www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 17 assigned permeability of 1.0 x 10-5m/s, a typical fresh andesite formation permeability of 2.5 x 10-8m/s, and for weathered tuff of 1.0 x 10-7m/s. Both the andesite and tuff formations were set to be free draining and the fractures zones were set to seal after 14 days. The analysis resulted in a baseflow loss to the Mataura Stream of 15m3/d, an amount considered insignificant compared to overall streamflow. 5.1.3. OTAHU CATCHMENT CHM AND NUMERICAL ANALYSIS The Otahu CHM is shown in Figure 5-2 with the following features: • The tunnel passes beneath the Waiharakeke Stream at a depth of 1,150m, crosses the headwaters of a second branch of the Waiharakeke and Thompson Streams, terminating short of the Wharekirauponga Stream. • The upper reaches of the surface water catchment are steep with a high surface run-off ratio expected. • Stream baseflow is expected to be supported largely by interflow and from the shallow regolith soils, with low flows fed by lithologically controlled bedrock discharge (confined to rhyolite outcroppings of limited extent) [personal communication with Chris Simpson]. • As the tunnel face advances groundwater is intercepted and depressurised. • As with the Waihou tunnel section, dewatering of the surface formations does not take place due to the absence of a hydraulic connection in the low permeability andesite bedrock between groundwater at depth and surficial aquifers. • Groundwater inflows were determined using groundwater elevations calculated with an algorithm that uses the observed vertical hydraulic gradients at Willow Farm to derive heads based on surface elevation. • Hydraulic gradients are largely influenced by surface topography. • Recharge to the surficial, unconfined, valley-fill aquifers and consequent groundwater levels are maintained by rainfall. Figure 5-2: Access Tunnel CHM for Otahu Groundwater Catchment (after GWS) The effects of the tunnel on springs and streamflow have been undertaken using numerical modelling in SEEP/W. The simulation of the tunnel passing beneath the Waiharakeke Stream assesses stream losses that might occur prior to mitigation being put in place. A second model section was also developed simulating the plane of the fault that depletion affects along the length of the stream. The model results indicate a maximum of up to 520m3/d could be diverted before mitigation is put in place. In the context
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