9 Things You Should Know About Hydraulics: A Hydrology Review
Those taking the PE Civil exam should be familiar with the topics of Hydraulics and Hydrology. These topics are covered on the exam regardless of which depth version of the exam is selected. Aside from the Water Resources and Environmental depth exam, which covers these topics in depth, the other exams cover these topics with six to nine questions. Although at first glance, this may not seem to be a large number of questions, it is actually among the topics of the greatest coverage in the breadth portion of the exam. It is therefore especially important to be familiar with these topics to pass the exam.
The term hydraulics refers to the analysis and engineering of open channel and closed conduit flow systems on the basis of the principles of fluid mechanics. At the level of basic fluid mechanics, it's important to know that the behavior of fluids can be understood by the principles of the conservation of mass, the conservation of energy, and the conservation of momentum. In particular, exam-takers should be familiar with using the Bernoulli equation, which is the equation for the conservation of energy of a fluid. Essentially, it relates the pressure, speed, and height of a fluid at two different points within a changing cross-section of piping.
2. Open Channel Flow
With open channel flow, a fluid is not flowing under pressure within a closed conduit, and its behavior is consequently based primarily upon gravity and the slopes of the channels it flows within. The volumetric flow rate in an open channel is simply the average velocity of the fluid multiplied by the fluid's cross-sectional area. The velocity can be determined with one of two formulas: the Chezy-Manning equation or the Hazen-Williams equation. These formulas, the specifics of which will not be discussed here but with which the exam-taker should be familiar, take into consideration the effects of the friction which occurs between the fluid and the surfaces of the channel through which it flows. It should be noted that the hydraulic radius of a fluid in an open channel is the ratio of the fluid's cross-sectional area to its wetted perimeter (which is that portion of the channel section which is contact with the fluid).
3. Closed Conduit Flow
Closed conduit flow, in contrast to open channel flow, is characterized by a fluid's flow under pressure. Friction losses for closed conduits are calculated using the Darcy-Weisbach equation, which can be used for the laminar or turbulent flow conditions of any fluid, or the Hazen-Williams equation, which can be used for the turbulent flow conditions of water. The Darcy-Weisbach equation utilizes what is known as the Reynolds number as well as the roughness of the pipe to determine the friction loss. The Hazen-Williams equation also factors in the roughness of the pipe, utilizing constant for different pipe materials. Exam-takers should be familiar with these equations for determining friction losses in closed conduits.
Hydrology is the study of the distribution and movement of water over and underneath the earth's surface. For the purposes of study for the PE Civil exam, it is necessary to be familiar with storm characteristics, stormwater collection and drainage, runoff analysis, and the use of retention and detention ponds.
5. Storm Characteristics within Hydrology
Storm characteristics in hydrology include storm frequency as well as rainfall measurement and distribution. The concept of rainfall intensity is used in several equations in hydrology. It is the amount of rain in depth over a period of time and can be measured with a rain gauge. Storm frequency can be understood as the likelihood that a storm of particular intensity may occur in any given year. For example, a 20-year storm would have a probability of occurrence of 1/20 in a given year, and a 100-year storm would have a probability of 1/100. Historical data on rainfall intensity and storm frequency is often available on a regional graph of intensity-duration-frequency curves, which can be used to determine the time of concentration for a flow for a particular storm frequency. Time of concentration is the amount of time which it takes for water from the furthest point in a watershed area to reach the point of analysis, and thus it is the time needed for all areas within the watershed area to start contributing to the storm discharge.
6. Stormwater Collection and Drainage
Systems of stormwater collection and drainage include the components of surface drainage such as over street and gutters, culverts, stormwater inlets, and stormwater pipes. The design of the system is intended to provide for the efficient drainage of stormwater and reduce the risk of flooding. Where impervious surfaces carry surface flow, such as along paved areas, streets and gutters, the slopes along with the sectional cross slopes are designed to channel water into stormwater inlets such as catch basins or curb inlets. These collect the surface water into the underground drainpipes which connect to the main stormwater line. Culverts are conduits which allow for the flow of surface water below a roadway or an embankment.
7. Runoff Analysis
Runoff analysis utilizes hydrographs which are based on data from a storm in a particular area. There are different graphical methods which are used to separate the base flow conditions from the additional flow which is a result of a storm event. From this analysis a unit hydrograph can be plotted which isolates the excess flow from the storm event. It plots the discharge (typically in cubic feet per second) over the course of a storm event such that the graphical representation of the data allows for an understanding of a storm's peak flow and it when it occurs. This can be used to help predict the peak discharge for other future storms with different rainfall amounts.
8. Reading Hydrographs
It should be noted that the hydrograph indicates the particular time lag which occurs for peak flows given the particular topography, terrain, and other factors. A significant factor influencing the shape of the hydrograph curve is the amount of development within the catchment area and specifically the permeability of its surfaces. In permeable surfaces, there is a certain amount of time before the ground has reached maximum absorption and the surface begins to carry runoff. In contrast, paved areas will carry the storm water immediately as surface runoff. Thus, the amount of impermeable area within the catchment area will influence the shape of the curve represented in the hydrograph and the peak flow occurring during a storm event.
9. Retention Ponds
Retention ponds are structures which are used to retain water in the event of a storm so that the area's stormwater drainage system is not overwhelmed in the event of a large storm. They can be thought of as being similar to a water body held back by a dam which has a controlled outflow. The water level of the retention pond fluctuates based on precipitation amounts of storm events. Hydrographic analysis can be utilized to determine the necessary size of a retention pond to accommodate a storm of a particular frequency. Unlike retention ponds, which typically have some level of water in them in normal conditions, detention ponds are typically dry except during storm events. Both serve to reduce storm water discharge rates and the risk of flooding. They can also aid in allowing for the settling of suspended sediments, pollutants, and other particles present in the storm runoff, thus aiding in the improvement of water quality.
It is important for takers of the PE Civil exam to understand the basics of both hydraulics and hydrology as these topics are covered on all of the depth versions of the exam. The above summary serves as a short review of basic concepts which should be studied in more detail as these represent the topics for which there may be several questions on the exam.
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