6.9: Potable Water (Possible Sidebar)
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Potable water is water that is clean enough to drink safely. It does not contain harmful levels of chemical pollutants or microorganisms. Thanks to engineering efforts that began several thousand years ago, most residents of developed countries have access to safe, clean water. However, many residents of developing countries do not have such access, and struggle daily with sickness and other effects of bad water. Providing water to these people is a challenge to engineers and to the societies in which they work.
In this section, we trace some of the important historical engineering advances related to clean water in the context of their societies. We look at supplying water to large cities in the ancient Roman world, medieval and industrial Europe, and the modern western United States.
One common theme that runs through all of the engineering projects discussed in this section is that these projects were very large. Many stretched the technical capabilities of the civilizations that implemented them. Their implementation required not only good engineering, but also large commitments of funds by governments to pay large groups of laborers and provide significant amounts of material. Throughout history, the development of clean water supplies and sanitation systems has been primarily undertaken by governments and not by private individuals or corporations. Thus, the engineers that led these projects needed skills that extended far beyond the application of math and science; they needed to understand and be able to work with governments to obtain the resources for the projects, and they needed to understand the capabilities of the laborers who would work on the projects.
Many of the civilizations that preceded Rome developed important engineering techniques that were later adopted by the Romans. Jerusalem was one of the first large human settlements in which an engineered system supplied drinking water. Water from springs near the city was diverted through tunnels under the city to cisterns and underground reservoirs for storage. Neighboring civilizations, including those in present day Syria, Iraq, and Iran, used dams, aqueducts, tunnels, and quanats to supply water.
Initially, residents of Rome got drinking water from the Tiber River and local springs and wells. However, as Rome grew, some of these sources of water became polluted, and they did not provide enough water for the city. When Rome needed a reliable supply of water, Roman engineers could use these techniques that had been developed by earlier civilizations to supply water.
The Romans built aqueducts to move water from its source in springs or rivers to Rome. We are familiar with the arched bridges used to carry aqueducts across valleys; the aqueduct shown in the image (Figure below) is one such bridge. Perhaps less well known is that the Roman engineers avoided building these bridges whenever possible, preferring instead to use channels in the ground or tunnels to transport the water.
The Pont du Gard is an aqueduct in the South of France constructed by the Roman Empire.
The earliest aqueduct supplying Rome was the Appis, built in 312 BC. Eventually, by about AD 100, twelve aqueducts supplied water to the over one million people who lived in Rome; the aqueducts had a total length of about 300 miles, with only 40 miles on arched bridges. In addition to Rome, most other large Roman cities were supplied with potable water.
Similar to many engineering projects, the Roman engineers who planned and built the Roman water system did not invent all of the techniques they used, but did make improvements in these techniques. For example, the Romans developed a water-resistant cement that was used to line aqueducts. The Romans also developed the idea of storing potable water in reservoirs close to the water’s source, as opposed to reservoirs in the city.
Much of what we know about the Roman water system comes from the writings of Sextus Julius Frontinus (about 40–103), who was the Roman water commissioner about AD 100. He was clearly proud of the Roman water system and the engineering that had implemented it. He wrote “with such an array of indispensable structures carrying so many waters, compare if you will, the idle Pyramids or the useless, though famous works of the Greek.”
In addition to a supply of potable water, Rome had a sewer system. In this system, water from the aqueducts along with water from streams and springs flushed human waste and other undesirable substances through the sewers into the Tiber River. Unlike a modern sewage system, the waste was untreated and polluted the river. Only the wealthiest private houses were connected to the system. For those without indoor plumbing, public latrines were available for a small price. However, many people would empty chamber pots from upper story windows on to the street.
With the decline of the Roman Empire, many of the advances in supplying potable water and in dealing with wastes were lost, particularly in northern and western Europe.
London is located on the banks of the Thames River. In the thirteenth century, it had a population of about 40,000. By the seventeenth century, this population had grown to over one-half million. As in Rome, Londoners initially relied on water from the river Thames and springs and wells, but as the city grew, these resources became polluted and did not sustain the population.
Many engineering projects were developed to increase the water supply. In the mid-thirteenth century, the “Great Conduit” was the first of twelve conduit systems to be built. In these systems, water from a spring was stored in a large nearby cistern. This cistern was connected by a pipe to another cistern up to a mile away; this second cistern had spigots to dispense the water. From 1609 to 1613, the New River, a canal of almost 60 km, was built by Sir Hugh Myddleton (1560–1631). This canal is still an important source of water for London today.
As in Rome, the disposal of human and animal waste was also an issue in London throughout its history. Impure drinking water and poor sanitation were primary causes of the devastating plague epidemics that swept through Europe, including London, from the mid-fourteenth to the mid-seventeenth centuries. In spite of repeated efforts by the government, the Thames River was polluted by the sewage and other refuse that flowed into it.
In the mid-1840s, London’s Metropolitan Commission of Sewers ordered that cespits should be closed and that house drains should be connected to the sewer system that drained into the Thames. The increased pollution led to cholera outbreaks in 1848 and 1849. The image (Figure below) shows a caricature of commentary offered by Michael Faraday (1791–1867), a influential British scientist, on the state of the river in 1855. The summer of 1858 was unusually hot, and the Thames River, as well as many of the streams that flow through London into it, were extremely polluted with sewage. The resulting smell was so bad that it threatened to shut down the operation of the British government. This episode was labeled the “Great Stink.”
A caricature of commentary on the state of the River Thames offered by Michael Faraday in 1855.
The Great Stink was so bad that the Metropolitan Board of Works (which replaced the Metropolitan Commission of Sewers) authorized its chief engineer, Joseph Bazalgette (1819–1891), to redesign and rebuild the London sewer system. His design used 83 miles of brick-lined sewer tunnels to move the sewage downstream of London where it was released untreated into the Thames. The capacity of the sewer system was large enough that it is still in use today. The London sewer system was a massive public works program.
The Western United States
Much of the western United States is arid or semiarid land. Many of the West’s major metropolitan centers can sustain their current populations only because of large water conservation projects. Water conservation projects include dams to store water and canals to distribute this water.
Most of these large water conservation projects in the West were built in the first half of the twentieth century. Construction of these projects was a significant feat of engineering. They all involved large budgets, large work forces, and made use of the most advanced technology of their time. Three of these projects are the Salt River project in Arizona, the Los Angeles aqueduct in California, and Hoover dam on the Colorado River (on the Nevada/Arizona border). We briefly describe these three projects.
The Salt River Project
The Salt River Project was begun in 1904 with the start of construction on Theodore Roosevelt Dam. The Salt River flows from the mountains in eastern Arizona, through the Phoenix metropolitan area, then joins the Gila River on the way to the Colorado River. The Salt River is subject to both floods and droughts. Farmers whose crops were watered by the river needed a more reliable supply of water. So they created the Salt River Valley Water Users Association in 1903. The first major engineering project was the construction of the Theodore Roosevelt Dam shown in a photograph from 1915 in the image (Figure below). Begun in 1904 and completed in 1911, this dam was the highest masonry dam in the world at the time of its completion. It was 280 feet tall and stored 1.65 million acre feet (537 billion gallons) of water in the Theodore Roosevelt Lake (the reservoir created by the dam). The next image (Figure below) is a photograph of the dam’s dedication by Theodore Roosevelt, who was president of the United States at the time of its completion.
The Theodore Roosevelt Dam in 1915.
President Theodore Roosevelt speaking at the dam that bears his name.
Three more dams (Horse Mesa, Mormon Flat, and Stewart Mountain) were added on the Salt River below Theodore Roosevelt Dam between 1923 and 1930. Water stored by these dams is released into the Salt River when needed, and flows downstream to the Granite Reef diversion dam where it is channeled into canals that distribute the water throughout the Phoenix metropolitan area. The original purpose of the Salt River Project was to supply water for agriculture. Since the 1960s, this water has also made the rapid population growth of the Phoenix metropolitan area possible; the Phoenix metropolitan area has grown to more than 4 million people.
The Los Angeles Aqueduct
The Los Angeles aqueduct supplies the Los Angeles metropolitan area with water. The aqueduct transports water from the Owens River in Central California to Los Angeles. It was constructed from 1908 to 1913 by about 5000 workers at a cost of $23 million.
The engineer primarily responsible for the design and construction of the aqueduct was William Mulholland (1855–1935). An Irish immigrant born in 1855, he arrived in Los Angeles in 1877 and began work as a ditch maintainer. He had little formal education, but was mostly self-taught from mathematics and engineering textbooks. He eventually became the head of the Los Angeles Department of Water and Power, and it was in this position that he planned and built the aqueduct. His career as an engineer was abruptly ended in 1928, when the St. Francis Dam that he had designed and whose construction he had supervised collapsed, and the resulting flood killed almost 500 people.
The aqueduct was a significant engineering accomplishment at the time of its construction. It transports water for 226 miles. It has 142 tunnels whose total length is 43 miles; the longest tunnel is the Elizabeth, which is five miles long. The aqueduct uses siphons to cross several large valleys. The entry of the aqueduct into Los Angeles is by the cascades shown in this image (Figure below).
After flowing through the aqueduct, water enters Los Angeles through these cascades.
The Los Angeles aqueduct made the rapid growth of the Los Angeles area possible, particularly during the first half of the twentieth century. This came at a severe environmental cost: the Owens River Valley was changed into a desert. Owens Lake, originally fed by the Owens River, dried into an alkali salt flat, and dust from this flat today is an environmental hazard. Birds once used Owens Lake as a resting area while migrating; they no longer do so. As a result of a lawsuit settled in 2003, the Los Angeles Department of Water and Power (which operates the Los Angeles aqueduct) was required to start allowing some water to flow in the Owens River.
The Colorado River flows for 1440 miles from its source in the Rocky Mountains to the Gulf of California in the Pacific Ocean, and drains an area of 244,000 square miles. It has an average annual flow of 17.5 million acre feet; this flow varies tremendously from much lower in drought years to much higher in flood years. The Colorado River basin includes portions of seven states: Arizona, Colorado, California, Nevada, New Mexico, Utah, and Wyoming. The Colorado River supplies water to more than 24 million people living in communities inside and outside of its basin, including Los Angeles, Phoenix, Albuquerque, Las Vegas, Salt Lake City, Denver, and San Diego. It also provides irrigation water to about 2 million acres of land.
The Colorado River is one of the most regulated water sources in the United States, and each state’s share of water is determined by several federal laws. To provide this water, a system of dams and canals have been developed on the Colorado River and its tributaries. Hoover Dam was the first of these dams and one of the largest engineering projects in the United States.
Hoover Dam (originally called Boulder Canyon Dam) was constructed between 1931 and 1935. The dam and Lake Mead (the reservoir behind the dam) are shown (Figure below). At the time of its construction, it was the largest concrete structure in the world. It is 726 feet tall, and was the tallest dam in the world when constructed. The hydroelectric power plant at the base of the dam generates electric power; it was the largest hydroelectric power plant in the world from 1939 to 1949.
An aerial photograph of Hoover Dam.
The next image (Figure below) shows a plan of the dam and the surrounding canyon. It shows several of the techniques that were necessary to build the dam at the bottom of a deep canyon. Before construction could begin on the dam, the Colorado River was diverted away from the construction site. The river was diverted through four tunnels cut into the canyon walls. The tunnels were 56 feet in diameter with concrete linings that were three feet thick. After the tunnels were finished, two cofferdams were built, one upstream of the dam site and one downstream of the dam site. These diverted the river through the tunnels, leaving the dam site dry for construction.
A contour map of Hoover Dam and the surrounding canyon.
At the time of its construction, the dam was the largest concrete structure that had been built. This presented several challenges in the construction. One was moving the wet concrete to the proper location as the dam was built. Another was cooling the concrete as it hardened (concrete gives off heat as it sets, and if it becomes too hot, will not set properly).
Frank Crowe (1882–1946) was the engineer who directed the construction of the dam. He invented the techniques that were used to solve many of the construction problems. Born in 1882, he attended the University of Maine from 1891 to 1895, studying Civil Engineering. In 1905, he began work at the US Reclamation Service, and worked in dam construction for the next 20 years; it was in this period that he began to develop the construction techniques that would make it possible to construct Hoover Dam. In addition to his technical expertise, he was talented at getting along with different people on different levels. According to one co-worker, “One thing he knew was men.”
As with all developments of such magnitude, there are also issues associated with the dam. One is that Lake Mead is slowly filling up with sediment. The Colorado River carries a huge amount of rocks, sand, and silt that has been eroded from the land that it drains. As the river flow slows on entering Lake Mead, this sediment settles out of the water. Recent studies show that it is now between 30 meters and 70 meters deep. At the current rate of sedimentation, enough sediment will accumulate to fill Lake Mead entirely within the next few hundred years unless a method is devised to solve the sedimentation problem.