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MPP 655: Policy Making for Sustainable Urban Communities
Module 7: Solutions: Green Infrastructure – Part 1
Hello, and welcome to module #7, solutions, green infrastructure, which is part of
MPP 655, policy-making for sustainable urban communities. What we are going to
talk about today is the idea of green infrastructure. Green infrastructure is the
counterpoint to the idea of gray infrastructure. That is to say, gray infrastructure is
built infrastructure, roads, parking lots, sewage lines, electrical, heating, cooling,
those kinds of things, that we build to maintain the comfort that we have for urban
living.
So, three broad categories that emerge in this idea of green infrastructure: there is the
idea of urban heat island mitigation, which we will talk about in some detail. There
is the idea of urban forestry, and xeriscape and native vegetation, and habitat
conservation and restoration, having to do with the landscape, primarily. Those
kinds of solutions. Bioswales, things like that. Then we will talk about storm water
retention and groundwater recharge. What is called low-impact development, which
is a way of developing land that minimizes the amount of storm water runoff that you
get. So you have more porous pavement; things like that. And we will talk a little
bit about the idea of day-lighting streams and rivers. Many streams and rivers, in the
process of urbanization in many urban environments, such as Los Angeles, have been
paved over over the years. Streams have been put into culverts, have been put into
drainage ditches, sewer lines, and so on and so forth. So the idea of daylighting
streams and rivers is that you expose them to daylight. You open them up and allow
them free play across the landscape.
Let's take the idea of urban heat islands first and talk about this. What we have
realized over the years is that urbanized areas tend to be something like 4 and 8
degrees Fahrenheit hotter than the countryside. This is primarily because of darker
surfaces, roofs and paving, asphalt, things like that; the fact that there is less
vegetation usually in urbanized areas; and so you get less evaporative transpiration,
that is to say, plants sucking up moisture from the soil and releasing it into the
atmosphere, thus cooling the atmosphere; and then we have more heat-generating
machines, central air-conditioning, things like that. That generate heat in and of
themselves.
When you look at a profile of what an urban heat island looks like, [on the board], it
goes something like this. You have the rural areas are generally the lowest in terms
of temperature; and then as you get into suburban areas, the temperature goes up.
And then as you get into downtown areas it peaks sharply, and then as you move
outward back to rural areas, the temperature tends to drop, and peaks a little bit in
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suburban areas, drops particularly in parklands and things like that, and then drops
again in the rural areas. This is one way of thinking about the urban heat island. We
have measured evidence that this is true, and the question is, does it matter? How
does it matter? And what can we do about it?
It matters because heat islands tend to increase air pollution, energy consumption,
and storm water runoff. All three are concerns for us, and we build gray
infrastructure to compensate for these effects. My argument is that we could use
green infrastructure instead to countervail each of these three adverse impacts, and
this would lead us to an urbanized area that is more beneficial to humans, and less
costly for us to maintain.
So ozone formation, we will talk a little bit about how it is that ozone formation is
linked to the heat island effect. How could it be that just simply increasing the
temperature of a place, by putting down asphalt and other heat-absorbing surfaces
increases tropospheric ozone formation? Smog, is what it is called. Particulate
matter gets increased, and heat stress gets increased. Energy consumption, quite
obviously, with air-conditioning and so on and so forth, there is an increased load on
energy. If you could cool the urban areas by two to four degrees Fahrenheit you
could reduce energy consumption accordingly. And then storm water runoff – many
of the surfaces that are heat absorbing tend to be as well impervious surfaces, and
being impervious surfaces causes storm water runoff.
Here, [on the board], is a chart of what is called albedo, the heat reflecting property
of surfaces. You see asphalt here (bottom center), is close to zero. Pure black is 0
and pure white is 1. And that is the scale upon which albedo is measured, and so
trees are between 0.15 to 0.18. Porous colored paints, red and brown tiles, tar and
concrete, brick and stucco, concrete, and so on and so forth. So you get these various
albedo properties, and what you are looking to do is reduce the lower numbers and
increase the higher numbers. If you do that, you have reduced the albedo. I am
sorry. You have increased the albedo, and thus increased the heat-reflecting
properties of the area, and made it more conducive to lower temperatures.
Asphalt makes for higher temperatures, and the Lawrence Berkeley National
Laboratory is experimenting with various kinds of asphalt coating that might help
increase the albedo, and reduce the temperatures.
Here is a chart that shows the relationship between temperature -- I'm sorry.
Maximum daily ozone concentrations on the vertical scale, and temperature on the
horizontal scale. And for two areas, Atlanta, Georgia and New York City, you will
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see a difference in the pattern because the variation in temperature is much higher in
New York than it is in Atlanta. Atlanta being a southern city, and tends to have a
tighter cluster of temperatures. But in general, what you see is a trend upward. As
temperature increases, ozone concentrations increase. This is strictly a temperature
relationship, that does not have anything to do with emissions of gases and so forth.
Simply by increasing the temperature, you increase the amount of ozone that is
formed. What you have to understand about ozone is that it is not an emitted gas. It
is formed in the atmosphere from other gases that are emitted from various sources.
For instance, automobiles will emit volatile organic compounds and oxides of
nitrogen, and these mix in the lower atmosphere. When they are hit by sunlight,
there is a photochemical reaction that takes place. This photochemical reaction is
temperature-sensitive.
So for the same volume of gases that are present in the lower atmosphere, if you
increase the temperature, you increase the reactivity of those gases, and thus increase
the amount of tropospheric ozone that is formed.
There are two potential heat island mitigation strategies, broadly speaking. One of
them is surface albedo modification, and the other is urban revegetation. If you
increase the albedo, you increase the heat reflective properties of surfaces, and
therefore allow less absorption of heat, and so less heating of the lower atmosphere.
And if you increase urban revegetation, trees provide shade; but trees and shrubs and
other kinds of vegetation provide evaporative transpiration, and so they are cooling
the atmosphere.
So, lighter-colored materials and thermal materials. These are the two strategies
within surface albedo modification. The basic principle is that if you have an
ordinary roof, it absorbs heat and heats up the space within. If you have a heatreflecting roof, then it reflects the heat away and the room, the space below the roof
becomes cooler.
Here is a chart, [on the board], showing the relationship between temperature
differences between the roof surface and the air, and solar reactivity. White paint has
a high reflective property, and black paint has very low heat reflective properties. So
this makes for a cooler roof, and gives us a way in which we can begin to mitigate
urban heat island effects. White roofs are a classic example. Just paint the roof
white and that reduces the temperature within by a few degrees Fahrenheit or
centigrade. Green roofs are another kind of strategy that is used, [on the board],
where you plant shrubs and grasses and other kinds of vegetation on the roof itself.
There are many kinds of green roofs. We can talk about this in more detail. There
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are intensive roofs and extensive roofs. This, [on the board], is an example of an
extensive roof. Here is an example from Chicago. A green roof, where various
native plants are being grown to cool the spaces below, and reduce the amount of
heat that is absorbed by the roof itself.
There are air quality benefits from urban heat island mitigation, we find. As we saw
before, ozone formation is one way in which we improve air quality. But more than
that, there is a 10 to 15 percent reduction in volatile organic compounds from
running losses in mobile sources. That is to say, the amount of lost gases, volatile
organic compounds, that are put out by mobile sources are themselves reduced,
simply because the temperature, the ambient temperature has been reduced. There is
a 15 to 30% reduction in volatile organic emissions from parked vehicles. If you
provide shade trees in parking lots, and so on and so forth. There is a reduction in
heat exposure to ozone, and there are additional emission reductions from area and
stationary sources.
Here is an estimate of the total energy saving potential of light-colored roofs. In the
west, Los Angeles and Phoenix, these are high heat areas. Southern America, going
up to Chicago, and toward the Northeast, where there are also examples of savings
from white roofs.
Urban revegetation. Tree planting, replacement of trees that have died, landscaping,
xeriscaping, which is the planting of drought tolerant plants, and native plants which
are adapted to the local environment, and thus do not require much irrigation.
Those kinds of strategies ought to be embraced as part of an urban revegetation
campaign that is looking to mitigate heat islands. Trees act in various ways to give
us many kinds of benefits. The roots stabilize the soil; the trunk, branches and leaves
provide evaporative transpiration. The leaves filter dangerous pollutants from the
air. Acting as a net that catches dust and so forth. They provide shade; they absorb
sound and block erosion-causing rainfall; and entirely help habitat for birds, animals,
insects and microbes. Let's not forget microbes, which are very important to the
functioning of ecosystems.
Los Angeles has a “million trees” program, and you can go to this website and you
can find more out about this program and what it does. [on the board]. There are
various tree planting efforts around the southland that you can find out more about,
and you can participate in efforts to plant new trees, provide more shade, provide
increased evaporative transpiration, things of that sort.
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When it comes to tree planting, there are four benefit streams of large-scale treeplanting efforts. That is to say, millions of trees. If you plant millions of trees in
Southern California, you'll definitely end up improving the air quality, improving
energy efficiency, improving groundwater recharge, and increasing evaporative
transpiration. This is a certainty. This is what we mean by green infrastructure. All
these benefits being captured in the process of planting trees, besides the fact that
trees are ecologically beneficial, gives us humans a reason for supporting these kinds
of strategies.
Improvements in air quality, again, become important to this process. Shade trees in
parking lots prevent evaporative losses from automotive fuels in parked cars;
mitigate greenhouse gas emissions; and reduce heat loads within automobiles – if
you park in a shaded area, when you come back to your car it is not quite so hot, as
I’m sure you are all aware, as if you were to park in the sunlight. You will notice if
you have driven in Southern California at all, that almost every parking lot is bereft
of trees. There are very few tree-covered parking lots in Southern California, and if
we were to begin to address this, we would reduce air pollution significantly, reduce
the amount of air conditioning load that is needed when you start up your car, and
generally make for improved comfort.
So there are definitely improvements in energy efficiency. Strategically-chosen and
planted trees, because of the shading they provide to the built environment, cut the
thermal load. Cool roofs and thermal insulation reduce indoor temperatures, albedo
modification reduces urban heat island effects, and evaporative transpiration from
trees, shrubs and other vegetation cut the temperature significantly.
Here is a chart that you can survey at your leisure after we're done with this lecture.
But it basically sets out the strategies that we can adopt for heat island mitigation, the
categories across which they act, and the monetized benefits that accrue to us from a
cool roof, a cool community kind of strategy. You would see the total value in this
estimate, which is somewhat basic, is $535 million per year that we could be
generating, simply by doing these kinds of cool roofs, trees, cool pavements, and so
on and so forth. (left side).
Let's break here, and then we can come to groundwater recharge and storm water
management. [Session ends.]
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