PERIOD OF TRIAL AND ERROR
When we think about great hydrological engineering achievements, pride of place is normally given to familiar structures such as Hoover Dam, the California aqueduct, the Panama Canal, or less commonly to innovative examples such as the Falkirk Wheel (Figure 1). However, before we congratulate ourselves too profusely we should reflect on some truly astounding hydrologic achievements that were made thousands of years ago. Some of these ancient works were even more marvelous than our modern examples, particularly given the technology that was available in their day. It is best to reflect on this history before we launch into the subject of modern hydrology.
The “science of hydrology” has progressively advanced to promote the development and use of water resources. The most important challenges involved water supply, specifically: 1) identification of adequate water sources, 2) construction of systems for long-distance conveyance of water, 3) storage of the water if necessary, 4) treatment of the water if necessary for human consumption, 5) distribution of the water to local demand sites, and 6) capture of any waste generated from water storage, distribution, purification, and use. In addition, corollary challenges involved 7) flood control of rivers and storm runoff, and 8) river and coastal sea navigation. It can be readily demonstrated that the technology needed to solve most of these problems was developed more than 2000 years ago. Only in the case of water quality treatment has modern western technology made truly pioneering strides, with much help from the chemical and biological sciences. Below we discuss some remarkable examples of early water projects to highlight our debt to the ancient civilizations that developed the institutionalized knowledge that is essential to the modern hydrologist. The historical path of water resource development and its practice recapitulates the human journey from humble hunters and gatherers to technologically advanced societies. The variations in water availability have fostered much technological innovation and philosophical inquiry, even as increasing demands have caused chronic water shortages in many regions. These shortages stimulated efforts to increase water supplies, by long distance diversion, storage, pumping, or purification of polluted or saline sources. Of course, any viable method must meet the demand at affordable cost, or provide offsetting benefits. For example, the rapidly increasing demand for freshwater in the Middle East promoted the development of advanced membrane systems that can convert sea water into large quantities of potable freshwater. The financial investment required for this undertaking could not have been made without the oil wealth of this region. As another example, several huge, long-distance water diversions, from the Sacramento, Colorado, and Owens rivers to Los Angeles and other cities, have greatly fueled California’s rapid economic growth (Figure 2). Water resource development is clearly linked to human needs, progress and productivity. Figure 2. The California Aqueduct originates in central California and delivers 370 m3/s of water to dry areas such as Los Angeles located as much as 700 miles to the south. A series of vertical lifts raises the water by 587 m. (Photo: California Department of Water Resources) Of course, our great water achievements beg the question of how did the requisite knowledge and practice originate and evolve over time? How did people learn to manipulate water for their own means, rather than depend entirely on nature? How long ago did this manipulation begin? Unfortunately, available archeological evidence cannot completely answer these questions. However, even a casual reader understands the link of the ancient Egyptians to the Nile River, or has heard of their floodplain irrigation system. He may also know of the gravity-fed canals of ancient Sumeria that were used for flood control, irrigation, and navigation. Ancient China similarly tamed parts of the Yellow River during the Bronze Age. As discussed below, these ancient large-scale water projects were relatively advanced for their time and were based on institutionalized knowledge. Clearly, key knowledge must have developed over a long period of unrecorded history, which preceded these ancient civilizations. The Paleolithic -Neolithic Transition The first great transition relevant to water resource development was the advent of agriculture and domestication of animals, marking the beginning of civilization. This essential transition took place during ~12,000 to 6000 BC, and featured the earliest permanent habitations that distinguish late-Paleolithic and Neolithic cultures. Climate change was rapid at low to mid latitudes, particularly in the area known as the Levant, which is the area now centered on Israel, Syria and Jordan. The Levant was originally occupied by hunter-gatherers known as the Natufian, who had advanced lithic tools and semi-permanent stone dwellings. Abundant game negated any need for long-distance migration. No evidence proves that the Natufians consumed the wide variety of local, wild grain types, but subsequent inhabitants of this area clearly did. These first farmers built permanent, rectangular-shaped dwellings that contrast with the round structures of the Natufian. Sequential archeological evidence reveals that the use of wild grain was followed by use of a domesticated, hardier seed, which originated by inadvertent selection due to intentional harvesting before the grain fell naturally. Consequently, as the knowledge spread, regular grain planting and harvesting was forever woven into the socio-economic fabric of the Fertile Crescent. Long distance trade is documented by the wide use of obsidian, locally absent and probably from Turkey. Knowledge spread with trade, and so it was that permanent agriculture was established. Domestication of goats, sheep, and other animals soon followed. So how does water factor into these critical human developments? Dependable water sources were necessary throughout human history, and in the Levant the demand was originally met by perennial springs. Jericho, biblically-famous for its spring, was inhabited as early as the Natufian period. Evidence from nearly ten thousand years ago shows that Jericho spring was utilized by a few hundred permanent inhabitants whose village was surrounded by a stone wall. More than 8000 thousand years ago at El Knowm, located in the upper Euphrates River, houses had internal domestic water and waste water drains lined with plaster. About this time, down river in what now is Iraq, irrigation canals as wide as 2 meters coursed for hundreds of meters along topographic contours. This technological development may have been motivated by rising sea levels that inundated the lower Tigris and Euphrates rivers, causing their channels to change from braided to meandering. The climate dried at this time, and because the expanding population had for millennia incorporated agrarian subsistence in their daily lives, the need for reliable water to produce high crop yields undoubtedly fostered the development of irrigation canals. Of further note, evidence of hand dug wells appear in the archeological record around 7000 years ago, and plaster-lined underground cisterns were constructed to store water (Figure 3). Figure 3. A water well dating between 6400-6000 BC is exposed at Sha'ar HaGolan, a Neolithic settlement in the Levant belonging to the Yarmuk Culture. (Wikipedia; Photo by Yosef Garfinkel [CC BY-SA 3.0]) In summary, the period of incipient civilization featured the development of agriculture, permanent villages and population growth. People of this period had wells, irrigation canals, cisterns, and in some areas water supply lines and waste water drains. These societies required organization, management structures, and technology to harness essential resources, especially sustainable water supplies, and the demands could only grow. Necessity, persistence, and trial and error provided the required knowledge. WATER RESOURCE DEVELOPMENT The advanced level of water planning and engineering in ancient times is exemplified by a small city that emerged in modern day northern Jordan. The city Jawa was established before 3000 BC (>5000 years ago!) in the ephemeral drainage wadi Rajil, which drains the southwestern highlands of Syria (Figure 4). Annual flooding supplied sufficient water to sustain a population of nearly 5000. To ensure adequate water year round, a complex system of diversion and storage was constructed. This comprised one of the earliest known dams that created an artificial reservoir, plus a cascade of storage ponds, and subsurface cisterns. These facilities maintained a viable agricultural system and livestock supply that sustained the isolated population. Clearly, great sophistication was required to compute the water demand, and to plan, build and manage the structures required to meet this demand. This planning was likely based on long-term observations of annual wadi flows, the evaporation rates of stored surface water, and the consumption rates of water by crops, humans, and animals. Without these observations, planning would have been impossible and disastrous droughts or floods would have doomed the city’s development. The knowledge required for this effort probably was not developed at Jawa, and more likely was transferred from emerging city-states along the lower Tigris and Euphrates rivers and in the Nile Delta. Figure 4. Outline of the ancient town wall of JAWA located in the Wadi Rajil in northern Syria. This 5000 year old ruin likely had around 5000 people living an agricultural lifestyle supported by engineered water resources. These included surface water diversion structures feeding surface ponds and underground cisterns, all controlled by an upstream dam (not shown). (image: overlay on Google Earth; ML Davisson) Similar sophistication is also found in the upper Euphrates dating back to 3400 BC in an early city known as Habuba Kebira. Here paved streets and individual buildings were outfitted with dual plumbing, one for the capture rainwater and the other for discharge of waste water. The latter featured interlocking clay pipes sealed with clay and mud. Another impressive, ancient technology was the qanat. Originating in Persia around 5000 years ago, qanats transported groundwater via sloping tunnels, to steadily provide water to cities that were often tens of kilometers downgradient (Figure 5). Qanat construction involved digging a well in a distal, uphill alluvial plain. Saturated groundwater was not encountered in some of these wells until they were > 200 meters deep! Starting from the city, a tunnel was hand dug all the way to the well, so groundwater could flow by gravity toward the city. Every few hundred meters along the tunnel alignment, an intersecting shaft was dug to provide access and ventilation. The largest qanat, built 2700 years ago in Gonabad, Iran, had a 360 meter-deep well and a tunnel length of 45 km, and supplied water to nearly 40,000 people. The necessary surveying skills required for tunnel alignment, so as not to miss the upper well or shafts, was astounding, and almost certainly required repetitive, independent survey lines. Accuracy of a few centimeters was maintained for distances of tens of kilometers! The surveyors must have understood the precision required to achieve this feat. Figure 5. A qanat requires an initial well drilled to the water table. Then, starting from the point of use, a carefully surveyed tunnel is dug with access shafts (red arrows spaced ~100m apart). This Google Earth image shows excavation remains from shafts for Gonabad qanat in Iran. (image: diagram ML Davisson) Water resource technology spread from the Levant and Egypt to early Greek civilization, where people faced semi-arid mountainous terrain with limited water in any single location. Early diversion canals copied existing designs to exploit the topographic gradient. However, following Alexander’s great conquest and the emergence of the Hellenistic period, unique hydraulic innovations appeared. For example, the Greeks developed U-shaped reverse siphons, constructed of interconnected terracotta pipes that could convey water across valleys up to 200 meters deep. Pressures were highest in the valley bottoms, where massive, bored, interlocking stone blocks replaced the pipe. During its height the Roman empire governed an area of nearly five million square kilometers, and was responsible for meeting the daily needs of local population centers. Chief among these was Rome whose population exceeded one million. Water was needed to supply daily household requirements, as well as a multitude of public fountains and abundant public baths that were integral to the daily Roman ritual. The estimated per capita consumption rate was nearly 300 gallons per day! This rate rivals the most ostentatious lifestyles in the modern western USA. Roman aqueducts stand out as a high achievement in water resource development. Rome was the ultimate inheritor of water and engineering know-how in the ancient world. They were greatly influenced by Hellenistic Greece and in particular relied on the innovative work emerging from the Alexandria school. However, Romans were practical people who used existing methodologies to design their systems, and spent little effort trying to establish theoretical reasons for their success. A case in point is the wide use of the Roman water measurement unit known as the quinaria. A quinaria is equivalent to approximately 4.2 cm2 and used to measure a cross-sectional area of a conduit. However, all flow requirements in aqueduct design and all distribution pipe for water in Roman cities and towns was measured as quinaria! Note that quinaria are cross-sectional areas, not volumetric rates, but this seems to never have made a difference in Roman design. A Roman aqueduct was an engineered river that diverted flows of tens of cubic feet per second (or up to 2 m3/s), into a constructed channel from a few to >100 km long. For instance, Rome at its greatest population was served by 11 different aqueducts that provided a total flow of greater than 11.5 m3/s (450 cubic feet per second; Table 1). Many aqueducts are mostly underground, but the most prominent ones featured beautiful, towering archways winding across the landscape (Figure 6). Siphons were often used to cross valleys unless elevation differences were too great, and tall spans of bridgework were constructed. Above ground aqueducts were normally covered and many of them had more than one conduit. Intakes and outflows were constructed along its path. Average gradients ranged from less than 1 to greater than 10 meters per kilometer. Figure 6. The Roman aqueduct bridge in Segovia, Spain is part of a 15 km-long water import structure that has been used continuously since its construction nearly 2000 years ago. (photo: Cees W. Passchier, Driek van Opstal and/or Wilke D. Schram © 2004 – 2015 with permission - http://www.romanaqueducts.info) Roman ingenuity is best illustrated by the distribution of water to its final destination. For example, the Aqua Augusta supplied water to Pompeii, which was famously buried by ash during the catastrophic eruption of Mt. Vesuvius in 79 AD. Excavation of this remarkably preserved town has revealed the internal workings of its water distribution system. For instance, flow from the aqueduct, which is under a residual pressure head from its source area 140 km away, was first brought into a main water tower (Figure 7). The water elevation in this tower maintained pressure on the rest of the distribution system. Inside the tower was a device known as a castellum that split the flow into three parts, each controlled by check dams of three unequal heights. The height depended on the water usage rate. From there each flow was delivered by buried stone or lead conduit, with pressure being maintained by intermittent water towers. The most important use was public fountains where daily water needs were freely supplied to each individual. Figure 7. Diagram of the water distribution system in the ancient town of Pompeii. Much of the town was buried in volcanic ash that preserved many of these features (diagram: ML Davisson; overlain Google Earth image) LAND SURVEYING AND HYDROLOGY Large scale water diversions in ancient civilizations depended on the ability to survey land in order to utilize gravity as a conveyance force. Surveying is an ancient art dating back to pre-written history and whose accuracy is testimonial in great architectural achievements such as the pyramids of Giza. Ancient surveying methods required accurate means to measure distance and elevation gradients combined with scribing circles and right angles. Simplest means for measuring distance at the time was a knotted cord whose lengths were determined by standards administered and controlled by a central authority, much like our modern institutional standards. In ancient Egypt, such cords were ritualistically used to determine boundaries of irrigated land. One end of the rope may be tied with a peg as a means to pivot from one direction to another. It is thought Stonehedge was originally surveyed in pre-historic times using a peg and cord method. A peg and taught cord can also be used to scribe a circle. It has long been understood that right angles can be created using a triangle with sides having multiples of 3, 4, and 5, even though this was never mathematically formalized until Pythagorus in the 6th century BC. Establishing an elevation gradient required a method for leveling. In order to calculate a gradient, the elevation difference between two locations must be determined. This is accomplished by means of an observer peering down a line of sight at a fixed point in the distance. The observer must establish the height above the ground of his sight line, and this height must be measured exactly vertically. If the observer’s line of sight is level, then sighting to and marking the point of intersection at the distant location establishes where the ground elevation matches that of the observer’s sight. The gradient is the sight height divided by the distance between the observer and the remote location. Establishing a level sight line was the main challenge in this method, and this required some innovation. As documented in hieroglyphic records, the carpenter’s plumb line was used in ancient Egypt to establish level and vertical surfaces. However, large-scale leveling requirements preclude the use of this tool because of the great distances involved. Surveying methods using two graduated rods connected by known length of cord known as a mizan was likely used instead (Figure 8a). In the center of the cord was attached an accurately constructed device that established level. For example, a plate in the shape of an isosceles triangle hung from its base and had an accompanying plumb line suspended from the triangle’s center. Alignment of the plumb line with the triangle’s apex would establish level. This was achieved by adjusting one end of the cord along the rod until the plumb line was aligned with the apex. The difference in elevation measured on each rod was the elevation difference between the two locations. Figure 8. Drawing of a simple surveying tool known as a mizan. A cord is adjusted along one staff until leveling is achieved with a plumb line. Use of a sight tube eliminates the need for the adjusting cord. The topographic gradient is defined by the elevation difference read on the two ruled staffs, and by the distance between them. A more convenient method for mizan leveling used a sighting tube suspended from the cord at exactly the tube’s center point (Figure 8b). The tube would always maintain level even when there is a slope in elevation of the cord stretched between two locations. Siting back through the tube in each direction allowed the surveyor to read off the heights along each rod to establish elevation difference. This approach is thought to have been employed by Persian qanat builders and the methodology transplanted to ancient Greece and Rome in their use of the dioptra. The dioptra is the predecessor of the modern theodolite used in surveying today (Figure 9). The dioptra consisted of a tripod supporting a rotating circular disc with scribed angular graduations. More advanced versions also had a graduated, perpendicular half circle for measuring inclination, or allowed the disc to be positioned either vertically or horizontally. On the disc was attached at 180 degrees from one another sites that could be aligned to the object viewed at a distant location. Leveling was achieved by a variation of a plumb line tool. The dioptra allowed measurement of the elevation difference between two points, while establishing the angular direction. This device was also used to make astronomical observations. The Greeks likely used the dioptra for tunnel construction and aqueduct building, and Roman aqueducts and roads demonstrate that great accuracy could be achieved with it. Figure 9. Simple dioptra of the type used in ancient Greece and Rome. A level table supports a sight tube used to observe a ruled staff in the distance. Radial graduations define the angular direction. This design is the basis for the theodolite used in routine surveying today. (Illustration: ML Davisson) Early land surveying was a highly skilled profession, but its methods were rarely recorded and instead were passed along orally along generational lines. Nevertheless, archeological remains attest to the sophisticated knowledge and precision that must underlie this practice. This knowledge was never lost through humans’ tumultuous history over the past 2000 years. Precise surveys were essential to ancient and modern hydrological engineering works. Most modern hydrology concepts have a haunting parallel to knowledge acquired by the builders of ancient water works. LARGE SCALE IRRIGATION City-states emerged around 3000 BC in Sumeria and with them came the advent of long range surface water diversion for irrigation, flood protection, and navigation. Cultivation of the fertile soil of the lower Tigris and Euphrates valleys produced an abundant food supply that stimulated trade with neighboring states. Spring runoff and high water in both rivers was ill-timed with crop water demand, which was greatest in mid- to late-summer. In response, communities developed a complex network of diversion canals that corresponded to surveyed land holdings. Flow was administratively controlled and allocated. Disputes ultimately arose between adjoining city-states over amounts, timing, and payments. Eventually, Sumeria was unified under Sargon of Akkad and his successors. These rulers controlled the rivers to their headwaters in Syria, including reaches channelized for navigation, as well as navigable canals and large diversion projects. Maintaining access to port cities was essential for seafaring trade. Main canals that followed topographic gradients fed smaller canals that fed retaining basins near irrigated fields. Water levels in the canals and basins were maintained above the elevation of the irrigated fields to facilitate gravity flow. Also, water regulation originates from a Babylonian code of conduct, established during King Hammurabi’s reign (1792-1750 BC).
The Nile River was the lifeblood of ancient Egyptian civilization. The predictable rise and recession of the Nile regulated the timing of planting and irrigation. In contrast to the Sumerians, the Egyptians surveyed their land based on areas that flooded annually. They knew that, as the river annually rose, those flooded fields would be automatically provided with an initial watering prior to planting, then the water would recede. Irrigation could be delayed until summer, but then would require transport of river water to the fields, delivered by canals or from impounded flood waters. The Egyptians used the shaduf to lift water from canals to the fields. Although the Nile would predictably rise and fall, the delivery of irrigation water to the fields was less reliable. The Egyptians therefore were avid data collectors on river levels at different locations, utilizing increments of the nilometer (Figure 10). Nilometer units are equivalent to approximately half a meter in length, essentially the same as a cubit. Records are first seen around 2500 BC and reference levels are recorded for favorable river conditions upstream in Elephantine at 21 nilometers, 12-14 at Memphis, and finally in the Delta at 7. The observations were reported along the river length for public use. Figure 10. Photo of reproduced nilometer markings in a stone staircase. Ancient Egyptians used this device, essentially a staff gauge, to measure the level of the Nile River to predict flooding and plan seasonal irrigation. (Photo: Wikipedia, Olaf Tausch) Advanced irrigation practices independently emerged in Central America. The Mayan culture starting around 1000 BC built raised, terraced fields and elaborate gravity-fed irrigation canals to grow diverse food types. These advancements in agricultural practice paralleled their construction of elaborate stone temples and discoveries in metal refinement. As in the Levant, the Mayan’s independently developed mathematical methods that must have underpinned their surveying and construction accomplishments. FLOOD PROTECTION Evidence of farming and grain cultivation dating back before 3000 BC illustrates the likely transfer of knowledge between the Levant and Egypt regions and East Asia. However, the focus in China has mostly been on their long-term battle to tame river flow. East Asia geography lends itself readily to river floods because snowmelt from the world’s tallest mountains drains to the plains of eastern China. Note the gradient in the Yellow River averaged along its entire ~5500 km length of 0.002, or about two times greater than the Nile River. Gradients are steepest in the mountainous upper half of the Yellow River basin but abruptly flatten in the fertile plains where flooding was greatest. River capacity is exceeded during high precipitation years so adjacent farms are inundated. Several single catastrophic floods have killed hundreds of thousands to millions of Chinese people. The Yellow River in particular was prone to flooding almost annually, creating some of the greatest natural casualty rates in history. This frequent flooding was combatted starting a little after 3000 BC as recorded by the legends of Emperor Yu, who tamed the Yellow River by building dikes, dredging channels, and diverting flow. Sophisticated flood control projects also tamed the flow of the Euphrates near the ancient town of Babylon. Herodotus (484-425 BC) wrote that during Assyrian or early Babylonian times, a ruling queen ordered the lengthening of the river course above the city, achieved by introducing several meanders. This effort required raising levee heights to compensate for the decreased river gradient. In addition, a large basin was constructed and filled with diverted river water, creating a navigable flood causeway and a defensive barrier. This huge undertaking was clearly rooted in a profound understanding of fluvial mechanics, which must have been based on practical outcomes and not on theoretical ponderings. NAVIGATION The Egyptians undertook other bold projects that include an attempt to dam wadi Garawi southeast of Memphis (ca. 3000 BC), a tributary to the Nile with a steep slope and prone to violent flooding. The dam apparently wasn’t successful even though it was nearly 100 meters in thickness at its base. They also constructed canal bypasses to the cataracts at Aswan (ca. 2000 BC), near the location of a modern dam. These canals allowed navigation of building materials for projects downstream, and later on facilitated military excursions. In addition, their strategic location near the Mediterranean and Red Seas provided access to vital trade routes. However, a canal was required for a direct water connection between the Nile River and the Red Sea, and one was first dug around 1000 BC. The commercial metallurgy center of Mari, located 1-2 km off the middle Euphrates, was established around 3000 BC. Diverted river water along a 30 m-wide canal supplied the walled city, although the ruling palace had independent rainwater collection system and cistern storage. Drainage pipes directed waste water to remote cesspools. Mari also had a number of irrigation canals, but what stands out most is a 120 km long 8-11 m wide navigation channel. This remarkable feature has a carefully engineered drop of only 1-2 m for each 10 km section, an astoundingly gentle gradient given the surveying methods of the time. Starting after 1000 BC, impressive canals were built in China for navigational and military needs and to maintain regional unity. For example, the Yellow River was connected to the Huai River more than 2400 years ago. This was achieved by a series of artificial and natural waterways known as the Hong canal that stretched across 900 km of swampy terrain. Another audacious project known as the Han Guo or Grand Canal linked modern day Beijing to Hanzhou, via an ~1800 km north-south water course constructed of hand-dug canals linked to natural water bodies (Figure 11). Figure 11. The Grand Canal of China was constructed over many centuries and different dynasties. The canal has a total north-south length of approximately 1800 kilometers and an elevation difference of around 40 meters. (Google map overlay ML Davisson; cross-section after Viollet 2005). The Roman Empire once stretched from the wet, temperate British Isles to the dry desert of North Africa. Navigation on the Mediterranean Sea was the life-blood of interstate commerce at the time. Rome and other elite capitals had a high demand for imported commodities ranging from food to building materials, much of which was imported from Roman provinces. Adequate port facilities were essential to meet the commodity demand. Although examples are numerous, the Port of Trajan at the terminus of the Tiber River is used to illustrate their great engineering achievements. The Tiber carried a large alluvial bed-load, so sand and silt would build up rapidly on the coast and prevent large ships from reaching anchorage at the river mouth. During the reigns of Claudius (41-54 AD) and Nero (54-68 AD), two canals from the Tiber were dug to facilitate navigation to an excavated harbor that had two breakwater jetties (Figure 12). This harbor still suffered from silting, so a second, hexagonal-shaped harbor was dug during Trajan’s reign (98-117 AD) that avoided silt buildup by its minimal connection to the Tiber. Figure 12. The famous Roman port of Claudius and Trajan provided navigation between Rome and the Mediterranean via the Tiber River. Much of this area is now covered by landfill and an airport runway, but a surviving feature is the hexagonal harbor excavated during Trajan’s reign that prevented the inflow of the river silt. (Google overlay ML Davisson) WATER QUALITY Quality and chemistry of water were largely mysterious to early civilizations. A few relations were understood, such as saline water will kill crops, and water with too much suspended sediment will clog canals, but the connection between numerous diseases and water quality was not recognized until the late 19th century. Nevertheless, archeological evidence points to deliberate, early development of human waste disposal methods. As mentioned above, the early Sumerian city Habuba Kebira had elaborate plumbing systems made of interconnecting clay pipes designed to remove human waste. Similar internal plumbing dates back to the 8th century BC at El Kowm in central Syria, where drain pipes in houses led out to the streets. Elaborate plumbing networks for rainwater harvesting and sewage disposal are found in Harappan sites on the Indus Valley, as well in Babylonian cities such as Mari that featured bitumen-lined conduits. Sanitation and cleanliness became a lifestyle in ancient Greece and Rome. Bathing facilities and community toilets were common (Figure 13). Liquid waste was discharged away from the cities and collected in cesspools or released to the environment. The Romans built elaborately decorated facilities for both cold and hot bathing. Steam baths were constructed as double-walled convection chambers with a large gap between the walls, particularly under the floor where fires boiled water. Figure 13. Community toilets and a subsurface waste conduit in ancient Pompeii attest to the hygienic practices of ancient civilizations, a practice largely lost in post-Roman Europe. Waste removal may have prevented the plagues that devastated subsequent, Middle Age societies. (photo: Wikipedia, Fubar Obfusco) Although the knowledge of sanitation and water distribution survived the Roman Empire, much of it was cloistered in the monasteries where only priests enjoyed the benefits. Most domestic waste generated in cities and towns in Europe was directly discharged on public streets. Such practices and rapid urban growth fostered devastating plagues that at one time wiped out a quarter of Europe’s population. FLUID MECHANICS The development of fluid mechanics was stimulated by the need for civilized humans to manipulate water. Most of the underlying principles were discovered empirically, and as knowledge and skills accumulated they were put to practice thousands of years ago. And engineers they were; despite their great achievements, the leading Roman engineers seemed to not know how to make a discharge measurement! The flow of water in the Roman aqueduct was believed to be equal to the wetted, cross-sectional area of the channel, or Q=A. This misrepresentation of discharge didn’t matter because the aqueducts were likely proportioned to a standard design, already successfully built. Nevertheless, among the Greek philosophers in Alexandria, Heron (Hero of Alexandria; ca. 65AD) accurately described discharge. Although ignored in his day, he measured the time it took for spring water to fill a trough. This he related directly to the speed of the spring water flowing into the trough. Unfortunately the faulty relation Q=A was used as late as 1598 AD in Rome, as part of a flood survey of the Tiber river. A short time later Benedetto Castelli, who was influenced by Leonardo Da Vinci’s writings, formalized that velocity was essential to the discharge calculation, so that Q = vA where v is the water velocity. But by far development of fluid mechanics owes its origin to the great innovator Archimedes (287-212 BC), who first analytically demonstrated the principles of fluid statics. He reasoned that each part of a fluid is compressed by the fluid surrounding it, and showed any fluid must have a spherical surface. Furthermore, according to his most famous proposition, the weight of a boat must be compensated by a fluid displacement of equivalent weight. The great observationalist Leonardo Da Vinci (1452-1519) described accurately the principle of mass conservation. He described the flow of pedestrians in a narrowing alley, where the widest portion the crowd could take only one step for every two steps taken by pedestrians in the next part, and for every eight steps in the narrowest. Understanding the relationship between discharge and area requires the concepts of pressure and acceleration. Simon Stevin (1548-1620) showed that pressure only depends on the weight of an overlying body. Galileo Galilei (1565-1642) performed experiments that showed that the distance traveled by falling objects was proportional to time-squared, and he originated the concepts of momentum and inertia. Experiments with leaking vessels by Evangelista Torticelli (1608-1647) led to his famous relationship between elevation and velocity,
Blaise Pascal (1623-1662) lastly showed that pressure on one area of a fluid is distributed proportionally as pressure on other areas of different dimensions, illustrating the mathematical proof with his infinitesimal prism of fluid.
An additional key relation between pressure and velocity was achieved based on an experiment where a tube drained a large tank. Almost all modern hydrology books sum this work into the so-called Bernoulli equation:
Isaac Newton (1642-1727) provided the mathematical methods that underlie the complex analytical solutions to fluid dynamics problems. His second law of motion set the stage for analytical solutions to problems involving transient fluid flux. Finally, Leonhard Euler (1707-1783) constructed the mathematical framework for describing fluid mechanics as three-dimensional vector fields in terms of mass conservation. WHERE ARE WE TODAY? Three developments set modern water projects apart from those of ancient civilizations. The first is the ability to harness power. Although the water wheel existed in Roman times, no large-scale methods to generate and distribute power were available. Earlier devices that used water and steam power were only novelties, such as the Greek hydraulis or Heron’s steam ball. Large-scale power generation and distribution was not available until the industrial revolution, when the invention of powerful steam engines was quickly followed by electric motors and generators, gasoline engines, and many other devices. The second development is in water quality. Many ancient civilizations recognized the need to remove human waste, but the connection between disease and contaminated drinking water was not recognized until the mid-19th century. Subsequent advances in water treatment that utilize settling, filtration, and chlorination led to remarkable decreases in the incidence of waterborne disease, as safe drinking water became widely available. Generation of industrial wastes and chemical pollutants prompted the development of parallel treatment technologies, and fostered detailed study of the fate and transport of aqueous chemicals. Finally, mathematics has a special place in the hydrological sciences. Our advanced understanding of fluid mechanics has facilitated the construction of complex water projects, which enable the essential daily output of our industries and farms. Of course, computers have increased our ability to design systems, process data, and envision complex processes. Nevertheless, the most important lesson of this chapter is that observational hydrology is as old as human civilization. Second, our theoretical debts are to physicists, microbiologists, chemists, and mathematicians. Hydrologists used their hard-won observations to build complex water projects, but they did not create the important theories. Modern hydrology now emphasizes computer models over real example and illustrates important concepts with cartoons, while abundant real-world data are at our fingertips. In our view, future advancements of the hydrologic sciences will depend on a strong observational foundation consistent with its roots. This foundation enabled the large-scale water projects of ancient civilizations, and is equally essential today. Further Reading Helms, SW (1981) Jawa: Lost City of the Black Desert. Cornell University Press: Ithaca, NY, 270 pgs. Lewis, MJT (2001) Surveying Instruments of Greece and Rome. Cambridge University Press: United Kingdom, 389 pgs. Stiros, SC (2006) Accurate measurements with primitive instruments: the “paradox” in the qanat design. Journal of Archeological Science, 33, 1058-1064. Tokaty, GA (1971) A History and Philosophy of Fluid Mechanics. Dover Publications, Inc.: New York, 241 pgs. Viollet, P (2005) Water Engineering in Ancient Civilizations: 5000 years of History. International Association of Hydraulic Engineering and Research: Madrid, Spain. Translated 2007, 322 pgs.
where h is the elevation and g the acceleration of gravity deduced from Galileo’s kt2, where k is a proportionality constant.
where ρ is the mass density and p is the pressure. However, others have shown that this equation should be attributed to Louis de Lagrange (1736-1813).