urban forestry goals, action steps required to achieve those goals, and a detailed list of maintenance items and frequencies. Wilmington does not currently have an UFMP, but many of its codes and ordinances include typical UFMP components. These components can be divided into several sections including documentation of the community values of trees, outlining urban forestry goals and developing a maintenance item schedule.
  3. Work with developers to shrink the development footprint to minimize impervious surfaces. Holding a predevelopment conference, with all key staff in attendance, allows all parties to explore ideas for tree conservation before extensive funds are spent on land planning.
  4. Conduct a land cover assessment every four years to determine and allow for comparison of tree canopy coverage change over time. Keeping tree canopy coverages at levels that promote public health, walkability, and groundwater recharge for watershed health is vital for livability and meeting state water quality standards. Regular updates to land cover maps allow for this analysis and planning to take place and to spot and address negative trends and take preventative actions.
  5. Remove the exception for tree inventory requirements on lots with single-family homes that are two acres or smaller. Requiring tree inventories affords more opportunities for city-led tree save decisions.
  6. Increase the number of tree protection mechanism inspections and enforcement staff. Enforcement of tree-related codes and ordinances is more effective when there is adequate staff time available to enforce them. Hiring an additional urban forester to work with city staff on planning and zoning matters will help ensure that opportunities to save or add trees are realized.
  7. Perform tree risk assessments. Increase assessment intervals in densely populated portions of the city. Tree risk assessments minimize tree-related risks by actively managing the urban forest and can lessen the costs of removing trees after storms.
  8. Require tree canopy coverage percentages by land use. To assure quality of life for all in a community, add a requirement in Wilmington codes and ordinances for minimum tree canopy coverage by land use.
  9. Determine urban forestry data needs and which software will best collect the needed data. Implement the data collection process as part of the urban forestry program. Site-scale landscape changes are easily seen with the imagery but information about the urban forest that could be used in planning is lacking. Urban forestry data collection
    should provide detailed, quantifiable information.
  10. Use Silva Cells or other similar trade product, to provide adequate soil volume for trees in dense urban conditions. Silva Cells can increase survival rates for newly planted trees. They are expensive and as such, should be used only strategically, in commercial districts for example or in public plazas.
  11. Publicize Wilmington’s Right of Way (ROW) tree planting program and encourage more citizens to plant in ROWs near their homes. Trees shade streets and sidewalks, making walking and biking more comfortable in Southern urban locations. The City of Wilmington will plant trees in ROWs if adjacent homeowners request them, but many homeowners or renters do not know about this program.
  12. Revise city planting lists to reduce the number of nonnative invasive species listed. Develop a prohibited planting list. The current recommended planting list includes many non-native invasive species. Revise the list to include more natives and enjoy the ecological benefits these species provide.
  13. Adopt a complete green streets policy. Complete green streets allow for integration of stormwater management and aesthetic goals. By incorporating vegetation as an integral part of the design, they create and connect habitat, reduce urban heat island effect, help remove air pollutants, and promote walking and biking.
  14. Expand the application of the code that allows for variable space sizing to all city districts. Excessive parking standards have exponential negative effects on stormwater volume generation, especially in urban environments. It is good practice to ensure that parking requirements are consistent with demand.
  15. Offer stormwater fee reduction credits for tree plantings. Stormwater utility fees are a mechanism for funding stormwater management based on the amount of impervious surfaces generated for land cover by parcel and provide an incentive for reducing impervious areas to lessen the fee.
  16. Add a Geographic Information Systems (GIS) staff person to the Parks and Recreation Department. Effective management of the urban forest is data-driven and includes spatial analysis. Empower the Urban Forestry Program with data and allow for better management of the urban forest.
  17. Devote city resources to organization and training of a Wilmington tree stewards group. Tree stewards can carry out tree planting projects, provide tree care trainings, and increase the public’s awareness of the value and care of trees. In the past the city had a ‘pruning corps’ and this did not continue. Staff can investigate this past effort, as well as similar highly successful tree steward programs in other cities that are working well to determine the ingredients for what makes a successful program.
  18. Develop a forestry emergency response plan. The city does not have a plan for replacing trees lost to natural disasters such as hurricanes or other storms. This means that canopy will decrease over time. Given the many benefits that trees provide (increased groundwater infiltration, soil stability, and reduced runoff and flooding, shade and better air quality), the city should plan for funding and replacement tree plantings following natural disasters.

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