East St. Louis Action Research Project - Alta Sita Neighborhood 

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 URBAN ECOLOGY       - 

Problems
The rapid urbanization of U.S. cities during the past fifty years has been associated with a steady increase in downtown temperatures about 0.1degree to 1.1 degree Celsius per decade.  We can find the negative effect of Urban Heat island in the demand of cities for electricity increases.  About 3 to 8 percent of current electric demand for cooling is used just to compensate for this urban heat island effect.  Other implications of growing urban heat islands include increases in carbon dioxide emissions from power plants, municipal water demand, concentrations of smog, and human discomfort and disease.  Global warning, which may double the rate of urban temperature rise, could accentuate these environmental problems.
 

Analysis
Analysis of temporal differences shows that the intensity of the urban heat-island is greatest at night, primarily due to differences in urban –rural cooling.  At sunset, rural areas begin to cool rapidly while urban areas remain warm and then cool at a slower rate.  Different urban-rural cooling rates at sunset produce maximum heat-island intensities three to five hours later.  At sunrise, urban areas  begin to warm relatively slowly, sometimes producing urban “cool islands” during morning.
Warmer air temperature in cities compared to air temperatures in surrounding rural area is the principal diagnostic feature of the urban heat island.  Alterations of the urban surface by people result in diverse microclimates whose aggregate effect is reflected by the heat island.  Buildings, paving, and other physical elements of the urban fabric are active thermal interfaces between the atmosphere and land surface.
 

Light-colored surfaces
Increasing the albedo of building surfaces by whitewashing reduces solar heat gain and resulting heat storage.  Additionally, increasing the albedo of large areas of the city by using light-colored dyes or sand in paving materials will lower air temperature by reducing the absorption of shortwave radiation.
According to data from computer simulations for Sacramento, CA, a temperature drop of one to four degrees Celsius can be achieved by increasing citywide albedo from 25 to 40 percent.  Whitewashing residential buildings reduced 19 percent and peak cooling demand by 14 percent.  The combined effects of modifications in urban and building albedo resulted in savings of up to 62 percent for annual cooling and 35 percent for peak cooling.
 

Vegetated surfaces
Vegetation can mitigate urban heat islands directly by shading heat-absorbing surfaces, and indirectly through evapotranspirational cooling.  In most circumstance, the impact of one or several trees on ambient temperatures and cooling load are small compared to the shading effect.  Cool air produced in the tree crown is dissipated by the much larger volume of air moving through the tree.  However, large numbers of trees and expansive green spaces can reduce local air temperatures by one to five degrees Celsius.
 
 
 
 

                           Design Example
 

Street Canyons

Light-colored roof, wall, and paving surfaces further reduce the amount of absorbed radiation.  Tall, narrow deciduous trees shade the canyon walls and sidewalks, reduce downdrafts, and provide evapotranspirational cooling.  The deciduous trees lower summertime temperatures through ET cooling and provide increased winter irradiance when they drop their leaves.  The trees also have other benefits such as absorbing gaseous pollutants and carbon dioxide, emitting oxygen, intercepting particles, reducing noise, and enhancing scenic beauty.
 
 

Parking Lots

Issues of visibility, safety, screening, and access are central to traditional parking-lot design.  The design should incorporate these concerns as well as the needs for shade, buffering, and water harvesting.  Trees are aligned in north-south rows to shade as much pavement as possible during summertime.  Dense, broad-spreading tree crowns increase the amount of shade.  Ample growing space and soil moisture enhances tree survival, growth, and ET cooling effects.  Trees are pruned high enough for safe visibility and truck clearance.  Stormwater runoff is harvested in basins along the periphery, where structurally diverse plantings filter particulate, intercept rainfall, and screen the parking lot.
 
 
 
 
 
 
 
 
 
 

Residential Street

Most residential streets have paving that is too wide and planting strips that are too narrow.  This design example shows the wider green space along residential street, which shade the pavement and promote pedestrian travel within a more comfortable microclimate.  Narrowed traffic lanes force cars to travel slowly through residential neighborhoods.  Runoff harvested from streets and sidewalks reduces flooding downstream and conserves irrigation water.