Highway Hydrology

Hydrology is often defined as the science that addresses the physical properties, occurrence,
and movement of water in the atmosphere, on the surface of, and in the outer crust of the earth.
This is an all-inclusive and somewhat controversial definition as there are individual bodies of
science dedicated to the study of various elements contained within this definition. Meteorology,
oceanography, and geohydrology, among others, are typical. For the highway designer, the
primary focus of hydrology is the water that moves on the earth's surface and in particular that
part that ultimately crosses transportation arterials (i.e., highway stream crossings). A
secondary interest is to provide interior drainage for roadways, median areas, and interchanges.
Hydrologists have been studying the flow or runoff of water over land for many decades, and
some rather sophisticated theories have been proposed to describe the process. Unfortunately,
most of these attempts have been only partially successful, not only because of the complexity
of the process and the many interactive factors involved, but also because of the stochastic
nature of rainfall, snowmelt, and other sources of water. Hydrologists have defined most of the
factors and parameters that influence surface runoff. However, for many of these surface runoff
factors, complete functional descriptions of their individual effects exist only in empirical form.
Their qualitative analysis requires extensive field data, empirically determined coefficients, and
sound judgment and experience.
By application of the principles and methods of modern hydrology, it is possible to obtain
solutions that are functionally acceptable and form the basis for the design of highway drainage
structures. It is the purpose of this manual to present some of these principles and techniques
and to explain their uses by illustrative examples. First, however, it is desirable to discuss some
of the basic hydrologic concepts that will be utilized throughout the manual and to discuss
hydrologic analysis as it relates to the highway stream-crossing problem.
In highway engineering, the diversity of drainage problems is broad and includes the design of
pavements, bridges, culverts, siphons, and other cross drainage structures for channels varying
from small streams to large rivers. Stable open channels and stormwater collection,
conveyance, and detention systems must be designed for both urban and rural areas. It is often
necessary to evaluate the impacts that future land use, proposed flood control and water supply
projects, and other planned and projected changes will have on the design of the highway
crossing. On the other hand, the designer also has a responsibility to adequately assess flood
potentials and environmental impacts that planned highway and stream crossings may have on
the watershed.
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