Redusert trefall på kraftlinjer

Background: Wind-throw, snow breakage and snow-bent trees make up the major problem for distribution of electric power in Norway. Fallen trees cause stop in the delivery and high costs for the distributor in economic penalty and repair. The problem is expected to increase over time due to an increasing amount of forests and tall, old trees, as well as climate change. The problem can be reduced by appropriate forest management, by mapping the high-risk areas and by quickly detect fallen trees after storm and snow events in order to quickly remove them. Aims: The aim of the project has been to develop and implement efforts that can reduce the amount of tree fall on power lines. This shall be accomplished by developing a novel forest management along power lines, by developing risk models, by implementing a closer co-operation with forest owners, and by identifying suitable methods for fast mapping of fallen trees. Capacity building: The project has facilitated a capacity development in the net companies on the topic of forest management and forest stability against wind and snow. This has been achieved by on-site inspections and practical exercises; establishment of a new training course for people carrying out the practical forest management work, as well as publications with basic theory. Management belt: We have in the project developed a method for treating young forest areas along the lines. It is crucial to keep a long-term perspective on forest management. By establishing a management belt in a young forest, the needs for further management activities may be none during the remaining life span of the trees, i.e. the next 50-150 years. The belt should have a width of about 20m. The density of trees should be 8-10 per ha, and the trees should preferably make up a homogeneous forest by having trees preferably of the same species, the same height and evenly distributed in space. An agreement with the forest owner should be made, where the economic consequences of a reduced stand density and reduced production should be handled. Helicopter cutting: A helicopter can be used for topping and debranching of trees along the powerlines. This is a method that is likely to increase during the coming years. It is an effective measure, in the sense that many m of lines can be treated per hour, and provides easy access to remote areas. With topping, about 1/3 of the upper part of the crown is cut off. Even though this may appear to be a marginal treatment, it has a considerable effect with about 90% decrease in the probability of windthrow. While the windmoment on a tree is only moderately reduced and the corresponding critical wind speed is only moderately increased, the effect on windthrow probability is much larger due to the frequency of strong winds being disproportionately low. Topping of trees can lead to some rot and also to mulitple tops later, however; the economic losses for the forest owner is marginal. Debranching of trees with helicopter can be done on the forest edge facing towards the power line, by which the center of gravity in the crown is moved away from the power line. In addition to ensure compliance with the required distance between vegetation and power lines in the air, the measure has the effect that the probability of windthrow, snow breakage and snow-bending of trees is reduced. Mapping and monitoring fores talong powerlines: Monitoring of forest clear cuts is important in order to avoid remaining trees left close to the power lines. Such monitoring can be done with satellite data and track-log data from harvester machines, as well with airborne and drone campaigns. Concerning satellite data, an operational monitoring is provided by Global Forest Watch, and new and improved satellite-based monitoring services for this is expected during the coming years. Also for rapid mapping of fallen trees on powerlines after storms and heavy snow-events, there is a fast, ongoing development of new remote sensing technologies. In particular, new drone- and satellite-based applications is likely to develop. At present, however; a visual inspection with manual photography from helicopters or drones appears to be the most suitable method. An increasing amount of digital GIS data is available for mapping risk-factors along powerlines. Data on forest, soil, topography and climatic conditions are available, and can be used for estimation of the probability for trees falling on powerlines. Advanced processing of airborne laser scanning data at the single-tree level may to some extent help to identify risk trees. While the position and height of single trees can be well captured, and as well the size and potential asymmetry of the tree crown, a critical variable like the stem diameter to tree height ratio cannot be reliably derived from such data. Snow and wind damage and climatic conditions: The probability of trees falling on the line depends mainly on the tree properties, in particular height

Kompetanseoppbygging i nettsektoren Ulike metoder er utviklet for skogbehandling langs kraftlinjer Avtalemaler er laget for avtaler med skogbrukets aktører Kurs i skogbehandling langs kraftlinjer er etablert og i gang

Den overordnede idéen er at omfanget av trefall på kraftlinjene kan redseres betydelig og kostnadseffektivt. Dette er basert på tre forhold: 1) Trærnes evne til å motstå belastning fra vind og snø kan i stor grad økes gjennom endringer i skogbehandlingen. 2) Risikoen for trefall varierer med topografi, jordbunnsforhold og vindklima, og graden av endring i skogbehandlingen bør derfor varieres langs en kraftlinje slik at man legger kostnadene særlig der risikoen er størst. 3) En hurtigere opprydding av trefall etter storm og kraftig snøfall er mulig ved å kombinere risikomodeller og ny teknologi innen fjernmåling. Bakgrunnen for denne søknaden er at trefall på linjene er et stort og økende problem i distribusjonen av elektrisk kraft i Norge, og problemet ventes å øke framover i tid. Vi anser potensialet for å redusere problemet som betydelig, gjennom ny skogbehandling, sonering av tiltakene ved hjelp av risikomodellering, samt raskere kartlegging av trefall etter storm og snøvær.