Trees and Their Impact on Electric Reliability During and Following Tropical Storm Isabel

With Isabel, rainfall was less than expected and wind speeds were below those normally associated with significant tree failure. Despite this situation, tree failures and associated utility outages were massive and five days after the storm more than 2 million people in the Mid-Atlantic region remained without electricity. The Maryland Public Service Commission’s (PSC) inquiry into electrical service interruptions related to the storm considered what impact, if any, certain tree protection laws administered by the Department of Natural Resources (DNR) had on utility outages.

The storm was an act of God and the impacts were not accounted for by existing utility vegetation management practices. A preponderance of dead or damaged roadside trees in the Baltimore- Washington corridor coincided with the area having the majority of utility outages. The DNR laws regulating tree care require adherence to the industry-consensus standards for tree care operations. Adherence to proper tree care practice has been demonstrated to improve electric reliability. A new treatment paradigm—one that addresses trees outside of the traditional treatment envelope and focuses on amelioration of mechanical defects and storm forces on tree crowns using tree pruning, removal, and replacement— may reduce the severity of tree-related utility outages during storms. ..