Tiêu chuẩn quốc tế

• Số hiệu / Standard Number: AASHTO HDG CHAPTER 2:2007
• Năm ban hành / Publication date: 2007
• Tình trạng / Status: A - Còn hiệu lực
• Tên tiếng Anh / Title in English: HYDROLOGY
• Thay thế cho / Replace: AASHTO HDG VOL II:1999
• Số trang / Page: 63

Phạm vi áp dụng / Scope of standard

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).¹
It is also defined as the science 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 science 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 design 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 drainage
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 engineer 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 significant 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 stream's flood hydrograph (discharge
over time). 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 lowflood 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 aggregate of the lesser
damage from frequently occurring floods is intolerable.

Besides the peak discharges, 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 design 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 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 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 methods. 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, but 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.

¹ Italicized numbers in parentheses refer to
publications in "References" (Section 2.11).