1、 CHAPTER 2 HYDROLOGY 2007 by the American Association of State Highway and Transportation Officials. 2007 by the American Association of State Highway and Transportation Officials.CHAPTER 2 TABLE OF CONTENTS 2.1 INTRODUCTION. 2-1 2.2 FACTORS AFFECTING FLOOD RUNOFF 2-2 2.2.1 Physiographic Characteris
2、tics 2-2 2.2.1.1 Drainage Area 2-3 2.2.1.2 Shape Factor. 2-4 2.2.1.3 Slope 2-4 2.2.1.4 Land Use 2-4 2.2.1.5 Soil and Geology 2-5 2.2.1.6 Storage Area - Volume 2-5 2.2.1.7 Elevation 2-6 2.2.1.8 Orientation of the Basin . 2-7 2.2.1.9 Configuration of Channel and Floodplain Geometry. 2-7 2.2.1.10 Strea
3、m and Drainage Densities . 2-7 2.2.2 Site-Specific Characteristics 2-8 2.2.2.1 Aggradation and Degradation 2-8 2.2.2.2 Ice and Debris 2-8 2.2.2.3 Seasonal and Progressive Changes in Vegetation 2-9 2.2.2.4 Channel Modifications. 2-9 2.2.3 Meteorological Characteristics 2-9 2.2.3.1 Rainfall. 2-10 2.2.
4、3.2 Snow 2-11 2.2.3.3 Temperature, Wind, Evaporation and Transpiration 2-11 2.2.3.4 Mixed Population Floods. 2-12 2.3 DATA SOURCES . 2-12 2.3.1 Categories of Hydrologic Data 2-13 2.3.2 Sources of Hydrologic Data. 2-13 2.3.2.1 Runoff Data 2-13 2.3.2.2 Rainfall Data 2-15 2.3.2.3 Flood History and Hist
5、orical Floods 2-16 2.3.2.4 Flood History of Existing Structures 2-16 2.3.2.5 Paleoflood Data 2-17 2.4 ELEMENTS OF RUNOFF PROCESS. 2-17 2.4.1 Infiltration 2-18 2.4.2 Detention and Depression Storage. 2-20 2.4.3 Stream Flow and Flood Hydrograph 2-20 2.4.4 Hydrograph Parameters . 2-22 2.4.4.1 Time of C
6、oncentration . 2-22 2.4.4.1.1 Overland Flow 2-22 2.4.4.1.2 Swale, Ditch or Stream Channel Flow. 2-23 2007 by the American Association of State Highway and Transportation Officials.Highway Drainage Guidelines 2-iv 2.4.4.1.3 Storm Drain or Culvert Flow2-24 2.4.4.2 Lag Time, Rise Time and Time to Peak
7、.2-24 2.4.5 Unit Hydrographs .2-25 2.5 MEASUREMENTS OF FLOOD MAGNITUDES.2-25 2.5.1 Direct Measurements of Flood Magnitudes .2-25 2.5.2 Indirect Measurements of Flood Magnitudes.2-26 2.5.3 Ordinary Highwater and Mean Annual Flood2-26 2.6 FLOOD PROBABILITY AND FREQUENCY AS APPLIED TO HIGHWAY HYDROLOGY
8、.2-27 2.6.1 Concepts of Probability and Frequency Analysis.2-27 2.6.2 Floods Considered in Hydrologic and Hydraulic Analysis 2-28 2.6.2.1 Base Flood and Super Flood.2-28 2.6.2.2 Overtopping Flood2-28 2.6.2.3 Design Flood.2-28 2.6.2.4 Maximum Historical Flood.2-29 2.6.2.5 Probable Maximum Flood 2-29
9、2.6.3 Design Flood Frequency.2-29 2.6.3.1 Policy Alternative.2-30 2.6.3.2 Economic Assessment Alternative .2-30 2.6.3.3 Highway Classification.2-31 2.6.3.4 Flood Hazard Criteria .2-31 2.6.3.4.1 Sensitivity to Increased Flood Magnitude 2-32 2.6.3.4.2 Loss of Life.2-32 2.6.3.4.3 Property Damages.2-32
10、2.6.3.4.4 Traffic Interruption.2-33 2.6.3.4.5 Economics and Budgetary Constraints.2-33 2.7 METHODS FOR ESTIMATING FLOOD PEAKS, DURATIONS AND VOLUMES 2-33 2.7.1 Individual Station Flood Frequency Analysis 2-34 2.7.1.1 Development of Flood-Frequency Curve .2-34 2.7.1.1.1 Graphical Method.2-34 2.7.1.1.
11、2 Mathematical Method.2-35 2.7.1.2 Extrapolating Flood-Frequency Curves2-35 2.7.1.3 Transfer of Data2-36 2.7.2 Regional Flood-Frequency Analysis 2-36 2.7.2.1 Index-Flood Method .2-36 2.7.2.2 Multiple Regression AnalysisWatershed Characteristics .2-36 2.7.2.2.1 USGS-FHWA Urban Method 2-37 2.7.2.2.2 U
12、SGS Regional or Local Rural Methods .2-38 2.7.2.3 Multiple Regression AnalysisChannel/Characteristics Method .2-38 2.7.3 Empirical Hydrologic Methods 2-39 2.7.3.1 Rational Method .2-39 2.7.3.2 British Method2-40 2007 by the American Association of State Highway and Transportation Officials.Hydrology
13、 2-v2.7.3.3 NRCS T.R. 55 Method. 2-40 2.7.4 Unit Hydrograph Methods . 2-40 2.7.4.1 Finite Time Unit Hydrograph. 2-41 2.7.4.2 Synthetic Unit Hydrograph 2-41 2.7.4.2.1 Ten-Minute Unit Hydrographs. 2-42 2.7.4.2.2 Dimensionless Hydrograph 2-42 2.7.5 Regional Hydrographs . 2-42 2.7.6 Mathematical Models.
14、 2-43 2.7.6.1 HYDRAIN Computer System 2-44 2.7.6.2 HEC-1/HEC-HMS Models 2-44 2.7.6.3 NRCS TR-20 Method 2-45 2.7.6.4 The Stormwater Management Model (SWMM) 2-45 2.7.6.5 The Stanford Watershed or Hydrocomp (HSP) Model 2-45 2.7.6.6 Penn State Urban Runoff Model 2-46 2.7.6.7 The Massachusetts Institute
15、of Technology Catchment (The MITCAT) Model 2-46 2.7.6.8 USACE STORM Model 2-46 2.7.6.9 ILLUDAS Model . 2-46 2.7.6.10 USGS “Dawdy” Model. 2-47 2.7.7 Accuracy of Methods for Estimating Peak Discharges 2-47 2.8 CHARACTERISTICS AND ANALYSIS OF LOW FLOWS 2-48 2.9 STORAGE AND FLOOD ROUTING FOR STORMWATER
16、MANAGEMENT. 2-49 2.9.1 Storage Characteristics. 2-49 2.9.2 Storage Size and Location . 2-50 2.9.3 Determination of Storage Volume and Flood Routing Procedures . 2-51 2.10 DOCUMENTATION. 2-51 2.11 REFERENCES. 2-52 2007 by the American Association of State Highway and Transportation Officials.Chapter
17、2 Hydrology 2.1 INTRODUCTION 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 (1).1It is also defined as the scien
18、ce that deals with the processes governing the depletion and replenishment of the water resources of the land areas of the earth (84). It is concerned with the transportation of water through the air, over the ground surface, and through the strata of the earth. Although hydrology is a very broad sc
19、ience 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. Hydrologic analysis is the most important step prior to the hydraulic d
20、esign 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 draina
21、ge 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 engin
22、eer 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. Highway drainage facilities are designed to convey predetermined discharges to avoid a significan
23、t 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 streams flood hydrograph (discharge over t
24、ime). 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. Also of interest is the performance of highway drainage facilities during the frequently occurring low-flood flow periods. Because
25、 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
26、 of the lesser damage from frequently occurring floods is intolerable. 1Italicized numbers in parentheses refer to publications in “References” (Section 2.11). 2007 by the American Association of State Highway and Transportation Officials.Highway Drainage Guidelines 2-2 Besides the peak discharges,
27、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 de
28、sign 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. Hydrology is considered an interdisciplinary science because it borrows heavily from many other branches of science and
29、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
30、 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 (61) is useful when comparing hydrologic metho
31、ds. In some instances, certain Federal, State, or local agencies may require that a specific hydrologic method(s) be used for computing the runoff. In this chapter, key aspects of hydrologic information relevant to highway engineering are discussed. The chapter is not intended to be all inclusive, b
32、ut 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. 2.2 FACTORS AFFECTING FLOOD RUNOFF The hydraulics engineer should become familiar with the many factors or characteristics that affect flood runoff be
33、fore making a hydrologic analysis. The peak discharge and volume of runoff are considered to be affected by similar factors, although the degree of influence by any given factor may be different between these two runoff categories. Factors affecting flood runoff can be broadly classified as physiogr
34、aphic, site specific, and meteorological; however, the three classes are interrelated in their flood-producing effects. In addition, components within such classes are so interrelated that experience and judgment are necessary to properly evaluate the various factors that apply to a particular situa
35、tion. There have been numerous studies that establish that some factors are more important than others in affecting peak discharge or volume of runoff. The dominant factors may vary with each individual site and hydrologic method. Some of the major factors related to runoff are discussed in the foll
36、owing sections. Those factors responsible for floods attributed to dam failures, tidal action and similar events are not presented. The physiographic, site-specific, and meteorological characteristics that may be used for flood runoff analysis are detailed in References (35), (36), (53), and (68). 2
37、.2.1 Physiographic Characteristics Physiographic factors may be grouped into two categories: basin characteristics and channel characteristics (22). Basin characteristics include such factors as size, shape, and slope of drainage area, soil permeability, and capacity of groundwater formations, prese
38、nce of lakes and swamps, and land use. Channel characteristics are related mostly to the hydraulic properties of the channel that 2007 by the American Association of State Highway and Transportation Officials.Hydrology 2-3govern the movement of stream flows and determine channel storage capacity. In
39、 evaluating the importance of various hydrologic characteristics for determining flood runoff (Section 2.7), it is often necessary to compare drainage basins; therefore, the hydraulics engineer should be familiar with drainage basin characteristics and how they affect flood runoff. Surface and subsu
40、rface runoff are collected and conveyed through the stream channels. The natural or altered condition of these channels can materially affect the volume and rate of runoff, so these conditions should be considered in the hydrologic and hydraulic analyses. The relative importance of physiographic cha
41、racteristics varies between different hydrologic areas and geologic and geographic regions. 2.2.1.1 Drainage Area A drainage basin is commonly surrounded by a readily discernible topographic divide, which is a line of separation that divides the precipitation that falls on two adjoining basins so th
42、at the ensuing runoff is directed into one or the other channel system. The size or area of the drainage basin is considered to be the area that contributes the surface runoff and is bounded by all or portions of the topographic divide. The size of a drainage basin is an important parameter with res
43、pect to the response of the basin to rainfall. Flood runoff in the same geographical area can be generally shown to be proportional to some power of the drainage area (Section 2.7.1.3). However, the effect of other basin characteristics often obscures the effect of drainage basin size alone. Determi
44、ning the size of the drainage area that contributes to flow at the drainage structure site is a basic step in a hydrologic analysis. The drainage area, usually expressed in hectares acres, or km2mi2, is determined from field surveys, topographic maps, aerial photographs, or a combination of these it
45、ems. Topographic maps are valuable aids in obtaining the size of drainage areas. The most commonly used topographic maps are those of the USGS. Information concerning these can be obtained from the USGS Information Center, Box 25286, Federal Center, Denver, CO 80225 or over the counter at various US
46、GS Earth Science Information Centers (see www.usgs.gov). Field inspection of the drainage area, especially for small basins, is very desirable because topographic maps are not always current. Although the contour maps may show many areas as contributing to the runoff, a field inspection may show nat
47、ural or man-made depressions such as gravel pits, playa lakes, or natural sinks, which may intercept a portion of the runoff from the drainage area. There may also be subtle topographic features that divert runoff from one watershed into another or indistinct divides not apparent on topographic maps
48、. Once the boundaries of the contributing areas have been established, they should be delineated on a base map and the areas determined. For urban areas, a local agencys sewer maps may be a valuable source of drainage boundary information. Accurate aerial photography supplemented by vertical and hor
49、izontal control surveys provides a means of measuring the size of a drainage area. Although uncontrolled aerial photographs aid the engineer and are of generally acceptable accuracy for large areas, the determination of the boundary of a drainage area by the photographs should be supplemented with field verification. 2007 by the American Association of State Highway and Transportation Officials.Highway Drainage Guidelines 2-4 Digital elevation models (DEMs) or digital terrain models (DTMs) are becoming increasing popular within the field of digital topographical data
