Chapter 5: Hydrology
Anchor: #i1025392Section 1: Introduction
Anchor: #i1025397Description
For the purpose of this manual, hydrology deals with estimating flood magnitudes as the result of precipitation. In the design of highway drainage structures, floods are usually considered in terms of peak runoff or discharge in cubic feet per second (cfs) or cubic meters per second (m3/s) and hydrographs as discharge per time. Use peak discharge 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.
Fundamental to the design of drainage facilities are analyses of peak rate of runoff, volume of runoff, and time distribution of flow.
Errors in the estimates result in a structure that is either undersized, which could cause drainage problems, or oversized, which costs more than necessary. On the other hand, 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.
Because hydrology is not an exact science, different hydrologic methods developed for determining flood runoff may produce different results for a particular situation. Therefore, exercise sound engineering judgment to select the proper method or methods to be applied. In some instances, certain federal or state agencies may require (or local agencies may recommend) a specific hydrologic method for computing the runoff.
While performing the hydrologic analysis and hydraulic design of highway drainage facilities, the hydraulic engineer should recognize and evaluate potential environmental problems that would impact the specific design of a structure early in the design process.
Most complaints relating to highway drainage facilities stem from the impact to existing hydrologic and hydraulic characteristics. In order to minimize the potential for valid complaints, gather complete data reflecting existing drainage characteristics during design.
Anchor: #i1025463Peak Discharge versus Frequency Relations
Highway drainage facilities are designed to convey predetermined discharges in order to avoid significant flood hazards. Provisions are also made to convey floods in excess of the predetermined discharges in a manner that minimizes the hazards. Flood discharges are often referred to as peak discharges as they occur at the peak of the stream’s flood hydrograph (discharge over time). Peak discharge magnitudes are a function of their expected frequency of occurrence, which in turn relates to the magnitude of the potential damage and hazard. (All the methods described in this manual allow determination of peak discharge.)
The highway designer’s chief interest in hydrology rests in estimating runoff and peak discharges for the design of highway drainage facilities. The highway drainage designer is particularly interested in development of a flood versus frequency relation, a tabulation of peak discharges versus the probability of occurrence or exceedance.
The flood frequency relation is usually represented by a flood frequency curve. A typical flood frequency curve is illustrated in Figure 5-1. In this example, the discharge is plotted on the ordinate 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.
Figure 5-1. Typical Flood Frequency Curve
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 to design a drainage facility to convey a large, infrequently occurring flood with an acceptable amount of floodplain damage only to find that the accumulation of damage from frequently occurring floods is intolerable.
Anchor: #i1025514Flood Hydrographs
In addition to peak discharges, the hydraulics engineer is sometimes interested in the flood volume and time distribution of runoff. You can use flood hydrographs to route floods through culverts, flood storage structures, and other highway facilities.
By accounting for the stored flood volume, the hydraulics engineer can often expect lower flood peak discharges and smaller required drainage facilities than would be expected without considering storage volume. You can also use flood hydrographs for estimating inundation times of flow over roadways and pollutant and sediment transport analyses.
Anchor: #i1025543Unit Hydrograph
A unit hydrograph represents the response of a watershed to a rainfall excess of unit volume and specific duration. For department practice, the unit is 1 in. (1 mm) — that is, the volume associated with an excess rainfall of 1 in. (1 mm) distributed over the entire contributing area.
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. For example, the runoff hydrograph resulting from a two-unit pulse of rainfall of a specific duration would have ordinates that are twice as large as those resulting from a one-unit pulse of rainfall of the same duration. Also, the hydrograph resulting from the sequence of two one-unit pulses of rainfall can be found by the superposition of two one-unit hydrographs. Thus, by determining a unit hydrograph for a watershed, you can determine the flood hydrograph resulting from any measured or design rainfall using these two principles.
Anchor: #i1025567Interagency Coordination
Because many levels of government plan, design, and construct highway and water resource projects that might affect each other, interagency coordination is desirable and often necessary. In addition, agencies can share data and experiences within project areas to assist in the completion of accurate hydrologic analysis. (See the Environmental Procedures in Project Development Manual for more information on interagency coordination.)