Engineering the City
How Infrastructure Works: Projects and Principles for Beginners
By Matthys Levy, Richard Panchyk
Chicago Review Press Incorporated Copyright © 2000 Matthys Levy and Richard Panchyk
All rights reserved.
Water, Water Everywhere
There would be no life without water. As we look up to the heavens and particularly toward our nearest neighbors, the planets, one of our first questions is usually: Is there any water there? For centuries, the lines crisscrossing the surface of Mars that can be seen through a telescope were thought to be canals, perhaps constructed by intelligent beings. Sadly, this turns out not to be true. When a spacecraft landed on Mars recently and sent back signals that water may exist under the surface of the red planet, astrophysicists, scientists who study the composition of the universe, were wildly excited. It was the first evidence that life may exist outside of Earth. The first clue that other forms of life exist somewhere in the universe will most likely be the existence of water — the building block of life.
What Is Water?
Although water is a liquid, it is a compound of two gases: one part oxygen mixed with two parts hydrogen. If you look at a globe of Earth, you will notice that the color blue predominates. Blue is typically used to designate water, which covers 71 percent of the globe. This ocean water tastes salty and contains as many as 32 different salts and minerals. If you were to drink it, you would get sick. Actually, too much saltwater could easily kill you. On the other hand, if you were to put a fish from the ocean into fresh water, it would swell up (by a process called endosmosis) and die. Fortunately for us, seawater is heated by the sun and evaporates, leaving all the salts and minerals behind. It then condenses into clouds that float around the sky and, through precipitation, release their water as rain or snow (Figure 1.1). When rain or snow falls on the land, it seeps into the ground and forms springs, rivers, and lakes that all, eventually, flow back into the ocean. The water that we use comes from these springs, rivers, or lakes and is relatively pure. This cycle of evaporation, condensation, precipitation, seepage, and flow is the natural water cycle without which life on Earth would not be possible.
The First People
Even the earliest humanlike creatures, called Australopithecines, who lived 4,000,000 years ago, knew how and where to find water. When our human ancestors first left their cave dwellings more than 10,000 years ago (at the end of the Ice Age or Paleolithic period), they gathered in villages located near streams, rivers, or lakes. They recognized that water was their most important commodity. After all, about 70 percent (actually somewhere between 67 and 78 percent) of the human body consists of water that must constantly be replaced. Water in the body is used to help drive the digestive system, to lubricate the body's joints, to cushion the internal organs, to cleanse the body both inside and out, and to control the skin's temperature through evaporation as we perspire.
Our ancestors usually sited their dwellings close to water. From their round huts of woven vines or reeds covered with thatch, these early people looked out onto a river or lake and drank its water, washed in it, and pulled out buckets of it to irrigate their gardens.
Three thousand years ago, on the banks of the Danube River and the lakes of Switzerland, Stone Age civilizations of hunters and agriculturists built platforms on poles set into the river or lake bottom. On these platforms, they built clay floors with raised hearths. They framed their houses with steep thatch roofs and triangular gables and walls of mud plaster or logs sharpened to fit into vertical grooved posts (Figure 1.2). From these houses, the people would lift buckets of water from the river or lake for their drinking and cooking needs. They would also dump their waste down to the lake or river.
At that time, no one thought of separating the drinking water from that used for cleaning. After all, the lake was so large or the river so swift that one person or even a family couldn't possibly become ill from drinking the same water in which one washed.
However, the first families multiplied over time. The village became a town and, much later, a city. After a while, the crystal-clear lake or river gradually turned cloudy and people became ill from drinking its smelly water.
People noticed that water that flowed from a spring in the ground was usually clear. So they began to dig wells in the ground as deep as necessary to find this clear water (Figure 1.3). Water that flows into wells originates as rain or snow that seeps underground through the earth, which acts as a filter to remove dangerous organisms.
Since older wells were dug by hand, they were made about 3 feet (1 m) in diameter, just big enough for a man to stand in the hole while digging it. These early wells were lined with stone to keep earth from falling into the hole. The Indus Valley in present-day Pakistan had this type of well 5,000 years ago. Later civilizations used clay bricks to line their wells (Figure 1.4). Today wells are dug by machine and are no bigger than the 2- to 4-inch (50- to 100-mm) diameter steel pipe that is drilled into the ground. Since modern drills can go through rock as well as soil, wells can recover water that flows through cracks in the rock hundreds of feet below the surface.
The type of earth or rock through which underground water flows limits the amount of water that flows into a well. To supply water to larger families and villages, larger and deeper wells were needed. Such town wells can still be seen in the central squares of many old villages in Europe and Asia.
Many towns today rely on wells for their water supply. But these wells reach down past several layers of soil, sand, and gravel into what is called an aquifer, or layer of porous, water-holding rock. These aquifers were originally pure, but as people used fertilizers and chemicals on their land, the chemicals began to seep down into the aquifers. Thousands of wells all over the world are now too polluted to drink from. Only the deepest aquifers remain unpolluted today.
Eventually, even in ancient times, villages grew to the size of towns whose increasing population needed more water than wells could supply. "Why not bring the water down from mountain springs?" thought the people of these ancient civilizations. But first a problem had to be solved: because water only flows downhill, how do you create a channel that slopes from the mountain spring down to the city, crossing hills and valleys? An aqueduct, a conduit, originally lined in stone, provided the answer. To cross valleys, arched bridges were built (Figure 1.5); and to penetrate hills, tunnels were bored. The first aqueducts were built almost 3,000 years ago in the countries around the Mediterranean Sea. Although they were not the first aqueduct builders, the clever Romans developed the idea masterfully, building many stone aqueducts to supply the cities of their growing empire. For instance, there were 11 aqueducts leading into ancient Rome with a total length of 310 miles (500 km) of which 260 miles (420 km) were in tunnels and the rest in arched structures. These satisfied the needs of the city's occupants for domestic use, public baths, and almost 200 public fountains. They were so well built that many of Rome's aqueducts survive today, although underground pipes have replaced their function.
Find the Centerline
Discover what it was like for early engineers to build straight tunnels. Try this experiment with four friends!
2 empty cans
2 short sticks
Lots of pebbles, pennies, paper clips, or other objects to use for markers
* Give two of your friends each an empty can and a short stick. Have them stand in opposite corners of a room or outside on a lawn. Next, blindfold them and make certain they don't peek. Be sure to tell them to be silent. Turn them around a few times so they don't know which way they are facing. Ask each of them to start walking slowly and at the same time hit their tin cans every 10 seconds. Explain to them that they should walk toward the sound of the other can. Have two other friends walk behind each of the first two, marking with pebbles (or any other small objects) the paths being followed. The game ends when the first two friends reach each other.
Notice the path followed by each of the two as they walked toward each other using only sound as their cue. See how difficult it is to follow a straight line.
Aqueducts are still built today to feed cities' need for water. Instead of stone, modern aqueducts are built of sealed pipes of steel or concrete reinforced with steel bars. These pipes are strong enough to withstand the pressure of the water pushing against their sides, and are usually circular except when they sit on the ground.
How the Pressure Exerted by Water Shapes Conduits
Flexible plastic tube with soft sides and a sealed end, or a long, thin balloon
Duct tape or electrical tape
* Fill the tube or balloon with water and hold vertically. If you use a balloon, don't fill it to bursting. Close the open end by tying or taping it. Notice that the shape of the tube is circular (Figure 1.6). Now lay the tube on a table and notice what shape it takes. Compare it to the drawing of the Croton Aqueduct, built in 1842 to provide water to New York City (Figure 1.7).
Instead of sloping continually from the water source to the city, modern steel or concrete aqueducts can follow the terrain of the land. They are able to do this because they are sealed and because they take advantage of the siphon principle, discovered by Hero, a Greek who lived around 150 B.C. Hero found that he could make water flow uphill in a sealed tube. Of course, there was a trick involved in his method.
How Does a Siphon Work?
2 cooking pots or large containers to hold water
Rubber or plastic tube, or piece of clean, small diameter garden hose about 3 feet (1 m) long
* Fill both pots half full with water and place one on a counter and the other on a chair below the counter. Take the tube (make sure it is long enough to reach from the counter to the chair) and place one end in the upper pot. Take the other end in your mouth and suck until water rises up in the tube. Now pinch the tube to keep the water from sliding back into the upper pot and place the open end of the tube into the lower pot. Release the tube and notice that water flows from the upper to the lower pot. In doing so, water first has to flow up the tube (to get over the edge of the pot) before it flows down the other end. This only works if the second pot is below the first one (this is the trick). What do you think causes water to flow uphill? Is it any different than sucking liquid through a straw?
The scientific explanation is that the air all around us exerts pressure, a force acting on every square inch (mm) of surface it touches. This air pressure represents the weight of the column of air above the earth's surface all the way up to the stratosphere. Since the air becomes thinner — and therefore lighter — the higher we go, the air pressure is less on the top of a mountain than it is in the valley (Figure 1.8).
As we suck on a straw, we are removing the air in the straw (creating a vacuum) and allowing the outside air pressure to push the liquid up the straw. This is the same action that takes place in the siphon. It is also the same action that takes place as a pump pulls water up from a well. Unfortunately, there is a limit to how high the water can be lifted in a siphon. When the outside air pressure equals the weight of the column of water in the tube, the water can go no higher. This happens when the tube of water is about 33 feet (10 m) above the surface of the upper water reservoir at sea level (and less if it is on the top of a mountain where air pressure is lower).
In places where water is obtained from underground aquifers more than 30 feet (10 m) below the ground, pumps must be placed at the bottom of the wells. Instead of sucking up the water, the pumps push the water up the pipe. Such pumps can operate hundreds of feet below the ground.
Many towns have storage tanks on top of towers that are higher than the highest building (Figure 1.9). Water can flow from the storage tank down a pipe, through an underground pipe, and then back up to the top floor of any building in town. Under a modern city a maze of pipes distributes water from the reservoir, or storage tank, through large pipes and then smaller pipes to the thousands of buildings in which we work and live (Figure 1.10).
Next time you turn on a faucet, imagine how far the water has traveled and through what aqueducts or pipes and from what reservoirs or storage tanks.
We have come a long way from being able to sip clear water from streams and rivers. In most places, water is treated with the chemical chlorine to purify it before it enters our homes. Where the untreated water is not very safe, the chlorine taste can be pretty strong, but it won't do you any harm. In some towns, fluoride is also added to the water. Fluoride is a chemical that is good for our teeth and helps prevent cavities.
Although the water we use may come from far away, thanks to our high-technology testing and treatment systems, most of the water that comes into our homes is safe to drink.
* Draw a time line showing the dates when events identified in this chapter took place. A time line is a linear graph that you divide into parts (years, in this case) on which you identify significant events.
* Think about how the ideas in this chapter apply to the workings of your body. What organs or organ systems most closely relate to the story of water? Write your ideas down in a journal or notebook.
* Find out where the water you use in your house comes from. Does it come from a reservoir, well, or river? How far away is the source? In some places water may come from a lake or the ocean. In Chicago, for instance, water is pumped from Lake Michigan through a filtration plant where chemicals are added to purify the water. In Kuwait, on the Arabian Gulf, water is pumped from the ocean through a desalinization plant that removes the salts and purifies the water.
The story of cities starts with water. As you have seen in the previous chapter, early people settled along bodies of water such as streams, rivers, or lakes. These early settlements grew, and many became cities. If you were asked to name the great cities in your country and the world, you would certainly mention a city along a river or lake. For example, Rome is on the Tiber, Paris is on the Seine, London is on the Thames, New York City is on the Hudson, and Cairo is on the Nile.
Cities and Water
* On your map, use your highlighter to draw lines along the rivers and around lakes. Now identify cities that lie along the waterways and write a list of the largest ones and their respective countries. You will notice that it is almost impossible to find a major city that is not next to or near a body of water. A city such as Las Vegas that is far from any body of water is totally dependent on water being piped down from faraway mountain reservoirs.
The earliest people probably didn't know much about what existed beyond the immediate area in which they lived and hunted. For them, leaving the immediate neighborhood was probably almost as frightening as being visited by strangers. But as cities developed almost 7,000 years ago, their occupants began to travel outside their immediate environs. There were many reasons for them to look beyond their city's borders. They had to find agricultural land to feed a growing population; they needed natural resources such as iron and copper to make tools and containers; and they wanted to expand their influence to other cities or peoples. When people built up their courage and decided to explore the world beyond their territory, it was natural for them to think of using rivers as roads. But to travel on the river they first needed boats. (Continues...)
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