Wind Risk Assessment in Urban Environments: the Case of Falling Trees During Windstorm Events in Lisbon
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Střelcová, K., Škvarenina, J. & Blaženec, M. (eds.): “BIOCLIMATOLOGY AND NATURAL HAZARDS” International Scientific Conference, Poľana nad Detvou, Slovakia, September 17 - 20, 2007, ISBN 978-80-228-17-60-8 Wind risk assessment in urban environments: the case of falling trees during windstorm events in Lisbon A. LOPES (1), S. OLIVEIRA (1), M. FRAGOSO (1), J. A. ANDRADE (2) and P. PEDRO (3) (1) Centre of Geographical Studies, University of Lisbon (e-mail: antlopes@fl.ul.pt) (2) Geosciences Department, University of Évora (e-mail: [email protected]) (3) Lisbon City Council, RSBL (e-mail: [email protected]) Abstract Trees bring many benefits to the urban environment. However, they may also cause hazards to human population, being the major causes of injuries and infrastructural damage during strong wind events. In the city of Lisbon, strong winds rather frequently result in tree falls, depending on the season and meteorological conditions. This paper presents a methodology to analyse tree damage due to strong wind events in urban environments. Each occurrence has been recorded by the Lisbon Fire Brigade and Rescue Services (Regimento de Sapadores Bombeiros de Lisboa - RSBL). Information provided by RSBL relating to the period of 1990-2005 was considered along with hourly wind speed and direction, species, fitossanitary conditions and urban parameters. To ensure that the fallen trees were caused by strong winds, only days with three or more occurrences of fallen trees were selected. It was found that in summer, northerly winds are responsible for 11% of tree falls, with winds from other directions (west, southwest and south) responsible for 5%. From autumn to spring, perturbed weather conditions originating from the west, southwest and south are responsible for 84% of fallen trees. The majority of tree falls occurred when wind speed surpassed 7 m/s in the six hours prior to their fall. Some recommendations to the Civil Protection Agency and the Fire Department are presented to improve the mission of collecting information. This research is a contribution to the assessment of wind risk in Lisbon. Key words: strong winds, trees in urban streets, urban environment, Lisbon Introduction revealed that in three European countries (Austria, France and Switzerland), more people died prematurely Planting trees brings many benefits to the urban due to the effects of pollution from vehicle emissions environment (Fabião, 1996; Jim and Liu, 1997; Nilsson than due to car accidents. Since a large proportion et al, 2000; Saebe et al, 2003; Soares and Castel-Branco, 2007). of the population is expected to continue living in urban Trees have a moderating effect on the urban microclimate areas, where current EU standards for PM and long-term and improve the physical, biological and chemical aspects of average nitrogen dioxide are exceeded, owing mainly to road- the urban environment, namely by reducing the urban “heat traffic emissions (Krzyzanowski et al., 2005), the plantation island” effect (Alcoforado, 1992), by acting as a barrier against of trees in these areas must be strongly encouraged. However, strong wind channelling (Lopes, 2003), by protecting urban excessive plantation of trees and obstructive “green masses” surfaces from direct sunlight (McPherson and Muchnick, in ventilation paths should be avoided, because it can reduce 2005), by capturing air pollutants and dust in urban areas mean wind speed and deplete dispersion conditions (Lopes, (Freer-Smith et al, 2004) and by increasing the biodiversity, 2003; Alcoforado et al., 2005). providing habitat for birds and small mammals (Clergeau, On the other hand, trees’ benefits for the urban 1996). Trees also protect urban surfaces by reducing environment are frequently accompanied by hazards caused the impact of rainwater, while their roots remove nutrients to the human population and infrastructure, particularly that can be harmful to the water in urban soils, hence during strong wind events, due to total or partial tree falls in improving urban hydrology and controlling erosion. public and private open spaces. This is one of the major causes In addition, trees in urban settings sequester carbon, of human injuries (and, sometimes, death) during extreme providing a helpful hand to combating climate change, events. For example, the extreme windstorms of 26-28 reduce the energetic demand of the city, influence thermal December 1999 were directly responsible for killing 95 and mechanic comfort and foster citizens’ wellbeing people, injuring 11 and affecting approximately 3,400,000 (e.g., affecting physical and mental health, aesthetic people in France; killing 15 in Germany; 11 in Switzerland; and socio-economic values, common heritage, recreation 11 in the United Kingdom; and 5 in Spain. The total cost benefits, etc). A 1999 World Health Organisation report of the disaster was estimated at more than USD 10 billion A. Lopes et al. Střelcová, K., Škvarenina, J. & Blaženec, M. (eds.): “BIOCLIMATOLOGY AND NATURAL HAZARDS” International Scientific Conference, Poľana nad Detvou, Slovakia, September 17 - 20, 2007, ISBN 978-80-228-17-60-8 (EM-DAT: the OFDA/CRED International Disaster the ability of trees to resist these environmental factors must Database). During this event, wind speed exceeded be taken into account both by selecting wind-resistant species 160 km/h in many places along the French coast and along and evaluating the damage to trees caused by windstorms. the 49ºN parallel. Near Paris, in Versailles, more than A simple relationship between the observed damage 4,000 trees were windthrown as a result of the storm and the measured wind speed is not enough to evaluate and the French Office National des Forêts reported that wind damage to trees. an estimated 1,300,000 trees were blown away throughout the country (http://www.onf.fr/reg/Ile-de-France/versailles/ Wind damage to urban trees tempet.htm). The effects of windstorms on urban trees also depend The constraints of trees in the urban environment on the inherent characteristics of each of the species (e.g., age, size, height, bearing, density of the foliage, Windstorms damages greatly depend on the physical width of the fustic, density of the wood, etc.), as well as conditions imposed by the urban environment. In fact, its cultural and health conditions. After two hurricane the environment in urban areas is generally unfavourable events in Florida in the United States (Hurricane Erin, for most arboreous species because climate, sun light, 2 August 1995, with 37 m/s sustained wind and peak gusts soil conditions and air quality are more restrictive than of 45 m/s, and Hurricane Opal, 4 October 1995 with 56-65 in the rural ones. The severity of urban conditions is not m/s, respectively), Duryea (1997) noted that “species that uniform, since it depends on the location of urban trees grow into large trees were more likely to cause property (Fabião, 1996; Nilsson et al, 2000): trees planted in paved damage than small trees”. Sand pine (Pinus clausa), slash areas (street trees) are exposed to a relatively higher level pine (Pinus elliottii), longleaf pine (Pinus palustris) and laurel of stress than those planted in urban parks (park trees) oaks (Quercus laurifolia) were more likely to cause damage or in small urban forests (urban woodlands). Therefore, if they fell than the smaller Southern red cedar (Juniperus the average life span of street trees is generally shorter than silicicola), Carolina laurelcherry (Prunus caroliniana) those of parks trees or trees planted in urban woodlands. and Chinese tallow (Sapium sebiferum). Besides Furthermore, the number of species used in urban planning the size there is a predisposition to Florida native trees, like in paved areas is fewer than those used in the other urban the dogwood (Cornus florida), sand live oak (Quercus environments. For example, about 55% of all trees planted geminate), live oak (Quercus virginiana), sabal palm (Sabal in Spanish paved areas (Garcia-Martin and Garcia- palmetto), and Southern magnolia (Magnolia grandiflora), Valdecantos, 2001) or in Lisbon streets (Soares and Castel- to tolerate hurricane-force winds extremely well. Some Branco, 2007) are represented by only five genera. foreign species have the predisposition to be affected by insects and disease after experiencing strong winds Trees planted in the urban environment, namely street and are less resistant (Duryea, 1997). trees, must be adapted to higher temperatures (heat island effect) and higher shadow levels than those observed Trees in natural environments are optimised structures outside urban areas. Building construction and paving are which are subjected to static and dynamic loads (Mattheck responsible for some important changes in soil conditions, and Breloer, 1994). According to James (2003), the strength such as the lowering of the phreatic water table, the removal of each structural member depends on the size of the cross- of the soil surface layer and the soil compaction. This, section (the base of the trunk being the strongest part thus, reduces the soil volume available for root expansion, where static loads are the greatest), the shape of the cross- the available nutrient content and soil permeability in section (a circular section being best adapted to torsional addition to restricting the water and air uptake by the roots. forces) and the strength of the material (young wood Street trees are also damaged by the effects of air pollution, being more flexible than old wood). Trees also show some mainly by gas emissions from industries and cars, poisoned physical features of adaptive growth against wind loads. by salts, gas, oil and various piped chemical products For example, the growth rings are thicker on the side from industries. Furthermore, they are damaged by severe where the dynamic (wind) load is greatest and thinner and inaccurate pruning practises or by occasional events like on the opposite side, while the base of a trunk or branch is windstorms, hail fall and physical impact caused by cars and massive due to the addition of wood and, thus, represents building materials.