Air Ventilation Assessment (AVA) System for High Density Planning and Design September 2006 PLEA2006
An ideal scene Hong Kong? Who needs to worry about design for city ventilation if we all have a view like this outside our home
High Density Hong Kong Bulky buildings Narrow streets Obstructions
Congested Hong Kong Hot and humid weather crowdedness Stagnant air High pedestrian level activities Dark/hot materials
Team Clean After SARS of 2003 To promote better layout of building blocks in the city, we are examining the practicality of stipulating air ventilation assessment (AVA) as one of the considerations, similar to traffic and infrastructure capacities, for all major development or redevelopment proposals and in future planning. a t
Objectives Explore the feasibility of establishing some protocols to assess the effects of major planning and development proposals on external air movement for achieving an acceptable macro wind environment.
Towards a A quality urban environment Openness Breeze Shade Greenery Cool materials
Wind for what? Ng and Cheng (2004), all rights reserved
Evaluation Find the problem, otherwise one risks solving the wrong one Mat Santamouris, U of Athens (Urban urban physics, urban climatology & air ventilation design Lutz Katzschner, Kassel U, Germany (urban climatology and Urban Climatic Mapping) Baruch Givoni,UCLA, USA (thermal comfort, urban climatic design & human physiology ) Shuzo Murakami,Keio U, Japan (wind studies, urban heat island & assessment methodology ) Wong Nyuk-Hien,NUS, Singapore (thermal comfort, wind studies & urban heat island )
Issues Large podium and congested building masses Tight site, narrow streets and alleys Small street width to building height ratio Uniform building heights Buildings with very minimal or literally no setbacks Towers built on podiums First row buildings of similar height perpendicular to the prevailing wind direction Building layout creates stagnant zones on building facades Minimal or no planting of trees
International Experts view The Goal of AVA for Hong Kong High density living Congested human activities Sub-tropical living more the better
Urban Acupuncture An underlying spirit of AVA is to promote more scientific based urban interventions conceptually described as a form of urban acupuncture. The study focuses on the fundamental mission: How to design and plan our city fabric effectively for better natural air ventilation?
The focus of AVA to allow a simple assessment mechanism to aid design and to optimize built forms, so as to promote better air ventilation through the city fabric.
Velocity Ratio as an indicator VR w = V V p (Wind at pedestrian level) (Wind potential / availability) Wind Velocity Ratio (VR w ) indicates how much of the wind availability of a location could be experienced by pedestrians on ground taking into account the surrounding buildings. As VR w is solely affected by the buildings of the location, it is a simple indicator to assess the effects of designs. The higher the value of VR w, the lesser the impact of the design on wind availability.
The concept of VR I
The concept of VR II VR VR i w = = V V 16 i= 1 pi i F i VR i Wind from direction i Vpi is the pedestrian wind velocity of the location when wind comes from direction i. V i is the available wind velocity of the site when wind comes from direction i. VRi is the velocity ratio of the location when wind comes from direction i. Fi is the frequency occurrence of wind from direction i. 16 directions are considered. VRw is the wind velocity ratio
The concept of VR III AVA reports two velocity ratios: Site Velocity Ratio (SVRw) is the spatial average wind velocity ratio of all the perimeter test points. It gives an indication of how the proposal affects the immediate surroundings. Local Velocity Ratio (LVRw) is the spatial average wind velocity ratio of all the perimeter test points and all the overall test points. It gives an indication of how the proposal affects the overall surroundings.
Tools For AVA, Wind tunnel is a preferred and recommended tool. CFD may be used for initial studies only.
Methodology Find out how well the design is doing Design new Compare designs Evaluate, select, alter and improve design Report SVR & LVR SVR site spatial average VR LVR local spatial average VR Model Work out VR Further studies Test points Perimeter test points Overall test points Special test points Testing Use appropriate wind profiles and characteristics, test 16 directions
Example
Qualitative guidelines for design Hong Kong Planning Standards and Guidelines Chapter 11 It is useful to have an idea qualitatively how to optimise designs for air ventilation in the early design stages. By following these guidelines, the proposal has a better possibility to achieve better air ventilation. This could then be confirmed with the AVA later.
Qualitative guidelines I Breezeway / Air path It is important for better urban air ventilation in a dense, hot-humid city to let more wind penetrate through the urban district. Breezeways can be in forms of roads, open spaces and low-rise building corridors through which air reaches inner parts of urbanised areas largely occupied by highrise buildings. Projecting obstructions over breezeways/air paths should be avoided to minimize wind blockage
Qualitative guidelines II Orientation of Street Grids An array of main streets, wide main avenues and/or breezeways should be aligned in parallel, or up to 30 degrees to the prevailing wind direction, in order to maximize the penetration of prevailing wind through the district.
Qualitative guidelines III Linkage of Open Spaces Where possible, open spaces may be linked and aligned in such a way to form breezeways or ventilation corridors. Structures along breezeways/ventilation corridors should be low-rise.
Qualitative guidelines IV Non-building Area Compact developments on large sites are particularly impeding air movement. Development plots should be laid out and orientated to maximize air penetration by aligning the longer frontage in parallel to the wind direction and by introducing nonbuilding areas and setbacks where appropriate.
Qualitative guidelines V Waterfront Sites Waterfront sites are the gateways of sea breezes and land breezes due to the sea cooling and sun warming effects. Buildings along the waterfront should avoid blockage of sea/land breezes and prevailing winds.
Qualitative guidelines VI Scale of Podium For large development sites particularly in the existing urban areas, increase permeability of the podium structure at the street levels by providing some ventilation corridors or setback in parallel to the prevailing wind. Where appropriate, a terraced podium design should be adopted to direct downward airflow, which can help enhance air movement at the pedestrian level.
Qualitative guidelines VII Building Heights Height variation should be considered as much as possible with the principle that the height decreases towards the direction where prevailing wind comes from. The stepped height concept can help optimize the wind capturing potential of the development itself.
Qualitative guidelines VIII Building Disposition Where practicable, adequately wide gaps should be provided between building blocks to maximize the air permeability of the development and minimize its impact on wind capturing potential of adjacent developments. The gaps for enhancing air permeability are preferably at a face perpendicular to the prevailing wind.
Qualitative guidelines XI Shading and greenery Tall trees with wide and dense canopy should be planted along streets / entrance plazas / setback areas for maximizing pedestrian comfort reducing urban heat island effect while avoiding blocking air movements at the pedestrian level.
A design example The Hanoi Tokyo Project An example of Design Research by Prof. Murakami to illustrate the relationship between porosity in high density design and the air ventilation environment.
A Design Example Tseung Kwan O
Stage A of AVA A soft, gentle launch with the government taking a lead is strongly recommended At the initial stage (Stage A), The Govt and quasi-govt organizations should undertake AVA during preparation/review of town plans and for major projects. The private sector are encouraged to conduct AVA on a voluntary basis. The AV impact of options may be compared with a view to achieving a favourable urban form.
Examples of Conditions recommending AVA Preparation of new town plans and major revision of such plans Development that deviates from the statutory development restriction(s) Erection of building structure within a designated breezeway Urban renewal development that involves agglomeration of sites together with closure and building over of existing streets Large-scale development with a high density, e.g. site area over 2 hectares and an overall plot ratio of 5 or above, development with a total GFA of 100,000 sq.m. or above Development with shielding effect on waterfront, particularly in confined airsheds (For developments situated in an exposed location, an assessment of windy conditions that may affect the safety of pedestrians should also be included.)
Actions Taken prepare and promulgate a government technical circular setting out the responsibilities and procedures for carrying out AVA for all major government proposals and projects. amend HKPSG to incorporate the qualitative design guidelines for better air ventilation. initiate Stage B (urban climatic mapping) and Stage C (benchmarking standards) research studies.
Stages for Implementing AVA
Stage B - Urban Climatic Mapping An experience from Germany (Katzschner)
Stage B - Urban Climatic Mapping
Stage C Ventilation Standard Wind availability experiments Benchmarking (wind tunnel) tests Numerical Simulations Field measurements User surveys Criteria and (wind-ventilation) standard Refine AVA technical methodology & implementation strategies
Stage D - Quantitative Guidelines Not yet funded Quantitative guidelines can be in the form of simple ratios, indexes. When applied, development proposal would be deemed to satisfy air ventilation requirements without the need to go through the testing process. For example: An air path or air paths along the direction of the prevailing wind should be provided on site equal to 25% of the site area.
END Thank you