The primary objectives of vegetation management are to:
- Minimize the risk of trees or branches falling into transmission lines and causing electric service outages
- Maximize the safety of all people who, for any reason, need to be in the vicinity of Platte River's transmission lines
- Maintain sufficient clearance on rights-of-way and access routes to allow adequate inspection of facilities and emergency response
- Meet all federal regulatory guidelines for vegetation management
Platte River Power Authority accomplishes these objectives through: management zones, inspections, tree removal and pruning.
Platte River acknowledges two zones in managing vegetation near high voltage power lines: wire zone and border zone.
The wire zone includes vegetation both under and to the sides of high voltage transmission wires - up to the edges of rights-of-way. Only low-growing ornamental trees, shrubs and grasses are allowed to flourish in this zone. Vegetation exceeding 15 feet in height within the right-of-way will be removed — unless an exception exists as described below.
Exceptions to the clear-cut wire zone exist where a transmission line is routed through an urban area or high-elevation spans over canyon bottoms.
- In urban areas, the trees are maintained at a maximum 15 feet height within the right-of-way, depending on tree species, growth rate, species failure characteristics, local climate and rainfall patterns, line terrain, wire elevation and location of the vegetation within the span.
- In cases of high-elevation spans over canyons, vegetation is evaluated on a case-by-case basis and managed selectively depending on growth rates, span location, local climate and rainfall patterns and maximum expected species growth. Inspections shall occur more frequently in these areas. No overhanging tree branches are allowed on the transmission system.
The next zone outward is the border zone where trees are managed using a 45-degree rule. Starting at the edge of the wire zone, trees are pruned at a 45-degree angle up and away from the wire zone. Vegetation is managed in this manner unless it is deemed a hazard. All trees deemed a hazard are removed. This type of management prevents trees from falling into and contacting transmission lines. All mature plant species are allowed to grow beyond the border zone. The entire right-of-way becomes a transition zone between the mature forest and the vegetation found within the right-of-way.
Inspection and access requirements
Platte River requires that access to its rights-of-way and facilities allow safe and effective inspection of the facilities. To assure safety and effectiveness, the following guidelines are followed:
- Inspection of structures: Brush and trees must be completely removed at least ten feet inward and outward from the outer circumference of transmission structures and down guys.
- Vehicular access: Access lanes of at least 10 to 12 feet in width must be kept clear of interfering vegetation.
- Structure-To-Structure line-of-sight: Brush and trees will be removed to allow visual inspection from the ground of conductors and structures, one span in either direction.
- Fire danger: In critical areas, heavy vegetation must be removed from the right-of-way if damage to facilities is possible from wildfires.
Transmission line, vegetation and right-of-way inspections occur a minimum of once per calendar year with no more than 18 calendar months between inspections. This schedule is based on the historical anticipated growth of vegetation, as well as other environmental and operational factors that impact the relationship of vegetation to Platte River's transmission lines. Trees classified as fast growing (expected growth of 24 inches per year) include, but are not limited to: Cottonwood, Silver Leaf Maple and Weeping Willow. Fast growing trees are noted during inspections and line sections are prioritized for more frequent, routine inspections than for the whole line, if vegetation conditions warrant.
Aerial or ground patrols are conducted at least once per calendar year on the entire system. Additional patrols are conducted on an as-needed basis when system conditions warrant. Some aerial patrols inspect all transmission facilities as well as vegetation on rights-of-way.
The timelines for inspections are flexible and can deviate from scheduled inspections to adjust for changing conditions.
Tree removal and pruning
Fast growing trees within, and adjacent to, transmission line rights-of-way can interfere with electrical service and cause power outages. Platte River recognizes that in such situations, tree removal is generally superior to trimming, especially for trees deemed a high risk to electrical service or to the general public. A Platte River representative will notify property owners regarding the need for pruning or removal. When Platte River removes a tree, the preferred method is to apply herbicide immediately to the cut stumps that are likely to re-sprout; this action depends on safety concerns, landowner permission and any environmental restrictions.
Hazard trees within the right-of-way will be promptly removed. Hazard trees adjacent to Platte River rights-of-way will be managed through documented landowner contact and negotiation for removal or trimming. ANSI A300 IVM defines a hazard tree as "any tree located on or adjacent to a transmission line right-of-way or substation that could damage the facilities should it fall where (i) the tree leans toward the right-of-way or (ii) the tree is defective because of any cause, such as heart rot, shallow roots, excavation, bad crotch, dead or with dead top, deformity, cracks or splits, or another reason that could result in the tree or a main lateral of the tree falling". Extreme conditions such as snow loading, high winds and saturated soils will be taken in to account when evaluating trees for potential hazards.
When trees are not removed, the maximum amount of clearance within the right-of-way will be obtained by pruning.
Landowner notification and permission
Platte River always attempts to notify landowners of anticipated tasks to be performed on a right-of-way. This gives property owners time to raise any concerns they may have. When federal regulatory violations are imminent, Platte River will perform work if attempts to contact property owners are unsuccessful.
Platte River prefers to take a preventative approach to vegetation management rather than a reactive approach by recommending that any new trees planted on, or near, transmission rights-of-way have mature heights that comply with the height requirements mentioned earlier.
Planting according to mature height will reduce the likelihood that trees will eventually need to be pruned or removed.
For a guide to trees and their mature heights, go to Colorado State University's Cooperative Extension web site: www.ext.colostate.edu/ptlk/1715.html.