Chapter 5: Hydrology

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Section 1: Introduction

Description

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.

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Peak 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 typ