Southwest Environmental Limited

Downwash Analysis

Downwash Analysis often gives very unexpected results. This is why the Downwash Analysis is such a useful tool, it takes away some of the guess work of what would otherwise be a hopeless task, trying to estimate environmental conditions and stack heights to mitigate against those conditions.

Typically the more irregular the surrounding area the more complex and variable the downwash will be. Shapes and heights of buildings and their orientation relative to prevailing winds, will determine the required stack height in order to avoid downwash effects, that is the concentration of emissions is certain areas owing to eddie currents and the likes.

We use a computer model to work out the require stack height in order to avoid negative down wash effects.

Downwash Theory is quite old, most the the maths was thought up 20 years ago. The model we use builds on this tried and tested calculation method, and allows for very complex modeling. 

The affect that downwash will have on emissions plumes will depend on the various variables we introduce in to the Downwash Analysis model.

Downwash Analysis Variables

The affect that downwash will have on emissions plumes will depend on:

• existing velocity of emissions
• temperature of emissions

Downwash Analysis Consultancy

If you have concerns regarding adequate stacks heights, or the effects that new building will have on downwash from an existing stack, then please contact us and we can crunch the numbers. Initial consultations are always free, and we will move quickly to provide a quotation and deliver a thorough and robust report.

The below section was written by a machine.

Technical Insight: Understanding Building Downwash in Air Quality Modeling

Building downwash is a critical phenomenon in atmospheric science that occurs when the wind flow is disturbed by structures near an emission source. When a plume is emitted from a stack, the aerodynamic wake created by nearby buildings can pull the pollutants toward the ground much sooner than they would naturally disperse. Failure to account for these effects can lead to significant underestimation of ground-level concentrations in a professional air quality assessment.

The PRIME Algorithm and Modeling Accuracy

Modern dispersion models, such as AERMOD and ADMS, utilize sophisticated algorithms like PRIME (Plume Rise Model Enhancements) to simulate these complex interactions. This analysis is an essential component of any high-level atmospheric modelling exercise. Key factors considered during a downwash study include:

• Building Geometry: The height, width, and orientation of structures relative to the prevailing wind and the emission point.
• Wake and Cavity Regions: Defining the areas of turbulent flow where pollutants can become trapped or rapidly recirculated.
• Stack Height Optimization: Determining the minimum effective chimney height required to clear the "building influence zone" and prevent localized air quality breaches.
• BPIP Processing: Utilizing the Building Profile Input Program to provide the necessary cross-sectional data for the dispersion model.

Regulatory Compliance and Environmental Permitting

For industrial operators and developers, downwash analysis is often a mandatory requirement to satisfy the Environment Agency or local planning authorities. This is particularly true for applications involving ammonia emissions modelling or bespoke environmental permitting. By accurately predicting how structures will impact emissions, we can design effective mitigation strategies, such as increasing stack heights or relocating emission points, to ensure compliance with National Air Quality Objectives.

Role in Environmental Impact Assessment

A robust downwash analysis forms a defensive pillar within a wider Environmental Impact Assessment (EIA). It ensures that the "worst-case" scenarios are scientifically evaluated, providing planners with the technical authority needed to grant consent. At Southwest Environmental Limited, we combine geospatial building data with meteorological observations to deliver high-precision dispersion studies. With over 15 years of industry experience and 900+ projects delivered across the UK, we provide the expert insight (IES/IAQM) required to navigate complex air quality challenges.