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 18 of 30 The details of the temporary and permanent CWDs (including stream re-alignments) are described in the respective SSESCPs. 6.3.3. Sediment Control – Sediment Retention Ponds Sediment retention ponds (SRPs) are impoundment devices that provide time for suspended solids to settle out before the runoff is discharged to the receiving environment. The key design criteria for all SRPs that are to be utilised across the Project are set out below. • In general13, a minimum storage volume sized at a ratio of at least 3% of the contributing catchment (i.e. 300m3 / ha of catchment) and an ideal length to width ratio of 3:1, but not exceeding 5:1. The TR2009/02 recommended maximum contributing catchment of 5ha is acknowledged and adopted where practicable. In some cases, such as the Gladstone Pit, the contributing catchment exceeds that recommendation, but is considered the best practicable option and will achieve the same level of treatment as anticipated by a 5ha catchment pond. • Spillways formed to safely convey the 1% annual exceedance probability (AEP) rain event. • T-bar floating decants with a mechanism to control outflow such as a manual decant pulley system or screw on end cap. • Chemical treatment in accordance with the CTMP. 6.3.4. Decanting Earth Bunds Decanting earth bunds (DEBs) will be utilised to treat sediment laden runoff from areas of up to 3000m2. The DEB volumes will be sized at a ratio of 2% or 3% based on the slope steepness and slope length of the contributing catchment. DEBs will be chemically treated in accordance with the CTMP. 6.3.5. Silt Fences and Super Silt Fences Silt fences and super silt fences will be used to treat sediment-laden runoff from small areas that cannot be diverted to an alternative device. 6.3.6. Chemical Treatment To maximise their efficiency, chemical flocculant will be added to the inflow of all SRPs and DEBs using a rainfall activated or flow activated system. A Chemical Analysis Reactivity Test (CART) report has been provided with the CTMP in Appendix A. Bench testing has been completed on supplied soils from the footprint of the TSF3 and the NRS. Results indicate that chemical treatment, using Polyaluminium chloride (PAC), enhances the sediment removal efficiency and increases water clarity and turbidity over the control tests to an extent that justifies the implementation of chemical treatment. Chemical selection and recommended dose rate is based on the bench testing of soils and is discussed further in the CTMP and CART. The set-up for each device will be provided with the as-built documents and will include details of the chemical type and dosing rate. 6.3.7. Dewatering and Pumping Wherever possible, gravity flow into the various sediment retention devices will be used in preference to pumping. However, it may not always be possible to achieve gravity flow to sediment control devices during construction. Floating decants incorporated in the design of each sediment retention pond will be fitted with a mechanism to control outflow, such as a 13 The TFS3 and NRS sites require an alternative approach to SRP impoundment sizing. That is described in the SSESCP for that site as provided in Appendix C.
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