Severe Storms In 2006 and 2007 Knocked Out Power to More Than 1.5 Million Utility Customers in the Ameren (St. Louis, Missouri) service territory. These storms had been unmatched in severity and frequency in the utility's history and cost $223 million in 2006 alone.

In July 2006, back-to-back storms with wind speeds in excess of 90 mph affected 950,000 customers in Illinois and Missouri. In November and December 2006, and in January 2007, severe weather struck again, this time in the form of ice storms. These storms were the worst to hit Ameren's Illinois and Missouri region in the last 30 years. The 2006 ice storm affected 520,000 customers, and the 2007 storm affected 350,000 customers. To a large extent, these storm systems affected the same customers and a similar geographical area.

As a result of this severe weather, Ameren's Illinois and Missouri utilities launched initiatives to continue to provide safe, reliable power to its customers. These programs target electric lines built over the past four decades to serve new developments. Many new trees planted during that time now jeopardize the system in severe weather. The utility will continue to work with local governments on ways to handle the tree issues both on public lands and private property.

At AmerenUE, Ameren's Missouri-based regulated utility, these projects began in 2005 and continue today with the recently announced Project Power On in 2007. Project Power On is a three-year, $1 billion AmerenUE initiative to improve service reliability, upgrade power-delivery systems and enhance the environmental performance of the utility's power plants. Vegetation management is one component of this broad program, with annual expenditures of $50 million for tree trimming (more than double previous years' budgets) and planned expenditures of more than $150 million in the next three years. AmerenUE also is pursuing a more aggressive tree-removal and tree-trimming plan.


In 2005, Ameren increased its annual vegetation budget by more than 25%, with the goal of obtaining a shortened cycle length on the entire distribution system by the end of 2008. AmerenUE serves 1.2 million customers in Missouri through more than 21,000 miles of overhead distribution wires. The service area has a wide range of variability including highly urbanized, rear-lot easements in the greater St. Louis area to very remote easements in outlying areas with lower customer densities, interspersed with larger towns and cities throughout.

To address this variability and focus tree-trimming resources, Ameren developed a feeder classification to evaluate cycle lengths. On average, an urban feeder has 35 or more customers per overhead/underground mile and is trimmed every four years. A rural feeder is defined as less than 35 customers per mile and trimmed every six years. By using this definition, in effect, 37% of the total distribution miles will be considered urban; however, these circuits provide service to 80% of the total customers. The rural feeders, based on the typically wider easements and the ability to more readily manage with the use of herbicides and specialty equipment, will be trimmed on the longer six-year cycle.

By the end of 2007, AmerenUE was ahead of schedule to complete the vegetation plan of reduced cycle lengths, based on classification, on or before December 2008. The city of St. Louis was completed one year ahead of the initial goal of 2008, and the System Average Interruption Frequency Index (SAIFI) attributed to trees — excluding major events per IEEE Standard 1366 — has been reduced by half in 2007 from 2006. AmerenUE's entire distribution system Tree SAIFI improved 12% for the same period.


AmerenUE has historically trimmed to industry standards, allowing varying amounts of overhang above the conductors on distribution lines, especially in highly urbanized areas. The amount of overhang allowed varied based on the species, the voltage of the line (three-phase or single-phase) and the position of the tree relative to the line. For example, a soft-wooded tree, such as a silver maple or sycamore, typically would have more overhang removed on both three-phase and single-phase than a hard-wooded tree, such as an oak or hickory.

During the spring of 2006, Ameren performed an internal study to analyze tree-related outages to see what it could learn from the 2001 to 2006 outage data. In addition to continuing to work toward the desired cycle length, the data showed a need to broaden clearances, especially on the three-phase sections of circuits, and to identify upcoming feeders with significantly higher Tree SAIFI issues relative to the other circuits from the substation and perform a more prescriptive trim.

One of the findings from the tree-related outages study showed that from 2001 to 2006, the total feeder outages attributed to tree issues was fairly consistent between 45% and 50% of all tree-related outages on the distribution system. It made sense for AmerenUE to focus on the three-phase sections in terms of eliminating or reducing overhanging branches and increasing the amount of removals both on and off rights-of way (ROW).

Starting in 2007 and going forward, all overhang above the three-phase section of each circuit is removed with only very limited exception. An example of a permissible exception would be a mature oak with structurally sound branches in an urban setting that, in removing the overhanging branches, would likely cause the tree to decline.

Concurrent with the broadening of the three-phase clearances, AmerenUE also has been much more aggressive in pursuing removals, both on and off the ROW. In 2003, AmerenUE began using contractor general foremen to assist its vegetation supervisors in planning the upcoming work in terms of manpower and specialty equipment beyond the typical bucket and split dumps. AmerenUE requires its general foremen to obtain ISA-certified arborist status. In 2006, the utility further expanded this program with training of all tree crews on tree risk awareness. Although this program of hazard recognition was initially for the tree trimmers' safety, it has also been used with preplanning work for identification of removal candidates ahead of the crews.

In addition, the utility is making a more concerted effort to contact property owners on removals, especially along the three-phase sections that will need trimming each cycle. In 2007, relative to 2006, an additional $3.3 million was spent on removals during the normal maintenance tree-trimming cycle.


The second finding of the study showed a relatively small number of circuits scheduled to be trimmed in 2007 were contributing a higher-than-average Tree SAIFI at the feeder level. Of 262 feeders in one operating division, 21, or 8%, of the total feeders contributed more than 20% of the customer interruptions during the previous six-year period. This 8% experienced a total feeder outage due to trees at least three times and, in some cases, more often. Since all feeders out of a substation typically have been trimmed at the same time, tree-trimming cycle length could not be the major factor.

An initial pilot program was put into place in late 2006 to perform a field investigation to see if anything could be done beyond the changes in increased clearances and removals. The backbone section (from the substation out to the first protective device) was patrolled by foot — in most cases with an AmerenUE vegetation supervisor and contractor supervisor — to identify any overhead or vegetation issues.

The overhead issues, for the most part, consisted of working with engineering groups to see if the utility could fuse tap to protect and shorten the total backbone feeder exposure. Not surprisingly, the majority of high Tree SAIFI feeders tended to have longer backbone sections and therefore more exposure.

If for any reason a single-phase tap cannot be fused, the line should be trimmed for three-phase clearances. In addition, any overhead equipment damage (such as bad poles, insulators or transformers) are identified for the construction group follow up.

The vegetation issues consisted of identifying:

  • Higher-than-normal problem species along the route, such as silver maples, elms and cottonwoods


    Past topping of trees by homeowners, leading to weak branch attachments

  • A section of line with a challenging location, such as creek or hillside

  • Dead/dying trees due to new construction, insects and disease.

Since the vast majority of these issues require going well beyond typical easement rights, it is critical to meet with property owners and municipals to discuss proposed work. Ameren's experience to date has been that most property owners are very receptive, once a thorough explanation is given. The goal of the program is to monitor these feeders over the next four to six years with the expectation that the Tree SAIFI on these prescriptive feeders approaches the overall average of the substation. The cost of this additional vegetation work has varied from $5000 to an excess of $100,000 per circuit.


The Vegetation Rule, which will be enacted this year by the Missouri Public Service Commission, features many components that will affect all Missouri investor-owned utilities such as defined cycle lengths based on feeder classification with the time line to achieve and clearance specifications by voltages. In addition, there are requirements for notification of property owners, municipalities and county officials of planned maintenance trimming and mid-cycle vegetation patrols.

In anticipation of the rule making, AmerenUE has started the notification and mid-cycle vegetation patrols. The utility has used the opportunity of the notification to include a pamphlet to educate its customers on why tree trimming is necessary, how the trees will be trimmed and recommendations on proper tree planting near electrical lines.

The issues all utility arborists face today, in large part, are a result of poor planting decisions by property owners. AmerenUE has and will continue to address those past decisions where possible with tree removal and, in some cases, a replacement program. AmerenUE is hopeful that going forward the educational efforts on “The Right Tree in the Right Place” with customers and municipalities will lessen the need for a tree-removal program.

Rick Schenk is the superintendent of vegetation management for AmerenUE. Schenk is an ISA-certified arborist and utility specialist. He is a member of the Midwest Chapter of ISA and a member of the Utility Arborist Association. Schenk has a BS degree in forestry management from the University of Missouri-Columbia.


St. Louis, Missouri, experienced more severe weather than any other office in the National Weather Service in 2006. By the end of the year, the city logged a total of 723 severe weather events. Here are some statistics on the major storms and how Ameren responded with the restoration effort.

July 2006 Storms

  • Back-to-back storms on July 19 and 21 affected electric service for nearly 950,000 customers in Illinois (301,700) and Missouri (646,200).

  • The numbers were very large compared to the August 2005 storm, which was the utility's last bad storm, when 217,000 customers were out of power. More than 500 distribution feeder circuits were out — more than triple the norm for even major storms.

  • Ameren companies replaced 1316 transformers, 1550 poles and 2 million ft of wire and cable.

  • More than 5300 people worked to repair the damage, including 2650 linemen and 1150 tree crews.

  • Utilities in 12 states sent crews to help, 25 electrical construction contractor companies sent crews, and 22 companies responded from the Association of Rural Electric Cooperatives (97 coop linemen helped).

  • Ameren companies answered 1.2 million customer calls.

  • Nearly 463,000 customers of the total 950,000 were restored in three days.

  • All power was restored in nine days; 93% had been restored by the eighth day.

November/December 2006 Storms

  • On Nov. 30/Dec. 1 the worst ice storm in at least 30 years affected electric service for 520,000 customers, including 290,000 in Missouri and 234,000 customers in Illinois. More than 5500 workers came from 14 states to help repair the damage.

  • Again, trees knocked onto power lines.

  • During this storm, AmerenUE replaced 392 poles in Missouri and installed 214 miles of new cable and conductor.

  • AmerenUE restored power to all customers in eight days. A review of other utilities' restoration times shows that most took between eight and 10 days to restore the bulk of their customers during ice storms.

January 2007 Storm

  • Due to a Jan. 13 ice storm, 350,000 customers in Illinois and Missouri lost power; they were all restored in less than five days.

  • More than 4200 linemen and 1500 tree-removal/vegetation management personnel worked to restore power.

  • Contractor and other utility crews came from 16 states to help restore power.