Chapter 2: Vegetation Management Flashcards
momentary interruptions
loss of electricity supply for small amounts of time that result from the fuses opening and then closing again as a result of the voltage changes not large enough to completely shut down the power
sustained interruptions
loss of power supply for a prolonged period ranging from a few minutes to hours or days; fairly long periods without power
energized conductors
conductors through which electricity is flowing
customer minutes lost
CML; a measure of the performance of a utility in terms of the length of time customers are off line as a result of a service interruption; it is simply the length of the interruption in minutes times the number of customers off line
system average interruption frequency index
SAIFI; a measure of the number of interruptions that occur on a utility’s system
customer average interruption duration index
CAIDI; a measure of how long customers have been without power over a given period of time
What are the ways that regulating agencies assess the performance of utilities in terms of outage frequency and outage duration?
customer minutes lost (CML), system average interruption frequency index (SAIFI), and customer average interruption duration index (CAIDI)
vegetation management
the control of wanted and unwanted vegetation on utility rights-of-way for various objectives
transmission line
conductors used to transmit electricity from the generating station to the distribution network that usually carry voltages in excess of 100,000 volts
distribution line
electricity lines that carry the power that is to be delivered to customers
How much does the Electric Power Research Institute (EPRI) estimate that electric utilities in the United States spend annually on vegetation management?
billions of dollars
What are the benefits of proper cost-effective vegetation management?
Hold or reduce maintenance costs while minimizing service interruptions and maximizing public and worker safety.
How is managing vegetation different than the engineering or hardware components of the system?
trees are dynamic, they change and grow over time, and the growth rate, form, and habit for every tree species are different and every individual tree is constantly changing; pylons, towers, poles, transformers, and conductors are static, they do not grow, change size, or change shape.
principles for successful and comprehensive vegetation management programs
- constant yet flexible to respond effectively to natural variability
- long-term commitment from upper management
- proactive rather than reactive
- utilize proper arboricultural practices to minimize costs and maximize clearance effectiveness
- based on the principles of integrated vegetation management (IVM) for efficiency and environmental sustainability and stewardship
- sound and efficient record-keeping system that facilitates managing productivity and measuring long-term success
- designed, administered, and supervised by professionals with sound arboriculture knowledge, experience, and technical expertise
right-of-way
area of land under and just adjacent to overhead conductors over which the utility has a negotiated right to enter and maintain vegetation when required
integrated vegetation management
IVM; a process in which vegetation is pruned or removed using a variety or combination of methods including pruning, herbicides, mowing, and biological control
How does IVM manage right-of-way vegetation?
- Problem trees and vegetation are identified
- Action thresholds are considered
- Various management options are evaluated
- Selected management treatments are implemented
What are the types of IVM management options?
biological, chemical, cultural, manual, and mechanical techniques
How do you choose the type of IVM techniques to use?
choice based on effectiveness, environmental impact, site characteristics, worker and public health and safety concerns, and economics
What is the desired outcome of IVM?
sustainable shrub-grass-forb communities that do not interfere with overhead power lines, pose a fire hazard, or hamper access; creates an ecosystem of low-growing plant communities such as grasses, herbs, forbs, wildflowers, and shrubs; provide natural habitat for rare and endangered plants
Benefits of IVM
low-growing vegetation can eventually dominate the right-of-way and inhibit tall-growing vegetation, thus providing cultural and biological management of right-of-way incompatible species and reducing the need for future treatments; reduced erosion; enhanced plant diversity; sustainable supply of forage and cover for wildlife; corridors for wildlife movement and wildlife viewing opportunities
What is the first step in vegetation management?
obtaining detailed information on the tree and brush workload to be managed; vegetation can be managed only when the extent and nature of the situation are known
brush
tall-growing trees less than some defined minimum diameter, often 4 inches
survey
a method to determine vegetation conditions throughout the system
inventory
a list of trees and vegetation present on the system
100% inventory
catalogs all the vegetation conditions on the entire overhead network; requires counting every tree and area of brush on the system
Pros and Cons of 100% inventory
Pro: most accurate approach
Con: very time consuming and expensive
statistically valid, random sample survey
can accurately project the workloads at a much lower cost than a 100% inventory; involves surveying sections of a larger system and collecting enough data to facilitate accurate projections of the entire system
What is a common level of accuracy used in utility vegetation surveys?
+/- 10% at a 90% level of confidence; which means if a survey projects an estimated total of 500,000 trees, one can be 90% confident that the actual figure is between 450,000 and 550,000 trees; if a higher level of confidence is required then more system sites must be surveyed
high-voltage lines
lines with voltages greater than a specified amount, depending on the utility; typically in excess of 480 volts
low-voltage lines
the measure of electrical potential, which is not greater than 1,000 volts between phases
Any management based on inadequate or inaccurate information can only be what?
Reactive; responding only to problems and not addressing the problem at the core
cycle
the length of time between each maintenance pruning for a given circuit or area; management and budgeting tool to guide the program; may be adjusted - should be flexible
How to determine the pruning cycle?
frequency of pruning operations must be based on the growth rates of the common species and the clearances that can reasonably be achieved while adhering to appropriate arboricultural practices; knowing the species composition and their regrowth rates following pruning is essential for establishing pruning cycle frequency for specific clearance distances
When does the traditional cycle schedule maintenance?
just before trees begin to grow into the conductors, increasing reliability, safety, and effectiveness of previous maintenance expenditures
How to schedule vegetation management operations in a cost-effective way?
on a circuit or geographic area basis
conventional multiyear cycle
cycle (length of time between pruning events) based on growth rates and achievable clearances that presumes that trees, once pruned, will continue to grow toward and into the conductors at a predictable rate
Pros and Cons of conventional multiyear cycle pruning
Pros: generally the easiest to manage; allows the utility to allot the appropriate level of funding so that all the trees and vegetation on the system will be maintained within the length of the cycle
Cons: not necessarily the most efficient approach
five-year cycle
20% of the system will be scheduled each year, and 100% of the system will be completed over the five years
What is an optimal cycle?
long enough cycle so that the clearance provided will not be eroded by regrowth before the next scheduled pruning event; depends on the frequency of occurrence of individual tree species throughout the entire service area of the utility and the amount of clearance that can be achieved in different divisions of the service area
just-in-time cycle
all circuits are maintained each year, but only those trees within a prescribed clearance standard are pruned or removed; targets maintenance resources directly at the problem trees before outages occur and technically provides the greatest reliability
Pros and Cons of just-in-time cycle pruning
Pros: resources targeted directly at problem trees before outages occur; provides greatest reliability
Cons: requires that crews spend a lot of time traveling between problem trees rather than actually pruning; significant costs are involved in patrolling, surveying, recording, cataloging, and mapping the trees that need maintenance
conventional multiyear cycle with mid-cycle maintenance
involves establishing a regular cycle but includes repruning of fast-growing species midway through the cycle
cycle busters
fast-growing trees that need to be pruned more frequently than the set cycle; problematic fast-growing species because their growth rates compromise even the most efficient cycles; growth rates could compromise reliability well before the end of the cycle if left to the defined cycle
Why would you implement mid-cycle pruning for the cycle busters?
mid-cycle pruning for the cycle busters may result in lower annual costs than the alternative, which would be to introduce a shorter cycle; only used to prune those trees that are likely to encroach into the conductor’s defined security zone before the normal maintenance cycle of pruning is due to occur; provides an opportunity to identify and assess hazard trees and to implement hazard abatement measures such as removal or pruning, or both, to remove branches that are likely to fail
reliability-centered management
RCM; a method to minimize tree-caused outages by conducting a detailed analysis of the system, vegetation conditions, outage history, infrastructure, and other factors, and identifying a range of preventative maintenance tasks that have the potential to provide greater reliability and cost efficiency than pruning and removal alone; might incorporate changes in over-current protection, changes in infrastructure, and changes in operating practices in addition to tree pruning and removal
What is the objective of reliability-centered management (RCM)?
to minimize tree-caused interruptions but not exclusively through traditional vegetation management (pruning and tree removal) methods and fixed-period cycles; relies on conducting a detailed analysis of the system, vegetation conditions, outage history, infrastructure, etc. and identifying a range of preventive maintenance tasks that have the potential to provide greater reliability and cost efficiency than pruning and removal alone
infrastructure
the machinery used to transmit and distribute electricity, including poles, towers, conductors, transformers, substations, etc.
What are the two historical approaches to vegetation management within the utility industry?
preventative (proactive maintenance) and reactive (corrective maintenance)
preventative/proactive maintenance
the management of trees and vegetation on the overhead lines following a survey or assessment of the problems and scheduling maintenance before vegetation causes any interruptions; identifies and addresses the reasons for outages and safety hazards through systematic and consistent management; based on an inventory approach; provides maximum benefit from the resources expended and maximizes cost effectiveness
reactive/corrective maintenance
management of trees and vegetation in response to interruptions caused by vegetation; relies on responding to problems as they occur; hotspotting; ineffective and inefficient; work on trees and brush is scheduled only when they have caused outages or represent significant safety hazards in response to requests from property owners for maintenance or in response to patrol reports
hotspotting
a process by which limited tree work is scheduled on a specific area of the network where shorts or outages are occurring
Which is the better maintenance approach?
preventive maintenance is the better approach to improving reliability and safety by reducing tree-related outages; cost-effective in the long term as it optimizes the results from the resources allocated; cost of each successive cycle becomes lower as the vegetation is brought under control and populations are stabilized
What happens when utilities pursue a policy of reactive maintenance?
the frequency of tree-related outages increases; safety related issues become more prevalent and may result in increased spending; trees and other vegetation on the network grow into and beyond the conductors - the closer trees are to conductors, the higher the cost and hazard of pruning them to provide the required (desired) clearances; deferring electric utility tree maintenance results in increased maintenance costs
What did The Economic Impacts of Deferring Electric Utility Maintenance (Journal of Arboriculture) find?
deferring maintenance beyond the appropriate cycle produces a significant increase in the cost per tree pruned; the higher cost is caused by an increased number and size of branches that need to be pruned and the increased difficulty associated with removing branches that have grown into or beyond the conductors; increase costs are also associated with the disposal of larger volumes of wood, waste, and debris; the longer the time between pruning events, the more the trees will have grown and the higher the cost associated with pruning and disposal
hazard tree
a tree whose condition is such that it could fail and cause damage to conductors or other hardware; a tree that has the potential to fail due to structural defects or conditions within the tree and if there is an identifiable target that the tree could hit if failed (if no potential target exists that could be damaged, a tree cannot be regarded as hazardous)
structural defects
decay, points of weakness such as poor branch attachment, and/or large regrowth from an old topping point, and large deadwood throughout the crown of a tree
target
any object that may be damaged or hit by a hazard tree
Types of hazard tree failures
shedding parts, snapping of the trunk, total collapse
What happens when a branch contacts across conductors? How is it remedied?
a fault is caused by the branch bridging the two conductors that are normally insulated from each other by air; the situation can be remedied by removing the branch and replacing the fuse or resetting the reclosure that has opened as a result of the fault
What are the basic concepts of hazard tree evaluation?
- failures are not random events; they usually result from a combination of identifiable defects and unfavorable conditions; some failures appear random because the defects may not be evident at first, and it may take some investigation and analysis to determine the defect
- health and hazard are not the same; trees can have a full crown of green leaves with no obvious branch or trunk decay and appear healthy to the untrained person; yet the same tree may have narrow branching angles with included bark and symptoms of internal decay that are indicative of structural defects to the trained professional
- the level of risk associated with a hazard tree is directly related to the severity of the defect(s) and the amount of damage that would result if the tree failed; the evaluation systems available facilitate the rating of both failure potential and the potential severity of any resultant damage
tree growth regulators
TGR; a chemical that can be applied to trees that slows extension growth by shortening cell elongation
benefits of TGRs for utilities
can extend pruning cycles and reduce maintenance costs; can reduce the need for mid-cycle pruning of fast-growing trees (cycle busters) and reduce the volume of regrowth that must be pruned and discarded; helpful on trees that cannot be heavily pruned because of property owner concerns or local legislation; other aesthetic benefits as well
soil drench application
a method of applying herbicides or tree growth regulators to trees through the soil by excavating a shallow trench in the soil around the base of the trunk and pouring the compound into the trench; involves excavating a shallow (1 to 2 inch) trench in the soil around the base of the trunk, then diluted compound is poured into the trench, which is refilled when the compound has been absorbed into the soil
soil injection application
a method of applying herbicides or tree growth regulators to trees through the soil via injection directly into the root zone; requires an injection tool that places the compound directly into the root zone at about 6 inches below the surface
How do the most common TGRs work?
they inhibit the production of gibberellin; when gibberellin is suppressed in the crown of a tree, shoot extension is significantly reduced, but other natural processes seem unaffected
gibberellin
a plant growth substance or hormone that controls cell elongation
What are the effects of TGRs?
trees with shorter internodes, smaller leaves, dark green foliage, and enhanced flowering, all of which are generally considered positive characteristics
previous TGR application methods
trunk injection and trunk implant; seldom used because they caused trunk damage and left the equipment on the tree where children and animals could come into contact with it
When should you use TGR?
application should be made when the tree is actively growing and two to three months before pruning; this timing allows the TGR to be absorbed by the tree and translocated to the shoots in the crown so that the active compound is distributed and in place to slow extension growth after pruning; or you can treat and prune at the same time, a period of “normal” resurgent growth may take place until the TGR reaches the crown, but it can reduce the cost of application if the crews are already on site to prune the trees
What species do TGRs not work on?
TGRs are effective in retarding growth on a variety of tree species, but generally less effective on conifers than hardwoods; local testing is important to determine which species can be managed cost effectively by TGRs
Types of mechanical vegetation management
pruning and mowing are the primary ways of mechanically managing right-of-way vegetation
mowing
essentially removing brush with a cutting machine attached to a vehicle, which is tracked or has high flotation tires
Where is mowing primarily used?
right-of-way where brush is dense and where defined easements permit management of obstructing vegetation
What types of mowing equipment does the the utility sector generally use?
rotary cutting head and flail-type cutters (flail mowers); various sizes of each type exist; most are easily capable of cutting brush/vegetation up to 8 inches, but some speciality machinery is available that can cut larger material
rotary cutting head
a single or multiple-blade arrangement that rotates horizontally and cuts or shreds vegetation; head usually consists of one or more blades
flail-type cutters
machines designated to cut vegetation by means of a moving blade, chain, or other cutting device; have metal teeth, chains, or anvil cutting heads attached to a rotating drum that knocks down and shreds vegetation
desirable vegetation
plants that do not grow to heights that would cause them to impact the conductors and that may suppress the establishment of undesirable plants under conductors; woody shrubs, grasses, and forbs
How to maximize effectiveness of mowing?
mowing is very effective in the short term; to maximize for the long term mowing should be followed by application of herbicides to inhibit resprouting of tall-growing vegetation; if herbicides cannot be used regular mowing is the recommended control method for areas with moderate to heavy amounts of brush that are reasonably flat and free of rock and debris
What are the disadvantages of mowing?
- unless mowing is combined with herbicide treatment, stem densities tend to increase, which increases the workload
- there is little, if any, opportunity to retain desirable species
- aesthetics and physical site disruption are often problems
Also not recommended to mow on wet sites where the machinery could cause significant damage or disruption of the site
herbicide
chemical compound that kills vegetation;
What are the benefits of herbicides on electric utility systems?
reduce long-term costs and maximize the benefits of tree and brush removal; can effectively control undesirable vegetation and encourage the retention and expansion of desirable (dense, low-growing) plant communities when used properly
What are the benefits of desirable vegetation?
as desirable plants spread over larger areas, the undesirable vegetation is unable to effectively re-establish or compete and future maintenance costs are reduced; low-growing plant cover such as grasses, forbs, and woody shrubs will impede the establishment of taller-growing brush and will reduce the volume of material and labor required for later phases of maintenance and subsequently reduce costs
What is the goal of every utility brush-control program?
to establish communities of low-growing, desirable plants; this goal marks a transition from chemical to biological control
How to retain desirable vegetation with a herbicide program?
by using selective application or selective herbicides
selective application
directs herbicides at specific undesirable plants, avoiding desirable plants; directing herbicides specifically at target (undesirable) plants, leaving desirable plants untreated so that they remain
selective herbicides
herbicides that are effective only on specific plant species; predominantly effective on certain plant species, typically the undesirable ones, leaving low-growing desirable plants unaffected; typically broadcasted or applied to all vegetation in an area so all plants receive treatment but only sensitive/undesirable plants are controlled
How to selectively control vegetation with nonselective herbicides?
direct application of the material onto the target plants
directed application
applying herbicide to specific plants or parts of plants
Types of directed herbicide application
foliar application (high-volume treatment), basal bark application (low-volume treatment), and cut-surface application (stump treatment)
foliar application
application of a fertilizer or other substance by direct spray on the foliage; high-volume treatment; technique that utilizes a vehicle with a large tank to transport and apply the herbicide solution; material directly applied to the foliage of the plant using a handheld, high-volume applicator (spray gun); ideally targeted at low brush (less than 12 ft)
basal bark application
application of herbicides mixed with penetrating oil to the lower 12 to 15 inches of the main stems; low-volume treatment; mixture is applied to the bark of the main stems of target species
cut-surface application
a method of placing herbicide on stumps immediately after cutting so that the herbicide is absorbed and translocated to the roots; stump treatment
broadcast application
a method of applying herbicides in which they are spread over a wide area and not aimed at specific plants or locations; set to a fixed rate (volume) per unit area (acre or hectare) and are independent of brush density
Where are broadcast applications typically made?
to foliage and cut stubble
foliage
the leaves of a plant
cut stubble
vegetation that has recently been mowed and before regrowth
What is the concentration of active herbicide used in high-volume foliar application?
low, ranging from 0.5 to 1.5%; total mixed volume per acre typically ranging from 100 to 400 gallons per acre (1,000 to 4,000 liters per hectare)
What pressure should be used in foliar herbicide application?
set to the minimum pressure necessary to achieve plant coverage
How to minimize drift?
thickening agents can be mixed in to minimize the presence of fine droplets that may drift onto nontarget plants; nozzles that produce coarse droplets can also be used to minimize drift
When should high-volume foliar herbicide application be done?
can be applied throughout the growing season after leaves are fully expanded, usually from late spring to early fall
What is the concentration of active herbicide used in low-volume foliar application?
high; ranging between 3 and 10%; higher concentration of herbicide than directed high-volume application.
How to achieve selectivity in low-volume foliar herbicide application?
very controlled, close application of coarse droplet sprays directed at individual stems or clumps of undesirable species while avoiding desirable vegetation; target low brush (less than 12 ft) which is not too dense
application types of foliar herbicide
hand pumps and powered backpack sprayers
When should low-volume foliar herbicide application be done?
can be applied at any time during the active growing season after the leaves are fully formed; the upper 65 to 75% of the crown is treated to wet the leaves but not to the point of runoff
When should low-volume basal bark herbicide application be done?
can be applied during the dormant and growing season; more flexibility in timing than foliar treatments; dormant-season application avoids brownouts that occurs when applications are made during the growing season
brownout
a term used to describe the brown appearance of dead foliage following the application of herbicide
What is the concentration of active herbicide used in low-volume basal bark application?
relatively concentrated mixture of herbicide (20 to 30% by volume) and penetrating oil
What types of carrier oils are used in low-volume basal bark herbicide application?
kerosene or diesel; or refined mineral or plant-derived oils when preferred for reasons of environmental sensitivity
When should directed cut-surface herbicide application be done?
hand-cutting stems and immediately (usually within an hour) applying an herbicide to the freshly exposed surface; timing can be extended for several months if a penetrating oil mixture is used; application during heavy rain should be avoided; possible all year round provided the stumps are not concealed beneath snow or water
How is cut-surface herbicide application done?
delivered to the cut surface of stumps from a handheld spray bottle; only a small amount of herbicide is applied directly to the stump from close proximity so runoff and contamination are minimized; adding a dye to the herbicide solution helps to identify those stumps that have been treated
When is cut-surface herbicide application most appropriate?
where the density of target species is low and environmental or visual sensitivity to herbicide is high
What is the concentration of active herbicide used in broadcast foliar herbicide application?
uses a fixed herbicide rate per unit area (acre or hectare) applied in a water solution over the entire target area; typically 20 gallons or more per acre (200 liters per hectare) are applied; higher volumes are applied to ensure penetration through the leaf canopy in areas where brush is high and dense
How can herbicides be delivered in broadcast foliar herbicide applications?
aircraft, trucks and trailers, tractors, ATV-mounted booms
Pros and Cons of aerial application in broadcast foliar herbicide application?
Pros: can be used with to control brush of any height; cost-effective way of initially controlling dense brush, particularly over large areas
Cons: creates a general brownout effect that is not aesthetically desirable in areas of high visibility
Where is aerial application of broadcast foliar herbicide application most suitable?
better suited for lower-growing (6 to 8 ft) vegetation in rural areas
What is an alternative to treating tall brush with a broadcast foliar application of herbicide?
mow the vegetation and immediately broadcast a soil-active herbicide, which prevents resprouting or regrowth of woody species
Benefits of cut-stubble applications of herbicides
suitable for visually sensitive areas because essentially no foliage remains after mowing to “brownout”
What should be considered before selecting an herbicide?
- use selective herbicides that will predominantly control the undesirable vegetation and not affect the low-growing desirable vegetation
- use directed or selective application techniques wherever practical; doing so ensures that only specific undesirable vegetation is treated and desirable vegetation is retained
label
the description of the active ingredients, dilution, and rates of application of herbicides and the uses to which they can be put
What responsibilities do personnel who apply herbicides have?
- must be trained (and in some states licensed)
- must ensure the herbicides are handled and applied correctly in accordance with label directions
- avoid environmental contamination
