Sediment dredging is the partial or complete removal of sediment layers rich in nutrient and organic matter. This technique can be multipurpose, resulting in control of both phytoplankton and macrophytes. Dredging aims at (1) deepening of shallow lakes, (2) eliminating accumulated toxic substances, (3) reducing the release rate of P, and (4) controlling rooted macrophytes. Additionally, dredging serves to sustain several technical functions of lakes or reservoirs (pre dams, flood protection, shipping lane).
Scientific background. The accelerated deposition of sedi ments in lakes is explained by high input of eroded material or by intensified primary production and leads to undesired effects on the water quality. At a certain P loading, shallow lakes are more productive than deeper lakes. The enrich ment of organic sediments and the lower volume of lake water have a negative impact on the oxygen budget. The accumulated nutrients and harmful substances in the sedi ments can be mobilized either by resuspension or due to changed chemical conditions. Especially, the mobilization of accumulated P can lead to a delayed response to external P load reduction. The development of macrophytes may be positively or negatively influenced by dredging due to an increased stability of the sediment, reduction of the sludge layer above a viable seed bank, or removal of the seed bank. Dredging may also have adverse effects on the lake and surrounding area. Sediment removal can, for example, create considerable turbidity and lead to the mobilization ofheavy metals by oxidation, which can also negatively affect down stream areas. Dredging represents a major re engineering of a lake, and should not be undertaken without clear recogni tion of its full impact.
Techniques. A lake can be dredged by dry or wet exca vation, hydraulic and pneumatic dredging. For dry excavation, the lake is drained and the sediments are dewatered before they are removed with conventional excavation equipment such as backhoes, bulldozers, or draglines. For wet excavation, amphibious excavators or bucket dredges mounted on cranes are used. Hydraulic dredges are typically equipped with a cutter head to loosen sediments that are then mixed with water and pumped as slurry through a pipeline extending to the disposal area (e.g., polder). Polymer addition and mecha nical dewatering allow transport and further utilization. Pneumatic dredging is a new technique that uses air pressure to pump sediment out of the lake. Dredging is a very expensive method to remove the phosphorus from a lake. In many cases it did not successfully lower eutro phication by P control because the pool of temporary stored P in the sediment was low or the internal P cycle was determined by newly settled P.
Control of Physical and Biological Structure Physical manipulation
Light and nutrient availability for phytoplankton growth can be influenced by changes in the physical structure, such as destratification.
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