Theory and Design
Siltation basins
A siltation basin is used to detain incoming water long enough to allow
Water that flows
significant deposition of nutrients and particulate matter.
to the main reservoir should have greatly increased quality as a result, while
materials deposited in the siltation basin can be
The
design of a siltation basin will be site-specific in that management personnel
must know the rates of water, nutrient, and silt income and then calculate the
size of basin appropriate to detain water long enough to have significant
deposition.
In some cases, spring
summer flows may be the only tar-
get, since this.may be the water that is most stimulatory to algae or that
causes, in other ways, the greatest problems in water quality.
A basin to
intercept summer low flows would be much smaller than a basin to handle runoff
from wet seasons.
The reports by Jones and
(1978); Canfield, Jones,
and Walker (1987b) are useful in designing a basin that
and
will allow significant sedimentation of phosphorus.
Prereservoir phosphorus removal
Since upstream treatment of runoff or effluents may be impossible or
inadequate, a significant fraction of the incoming phosphorus could be precip-
itated in the stream or at the head of the reservoir through the addition of
iron, calcium, or aluminum salts.
This reservoir protection procedure has had
very few published applications, and further research is needed.
Iron is added in the ferric
form, usually as ferric chloride..
Iron in this state will precipitate as
or as
with inorganic
phosphorus sorbed to it (Stumm and Lee 1960, Wetzel 1983) and will be carried
in the streamflow and deposited in the stream and upper end of the reservoir.
Phosphorus will remain bound in iron complexes as long as the
potential
in the sediments remains high.
Unfortunately, the
potential can be very
low in the anoxic hypolimnia of eutrophic reservoirs.
A low
potential
reduce iron, and phosphorus will be released as iron complexes become
This
soluble (Mortimer 1941, 1942).
internal phosphorus release may be high
enough to stimulate algal growth.
If dissolved oxygen is present at the
sediment-water interface, or if dissolved oxygen is introduced through natural
or artificial circulation, then iron should remain in an oxidized state and
phosphorus will remain sorbed to it.
Thus, for an iron addition to be
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