Determination of sediment removal depth
One objective of sediment removal can be the control of internal
nutrient release through the removal of nutrient-rich sediments.
sediments may have a sharp gradient of nutrient concentrations with depth into
the sediments, or a horizontal gradient over the reservoir.
A map of this
vertical and horizontal gradient should be made, as described in earlier para-
Lake Trummen, Sweden, is an example.
It was found (Bjork 1972) that
40 cm of silt had accumulated between 1940 and 1965, an interval during which
effluents from a flax mill and a wastewater treatment plant discharged to the
Sediments below this layer, under both aerobic and anaerobic condi-
tions, had distinctly lower phosphorus release rates. Thus, the depth of sed-
iment removal was judged to be 40 cm.
Stefan and Hanson (1979, 1980) described another method for determining
depth of sediment removal to control internal phosphorus release.
observed that in shallow Minnesota lakes, brief periods of summer thermal
stratification produced a sharp loss of dissolved oxygen in the hypolimnion,
followed by a high rate of phosphorus release from the anoxic sediments.
shallow, polymictic reservoirs (see Gaugush 1984 for case history), summer
wind storms disrupted the thermal stratification, mixed the lakes, and intro-
duced nutrient-rich water to the whole water column.
An algal bloom then
Stefan and Hanson calculated the depth that was required for the
lake to remain stratified for the entire
This depth became the
target depth for sediment removal.
This approach, however, requires a massive
volume of sediment removal.
Cooke et al. (1986) recommend the approach used
at Lake Trummen.
There is a direct relationship between transparency and the maximum
depth of colonization by submersed macrophytes.
each plant species may
have different light requirements and thus different depths to which it can
grow, it is possible to estimate the depth to which a reservoir would have to
be dredged in order to control nuisance submersed macrophyte growth through
et al. (1985) provide the following equations to
determine the maximum depth (in meters) of submersed macrophyte coloniza-
for Florida and Wisconsin lakes: