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Repair of the dam crest

A 1.5 metre wide section of the crest has recently broken away from the main buttress of the dam on the TRHS lowering the water level by some 600mm and concentrating the overflow to this side of the dam face. The missing section of dam crest is lying exposed on a small boulder beach immediately downstream from the dam face.

Repair to the dam crest will be with materials matching those of the original construction with the new section of concrete and river cobbles keyed into that existing by drilling and grouting 25mm rebar at 300 mm centers. Access is possible from the TLHS and the work can be completed using only hand-carried tools.

Repair of the valve system requires the draining of water behind the dam and the exposure of the ends of the existing pipe work by removal of covering river gravels. This can be achieved by providing a temporary bypass diversion for the river flow and pumping down of the contained water behind the dam during a period of low river flow. The use of a hydraulic excavator to excavate to expose the ends of the diversion system is required and for other general lifting purposes on the site. Excavated river gravel material can be used for roading or foundation purposes or cast to waste over the dam crest. This aspect of the work is expected to be completed within a 10 day period of suitably fine weather.

Care will be exercised to preserve the one remaining vertical steel pipe to which a lifting system controlled water flow through the diversion. At least one of the other two original pipes is lying immediately adjacent to that standing below water level.

The possibility exists of re-instating all three pipes to their original position assuming that they are in a serviceable condition. Manually operated valves will be established over the up stream end of the two diversion pipes to be controlled above water level from the refurbished and remounted stand pipes.

Plate VI. Parapara Flat. General line of the diversion Channel. Headworks at right distance.

Environmental Impacts / Mitigation

The environmental impact of this work is expected to be minor. Any fish exposed during water level lowering operations will be temporarily re-placed upstream. 5mm aperture screens will be maintained around the pump suction intakes. Suitable first class controls will be established to avoid any discharge of any outside contaminants such as seeds, fuels, oil or cement into the environment while the work takes place. Only late-model, well maintained machinery and equipment will be on site and containment and removal procedures in the event of any spillage will be considered, documented and practiced prior to the commencement of any work.

All new work exposed above the water will be painted or surfaced to match that of the existing surfaces as closely as possible.

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