Restoration and Management of Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 474

17.4.4 d redGInG
The dredging of lake sediments physically removes sediments and the internal nutrient source they
represent by removing the organics stored in sediments that can contribute to nutrient releases and
oxygen demands as they decay (sediment diagenesis). Dredging also removes the stored nutrients.
Removal via dredging may have long-term beneits if it is conducted in conjunction with the con-
trol of external sources. Some of the issues involved in dredging include costs, short-term impacts
on benthic biota, and the disposal of potentially contaminated sediments. Examples of dredging
approaches are illustrated in Figure 17.4.

17.4.5 d ILutIon and f LuSHInG
Some of the same methods described in the previous section for sediment routing (bypass or pass-
through) may also be applicable to the management of sediments. Nutrient-poor waters may also
be diverted to lakes to aid in diluting and lushing nutrients. NALMS (1990) indicated that there
are relatively few documented cases of the effective use of dilution and lushing, primarily due to
the lack of availability or the expense involved in developing a suitable water source of suficient
volume. Well-documented cases include Green Lake in Seattle and Moses Lake in Grant County,
Washington. For Moses Lake, low-nutrient water from the Columbia River was diverted through
the lake at a rate of about three times the normal rate on an annual basis, to achieve a reduction of
nutrient concentrations on the order of 50%. For Green Lake, water from the municipal water source
was added, which resulted in a 90% decline in chlorophyll concentrations (Welch 1981).

Bucket
dredge

“Leakproof”
truck

Dragline

“Wet” dredging
Containment
area

Water

To
containment

Sediment

“Dry” dredging

Front-end
loader

Backhoe

To
containment

To
containment

Sediment

To
containment

Barge with
pump

Cable for navigation

Pipeline

Water

Suction
cutterhead

Sediment Hydraulic

dredging

FIGURE 17.4 Examples of dredging approaches. (From Holdren, C., Jones, W., and Taggart, J., Managing
Lakes and Reservoirs , North American Lake Management Society and Terrene Institute, in cooperation with
the Ofice of Water Assessment and Watershed Protection, U.S. Environmental Protection Agency, Madison,
W I, 20 01.)