Environmental Engineering Reference
In-Depth Information
Land subsidence results from excessive groundwater extraction for water supply on a
regional basis, and from dewatering for excavations on a local basis, as described in
Section 10.2.
Groundwater and seepage control is required for a large number of situations including:
Excavations, to enable construction to proceed in the “dry” and to reduce exces-
sive pressures on the walls and bottom
Structures, to provide for dry basements and to prevent hydrostatic uplift on
slabs
Pavements, to provide protection against “pumping” and frost heave
Slopes, to provide for stabilization in either natural or cut conditions
Dams, to protect against excessive seepage through, beneath, or around an
embankment, which reduces stability and permits excessive storage loss. Dam
construction can also have a significant effect on the regional groundwater
regime, sometimes resulting in instability of slopes or surface subsidence.
Water quality is of concern in its various consumptive uses as well as for its possible dele-
terious effect on construction materials, primarily concrete.
8.2
Surface Water
8.2.1
Surface Hydrology
The Hydrological Cycle
Precipitation, in the form of rainfall or snowmelt, in part enters the ground by infiltration
to become groundwater, in part remains on the surface as runoff, and in part enters the
atmosphere by evaporation and transpiration to become a source of precipitation again.
The hydrological equation relating these factors can be written as
Infiltration
evaporation)
The conditions influencing the factors in the hydrological equation include climate,
topography, and geology.
precipitation
(runoff
transpiration
Climate affects all of the factors. In moist climates, precipitation is high but when
the ground is saturated runoff will also be high, and when vegetation is heavy,
transpiration will be high. In arid climates, loss of standing water through evap-
oration exceeds precipitation.
Topography impacts most significantly on runoff and evaporation. Steep slopes
encourage runoff and preclude significant infiltration. Gentle to flat slopes
impede runoff or result in standing water, permitting evaporation to occur.
Geologic conditions impact significantly on runoff and infiltration. Surficial or
shallow impervious materials result in high runoff and relatively little infiltra-
tion. Pervious surficial materials result in low runoff and high infiltration.
Precipitation
In some engineering applications, precipitation data are applied in runoff analyses to eval-
uate groundwater recharge and flood-prone zones, and to design drainage improvements,
such as culverts and channels, and spillways for dams.
Search WWH ::




Custom Search