Source: Translated and adapted from: "MACHU PICCHU: A CIVIL ENGINEERING MARVEL". Published in: CIVIL ENGINEERING MAGAZINE. January 2001

When the American hydraulic engineer Ken Wright saw some Machu Picchu photographs, taken by his wife during a visit to Peru in 1974, and heard her comments about the pools and other structures built by the Incas for water supply, he was motivated to visit the famous pre-Columbian ruins and started on-site research on the topic.

Yet, twenty years passed before the Peruvian government consented to research in Machu Picchu. Since 1994 and for the following six years, together with his team, Wright visited the place from one to three times each year while working full time for his own company, WRIGHT WATER ENGINEERS.

During the years of work in Peru, Wright collaborated closely with Alfredo Valencia Zegarra, a Peruvian archaeologist who devoted an extensive part of his professional career to studying Machu Picchu. In the year 2000 they both published the book Machu Picchu: A Civil Engineering Marvel, from which some of the following paragraphs have been taken.

"The city of Machu Picchu, once the royal state of the powerful Inca Empire, lay hidden in Peruvian mountains until 1911, when Hiram Bingham, Professor of History at Yale University, discovered its ruins. From then onwards, it has become South America's most important archaeological site.

In 1450 the Incas reached this place, a 2,440 meter-high ridge in the Andes range, with a goal in mind: to build a state for their emperor Pachacutec. They had "the perfect site", Wright notes, but one whose suitability must have been only obvious to an experienced engineer. The slopes were very steep and sheer. How could the buildings be prevented from sliding downhill in the middle of heavy rains? How would drinking water be made accessible? And from what source would the water come?"

Wright's research obviously revealed that they must have carefully planned the city before building it. First, engineers had to determine the spring's exact location and whether it would supply the population's needs beforehand. Wright's team found that the fountain, on the steep slope of a mountain to the north of Machu Picchu, is fed by a 16.3-hectare tributary drainage basin. After conducting an inflow-outflow evaluation, the team also concluded that the spring draws on drainage from a much larger hydro-geologic basin.

The Incas enhanced the spring's yield by building a spring collection system on the hillside. The system consists of a stone wall about 14.6 meters long and up to 1.4 meters high. Water from the spring seeps through the wall into a rectangular stone trench about 0.8 meters wide. Water from a secondary spring enters the channel about 80 meters west of the primary spring. The Inca also built a 1.5 to 2.0 meter-wide terrace to allow easy access for operating and maintenance work. The spring's condition surprised Wright. "The spring works were still intact, still working and yielding water after all these centuries of abandonment."

However, before the city could be built, Inca engineers had to plan how to channel the water from the spring, at an altitude of 2,458 meters, to the proposed site. They decided to build a 749 meter-long channel with a nearly 3 percent gradient. Inside the city walls, the water would be made accessible through a series of 16 fountains, the first of which would be reserved for the emperor. This is how the channel design, says Wright, determined the location of the emperor's residence and the layout or planning of the entire city of Machu Picchu.

The Incas built the water supply channel at a relatively steady level, depending on gravity flow to take the water from the spring to the city center. They used stone blocks to build a channel that ranged from 10 to 16 centimeters deep and 10 to 12 centimeters wide at the bottom. Wright's team concluded that the channel's nominal design capacity was about 300 liters per minute- more than twice the typical yield of 25 to 150 liters per minute of the primary and secondary springs.

The channel descends the mountain slope, enters the city walls, goes through the agricultural sector, then crosses an inner wall and enters the urban sector, where it feeds a series of 16 fountains known as the "Escalera de las Fuentes" (stairway of the fountains). The fountains are publicly accessible and partially enclosed by walls about 1.2 meters high, except for the lowest, which is a private fountain for the "Templo del Cóndor" (Condor's Temple), having higher walls. At the head of each fountain, a cut stone conduit channels the water to a rectangular spout, which was so shaped to create a water jet suitable for filling an aryballo, a typical Inca clay water jug used by the Incas for water collection and transportation. The water is collected in a stone basin carved into the rock of the mountain. Then, it enters a round drain that delivers it to the access channel for the next fountain.

Wright and his team studied the fountains extensively, conducting hydraulic flow tests and measuring the channels and outlets. They concluded that the Incas designed the fountains to operate optimally with a flow of about 25 liters per minute, but the fountains could operate with such low flows as 10 liters per minute and could handle a maximum flow of 100 liters per minute. The team found water control points at two spots along the channel, where excess water would have spilled over onto the agricultural terraces or into Machu Picchu's main drain before reaching the fountains.

Wright's study of Machu Picchu's hydrology and hydraulic engineering led him to conclude that the Incas understood the importance of having drinking water. The surface drainage system generally drove the agricultural and urban storm water runoff away from the water supply canal. Wright also noted that the Inca apparently did not use the fountains for bathing. The emperor, for example, had a bathing room with a separate drain. Therefore, water used for bathing did not re-enter the water supply.

In 1998, Wright's team discovered another, previously unknown series of fountains on the eastern side of the ridge, downhill from Machu Picchu. These fountains received their water not from the channel but from intercepted groundwater drainage. While elaborate spring works were not necessary here, Wright says, "the Inca would have had to identify the groundwater flow locations during dry-weather and concentrate this flow for use in the fountains."
Adjacent to some of the fountains, an important trail, also discovered by Wright's team, connected Machu Picchu to the Urubamba River, in the valley, at the height of the dense forest. The team restored the water flow to this second series of fountains probably for the first time in 450 years.

How successful were the Incas in planning their water supply? Observers have suggested different theories to explain why the Incas abandoned Machu Picchu. Some suggested that a water shortage forced the Incas to leave. Wright says his research rejects that theory.

A hydrological analysis showed that the yield of the primary spring was related to the amount of rainfall. To determine rainfall levels during the time the Incas occupied Machu Picchu (from 1450 to about 1540), Wright analyzed ice core data coming from a glacier found 250 km to the southeast. The analysis suggested that Machu Picchu annually received nearly 2,000 centimeters of rainfall, and that in the final decade of occupancy rainfall actually increased.

Wright determined that a 10 liters per minute flow for the fountains during the dry months would have been enough to meet the needs of the population, which may have varied from 300 to a maximum of 1,000 people when the emperor was in residence. In a dry year's winter, Wright says, the Incas may have experienced a temporary water shortage. But his discovery of the trail that leads to the Urubamba River seemed to confirm that the Incas in this case would have used the river as a secondary water source. Consequently, Wright concluded that a water shortage was not the explanation for Machu Picchu's abandonment.