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HYDROLOGY

For ENGINEERS

Students: Tamayo, John Railey P. Laurio, Meljun R. Fuentes, Salustiano IV L. Instructor: Engr. Mark Anthony V. Lobrigo

Southern Bicol Colleges College of Engineering Module in CE 421 (Hydrology) Course Title: CE 421 – Hydrology Course Pre-requisite: Math 227 – Integral Calculus Credit Unit/s: 3 units lecture (3 lecture hours per week Course Description: The course deals on the hydrologic cycle and the different processes such as precipitation, evaporation, overload flow, ground water flow, and surface runoff generation.

A number of factors contribute to the ineffectiveness of these engineering designs. First, the occurrence of rainfall cannot be predicted with certainty—that is, it is not possible to predict exactly how much rain will occur in one time period (e.g., a day, month, or year). The uncertainty of extreme variation in rainfall amounts is even greater than the uncertainty in the rainfall volumes occurring in the more frequent storm events. It is difficult to design engineering works that will control the water under all conditions of variation in both the time and spatial distributions. Second, even if we had perfect information, the cost of all of the worthwhile projects needed to provide the optimum availability of water is still prohibitive. Therefore, only the most efficient and necessary projects can be constructed. Third, hydrologic processes such as rainfall and runoff are very complex, and a complete, unified theory of hydrology does not exist. Therefore, measurements of observed occurrences are used to supplement the scant theoretical understanding of hydrologic processes that exists. However, given the limited records of data, the accuracy of many engineering designs is less than we would like. These three factors (hydrologic uncertainty, economic limitations, and lack of theory and observed data) are just some of the reasons that we cannot provide solutions to all problems created by undesirable variations in the spatial and temporal distributions of water. In spite of the inherent uncertainty in precipitation, the economic constraints, and the bounds on our theoretical understanding of hydrometeorological processes, solutions to the problems that are created by the temporal and spatial variations in water availability must be provided. Estimates of hydrologic quantities such as stream flow are required as input to engineering designs, which represent the engineer’s attempt to solve the problem. All engineering designs should, at the minimum, be rational. An understanding of the physical processes is a necessary pre-requisite to the development of rational designs.

1.2 THE HYDROLOGIC CYCLE

The physical processes that control the distribution and movement of water are best understood in terms of the hydrologic cycle. Although a real beginning or ending point of the hydrologic cycle cannot be identified, we can begin the discussion with precipitation. For the purposes of this discussion, we will assume that precipitation consists of rainfall and snowfall. A schematic of the hydrologic cycle for a natural environment is shown in Figure 1.1. Rain that falls on Earth may enter a water body directly, travel over the land surface from the point of impact to a watercourse, or infiltrate into the ground. Some rain is intercepted by vegetation. The intercepted water is temporarily stored on the vegetation until it evaporates back to the atmosphere. Some rain is stored in surface depressions, with almost all of the depression storage infiltrating into the ground. Water stored in depressions, water intercepted by vegetation, and water that infiltrates into the soil during the early part of a storm represent initial losses. The loss is water that does not appear as runoff during or immediately following a rainfall event. Water entering the upland streams travels to increasingly larger rivers and then to seas and oceans. The water that infiltrates into the ground may percolate to the water table or travel in the unsaturated zone until it reappears as surface flow. The amount of water stored in the soil determines, in part, the amount of rain that will infiltrate during the next storm event. Water stored in lakes, seas, and oceans evaporates back to the atmosphere, where it completes the cycle and is available for rainfall.

These changes to the processes of the hydrologic cycle, which are shown schematically in Figure 1.2, cause significant changes in runoff characteristics. The reduced storage results in increased volumes of surface runoff. The reduced surface roughness decreases the travel time of runoff. The reductions in both storage and travel time result in increased peak rates of runoff, which increase both flood damages due to overbank flows and channel erosion. In an attempt to compensate for lost natural storage, many localities require the replacement of lost natural storage with human-made storage. While the storm water detention basin is the most frequently used method of storm water management, other methods are used, such as infiltration pits, rooftop and parking lot storage, and porous pavement. The current trend is to design newly developed areas to have minimal hydrologic effects. These engineering works do not always return the runoff characteristics to those that existed in the natural environment. In fact, poorly conceived methods of control have, in some cases, made flood-run off conditions worse.

SUMMARY

Hydrology is the scientific study of water, encompassing its properties, distribution, and effects on Earth. While various professions view water from different perspectives, hydrologic engineering focuses on the design and operation of projects for water control and use, particularly concerning infrastructure. Despite Earth's abundant water, spatial and temporal variations in its distribution create problems like water scarcity, flooding, and inconsistent supplies for agriculture and manufacturing. Engineers and hydrologists strive to predict water availability to address these issues, but challenges arise due to the uncertainty of rainfall, economic limitations, and the complexity of hydrologic processes. Overcoming these hurdles requires rational engineering designs based on an understanding of physical processes to solve problems caused by variations in water availability. The hydrologic cycle describes the continuous movement and distribution of water. Precipitation, in the form of rain and snow, either enters water bodies directly, flows over land, or infiltrates into the ground. Initial losses include interception by vegetation and storage in surface depressions. Water travels through streams, rivers, and oceans, eventually evaporating back into the atmosphere. The landscape is constantly changing due to storms, erosion, droughts, and fires, all of which affect runoff. Human activities such as agriculture and urbanization significantly alter the hydrologic cycle by reducing vegetation, storage, and infiltration, leading to increased runoff and erosion. Attempts to mitigate these effects with engineered solutions like detention basins aim to manage stormwater, but can sometimes worsen runoff conditions if poorly implemented.