Environmental Engineering Reference
In-Depth Information
Fig. 3.45 Half-barrel seepage meter installed in sandy sediment. Note side port, to which the hose
is connected, and top port with cap and vent hole with rubber stopper
The seepage bag, used to measure the volume of water that flows across the
sediment-water interface covered by the seepage cylinder, also can be made from a
variety of materials. A convenient bag volume is 3-4 L and thin-walled, flexible
bags are preferable. Lightweight freezer-storage bags have often been used. Avoid
using bags with thicker walls, such as medical intravenous (IV) bags or solar-
shower bags; these bags have a substantial resistance to expansion and contraction
in response to being filled or emptied. Use of these bags will substantially reduce
the volume of water that otherwise would flow across the bed covered by the
seepage cylinder. The opening of the bag can be gathered together around a hose
and taped to the hose so the fitting does not leak. Another option is to weld or
otherwise seal the bag opening and cut a small slit in one of the corners of the bag,
through which you will insert a hose or tube and tape the bag to the hose or tube. As
with the seepage cylinder, the bag and fittings should not leak under small pressures
but the assembly does not withstand large pressures. It is convenient to install
hardware that includes a valve that can be closed while the bag is being transported,
attached or removed from the seepage cylinder, and during subsequent handling
prior to being weighed or measured.
The bag should be placed in a shelter for several reasons: (1) to prevent the bag
from being exposed to currents, (2) to maintain the bag in a proper orientation, and
(3) to protect the bag from fish or mammals or waterfowl, a particularly important
consideration in many wetland settings. Many different types of bag shelters have
been used; examples are provided in Figs.
3.44
and
3.45
. Design and build a bag
shelter of your choosing, including a section of tubing or hose that will connect to
the side opening on the seepage cylinder. The hose or tubing should be approxi-
mately 1-2 m long, which ensures that you will not disturb the seepage cylinder
while attaching or removing the seepage-collection bag.
