report: sidewalks and street trees 09.15.2010

DRAFT updated 09/15/2010 SIDEWALKS AND STREET TREES

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PHOTO: Large Oak Tree. Roots Cause Sidewalk Damage

Report by: Planning and Growth Management Department and Public Works Department September 2010


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Contents Page Executive Summary 4 The Assignment 4 The Issues 4 The Recommendations 5 Background 6 July 2008 amended Comprehensive Plan’s FLUE 7 Land Development Code Requirements 8 Potential Conflicts with Trees 9 Subdivisions 11 County Sidewalk Maintenance Costs 13 Mitigation Strategies 17 Right-of-Way Tree Evaluation 17 Tree Root Development 18 National Issue 20 Sunnyvale, California: Concrete and Tree Root Mitigation 27 Local Recommendations 29 Canopy and Tree Location 29 Curb Design and Sidewalk Location 31 Right-of-Way Use Options 32 Benefits 35 Summary 37 Species Selection 38 Recommended Minimum Design (New Subdivisions) 38 Enhanced Sidewalk Design 39 Repairs: Best Management Practices 41 Grinding and Patching 41 Root Pruning 41 Enhanced Sidewalk Design (Replacement) 42 Revisions to Land Development Code 42 Responsibility of the Engineer of Record 43 Contributors and References 44


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List of Tables Table 1: Transportation Maintenance Division Costs 13

between 1 Oct 1999 and 30 Sep 2009 Table 2: TOTAL COSTS SINCE 2002 / Engineering Division 14 Table 3: Projected Costs for Sidewalks and

Percent Replacement in Carrollwood Village Area 14 Table 4: Public Works O & M Comparison 15 Table 5: Radius of Root Influence 17 Table 6: Soil Area Required 23 Table 7: The Survey and Topics of Importance 35 Table 8: Comparative Benefits Matrix 36 List of Figures Figure 1: Recommended Utility Locations in Subdivisions 11 Figure 2: Typical Road in Subdivisions 12 Figure 3: Typical Sidewalk Repairs 15 Figure 4: Repairs and Funding 16 Figure 5: Mature Tree Development Profile 19 Figure 6: Increase Tree Planting Space 21 Figure 7: Chapter 6 by Edward Gilman 22 Figure 8: Unsuitable Growing Area 22 Figure 9: Root Barrier 24 Figure 10: Herbicidal Root 25 Figure 11: Reinforced Concrete Sidewalk with Footers 26 Figure 12: Root Control Barrier Installation at Sidewalk 27 Figure 13: Closed Canopy 29 Figure 14: Open Canopy 29 Figure 15: The Recommended Tree Location 30 Figure 16: Miami Curb Neighborhood 31 Figure 17: Barrier Curb Neighborhood 31 Figure 18: Option A 32 Figure 19: Option B1 33 Figure 20: Option B2 33 Figure 21: Option C1 34 Figure 22: Option C2 34 Figure 23: Sidewalk Design with Root Barrier and Crushed Base 38 Figure 24: ADA Requirements 40 Figure 25: Root Pruning Results 42


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Executive Summary The Assignment During the Board of County Commissioner’s March 18, 2009 meeting, Commissioner Ferlita requested a staff report addressing the location of street trees and their interaction with sidewalks within County maintained right-of-ways. Staff’s analysis of sidewalks and street trees defines this subject matter into two categories: (1) remedial actions addressing existing disturbances to sidewalks, and (2) preventative actions to lessen the need for future sidewalk repairs and to significantly reduce maintenance costs. The Issues There are three issues, nationally and locally.

• Sidewalks are being destroyed by ‘Street Trees’. Sidewalks must meet the Americans with Disabilities Act (ADA) criteria.

• ‘Street Trees’ are damaged by maintenance.

• Repair costs exceed funds. Sidewalk repairs in Hillsborough County will cost $30

Million by the year 2020. Sidewalks are damaged by tree roots because the Land Development Code instructs people to plant the trees three feet from the sidewalk. The effect is increasing costs due to more of the same damages from past plantings and future tree growth. In the future there will be more needs from past plantings and less money for repairs. The total cost needed for repairs in 2009 was $9 Million, but only $4.5 Million was funded for repairs by Hillsborough County. This does not account for the ‘Engineering Costs’ for the four community areas estimated to be $5 Million per community area. Potential costs for sidewalk reconstruction and repairs are approximately $30 Million by 2020. Remedial actions addressing existing disturbances to sidewalks caused by tree roots from past plantings are difficult and costly. Although this report describes best management practices available to reduce some of the sidewalk repairs, current design standards for the location of street trees hinder more effective techniques.


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The Recommendations The good news is that in this report you will see that you can have sustainable communities, with safe sidewalks and healthy trees! The bulk of this report focuses on the successful discovery of an alternative to lower repair costs for future sidewalks almost to zero. The intent of this report is to describe different design techniques sufficient to prevent a large majority of the sidewalk disturbances caused by maturing street trees. An alternative tree location and additional design features to extend the useful life of the sidewalk are recommended. The recommended tree location is behind the sidewalk. The strategy includes vision and mission statements, followed by the goal and objectives. Vision: For our residential communities to be ‘SUSTAINABLE’, with safe sidewalks

and healthy trees. Mission: For our sidewalks and our ‘Street Trees’ to be maintained with limited funds. Goal: Create ‘Sustainable’ safe sidewalks that are more pleasant for users and a

higher quality of life; provide ‘better’ healthier trees with more room to grow, and more area shaded for more energy efficient homes; and lower repair costs, almost to zero.

Objective: Plant the ‘Right Trees in the Right Place’. Objective: Stop sidewalks from being destroyed by ‘Street Trees’. Objective: Stop ‘Street Trees’ from being damaged by maintenance. Objective: Lower repair costs, almost to zero. Objective: Protect ‘Street Trees’ by locating trees in an easement or right-of-way. Objective: If right-of-way is increased, have no loss of subdivision lots. The recommended tree location provides ‘better’ healthier trees with more room to grow, and more area shaded for more energy efficient homes, and lower repair costs. It creates ‘sustainable’ safe sidewalks that are more pleasant for users, and a higher quality of life. All the objectives are met by the alternative tree location and additional sidewalk design features.


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In our recommendations, there are three parts to stop the uplifting of sidewalks:

• 10 feet of Separation from the tree to the sidewalk.

• Root Barrier. Root barrier is a thick piece of plastic that is located along the sidewalk, 12 to 24 inches deep. It redirects the tree roots downward away from the sidewalk.

• Crushed Base. For sidewalks less than 10 feet from the tree, ‘crushed base’ is needed

under the sidewalk, and the root barrier. This design is strongly supported by data and literature. There are options for the local street design and for the protection of the street trees in additional right-of-way or in an easement. There will be no loss in the number of subdivided lots. Recommended actions include:

• Revisions to the Land Development Code • Revisions to the Transportation Technical Manual • Preparation of a Tree Maintenance Guide

Background During the Board of County Commissioner’s March 18, 2009 meeting, Commissioner Ferlita requested a staff report addressing the location of street trees and their interaction with sidewalks within County maintained right-of-ways. A brief report was presented to the BOCC on April 15, 2009 from PGMD addressing Commissioner Ferlita’s request on street trees and sidewalk conflicts. The April 15, 2009 report was inconclusive as this subject matter encompasses considerable financial expenditures and therefore warranted thorough research and coordination between PGMD and Public Works. Therefore, the report recommended that Public Works and Planning and Growth Management Departments come back to the BOCC with a thorough report defining the options to address the issues. Staff’s analysis of sidewalks and street trees defines this subject matter into two categories: (1) remedial actions addressing existing disturbances to sidewalks, and (2) preventative actions to lessen the need for future sidewalk repairs and to significantly reduce maintenance costs.


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The first category involves the repairs of existing sidewalks and maintenance of trees in County right-of-way. Although some repair and maintenance in County right-of-way is expected, the frequency and degree of damage caused by tree roots indicate that there may be other management practices to minimize these disturbances. The intent of this report is to describe the best management practices available to reduce sidewalk repairs caused by tree roots from past plantings. The second category addresses optional design techniques for future sidewalk construction and their location relative to street tree plantings along the County right-of-way. Although street trees are required by the County’s Land Development Code (LDC), the current planting locations required for street trees appear to be incompatible as the tree matures. The intent of this report is to describe different design techniques sufficient to prevent a large majority of the sidewalk disturbances caused by maturing street trees. The placement of street trees within road right-of-ways retaining sidewalks is a national issue. Sidewalks are essential for pedestrian safety, safe routes to school for children, and for the mobility of those with disabilities. As Hillsborough County continues to create more multi-modal transportation systems, pedestrian walking ability and sidewalk issues become greater. As the number of subdivision sidewalk miles increase, so does the number of street trees. Sidewalks are intended to transport people safely and efficiently. Trees are intended to make the conditions more pleasurable; however, safety is paramount. Sidewalk displacements are unsafe. Street tree roots are responsible for 95 percent of all sidewalks and curb displacements (City of Sunnyvale: Report to Mayor and Council, 2007). Millions of dollars are spent each year to repair sidewalks damaged by tree roots. This report is intended to provide innovative sidewalk design techniques and their relationship with street trees to reduce costs for sidewalk repairs and replacements. Design options are addressed. Revisions to County design standards and code amendments are recommended. Tree establishment within County maintained road right-of-way occurs from various initiatives. These initiatives range from Land Development Code (LDC) mandates to volunteer plantings created by people or by nature. Of these initiatives, the most contributing factor in the establishment of shade trees in the right-of-way is by the LDC mandates. The LDC requirements for street trees evolved from the Comprehensive Plan policies addressing livable roadway concepts and community design considerations. July 2008 amended Comprehensive Plan’s Future land Use Element (FLUE) (WEBlink:www.theplanningcommission.org/hillsborough/comprehensiveplan/Future%20Land%20Use%20Element.pdf/view ) The County’s July 2008 amended Comprehensive Plan continues to provide for the planting of street trees.

http://www.theplanningcommission.org/hillsborough/comprehensiveplan/Future%20Land%20Use%20Element.pdf/view

http://www.theplanningcommission.org/hillsborough/comprehensiveplan/Future%20Land%20Use%20Element.pdf/view


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The FLUE includes the Community Design Component. “The purpose of the Community Design Component is to support the County’s programs for growth management and it provides a specific vision for livability.” Section 6.0 ROADWAY LEVEL DESIGN describes livable roadway concepts. “Livable Roadways is an approach to roadway planning and design that integrates transportation safety and function with other considerations.”

• In August of 2006, the “Livable Roadway Guidelines” document was adopted by the Metropolitan Planning Organization (MPO). These guidelines are further identified and included as part of Section 6.0 Goals, Objectives, and Policies.

• The purpose of the “Livable Roadway Guidelines” is to set forth “best practices of

roadway design. However, some changes are needed to employ all of the best practices. The intent is to inspire a new approach to roadway design.”

Section 6.13 LANDSCAPE DESIGN specifically addresses Goal 16.

• Goal 16: Provide aesthetically-pleasing landscape treatment along roadways and in the right-of-way. Support its creation and maintenance within adjacent developments. More specifically:

• Objective 16-1 states that the County is to “ensure that publicly owned land and right-of-

way is attractively and appropriately landscaped.” This objective is supported by “Policy 16-1.7: which indicates that regularly spaced shade trees are to be placed along the roads and adjacent to sidewalks.” The intent of Policy 16-1.7 had been codified in 1992 and currently exists as a standard in the County’s Land Development Code (LDC) Section 6.06.05 entitled Street Trees.

Land Development Code Requirements (WEBlink: www.municode.com/resources/gateway.asp?pid=12399&sid=9 ) In 1992, the Board of County Commissioners adopted Land Development Code Section 6.06.05 requiring all new developments to plant ‘Street Trees’ along all subdivision local and collector roadways. LDC Section 6.06.05, entitled Street Trees addresses when street trees are required, the type to be specified, and their specific location. The placement component of the street tree standard (LDC Section 6.06.05.C.1.) indicates the street tree is to be located “within the road right-of-way a minimum of five feet from the back of curb and a minimum of three feet from the inside edge of a proposed or established sidewalk.” This specification was based upon Section 6.13 LANDSCAPE DESIGN of the FLUE in the Community Design Component, specifically Goal 16: Provide aesthetically-pleasing landscape treatment along roadways and “in the right-of-way”.

http://www.municode.com/resources/gateway.asp?pid=12399&sid=9


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Points of Interest are:

• The minimum of three feet from the inside edge of a sidewalk to a large shade tree does not provide a suitable horizontal distance to avoid some degree of long term disturbance to a sidewalk from the roots of a large growing shade tree.

• Keeping the County’s current ‘Street Tree’ requirements on new development will result

in a continued maintenance obligation including costs to address long term sidewalk damages caused by trees if optional sidewalk designs are not considered.

• Landscaping must be consistent with roadway planning and design that integrates

transportation safety and function with other considerations such as the useful life of the road and sidewalk. “When tree characteristics and site characteristics match, the result is the right tree in the right place,” an internationally recognized arboricultural standard.

Potential Conflicts with Trees Land Development Code (LDC) Part 6.06.00 Landscaping, Irrigation and Buffering Requirements provide for the planting of trees within road right-of-way and new developments. These requirements are referenced under LDC sections entitled Road Right-of-Way (Section 6.06.03.H), Scenic Roadways (Section 6.06.03.I), and Street Trees (Section 6.06.05). The Over-riding Criteria The decision to locate trees in Road Right-of-Way as described in LDC Section 6.06.03.H. is subject to the over-riding criteria: “Public and private road right-of-way may contain trees and other landscaping material provided their location does not present a traffic hazard, impede drainage, or adversely interfere with the use of the right-of-way by utilities.” ‘Street Trees’ must not present a ‘traffic hazard’. For example, an errant car must be provided ample separation from the edge of pavement to the nearest obstruction such as a tree in order to safely recover. This critical element of roadway design is called the ‘clear zone’ which may be as much as twenty feet or more. Newly planted street trees must not be located within the clear zone. Another critical element of roadway design is called ‘sight distance’. For example, sight distance is the driver’s ability to see oncoming traffic while stopped at an appropriate distance from the intersection. Street trees must not obstruct the driver’s view of opposing traffic oncoming from the driver’s left or right.

• Any decision to locate trees in road right-of-way must not violate the over-riding criteria. Public safety is paramount, followed by the concerns for drainage and utilities.


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As are required for subdivisions, the planting of street trees for Scenic Roadways as described in LDC Section 6.06.03.I. are required.

• The planting of ‘Street Trees’ within road right-of-way for Scenic Roadways must not violate the over-riding criteria.

• Likewise, the planting of ‘Street Trees’ within road right-of-way for subdivisions must

not violate the over-riding criteria. Additionally, to locate trees in Road Right-of-Way where there are power lines, other restrictions apply. For the planting of trees near power lines, more specific restrictive conditions are provided in LDC Section 6.06.03.A.5. as follows:

• “To avoid a power line conflict, vegetation that exceeds 25 feet in height at maturity shall not be planted closer than 30 feet of the vertical plane of an existing power line, excluding service wires. The Administrator may grant an exemption to this requirement upon the applicant’s ability to demonstrate the canopy growth structure of a proposed species will not create conflict with an existing power line. Consultation with the local utility representative should occur for assistance on selecting suitable vegetative species.”

This overhead utility standard prohibits the planting of large growing trees within 30 feet of overhead lines. This restriction contributes to lessen the maintenance costs for these utilities in addition to controlling adverse trimming to a tree’s canopy. Although the overhead utility line standard addresses new developments, it does not address vegetation conditions existing prior to the adoption of this LDC provision. Points of Interest are:

• Since the locations for Street Trees are explicitly described in LDC Section 6.06.05.C. as in the road right-of-way, there are potential conflicts with the over-riding criteria.

• To comply with the over-riding criteria, the LDC Street Tree requirements must be

sensitive to clear zones and minimum sight distances at intersections.

• To comply with the over-riding criteria, the LDC Street Tree requirements must be sensitive to unobstructed drainage conveyances.


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• Trees are an extremely important component of urban environments providing shading

and cooling during warm weather, screening of unsightly views, and separation for inconsistent land uses. They function as valuable components of urban ecosystems by absorbing carbon dioxide and other pollutants, while at the same time generating life-giving oxygen. Mature trees improve the aesthetic environment, absorb noise, are traffic calming, reduce stress and crime, and create an inviting place to relax and socialize. However, large ‘Street Trees’ located in close proximity to roads and sidewalks are often inconsistent with transportation safety and function, particularly if measures are not employed to address this inconsistency.

Subdivisions The County’s standard designs for Recommended Utility Locations (see Figure 1) and Local Urban Roads (see Figure 2) are provided in the County’s Transportation Technical Manual. Figure 1: Recommended Utility Locations in Subdivisions


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Figure 2: Typical Road in Subdivisions

As Figure 2 identifies, eight (8) feet is noted between the back of curb and the inside edge of a proposed or established sidewalk on each side of the road within the noted 50 feet of right-of-way. From within the 8 feet between the curb and sidewalk, the same area is recommended for utilities (storm sewer, street light, water, reclaimed water, and wastewater) as shown on Figure 1. Points of interest are:

• From within the 8 feet between the curb and sidewalk, the separation from a sidewalk to a large tree does not provide a suitable horizontal distance to avoid some degree of long term disturbance to a sidewalk from the roots of a large shade tree.

• Keeping the current requirements on new development will result in the continued

maintenance obligation including costs to address long term sidewalk damages caused by tree roots.

• The Transportation Technical Manual’s current Local Urban Road section allows for

sidewalk disturbance caused by roots from trees located outside the road right-of-way.


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County Sidewalk Maintenance Costs The following information is provided by the Public Works Department. This information provides a detailed overview of sidewalk maintenance expenditures within the past decade. This information provides insight into the fiscal impact of sidewalk damages upon the County, as well as the extent and frequency of existing and projected repairs. Table 1: Transportation Maintenance Division Costs between 1 Oct 1999 and 30 Sep 2009

$7,529,575 / 29 miles / 5280 feet per mile = $49.2 per linear foot for sidewalk The average cost for repairs is $49.2 per linear foot of sidewalk (see Table 1). This per foot cost includes all of the costs incurred in the removal and replacement of sidewalks averaged for a year. It includes the cost to not only replace the concrete, but also for grading, sod, mobilizing, tree removal, root barrier, and extra footage to meet ADA (Americans with Disabilities Act) criteria, so that the sidewalks are ADA compliant when the repairs are completed.


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Table 2: TOTAL COSTS SINCE 2002 / Engineering Division

Since 2002, $7.2 Million was spent on ‘replacement’ of sidewalk (see Table 2). Table 3: Projected Costs for Sidewalks and Percent Replacement in Carrollwood Village Area

In 2009, 11.9 percent of the sidewalks require ‘replacement’ for the area inspected (see Table 3).


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Figure 3: Typical Sidewalk Repairs (Photo Left: Grinding, Photo Right: Replacement)

In 2009, 55 miles of sidewalks required repairs. The total cost needed for repairs in 2009 was $9 Million, but only $4.5 Million was funded for repairs by Hillsborough County. Before sidewalk replacement, damages are inventoried and identified for grinding or replacement (see Figure 3). Most initial damages can be temporarily corrected by ‘grinding’ to eliminate the displacement between adjacent sections of sidewalk. Since grinding is frequently the corrective action where noted damages lead to eventual sidewalk replacement, the noted increase in ‘grinding’ is a direct indicator for an expected increase in sidewalk replacement. Table 4: Public Works O & M Comparison

Since 1999, 458 miles of sidewalk were added (see Table 4).


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Figure 4: Repairs and Funding

Comparison of Miles of Repairs Relative To Funding

0

5

10

15

20

25

30

35

FY '10 FY '11 FY '12

Mile

s of

Sid

ewal

k

$0

$1,000,000

$2,000,000

$3,000,000

$4,000,000

$5,000,000

$6,000,000

$7,000,000

Dol

lars

of

Fund

ing

Estimated Repair Total Un-repaired Backlog Estimated New Defects

6

18

2

7

22

2

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18

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18

$1.2 M

$0.4 M

$1.4 M

Funding

Assuming funding remained at $400 Thousand and costs remained at Fiscal Year 2012 levels (see Figure 4), at least 20 miles of sidewalk will need to be replaced by Fiscal Year 2020. This does not account for the ‘Engineering Costs’ for the four community areas at an estimated $5 Million per community area. Potential costs are approximately $30 Million by 2020.


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Mitigation Strategies Right-of-Way Tree Evaluation Due the extensive and reoccurring sidewalk damages caused by trees, Public Works conducted a survey of types of trees causing sidewalk damage within the right-of-way. The survey confirms that the ‘Radius of Root Influence’ far exceeds the sidewalk separation in all cases. The ‘Radius of Root Influence’ is the ‘average’ of the maximum distances measured from the tree trunk flare to the sidewalk segment that has incurred disturbance from tree roots. Consequently, substantial sidewalk damage is expected in all cases unless special design techniques or other best management practices are implemented. The results of the survey are presented in Table 5.

Table 5: Radius of Root Influence

Public Works 2008 SurveyAverage Radius of Root Influence in Feet

Trunk Diameter

Type Tree 10 - 20 inches > 20 inches

Laurel Oak 11.3 13.0Live Oak 7.9 17.1

Sycamore 8.9 16.0

Maple 10.4

Pine 8.4

Elm 14.7

R


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PHOTOS: Large Oak Tree with 30 inches diameter trunk. Roots Cause Sidewalk Damage Tree Root Development To understand the problem of tree roots displacing concrete, one must understand tree root growth and development. Tree root growth is a function of genetics of a particular species, soil, environmental conditions and physics. In general, tree root development is predictable. For trees, roots have two primary functions – water and mineral absorption and physical support. As a tree matures the support function of roots is critical. Indiscriminant removal of tree roots can lead to overall tree failure which for street trees has a high probability for serious property damage. Tree root systems are quite variable but have a general pattern of development. Because street trees are planted from nursery containers, the primary roots that develop from container grown trees are lateral roots with a radial pattern around the trunk of the tree. The lateral roots cause most of the uplifting of sidewalks. Typically, tree root systems of mature tree roots are in the top three feet of soil with most in the top two feet (see Figure 5). Root growth depth is influenced by the soil’s oxygen and moisture content. Better drained soils equal deeper roots and vice versa for poorly drained soils.


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As tree roots grow in diameter, the force generated underneath the sidewalk may increase to 500 pounds per square inch which equals 72,000 pounds per square foot. The upward force is resisted by the weight of the unreinforced concrete which is 50 pounds per square foot for a four inch thick sidewalk. Because unreinforced concrete is weak in tension (tension is the bending force caused by uplifting), the force of a growing tree can easily fracture or displace a four inch thick sidewalk. To withstand such force, theoretically the concrete sidewalk would need to be 5 feet thick for a five-foot wide and 10 feet long section of sidewalk. Figure 5: Mature Tree Development Profile

Considerations for Change:

• For new street trees, adjustment to the location of street trees with greater separation between the sidewalks and trees is essential to minimize potential sidewalk damage.

• For new street trees, where the ‘Radius of Root Influence’ is expected to exceed the

sidewalk separation, special design techniques are needed such as root barriers, reinforced concrete, or modification to sidewalk location design.


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• For existing street trees, where the ‘Radius of Root Influence’ is expected to exceed the

sidewalk separation, other best management practices are needed such as root pruning and root barriers, and reinforced concrete design as shown in this report.

National Issue An international symposium entitled “Strategies to Reduce Infrastructure Damage by Tree Roots” convened in March 2000 at the University of California. This symposium congregated nationally and internationally renowned researchers, educators, tree managers, consultants, landscape architects, infrastructure engineers and urban planners to consider the tree impacts on sidewalks. The symposium resulted in a publication entitled “Reducing Infrastructure Damage by Tree Roots – A Compendium of Strategies”. This compendium identifies strategies to reduce infrastructure damage potential, and contains a comprehensive list of preventive and remedial actions that could be considered to diminish sidewalk maintenance expenditures. In the compendium, strategies are presented in three groups: (1) Tree Based, (2) Infrastructure Based, and (3) Root Based.

1) Tree-Based

Tree-based strategies include species selection and root pruning. Species selection is a preventative strategy only. Although some species are considered to be deep-rooted, limitations such as impermeable layers and high water table often found in Florida will cause deep-rooted species to become shallow-rooted with an increased ‘Radius of Root Influence’ identified in Table 5. Therefore, species selection is of primary importance in narrow planting strips. Root pruning is a remedial strategy only. Special care must be taken with consideration to the amount of root loss, pruning distance from the trunk, the timing of the pruning, and many other factors. Municipalities may establish pruning limitations, such as no more than four (4) roots of four (4) inches in diameter within four (4) feet of the trunk. Roots anchor the tree. Another recommendation is: “Never cut roots closer than the distance of 5 times the trunk diameter.” (Source: Chapter 5, Wind and Trees: Lessons Learned from Hurricanes, Publication FOR #118) Root pruning is an injurious practice that can damage the strength and health of trees.


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2) Infrastructure-Based Infrastructure-based strategies include design approaches that increase tree space. The larger the planting space, the less potential there is for sidewalk damage. Suggested distances between trees and sidewalks vary greatly from 5 to 15 feet depending upon the species and soil structure. Based on the results of our studies presented in Table 5, where the ‘Radius of Root Influence’ exceeds the sidewalk separation, for a four inch thick concrete sidewalk with less than 10 feet of separation for large mature trees, damage is almost guaranteed. Other infrastructure-based strategies include increasing right-of-way, curving sidewalks around trees, eliminating sidewalks, creating a bridge over roots or a ramp, planting pits, gravel layers, and enhanced sidewalk design. Increasing tree space by increasing the right-of-way or planting trees on private properties are viable options (see Figure 6). Figure 6: Increase Tree Planting Space (From In Front to Behind Sidewalk)

Here are the results of lessons learned. Large trees in narrow planting spaces cause damage to sidewalks. Large trees in large planting spaces are the ‘Right Trees in the Right Place’ and can be accommodated by reasonable design techniques. Planting the trees behind the sidewalks with more room to grow is recommended. The alternative tree location would affect LDC Section 6.06.05 Street Trees which addresses the planting of regularly-spaced shade trees along the roadway and adjacent to sidewalks, and may affect the FLUE Policy 16-1.7.


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From another perspective, Dr. Edward Gilman, world renowned tree expert and professor, draws the same conclusions. Figure 7: Chapter 6 by Edward Gilman (http://hort.ifas.ufl.edu/treesandhurricanes/preventive_design.shtml)

In his Chapter entitled Urban Design for A Wind Resistant Urban Forest (see Figure 7), he identifies a strong root system as essential to withstand forces of strong winds. Figure 8: Unsuitable Growing Area

Unsuitable Growing Area Leads to Unstable Trees

Photograph by Chuck Lippi

http://hort.ifas.ufl.edu/treesandhurricanes/preventive_design.shtml


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A strong root system is one of the most critical factors that allow trees to withstand hurricane-force winds. Tree Roots deflected near curbs affect tree stability (see Figure 8).

Dr. Gilman describes three conditions needed for trees to withstand hurricane-force winds: • Sidewalk 10 Feet From Tree

• Trees Behind Sidewalk

• Roots To Be Straight

The planting space needed depends upon tree size at maturity as presented in Table 6.

Table 6: Soil Area Required

Soil required for trees by size at maturity

10’30’ x 30’Large > 50’

6’20’ x 20’Medium < 50’

2’10’ x 10’Small < 30

Distance from Paved Surface

SoilArea

Height or Spread

Source: Urban Design for a Wind Resistant Urban ForestUniversity of Florida


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For infrastructure-based strategies, sidewalk materials and design modifications can include reinforced concrete, expansion joints with dowels, and thicker slabs to resist root pressure. For sidewalks located within 10 feet of large shade trees, an enhanced sidewalk design with thicker slabs at least 6 inches thick with reinforced concrete can be considered. For a properly designed and constructed sidewalk, the average useful life could be 50 years. For this expectation, it is imperative that sidewalk design include consideration of the tree species and planting location, or else all sidewalks must be constructed to a higher standard.

3) Root-Based Root-Based strategies include root guidance systems such as root barriers. Root barriers are both a preventative and remedial strategy. The most common barriers are linear plastic deflectors (see Figures 9 and 10). Roots growing laterally are deflected down below the depth of the barrier. Although not common, concrete footers can be extended downward next to the edge of the tree root zone as shown in Figure 11 (Source: “Reducing Infrastructure Damage by Tree Roots – A Compendium of Strategies”). Linear root barriers installed along sidewalks as part of construction and after root pruning are recommended.

Root barriers are recommended where there is (or will be) a tree within 15 feet of the sidewalk. Installation before tree growth is always preferred for redirecting roots and to avoid root pruning.

Figure 9: Root Barrier

Root barrier is a thick piece of plastic that is located along the sidewalk, 12 to 24 inches deep. It redirects the tree roots downward away from the sidewalk.


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Figure 10: Herbicidal Root Barrier (Source: Hillsborough County Transportation Technical Manual)


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Figure 11: Reinforced Concrete Sidewalk with Footers

Mr. George Gonzalez, the chief forester of the Urban Forestry Division for the City of Los Angeles confirmed that the detail presented in Figure 11 is used in ‘high value areas for high value trees’ as a sidewalk replacement. He described ‘rubber sidewalk’ as another alternative to be considered. Both the reinforced concrete and rubber sidewalk alternative were described as rarely used and cost prohibitive. The details provided by Mr. Gonzalez are published in the referenced “Reducing Infrastructure Damage by Tree Roots – A Compendium of Strategies.”


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Sunnyvale, California: Concrete and Tree Root Mitigation In Sunnyvale there are several methods and procedures used to mitigate concrete and tree root conflicts. For new trees, a root barrier is used and consists of a strip of 40 mil high density polyethylene (plastic about the thickness of a dime) twelve inches wide. The vertical root barrier (see Figure 12) inhibits lateral roots from growing into the sidewalk. Research has demonstrated that roots do not stay down once they pass under a barrier and may reemerge within 5 feet of the root barrier, therefore root barrier is kept near the sidewalk and not the tree. Figure 12: Root Control Barrier Installation at Sidewalk


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Mr. Leonard Dunn, the manager of the Street Tree Services and Concrete Maintenance for the City of Sunnyvale, confirmed that the detail presented in Figure 12 is effective and continues to be used for the construction of new sidewalks and replacement sections. Mr. Dunn is a contributing author on the subject of sidewalks and street trees. His work is published in the aforementioned “Reducing Infrastructure Damage by Tree Roots – A Compendium of Strategies.” As temporary remedial treatments where mature trees have lifted concrete sidewalk, grinding and wedges are used in Sunnyvale to reduce the tripping hazard. Grinding is used where a lifted concrete sidewalk displacement of less than one inch can be ground down to remove the lifted edge for a smooth surface between adjacent concrete slabs. For a lifted concrete sidewalk displacement greater than one inch, an asphalt ramp patch is installed. Ramp patched sidewalk is scheduled for future replacement. At the time of sidewalk replacement, the displaced sidewalk is removed and the tree roots are examined by the Sunnyvale City Arborist. The arborist decides which roots can be safely cut and removed and which must remain. The root evaluation requires an International Society of Arboriculture (ISA) certified arborist with special training in tree roots. Roots that can be cut are severed cleanly at a root branch intersection similar to how above ground tree branches are pruned.


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Local Recommendations Canopy and Tree Location Figure 13: Closed Canopy

Hillsborough County’s current right-of-way design encourages the growth of a ‘closed canopy’ rather than an “open canopy’ (see Figures 13 and 14). From within the eight (8) feet between the curb and sidewalk for Hillsborough County’s current design standard in 50 feet of right-of-way, the separation from a sidewalk to a street tree does not provide a suitable horizontal distance to avoid some degree of long term disturbance to a sidewalk from the roots of a large shade tree. Keeping these requirements on new development will result in a continued maintenance obligation including costs to address long term sidewalk damages caused by trees. The location of large shade trees must have much greater separation between the sidewalks and the trees to lessen potential sidewalk damage. Therefore, a slightly ‘open canopy’ may be preferred, with the trees located behind the sidewalk further from the local roads in subdivisions. Figure 14: Open Canopy


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Figure 14 shows trees 30 years old located behind the sidewalks, separated by 70 feet from one side of the street to the other. The ‘Recommended Tree Location’ is 58 feet, 12 feet closer for more street shade (see Figure 15). Figure 15: The Recommended Tree Location

Solution: An Alternative Tree LocationBetter Trees and More Shade

Now34 Feet

Recommended58 Feet

Here are the results of lessons learned. Trees located at 34 feet cause damage. Trees at 58 feet are the ‘Right Trees in the Right Place’. The recommended tree location provides ‘better’ healthier trees with more room to grow, and more area shaded for more energy efficient homes, and lower repair costs. It creates ‘sustainable’ safe sidewalks that are more pleasant for the users, and a higher quality of life.


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Curb Design and Sidewalk Location Figure 16: Miami Curb in Neighborhood

Figure 17: Barrier Curb in Neighborhood

To minimize sidewalk disturbances and provide for better healthier trees, a revision of the County’s standard design for Local Urban Roads (Drawing #TS-3, see Figure 2) is recommended. The revisions are presented as part of the ‘Right-of-Way Use Options’.


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Right-of-Way Use Options There are three ‘Right-of-Way Use Options’ to represent design variations. Option A is the ‘Status Quo’, the current County standard (see Figure 18). Options B and C are the viable options (see Figures 19, 20, 21, 22). B and C have more choices.

• A: The tree is in the narrow planting strip, 3 feet from the sidewalk

• B1: The tree is behind the sidewalk in an easement, 8 feet from the sidewalk

• B2: The tree is behind the sidewalk in an easement, 11 feet from the sidewalk

• C1: The tree is behind the sidewalk in the right-of-way, 8 feet from the sidewalk

• C2: The tree is behind the sidewalk in the right-of-way, 11 feet from the sidewalk Figure 18: Option A

Status Quo 50 Feet Right of Way

10’ 5’ 3’ 2’ 10’Utility

EasementSide-walk

Traffic Lane

MiamiCurb

Option A

CLR/W Line 25’ Half of ROW

5’


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Figure 19: Option B1

50 Feet ROW and Tree Easement

Option B1 – Miami Curb

8’

5’Sidewalk

10’Traffic Lane

2’Curb

4’Sod10’

Utility Easement

5’Tree

Easement


Figure 20: Option B2

50 Feet ROW and Tree Easement

Option B2 – Barrier Curb with 6’ Sidewalk

6’Sidewalk

10’Traffic Lane

2’Curb

11’

5’Tree

Easement

10’Utility

Easement



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Figure 21: Option C1

60 Feet ROW with Tree

Option C1 – Miami Curb

5’Sidewalk

10’Traffic Lane

2’Curb

4’Sod

30’ Half of ROWCLR/W Line

10’Utility

Easement

8’

Figure 22: Option C2

60 Feet ROW with Tree

Option C2 – Barrier Curb and 6’ Sidewalk

6’Sidewalk

10’Traffic Lane

30’ Half of ROW

2’Curb10’

Utility Easement

11’

CLR/W Line


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Benefits The topics of importance are presented as part of a survey in Table 7. The benefits of each option are presented in the Comparative Benefits Matrix (see Table 8). Table 7: The Survey and Topics of Importance

Rate importance of each factor

3 – High2 – Medium1 – Low

Value WeightingSurvey

Environmental Numerical Value

• Tree Stability in Storms ____• Minimizing Tree Canopy Impacts ____• Minimizing Tree Root Damage ____• Tree Health ____

Costs

• Construction (New) ____• Tree Maintenance ____• Sidewalk Maintenance ____• Liability Claims ____

Livable Roadways

• Shading of Sidewalks and Streets ____• Sidewalks along Both Sides of Street ____• ADA Compliance ____• Roadway Aesthetics ____

Sustainability

• Utilities ____• Sidewalks ____• Tree Resource ____• Fiscal Responsibility ____


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Table 8: Comparative Benefits Matrix

Benefits Legend:Benefits Legend: Option AOption A Option B1Option B1 Option B2Option B2 Option C1Option C1 Option C2Option C2H = HighH = HighM = Moderate M = Moderate L = LowL = Low

Environmental ConsiderationsEnvironmental ConsiderationsTree Stability in StormsTree Stability in Storms LL HH HH HH HHMinimizing Tree Canopy ImpactsMinimizing Tree Canopy Impacts LL HH HH HH HHMinimizing Tree Root Damage Minimizing Tree Root Damage LL HH HH HH HHTree HealthTree Health LL HH HH HH HH

Cost ReductionCost ReductionConstruction CostsConstruction Costs HH LL LL LL LLTree Maintenance CostsTree Maintenance Costs LL HH HH MM MMSidewalk Maintenance CostsSidewalk Maintenance Costs LL HH HH HH HHLiability Claims CostsLiability Claims Costs LL HH HH HH HH

Livable Roadway ConsiderationsLivable Roadway ConsiderationsShading of Sidewalks and StreetsShading of Sidewalks and Streets HH MM MM MM MMSidewalks along Both Sides of StreetSidewalks along Both Sides of Street LL HH HH HH HHADA ComplianceADA Compliance LL HH HH HH HHRoadway Aesthetics Roadway Aesthetics HH HH HH HH HH

SustainabilitySustainabilityUtilitiesUtilities MM MM MM MM M M SidewalksSidewalks LL HH HH HH HHTree ResourceTree Resource MM HH HH HH HHFiscal ResponsibilityFiscal Responsibility LL HH HH HH HH

All of the new options (Options B1, B2, C1, C2) have almost equal benefits while solving the problems.


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Summary Although street trees are specifically required by the County’s Land Development Code (LDC), the locations required for planting the large growing shade trees intended as ‘Street Trees’ appear to be incompatible with current County design standards. Elements of the design including the sidewalks and roadway safety have been compromised. Revisions to County design standards and code amendments are recommended. Landscaping must be consistent with roadway planning and design that integrates transportation safety and function with other considerations such as the useful life of the road and sidewalk. When tree characteristics and site characteristics match, the result is “the right tree in the right place”, an internationally recognized arboriculture standard. Public safety is paramount. Street trees must not present a ‘traffic hazard’. Large ‘Street Trees’ located in close proximity to roads and sidewalks are generally inconsistent with transportation safety and function. There are currently 2,339 miles of sidewalks within unincorporated Hillsborough County. Potential costs for sidewalk reconstruction and repairs are approximately $30 Million by 2020. With recommended revisions to County standards, the upward spiraling costs for sidewalk repairs can be reversed. For remedial actions addressing existing disturbances to sidewalks, an immediate reduction in annual expenditures is an unreasonable expectation. The existing conditions will continue to be problematic. Recurring repairs are expected until recommended best management practices can be routinely implemented. For some locations the difficult decision must be made to remove the imposing street trees, an undesirable option affecting the myriad of values these large growing trees contribute to our urban environment. Gradually as the older street trees decline and are removed, preventative measures as illustrated in this report can be employed when replacement trees are installed. Of the 2,339 miles of sidewalks, most are within residential subdivisions. Therefore if this trend continues, the most efficient way to reduce costs for repairs is to modify the County’s design standards for residential developments and amend County codes to implement these improved designs. Since the sidewalk and curb displacements are tree root related, the most effective way to reduce future costs for repairs is to require enhanced sidewalk designs within all future residential subdivisions where there are (or will be) ‘Street Trees’ near the sidewalks. Implementing modified design standards is the most efficient and effective way to prevent most of the sidewalk damages caused by ‘Street Trees’ along County right-of-way.


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Species Selection Species selection is of primary importance, especially in narrow planting strips. Every tree species has cultural requirements that need to be met for it to survive and thrive. When tree characteristics and site characteristics match, the result is the “Right Tree in the Right Place.” For example, large trees at maturity require a 30 feet by 30 feet area (Source: Chapter 6, Urban Design for a Wind Resistant Urban Forest). Recommended Minimum Design (New Subdivisions) In our recommendations, there are three parts to stop the uplifting of sidewalks:

• 10 feet of Separation from the tree to the sidewalk.

• Root Barrier. Root barrier is a thick piece of plastic that is located along the sidewalk, 12 to 24 inches deep. It redirects the tree roots downward away from the sidewalk.

• Crushed Base. For sidewalks less than 10 feet from a large growing shade tree, ‘crushed

base’ is needed under the sidewalk, and the root barrier. Figure 23 shows the location of the crushed base and root barrier for a length of twenty feet centered at the tree along the nearest edge of the sidewalk.

Figure 23: Sidewalk Design with Root Barrier and Crushed Base

4” 12”

Sidewalk Street Tree

4”

Crushed Stone Base

Root Barrier4”

If Less Than 10 Feet Separation

10’ 10’ 8’

Tree

Curb

Sidewalk

Side View

Plan View


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Enhanced Sidewalk Design For sidewalks located within 10 feet of large growing shade trees, an enhanced sidewalk design with thicker slabs at least 6 inches thick with reinforced concrete can be considered. Sidewalk materials and design modifications may include reinforced concrete, expansion joints with dowels, and thicker slabs to resist root pressure. For a properly designed and constructed sidewalk, the average useful life could be 50 years. For this expectation, it is imperative that sidewalk design include consideration of the tree species and planting location, or else all sidewalks must be constructed to a higher standard. Root-Based strategies include root guidance systems such as ‘Root Barriers’. The most common barriers are linear plastic deflectors. Roots growing laterally are deflected down below the depth of the barrier. Root barriers are both a preventative and remedial strategy. Linear barriers should be installed along sidewalks where there will be tree roots expected from large growing trees. All sidewalks and crosswalk ramps must be ADA compliant (see Figure 24). Therefore, it is imperative that the design of sidewalks and crosswalk ramps must comply with all ADA requirements and must remain compliant for a reasonable period before repairs become necessary. For this purpose, and with the recommended mitigation strategies, a useful life of 50 years should be expected. Infrastructure such as sidewalks should be managed on a ‘life-cycle’ basis to accommodate the street trees, not simply built then replaced when it wears out by being damaged by tree roots. Implementing this infrastructure management approach requires determination to see projects built to a more robust standard which could prove to be one of the simplest ways for governments to same money (Source: PE, by NSPE, October 2009).


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Figure 24: ADA Requirements


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Repairs: Best Management Practices Although some repair and maintenance of sidewalks and street trees along County right-of-way is expected, the frequency and locations of recurring sidewalk repairs and street tree maintenance indicate changes are needed. There are some locations requiring multiple recurring sidewalk repairs which are inconvenient, costly, and may be prevented by adhering to best management practices. Recommendations are provided for revisions to the County design standards, for methods of repairs, and for tree maintenance. Grinding and Patching As temporary remedial treatments where mature trees have lifted concrete sidewalk, grinding and asphalt wedges are used to reduce the tripping hazard (“Reducing Infrastructure Damage by Tree Roots – A Compendium of Strategies”). Grinding is used where a lifted concrete sidewalk displacement of less than 1 inch can be ground down to remove the lifted edge for a smooth surface between adjacent concrete slabs. For a lifted concrete sidewalk displacement greater than 1 inch, an asphalt wedge or ramp is installed. Ramp-patched sidewalk is scheduled for replacement. Root Pruning At the time of replacement the displaced sidewalk is removed and the tree roots should be examined by an arborist. The arborist decides which roots can be safely cut and removed and which must remain. The root evaluation requires an International Society of Arboriculture (ISA) certified arborist with special training in tree roots. Where the arborist has determined which roots can be cut and which must remain, root pruning and root protection is performed. Roots that can be cut are severed cleanly as practical at a root branch intersection similar to how above ground tree branches are pruned. Root pruning is an injurious practice that can damage the strength and health of a trees. Tree strength and stability must not be compromised. Trees need to resist strong winds to prevent property damage caused by fallen trees. Public safety is paramount. Special care must be taken with consideration to the amount of root loss, pruning distance from the trunk, the timing of the pruning, and many other factors. Municipalities may establish pruning limitations such as no more than 4 roots of 4 inches in diameter within 4 feet of the trunk. Another recommendation is: “Never cut roots closer than the distance of 5 times the trunk diameter” (Source: Chapter 5, Wind and Trees: Lessons Learned from Hurricanes, Publication FOR #118). All roots under proposed sidewalk within 12 inches of the surface should be removed, or the proposed sidewalk should be elevated for greater separation to reduce future damage to the sidewalk by tree roots.


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Figure 25: Root Pruning Results

Source: Chapter 5, Wind and Trees: Lessons learned From Hurricanes, University of Florida, http://hort.ifas.ufl.edu/treesandhurricanes/wind_and_trees.shtml The roots on these trees were cut to build new sidewalk making the tree vulnerable to hurricane-force winds. Enhanced Sidewalk Design (Replacement) For sidewalks located within 10 feet of large shade trees, crushed base and root barrier is recommended. Thicker slabs at least 6 inches thick with reinforced concrete can be considered. Sidewalk materials and design modifications can include reinforced concrete, expansion joints with dowels, and thicker slabs to resist root pressure. For the 6 inches thick reinforced sidewalk replacements, monolithic sections 10 feet in length should be considered. Root barriers should be installed at the earliest opportunity along all sidewalks that will be within 15 feet of tree roots. All enhanced sidewalk designs should include root barriers. The most common barriers are linear plastic deflectors (see Figures 9, 10, 12). Roots growing laterally are deflected down below the depth of the barrier. Although not common, concrete footers can be extended downward next to the edge of the tree root zone (see Figure 11). Root barriers are both a preventative and remedial strategy. Linear barriers installed along sidewalks after root pruning are recommended.

Revisions to Land Development Code In addition to revisions to County design standards, code amendments are recommended. The recommended revisions provide clarification for responsible design entities and related technical criteria that must be adhered to.

http://hort.ifas.ufl.edu/treesandhurricanes/wind_and_trees.shtml


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Sec. 6.06.05. Street Trees [SUGGESTED REVISIONS] C. Location and Placement 1. Street trees shall be planted within or adjacent to the road right-of-way a minimum of five feet from the back of curb and a minimum of ten three feet from the inside edge of a proposed or established sidewalk, unless otherwise approved by the County Administrator. When feasible possible, street trees should be centered and evenly spaced along the in front of the lot and evenly spaced along on roadways not fronted by lots. To minimize sidewalk damage, root barriers shall be required adjacent to the sidewalk along the side closest to the tree for a distance of twenty feet centered at the tree. 2. When not feasible to plant trees in the road right-of-way (such as for reasons of public safety or concerns for drainage or utilities), as determined by the Administrator, plantings shall be required permitted on the lot, provided the trees are installed within twenty five feet of the right-of-way. 3. Street trees shall be planted a minimum of ten feet from any above ground utility, such as transformer pads and fire hydrants. 4. Public and private road right-of-way may contain trees and other landscaping material provided their location does not present a traffic hazard, impeded drainage, or adversely interfere with the use of the right-of-way by utilities. Maximum flexibility shall be given to the placement of street trees to accommodate improvements such as driveways, utilities, lighting, etc. The Utility Notification Center should be notified to verify the location of utility lines prior to planting. 5. Street tree plantings shall be in compliance with the planting standards and roadway landscaping guidelines described in Livable Roadways Guidelines August 1, 2006 and subsequent revisions or replacements when needed, and consistent with the criteria contained in the County’s transportation, stormwater management, and water and wastewater technical manuals. Guidelines for Landscaping Hillsborough County Roadways 6. Underground facility damage shall be prevented. In accordance with the Underground Facility Damage Prevention and Safety Act (Chapter 556, F.S.), the Contractor shall call the Sunshine State One Call of Florida (SSCOF) at 811 or 1-800-432-4770 forty eight hours in advance of any excavation, including but not limited to excavations for tree plantings. Responsibility of the Engineer of Record Although construction plans are submitted to the County for review and project approval, as with all projects, the Engineer of Record shall be solely responsible for all design data and calculations submitted, and for ensuring that the construction plans submitted adhere to the criteria contained in the technical manuals (4.1.4.1.2.2.2.D.1., DRPM). Other related laws require the same including Florida Administrative Code 61G15 and Florida Statute Chapter 471. Although County staff is available to answer questions and to provide technical assistance at each step in the approval process, it is incumbent for the Engineer of Record to maintain responsibility for each project and to provide designs that will endure the expected useful life of sidewalks located in the vicinity of street trees.


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Contributors There have been 100s of hours of research and development spent on this initiative by the staff of the Hillsborough County Planning and Growth Management Department and others. Hillsborough County Planning and Growth Management Department

• Kim Ford • Bob Campbell • Carmen Telesca • John Schrecengost

Hillsborough County Public Works Department

• Roger Cox • John Newton • Mike Williams • Richard Sanders • Mike McClelland

Hillsborough County ADA Liaison

• Sandra Sroka Hillsborough County Communications

• Stacy Williams Liz Ellis, City of Seattle Leonard Dunn, City of Sunnyvale George Gonzalez, City of Los Angeles John Oldenburg, City of Miami Beach References

• Reducing Infrastructure Damage By Tree Roots: A Compendium of Strategies, 2003 • Hillsborough County Transportation Technical Manual • United States Access Board, http://www.access-board.gov/index.htm • Hillsborough County Comprehensive Plan, 2008 • Hillsborough County Land Development Code • Livable Roadway Guidelines, 2006, Metropolitan Planning Organization • Chapter 6, Urban Design for a Wind Resistant Urban Forest, Edward Gilman

report: sidewalks and street trees 09.15.2010

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