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Chapter 4
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Design Criteria
4.14 Street Trees and Landscape Architectural Standards

Street trees are required by municipal code and standards set by SDOT designed to provide maximum public benefit and compatibility with other infrastructure in the street right-of-way.  Street trees are to be protected during construction and otherwise routinely maintained for optimum health, longevity, and public safety. SDOT Urban Forestry advises applicants on tree selection, installation and protection measures to preserve the functional, environmental, social, and economic benefits of trees and to support the citywide urban forestry goal to increase canopy cover.

Like other street design standards, street trees are reviewed and approved by SDOT according to established guidelines and standards (see the SDOT Recommended Street Tree List for spacing recommendations and the Standard Plan # 030 for clearances and installation details).  However, because trees are living infrastructure, growing in size over time, each project is reviewed and inspected on a site specific basis to ensure the optimum outcome for the project, the neighborhood, and the city as a whole.

The street tree design criteria in this section are based on transportation safety requirements and on minimum requirements for street trees to achieve a mature canopy for effective reduction of both storm water runoff and reflected heat from paved street and sidewalk surfaces.
4.14.1 Links to Standard Plans and Specifications

Std Specification 1-07.16(2)   Tree Vegetation and Soil Protection
Std Specification  8-01.3(2)B  Tree Vegetation and Soil Protection Plan (TVSPP)
Std Specification 8-02   Landscape Construction
Std Specification 8-14  Cement Concrete Sidewalk
Standard Plans  030  through 142
Standard Plans Section 100: Landscape Planting (includes standard plans for trees, shrub and ground cover, irrigation, tree protection, soil preparation and grading)
Standard Plan 424a and b: Tree Pit Detail

4.14.2 Design Criteria

Clearances from street trees: Street trees require access to air and water, space for growth and must be located, installed and managed for compatibility with the built environment.  Standards for layout of trees and other infrastructure within a transportation corridor described in this manual are established to establish corridors, setbacks and depths for various elements including utilities (i.e. storm drains, sanitary sewer lines, water lines, etc.) to ensure that minimum clearances from trees and other objects in the right-of-way can be met.

Clearances from street trees—at grade: With limited right-of-way, SDOT will evaluate site conditions and may permit one or both of the following:

  • variances from the standard clearances from face of curb and/or sidewalk edge based on the street classification (arterial or non-arterial), tree species, curb lane use (parking or travel lane) and recorded or projected information about pedestrian volumes for the corridor; and
  •  variances from the typical planting strip and/or tree pit location within the street cross section (i.e. tree planting in bulb outs within a parking lane, behind the sidewalk, etc.)

Clearances from street trees—below grade: The design of street improvements must consider underground utilities in relation to standard utility corridors. The location of private service connections must also consider clearances from street trees.  Though less than optimum for both utilities and trees, a minimum standard of 5’ lateral clearance is required.  Where right-of-way width allows allocation of more than 5’, the investment to provide additional space up front often provides a long-term benefit through reduced impact on trees due to utility line maintenance and repair.

  • Where both utilities and street improvements are proposed by a project, the design must follow standards, including the 5’ clearance standard between service connections and street trees.
  • Where street improvements are proposed in a ROW with existing utilities that do occupy standard corridors, the standard 5’ clearance between street trees and service connections is required. 
  • Where utilities are proposed within a ROW with existing street trees, the design must follow standards with regard to placement of utilities within designated utility corridors and the 5’ standard clearance for service connections.
  • Where street improvements are proposed within a ROW with existing service connections and the 5’ clearance standard conflicts with proposed street trees in standard planting strips or tree pits, Street trees will still be required with additional mitigation measures to help protect both the trees and the service connection.  The mitigation measure must be approved by SDOT.
  • Where street improvements are proposed within a ROW with existing utilities that do not occupy standard corridors, Street trees will still be required with additional mitigation measures provided to protect the street tree and public utility.  The mitigation measures are subject to approval by SDOT and the public utility.

In some cases, depending upon the age, depth, and material of the utility, mitigation may not be possible, and the utility may be required to relocate if trees are required.
Possible Mitigation Measures – Other mitigation measure may be considered as new technologies become available and are assessed as to their feasibility for the project.

  • Vertical Root Barrier
  • Horizontal Root Barrier
  • Ductile Iron Pipe
  • Concrete Pipe with Rubber Gaskets (post 1960) after review by utility owners.
  • Utility Line Relocation

Planting strip dimensions and clearances: Maintain 5 feet 0 inches clearance between the back of the sidewalk and inside edge of the curb (5 feet 6 inches to the curb face) to allow a setback for street trees measuring 3 feet 6 inches from the centerline of the tree to the curb face and 2 feet 0 inches from the centerline of the tree and sidewalk edge.

Planting strip treatments: Plants in planting strips vary greatly in their potential to provide optimum pedestrian and environmental benefits. Though SDOT allows the installation of grass the department encourages the installation of low (24-30 inches) shrubs, perennial or groundcover plantings that provide a superior degree of separation between the sidewalk and street at reduced maintenance costs. Under some conditions, a combination of the plantings and grass or plantings and pavers may be appropriate depending on the street classification and need to accommodate parking in the curb lane.

Refer to Chapter 6.4 Green Stormwater Infrastructure for guidance on integrating stormwater conveyance and treatment into the planting plan.

Planting strips—grading: The final grade of soil surfaces in planting strips must accommodate runoff from sidewalk surfaces cross-sloped to drain toward the street. In cases where a swale or depressed planting strip is proposed, see Chapter 6.4 Green Stormwater Infrastructure.  In cases where  a mounded planting strip is proposed to provide a more effective separation between the sidewalk and street, a centerline height of 6” above the adjacent sidewalk grade is typical and gaps between mounded areas must be provided so that backup of runoff and ponding does not occur on the paved sidewalk.

Planting strip paving: Up to 40% of the area in planting strips may be paved if the following conditions are met:

  • Paving is done in combination with street trees;
  • Related landscape architectural features pose no public safety concern; and
  • The combination of paving and trees provides an equal or better balance of functional and environmental benefits than a fully planted condition.
  • Paved area is not used for parking.

Pervious pavement materials are encouraged to allow for the infiltration of runoff from the sidewalk before it enters the street to maximize the water quality entering the planting strip. Paving materials and installation details are subject to approval by SDOT.

Tree pits: are typically used as an alternative to planting strips in business districts where additional sidewalk width is important to accommodate pedestrian volumes.
When permitted as an alternative to planting strips, tree pits shall be constructed per Standard Plan 424, dimensioned to meet or exceed the minimum size required to meet standards. The minimum square footage for a tree pit is 24sf of open area (typically 4 feet x 6 feet or 5 feet x 5 feet). Any proposal dimensioned below minimum standards shall be subject to site-specific review to ensure that:

  • conditions justify the substandard size;
  • the design meets public safety standards; and
  • the design provides adequate conditions to support trees.

Tree pits—grading : Shall be graded to provide a soil surface 2 inches below the adjacent sidewalk and curb elevation and be top dressed with bark, wood chips, cinders, or crushed angular aggregate material that is routinely maintained to minimize the grade differential between the sidewalk and open pit area.

Street Tree Permit:  Please refer to Chapter 2.9.2 of the Right-of-Way Manual.

Tree grates: Often proposed as an architectural design element and/or as a means to maximize the pedestrian accessible area in the right-of-way, tree grates may be permitted by SDOT, but are not recommended.  When permitted, tree grates shall be maintained routinely by the property owner to ensure a flush condition between the grate surface and surrounding pavement, to replace broken segments, and to expand the opening as appropriate to accommodate the growth of the tree.

Tree and plant material—selection :    See SDOT Recommended Street Tree List

Tree and plant materials--installation and maintenance responsibilities: It is the responsibility of the property owner to ensure that installation and maintenance of grass, plantings and related improvements in planting strips meet public safety and industry standards. Street tree and landscaping improvements required by the Land Use Code must be maintained to meet public safety standards for the life of the project. This includes:

  • watering to ensure establishment of plant material;
  • mulching to minimize water use, discourage weeds and protect against erosion
  • pruning low shrubs and groundcover to control overgrowth onto sidewalks
  • pruning street trees(after first obtaining a street use permit) to ensure appropriate clearances over streets and sidewalks.

Tree protection and maintenance: Removal or pruning of street trees proposed requires a permit and is subject to review and, when approved, subject to inspection by the SDOT Landscape Architect’s Office.

Permit applications may be required to include public notification and/or mitigation for the lost value of the tree(s) proposed for removal. In cases where the applicant is not the owner of the property abutting the proposed work, applications must include signatures of adjacent property owners. The extent of notification is determined on a case by case basis to ensure public safety and awareness and/or approval of the project. Names and addresses of contacts may be submitted for approval or provided as a component of the permit review process by SDOT Urban Forestry. All permitted work must be completed within 60 working days unless otherwise defined by the permit.
4.14.3 Design Considerations
  • Trees and related landscape architectural treatments that are strategically planned for maximum public benefit within the often limited space within the street right-of-way provide a “sense of place” critical to the vitality of neighborhoods and their business districts.
  • The value of street trees to the urban environment is enhanced when they are combined with understory planting, specialized pavements, street furniture and public art.
  • Planting strips serve a number of important functions including:
    • pedestrian safety--they provide a buffer between the sidewalk and roadway;
    • reduction of runoff by providing area for rainfall to infiltrate;
    • water quality by infiltrating runoff from sidewalks before it enters the street; and
    • the growth and longevity of street trees.
  • Tree health and maintenance: because the standard, whether it be a 5 feet wide planting strip or 5 feet x 5 feet tree pit is not optimum to meet the horticultural requirements of a typical street tree, allocating larger areas to accommodate trees is encouraged whenever right-of-way space allows. Allocation of space is a key factor in the management of trees for compatibility with adjacent paved surfaces and other improvements, with the investment of more space up front often providing a substantial payoff in terms of reduced need for replacement or repair of paved surfaces and related improvements.
continue to section 4.15»   
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Detailed Table of Contents
Chapter 4
Design Criteria
4.1 Introduction
4.2 Street Classifications and Street Types
4.3 Design Criteria General Notes
4.4 Grading
4.5 Design Cross Section
4.6 Roadway Width
4.7

Roadway Pavement

4.8 Intersections
4.9 Driveways
4.10 Curbs
4.11 Sidewalks
4.12 Crosswalks
4.13 Bicycle Facilities
4.14 Street Trees and Landscape Architectural Standards
4.15 Introduction to Utilities Design Criteria
4.16 Street Lighting
4.17 Street Drainage, Storm Drains and Sewers
4.18 Water Mains
4.19 Fire Protection
4.20 Seattle City Light
4.21 Clearances
4.22 Structures in the Right-of-Way
4.23 Culdesacs and Turnarounds
4.24 Traffic Operations
4.25 Transit Zones
4.26 Street Furniture, Public Art and Unique Objects in the Public Right-of-Way
4.27 Access Easements
4.28 Contact Information
   
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