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Chapter 7 - Hydrology Publication 584 2010 Edition CHAPTER 7 Chapter 7 - Hydrology Publication 584 2010 Edition CHAPTER 7 HYDROLOGY 7.0 INTRODUCTION TO HYDROLOGY A. General - Hydrology. Hydrology is generally defined as a science dealing with the interrelationship between water on and under the earth and in the atmosphere. For the purpose of this manual, hydrology will deal with estimating flood magnitudes as the result of precipitation. In the design of highway drainage structures, floods usually are considered in terms of peak runoff or discharge in cubic feet or meters per second and hydrographs as discharge per time. Peak discharge is used to design facilities such as storm drain systems, culverts and bridges. For systems that are designed to control the volume of runoff, like detention storage facilities, or where flood routing through culverts is used, the entire discharge hydrograph will be of interest. The analysis of the peak rate of runoff, volume of runoff and time distribution of flow is fundamental to the design of drainage facilities. Errors in the estimates will result in structures that either are undersized and cause more drainage problems or oversized and cost more than necessary. On the other hand, one must realize that any hydrologic analysis is only an approximation. Although some hydrologic analysis is necessary for all highway drainage facilities, the extent of such studies should be commensurate with the hazards associated with the facilities and with other concerns, including economic, engineering, social, and environmental factors. Since hydrologic science is not exact, it is possible that different hydrologic methods developed for determining flood runoff may produce different results for a particular situation. Sound engineering judgment must be exercised to select the proper method or methods to be applied. In some instances, certain federal, state, or local agencies may require that specific hydrologic methods be used for computing the runoff. While performing the hydrologic analysis and hydraulic design of highway drainage facilities, the hydraulic engineer should recognize potential environmental problems that would impact the specific design of a structure. This area of concern should be evaluated early in the design process. Considerations for hydrologic analysis and several of the most widely used hydrologic methods are outlined in this chapter. The omission of other hydrologic methods from this manual does not preclude their use; however, the designer should ensure the method chosen is appropriate for local conditions and acceptable to PennDOT. B. Peak Discharge versus Frequency Relations. Highway drainage facilities are designed to convey specific predetermined discharges in order to avoid significant flood hazards. Provisions also are made to convey floods in excess of these discharges in a manner that minimizes the damage and hazard. These discharges are often referred to as peak discharges because they occur at the peak of the stream's flood hydrograph. These flood discharge magnitudes are a function of their expected frequency of occurrence, which in turn relates to the magnitude of the potential damage and hazard. The highway designer's chief interest in hydrology rests in estimating runoff and peak discharges for application in the design of highway drainage facilities. The highway drainage designer is particularly interested in the development of a flood magnitude versus frequency relationship. A flood frequency relation is a tabulation of peak discharges versus their probability of occurrence or exceedance. Peak discharges and probabilities of flooding will be discussed in this chapter. A typical flood frequency curve is illustrated in Figure 7.1. In this figure the discharge is plotted on the ordinate (y- axis) on a logarithmic scale and the probability of occurrence or exceedance is expressed in terms of return interval and plotted on a probability scale on the abscissa (x-axis). Also of interest is the performance of highway drainage facilities during the frequently occurring low flood flow periods. Because low flood flows do occur frequently, the potential exists for lesser amounts of flood damage to occur more frequently. It is entirely possible but not desirable to design a drainage facility to convey a large, infrequently occurring flood with an acceptable amount of flood plain damage only to find that the accumulation of damage from frequently occurring floods is intolerable. 7 - 1 Chapter 7 - Hydrology Publication 584 2010 Edition Figure 7.1 Typical Flood Frequency Curve C. Flood Hydrographs. Besides the peak discharges, the hydraulic engineer may be interested in the flood volume and time distribution of runoff. A flood hydrograph is a plot or tabulation of discharge with respect to time. Flood hydrographs can be used to route floods through culverts, flood storage structures, and other highway facilities. By accounting for the stored flood volume, the hydraulic engineer often can expect lower flood peak discharges and smaller required drainage facilities than would be expected without considering storage volume. Flood hydrographs are also useful for estimating design values of inundation times of flow over roadways as well as pollutant and sediment transport analysis. D. Unit Hydrograph. Sherman, Snyder, and Clark developed the theory of unit hydrographs as a tool to estimate a flood hydrograph for any rainfall event. A unit hydrograph represents the response of a watershed to a unit rainfall excess having a specific duration. Excess rainfall is defined as the total rainfall minus the hydrologic abstractions (losses) and is equal to the direct runoff. For PennDOT practice, the unit is 1 mm (1 in). That is, the volume associated with an excess rainfall of 1 mm (1 in) distributed over the entire contributing area. Therefore, a unit hydrograph is a hydrograph of the runoff resulting from a hypothetical storm that has a specified duration, e.g., 1 hr, and that produces a response runoff hydrograph, resulting from the unit depth of excess rainfall over the drainage area. For example, when a unit hydrograph is shown with units of cubic meters per second, it is implied that the ordinates are cubic meters per second per millimeter of direct runoff. The response of a watershed to rainfall is considered to be a linear process. This has two implications that are useful to the designer; the concepts of proportionality and superposition apply. For example, the runoff hydrograph resulting from a two-unit pulse of rainfall of specific duration would have ordinates that are twice as large as those resulting from one-unit pulse of rainfall of the same duration. Also, the hydrograph resulting from the sequence of two one-unit pulses or rainfall can be found by the superposition of two one-unit hydrographs. Thus, if a unit hydrograph for a watershed is known or can be determined, the flood hydrograph resulting from any measured or design rainfall can be determined using these two principles. Unit hydrograph applications are discussed later in this chapter. 7 - 2 Chapter 7 - Hydrology Publication 584 2010 Edition E. Site Investigation. Every problem is unique, and reliance upon strict application of a standardized procedure is risky without due appreciation of the characteristics of the particular site. A field survey or site investigation always should be conducted except for the most preliminary analysis. The need for a field survey to collect and appraise site-specific physical characteristics, as well as hydrologic and hydraulic data, cannot be overstated. Most complaints relating to highway drainage facilities result from changes to existing hydrologic and hydraulic characteristics. In order to minimize the potential for valid complaints, complete data reflecting existing drainage characteristics should be gathered and considered during design. Typical data which should be collected during field surveys include the following: • Highwater marks. • Performance assessments of existing and nearby drainage structures. • Assessment of stream stability and scour potential. • Location and nature of important physical and cultural features which could affect or be affected by the proposed structure. • Significant differences in land use from those indicated on available topographic maps. • Other equally important and necessary items of information which could not be obtained from other sources, such as man-made features that affect the hydrology and hydraulics. • Local residents, local landowners, and local or PennDOT highway maintenance personnel should be consulted. The individual responsible for the drainage aspects of a field survey should have a general knowledge of drainage design. Field surveys should be well planned and a systematic approach should be employed to maximize efficiency and reduce wasted effort. Data collected should be well documented with written reports and photographs. F. Interagency Coordination. Since many levels of government plan, design, and construct highway and water resource projects and because these projects often affect each other, interagency coordination is desirable and necessary. In addition, agencies can share data and experiences within project areas to assist in the completion of accurate well-coordinated hydrologic analyses. 7.1 FACTORS AFFECTING FLOODS A. Rainfall versus Runoff Quantity/Volume. Runoff Quantity or Volume from a watershed is influenced by two main factors: precipitation and hydrologic abstractions (losses). A discussion of these factors follows. 1. Precipitation. Precipitation in Pennsylvania
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