AASHTO HDG CHAPTER 2

Abstract

<strong>INTRODUCTION</strong> <br>Hydrology is the science that treats the waters of the earth, their occurrence, circulation and distribution, their chemical and physical properties, and their reaction with their environment, including their relation to living things (<em>1</em>).¹ It is also defined as the science that deals with the processes governing the depletion and replenishment of the water resources of the land areas of the earth (<em>84</em>). It is concerned with the transportation of water through the air, over the ground surface, and through the strata of the earth. <br>Although hydrology is a very broad science encompassing many disciplines relating to water, the hydraulics engineer is more concerned with estimating runoff than any other hydrologic problem. The scope of this chapter will be primarily limited to surface hydrology. <br>Hydrologic analysis is the most important step prior to the hydraulic design of a highway drainage structure regardless of its size or cost. Such an analysis is necessary to determine the discharge (rate of runoff) and volume of runoff that the drainage facility will be required to convey or control. Although some hydrologic analysis is necessary for all highway drainage facilities, the extent of such studies should be commensurate with the hazard associated with the facilities and with other economic, engineering, social, and environmental concerns. While performing the hydrologic analysis and hydraulic design of highway drainage facilities, the hydraulics engineer should be cognizant of potential environmental problems that would impact the specific design of a structure. This area should be evaluated before spending a large amount of time in detailed design. <br>Highway drainage facilities are designed to convey predetermined discharges to avoid a significant flood hazard. Provision is also made to convey floods in excess of these discharges in a manner that minimizes the damage and hazard to the extent practicable. These discharges are often referred to as peak discharges because they occur at the peak of the stream's flood hydrograph (discharge over time). These flood discharge magnitudes are a function of their expected frequency of occurrence that in turn relates to the magnitude of the potential damage and hazard. <br>Also of interest is the performance of highway drainage facilities during the frequently occurring lowflood 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 to design a drainage facility to convey a large, infrequently occurring flood with an acceptable amount of floodplain damage only to find that the aggregate of the lesser damage from frequently occurring floods is intolerable. <br>Besides the peak discharges, the hydraulics engineer is sometimes interested in the flood volume associated with a flood hydrograph. Flood hydrographs can be used to route floods through culverts, flood storage structures, and other highway facilities. By considering the stored flood volume, the hydraulics engineer can often design a storage structure to decrease the flood peak discharge and thus the size of the drainage facility. Flood hydrographs are also useful in environmental and land use analyses. <br>Hydrology is considered an interdisciplinary science because it borrows heavily from many other branches of science and integrates them for its own interpretation and uses. The supporting sciences required for hydrologic investigations include such things as physics, chemistry, biology, geology, fluid mechanics, mathematics, statistics, and the related research. Because 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. To this end, sound engineering judgment must be exercised to select the proper method or methods to be applied. Reference (<em>61</em>) is useful when comparing hydrologic methods. In some instances, certain Federal, State, or local agencies may require that a specific hydrologic method(s) be used for computing the runoff. <br>In this chapter, key aspects of hydrologic information relevant to highway engineering are discussed. The chapter is not intended to be all inclusive, but an effort has been made to cover as broad a spectrum of the subject as deemed appropriate, and references are cited for more detailed information. <br>¹ Italicized numbers in parentheses refer to publications in "References" (Section 2.11).