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3) Headworks and Diversion Channel & 4) Tunnel Construction
The PHSC diversion channel from the Parapara River to the headworks of the tunnel is required to be re-dug. A short section of this channel on the flat can be viewed at its northern limits extending into a low lying terrace of alluvial gravels for some 30 metres. The remainder of the 150 metres section through the Flat to the Parapara River has been filled with sediments over the past 100 years. This section of the proposed channel will be dug approximately on the same line as that of the original with dimensions of four metres width at surface level with 2:1 batters on the sides to give a 2 metre wide base 2 metres deep.
The excavated material is proposed to be placed on the western side of the excavation to facilitate access for future cleaning or for maintenance should the channel become blocked by flood debris from the Parapara River or through sediment build up over time. This track of three metres in width and approximately one metre high will also provide walking access to the pool at the head of the dam for recreation purposes. The present access is restricted to persons equipped for bush travel.
At the head of this channel, within the alluvial terrace at GR 81104915, concrete wing walls will feed water into the power system intake. This will include automated valves to control the flow input, an inclined screen to remove larger debris with an automated cleaning function and fluming into the tunnel portal, all contained within a structure of approximately 36m2 surface area set into the ground. On the southern side of this structure a vehicle car park is required. Initially this will be required for construction purposes and then for a maintenance vehicle and to remove debris from the screen. Provision can be made for additional parking spaces for recreational users use. Vehicle access to the embankment track to the Parapara River will be restricted by way of a locked vehicle gate with a pedestrian style.
Environmental Impacts / Mitigation
This work will involve the use of a hydraulic excavator to excavate and place material to form the channel embankment system and excavate for the control structure. Vegetation in the path of the channel on the Flat is not identified as unique or endangered. (Simpson P. 07.03) This channel is proposed to have a curved alignment for a pleasing visual effect.
The material to be removed has been placed in its current position over the past 100 years by fluviatile processes as a result of the dam construction. The release of a small proportion excavated material into the waterway during construction is thus not seen as a significant effect. Excavation of the channel will be programmed for the period in which the water level behind the dam is lowered for reconstruction purposes to minimise sediment contamination.
The work area will be the minimum required to complete construction and disturbance of vegetation will be limited to no more than one metre from the finished work profile. Locally excavated topsoil will be saved for re-spreading over all disturbed surfaces which are not required to be metalled for vehicle access. All excavated material from the channel is proposed to be utilised on site for foundation or landscaping purposes.
The water table in this area of Richmond Flat in the immediate vicinity of the channel will be lowered with potential impact on fish habitat. This impact is not seen as significant.( Deans N. 07.03) and the channel itself will provide an area of habitat of a different form as compensation.
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