Landscape Design Strategies to Reduce Heat Island Effect - Part 01

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1. Heat Island Effect

The term “heat island effect” simply use to describe that urban areas or metropolitan areas hotter than nearby rural areas. Comparing to the day time, evening and night time temperature is higher than its surroundings. Therefore, “Heat Island Effect” simply use as “Urban Heat Island Effect”.

1.1 Impacts of heat island effect

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In the days which have less rain roof and pavement surface temperatures can be 50–90°F (27–50°C) hotter.  This high temperature has caused multiple impacts to the environment and contribute indirect impacts as well.
Increased energy consumption, elevated emissions of air pollutants and greenhouse gases, compromised human health and comfort, impaired water quality are the major constraints generated due to Heat Island Effect.

1. Increased energy consumption
Urban heat islands increase overall electricity demand, as well as peak demand. Offices, residences, commercial areas need more electricity to lights, cooling systems and other cooling applications than rural areas due to HIE.

2. Elevated emissions of air pollutants and greenhouse gases
As discussed above, urban heat islands raise demand for electrical energy. Companies that supply electricity typically rely on fossil fuel power plants to meet much of this demand, which in turn leads to an increase in air pollutant and greenhouse gas emissions.
As examples sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM), carbon monoxide (CO) and mercury (Hg) kind of pollutants cause to health issues and contribute to air quality problems.

3. Compromised human health and comfort
HIE affects human health by contributing to general discomfort, respiratory difficulties, heat cramps and exhaustion, non-fatal heat stroke, and heat-related mortality.

4. Impaired water quality
High pavement and rooftop surface temperatures can heat storm water runoff. This heated stormwater generally becomes runoff, which drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes.

1.2 Sources to Urban Heat Island

1) paved areas (which can account for 1/3 or more of a city’s land area) and dark-colored rooftops absorb more sunlight and re-emit more heat than would an area’s native soils and vegetation.
 2) motor vehicles, buildings and machinery produce waste heat that is not quickly removed from the area.
3) loss of vegetation eliminates natural cooling from evapotranspiration and shading.

2. Strategies and technologies to Reduce Heat Island Effect through a Landscape Design

Increasing tree and vegetation cover lowers surface and air temperatures by providing shade and cooling through evapotranspiration. Trees and vegetation can also reduce storm water runoff and protect against erosion.

1. Plant selection and plant placing

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Increasing tree and vegetation cover lowers surface and air temperatures by providing shade and cooling through evapotranspiration. Trees and vegetation can also reduce storm water runoff and protect against erosion.

• Create a landscape plan that considers the sun’s path.

• Protect existing trees and shrubs when possible. If not, consider transplanting as an alternative to complete removal. Transplanted trees require plenty of water until they are established, typically through at least one season, depending on size.

Site Analysis
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• Install windbreaks a distance away from the home of 2 to 5 times the mature height of the trees being used for maximum protection.

• Keep foundation plants several feet away from the home to improve airflow.

• Balance shading with solar access, especially when systems such as solar panels that require sunlight are present.

• Place medium to large-sized trees at least 20 feet away from the building.

• Trees that are smaller when mature can be placed closer to the building, but consider the breadth of the full-grown crown of a tree to maintain both energy and aesthetic value.

• Place trees, shrubs, hedges, or trellises to shade pavements and other hardscapes such as driveways, patios, and sidewalks that absorb energy from the sun and give off heat.

• Provide shading for an air conditioning unit to increase its efficiency by as much as 10%.

Plant selection tips

• Review the Native Plants strategy that provides information on plant selection.

• Select low-growing shrubs and herbaceous plants to avoid blocking windows allowing for visibility to the outdoors and adequate air circulation when windows are open.

• Select site-appropriate plants including native trees that may reduce watering needs once they are established, though all newly installed plants require watering.

• Smaller trees adapt much faster to site conditions and are generally cheaper than large trees.

• Evergreen trees and large shrubs provide greater protection from wind and noise as well as year-round privacy.

• Consider genetic diversity in the landscape design to reduce vulnerability to plant disease and pests.

2.2 Materials

Proper material usage and design standards direct to reduce UHI.

Green roofs

Roofs can be designed and built as practical heat reflectors.
Roofing materials that have a high solar reflectance are known as “cool roofs.” This attribute decreases heat transfers to the indoor environment and can also improve the durability of roofs. Cool roofing materials may also possess the characteristic of high emittance, that is, releasing a large percentage of the solar energy they absorb.

Pavements and roads
Pavement-ving materials that lower surface temperatures are available and can be used in establishing roads, streets, parking lots and other facilities that require paved surfaces. Cool pavement options are available for traditional materials like asphalt and concrete as well as unpaved surfaces.
There are two cool pavement alternatives:
(a) pavements with higher solar reflectance that are relatively cooler in the sun.
(b) porous, or permeable pavements that benefit from the cooling effect of evaporation.

Conclusion

Densely developed urban areas warmer than they would be if left in an undeveloped condition is termed as “Heat Island Effect”. UHI causes many problems such as high energy consumption, health issue, Emitting greenhouse gases etc. Except that it leads to many indirect impacts to the earth.
Human activities are the main reason for the heat island effect and it can control through a good landscape design. Plant selection, placement of plants, water efficient landscaping, choosing high Solar reflectance materials, using permeable pavings for the pathways are some strategies to reduce the heat island effect.