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Wetted works - dumping the filter-bed

After extensive research it was considered feasible to dump the various filter materials on the river bed. This procedure involved two risks:
  • the risk of separation of the filter material (with larger particles becoming separated from smaller), thus potentially reducing the stability of the filter-bed.
  • the risk of mixing in sand when the filter material was dumped; the deposition of sludge between the various layers also reduces the stability of the filter-bed.
Special measuring techniques were used in the dumping-process. It proved possible to dump the various layers on each other to within 10 centimetres of the required thickness.

Various vessels were used in installing the filter-bed. A dredge was fitted out in order to suck up sand and to dump coarse sand and gravel. Another vessel (Jetsed) squirted air and water jets to prevent any sludge from being deposited between the various layers. The Jetsed is a self- propelling vessel that removes sediment deposits in the cunette between the filter layers. This is achieved by vertical air and water injection, thus throwing up the disturbed sediment, which is then borne away in the tidal currents.