Street trees and new or second vehicular crossings

In order to protect the city’s street tree population, the application for a new vehicular crossing in the vicinity of a street tree will undergo thorough investigation by Council officers. When applying for construction of a vehicular crossing in the vicinity of an existing street tree, Council will not accept or accommodate non-disclosure or incorrect plotting of street trees on a site-plan.

How Council arborists assess trees for preservation near vehicular crossings

Street trees are highly valued by the community and Council is committed to the retention and enhancement of the Urban forest within the City of Onkaparinga. Street trees are valued for more than their aesthetic appeal as they also promote economic development in commercial areas, increase residential property values, energy savings, improvement to air quality and mitigation of the impact of storm water.

Process

Application for the construction of a vehicular crossing must be in accordance with Council's Permits process which includes tree assessment.

First invert / crossover

Trees will only be removed after all alternatives to retain the tree have been exhausted. This applies to all reasons for removal, including as a consequence of the development or redevelopment of a property, such as facilitating a driveway installation. Our City Arborist (CA) will inspect each request once received from a planning officer and will provide comment back as part of the development process, Planning officers will then negotiate changes if tree is to be retained or can approve the application if no alternative but to remove the tree for a fee based on removal and replacement cost and inform us of the decision so we can schedule in the works.

Second invert / crossover

Second invert/crossover is for an applicant to complete the required council application form together with payment of the fee (in accordance with council’s fees and charges schedule).  Once an application is received it is considered by council’s civil maintenance technical officer who inspects the proposed location and details any requirements or changes required to the application. If trees will be impacted by the installation of the crossover, the civil maintenance officer will notify council’s Permit Officer who will forward to the applicant a tree assessment form to be completed and returned to council.  Council will then inspect applying council’s tree assessment criteria.  If the tree meets the assessment criteria the tree will approved for removal, the council will remove the tree at council’s cost.  If the tree is not approved for removal, the council will consider an alternative location for the invert/ crossover or if no other location is suitable the applicant is required to pay a fee based on the removal and replacement costs of the individual tree or the application will be declined with the relevant reasons.

Root system development

Tree health and stability can be affected by excavation to construct a vehicular crossing as tree roots are generally shallow in the soil and can travel well beyond the dripline of the trees crown. Tree root growth is opportunistic and occurs where the essentials for growth are present and is not always symmetrical in form and depth. The base of the tree trunk typically flares out into buttresses which extend into the main lateral structural roots. The main structural roots rapidly divide into smaller roots which anchor the tree into the soil and serve as a conduit for water and nutrients.

Diagram 1: tree root growth
Diagram 1: Tree root growth

It is often assumed that tree roots extend deep underground and out to the crown dripline. In the majority of cases, tree roots are actually shallower in the soil profile and can extend well beyond the crown dripline. As a result, tree roots are vulnerable to surface disturbances during development projects.

Trees planted within linear street road reserves typically develop asymmetrical root systems because of the restricted rooting area and obstacles such as heavy curbing and pavement, creating a concentrated mass of roots in this area.

Construction of new driveway crossings

In general, a vehicle crossing must be constructed in a material which is consistent with the prevailing crossing construction material in the street. Minimum tree protection distances (separation) are to be maintained.  Tree protection distances will be allocated by Council’s Senior Arborist or delegated officer(s).

 

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