SECTION 4 Groundwater Effects Assessment – Willows Farm Access Tunnel 15 4. Groundwater Effects Assessment – Willows Farm Access Tunnel 4.1 Characterisation of Tunnel Alignment 4.1.1 Physiography Figure 10 shows the proposed access tunnel in relation to the site topography. The site is shown to slope north eastwards towards the Mataura Stream. The slope is cut by one prominent north east trending side gully and several smaller gullies. Slopes range from approximately 1v:1.5h (33 degrees) to 1v:7.7h (8 degrees). The steeper slopes occur in the gullies while the shallow slopes occur closer to the Mataura Stream. The portal would be initiated on slopes up to approximately 1v:2.7h (22 degrees); the shaft on slopes up to 1v:3.5h (16 degrees); and the infrastructure on slopes up to 1v:5.8h (10 degrees). Figure 10 Willows Farm Site Topography 4.1.2 Hydrology The location of the Mataura surface water catchment is shown in Figure 11. This catchment is 6.5 km2 in size and drains southeast to join the Ohinemuri River. The Willows Farm property occupies approximately one third of the lower end of the catchment. The upper reaches of the catchment are steep and high run-off resulting in high stream flows is observed during and after rainfall. Stream baseflow is expected to be mostly sourced from the shallow regolith soils, with low flows fed by rockmass discharge. The tunnel crosses beneath the Mataura Stream in andesite at a depth of approximately 225 m and the position of the Mataura Stream where the tunnel passes beneath it is shown in Figure 11.
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