Geoscience Reference
In-Depth Information
the water level in the well no longer changes and remains constant. At this time we
can start to measure and evaluate the fl ow of the groundwater. The difference of the
water levels in our two observation wells divided by their horizontal separation
distance is the driving force of the fl ow, or more exactly expressed, it is the potential
gradient. We assume that the groundwater level has a smooth, continuous slope. The
groundwater fl ows in the direction of this slope.
When we are in the valley of a river and the slope of the groundwater level leads
from the river into the sediments of the valley, we know that the river is supplying the
surrounding sediments and soils with water. This process may be especially important
during a long-lasting rainless period of time if the vegetation in the valley can be
adequately supplied with water coming from the river by groundwater fl ow. The
accessibility of the river water to plant roots depends upon the depth of the ground-
water level. We will deal with this aspect in detail later on. An opposite situation exists
whenever the groundwater level is inclined from the surrounding alluvial sediments
toward the riverbed. Groundwater would then fl ow from the valley sediments to the
river. Hence, with the river being supplied by groundwater, the valley sediments are
being drained by the river. We could easily verify this situation from our two observa-
tion wells because the water in the well close to the river would be lower than the
water in the well more distant from the river. A combination of both situations also
happens sometimes when one portion of the valley is drained by the river and another
portion of the region is supplied by groundwater streaming from the river.
The fl ow rate of the groundwater depends upon two factors. The fi rst is the slope
of the groundwater level and we wrote about it. In practice for a given soil, the
greater slope of the water level, the higher is the fl ow rate. We know already that if
the words water level are replaced by the word
potential
and if the slope of the water
level is substituted by
potential gradient
, we are more exact. Later on, we shall
show the advantage and real benefi t of using the second term when we write about
unsaturated fl ow. Now back to saturated fl ow. The second important factor is the
property of soil to allow water to penetrate into it or be conducted through it - a
characteristic denoted by the expression, by saturated hydraulic conductivity, or
sometimes simply by conductivity. It has the dimension of velocity (frequently m/
day or cm/h) and is equal to the fl ow rate when the hydraulic slope is unity.
Basic equations were formulated in the middle of the nineteenth century by the
French engineer Henry Darcy (1803-1858). After graduation at the most famous
engineering school of that time, School of Bridges and Roads (
L'Ecole des Ponts et
Chaussées
), he spent the majority of his life in Dijon, the town in Cote d'Or in the
Burgundy region. Today, the region is famous for its wine. Much earlier when deci-
mated by the plague, Dijon was well known in all of Europe for its extremely bad
water quality. During that time period, Darcy decided to try to construct a new,
modern, sanitary, and effi cient water supply available to all inhabitants of the town.
He studied theory of hydraulics and performed many experiments to test his new
theoretical developments on fl ow of water in pipes throughout the town distribution
system. Among other new approaches, he discovered the law how water fl ows in
sediments and in sand fi lters used for improvement of river water quality. He con-
sidered the groundwaters in springs located in alluvial sediments as the best source
