From the 20th century, environmental issues caused by climate change and energy shortages have attracted much attention in the urban study field, and climate adaptation measures are increasingly considered in urban planning and design. Experts do extensive modelling of temperature and precipitation, given they are dominant factors in the environmental system. On the contrary, the wind is perceived as a key component of climate and affects daily experience, it is usually overlooked in urban climate analysis. Storm-force winds, a known hazard, given their capacity to increase wildfire risks and aggravate drought, are regarded as macro research targets. The winds below the extreme warning threshold but beyond comfortable range have largely fallen off the research radar, good ventilation, corresponding to a certain wind speed, is regarded as good news for energy exchange, however, the side effects brought by the strong winds on industries, agriculture, the safety of the living environment could not be ignored. Starting with an examination of the consequences of two major events with strong wind that occurred in Milan, Italy, in 2022 and 2023, as recorded by weather stations and the database of urban trees (including species, height, diameter, age, health conditions of trees), the study conducts a systematic analysis of urban environmental conditions that can be assumed to induce a greater likelihood of trees falling. In particular, the paper first examines the distribution and characteristics of trees damaged or removed during the two events and advances a preliminary hypothesis on the vulnerability factors influencing the risk of falling trees. Secondly, it performs a spatial analysis that attempts to generalise and predict future risks associated with wind hazards at a relatively small scale, leveraging a multilayer urban canopy model that includes the height distribution of the buildings, tree locations, street configurations (aspect ratio and orientation), and land use factors affecting wind speed and flow throughout the urban canopy layer. Ultimately, this study proposes a wind risk index that cross-references wind hazards (determined by speed), exposure (population density), and vulnerability factors (by tree age, assumed as the key to describe tree growth condition). The puzzles of research on wind impacts in the urban study field are, on one hand, the complicated mechanism behind the changing of near-surface wind which is previously hypothesized as determined by vegetation growth and/or urbanization, on the other, the availability and reliability of data collected from sensing points being processed for extensive urban areas. Accordingly, through the focus study of Milan city in terms of urban wind safety, this work outcome two aspects as complementary to urban wind and climate research. One is the systemic risk assessment through quantitative and qualitative analysis of the existing and historical damage. Another is linking the analytical results to urban responsive mechanisms and strategies with the guide of the risk assessment, combining 3-D modelling simulation of different wind conditions. These two outcomes may serve as an explorative trial for a wider range of applications, as examples for other areas, promoting wind resilience in the broad context of climate change and the need for refined management of cities.
A partire dal XX secolo, le questioni ambientali causate dal cambiamento climatico e dalla carenza energetica hanno attirato molta attenzione nel campo degli studi urbani, e le misure di adattamento climatico sono sempre più prese in considerazione nella pianificazione e nella progettazione urbana. Gli esperti realizzano modelli estesi della temperatura e delle precipitazioni, dato che sono dominanti. fattori nel sistema ambientale. Al contrario, il vento è percepito come una componente chiave del clima e influenza l’esperienza quotidiana, di solito viene trascurato nell’analisi del clima urbano. I venti di tempesta, un pericolo noto, data la loro capacità di aumentare il rischio di incendi e aggravamento della siccità, sono considerati obiettivi di ricerca macro. I venti al di sotto della soglia di allarme estremo ma oltre la portata confortevole sono in gran parte scomparsi dai radar della ricerca, una buona ventilazione, corrispondente a una certa velocità del vento, è considerata una buona notizia per lo scambio energetico, tuttavia , non potevano essere ignorati gli effetti collaterali portati dai forti venti sulle industrie, sull'agricoltura, sulla sicurezza dell'ambiente di vita. Partendo dall’esame delle conseguenze di due grandi eventi con forte vento verificatisi a Milano, in Italia, nel 2022 e nel 2023, registrati dalle stazioni meteorologiche e dal database degli alberi urbani (tra cui specie, altezza, diametro, età, condizioni di salute degli alberi) alberi), lo studio conduce un'analisi sistematica delle condizioni ambientali urbane che si possono ipotizzare inducono una maggiore probabilità di caduta di alberi. In particolare, il lavoro esamina dapprima la distribuzione e le caratteristiche degli alberi danneggiati o rimossi durante i due eventi e avanza una prima indagine ipotesi sui fattori di vulnerabilità che influenzano il rischio di caduta di alberi.In secondo luogo, esegue un'analisi spaziale che tenta di generalizzare e prevedere i rischi futuri associati ai pericoli del vento su una scala relativamente piccola, sfruttando un modello di copertura urbana multistrato che include la distribuzione in altezza degli alberi edifici, posizione degli alberi, configurazioni stradali (proporzioni e orientamento) e fattori di utilizzo del suolo che influenzano la velocità del vento e il flusso attraverso lo strato della chioma urbana. In definitiva, questo studio propone un indice di rischio del vento che fa riferimenti incrociati ai pericoli del vento (determinati dalla velocità), esposizione (densità di popolazione) e fattori di vulnerabilità (per età degli alberi, assunta come chiave per descrivere le condizioni di crescita degli alberi). Gli enigmi della ricerca sugli impatti del vento nel campo degli studi urbani sono, da un lato, il complicato meccanismo alla base del cambiamento del vento in prossimità della superficie, precedentemente ipotizzato come determinato dalla crescita della vegetazione e/o dall’urbanizzazione, dall’altro, la disponibilità e affidabilità dei dati raccolti dai punti di rilevamento in fase di elaborazione per estese aree urbane. Di conseguenza, attraverso lo studio focalizzato sulla città di Milano in termini di sicurezza dal vento urbano, questo lavoro ha portato alla luce due aspetti complementari alla ricerca sul vento urbano e sul clima. Uno è la valutazione del rischio sistemico attraverso l’analisi quantitativa e qualitativa del danno esistente e storico.Un altro è collegare i risultati analitici ai meccanismi e alle strategie di risposta urbana con la guida della valutazione del rischio, combinando la simulazione della modellazione 3D di diverse condizioni del vento.Questi due risultati possono servire come un sperimentazione esplorativa per una gamma più ampia di applicazioni, come esempi per altre aree, promuovendo la resilienza al vento nell’ampio contesto del cambiamento climatico e della necessità di una gestione raffinata delle città.
Urban wind and trees : wind risk index and co-maintenance strategies for urban trees to resist wind-induced damages with the case of Milan
Yan, Weixi
2022/2023
Abstract
From the 20th century, environmental issues caused by climate change and energy shortages have attracted much attention in the urban study field, and climate adaptation measures are increasingly considered in urban planning and design. Experts do extensive modelling of temperature and precipitation, given they are dominant factors in the environmental system. On the contrary, the wind is perceived as a key component of climate and affects daily experience, it is usually overlooked in urban climate analysis. Storm-force winds, a known hazard, given their capacity to increase wildfire risks and aggravate drought, are regarded as macro research targets. The winds below the extreme warning threshold but beyond comfortable range have largely fallen off the research radar, good ventilation, corresponding to a certain wind speed, is regarded as good news for energy exchange, however, the side effects brought by the strong winds on industries, agriculture, the safety of the living environment could not be ignored. Starting with an examination of the consequences of two major events with strong wind that occurred in Milan, Italy, in 2022 and 2023, as recorded by weather stations and the database of urban trees (including species, height, diameter, age, health conditions of trees), the study conducts a systematic analysis of urban environmental conditions that can be assumed to induce a greater likelihood of trees falling. In particular, the paper first examines the distribution and characteristics of trees damaged or removed during the two events and advances a preliminary hypothesis on the vulnerability factors influencing the risk of falling trees. Secondly, it performs a spatial analysis that attempts to generalise and predict future risks associated with wind hazards at a relatively small scale, leveraging a multilayer urban canopy model that includes the height distribution of the buildings, tree locations, street configurations (aspect ratio and orientation), and land use factors affecting wind speed and flow throughout the urban canopy layer. Ultimately, this study proposes a wind risk index that cross-references wind hazards (determined by speed), exposure (population density), and vulnerability factors (by tree age, assumed as the key to describe tree growth condition). The puzzles of research on wind impacts in the urban study field are, on one hand, the complicated mechanism behind the changing of near-surface wind which is previously hypothesized as determined by vegetation growth and/or urbanization, on the other, the availability and reliability of data collected from sensing points being processed for extensive urban areas. Accordingly, through the focus study of Milan city in terms of urban wind safety, this work outcome two aspects as complementary to urban wind and climate research. One is the systemic risk assessment through quantitative and qualitative analysis of the existing and historical damage. Another is linking the analytical results to urban responsive mechanisms and strategies with the guide of the risk assessment, combining 3-D modelling simulation of different wind conditions. These two outcomes may serve as an explorative trial for a wider range of applications, as examples for other areas, promoting wind resilience in the broad context of climate change and the need for refined management of cities.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/219202