The wind tunnel and Lawson criteria results section

The results section of a wind microclimate assessment has one job: to tell a case officer, location by location, whether the space outside the building will be comfortable for what it is meant to be used for, and whether anywhere is unsafe. Everything else in the report is method. The lawson criteria wind assessment framework does that by mapping wind speeds, and the frequency with which they are exceeded, onto activity categories, so that a seating area and a service yard are judged against different thresholds.

Where reports go wrong is in presentation rather than in modelling. Colour plans with no receptor schedule, mitigation asserted rather than tested, and no statement of which locations fail.

What the comfort and safety criteria describe

The Lawson comfort criteria group wind conditions into categories of use, running from sitting, through standing, strolling and business walking, to conditions unsuitable for pedestrians. Each category is defined by a wind speed exceeded for a stated small proportion of the time, so the criterion combines a speed and a frequency rather than a single peak.

The safety or distress criterion is separate and more demanding in character. It uses a gust related measure, exceeded very infrequently, to identify locations where a person could be blown off balance. Safety exceedance is not a comfort judgement to be balanced; if a location fails it, the scheme has to change.

Both are applied to the intended use of each location. A bench, a play space, a cycle stand, a bus stop, a main entrance and a roof terrace all attract different target categories, and the assessment should state the target for each rather than reporting bare conditions. A Wind Microclimate Assessment is scoped around that list of locations.

Test configurations and the seasons modelled

A defensible assessment tests more than the proposed scheme. The normal configurations are the existing situation, the proposed development alone, and the proposed development with surrounding consented or emerging schemes included, which is the cumulative case.

Wind data comes from long term records at a nearby meteorological station, corrected for terrain and for the site's surroundings, and analysed directionally so that the exposure of each location reflects the directions the wind actually comes from. Results are usually reported for the windiest season and for the annual condition, and sometimes for summer separately, because a terrace that works in July and is unusable in February needs both numbers.

Say which configurations were tested and why any were omitted. An assessment of the proposed scheme with no existing baseline cannot show whether the building improves or worsens conditions, which is the comparison officers and objectors both want.

Reporting comfort categories by location

The readable format is a schedule, not a plan. Number every test location, state its intended use, the target category, the measured or modelled category for each configuration and season, and whether it passes.

A plan with coloured contours supports the schedule and does not replace it, because contours invite the reader to interpolate at exactly the points that matter. Put the receptor numbers on the plan so the two documents read together, and keep the colour scheme consistent across configurations so a reader can compare them by eye. The same discipline governs the presentation of other spatial results, as our note on shadow plots and dates in an overshadowing study sets out.

Where a location changes category between configurations, say so in words. "Location 14, the main residential entrance, moves from standing to business walking in the proposed condition and returns to standing with the canopy in place" is the sentence the officer needs.

Distress exceedance and what follows

Treat safety exceedances as a separate, short section. List every location that fails, with its use, and then the design response and the retested result.

There is no version of a wind report where an unmitigated safety exceedance at a public entrance is acceptable, so the report should not bury it in a table of thirty locations. Where a failure occurs at the edge of the site or on the highway, the highway authority may become a consultee on the mitigation, and where it occurs at a school gate or a crossing point, expect the objection to be specific.

The honest reports name the failures in the summary. The ones that lose credibility are the reports where a reader finds the exceedance in appendix C.

Mitigation through massing, canopies and planting

Mitigation works in a hierarchy, and the order matters to a design officer:

1. Massing changes, such as podium setbacks, chamfered or rounded corners, breaking up long facades and avoiding a gap between two tall elements that accelerates flow.
2. Built screening, including canopies at entrances, colonnades, solid screens at terrace edges and porous balustrades.
3. Landscape, meaning trees and planting of an appropriate species and density.
4. Management, such as a policy of closing a terrace in high winds, which is the weakest and is rarely accepted alone.

Landscape mitigation carries a maturity problem. A scheme that relies on a row of trees to bring an entrance within its target category is relying on conditions ten or fifteen years after occupation, and the report should test the unplanted condition as well. Deciduous planting also does less in winter, which is when the wind data is worst.

Every mitigation measure claimed in the report should have been tested in the model, with the tested result reported. Mitigation asserted from experience is the commonest reason a consultee asks for a revised assessment.

Wind tunnel testing against computational modelling

Boundary layer wind tunnel testing with a physical model of the scheme and its surroundings remains the reference method, and for tall or complex schemes many authorities expect it. Computational fluid dynamics is cheaper, iterates faster during design development, and is well suited to comparing options.

The limitation to state plainly is that steady state computational approaches predict mean flow better than they predict gusts, and the safety criterion is gust based. Where the assessment relies on computational modelling for a scheme with safety sensitive locations, the report should explain the approach taken to gusts and its uncertainty rather than presenting a single figure with false precision.

A sensible programme uses computational modelling during design to test options, then tunnel testing to verify the submitted scheme and its mitigation. Say in the report which tool produced which result.

Presenting results a case officer can read

The structure that works runs: a one page summary naming every location that fails and what fixes it, the receptor schedule, the plans, the mitigation with tested results, the limitations, then the technical appendices.

Write the summary so that it can be lifted into the officer's report. That is what happens to it, and a summary that hedges gets rewritten by someone with less information than you have. Where a location remains marginal after mitigation, say so and explain the residual position rather than rounding it into a pass. The report types by category page shows how the wind assessment sits alongside the other environmental documents on a tall building application.

One thing worth remembering: the wind assessment is one of the few technical documents that can change the massing. Commission it early enough to matter, because a report that arrives after the facade is designed becomes an argument about canopies.

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