- Dr. Arpita Mondal
I am into fourth year of my teaching Applied Hydraulic Engineering to Civil Engineering BTech students at IIT Bombay – and this experience gives me some confidence to attempt to explain what hydraulic engineering is and why civil engineers should study it. Loosely put, hydraulic engineering is fluid mechanics put to engineering use. However, it is worth noting right at the outset that much of hydraulic engineering historically developed as empirical science. What is empirical science? Science that is based on observed data and that works for our practical purpose. Therefore, right from the outset of modern, urban-and-industry centric civilization, out of necessity more than anything else, engineers proudly continued to build water systems such as conduits, canals, dams or reservoirs based on empirical relationships derived from real-life observations without bothering too much about the physics of the problem. Many of those systems withstood the test of time and function perfectly even to this day. However, what is fascinating is the link between such empirical relationships and theoretical principles of fluid mechanics, that got established much later. For example, since late eighteenth century, it wasn’t uncommon to design drainage channels using empirical relationships between velocity of flow and frictional resistance. It was only much later in the twentieth century that it was shown that such relationships are linked with theoretical resistance laws that have roots in the boundary layer theory. Hydraulic engineering, therefore, offers an exciting and intriguing synergy of physical laws and practical wisdom.
Let us begin with a large water resources system. For purposes of municipal or industrial water supply, irrigation, hydropower generation, flood control, recreation or one or more of these, a dam is to be built along the course of the river. Due consideration has to be given to ecological sustainability both upstream and downstream of the dam. The very first question then faced is: how much upstream area will be flooded because of the construction of the reservoir behind the dam? How would the depth of water upstream or downstream of the dam change? What volume of water should the reservoir retain? Discussions on uniform and gradually varied flows as part of open channel flow, an essential component of hydraulic engineering, helps answer those questions.
Moving along, for municipal or industrial water supply, intake structures are erected to extract water that is then treated and distributed through supply networks. Such networks are complex connections of pipes ranging a few meters to sometimes several hundred kilometres. Where should the storage reservoirs be located, what should the diameters of the pipes be, whether and where should pumps be installed to make sure the water overcomes head loss to reach the target and what should the pump capacities be? Flow through pipes is a detailed topic in hydraulic engineering that determines the answers.
If the dam supplies water for agriculture through irrigation canals, it is imperative to know what should be the material, cross sectional shape and size of such canals to satisfy the irrigation demands. This is particularly important for India since our economy is heavily dependent on agriculture. Remember, water should reach where it has to, and like all engineering design, the system must be ‘efficient’. On the way, the channel may face constrictions or expansions, humps or obstructions which lead to an interplay of adjustments in the energy and momentum of flow leading changes in water level and velocity. Growth of vegetation or silt deposition in such channels, as they age, may also influence the roughness or resistance to flow. Sometimes, flow transitions may happen quickly, sometimes they are gradual while at other points flow may not change at all. These issues are also discussed in the topic of open channel flow.
Reservoir water that accumulates behind a dam are typically at an elevation. That potential energy may be harvested to supply electricity through hydropower generation plants. While skeptics may argue, hydroelectric power is believably greener as compared to thermal power that relies on fossil fuel burning. Further, the historical role of hydropower in providing electricity during the course of modern human civilization is undeniable. Water flows at large speeds through special ducts called penstocks to reach the turbine that is rotated by the force of water to convert the mechanical energy of water to electrical energy by means of a generator. Valves at the end of the penstocks are opened or closed depending on the load on the turbine which in turn depends on the power demands. How long and of what material and diameters should the penstocks be? How fast or slow should the valves be opened and closed such that the excess pressure doesn’t damage the pipes? How does the excess pressure due to valve closure vary along the length of the penstock and with time? What measures can we take to dissipate the excess pressure? Water hammer analysis, taught in hydraulic engineering, will answer all these questions for you.
Finally, before an impending flood (or not), excess water needs to be released from the dam through specific structures known as spillways. At what rates must water be released and how are such rates related to the reservoir level? The sudden gush of released water will have a lot of excess energy that is typically dissipated through a phenomenon called hydraulic jump where the flow depth in the channel undergoes change through a highly turbulent, rapidly varying phase. Civil engineers are interested in the energy loss and also the location of the jump after the spillway, for, a flood-related spillway failure may result in massive destruction of lives and properties. The topic of hydraulic jump, as part of open channel flow, forms one of the most important parts of hydraulic engineering. In most competitive exams at the graduate level, you may expect a few questions on this topic. The process of release of flood waters also targets an attenuation of flood peak, that is, the highest discharge downstream of the dam must be significantly lower, and at a later time than that at the upstream of the dam. Achieving this target may involve simple mass balance-based ‘flood routing’, or more complex methods relying on numerical solutions of partial differential governing equations. We learn these in hydraulic engineering. It is worth noting that governing equations form a general and recurring idea in hydraulic engineering and applies to several topics in this course. We talk about them in details for gradually varied flow computations along a river in open channel flow (and not so much in details for rapidly varying flows), and also in the context of water hammer where pressure and velocity varies along the length of the pipe and also along time.
With all of this background, civil engineers are now capable of building several components of the large water resources system that we talked about. What about their performance and maintenance? Calculations done prior to design, on paper and on the computer, must be complemented by physical studies in the laboratory. Models tested in the laboratory simulate how the prototype structure is likely to behave in real life. But what ratios of lengths or forces shall be adopted in the lab to get a realistic picture? Are such ratios physically feasible to be achieved in a lab? What can we do to circumvent such infeasibility problems? Which parameters are most important for the problem at hand, and how can we extrapolate our findings to other problems? Dimensional analysis and model studies aid in finding answers to these questions. And while we study dimensional analysis as part of hydraulic engieering, we also learn about their contextual use in other branches of engineering such as mechanical, chemical or aerospace engineering, as well as astrophysics and space science. We also do some fun examples that show how scientifically unsound the book/movie Jurassic Park was!
How about the concepts that we don’t detail in this subject, but perhaps should? We don’t cover hydraulic machines. And although we briefly touch upon flow measurement and calibration, this is another area that needs more attention. For the sake of classroom teaching, examples and fundamental concepts in hydraulic engineering are usually restricted to simple cases that are appreciable at the undergrad level. Though we discuss pioneering charts (Moody’s/Stanton’s diagram, Allievi Charts, etc.) and demonstrations of several open-source computational packages (JalTantra, EPANET, Allievi, HEC-RAS) it would be ideal to include more hands-on exercises at par with industry standards. Some of the latest research themes and publications in related areas could also be discussed.
What is the future of teaching hydraulic engineering for civil engineers? Finally, I will end here with an emphasis on the need for an interdisciplinary approach to make hydraulic engineering more inclusive and comprehensive. Civil engineering deals with coupled human-natural systems and understanding human behaviour governing such systems is as important as understanding physiographical processes. In several topics listed above, the human and societal component is significant – be it in terms of inundated area due to construction of a dam, irrigation water supply to farmers, hydropower demands, flood related loss and damage or municipal water supply networks. I hope that in future, classroom teaching of hydraulic engineering will also include a flavour of social science.
Dr. Arpita Mondal is an Assistant Professor specialised in Water Resources Engineering in the civil engineering department at IITB. She is involved in teaching (among other courses) Hydraulic Engineering to 2nd year civil engineering undergraduates. Her main research interests are modelling of extreme events, climate change, and detection & attribution.
Link to her profile: https://sites.google.com/site/arpita567/home
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