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Urban design for hot humid regions
Renewable Energy, Vol.5, PartlI pp. 1047-1053, 1994 Elsevier Science Ltd Printed in GreatBritain
~l'g~llOI!
0960-1481/94 $7.00+0.00
URBAN DESIGN FOR HOT HUMID REGIONS B. Givoni Graduate School of Architecture and Urban Planning University of California, Los Angeles, USA ABSTRACT The paper discusses the design objectives and the urban design elements which affect and can modify the urban micro-climate in hot humid regions. The design elements discussed are: Location of towns in a region; Density of the built-up area; Orientation and width of streets; Building design details affecting the comfort of people outdoors, and the Design details of public and private "green" areas. Urban design details are suggested for hot humid and for hot dry regions. The paper is based on a recent book of the author (Givoni 1994). KEYWORDS Urban design; Urban climatology; Urban density; Design guidelines; Hot humid climate; Hot dry climate, Town's location; Street orientation; Urban green areas.
~ntroduction Many features of the physical structure of the city can affect the urban climate. As the structure of a city can be controlled by urban planning and design it is possible to modify the urban climate through design. With such modifications it is possible to improve the comfort of the inhabitants outdoors and indoors, and to reduce the
1047
1048 e n e r g y d e m a n d of the b u i l d i n g s for c o o l i n g in summer.
for h e a t i n g
in w i n t e r and
The effects of the f o l l o w i n g physical features of the u r b a n area on the urban climate in hot h u m i d regions will be d i s c u s s e d in this paper: L o c a t i o n of a town w i t h i n a region D e n s i t y of the u r b a n area O r i e n t a t i o n and w i d t h of the streets Heights, and relative heights, of b u i l d i n g s U r b a n Parks and p r i v a t e green areas
Location Considerations in Hot-Humid Regions A m a j o r factor d e t e r m i n i n g the level of thermal stress in a hot-humid region is the street level air speed. W i n d w a r d slopes are p r e f e r a b l e to l e e w a r d slopes from the v e n t i l a t i o n viewpoint. V a l l e y slopes m a y e x p e r i e n c e downslope air c u r r e n t s d u r i n g w i n d l e s s nights, thus p r o v i d i n g m o r e c o m f o r t a b l e c o n d i t i o n s as c o m p a r e d w i t h the v a l l e y floor area (Lyons 1984, Givoni 1989). S e a s h o r e and large l a k e s h o r e areas m a y benefit from the d a y t i m e sea and n i g h t t i m e land breezes. Flat areas along rivers or near outlets of rivers m a y be s u b j e c t e d to severe floods w h e n h e a v y rains, o f t e n in far a w a y c a t c h m e n t areas, raise the w a t e r level above the "normal" flood level. M a n y towns in h o t - h u m i d coastal r e g i o n s are in areas prone to h u r r i c a n e (or c y c l o n e as t h e y are c a l l e d in S o u t h e a s t Asia) hazards. The storms are a c c o m p a n i e d by v e r y h i g h rainfall. O f t e n such areas are attractive for naturally irrigated agriculture b e c a u s e of the f e r t i l i t y of the land, or for p r o v i d i n g opportunities for fishing, coastal trade, etc. This e c o n o m i c a t t r a c t i v e n e s s should be w e i g h t e d against the risks to life and p r o p e r t y losses d u r i n g c y c l o n i c floods.
Min~mlzinq Flood Hazards b 7 Urban Design Features D u r i n g rains the city itself increases the excess of w a t e r run-off. The a b i l i t y of the g r o u n d in an u r b a n area to a b s o r b w a t e r is reduced, as a result of the c o v e r a g e of the land b y buildings, roads, p a r k i n g areas, etc. This factor i n c r e a s e s the risks from floods in low lying flat u r b a n areas. The risk of floods from excess rain w a t e r can be m i n i m i z e d by details of u r b a n design. The
1049
following design details can be applied to achieve this goal: Increasing rain absorption in soil in the urban area, by the type of covering, thus reducing run-off. Preserving land features of natural drainage such as interconnected valley systems. The network of valleys which naturally drain off the rain water from the town's area can serve as urban parks and other open spaces. With proper planting they can be reused for such functions after the rain water subsides. Thus they can serve socio-functional needs of the people together with this climate related function.
Layout
(Orientation)
of the Street's Network
In hot-humid regions the main climatic objectives concerning the street layout would be to provide good ventilation conditions for pedestrians in the streets and good potential for ventilation of the buildings along the streets. The "ideal" design solutions for these two objectives are not the same and a compromise may be called for. Another objective is to provide shade over sidewalks in streets with large concentrations of pedestrians. Such shade can be provided by trees along sidewalks, as well as by special details of the buildings. The best ventilation within the streets and the sidewalks is achieved when the streets are parallel to the direction of the prevailing winds during the afternoon hours (when the urban temperature reaches its maximum). However, when streets are parallel to the wind direction and the buildings along them are very close together and face the street, the ventilation potential of the buildings is compromised. The reason is that with this orientation, all the walls of the building are in "suction" zones. Effective indoor cross-ventilation can occur in a building only when at least one of its walls (and windows) are in a "pressure" zone. Streets perpendicular to the wind direction, with closely spaced long buildings along them, as is common in many town in Developing Countries, may block the wind in the whole urban area and do not contribute at all to the urban ventilation. A good street layout from the urban
1050 ventilation aspect in a hot-humid region is when wide main avenues are oriented at an oblique angle to the prevailing winds (e.g. between 30 and 60 degrees). This orientation enables penetration of the wind into the heart of the town. The buildings along such avenues are exposed to different air pressures on their front and back facades. The upwind wall is at a pressure zone while the downwind wall is at a suction zone, thus providing the potential for natural ventilation.
Urban Density and Buildinq Heiqhts in Hot-Humid Reaions Urban density is among the major factors which determine the urban ventilation conditions, as well as the urban temperature. Generally speaking, the higher the density of the buildings in a given area the poorer will be its ventilation conditions. However, for a given density level there can be different urban ventilation conditions, depending on the particular configuration of buildings by which this density is obtained. An urban configuration to be avoided as much as possible in hot humid regions is that of high long buildings, of about the same height, perpendicular to the prevailing wind direction. This configuration blocks the wind and creates poor ventilation conditions both in the streets and for the buildings. An urban profile of variable building height, where buildings of different heights are placed next to each other, and when the long facades of the buildings are oblique to the wind, enhances urban ventilation.
Characteristics of Different BuildinqTypes Reqions
in Hot-Humid
As most countries in the hot-humid areas are developing countries, the special social problems and needs of the population should be a critical factor in choosing a given building type. However, as this paper is limited to dealing with the climatic aspects, the social impacts of the different building types will not be discussed here, except for some comments when seemed appropriate.
Detached Single Family Houses By definition, detached houses are exposed to the outdoor air on all sides. This feature provides good potential
1051
daytime hours would not be worse than in more compact buildings. Furthermore, during the evening and night hours, when the outdoor wind usually weakens or subsides, a detached house will cool down faster than other types of buildings, providing better comfort. This building type is the least sensitive to the orientation of the building from the ventilation aspect. In summary, a detached house is the most suitable in a hot-humid climate. This feature should be taken into account by urban planners, especially since this type is also more suitable socially and functionally to the needs of large low income families.
Townhouses
(Row Buildings)
Townhouses are comprised of a set of several single family units, attached to each other on their sidewalls, thus forming a "row" of dwelling units. The units can range in height from a single story up to three stories. In hot-humid regions townhouses provide the next best building type for mass housing, from the climatic aspect, after detached houses. However, townhouses are much more sensitive than single family houses, from the ventilation aspect, to their orientation with respect to the wind direction. In m u l t i - s t o r i e d townhouses, internal staircases can form natural shafts for vertical air flow. When these shafts are large enough, they transform the whole space of the house into an integrated space from the air flow aspect.
Multi-Story Apartment Buildings Multi-storied apartment buildings may present difficulties in providing cross ventilation, unless they are planned with staircases (or elevators in buildings with too many stories for walkup) serving only two units per floor. Such buildings are also much more sensitive than single family houses, from the ventilation aspect, to their orientation with respect to the wind direction. The worst conditions would be in buildings with internal, double-loaded corridors. Therefore such apartment buildings should be avoided in hot-humid regions, in spite of being the least expensive, especially for buildings with elevators. When the corridor is external and open (like a balcony), it is theoretically possible to have cross ventilation in the apartments along it. In order to have such ventilation, doors and windows in the walls along the
1052
corridor should be left open. This, of course, severely interferes with the visual and acoustical privacy of the occupants When a staircase serves only two units per floor, each apartment is exposed to at least two external walls. With adequate internal organization of the indoor space it is then possible to insure good ventilation conditions to all the dwelling units. To achieve this goal, the building should be so oriented that the wind will be either perpendicular or oblique 9ith an angle of not more than 60 degrees from the perpendicular position.
Highrise "Tower" Buildings This type of building, which requires sophisticated structure, elevators and other mechanical systems, is applicable only to relatively high income people and therefore can not solve the mass housing problems of the developing countries. However, it has some interesting features from the climatic aspect in hot-humid regions and therefore deserves attention. When high narrow buildings are placed relatively far apart, they do not reduce the air speed near the ground level. In fact such buildings can increase appreciably the low level air speed, thus improving the ventilation potential for lower buildings between them and in the streets. The occupants of the high stories enjoy lower temperature, as well as lower humidity (as the vapor is generated at the ground level). This is in addition to the better ventilation potential and the view offered from the high stories. Providing high standard, although expensive, housing in a city can also increase its attractiveness for professionals and high income population groups, an attraction which is an asset for towns of developing countries.
Desiqn Details Reqions
of
Open
"Green"
Spaces
in
Hot-Humid
The climatic objectives in planning public urban open "green" areas (parks, playing grounds, etc.) in hot-humid regions should include, inter alia: Providing shade for the users of these areas. Minimizing blockage of the wind within the open spaces and in the built-up areas around the open spaces.
I053 sensitive
Minimizing urban
damage of areas.
floods
in
the
more
B e c a u s e of the h i g h p r e c i p i t a t i o n in h o t - h u m i d regions, local p l a n t s do not n e e d i r r i g a t i o n most of the year. Therefore, it is p o s s i b l e to m a i n t a i n "green" areas w i t h less m a i n t e n a n c e w o r k and expenses than it w o u l d be in a r i d regions.
The impact of plants on human comfort in h o t - h u m i d areas can be a m i x e d bag. The shading p r o v i d e d b y trees is always welcomed. However, the b l o c k a g e of the w i n d and the c o n t r i b u t i o n to the h u m i d i t y level b y e v a p o r a t i o n from the leaves increases human discomfort. Trees w i t h a h i g h trunk and wide c a n o p y are the most e f f e c t i v e p l a n t s in p r o v i d i n g usable shade. If d e n s e l y placed, t h e y of course block the wind. Therefore, the best s t r a t e g y w i t h such trees is to have t h e m o n l y at the spots where their shade will be utilized, such as n e a r benches, c h i l d r e n p l a y areas, along pathways, etc. H i g h shrubs block the w i n d and "contribute" to the h u m i d i t y level without p r o v i s i o n of shade. Therefore, t h e i r i n t r o d u c t i o n should be minimal in p a r k s in hothumid regions, although they have aesthetic and f u n c t i o n a l uses as area dividers, etc. A c o m b i n a t i o n of grasses, low flower beds and shade trees w i t h h i g h trunks is thus the most a p p r o p r i a t e plants c o m b i n a t i o n in p a r k s in this climate. L o w lying areas prone to floods can be g r a s s e d and p l a n t e d w i t h trees w h i c h can w i t h s t a n d flooding. If the v e g e t a t i o n can w i t h s t a n d a given height of w a t e r for a few days, then such areas can be u t i l i z e d as flood c o n t r o l s d u r i n g and i m m e d i a t e l y after rain storms. D u r i n g the p e r i o d s b e t w e e n the storms, such areas can a g a i n be u s e f u l p u b l i c green spaces for recreation, grazing, etc.
References
i. Givoni, B. (1994) : B u i l d i n q AND Urban D esiqn G u i d e l i n e s FOR D i f f e r e n t Climates. Van N o s t r a n d Reinhold. N.Y.
~l'g~llOI!
0960-1481/94 $7.00+0.00
URBAN DESIGN FOR HOT HUMID REGIONS B. Givoni Graduate School of Architecture and Urban Planning University of California, Los Angeles, USA ABSTRACT The paper discusses the design objectives and the urban design elements which affect and can modify the urban micro-climate in hot humid regions. The design elements discussed are: Location of towns in a region; Density of the built-up area; Orientation and width of streets; Building design details affecting the comfort of people outdoors, and the Design details of public and private "green" areas. Urban design details are suggested for hot humid and for hot dry regions. The paper is based on a recent book of the author (Givoni 1994). KEYWORDS Urban design; Urban climatology; Urban density; Design guidelines; Hot humid climate; Hot dry climate, Town's location; Street orientation; Urban green areas.
~ntroduction Many features of the physical structure of the city can affect the urban climate. As the structure of a city can be controlled by urban planning and design it is possible to modify the urban climate through design. With such modifications it is possible to improve the comfort of the inhabitants outdoors and indoors, and to reduce the
1047
1048 e n e r g y d e m a n d of the b u i l d i n g s for c o o l i n g in summer.
for h e a t i n g
in w i n t e r and
The effects of the f o l l o w i n g physical features of the u r b a n area on the urban climate in hot h u m i d regions will be d i s c u s s e d in this paper: L o c a t i o n of a town w i t h i n a region D e n s i t y of the u r b a n area O r i e n t a t i o n and w i d t h of the streets Heights, and relative heights, of b u i l d i n g s U r b a n Parks and p r i v a t e green areas
Location Considerations in Hot-Humid Regions A m a j o r factor d e t e r m i n i n g the level of thermal stress in a hot-humid region is the street level air speed. W i n d w a r d slopes are p r e f e r a b l e to l e e w a r d slopes from the v e n t i l a t i o n viewpoint. V a l l e y slopes m a y e x p e r i e n c e downslope air c u r r e n t s d u r i n g w i n d l e s s nights, thus p r o v i d i n g m o r e c o m f o r t a b l e c o n d i t i o n s as c o m p a r e d w i t h the v a l l e y floor area (Lyons 1984, Givoni 1989). S e a s h o r e and large l a k e s h o r e areas m a y benefit from the d a y t i m e sea and n i g h t t i m e land breezes. Flat areas along rivers or near outlets of rivers m a y be s u b j e c t e d to severe floods w h e n h e a v y rains, o f t e n in far a w a y c a t c h m e n t areas, raise the w a t e r level above the "normal" flood level. M a n y towns in h o t - h u m i d coastal r e g i o n s are in areas prone to h u r r i c a n e (or c y c l o n e as t h e y are c a l l e d in S o u t h e a s t Asia) hazards. The storms are a c c o m p a n i e d by v e r y h i g h rainfall. O f t e n such areas are attractive for naturally irrigated agriculture b e c a u s e of the f e r t i l i t y of the land, or for p r o v i d i n g opportunities for fishing, coastal trade, etc. This e c o n o m i c a t t r a c t i v e n e s s should be w e i g h t e d against the risks to life and p r o p e r t y losses d u r i n g c y c l o n i c floods.
Min~mlzinq Flood Hazards b 7 Urban Design Features D u r i n g rains the city itself increases the excess of w a t e r run-off. The a b i l i t y of the g r o u n d in an u r b a n area to a b s o r b w a t e r is reduced, as a result of the c o v e r a g e of the land b y buildings, roads, p a r k i n g areas, etc. This factor i n c r e a s e s the risks from floods in low lying flat u r b a n areas. The risk of floods from excess rain w a t e r can be m i n i m i z e d by details of u r b a n design. The
1049
following design details can be applied to achieve this goal: Increasing rain absorption in soil in the urban area, by the type of covering, thus reducing run-off. Preserving land features of natural drainage such as interconnected valley systems. The network of valleys which naturally drain off the rain water from the town's area can serve as urban parks and other open spaces. With proper planting they can be reused for such functions after the rain water subsides. Thus they can serve socio-functional needs of the people together with this climate related function.
Layout
(Orientation)
of the Street's Network
In hot-humid regions the main climatic objectives concerning the street layout would be to provide good ventilation conditions for pedestrians in the streets and good potential for ventilation of the buildings along the streets. The "ideal" design solutions for these two objectives are not the same and a compromise may be called for. Another objective is to provide shade over sidewalks in streets with large concentrations of pedestrians. Such shade can be provided by trees along sidewalks, as well as by special details of the buildings. The best ventilation within the streets and the sidewalks is achieved when the streets are parallel to the direction of the prevailing winds during the afternoon hours (when the urban temperature reaches its maximum). However, when streets are parallel to the wind direction and the buildings along them are very close together and face the street, the ventilation potential of the buildings is compromised. The reason is that with this orientation, all the walls of the building are in "suction" zones. Effective indoor cross-ventilation can occur in a building only when at least one of its walls (and windows) are in a "pressure" zone. Streets perpendicular to the wind direction, with closely spaced long buildings along them, as is common in many town in Developing Countries, may block the wind in the whole urban area and do not contribute at all to the urban ventilation. A good street layout from the urban
1050 ventilation aspect in a hot-humid region is when wide main avenues are oriented at an oblique angle to the prevailing winds (e.g. between 30 and 60 degrees). This orientation enables penetration of the wind into the heart of the town. The buildings along such avenues are exposed to different air pressures on their front and back facades. The upwind wall is at a pressure zone while the downwind wall is at a suction zone, thus providing the potential for natural ventilation.
Urban Density and Buildinq Heiqhts in Hot-Humid Reaions Urban density is among the major factors which determine the urban ventilation conditions, as well as the urban temperature. Generally speaking, the higher the density of the buildings in a given area the poorer will be its ventilation conditions. However, for a given density level there can be different urban ventilation conditions, depending on the particular configuration of buildings by which this density is obtained. An urban configuration to be avoided as much as possible in hot humid regions is that of high long buildings, of about the same height, perpendicular to the prevailing wind direction. This configuration blocks the wind and creates poor ventilation conditions both in the streets and for the buildings. An urban profile of variable building height, where buildings of different heights are placed next to each other, and when the long facades of the buildings are oblique to the wind, enhances urban ventilation.
Characteristics of Different BuildinqTypes Reqions
in Hot-Humid
As most countries in the hot-humid areas are developing countries, the special social problems and needs of the population should be a critical factor in choosing a given building type. However, as this paper is limited to dealing with the climatic aspects, the social impacts of the different building types will not be discussed here, except for some comments when seemed appropriate.
Detached Single Family Houses By definition, detached houses are exposed to the outdoor air on all sides. This feature provides good potential
1051
daytime hours would not be worse than in more compact buildings. Furthermore, during the evening and night hours, when the outdoor wind usually weakens or subsides, a detached house will cool down faster than other types of buildings, providing better comfort. This building type is the least sensitive to the orientation of the building from the ventilation aspect. In summary, a detached house is the most suitable in a hot-humid climate. This feature should be taken into account by urban planners, especially since this type is also more suitable socially and functionally to the needs of large low income families.
Townhouses
(Row Buildings)
Townhouses are comprised of a set of several single family units, attached to each other on their sidewalls, thus forming a "row" of dwelling units. The units can range in height from a single story up to three stories. In hot-humid regions townhouses provide the next best building type for mass housing, from the climatic aspect, after detached houses. However, townhouses are much more sensitive than single family houses, from the ventilation aspect, to their orientation with respect to the wind direction. In m u l t i - s t o r i e d townhouses, internal staircases can form natural shafts for vertical air flow. When these shafts are large enough, they transform the whole space of the house into an integrated space from the air flow aspect.
Multi-Story Apartment Buildings Multi-storied apartment buildings may present difficulties in providing cross ventilation, unless they are planned with staircases (or elevators in buildings with too many stories for walkup) serving only two units per floor. Such buildings are also much more sensitive than single family houses, from the ventilation aspect, to their orientation with respect to the wind direction. The worst conditions would be in buildings with internal, double-loaded corridors. Therefore such apartment buildings should be avoided in hot-humid regions, in spite of being the least expensive, especially for buildings with elevators. When the corridor is external and open (like a balcony), it is theoretically possible to have cross ventilation in the apartments along it. In order to have such ventilation, doors and windows in the walls along the
1052
corridor should be left open. This, of course, severely interferes with the visual and acoustical privacy of the occupants When a staircase serves only two units per floor, each apartment is exposed to at least two external walls. With adequate internal organization of the indoor space it is then possible to insure good ventilation conditions to all the dwelling units. To achieve this goal, the building should be so oriented that the wind will be either perpendicular or oblique 9ith an angle of not more than 60 degrees from the perpendicular position.
Highrise "Tower" Buildings This type of building, which requires sophisticated structure, elevators and other mechanical systems, is applicable only to relatively high income people and therefore can not solve the mass housing problems of the developing countries. However, it has some interesting features from the climatic aspect in hot-humid regions and therefore deserves attention. When high narrow buildings are placed relatively far apart, they do not reduce the air speed near the ground level. In fact such buildings can increase appreciably the low level air speed, thus improving the ventilation potential for lower buildings between them and in the streets. The occupants of the high stories enjoy lower temperature, as well as lower humidity (as the vapor is generated at the ground level). This is in addition to the better ventilation potential and the view offered from the high stories. Providing high standard, although expensive, housing in a city can also increase its attractiveness for professionals and high income population groups, an attraction which is an asset for towns of developing countries.
Desiqn Details Reqions
of
Open
"Green"
Spaces
in
Hot-Humid
The climatic objectives in planning public urban open "green" areas (parks, playing grounds, etc.) in hot-humid regions should include, inter alia: Providing shade for the users of these areas. Minimizing blockage of the wind within the open spaces and in the built-up areas around the open spaces.
I053 sensitive
Minimizing urban
damage of areas.
floods
in
the
more
B e c a u s e of the h i g h p r e c i p i t a t i o n in h o t - h u m i d regions, local p l a n t s do not n e e d i r r i g a t i o n most of the year. Therefore, it is p o s s i b l e to m a i n t a i n "green" areas w i t h less m a i n t e n a n c e w o r k and expenses than it w o u l d be in a r i d regions.
The impact of plants on human comfort in h o t - h u m i d areas can be a m i x e d bag. The shading p r o v i d e d b y trees is always welcomed. However, the b l o c k a g e of the w i n d and the c o n t r i b u t i o n to the h u m i d i t y level b y e v a p o r a t i o n from the leaves increases human discomfort. Trees w i t h a h i g h trunk and wide c a n o p y are the most e f f e c t i v e p l a n t s in p r o v i d i n g usable shade. If d e n s e l y placed, t h e y of course block the wind. Therefore, the best s t r a t e g y w i t h such trees is to have t h e m o n l y at the spots where their shade will be utilized, such as n e a r benches, c h i l d r e n p l a y areas, along pathways, etc. H i g h shrubs block the w i n d and "contribute" to the h u m i d i t y level without p r o v i s i o n of shade. Therefore, t h e i r i n t r o d u c t i o n should be minimal in p a r k s in hothumid regions, although they have aesthetic and f u n c t i o n a l uses as area dividers, etc. A c o m b i n a t i o n of grasses, low flower beds and shade trees w i t h h i g h trunks is thus the most a p p r o p r i a t e plants c o m b i n a t i o n in p a r k s in this climate. L o w lying areas prone to floods can be g r a s s e d and p l a n t e d w i t h trees w h i c h can w i t h s t a n d flooding. If the v e g e t a t i o n can w i t h s t a n d a given height of w a t e r for a few days, then such areas can be u t i l i z e d as flood c o n t r o l s d u r i n g and i m m e d i a t e l y after rain storms. D u r i n g the p e r i o d s b e t w e e n the storms, such areas can a g a i n be u s e f u l p u b l i c green spaces for recreation, grazing, etc.
References
i. Givoni, B. (1994) : B u i l d i n q AND Urban D esiqn G u i d e l i n e s FOR D i f f e r e n t Climates. Van N o s t r a n d Reinhold. N.Y.