Geoscience Reference
In-Depth Information
Atmospheric Water
Clouds
Clouds
Evapotranspiration
(from plants and
inland waters)
Precipitation
Transpiraton
Foliage
Hills
Hills
River
Evaporation
Hills
Lake
Estuary
OCEAN
FIGURE 25.1
Natural water cycle.
The hydrologic cycle is complex, and to simulate just a small portion of it, such as the relationship
between precipitation and surface runoff, can be an inexact science. Many variables and dynamic
relationships must be accounted for and, in most cases, reduced to basic assumptions. However,
these simplifications and assumptions make it possible to develop solutions to the flooding, erosion,
and water quality impacts associated with changes in land cover and hydrologic characteristics.
Proposed engineering solutions typically involve identifying a storm frequency as a benchmark
for controlling these impacts. The 2-year, 10-year, and 100-year frequency storms have tradition-
ally been used for hydrological modeling, followed by an engineered solution designed to offset
increased peak flow rates. The hydraulic calculations inherent in this process are dependent upon
the engineer's ability to predict the amount of rainfall and its intensity. Recognizing that the fre-
quency of a specific rainfall depth or duration is developed from a statistical analysis of historical
rainfall data, the engineer cannot presume to accurately predict the characteristics of a future storm
event. This section provides guidance for preparing acceptable calculations for various elements of
the hydrologic and hydraulic analysis of a watershed.
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