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164 Once the ducting is installed beneath the flume pipe the trench will be backfilled first with subsoil and secondly with the stored watercourse materials such that it is level with the rest of the bed of the watercourse. The watercourse banks will be reformed to their original profile with the excavated material and/or stone pitching and/or soil filled bags or other protection where necessary.


165 On completion of installation and backfilling, the flume pipes will be carefully removed to allow the river to flow over the excavated area as slowly as possible to minimise sediment generation.


11.6.3 Over-Pumping Technique 166


167


In this method, a temporary dam is constructed and the water is pumped around the trench.


The watercourse will be dammed at both sides of the proposed excavation with sand bags and water will be pumped over the area to be excavated. To prevent overland flood-flow, the top of the sandbag dams will be set lower than the bank-top so that during the works, in the event of pump failure water would return to the channel downstream of the work. Straw bales will be placed downstream to capture sediment, being replaced as required until reinstatement is complete.


168


The area of water between the dams will be inspected for fish and other aquatic life; any wildlife present will be carefully relocated to the other side of the dam, as set out in the Outline Landscape and Ecological Management Strategy. This area will then be pumped dry allowing the excavation to start in the dry river/ditch bed. Over pumping will require pumps with sufficient capacity for dealing with twice the volume of water should any sudden rise in the water level or flash flooding occur. Proposals for temporary dams will be subject to prior assessment of weather and water flow conditions.


169 Pumps will be positioned at the maximum practicable distance from the watercourse. Standby pumps will also be provided where the pumps are operated continuously. Where relevant, in consultation with the Environment Agency/Internal Drainage Board, pumps will be fitted with grills to prevent fish entering them. The pumped water is filtered, where practical, to remove sediment before being discharged back into the watercourse. Straw bales will be used to catch sediment. Sediments, break tanks or similar will also be used, where necessary, to break the fall of the released water in order to minimise scouring or sediment generation.


170 Small crossings will be excavated using a single machine generally digging from one side to the other. The trench excavation will be checked for sufficient depth of cover through the crossing.


Outline Code of Construction Practice. Version 3. Page 51


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