Urban wind comfort and safety is becoming an increasingly important issue as increasing pressure on land spaces forces new developments to be built higher and larger. As a wind gust approaches a large building, the air flow is simultaneously forced around the building and speeds up. This is because proximity between buildings creates low pressure causing the wind to accelerate at the base of buildings. Especially if the air flow is forced into a ‘canyon’ between buildings, this increased wind speed can cause comfort and even safety issues at ground level on the pavement.
To mitigate this, specifiers and developers commission wind comfort and safety studies when planning a new development or city quarter. In fact, some cities in the UK such as London and Leeds require wind microclimate reports to be filed at planning stage by law. Although most cities don’t require this to be done, it is recommended practise and nearly all tall buildings have a wind microclimate report included as part of the proposal. Wind microclimate studies are conducted to predict, assess and suggest mitigation methods for the impact of a new building on pedestrian wind comfort and safety.
What part does Lawson’s Criteria play in Wind Microclimate assessments?
There is no one standard by which to assess wind comfort, but the Lawson criteria is the most widely used and is considered to be an effective criterion. It determines the acceptability of wind conditions for pedestrian safety and comfort. Its important to note that the Lawson criteria not only considers the velocity of wind at a set location but also the frequency of occurrence of these velocities.
Confusingly, there is no one Lawson criteria. Within the Lawson standard, there are different criteria to accommodate different cultural perceptions and geographical locations. The 3 main subtypes to be aware of is the LDDC criteria, 2001 criteria and 1970 criteria. For the purpose of simplicity, in this article we will only look at the LDDC criteria which is commonly found in Wind Microclimate studies. The LDDC criteria is given extra credibility as it is the variant required by the City of London planning department for wind studies.
|Category||Comfort Category||Threshold Wind Veloctiy (m/s)||Percentage of Exceedance|
|I||Sitting||0 – 4 m/s||5%|
|II||Standing||4 – 6 m/s||5%|
|III||Walking||6 – 8 m/s||5%|
|IV||Business Walking||8 – 10 m/s||5%|
There is also another category called the safety category rather than the comfort category. This determines that wind speeds over 15 m/s are considered unsafe and will pose a safety risk particularly for more vulnerable pedestrians. Within the safety category, the percentage of exceedance is 0.022%, effectively once a year. Obviously, wind mitigation measures are required for areas with a chance of unsafe winds.
The Lawson criteria is assessed by a specialist wind engineer who will use either wind tunnel modelling or more frequently, computational fluid dynamics (CFD) simulations.
To give a brief outline, a reference wind speed is taken from a nearby meteorological station, usually a local airport. This wind speed then goes through a series of computations taking into account surface roughness and statistical frequency before the wind engineer is able to integrate all data into a combined wind contour map and pedestrian safety and comfort can be analysed.