Hydraulic Engineering

Summary

Hydraulic engineering is a branch of civil engineering concerned with the properties, flow, control, and uses of water. Its applications are in the fields of water supply, sewerage evacuation, water recycling, flood management, irrigation, and the generation of electricity. Hydraulic engineering is an essential element in the design of many civil and environmental engineering projects and structures, such as water distribution systems, wastewater management systems, drainage systems, dams, hydraulic turbines, channels, canals, bridges, dikes, levees, weirs, tanks, pumps, and valves.

Definition and Basic Principles

Hydraulic engineering is a branch of civil engineering that focuses on the flow of water and its role in civil engineering projects. The principles of hydraulic engineering are rooted in fluid mechanics. The conservation of mass principle (or the continuity principle) is the cornerstone of hydraulic analysis and design. It states that the mass going into a control volume within fixed boundaries is equal to the rate of increase of mass within the same control volume. For an incompressible fluid with fixed boundaries, such as water flowing through a pipe, the continuity equation is simplified to state that the inflow rate is equal to the outflow rate. For unsteady flow in a channel or a reservoir, the continuity principle states that the flow rate into a control volume minus the outflow rate is equal to the time rate of change of storage within the control volume.

Energy is always conserved, according to the first law of thermodynamics, which states that energy can neither be created nor be destroyed. Also, all forms of energy are equivalent. In fluid mechanics, there are mainly three forms of head (energy expressed in units of length). First, the potential head is equal to the elevation of the water particle above an arbitrary datum. Second, the pressure head is proportional to the water pressure. Third, the kinetic head is proportional to the square of the velocity. Therefore, the conservation of energy principle states that the potential, pressure, and kinetic heads of water entering a control volume, plus the head gained from any pumps in the control volume, are equal to the potential, pressure, and kinetic heads of water exiting the control volume, plus the friction loss head and any head lost in the system, such as the head lost in a turbine to generate electricity.

Hydraulic engineering deals with water quantity (flow, velocity, and volume) and not water quality, which falls under sanitary and environmental engineering. However, hydraulic engineering is an essential element in designing sanitary engineering facilities such as wastewater treatment plants.

Hydraulic engineering is often mistakenly thought to be petroleum engineering, which deals with the flow of natural gas and oil in pipelines, or the branch of mechanical engineering that deals with a vehicle's engine, gas pump, and hydraulic braking system. The only machines that are of concern to hydraulic engineers are hydraulic turbines and water pumps.

Background and History

Irrigation and water supply projects were built by ancient civilizations long before mathematicians defined the governing principles of fluid mechanics. In Peru's Andes Mountains, remains of irrigation canals were found dating from the fourth millennium BCE. The first dam, for which there are reliable records, was built before 4000 BCE on the Nile River in Memphis, ancient Egypt. Egyptians built dams and dikes to divert the Nile's floodwaters into irrigation canals. Mesopotamia (now Iraq and western Iran) has low rainfall and is supplied with surface water by the Tigris and the Euphrates. These major rivers are much smaller than the Nile but have more dramatic spring floods. Mesopotamian engineers, concerned with water storage, flood control, and irrigation, built diversion dams and large weirs to create reservoirs and supply canals carrying water for long distances. In the Indus Valley civilization (now Pakistan and northwestern India), sophisticated irrigation and storage systems were developed.

One of the most impressive dams of ancient times is near Marib, an ancient city in Yemen. The 1,600-foot-long dam was built of masonry strengthened by copper around 600 BCE. It holds back some of the annual floodwaters coming down the valley and diverts the rest of that water out of sluice gates and into a canal system. The same sort of diversion dam system was independently built in Arizona by the Hohokam civilization around the second or third century CE.

In the Szechwan region of ancient China, the Dujiangyan irrigation system was built around 250 BCE and still supplies water in modern times. By the second century CE, the Chinese used chain pumps, which lifted water from lower to higher elevations, powered by hydraulic waterwheels, manual foot pedals, or rotating mechanical wheels pulled by oxen.

The Minoan civilization developed an aqueduct system in 1500 BCE to convey water in tubular conduits in the city of Knossos in Crete. Roman aqueducts were built to carry water from large distances to Rome and other cities in the empire. Of the 800 miles of aqueducts in Rome, only 29 miles were above ground. The Romans kept most of their aqueducts underground to protect their water from enemies and diseases spread by animals.

The Muslim agricultural revolution flourished during the Islamic golden age in various parts of Asia and Africa, as well as in Europe. Islamic hydraulic engineers built water management technological complexes, consisting of dams, canals, screw pumps, and norias, which are wheels that lift water from a river into an aqueduct.

The Swiss mathematician Daniel Bernoulli published Hydrodynamica (1738; Hydrodynamics by Daniel Bernoulli, 1968), applying the discoveries of Sir Isaac Newton and Gottfried Wilhelm Leibniz in mathematics and physics to fluid systems. In 1752, Leonhard Euler, Bernoulli's colleague, developed the more generalized form of the energy equation.

In 1843, Adhémar-Jean-Claude Barré de Saint Venant developed the most general form of the differential equations describing the motion of fluids, known as the Saint Venant equations. They are sometimes called Navier-Stokes equations after Claude-Louis Navier and Sir George Gabriel Stokes, who were working on them around the same time.

The German scientist Ludwig Prandtl and his students studied the interactions between fluids and solids between 1900 and 1930, thus developing the boundary layer theory, which theoretically explains the drag or friction between pipe walls and a fluid.

How It Works

Properties of Water. Density and viscosity are important properties in fluid mechanics. The density of a fluid is its mass per unit volume. When the temperature or pressure of water changes significantly, its density variation remains negligible. Therefore, water is assumed to be incompressible. Viscosity, on the other hand, is the measure of a fluid's resistance to shear or deformation. Heavy oil is more viscous than water, whereas air is less viscous than water. The viscosity of water increases with reduced temperatures. For instance, the viscosity of water at its freezing point is six times its viscosity at its boiling temperature. Therefore, a flow of colder water assumes higher friction.

Hydrostatics. Hydrostatics is a subdiscipline of fluid mechanics that examines the pressures in water at rest and the forces on floating bodies or bodies submerged in water. When water is at rest, as in a tank or a large reservoir, it does not experience shear stresses. Therefore, only normal pressure is present. When the pressure is uniform over the surface of a body in water, the total force applied to the body is a product of its surface area times the pressure. The direction of the force is perpendicular (normal) to the surface. Hydrostatic pressure forces can be mathematically determined on any shape. Buoyancy, for instance, is the upward vertical force applied on floating bodies (such as boats) or submerged ones (such as submarines). Hydraulic engineers use hydrostatics to compute the forces on submerged gates in reservoirs and detention basins.

Fluid Kinematics. Water flowing steadily in a constant-diameter pipe has a constant average velocity. The viscosity of water introduces shear stresses between particles that move at different velocities. The velocity of the particle adjacent to the wall of the pipe is zero. The velocity increases for particles away from the wall, and it reaches its maximum at the center of the pipe for a particular flow rate or pipe discharge. The velocity profile in a pipe has a parabolic shape. Hydraulic engineers use the average velocity of the velocity profile distribution, which is the flow rate over the cross-sectional area of the pipe.

Bernoulli's Theorem. When friction is negligible and there are no hydraulic machines, the conservation of energy principle is reduced to Bernoulli's equation, which has many applications in pressurized flow and open-channel flow when it is safe to neglect the losses.

Applications and Products

Water Distribution Systems. A water distribution network consists of pipes and several components, like reservoirs, pumps, elevated storage tanks, valves, and other appurtenances such as surge tanks or standpipes. Regardless of its size and complexity, a water distribution system transfers water from one or more sources to customers. There are raw and treated water systems. A raw water network transmits water from a storage reservoir to treatment plants via large pipes called transmission mains. The purpose of a treated water network is to move water from a water-treatment plant and distribute it to water retailers through transmission mains or directly to municipal and industrial customers through smaller distribution mains.

Some water distribution systems are branched, whereas others are looped. The latter type offers more reliability in case of a pipe failure. The hydraulic engineering problem is to compute the steady velocity or flow rate in each pipe and the pressure at each junction node by solving a large set of continuity equations and nonlinear energy equations that characterize the network. The steady solution of a branched network is easily obtained mathematically. However, the looped network initially offered challenges to engineers. In 1936, American structural engineer Hardy Cross developed a simplified method that tackled networks formed of only pipes. In the 1970s and 1980s, three other categories of numerical methods were developed to provide solutions for complex networks with pumps and valves. In 1996, engineer Habib A. Basha and his colleagues offered a perturbation solution to the nonlinear set of equations in a direct, mathematical fashion, thus eliminating the risk of divergent numerical solutions.

Hydraulic Transients in Pipes. Unsteady flow in pipe networks can be gradual. Therefore, it can be modeled as a series of steady solutions in an extended period simulation, mostly useful for water-quality analysis. However, abrupt changes in a valve position, a sudden shutoff of a pump because of power failure, or a rapid change in demand could cause a hydraulic transient or a water hammer that travels back and forth in the system at high speed, causing large pressure fluctuations that could cause pipe rupture or collapse.

The solution of the quasi-linear partial differential equations that govern the hydraulic transient problem is more challenging than the steady network solution. The Russian scientist Nikolai Zhukovsky offered a simplified arithmetic solution in 1904. Many other methods—graphical, algebraic, wave-plane analysis, implicit, and linear methods, as well as the method of characteristics—were introduced between the 1950s and 1990s. In 1996, Basha and his colleagues published another paper solving the hydraulic transient problem in a direct, noniterative fashion, using the mathematical concept of perturbation.

Open-Channel Flow. Unlike pressure flow in full pipes, which is typical for water distribution systems, flow in channels, rivers, and partially full pipes is called gravity flow. Pipes in wastewater evacuation and drainage systems usually flow partially full with a free water surface subject to atmospheric pressure. This is the case for human-built canals and channels (earth or concrete-lined) and natural creeks and rivers.

The velocity in an open channel depends on the area of the cross-section, the length of the wetted perimeter, the bed slope, and the roughness of the channel bed and sides. A roughness factor is estimated empirically and usually accounts for the material, the vegetation, and the meandering in the channel.

Open-channel flow can be characterized as steady or unsteady, uniform or varied, or gradually or rapidly varied. The hydraulic jump is a famous example of a rapidly varied flow.

When high-energy water, gushing at a high velocity and a shallow depth, encounters a hump, an obstruction, or a channel with a milder slope, it cannot sustain its supercritical flow (characterized by a Froude number larger than one). It dissipates most of its energy through a hydraulic jump, a highly turbulent transition to a calmer flow (subcritical flow with a Froude number less than one) at a higher depth and a much lower velocity. One way to solve for the depths and velocities upstream and downstream of the hydraulic jump is by applying the conservation of momentum principle, the third principle of fluid mechanics and hydraulic engineering. The hydraulic jump is a very effective energy dissipater that is used in the designs of spillways.

Hydraulic Structures. Many types of hydraulic structures are built in small or large civil engineering projects. The most notable by its size and cost is the dam. A dam is built over a creek or a river, forming a reservoir in a canyon. Water is released through an outlet structure into a pipeline for water supply or into the river or creek for groundwater recharge and environmental reasons (sustainability of the biological life in the river downstream). During a large flood, the reservoir fills up, and water can flow into a side overflow spillway—which protects the integrity of the face of the dam from overtopping—and into the river.

The four major types of dams are gravity, arch, buttress, and earth. Dams are designed to hold the immense water pressure applied on their upstream face. The pressure increases as the water elevation in the reservoir rises.

Hydraulic Machinery. Hydraulic turbines transform the drop in pressure (head) into electric power. Also, pumps transform electric power into a water head, moving the flow in a pipe to a higher elevation.

There are two types of turbines—impulse and reaction. The reaction turbine is based on the steam-powered device developed in Egypt in the first century CE by the Hero of Alexandria. A simple example of a reaction turbine is the rotating lawn sprinkler.

Pumps are classified into two main categories—centrifugal and axial flow. Pumps have many industrial, municipal, and household uses, such as boosting the flow in a water distribution system or pumping water from a groundwater well.

Careers and Course Work

Undergraduate students majoring in civil or environmental engineering usually take several core courses in hydraulic engineering, including fluid mechanics, water resources, and fluid mechanics laboratory. Advanced studies in hydraulic engineering lead to a master’s of science or a doctoral degree. Students with a bachelor's degree in a science or another engineering specialty could pursue an advanced degree in hydraulic engineering. Still, they may need to take several undergraduate-level courses before starting the graduate program.

Graduates with a bachelor's degree in civil engineering or advanced degrees in hydraulics can work for private design firms that compete to be chosen to work on the planning and design phases of large governmental hydraulic engineering projects. They can also work for construction companies that bid on governmental projects to build structures and facilities that include hydraulic elements or for water utility companies, whether private or public. Some common areas for hydraulic engineers to work in are stormwater management, sediment transport, and canal creation for irrigation and transportation.

Teaching or conducting research at a university or a research laboratory requires a doctoral degree in one of the branches of hydraulic engineering.

Social Context and Future Prospects

In the twenty-first century, hydraulic engineering is closely tied to environmental engineering. Reservoir operators plan and vary water releases to keep downstream creeks wet, thus protecting the biological life in the ecosystem. They also conceptualize and build water management systems. Hydraulic engineers can also be involved in coastal engineering, which includes fighting erosion through sediment delivery and flood management. Their skills can also be applied to other river or ocean conservation projects.

Clean energy is the way to ensure the sustainability of the planet's resources. Hydroelectric power generation is a form of clean energy. Energy generated by ocean waves is a developing and promising field, although wave power technologies still face technical challenges. As sea levels rise and water becomes a valuable resource, hydraulic engineers will become invaluable. Other modern trends in hydraulic engineering include fluid-hydraulic pumps and hydraulic-powered Autonomous Mobile Robots (AMRs).

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