Chapter 4: Hydrology

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Section 1: Hydrology’s Role in Hydraulic Design

In the context of hydraulic design, hydrologic analysis provides estimates of flood magnitudes as a result of precipitation. These estimates consider processes in a watershed that transform precipitation to runoff and that transport water through the system to a project’s location.

The design of drainage facilities requires the designer to:

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  • Select the level of protection desired, specified in terms of probability of capacity exceedance.
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  • Find the corresponding flow rate and/or volume, computing in many cases the corresponding water surface elevation.
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  • Use that as a basis for design.

In the design of facilities such as storm drain systems, culverts, and bridges, floods are usually considered in terms of peak runoff or discharge in cubic feet per second or cubic meters per second. For systems that are designed to control the volume of runoff, such as detention storage facilities, or where flood routing through culverts is used, the discharge per time will be of interest. Thus, depending on the needs of a particular project, the hydrology study may provide:

These results may be obtained through statistical analysis of historical observations or through empirical or conceptual models of the relevant watershed and channel processes.

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