Erosion and Sediment Control Assessment Report Rev. 0 This is an uncontrolled copy if photocopied or printed from the Intranet. Copyright © 2016, SouthernSkies Environmental Ltd. All rights reserved. Page 24 of 30 9.1.4. Gladstone Open Pit The establishment of the Gladstone Open Pit (GOP) will include the construction of two SRPs. One SRP will be constructed to service a stockpile area to the southeast of the GOP with a catchment area of approximately 1.2ha. The other SRP is proposed to be constructed in between Gladstone Hill and Winner Hill and capture runoff from a total catchment area of up to 7.0ha. This catchment area exceeds the recommended maximum catchment area in TR2009/02. The reason for deviating from the guidelines for the initial stripping and earthworks area is to take advantage of existing contours and the site low point. This will also simplify site management. This SRP has been sized to provide a minimum storage volume of 3% of the contributing catchment and is anticipated to achieve the same performance standard required in TR2009/02. The GOP portal catchment area (approximately 1ha) will be managed on site until such time as the runoff can be contained within the GOP. The details for how the portal will be constructed as to be confirmed and the required ESC measures will be confirmed as part of the SSESCP. Once the GOP becomes inverted and all runoff is contained within the pit, water will be pumped to the existing WTP, and the SRP will be decommissioned. For completeness, it is reiterated that all other ESCs associated with the GOP will be designed, constructed, and maintained in accordance with TR2009/02. 9.2. Estimate of Sediment Yield Typically, an estimate of sediment yield is undertaken to assist in the assessment of potential sediment-related effects of significant earthworks activities for which consent is sought. Sediment yield is the amount of sediment that is discharged to the receiving environment. If no ESCs are implemented, sediment yield will be high. With the adoption of appropriate ESCs the sediment yield will be acceptably low. The estimate of sediment yield is most commonly generated using the Universal Soil Loss Equation (USLE) which takes account of the following variables: • Slope length and gradient • Soil type • Rainfall • Surface cover • Surface roughness • Earthworks area • Time of exposure Undertaking an estimate of sediment yield for this Project is complex because as soon as PAF material is exposed and / or imported into a site, all runoff from those areas will be diverted to the Water Treatment Plant and will not contribute to a potential elevated sediment yield. Rather, any such areas will result in a reduction in sediment yield from the corresponding area, including when assessed against the existing vegetated land use. An estimate of sediment yield has been undertaken for two sites, the NRS and TSF3, which has taken account of these specific characteristic of the development. Comment on Wharekirauponga Project works have not been included in the indicative estimate of sediment yield, due to the significant excess storage that will be achieved by treated runoff from much of those works areas through the Collection Pond and Retention Pond.
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