Inca Water Quality, Conveyance, and Erosion Control

Transcription

1 Inca Water Quality, Conveyance, and Erosion Control D. Apt 1 * 1 RBF Consulting, Alton Parkway, Irvine, CA, USA *Corresponding author, dapt@rbf.com ABSTRACT Machu Picchu is an extraordinary place for its vistas and architecture and history of the Incan culture. The Incas had an advanced understanding of engineering and building practices to build such a place that has stood the test of time over many centuries. Part of these building practices incorporate erosion control and water quality treatment that look very similar to what is known as Low Impact Development in the area of stormwater management today. The terraces at Machu Picchu were designed to ensure stability of the slope but they were also deigned to filter water and are similar to modern bioretention systems. The site of Machu Picchu was designed to convey and naturally treat water throughout the site while maintaining the stability of the structures and terraces, which are the foundation of the site that has lasted for centuries. The paper will explore the water quality and erosion control design features of Machu Picchu as well as other Incan sites and compare them to modern Low Impact Development techniques. KEY WORDS Low Impact Development, erosion control, water conveyance, drainage, integrated water management, Inca, Machu Picchu. INTRODUCTION The Inca civilization began in the area of Cuzco, Peru, which is in the Peruvian Andes, in approximately 1200 A.D. Machu Picchu, which is approximately 80 kilometers northwest of Cuzco, was built in approximately 1400 A.D. Machu Picchu sits on a mountain ridge in the Urubamba River Valley at an elevation of 2430 meters above sea level. The site is commonly believed by archaeologists to be built as an estate for the Inca emperor Pachacuti. The site was believed to be abandoned by the Inca at the time of the Spanish conquest in the 1500s. The site was brought to international attention by American historian Hiram Bingham in Machu Picchu is commonly recognized as an amazing feat of engineering. The Incas were extraordinary engineers, especially in the areas of drainage, erosion control, and water quality. Machu Picchu contains a complex drainage system that was designed to ensure that the site would endure the test of time. The elements of the drainage system include a system of terraces, drains, and a central underground aquifer to ensure minimization of erosion of the very steep site. The site also contains an elaborate water supply system with the source of water from mountain spring, canals, and fountains. The design of elements of the drainage system at Machu Picchu including the terraces and the central underground aquifer are very similar to modern erosion control and Low Impact Development techniques. The purpose of this paper is to explore the drainage, water quality, erosion control and conveyance elements of the Machu Picchu site and their similarities to modern erosion control and Low Impact Development techniques. Apt 1

2 METHODS The methodology for the study included a site visit and tour of Machu Picchu and the sites of Ollantaytambo and Sacsayhuaman August of During the tours of the site the complex drainage and water supply systems of the site were explored. The tours of the sites was lead by an indigenous guide of Inca decent that had a thorough understanding of the sites history and layout including the details of the engineering of the site and specifically the drainage and water supply systems. Aspects of the drainage and water supply systems were documented and photographed. The next step in methodology was detailed research based on what was learned during the tour and site visit. The research focused on the drainage and water supply studies that had been performed for Machu Picchu. Specifically the design of the terraces and central underground aquifer were investigated through a comprehensive literature review. The designs and layout of the drainage system was then compared to modern erosion control and Low Impact Development practices to identify similarities. RESULTS AND DISCUSSION Geography and Climate Machu Picchu is located in the Peruvian Andes and the site is located about 13 degrees south of the equator. The site sits on a mountain ridge in the Urubamba River Valley at an elevation of 2430 meters above sea level and is approximately 80 kilometers northwest of Cuzco. The site sits on a mountain ridge saddle between two mountains of Machu Picchu and Huayna Picchu. A photo of the site is provided in Figure 1. The mountain ridge the site sits on has about a 50% slope. The climate at Machu Picchu is tropical with the wet season from October through April and the dry season from May to September. The average annual rainfall is approximately 70 inches. Figure 1. Photo of Machu Picchu site. Drainage With the mountain ridge of where Machu Picchu is located having a 50% slope, drainage and erosion was a significant concern in its construction. The solution the Incas developed was a series of terraces designed to support the city and ensure that erosion was minimized. These terraces are a fundamental part of the design of the site as the form the foundation for building in such a steep location. Although some were also used for agriculture they were fundamental for the building of the site on the steep slope. There are approximately 4.9 ha of terraces at 2 Inca Water Quality, Conveyance, and Erosion Control

3 the Machu Picchu site (Brown). A photo of the Machu Picchu terraces is provided in Figure 2. Figure 2. Machu Picchu terraces. Investigations have shown that the terraces were designed not only as structural support and erosion control for the city, but they were integrated as part of the drainage system to ensure their stability. The terraces incorporated subsurface drainage to allow the significant amount of rainfall to infiltrate into the subsurface and then slowly infiltrate into the ground below the terraces. This would ensure that there was not a significant amount of surface runoff, which can cause erosion of the slope. Subgrade analysis was performed on the terraces, which identified that the terraces were designed for efficient drainage and infiltration. The bottom layer of each terrace was made up of large stones, the next layer up was made up of gravel, the next highest layer was made up of sandy material, and the top layer consisted of topsoil with stone retaining walls holding the terraces in place, as is identified in Figure 3. Figure 3. Cross section of Machu Picchu terraces. (Ghosts, 2010) The design of the terraces provided highly efficient drainage with the subgrade of the terraces acting as detention allowing water to slowly infiltrate into the soil. Soil test results of the terraces that the topsoil had an infiltration rate of 10 cm/hour and the stone drainage layer had permeability of 12L per day per square centimeter (Wright, 1999). Redundancy was also incorporated into the drainage for the terraces as the slope of the terraces was directed towards a system of drainage channels and drains that are integrated into stairways and other Apt 3

4 structures on the site for any high intensity rain events (Wright, 1999). More than 100 drains were incorporated into the site and are illustrated in Figure 4. Figure 4. Integrated drains at Machu Picchu. (Ghosts, 2009) The Machu Picchu site also incorporated a large underground central drainage system underneath the main plaza of the site. This system accepted flows from the surrounding urban area of the site which consisted of approximately 172 buildings covering an area of 8.5 ha. The urban area had a fairly high density with high runoff coefficients resulting from the granite construction, which resulted in increased runoff (Wright, 1999). The urban area was built with an intricate system of surface drainage channels, and subsurface drainage and subsurface chambers that fed into multiple underground drainage chambers including a large underground chamber underneath the central plaza of the site (Wright, 1999). This chamber was designed with topsoil as the upper layer, a dirt gravel mixture in the middle layer, and white granite chips as the bottom layer as identified in Figure 5. Figure 5. Cross-section Machu Picchu underground central drainage chamber. (Ghosts, 2010) The white granite chips used in the bottom layer of the central underground drainage chamber are thought to be the left over materials from all the stone cutting to build the buildings and walls of the Machu Picchu site. The depth of the central underground drainage chamber is approximately 9 feet covering an area of several acres (Ghosts, 2009). The chipped granite layer was approximately 1 meter thick with an estimated permeability of 160 m/day with a detention capacity of about 150 L/m 3 (Wright, 1999). The central underground drainage 4 Inca Water Quality, Conveyance, and Erosion Control

5 chamber also included subdrains, as identified in Figure 6 that directed flows directed to large east-west main drain that takes flow away from the entire site. Figure 6. Under drains for underground central drainage chamber. (Ghosts, 2010) Other drains on the site as well as flows from the terraces were also directed toward the large east-west main drain. The primary advantage of the central underground drainage chamber and the other drainage features on the site was that major rainfall events could be temporarily detained underground with some water infiltrating into the ground and the remainder of the flow being slowly released into the drainage system thus mitigating the erosive power of significant rainfall events. Water Supply and Conveyance Machu Picchu and other Inca sites were designed with an understanding of the water supply and conveyance needs of a site. At Machu Picchu water is supplied to the site from a spring on the north slope of Machu Picchu at an elevation of 2,458m (Wright, 1997). The spring is intercepted by a water collection trench and then it is conveyed to the center of the city through a 749 meter long canal (Wright, 1997). The water supply was then conveyed 16 fountains spread throughout the site that were designed to not only provide domestic water supply to the entire site, but were also laid out to enhance the urban environment with the site and sound of flowing water and display the power of the Inca ruler Pachacuti (Wright, 1997). A photo of one of the fountains is provided in Figure 7. Figure 7. Machu Picchu fountain. Apt 5

6 Modern Low Impact Development and Erosion Control Techniques Comparison There are several modern LID and erosion control practices where their designs have a striking comparison to the techniques that the Incas used at Machu Picchu. The similarities to the practices the Incas used at Machu Picchu and modern practices are provided below. Terraces for Erosion Control. In the modern area of erosion control gradient terraces are identified as a usual requirement for development located on steep slopes. Many jurisdictions require these require gradient terraces for slopes greater than 25%. Terraces intercept and slow runoff velocity and provide both infiltration and detention of water, which helps to reduce the erosive nature of high intensity storm events. Terraces protect against erosion and reduce sediment transport. Terraces are a modern permanent erosion control practice, which are used throughout the world in mountainess terrain. The terraces that the Inca implemented at Machu Picchu serve the same fundamental role of modern terraces, which is permanent erosion control. Multiple Drain System for Slope Erosion Control. Implementing an adequate drainages system on steep slope developments is a critical permanent erosion control practice. By implementing multiple drains instead of a singular drain the volume of water in any one conveyance is reduced which reduces the possibility of erosion on the slope. The Incas understood this concept and designed a similar multiple drain system in to the terraces as identified in Figure 4. Low Impact Development Bioretention. Bioretention is one of the many integrated site design practices used for implementing Low Impact Development. Bioretention functions as a soil and plant-based filtration device that removes pollutants through physical, biological, and chemical treatment processes (PGC, 1999). Bioretention systems are usually incorporated into the landscape of a development site and accept runoff from the surrounding drainage area. Bioretention systems assist with both reduction of stormwater pollutants and reduction in the volume of runoff from a site. Bioretention systems can be developed with or without an underdrain depending on the infiltration rates of the underlying soils. A cross section of a modern bioretention system is provided in Figure 8. Figure 8. Bioretention Cross-section. 6 Inca Water Quality, Conveyance, and Erosion Control

7 In comparing the bioretention cross section identified in Figure 8 and the cross sections of both the Machu Picchu terrace and the underground central drainage chamber there is a striking similarity. The bioretention system and the Machu Picchu terrace and underground central drainage chamber have topsoil as the upper layer, a sand layer in the middle layer, and gravel as the bottom layers. Essentially the terrace and the underground central drainage chamber at Machu Picchu are the same design as the modern bioretention system. Underdrains were incorporated into the underground central drainage chamber at the Machu Picchu site which helped to convey flows from large storm events. Underdrains in modern bioretention systems function in the same manner as they also help to convey the storm events that cannot be infiltrated into the soil. The result of this comparison shows that the Incas had a remarkable understanding of hydrology and infiltration and it can be said that they may be the original creators of what we now know as bioretention. Water Quality Treatment Bioretention. Bioretention systems as previous identified are filtration systems that remove pollutants through physical, biological, and chemical treatment processes. The primary source of potable water at the Machu Picchu site was the mountain spring previously identified, however there is evidence that shows that runoff from terraces and the underground central drainage basins were conveyed to fountains in the lower terraces downhill of the Machu Picchu site (Wright, 1997). The Incas understood that the soil, sand, and gravel layers of the terraces and the underground central drainage system acted as a filter treating the water enough where it could be conveyed to fountains for potable use on the lower terraces. Although the terraces and central underground drainage basins primary function was drainage the Incas understood there was a secondary benefit with water quality treatment, otherwise this water would not have been conveyed to the fountains on the lower terraces. Low Impact Development Site Design. One of the fundamental LID site planning concepts is using hydrology as the integrating framework for designing a site. The primary goal of Low Impact Development is to mimic pre-development hydrology in a developed site, through the use of site design techniques that infiltrate, evaporate and detain runoff. Low Impact Development seeks to mimic the natural drainage functions of a site and uses site planning and integrated hydrologic design to accomplish this goal. Integration of hydrology into the site design is achieved through reducing or minimizing impervious areas on a site, disconnecting impervious areas, and integrating bioretention areas and infiltration devices on a site. Hydrology is a fundamental site design principle for Low Impact Development. The Incas planned Machu Picchu using hydrology as the fundamental design principle. It is estimated that approximately 60% of the structure of Machu Picchu is underground and it is all based on an effective and sustainable drainage system (Wright, 1999). The Incas integrated hydrology and drainage into the planning and design of the site and if it were not for this effective hydrologic planning the site would likely not be in existence today. The Incas essentially implemented a Low Impact Development approach at Machu Picchu as the planned and designed the site around drainage and hydrology, they disconnected impervious surfaces through the implantation of a large network of underground detention and drainage systems, the minimized impervious areas with the terraces as they were pervious with top soil and vegetation, and they incorporated essentially bioretention and infiltration devices throughout the site with the terraces and underground drainage system with detention and infiltration chambers thus helping to mimic the predevelopment hydrology of the site. By implementing this hydrologically functional design, the Incas essentially helped to define Apt 7

8 sustainable Low Impact Development as it is the primarily reason why the site has stood the test of time and the significant rainfall in the region over the centuries. CONCLUSION The Incas had an extensive understanding of hydrology and drainage and they used hydrology as a fundamental design principle in panning and designing the site of Machu Picchu. Without this understanding of hydrology and drainage the building of Machu Picchu would not have been possible. The Incas understood that building on such steeps was possible but only if they could stabilize the development and that this was only possible by incorporating permanent erosion control measures. In review of the design of the terraces built to support and stabilize the site and the central underground drainage basins, the design is very similar to the design of modern bioretention systems. It can be argued that the Incas should be recognized as the true inventor of modern bioretention as the terraces and underground drainage basins are essentially mimicked in modern bioretention design. With the use of bioretention type systems the Inca understood that water that had been filtered in these systems could be used as a potable source of water as flows form these systems were conveyed to fountains on the lower terraces. Although the primary water supply for Machu Picchu was a mountain spring the Inca understood the value of water that had been treated through their terrace and underground drainage basins and so they in effect implemented water harvesting and an integrated water resources approach. Finally the Incas understood sustainability, as hydrology was a fundamental element in the design of the site and with out allocating significant effort to planning the site around hydrology the site would no longer exist. In using hydrology as a fundamental principle of the planning and design of the site the Inca implemented the fundamental principle of Low Impact Development of a hydrologic functional design that mimics pre-development hydrology. Machu Picchu and the Incas should be recognized as pioneers in hydrologic design as: The Incas implemented effective sustainable permanent erosion controls at Machu Picchu that have lasted for centuries. The Incas are the true creators of bioretention as it was integrated into the terraces and underground drainage systems of Machu Picchu The Incas implemented an integrated water resources approach through an extensive water supply system and conveying runoff from the terraces and underground basins that had effectively received water quality treatment to fountains on the lower terraces. The Incas implemented the a hydgologically functional design by using hydrology and drainage as a fundamental planning and design element thus implementing a Low Impact Development approach centuries before LID was created. REFERENCES Ghosts of Machu Picchu (2010), PBS, NOVA Montgomery County, Maryland, Department of Environmental Protection (2011), Bioretention Garden Cross Section Prince George s County (1999), Low Impact Development Design Strategies An Integrated Design Approach, Department of Environmental Resources, Programs and Planning Division Wright, Kenneth R. (1999); Valencia, Alfredo; Lorah, William L.; Ancient Machu Picchu Drainage Engineering, Journal of Irrigation and Drainage Engineering, Vol. 125, No. 6, November/December, ASCE, ISSN /99/ , Paper No Wright, Kenneth R. (1997); Kelly, Jonathan M.; Zegarra, Alfredo Valencia; Machu Picchu: Ancient Hydraulic Engineering, Journal of Hydraulic Engineering, October Inca Water Quality, Conveyance, and Erosion Control