Aqueducts

 

An aqueduct is a channel or pipe used to transport water from a remote source to a desired location, such as a town, city, or agricultural area. The simplest, most primitive form of aqueduct was used in Egypt, Assyria, and Babylon. It consisted of nothing more than an open canal dug between the river and the city. Water from the river flowed into the city by the force of gravity.

More complicated aqueducts were later developed in Persia (modern-day Iran). Called qanats, they consisted of tunnels that began in the foothills of mountains and wound gradually downhill to bring water to population centers. The qanats were hand-dug, so vertical shafts had to be placed every 20 to 30 meters along the length of the qanat in order to remove excavated material, as well as to provide ventilation and access for repairs. The excavated soil was used to build up mounds around the shaft openings, which prevented surface runoff from entering the qanat and contaminating the water with silt and other material. When the soil in which the qanat was dug was loose, reinforcing rings made of burnt clay were installed to prevent cave-ins. To this day, qanats are still used in Iran, and they continue to be built in much the same way.

 

The Romans are perhaps the most famous aqueduct builders of the ancient era. In fact, the word "aqueduct" is derived from the Latin words aqua ("water") and ducere ("to lead"). Within a period of about 500 years, the Romans constructed 11 major aqueducts to supply Rome with water. The first Roman aqueduct, Aqua Appia, was built around 312 BC. By the time the 11th aqueduct, Aqua Alexandrina, was completed in 226 AD, Rome was being watered by 359 miles of aqueducts and was receiving about 50 million gallons of water each day. In addition to building aqueducts for Rome, the Romans also built aqueducts for regions throughout their empire, including France, Spain, and Northern Africa. Remains of most of these aqueducts still exist, and a few, such as the one in Segovia, Spain, are still in use.

Like qanats, the Roman aqueducts relied mainly upon gravity for their operation. They were built at ground level and underground where possible, with a very gradual slope of 1/200 or less. When large valleys had to be crossed, they constructed arcades, or arched bridges, to maintain the desired height for the water. One of the more interesting features of Roman aqueducts was the occasional use of inverted siphons to cross low points. An inverted siphon consisted of a closed pipe laid along the valley floor. Because the water at one end of the pipe was at a higher elevation than the water at the other end, a pressure difference forced the water through the pipe to the lower end, enabling the flow to continue uninterrupted, and relieving the need to build an arcade.

When pipes were used in the Roman water systems, they were often made of lead, which has led many to wonder whether the fall of the Roman Empire may have been at least partially due to lead poisoning. However, there are two important facts that tend to refute this hypothesis. First, to become contaminated by lead, water has to sit in the pipes for a period of time. The water in the Roman pipes was almost always flowing, so the water never had time to settle and become contaminated. Second, most of the water flowing into Rome was hard water, which meant that it carried dissolved minerals. These minerals would form encrustations along the pipes, thus providing a buffer between the lead and the water. In fact, the encrustations would occasionally become so thick that they had to be chipped off to enhance the water flow.

After the fall of the Roman Empire and the descent into the Dark Ages, many of the Roman aqueducts continued to be used until they fell into disrepair. Because sufficient repairs typically were not capable of being made, most of the aqueducts eventually fell into disuse. During this era, aqueduct building was conducted mainly by churchmen. However, the aqueducts they built were often for the use of monasteries and convents only, and were relatively insignificant compared to the magnificent aqueducts of the Romans. Because many city water supplies were insufficient and contaminated, diseases such as typhoid, dysentery, and cholera were common during this time.

With the Renaissance came a renewed interest in aqueducts and water systems. The first "modern" waterworks is believed to have been built in London in 1582. This system used pumps to fill an elevated reservoir. Pressure created by gravity then forced the water through the wooden mains. The first American waterworks were built in Boston in 1652.

These early distribution systems used hollow wooden logs to contain the water. The logs were tapered on one end, so the tapered end of one fit snuggly into the hollow end of the next. Pressure in the system was so low that the water could not be raised above the ground floor of most houses. Additionally, these waterworks operated during only a fraction of the day, so water could only be drawn from the system at certain times. Because the mains were made of wood, contaminated water could very easily seep into them and spread the contamination.

Many early waterworks used horse-driven pumps to move the water. Therefore, the invention of the steam engine greatly improved the productivity of waterworks. The very first steam piston engine, developed by the French physicist Denis Papin in 1690, was used for pumping water. Later steam engines, such as the Newcomen engine (first built in 1712), also saw much use as water pumps in early waterworks. Additionally, iron pipes began to replace the wooden mains, enabling higher water pressures in the system and reducing the risk of contamination.

Modern aqueducts and waterworks typically employ enormous pumps that are driven by electricity or turbines. Thus, modern water systems can be built with little regard for the terrain or the force of gravity.